Somatotropin-induced protein anabolism in hindquarters and
portal-drained viscera of growing pigs
Jill A.
Bush1,
Douglas G.
Burrin1,
Agus
Suryawan1,
Pamela M. J.
O'Connor1,
Hanh V.
Nguyen1,
Peter J.
Reeds1,
,
Norman C.
Steele2,
Johannes B.
Van Goudoever1, and
Teresa A.
Davis1
1 United States Department of
Agriculture/Agricultural Research Service Children's Nutrition
Research Center, Department of Pediatrics, Baylor College of
Medicine, Houston, Texas 77030; and 2 United States
Department of Agriculture/Agricultural Research Service Growth
Biology Laboratory, Beltsville, Maryland 20705
 |
ABSTRACT |
To
differentiate the effect of somatotropin (ST) treatment on protein
metabolism in the hindquarter (HQ) and portal-drained viscera (PDV),
growing swine (n = 20) treated with ST (0 or 150 µg · kg
1 · day
1)
for 7 days were infused intravenously with
NaH13CO3 and
[2H5]phenylalanine and enterally with
[1-13C]phenylalanine while in the fed state. Arterial,
portal venous, and vena cava whole blood samples, breath samples, and
blood flow measurements were obtained for determination of tissue and
whole body phenylalanine kinetics under steady-state conditions. In the
fed state, ST treatment decreased whole body phenylalanine flux,
oxidation, and protein degradation without altering protein synthesis,
resulting in an improvement in whole body net protein balance. Blood
flow to the HQ (+80%), but not to the PDV, was increased with ST
treatment. In the HQ and PDV, ST increased phenylalanine uptake (+44
and +23%, respectively) and protein synthesis (+43 and +41%,
respectively), with no effect on protein degradation. In ST-treated and
control pigs, phenylalanine was oxidized in the PDV (34-43% of
enteral and arterial sources) but not the HQ. In both treatment groups,
dietary (40%) rather than arterial (10%) extraction of phenylalanine
predominated in gut amino acid metabolism, whereas localized blood flow
influenced HQ amino acid metabolism. The results indicate that ST
increases protein anabolism in young, growing swine by increasing
protein synthesis in the HQ and PDV, with no effect on protein
degradation. Differing results between the whole body and the HQ and
PDV suggest that the effect of ST treatment on protein metabolism is
tissue specific.
growth hormone; protein synthesis; protein degradation; amino acid kinetics; muscle
 |
INTRODUCTION |
A PRIMARY GOAL OF EXOGENOUS SOMATOTROPIN (ST)
treatment is to increase lean body mass. This is, in part, accomplished
by the ST-induced increase in the overall efficiency with which dietary amino acids are used for protein deposition (18,
20). ST administration also decreases blood urea
nitrogen concentrations (4, 15, 45, 48) and whole body
leucine oxidation (45), suggesting a reduction in amino
acid catabolism.
Most research in ST-deficient (2, 19, 37), as well as
normal, mature animals and humans (1, 7, 18, 24, 25, 35)
suggests that ST treatment increases protein deposition by stimulating
whole body and muscle protein synthesis. For example, acute ST infusion
in adult humans increases limb protein synthesis (24, 25),
although differing results have also been reported (11).
Chronic ST treatment in cattle increases amino acid uptake by the
hindquarter (4) and protein synthesis in skeletal muscle (18). Less is known about its effects on protein
degradation, particularly at the tissue level. The equivocal findings
of the ST-induced effects on protein synthesis and protein degradation in previous studies may be due to different developmental stages of the
subject population (i.e., growing vs. mature), species studied,
nutrient status of the subject (i.e., fasted or fed), tissues analyzed,
and length and/or mode of ST treatment (4, 5, 17, 24, 25, 30, 37,
38, 42, 45).
Recently, studies in our laboratory have suggested that ST treatment
for 7 days in young, growing swine enhances metabolic efficiency by
minimizing protein loss during fasting and maximizing protein gain
during meal absorption (45, 46). These studies demonstrated that ST treatment improves protein balance by maintaining higher rates of whole body protein synthesis in the postabsorptive condition (45). Feeding increases whole body protein
synthesis, but the feeding-induced increase in protein synthesis is not
significantly greater in ST-treated than in control pigs
(46). Whole body protein degradation rates in the fed
condition are reduced with ST treatment, thereby enhancing whole body
protein balance. Thus the results suggest that ST treatment attenuates
the reduction in whole body protein synthesis that occurs with fasting,
further reduces whole body proteolysis that occurs with feeding, and
reduces amino acid catabolism in both the postabsorptive and
postprandial states. The conservation of amino acids results in an
improvement in protein balance and likely contributes to the reduction
in circulating amino acid concentrations that we have observed in fully
fed ST-treated pigs.
In the current studies, we wished to identify the tissue-specific
responses of protein synthesis and protein degradation to 7 days of
exogenous ST treatment in fully fed, young, growing swine. Amino acid
kinetics were measured in vivo in the hindquarter (HQ) and
portal-drained viscera (PDV) with a dual stable isotope tracer/mass
transorgan balance technique. Studies were performed in rapidly growing
pigs (~25 kg) in which protein intake and ST treatment were
rigorously controlled over a 7-day treatment period and during a 6-h
isotope infusion study to ensure fully fed and steady-state conditions.
The effects of ST treatment on whole body phenylalanine oxidation were
reported in a recent paper on ureagenesis (6), but for
completeness, oxidation data are presented herein together with data
for whole body phenylalanine turnover.
 |
MATERIALS AND METHODS |
Animals and dietary intake.
The protocol, previously described by Bush et al. (6), was
approved by the Animal Care and Use Committee of Baylor College of
Medicine and was conducted in accordance with the National Research
Council's Guide for the Care and Use of Laboratory Animals. Housing and care of the animals conformed to US Department of Agriculture guidelines. Twenty crossbred (Landrace × Yorkshire × Hampshire × Duroc) female pigs were purchased
from the Agriculture Headquarters at the Texas Department of Criminal
Justice, Huntsville, TX. The pigs were received at the Baylor College
of Medicine Animal Facility at 8-10 wk of age, weighing ~10 kg,
and were housed in individual cages. During the 2-wk acclimation
period, pigs were fed a 24% high-protein dry diet (Producers
Cooperative Association, Bryan, TX) at a rate of 6% of body weight per
day. Pigs were weighed every other day to calculate feed intake at 6%
body weight, thus ensuring that ~90% of ad libitum intake for pigs
of this age was consumed. Pigs were offered food in two equal amounts
(one-half the total amount of feed, twice daily) each day at 0800 and
1500. Pigs generally consumed all of the food presented to
them; any unconsumed food was accounted for when estimating daily food
intake and feed efficiency. Water was continuously available. The pigs received ~1,750
kJ · kg
1 · day
1
metabolizable energy in the dry matter diet, which consisted of protein
(263.5 g/kg from soybean meal), carbohydrate (470.9 g/kg from crude
fiber), fat (69.6 g/kg from soybean oil, rice, and corn), vitamins
(0.436 g/kg), minerals (17.264 g/kg), ash (76.010 g/kg), and moisture
(89.201 g/kg).
Surgery.
After the 2-wk orientation period, pigs were fasted overnight, and the
carotid artery, jugular vein, hepatic portal vein, inferior caudal vena
cava (inferior to the renal veins and superior to the common iliac
vein), and duodenum were catheterized using sterile techniques and
general anesthesia, as previously described (13). The
catheters were flushed with 100 U heparin/ml saline and tied off to
prevent discharge. Perivascular flow probes (Transonic Systems, Ithaca,
NY) were secured around the hepatic portal vein and inferior caudal
aorta, adjacent to the vena cava catheter insertion site, for
measurement of blood flow. Catheters and flow probes were externalized
and enclosed within a pocket of a specialized swine jacket (Lomir
Biomedical, Harvard Apparatus, Holliston, MA).
Postoperatively, pigs received intravenous nutrition (a nutrient
solution of 100 ml · kg
1 · day
1)
during the 2- to 3-day recovery period before returning to their normal
dietary regimen (dry matter diet). The elemental nutrient solution
consisted of glucose (104 g/l), lipid (21 g/l; Intralipid, Baxter
Healthcare, Deerfield, IL), a complete amino acid mixture (55 g/l;
Ajinomoto, Tokyo, Japan), electrolytes, and trace minerals sufficient
to meet or exceed the requirements for young, growing pigs (5,
33, 34, 39). Intravenous antibiotics (enrofloxacin, 2.5-5.0
mg/kg) were administered daily to prevent infection. Intramuscular injections of a mild pain reliever (butorphanol tartate, 0.01 mg/kg)
were given 1 day after surgery to reduce the sensation of pain from
surgical intervention.
Experimental design.
Pigs were weight-matched and randomly assigned to either the control
(saline, n = 10) or recombinant porcine ST group
(n = 10) (Southern Cross Biotech, Australia). The ST
was administered at a concentration of 150 µg · kg
1 · day
1
for a 7-day period. This dose has been shown to be effective in
increasing protein deposition and reducing blood urea nitrogen in
domestic animals (4, 9, 26, 45, 46, 48). The dose of ST
was divided into two equal daily injections and administered into
alternating HQ musculature concurrently with the feeding sessions.
Control pigs received equal volume injections of saline. To minimize
the confounding effect of differences in feed intake, control pigs were
pair-fed to the level of their respective weight-matched ST-treated
counterparts during the 7-day treatment period.
Infusions.
On the morning of infusions, overnight-fasted pigs were given
their final injection of ST (150 µg · kg
1 · day
1)
and secured in a swine hammock (Walter Terry Distributor,
Houston, TX). To ensure that pigs were in the fully fed state
throughout the infusion period, control and ST-treated pigs
were infused intraduodenally for 7 h with the nutrient
solution (11 ml · kg
1 · h
1;
37.5 kJ · kg
1 · h
1,
0.54 g amino
acid · kg
1 · h
1)
beginning 1 h before the onset of the tracer infusion. To estimate CO2 production rate, a primed (7.5 µmol/kg), continuous
(10 µmol · kg
1 · h
1)
intravenous infusion of NaH13CO3 (Cambridge
Isotope Laboratories, Andover, MA) was performed from 0 to 120 min.
Arterial whole blood (1.0 ml) and breath samples were obtained at
baseline and every 15 min throughout the 2-h NaH13CO3 infusion for analysis of steady-state
CO2 production rate.
To quantify phenylalanine kinetics in the HQ and PDV, a primed (10 µmol/kg), continuous (10 µmol · kg
1 · h
1)
intravenous infusion of [2H5]phenylalanine
(Cambridge Isotope Laboratories) and a primed (20 µmol/kg),
continuous (20 µmol · kg
1 · h
1)
intraduodenal infusion of [1-13C]phenylalanine (Cambridge
Isotope Laboratories) were performed from 120 to 360 min. Volume blood
flow rate (ml/min) measurements were obtained by using simultaneous
transit-time ultrasound and blood sampling by a dual-channel flowmeter
(T206, Transonic Systems).
Arterial, portal venous, and vena cava whole blood samples (1.0 ml)
were obtained at baseline and every 30 min throughout the 4-h
phenylalanine tracer infusions for analysis of steady-state isotopic
enrichment of [1-13C]- and
[2H5]phenylalanine and concentrations of
whole blood amino acid, glucose, and CO2. Arterial blood
(1.0 ml) was also taken at 0, 240, and 360 min for measurement of
plasma urea nitrogen (PUN) and insulin-like growth factor I (IGF-I).
Breath samples were obtained at baseline and every 30 min throughout
the 7-h infusion study for isotopic enrichment of expired
13CO2. At the end of the 7-h infusion study,
pigs under pentobarbital sodium anesthesia were killed via exsanguination.
Hormone and substrate concentrations.
Heparinized blood (1.0-ml) samples were obtained and centrifuged at
2,500 g for 15 min at 4°C, and the plasma was stored at
80°C until analyzed for IGF-I and PUN concentrations. Plasma IGF-I
concentrations were analyzed in duplicate via two-site
immunoradiometric assay with prior extraction (Diagnostic Systems
Laboratories, Webster, TX). PUN concentrations were analyzed in
duplicate via an end-point colorimetric assay in which urease reacts to
generate ammonia, which then reacts with bromophenol blue (Vitros
Chemistry Products, Johnson & Johnson Clinical Diagnostics, Rochester,
NY). Blood glucose concentrations were rapidly analyzed by a glucose oxidase reaction (YSI 2300 STAT Plus, Yellow Springs Instruments, Yellow Springs, OH).
Heparinized whole blood (1.0 ml) was obtained for analysis of amino
acid concentrations via reverse-phase HPLC, as previously described
(14). Briefly, whole blood spiked with methionine sulfone
(internal standard) was filtered through a 10,000 molecular weight
filter. Phenylalanine and tyrosine concentrations were precolumn-derivatized with phenyl isothiocyanate, separated on a
PICO-TAG reverse-phase column (Waters, Milford, MA), and detected on-line by spectrophotometry. Concentrations were calculated with the
use of an amino acid standard (Pierce Chemical, Rockford, IL).
Analysis of tracer enrichment.
Blood and breath samples for 13CO2 production
were analyzed using isotope ratio mass spectrometry (IRMS; ANCA,
RoboPrep-G, Europa Instruments, Crewe, UK). Briefly, to estimate
13CO2 production rate, an aliquot of whole
blood (1.0 ml) was placed in a 10-ml vacutainer (Becton Dickinson,
Franklin Lakes, NJ) containing 1.0 ml of perchloric acid (10% wt/wt),
gently mixed, and placed on ice for 60 s. Room air was filtered
via a soda lime filter (Sodasorb, Grace Container Products, Lexington,
MA) to obtain air devoid of CO2. The air (~8 ml) was
injected into the 10-ml vacutainer containing 1:1 whole
blood-perchloric acid. Eight to ten milliliters of gas were withdrawn
and transferred to a sterile 10-ml vacutainer for subsequent analysis
of isotopic enrichment of 13CO2 via continuous
flow GCIRMS. To analyze expired 13CO2, breath
samples were collected from the pig via a two-way valve breath bag for
30 s. The collected expired air was injected into a sterile 10-ml
vacutainer for subsequent analysis of isotopic enrichment of
13CO2 via IRMS.
Mass spectrometric analysis of whole blood [1-13C]- and
[2H5]phenylalanine was conducted via
heptafluorobutyric anhydride derivative (12).
Phenylalanine was isolated via cation exchange chromatography (AG-50W
resin, Bio-Rad, Hercules, CA). The isotopic enrichment of derivatized
[1-13C]- and [2H5]phenylalanine
was determined by negative chemical ionization GC-MS (Hewlett-Packard
5890 Series II GC equipped with a Europa Orchid 20/20 stable isotope
analyzer; Hewlett-Packard, Palo Alto, CA) by monitoring the
mass-to-charge ratio of ions at 383/384 and 383/388, respectively.
Calculations.
A schematic model of phenylalanine kinetics across the HQ and PDV is
shown in Fig. 1. Calculations for whole
body phenylalanine turnover and phenylalanine kinetics across the HQ
and PDV are provided in the APPENDIX.

View larger version (14K):
[in this window]
[in a new window]
|
Fig. 1.
Schematic representation of the metabolic fate of
systemic phenylalanine in the hindquarters (A) and systemic
and enteral phenylalanine in the portal-drained viscera (B).
A, arterial flux of phenylalanine; A', dietary intake of phenylalanine;
B, venous outflow of phenylalanine; C, tissue utilization of
phenylalanine; D, oxidation of phenylalanine; E, phenylalanine used for
protein synthesis; F, phenylalanine released from protein degradation;
G, nonmetabolized systemic phenylalanine; G', nonmetabolized
systemic and dietary phenylalanine; and H, recycled phenylalanine in
the systemic circulation derived from dietary phenylalanine intake.
|
|
Statistics.
Individual t-tests were performed on the data to detect
significant differences among treatment groups for phenylalanine
kinetics across the HQ and PDV. ANOVA with repeated measures was used
to detect changes with treatment during sampling over the 7-h infusion for hormone and substrate concentrations and isotopic enrichments. Three control pigs and one ST-treated pig died during the treatment period because of complications following surgery; therefore, the
resulting sample size was seven for the control group and nine for the
ST-treated group. Results are presented as means ± SD.
Probability values of P
0.05 were considered
statistically significant.
 |
RESULTS |
Animal growth.
Body weight did not differ significantly (P = 0.2)
between ST-treated and control pigs before the treatment period began
(17.1 ± 1.6 vs. 15.6 ± 1.7 kg, respectively). At the end of
the 7-day treatment period, the body weight of the ST-treated pigs was
significantly (P < 0.05) greater than that of the
control pigs (22.1 ± 2.2 vs. 19.5 ± 2.3 kg, respectively).
The weight-scaled average daily gain tended to be higher
(P = 0.1) in the ST-treated than in the control pigs
during the 7-day treatment period (33.4 ± 6.2 vs. 27.2 ± 11.5 g · kg
1 · day
1,
respectively). Feed efficiency, as reflected by the gain-to-feed ratio,
during the 7-day treatment period was not statistically different
(P = 0.2) between ST-treated (0.56 ± 0.10 g
gain/g intake) and control pigs (0.48 ± 0.18 g gain/g intake).
Hormone and substrate concentrations.
To verify the effectiveness of ST treatment in growing pigs,
circulating concentrations of IGF-I and PUN were determined. As
expected, IGF-I concentration was significantly (P < 0.001) higher (+464%) in the ST-treated group than in controls
(352.0 ± 126.2 vs. 62.3 ± 24.1 ng/ml, respectively). There
was a significant (P < 0.001) decrease (
46%) in PUN
concentrations after 7 days of ST treatment compared with controls
(9.7 ± 2.3 vs. 18.2 ± 2.7 mg/dl, respectively). Blood
glucose concentrations increased (+25%) significantly
(P < 0.04) in the ST-treated group compared with controls (151.4 ± 31.3 vs. 120.2 ± 9.2 mg/dl, respectively).
Phenylalanine and tyrosine concentrations.
Phenylalanine concentrations were lower in ST-treated than in control
pigs in the arterial (
8%), portal venous (
9%), and vena cava
(
11%) circulation (Table 1). Tyrosine
concentrations were also lower in ST-treated than in control pigs in
the arterial (
150%), portal venous (
170%), and vena cava
(
176%) circulation.
View this table:
[in this window]
[in a new window]
|
Table 1.
Phenylalanine and tyrosine concentrations in arterial, portal venous,
and vena cava circulation in fed ST-treated and control pigs
|
|
Whole body phenylalanine turnover.
Isotope-labeled tracers (i.e., [1-13C]- and
[2H5]phenylalanine and
NaH13CO3) were utilized to estimate whole body
phenylalanine turnover under steady-state conditions. There was no
significant difference in whole body phenylalanine flux when calculated
with either the [1-13C]- or the
[2H5]phenylalanine tracers. Therefore,
arterial [1-13C]phenylalanine was used to estimate whole
body phenylalanine flux. There was no significant difference
(P > 0.1) between the isotopic enrichment of
13CO2 in expired breath and that of
13CO2 in arterial blood in either treatment
group during metabolic steady state of NaH13CO3
infusion (75-120 min) (6). Therefore, we used
13CO2 in expired breath to estimate whole body
CO2 production rate (see APPENDIX) during
metabolic steady state of NaH13CO3 infusion
(75-120 min). Isotopic enrichments of
13CO2 in expired breath and
[1-13C]phenylalanine in arterial blood during the
phenylalanine infusion (270-360 min) plus the CO2
production rate were used to estimate whole body phenylalanine
oxidation (see APPENDIX).
ST treatment significantly (P < 0.05) reduced (
10%)
whole body phenylalanine flux (Fig. 2).
There was a significant (P < 0.01) decrease (
22%)
in whole body phenylalanine oxidation with ST treatment vs. control,
indicative of a reduction in the loss of carbon from the amino acid
pool. ST treatment significantly (P < 0.01) increased
(+34%) whole body protein deposition, largely due to a significant
(P < 0.05) decrease (
23%) in whole body protein
degradation without a change in whole body protein synthesis. The
efficiency with which dietary phenylalanine was utilized for protein
deposition was significantly (P < 0.01) increased
(+35%) with ST treatment (ST, 0.56 ± 0.07 vs. control, 0.36 ± 0.08 µmol · kg
1 · h
1
deposited for
µmol · kg
1 · h
1
intake). Both groups exhibited an increased retention of amino acids,
as indicated by higher whole body protein synthesis rates than whole
body proteolysis rates, as would be expected in the fed state.

View larger version (28K):
[in this window]
[in a new window]
|
Fig. 2.
Whole body phenylalanine turnover in fed
somatotropin-treated and control pigs. Values are means ± SD in
control (n = 7) and somatotropin-treated
(n = 9) pigs. * Different from control, P
0.05.
|
|
Phenylalanine kinetics across the HQ.
Isotope-labeled tracers (i.e.,
[2H5]phenylalanine and
NaH13CO3) and caudal aorta blood flow
measurements were utilized to estimate phenylalanine kinetics across
the HQ during isotopic steady state (Fig. 1). Isotopic steady state was
achieved in the artery and vena cava for
[2H5]phenylalanine (Fig.
3). Blood flow to the HQ was
significantly (P < 0.01) increased (+63%) with ST
treatment compared with controls (Table
2). ST treatment increased
(P < 0.03) mass balance in the HQ, and thus the
utilization of phenylalanine by the HQ was significantly
(P < 0.01) increased (+44%). The extraction of
phenylalanine by the HQ was similar between treatment groups (ST,
0.17 ± 0.04 vs. control, 0.23 ± 0.03 µmol · kg
1 · h
1
uptake for
µmol · kg
1 · h
1
input). The majority of the phenylalanine extracted by the HQ was
retained for protein synthesis and deposition, mediated largely by the
significant increase in blood flow, not extraction rate. Although we
did not quantify the rate of phenylalanine conversion to tyrosine, the
net release of 13CO2 by the HQ was negligible.
The rate of utilization of phenylalanine for protein synthesis in the
HQ was significantly (P < 0.01) greater (+43%) in the
ST-treated pigs than in controls. The calculated rate of proteolysis
was not significantly affected by ST treatment in growing pigs. ST
treatment increased (+120%; P < 0.03) protein deposition rates compared with controls. There was a positive net
protein balance in both the ST and control pigs, as expected in pigs
studied in the fed state, owing largely to the significant increase in
protein synthesis. Thus 7 days of ST treatment significantly increased
the utilization of amino acids by the HQ, resulting in an increase in
protein deposition.

View larger version (19K):
[in this window]
[in a new window]
|
Fig. 3.
Isotopic enrichments (IE) of arterial (A),
portal venous (B), and vena cava (C)
[1-13C]- and [2H5]phenylalanine
(Phe) at steady state during [1-13C]- and
[2H5]phenylalanine infusions. Duration of
tracer infusion was 120-360 min, with steady-state conditions
reached between 270 and 360 min. APE, atom percent excess.
|
|
Phenylalanine kinetics across the PDV.
Isotope-labeled tracers (i.e., [1-13C]- and
[2H5]phenylalanine and
NaH13CO3) and portal venous blood flow
measurements were utilized to estimate phenylalanine kinetics across
the PDV (Fig. 1). Portal and arterial isotopic steady state was
achieved for [1-13C]- and
[2H5]phenylalanine (Fig. 3). In
contrast to the HQ, portal venous blood flow was unaffected by ST
treatment in growing pigs (Table 2). Phenylalanine utilization by the
PDV was estimated from combined enteral and arterial sources of
phenylalanine (Fig. 1). The phenylalanine tracer was infused
intraduodenally and had the potential to recycle to the PDV via the
systemic circulation. Thus the phenylalanine kinetics across the PDV
were corrected for recycling of the [1-13C]phenylalanine
tracer by accounting for the fractional uptake of the
[2H5]phenylalanine tracer infused
systemically, assuming similarity in kinetics between the two different
phenylalanine tracers. In both ST-treated and control pigs,
35-45% of the enteral source of phenylalanine (206 µmol · kg
1 · h
1)
was utilized by the mucosa. The remaining 55-65% of enteral phenylalanine was absorbed and available for the remaining tissues in
the body. Approximately 10% of arterial phenylalanine influx or input
was utilized by the PDV. There was no significant effect of ST
treatment on PDV mass balance with ST treatment.
Total utilization of phenylalanine, as reflected by combined enteral
and arterial influx, by the PDV was increased (+23%) by ST treatment
(P < 0.05). In both treatment groups, ~42% of total
phenylalanine uptake was oxidized in the PDV and 58% was used for
protein synthesis. This estimate represents total phenylalanine oxidation by the PDV, because we were not able to differentiate phenylalanine oxidation derived from arterial vs. luminal input. ST
increased (+41%) the amount of phenylalanine utilized for protein synthesis by the PDV (P < 0.02). Release of
phenylalanine from protein degradation in the PDV was unaffected by ST
treatment. Because protein synthesis rates were higher than protein
degradation rates in all pigs, there was a positive net protein balance
and, hence, dietary phenylalanine was deposited as protein in the PDV. The increased utilization of phenylalanine for protein synthesis, and
therefore protein deposition, in the PDV of ST-treated pigs may have
contributed to the significantly (P < 0.02) increased (+21%) small intestinal weight-to-length ratio observed in ST-treated pigs (ST, 0.70 ± 0.11 vs. control, 0.58 ± 0.05 g
weight/cm length), despite no significant difference in small
intestinal mass per kilogram body weight with ST treatment (ST,
37.7 ± 3.5 vs. control, 36.1 ± 3.2 g weight/kg body wt).
 |
DISCUSSION |
The results of this study indicate that 7 days of exogenous ST
administration in growing pigs enhanced protein anabolism in the fed
state by altering protein turnover in the whole body and amino acid
kinetics in the HQ and PDV. ST treatment in growing pigs reduced whole
body phenylalanine flux, oxidation, and protein degradation without
altering whole body protein synthesis. In the HQ and PDV, ST treatment
increased protein deposition by increasing the utilization of
phenylalanine for protein synthesis, but it had no effect on protein
degradation or oxidation rates. Given the fact that protein synthesis
rates exceeded proteolysis rates, a positive net protein balance was
attained in both treatment groups at both the whole body and tissue-bed
level in young, growing swine in the fed state.
Whole body protein turnover effects of ST.
In the current study, ST treatment for 7 days in growing swine elicited
the metabolic responses that are characteristic of ST treatment in
domestic animals. ST treatment increased body weight and improved the
efficiency (+35%) with which dietary phenylalanine was utilized for
whole body protein deposition and growth, as demonstrated in numerous
studies (7, 9, 20, 21, 38, 45, 47). ST treatment also
produced the characteristic stimulation of the somatotropic axis
(4, 9, 19, 45, 47, 49), as indicated by a fivefold
elevation in IGF-I concentration, and a diabetogenic response
(10, 16, 46, 48), as indicated by the rise in plasma
glucose concentration.
ST administration reduced PUN concentration, indicating that ST
administration reduced amino acid catabolism in the fed state. Our
previous studies showed that the decrease in PUN is associated with a
reduction in liver urea cycle enzyme activity, ureagenesis, leucine
oxidation, and thus amino acid catabolism (6, 45). The
reduction in amino acid concentration in the systemic, PDV, and HQ
circulation and the decrease in whole body phenylalanine oxidation in
the current and previous (6, 40, 41) studies are
consistent with a reduction in substrate availability for both urea
production and amino acid oxidation. The reduction in amino acid
concentration with ST treatment suggests that ST induced an increase in
net removal of amino acids from the plasma pool, a reduction in net
release of amino acids from body protein into the plasma, or an
increase in net absorption of dietary phenylalanine. In fact, all three
are suggested from whole body and tissue kinetic data in ST-treated
pigs. Together, the results demonstrate that ST treatment in a growing
animal results in a more efficient use of dietary amino acids for growth.
Our current findings of the effects of ST on whole body accretion rates
and amino acid oxidation are in accord with the majority of previous
studies in animals and humans (4, 6, 17, 24, 25, 29, 37, 38, 45,
46). However, less is known about the specific effects of ST on
whole body proteolysis rates in the postabsorptive and postprandial
states. In fact, conflicting results of an increase (42),
decrease (45), and/or no change (24, 29) in
whole body proteolysis rates have been reported with ST treatment in
either postabsorptive (24, 29) or postprandial states
(42, 45). In the current study with phenylalanine tracers, as in our previous study using a leucine tracer (45), we
found that ST treatment in fed growing swine reduced whole body proteolysis.
Evaluation of the dual stable isotope tracer/mass transorgan
balance technique.
Several assumptions were made with this study design. First, we assumed
that the metabolism throughout the body of the [1-13C]-
and [2H5]phenylalanine tracers would not be
different simply because of their isotopic label (i.e., no isotope
effect). Second, we have assumed that all
[1-13C]phenylalanine taken up by the tissues (either HQ
or PDV) and not oxidized to CO2 was incorporated into
protein. A consequence of this assumption was that we might have
underestimated the rate of irreversible loss of phenylalanine, since
the rate of conversion to tyrosine was not quantified.
Amino acid kinetics in the HQ.
A major objective of this study was to identify the responses of
protein synthesis and degradation in the HQ to 7 days of exogenous ST
administration. In the process of measuring the phenylalanine kinetics
by the HQ, we observed that ST markedly increased blood flow to the HQ
by 80%. The substantial increase in blood flow contributed to the
increase in the amount of phenylalanine utilized by the HQ in
ST-treated pigs. This finding is consistent with previous studies and
is believed to be a function of local tissue metabolism (4, 16,
22, 24, 25, 33, 45). The localized increase in blood flow to the
HQ may be mediated also by the vasodilator properties of IGF-I
(28), which can stimulate the production of endothelial
nitric oxide and induce endothelium-dependent vasodilation (3,
23, 28).
The phenylalanine uptake by the HQ was partitioned largely for protein
synthesis in both control and ST-treated pigs. There are potentially
three main metabolic fates of phenylalanine within the HQ:
1) incorporation into protein, 2) conversion to
tyrosine, and 3) further metabolism of tyrosine to
CO2. Based on evidence of the absence of phenylalanine
hydroxylase in muscle tissue, it is generally assumed that oxidation of
phenylalanine does not occur in muscle tissue. We observed in our study
no significant net release of 13CO2 by HQ, thus
supporting the assumption that oxidation of phenylalanine is negligible
by the tissues of the HQ in vivo. This finding is similar to results
obtained by Harris et al. (27), in that they observed no
net release of 14CO2 across the HQ and no
venous accumulation of p-hydroxy-phenylpyruvate formed
during the hydroxylation of phenylalanine to tyrosine. As a result of
our observation in this respect, we have assumed that all
[1-13C]phenylalanine taken up by the HQ is incorporated
into protein, because the rate of conversion to tyrosine was not
quantified. Thus the marked increase in phenylalanine utilization in
response to ST was used for protein synthesis. This increase in
phenylalanine utilization was due largely to increased blood flow
(+63%) to the HQ with ST treatment and not to an increase in the
extraction of phenylalanine by the HQ. Moreover, nearly 47% of whole
body protein synthesis in ST-treated pigs was directed toward enhancing protein synthesis in the HQ. Several studies in both the postabsorptive (18, 24, 25, 38) and postprandial states (4,
46) have observed similar ST-induced increases in muscle protein
synthesis in the mature, adult human (24, 25) and growing
and mature animal (4, 18, 38, 46) models. On the other
hand, protein degradation in the HQ was not affected by ST treatment.
As expected in the fed state, net protein balance was positive,
regardless of treatment. However, ST treatment enhanced protein
deposition by 120%, likely contributing to the significantly greater
body mass (+16%) observed with ST over the 7-day treatment period.
Amino acid kinetics across the PDV.
A secondary objective of this study was to compare amino acid kinetics
in the HQ with those in the PDV. To determine amino acid metabolism in
the PDV, we infused two separate isotope tracers of phenylalanine via
the intraduodenal ([1-13C]phenylalanine) and intravenous
([2H5]phenylalanine) routes. We did this
because the intraduodenally infused [1-13C]phenylalanine
tracer, once absorbed through the small intestine, is recycled back to
the PDV through the systemic circulation.
In contrast to the results in the HQ, blood flow in the hepatic portal
vein was not significantly altered by ST treatment. After accounting
for recycling of the phenylalanine to the PDV through the systemic
circulation, total utilization, as reflected by the combined enteral
and arterial sources of amino acid during metabolic steady state, was
significantly (P < 0.05) greater (+22%) in
ST-treated pigs than in controls. Of the enteral phenylalanine (206 µmol · kg
1 · h
1)
provided to the mucosa, ~33-45% (control and ST, respectively) was utilized for metabolic processes within the mucosa (i.e., protein
synthesis or oxidation), with no difference between treatment groups.
However, there was a trend (P = 0.1) for an increase
(+35%) in utilization of phenylalanine from enteral sources with GH
treatment. Of the arterial phenylalanine available to the PDV, only a
small percentage was extracted (~10%) by the PDV, regardless of
treatment. However, a significantly (P < 0.01) greater
percentage of enteral (~40%) vs. arterial (~10%) input of
phenylalanine was utilized by the mucosa for metabolic processes, thus
implying a preferential use of dietary phenylalanine by the gut. This
may be a consequence of the transport capacity for amino acids on the
basolateral vs. brush border (apical membrane) of mucosal epithelium,
although in absolute terms, the amount of phenylalanine utilized by the PDV from the arterial and enteral input was similar.
Interestingly, ~40% of the total uptake of phenylalanine (arterial
and enteral sources) by the mucosa was oxidized in control and
ST-treated pigs. Oxidation of essential amino acids by the PDV has been
reported in studies with young animals (i.e., lysine, leucine)
(43, 44, 50, 51). Phenylalanine oxidation by the PDV
accounted for nearly 40% of whole body phenylalanine oxidation in the
current study. The values reported herein (~40% of whole body
oxidation) are slightly higher than those reported for lysine (~30%;
Ref. 43) and slightly lower than (~50%; Ref.
42) or equal to (~40%; Ref. 50) those
reported for leucine. This raises further questions about the
irreversible loss of phenylalanine. These values for phenylalanine
oxidation in the present study may be underestimated, because the first
irreversible loss of phenylalanine (i.e., its conversion to tyrosine)
was not measured in this study. To our knowledge, this is the first
report in the literature that the small intestine oxidizes
phenylalanine to any significant degree in vivo. However, ST treatment
had no effect on phenylalanine oxidation by the PDV, consistent with
our previously reported finding of a lack of effect of ST treatment on
urea cycle enzymes in the intestines (6).
Approximately 58-66% of the total phenylalanine extracted by the
PDV was utilized for protein synthesis, with a significantly (P < 0.02) greater (+66%) rate of protein synthesis
observed with ST treatment. Similar to the amino acid metabolism in the
HQ, ST treatment did not affect protein degradation rates. Regardless of treatment and as observed in the HQ, a positive net protein balance
was observed in the PDV, owing to higher protein synthesis vs. protein
degradation rates. The increased utilization of phenylalanine for
protein synthesis, and therefore protein deposition, in the PDV of
ST-treated pigs may have contributed to the significantly increased
small intestinal weight-to-length ratio observed with ST
administration. Study findings suggesting increased amino acid utilization by the intestine (5) and an increased
intestinal protein synthesis rate (18) in steers
chronically treated with ST are consistent with those of our current
study. The increased mucosal weight-to-length ratio is consistent with
previous studies indicating that ST has the capacity to stimulate
enterocyte growth and differentiation in rats (39) and
humans (8).
Perspectives.
Few in vivo studies have been performed to estimate amino acid kinetics
in different areas of the body by use of the dual stable isotope
tracer/mass transorgan balance technique that was utilized in this
current study. The technique allowed us to investigate the
tissue-specific metabolism of an individual essential amino acid in the
fed state in pigs administered ST for 7 days. On the whole body level,
we observed a decrease in total phenylalanine flux, oxidation, and
protein degradation with ST treatment that was similar to our previous
results with a leucine tracer (45). However, at the
individual tissue level, these observations were somewhat different.
The results show that ST increased the utilization of phenylalanine for
protein synthesis by the HQ and PDV. The increase in amino acid
utilization and protein synthesis in the HQ, but not in the PDV, was
mediated largely by the regional increase in blood flow. Interestingly,
recent studies have shown that adipose tissue protein synthesis is
highly responsive to nutrient stimulation (31, 32).
Because ST induces a repartitioning of nutrients away from adipose
tissue and toward lean tissue deposition (19), we
postulate that the lack of ST-induced stimulation of whole body protein
synthesis in the fed state may, in part, be due to an ST-induced
suppression of the feeding-induced stimulation of protein synthesis in
adipose tissue. However, other mechanisms may be involved. We further
found that ST treatment had no effect on protein degradation in either
the HQ or PDV. Whether the reduction in whole body degradation with ST
treatment can be accounted for by a reduction in proteolysis in liver
and other visceral tissues remains to be determined. Phenylalanine was
not oxidized by the HQ and thus was unaffected by ST treatment, and
although a significant portion of phenylalanine utilized by the mucosa
was oxidized, this was unaffected by ST treatment. Because ST treatment
does not alter urea cycle enzyme activity in the intestine but reduces urea cycle activity in the liver (6), we postulate that
the reduction in whole body phenylalanine oxidation is due largely to a
reduction in phenylalanine oxidation by the liver.
Thus the fact that the results in the PDV and HQ differ from those in
the whole body with regard to the effect of ST treatment on protein
synthesis, degradation, and amino acid oxidation suggests that ST must
have affected these processes in other tissues of the body differently.
This suggests that the effects of ST treatment on the processes that
regulate protein turnover, similar to those that regulate urea
production (6), are tissue specific. Furthermore, because
protein metabolism by the HQ represents that of individual skeletal
muscles, skin, bone, and adipose tissue and that by the PDV includes
the small intestine, large intestine, stomach, spleen, and pancreas,
the role of these individual tissues in the observed responses to ST in
the two tissue beds requires further study.
 |
APPENDIX |
Whole body protein turnover.
Whole body phenylalanine kinetics were determined by the following
standardized equations
|
(A1)
|
|
(A2)
|
|
(A3)
|
|
(A4)
|
where Ei is the isotopic enrichment of the infusate,
Ep is the plasma isotopic enrichment during metabolic
steady state of the phenylalanine infusion (270-360 min),
Eb is the 13CO2 isotopic enrichment
in expired breath during metabolic steady state of the
NaH13CO3 infusion (75-120 min), IR is the
infusion rate, and Q is the total flux of phenylalanine.
Intake is a function of the enteral nutrient solution plus
phenylalanine infusion rate. Degradation is equal to total flux minus
intake. Synthesis is equal to total flux minus oxidation. Deposition is
the difference in degradation and synthesis rates. Total flux was
calculated using the arterial isotopic enrichment of
[1-13C]phenylalanine tracer (Eq. A1), with no
significant difference observed in the flux when calculated using the
arterial isotopic enrichment of
[2H5]phenylalanine tracer.
HQ phenylalanine kinetics.
Phenylalanine kinetics in the HQ were determined via incorporation of
an arteriovenous difference calculation of the
[2H5]phenylalanine intravenous tracer into
these equations
|
(A5)
|
|
(A6)
|
|
(A7)
|
|
(A8)
|
|
(A9)
|
|
(A10)
|
where CA is arterial phenylalanine concentration,
CV is phenylalanine concentration in the vena cava,
CA1 is arterial CO2 gas concentration,
CV1 is CO2 gas concentration in the vena cava, EA1 is isotopic enrichment of arterial
13CO2, EV1 is isotopic
enrichment of venous 13CO2, EA2 is
isotopic enrichment of arterial
[2H5]phenylalanine, EV2 is
isotopic enrichment of vena cava
[2H5]phenylalanine, and BFH is
blood flow in the caudal aorta.
PDV phenylalanine kinetics.
Phenylalanine kinetics across the PDV were determined via incorporation
of an arteriovenous difference calculation of the intravenous
[2H5]phenylalanine and enteral
[1-13C]phenylalanine tracers into the equations to
follow. For estimations of PDV phenylalanine kinetics across the PDV, a
[1-13C]phenylalanine tracer was infused enterally. The
potential for recycling of the enteral
[1-13C]phenylalanine tracer in PDV from arterial sources
exists during steady-state conditions; thus the PDV phenylalanine
kinetics were corrected for the recycling of this tracer. We estimated
the percentage of [2H5]phenylalanine tracer
utilization across the PDV to correct for the recycling of the enteral
[1-13C]phenylalanine tracer reentering the PDV through
the arterial source, assuming that the kinetics of the two different
isotope-labeled tracers of phenylalanine would be similar.
|
(A11)
|
|
(A12)
|
|
(A13)
|
where the uptake of arterial [1-13C]phenylalanine
by PDV is corrected for arterial recycling of the
[1-13C]phenylalanine tracer by using the percent uptake
of the [2H5]phenylalanine tracer by the PDV.
|
(A14)
|
|
(A15)
|
|
(A16)
|
where absorption of phenylalanine is defined as the remaining
phenylalanine not utilized by the PDV and thus absorbed into arterial
sources, becoming available for utilization by the remainder of the
body.
|
(A17)
|
|
(A18)
|
|
(A19)
|
|
(A20)
|
|
(A21)
|
|
(A22)
|
where IRD is the infusion rate of the enteral
nutrient solution, IRP is the infusion rate of enteral
[1-13C]phenylalanine, CA is arterial
phenylalanine concentration, CP is phenylalanine
concentration in the portal vein, CA1 is arterial CO2 gas concentration, CP1 is CO2
gas concentration in the portal vein, EA1 is isotopic
enrichment of arterial 13CO2, EV1
is isotopic enrichment of portal venous 13CO2,
EA2 is isotopic enrichment of arterial
[2H5]phenylalanine, EP2 is
isotopic enrichment of [2H5]phenylalanine in
the portal vein, EA3 is isotopic enrichment of arterial
[1-13C]phenylalanine, EP3 is isotopic
enrichment of [1-13C]phenylalanine in the portal vein,
and BFP is blood flow in the portal vein.
 |
ACKNOWLEDGEMENTS |
We gratefully acknowledge the contribution of the late Dr. Peter J. Reeds to this paper.
 |
FOOTNOTES |
Deceased 13 August 2002.
We thank H. M. Mersmann, M. L. Fiorotto, M. C. Thieverge, and B. Stoll for helpful discussions; C. W. Liu,
J. F. Henry, and J. R. Rosenberger for technical assistance;
J. Cunningham, F. Biggs, G. Dailey, and J. Stubblefield for care of the
animals; L. Loddeke for editorial assistance; A. Gillum for graphics;
and J. Croom for secretarial assistance.
This project was supported by the US Department of Agriculture
National Research Initiative Grants 96-35206-3657 and
00-35206-9405; the US Department of Agriculture, Agriculture
Research Service under Cooperative Agreement 58-6250-6-001;
and National Institute of Child Health and Human Development Training
Grant T32-HD-07445.
This work is a publication of the USDA/ARS Children's Nutrition
Research Center, Department of Pediatrics, Baylor College of Medicine,
and Texas Children's Hospital, Houston, TX. The contents of this
publication do not necessarily reflect the views or policies of the US
Department of Agriculture nor does mention of trade names, commercial
products, or organizations imply endorsement by the US Government.
Address for reprint requests and other correspondence: T. A. Davis, USDA/ARS Children's Nutrition Research Center, Dept. of Pediatrics, Baylor College of Medicine, 1100 Bates St., Houston, TX
77030 (E-mail: tdavis{at}bcm.tmc.edu).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published October 15, 2002;10.1152/ajpendo.00309.2002
Received 11 July 2002; accepted in final form 4 October 2002.
 |
REFERENCES |
1.
Bell, AW,
Bauman DE,
Beermann DH,
and
Harrell RJ.
Nutrition, development, and efficacy of growth modifiers in livestock species.
J Nutr
128:
360S-363S,
1998[ISI][Medline].
2.
Bier, DM.
Growth hormone and insulin-like growth factor I: nutritional pathophysiology and therapeutic potential.
Acta Paediatr Scand Suppl
374:
119-128,
1991[Medline].
3.
Boger, RH.
Nitric oxide and the mediation of the hemodynamic effects of growth hormone in humans.
J Endocrinol Invest
22:
75-81,
1999[Medline].
4.
Boisclair, YR,
Bauman DE,
Bell AW,
Dunshea FR,
and
Harkins M.
Nutrient utilization and protein turnover in the hindlimb of cattle treated with bovine somatotropin.
J Nutr
124:
664-673,
1994[ISI][Medline].
5.
Bruckental, I,
Huntington GB,
Baer CK,
and
Erdman RA.
The effect of abomasal infusion of casein and recombinant somatotropin hormone injection on nitrogen balance and amino acid fluxes in portal-drained viscera and net hepatic and total splanchnic blood in Holstein steers.
J Anim Sci
75:
1119-1129,
1997[Abstract/Free Full Text].
6.
Bush, JA,
Wu G,
Suryawan A,
Nguyen HV,
and
Davis TA.
Somatotropin-induced amino acid conservation in pigs involves differential regulation of liver and gut urea cycle enzyme activity.
J Nutr
132:
59-67,
2002[Abstract/Free Full Text].
7.
Campbell, RG,
Johnson RJ,
King RH,
Taverner MR,
and
Mesiinger DJ.
Interaction of dietary protein content and exogenous porcine growth hormone administration on protein and lipid accretion rates in growing pigs.
J Anim Sci
68:
3217-3225,
1990[Abstract/Free Full Text].
8.
Canani, RB,
Bisceglia M,
Bruzzese E,
Mallardo G,
and
Guarino A.
Growth hormone stimulates, through tyrosine kinase, ion transport and proliferation in human intestinal cells.
J Pediatr Gastroenterol Nutr
28:
315-320,
1999[ISI][Medline].
9.
Caperna, TJ,
Komarek DR,
Gavelek D,
and
Steele NC.
Influence of dietary protein and recombinant porcine somatotropin administration in young pigs. II. Accretion rates of protein, collagen, and fat.
J Anim Sci
69:
4019-4029,
1991[Abstract/Free Full Text].
10.
Chung, CS,
Etherton TD,
and
Wiggins JP.
Stimulation of swine growth by porcine growth hormone.
J Anim Sci
60:
118-130,
1985[ISI][Medline].
11.
Copeland, KC,
and
Nair KS.
Acute growth hormone effects on amino acid and lipid metabolism.
J Clin Endocrinol Metab
78:
1040-1047,
1994[Abstract].
12.
Culea, M,
and
Hachey DL.
Determination of multiply labeled serine and glycine isotopomers in human plasma by isotope dilution negative-ion chemical ionization mass spectrometry.
Rapid Commun Mass Spectrom
9:
655-659,
1995[ISI][Medline].
13.
Davis, TA,
Burrin DG,
Fiorotto ML,
and
Nguyen HV.
Protein synthesis in skeletal muscle and jejunum is more responsive to feeding in 7- than in 26-day-old pigs.
Am J Physiol Endocrinol Metab
270:
E802-E809,
1996[Abstract/Free Full Text].
14.
Davis, TA,
Fiorotto ML,
and
Reeds PJ.
Amino acid compositions of body and milk protein change during the suckling period in rats.
J Nutr
123:
947-956,
1993[ISI][Medline].
15.
Dunshea, FR,
Bauman DE,
Boyd RD,
and
Bell AW.
Temporal response of circulating metabolites and hormones during somatotropin treatment of growing pigs.
J Anim Sci
70:
123-131,
1992[Abstract/Free Full Text].
16.
Dunshea, FR,
Boisclair YR,
Bauman DE,
and
Bell AW.
Effects of bovine somatotropin and insulin on whole-body and hindlimb glucose metabolism in growing steers.
J Anim Sci
73:
2263-2271,
1995[Abstract/Free Full Text].
17.
Eisemann, JH,
Hammond AC,
Bauman DE,
Reynolds PJ,
McCutcheon SN,
Tyrrell HF,
and
Haaland GL.
Effect of bovine growth hormone administration on metabolism of growing heifers: protein and lipid metabolism and plasma concentrations of metabolites and hormones.
J Nutr
116:
2504-2515,
1986[ISI][Medline].
18.
Eisemann, JH,
Hammond AC,
and
Rumsey TS.
Tissue protein synthesis and nucleic acid concentrations in steers treated with somatotropin.
Br J Nutr
62:
657-671,
1989[ISI][Medline].
19.
Etherton, TD,
and
Bauman DE.
Biology of somatotropin in growth and lactation of domestic animals.
Physiol Rev
78:
745-761,
1998[Abstract/Free Full Text].
20.
Etherton, TD,
Wiggins JP,
Evock CM,
Chung CS,
Rebhun JF,
Walton PE,
and
Steele NC.
Stimulation of pig growth performance by porcine growth hormone: determination of the dose-response relationship.
J Anim Sci
64:
433-443,
1987[ISI][Medline].
21.
Evock-Clover, CM,
Steele NC,
Caperna TJ,
and
Solomon MB.
Effects of frequency of recombinant porcine somatotropin administration on growth performance, tissue accretion rates, and hormone and metabolite concentrations in pigs.
J Anim Sci
70:
3709-3720,
1992[Abstract].
22.
Fong, Y,
Rosenbaum M,
Tracey KJ,
Raman G,
Hesse DG,
Matthews DE,
Leibel RL,
Gertner JM,
Fishman DA,
and
Lowry SF.
Recombinant growth hormone enhances muscle myosin heavy-chain mRNA accumulation and amino acid accrual in humans.
Proc Natl Acad Sci USA
86:
3371-3374,
1989[Abstract].
23.
Fryburg, DA.
NG-monomethyl-L-arginine inhibits the blood flow but not the insulin-like response of forearm muscle to IGF-I.
J Clin Invest
97:
1319-1328,
1996[Abstract/Free Full Text].
24.
Fryburg, DA,
and
Barrett EJ.
Growth hormone acutely stimulates skeletal muscle but not whole-body protein synthesis in humans.
Metabolism
42:
1223-1227,
1993[ISI][Medline].
25.
Fryburg, DA,
Gelfand RA,
and
Barrett EJ.
Growth hormone acutely stimulates forearm muscle protein synthesis in normal humans.
Am J Physiol Endocrinol Metab
260:
E499-E504,
1991[Abstract/Free Full Text].
26.
Harrell, RJ,
Thomas MJ,
Boyd RD,
Czerwinski SM,
Steele NC,
and
Bauman DE.
Effect of porcine somatotropin administration in young pigs during the growth phase from 10 to 25 kilograms.
J Anim Sci
75:
3152-3160,
1997[Abstract/Free Full Text].
27.
Harris, PM,
Skene PA,
Buchan V,
Milne E,
Calder AG,
Anderson SE,
Connell A,
and
Lobley GE.
Effect of food intake on hind-limb and whole-body protein metabolism in young growing sheep: chronic studies based on arterio-venous techniques.
Br J Nutr
68:
389-407,
1992[ISI][Medline].
28.
Haylor, J,
Singh I,
and
El Nahas AM.
Nitric oxide synthesis inhibitor prevents vasodilation by insulin-like growth factor I.
Kidney Int
39:
333-335,
1991[ISI][Medline].
29.
Horber, FF,
and
Haymond MW.
Human growth hormone prevents the protein catabolic side effects of prednisone in humans.
J Clin Invest
86:
265-272,
1990[ISI][Medline].
30.
Horber, FF,
Marsh HM,
and
Haymond MW.
Differential effects of prednisone and growth hormone on fuel metabolism and insulin antagonism in humans.
Diabetes
40:
141-149,
1991[Abstract].
31.
Lynch, CJ,
Hutson SM,
Patson BJ,
Vaval A,
and
Vary TC.
Tissue-specific effects of chronic dietary leucine and norleucine supplementation on protein synthesis in rats.
Am J Physiol Endocrinol Metab
283:
E824-E835,
2002[Abstract/Free Full Text].
32.
Lynch, CJ,
Patson BJ,
Anthony JC,
Vaval A,
Jefferson LS,
and
Vary TC.
Leucine is a direct-acting nutrient signal that regulates protein synthesis in adipose tissue.
Am J Physiol Endocrinol Metab
283:
E503-E513,
2002[Abstract/Free Full Text].
33.
Moller, N,
Jorgensen JO,
Schmitz O,
Moller J,
Christiansen J,
Alberti KG,
and
Orskov H.
Effects of a growth hormone pulse on total and forearm substrate fluxes in humans.
Am J Physiol Endocrinol Metab
258:
E86-E91,
1990[Abstract/Free Full Text].
34.
National Research Council.
Nutrient Requirements of Swine (10th ed.). Washington, DC: Natl Acad Sci, 1998.
35.
Pell, JM,
Elcock C,
Harding RL,
Morrell DJ,
Simmonds AD,
and
Wallis M.
Growth, body composition, hormonal and metabolic status in lambs treated long-term with growth hormone.
Br J Nutr
63:
431-445,
1990[ISI][Medline].
36.
Reeds, PJ,
Burrin DG,
Stoll B,
and
Jahoor F.
Intestinal glutamate metabolism.
J Nutr
130:
978S-982S,
2000[ISI][Medline].
37.
Russell-Jones, DL,
Bowes SB,
Rees SE,
Jackson NC,
Weissberger AJ,
Hovorka R,
Sonksen PH,
and
Umpleby AM.
Effect of growth hormone treatment on postprandial protein metabolism in growth hormone-deficient adults.
Am J Physiol Endocrinol Metab
274:
E1050-E1056,
1998[Abstract/Free Full Text].
38.
Seve, B,
Ballevre O,
Ganier P,
Noblet J,
Prugnaud J,
and
Obled C.
Recombinant porcine somatotropin and dietary protein enhance protein synthesis in growing pigs.
J Nutr
123:
529-540,
1993[ISI][Medline].
39.
Shulman, DI,
Hu CS,
Ducklett G,
and
Lavallee-Gray M.
Effect of short-term growth hormone therapy in rats undergoing 75% small intestinal resection.
J Pediatr Gastroenterol Nutr
14:
3-11,
1993[ISI].
40.
Stoll, B,
Burrin DG,
Henry JF,
Jahoor F,
and
Reeds PJ.
Phenylalanine utilization by the gut and liver measured with intravenous and intragastric tracers in pigs.
Am J Physiol Gastrointest Liver Physiol
273:
G1208-G1217,
1997[Abstract/Free Full Text].
41.
Stoll, B,
Henry JF,
Reeds PJ,
Yu H,
Jahoor F,
and
Burrin DG.
Catabolism dominates the first-pass intestinal metabolism of dietary essential amino acids in milk protein-fed piglets.
J Nutr
128:
606-614,
1998[Abstract/Free Full Text].
42.
Tomas, FM,
Campbell RG,
King RH,
Johnson RJ,
Chandler CS,
and
Taverner MR.
Growth hormone increases whole-body protein turnover in growing pigs.
J Anim Sci
70:
3138-3143,
1992[Abstract/Free Full Text].
43.
Van Der Schoor, SRD,
Van Goudoever JB,
Stoll B,
Henry JF,
Rosenberger JR,
Burrin DG,
and
Reeds PJ.
The pattern of intestinal substrate oxidation is altered by protein restriction in pigs.
Gastroenterology
121:
1167-1175,
2001[ISI][Medline].
44.
Van Goudoever, JB,
Stoll B,
Henry JF,
Burrin DG,
and
Reeds PJ.
Adaptive regulation of intestinal lysine metabolism.
Proc Natl Acad Sci USA
97:
11620-11625,
2000[Abstract/Free Full Text].
45.
Vann, RV,
Nguyen HV,
Reeds PJ,
Burrin DG,
Fiorotto ML,
Steele NC,
Deaver DR,
and
Davis TA.
Somatotropin increases protein balance by lowering body protein degradation in fed, growing pigs.
Am J Physiol Endocrinol Metab
278:
E477-E483,
2000[Abstract/Free Full Text].
46.
Vann, RC,
Nguyen HV,
Reeds PJ,
Steele NC,
Deaver DR,
and
Davis TA.
Somatotropin increases protein balance independent of insulin's effects on protein metabolism in growing pigs.
Am J Physiol Endocrinol Metab
279:
E1-E10,
2000[Abstract/Free Full Text].
47.
Wester, TJ,
Davis TA,
Fiorotto ML,
and
Burrin DG.
Exogenous growth hormone stimulates somatotropic axis function and growth in neonatal pigs.
Am J Physiol Endocrinol Metab
274:
E29-E37,
1998[Abstract/Free Full Text].
48.
Wray-Cahen, D,
Boyd RD,
Bauman DE,
and
Ross DA.
Effect of porcine somatotropin on the response of growing pigs to acute challenges of glucose, insulin, and epinephrine and during a hyperinsulinemic-euglycemic clamp.
Domest Anim Endocrinol
10:
103-115,
1993[ISI][Medline].
49.
Wray-Cahen, D,
Ross DA,
Baumann DE,
and
Boyd RD.
Metabolic effects of porcine somatotropin: nitrogen and energy balance and characterization of the temporal pattern of blood metabolites and hormones.
J Anim Sci
69:
1503-1514,
1991[Abstract/Free Full Text].
50.
Yu, YM,
Burke JF,
Vogt JA,
Chambers L,
and
Young VR.
Splanchnic and whole body L-[1-13C,15N]leucine kinetics in relation to enteral and parenteral amino acid supply.
Am J Physiol Endocrinol Metab
262:
E687-E694,
1992[Abstract/Free Full Text].
51.
Yu, YM,
Young VR,
Tompkins RG,
and
Burke JF.
Comparative evaluation of the quantitative utilization of parenterally and enterally administered leucine and L-[1-13C,15N]leucine within the whole body and the splanchnic region.
J Parenter Enteral Nutr
19:
209-215,
1995[Abstract].
Am J Physiol Endocrinol Metab 284(2):E302-E312