Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0615
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
We have previously shown that a selective
increase of 84 pmol/l in either arterial or portal vein insulin
(independent of a change in insulin in the other vessel) can suppress
tracer-determined glucose production (TDGP) and net hepatic glucose
output (NHGO) by ~50%. In the present study we investigated the
interaction between equal increments in arterial and portal vein
insulin in the suppression of TDGP and NHGO. Isotopic
([3-3H]glucose) and
arteriovenous difference methods were used in conscious overnight
fasted dogs. A pancreatic clamp was used to control the endocrine
pancreas. A 40-min basal period was followed by a 180-min test period,
during which arterial and portal vein insulin levels were
simultaneously and equally increased 102 pmol/l. Hepatic sinusoidal
glucagon levels remained unchanged, and euglycemia was maintained by
peripheral glucose infusion. TDGP was suppressed ~60% by the last 30 min of the experimental period. In contrast, NHGO was suppressed 100%
by that time. Coincidentally, hepatic glucose uptake (net hepatic
[3H]glucose balance)
increased significantly (~4
µmol · kg1 · min
1).
The effects of simultaneous equal increases in peripheral and portal
venous insulin were not additive in the suppression of TDGP. However,
they were additive in decreasing NHGO as a result of an increase in the
uptake of glucose by the liver.
conscious dog; hepatic glucose uptake
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
WE HAVE PREVIOUSLY SHOWN that a selective 84-pmol/l
increase in either arterial or portal vein insulin (with no change of insulin in the other vessel) can suppress tracer-determined glucose production (TDGP) and net hepatic glucose output (NHGO) by ~50% (27). A selective 84-pmol/l increase in portal vein insulin decreased
NHGO by 5.1 µmol · kg1 · min
1
(relative to a control study) as the result of a suppression of hepatic
glycogenolysis. A similar (84 pmol/l) selective increase in arterial
insulin suppressed NHGO by 3.9 µmol · kg
1 · min
1
(relative to a control study). In that case, suppression of
gluconeogenesis explained approximately one-third of the fall, as did
the increase in hepatic sinusoidal insulin, which occurred as a result
of the increase of insulin in the hepatic artery. The remainder of the fall was linked to a significant increase in lactate output from the
liver, which resulted from a diversion of glycogenolytically derived
carbon to lactate. This increase in net hepatic lactate output
correlated temporally with a significant suppression of net hepatic
nonesterified fatty acid (NEFA) uptake. On further investigation into
the role of NEFA in the suppression of NHGO, we found that, when the
NEFA levels were clamped during a selective 78-pmol/l increase in
arterial insulin, net hepatic lactate output did not increase, net
hepatic gluconeogenesis did not change, and the suppression of NHGO by
peripheral insulin was halved (28).
Selective increments in arterial and portal vein insulin thus bring about suppression of NHGO via different modes of action. Other studies investigating the effects of peripheral and portal vein insulin infusion on hepatic glucose production (HGP) were designed so that the level of the hormone changed at the liver and peripheral tissues simultaneously (1, 12, 13). This prohibited an analysis of the interaction of the two signals. The goal of our study, therefore, was to determine whether the effects of equal increments in peripheral and portal vein insulin would be additive in suppression of HGP.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Animal care and surgical procedures. Experiments were conducted on 11 conscious mongrel dogs (18-30 kg) of either sex that had been fed a meat and chow diet [34% protein, 46% carbohydrate, 14.5% fat, and 5.5% fiber based on dry weight; Kal Kan (Vernon, CA) beef dinner and Purina Lab Canine Diet No. 5006] once daily. The surgical facility met the standards published by the American Association for the Accreditation of Laboratory Animal Care, and the protocols were approved by the Vanderbilt University Medical Center Animal Care Committee.
Each dog underwent a laporatomy performed with the animal under general anesthesia (15 mg/kg pentothal sodium, presurgery, and 1% isoflurane inhalation anesthetic during surgery) 2 wk before the experiment. With the use of previously described sterile techniques (2), Silastic catheters (0.03-in. ID; Dow Corning, Midland, MI) were placed into a splenic and jejunal vein for intraportal infusions as required. Catheters (0.04-in. ID) for blood sampling were placed in the left common hepatic vein, the hepatic portal vein, and the femoral artery, as described previously (10). All catheters were filled with saline containing heparin (200 U/ml; Abbott Laboratories, North Chicago, IL), and their free ends were knotted before closure of the skin. Doppler flow probes (Instrument Development Laboratories, Baylor College of Medicine, Houston, TX) were placed around the hepatic artery and portal vein to determine hepatic blood flow, as previously described (24). The Doppler leads, along with the catheters, were placed in a subcutaneous pocket before closure of the abdominal skin. The positions of the catheter tips were confirmed with autopsy. Only dogs that had a leukocyte count <18,000/mm3, a hematocrit >35%, normal stools, and had consumed their daily food ration were used for a study. On the day of the experiment, after an 18-h fast, the catheters and flow probe leads were exteriorized, with the animal under local anesthesia (2% lidocaine; Astra Pharmaceutical, Worcester, MA). The contents of each catheter were aspirated, and the catheters were flushed with saline. The intraportal catheters (splenic and jejunal) were used for the infusion of insulin and glucagon (Lilly, Indianapolis, IN). Angiocaths (Deseret Medical, Becton-Dickinson, Sandy, UT) were inserted percutanously into the left cephalic vein for [3-3H]glucose (NEN, Boston, MA) plus indocyanine green (Becton-Dickinson, Cockeysville, MD) infusion and into a saphenous vein for somatostatin (Bachem, Torrance, CA) plus insulin infusion. An angiocath was inserted into the right cephalic vein for peripheral glucose infusion. Each animal was allowed to rest quietly in a Pavlov harness for 30 min before the experiment was begun.Experimental procedure.
Each experiment consisted of a tracer and dye equilibration period
(140 to
40 min), a basal period (
40 to 0 min), and
an experimental period (0 to 180 min). At
140 min, a priming
dose of [3-3H]glucose
(25 µCi) was given and a continual infusion of
[3-3H]glucose (0.21 µCi/min) was begun to allow assessment of HGP. Constant infusions of
indocyanine green (0.07 mg/min) and somatostatin (0.8 µg · kg
1 · min
1)
were started simultaneously (t =
140 min) via a leg vein to measure hepatic blood flow and to
inhibit the endogenous secretion of insulin and glucagon, respectively.
A constant intraportal infusion of glucagon (0.5 ng · kg
1 · min
1)
was given to replace endogenous glucagon secretion. A constant infusion
of insulin was given via a peripheral vein, and a variable insulin
infusion was given via the portal infusion catheters to replace
endogenous insulin secretion. The rate of the portal insulin infusion
was adjusted to maintain preexisting plasma glucose levels. Once the
plasma glucose level had been stabilized at a euglycemic value for 30 min, the basal sampling period was begun. The rate of
[3H]glucose infusion
during the experimental period in the test group was adjusted to clamp
the arterial plasma glucose specific activity at the value that
preexisted during the basal period. This was not done in the control
group because of the continued existence of a steady state for glucose.
Protocol 1: Combined simultaneous equal increase in peripheral and
portal vein insulin group.
During the basal period, the portal insulin infusion rate averaged 1.29 pmol · kg1 · min
1, whereas the
peripheral insulin infusion rate was 0.48 pmol · kg
1 · min
1.
At 0 min, the portal insulin infusion was raised 0.54 pmol · kg
1 · min
1
and the peripheral insulin infusion was increased 1.5 pmol · kg
1 · min
1.
On the basis of our earlier studies (27, 28), it was calculated that
these changes in the insulin infusion rates would increase arterial and
portal vein insulin levels ~84 pmol/l. These rates are in line with
those used in our earlier studies (27, 28), although, in the previous
paper, they were mistakenly reported to be
103 greater than they actually
were. Euglycemia was maintained during the experimental period using a
variable glucose infusion given through a peripheral vein. In this way
we were able to create simultaneous and equal increases in both the
arterial and portal vein insulin concentrations.
Protocol 2: Control group.
Control studies (Cont, n = 5) were
carried out as described in protocol 1 except that, on completion of
the basal period, no changes were made in the site or rate of insulin
delivery. The portal insulin infusion averaged 0.48 pmol · kg1 · min
1,
and the peripheral insulin infusion was 0.48 pmol · kg
1 · min
1.
Glucose infusion was not required. In this way we controlled for
changes in HGP, which might occur over time even in the absence of a
change in insulin. The data from this group have been presented elsewhere (27).
Analytic procedures.
The handling and immediate processing of blood samples has been
described in our previous paper (27). Blood samples were processed for
the later determination of acetoacetate, -hydroxybutyrate, glycerol,
and lactate and the gluconeogenic amino acids alanine, glutamine,
glutamate, glycine, serine, and threonine. Plasma samples were obtained
for immediate analysis of glucose using the glucose oxidase method with
a Beckman glucose analyzer (Beckman Instruments, Fullerton, CA). Plasma
samples were also processed for the later determination of
[3H]glucose,
immunoreactive glucagon and insulin, NEFAs, and cortisol. All samples
were kept in an ice bath during processing and then were stored at
70°C until they were assayed.
Tracer calculations. Rates of tracer-determined total HGP (Ra) and tracer-determined glucose utilization were measured using a primed, continual infusion of [3-3H]glucose. Data calculation was carried out using a two-compartment model described by Mari (19) using canine parameters reported by Dobbins et al. (11). TDGP was calculated as the difference between Ra and the exogenous glucose infusion rate.
Arteriovenous difference calculations.
The NHGO and the net hepatic balance of gluconeogenic substrates were
calculated using the formula [H (0.28A + 0.72P)] × HF, where H, A, and P are the substrate concentrations in the hepatic vein, femoral artery, and portal vein blood or plasma, respectively. HF is total hepatic flow of blood or plasma as estimated from indocyanine green, and 0.28 and 0.72 represent the approximate contributions of the hepatic artery and the portal vein, respectively, to total hepatic blood flow during somatostatin infusion (24). With
this calculation, a positive value represents net production by the
liver, whereas a negative value represents net hepatic uptake. Plasma
glucose values were multiplied by 0.73 to convert them to blood glucose
values for the net hepatic balance calculation (22). The data displayed
in RESULTS were calculated with the use of indocyanine green-determined blood flows. The two (artery and
portal vein) Doppler flow probes were both functional in only 6 of the
11 studies, precluding the use of Doppler-determined flows for the
entire data base. Nevertheless, the Doppler flow data that were
obtained confirmed that the ratio of arterial to portal blood flow that
we used was correct. In previous studies (24), we have shown that the
Doppler- and indocyanine green-determined blood flows are not
significantly different and, therefore, the method of flow
determination used has little effect on the net hepatic balance
calculation. Hepatic sinusoidal hormone concentrations were calculated
using the formula 0.28A + 0.72P, where A and P represent the hormone
levels in arterial and portal plasma, respectively. Maximal
gluconeogenesis from circulating precursors was calculated by summing
the net hepatic uptake rates of all of the gluconeogenic precursors and
dividing by two to account for the incorporation of the three carbon
precursor into the six carbon glucose molecule. Lactate and the
individual gluconeogenic amino acids were only considered for inclusion
in the gluconeogenic uptake calculation (or total amino acid uptake) if
net hepatic uptake was evident. The mean lactate and gluconeogenic
amino acid data, on the other hand, represent the entire data base
regardless of the sign of net balance. For calculation of total hepatic
glucose uptake by the liver, net hepatic
[3-3H]glucose uptake
[disintegrations · min
1
(dpm) · kg
1 · min
1]
was divided by the arterial glucose specific activity (dpm/µmol). The
calculation of hepatic glucose uptake assumes that uptake of glucose
occurs before production and the resulting dilution of glucose specific
activity across the liver is minimal. Because there is no difference
between arterial and portal vein glucose specific activity and a
minimal drop (6%) across the liver, the impact of this assumption is
negligible. Failure of the hepatic sampling catheter occurred in one
study in the combination group (Combo,
n = 6), thus precluding the
calculation of net hepatic balance data in that experiment. Therefore,
an n = 5 value was used in the net
hepatic balance calculations for each group, but the tracer data are
based on n = 6 for Combo and
n = 5 for Cont.
Statistics. The level of significance was P < 0.05 (2-sided test). The data were analyzed for differences on the basis of group-by-group comparisons and for changes from intragroup baseline values. Statistical comparisons between groups were calculated using two-way analysis of variance, and intragroup difference from baseline was calculated using one-way analysis of variance (Statview, Calabasas, CA). The Scheffé procedure and Fisher's protected least significant difference test for multiple comparisons were used post hoc when significant F ratios were obtained.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Effects of a combined increase in peripheral and portal vein
insulin.
When simultaneous increments in arterial and portal insulin were
brought about, the arterial insulin level rose from 66 ± 9 (basal)
to 168 ± 13 pmol/l by the last 30 min of the study (Fig. 1) and the portal insulin level increased
similarly (210 ± 47 to 312 ± 45 pmol/l). Sinusoidal
insulin therefore increased from ~170 (basal) to 272 pmol/l (last 30 min). The hepatic sinusoidal glucagon level fell slightly (~10%),
whereas the arterial plasma glucose concentration and plasma glucose
specific activity remained unaltered (Table
1). TDGP fell from 12.4 ± 0.7 to 7.9 ± 0.9 µmol · kg1 · min
1
by 30 min into the experimental period and was suppressed to 5.3 ± 0.8 µmol · kg
1 · min
1
by the last 30 min of the study (Fig. 2,
P < 0.05). NHGO fell from 9.8 ± 2.2 to 4.0 ± 1.2 by 30 min and to 0.0 ± 1.6 µmol · kg
1 · min
1
by the last 30 min of the study (Fig. 2,
P < 0.05). The rise in insulin
increased whole body glucose utilization by more than twofold (Table 1,
P < 0.05). Interestingly, total
hepatic glucose uptake (calculated from net hepatic
[3-3H]glucose balance)
increased significantly from 1.2 ± 0.1 (basal) to 4.3 ± 0.2 µmol · kg
1 · min
1
by 1 h into the experimental period (P < 0.05) and remained elevated for the rest of the study (4.2 ± 0.5 µmol · kg
1 · min
1,
last 30 min, Fig. 3).
|
|
|
|
|
|
|
|
|
Control group.
Neither arterial, portal, nor hepatic sinusoidal insulin levels changed
over the course of the control experiments (Fig. 1). Once again the
hepatic sinusoidal glucagon level fell slightly (~10%), whereas both
the arterial plasma glucose concentration and the arterial plasma
glucose specific activity remained unchanged (Table 1). TDGP declined
from 13.7 ± 0.8 to 11.0 ± 0.6 µmol · kg1 · min
1
by the last 30 min of the experimental period
(P < 0.05, Fig. 2). NHGO showed a
similar decline (P < 0.05) from 10.5 ± 0.7 (basal) to 8.4 ± 1.1 µmol · kg
1 · min
1
(Fig. 2), whereas whole body glucose utilization declined minimally over the course of the experiment (Table 1). Hepatic glucose uptake
(net hepatic
[3-3H]glucose balance)
did not change (2.5 ± 0.3, basal, to 2.4 ± 0.6 µmol · kg
1 · min
1,
last 30 min, Fig. 3),
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The combination of equal increases of 102 pmol/l in peripheral
and portal vein insulin suppressed TDGP ~60% and NHGO 100%. The
difference in the two estimates of suppression can be explained by the
ability of the combination of portal and peripheral insulin to increase
hepatic glucose uptake, which would only be apparent in NHGO. Such an
increase did not occur in response to a similar rise in arterial or
portal insulin alone (27). Hepatic glucose uptake must therefore have
increased either because the liver sinusoidal insulin level was higher
(60%) when the increments were brought about together or because the
combination of the rise in portal vein and peripheral insulin has a
unique effect. In response to the simultaneous and equal increases in
arterial and portal vein insulin, TDGP decreased 4.4 µmol · kg1 · min
1
over and above the fall evident in the control protocol. In our previous studies, selective and independent 84-pmol/l increases in
arterial and portal vein insulin suppressed TDGP by 5.2 and 4.4 µmol · kg
1 · min
1
(by 180 min), respectively, relative to a control study. Obviously, the
effects of combined increases in peripheral and portal vein insulin
were not additive in suppression of TDGP (relative to the control
study). When portal insulin was selectively raised in our previous
study, glycogenolysis was completely inhibited. During the selective
increase in peripheral insulin, 1.8 µmol · kg
1 · min
1
of the decrease in NHGO was due to an inhibition of glycogenolysis (resulting from the increase in hepatic sinusoidal insulin that resulted from the rise in arterial insulin) and 1.8 µmol · kg
1 · min
1
was due to the redirection of glycogenolytically derived carbon to
lactate. In the present experiments, neither of the latter would be
expected to occur in response to the increment in peripheral insulin,
since the increase in hepatic sinusoidal insulin resulting from the
concurrent increase in peripheral and portal insulin was greater (60%)
than the increase previously experienced and would have inhibited
glycogenolysis completely. Thus, in the Combo group, one would have
expected an inhibition of glycogenolysis (~4.4
µmol · kg
1 · min
1)
along with a fall in net hepatic gluconeogenesis (1.6 µmol · kg
1 · min
1),
which would have resulted from the rise in peripheral insulin. If the
drop in TDGP (2.7 µmol · kg
1 · min
1)
in the Cont group is taken into account, a fall of 8.7 µmol · kg
1 · min
1
from baseline would be predicted in the Combo group. The observed TDGP
fall in the Combo group was 7.1 µmol · kg
1 · min
1,
a value very close to that predicted.
The inability of small increases in insulin to fully suppress TDGP is
not an unexpected result. In humans, Katz et al. (16) and Hother-Nielsen et al. (15) found the 50% effective
dose for the suppression of HGP by arterial insulin to be
~95-165 pmol/l. Bevilacqua et al. (4) raised
arterial insulin 360 pmol/l in humans during euglycemia and found that
HGP was suppressed from 66 ± 7 to 15 ± 8 mg · m2 · min
1
by 120 min. In all of the above studies, however, glucagon was either
not measured or fell, and this change most likely contributed to the
decrease in glucose production. Boden et al. (5) found that an increase in arterial insulin from 30 to 420 pmol/l suppressed HGP from 12.5 to 1.6 µmol · kg
1 · min
1
even when the glucagon concentration did not change. In agreement with
our data, these previous studies demonstrated that glucose production
by the liver, as measured by tracer methodology, is not completely
inhibited, even by relatively high insulin concentrations.
NHGO in the Combo group was fully suppressed (9.8 to 0.0 µmol · kg1 · min
1)
by insulin infusion. Because NHGO represents the net movement of
glucose into and out of the liver, and since TDGP fell to only 5.3 µmol · kg
1 · min
1,
the suppression of NHGO must have resulted in part from an increase in
hepatic glucose uptake. The difference between TDGP (5.3 µmol · kg
1 · min
1)
and NHGO (0 µmol · kg
1 · min
1)
in the last 30 min of the study provides an estimate of the net amount
of glucose taken up by the liver (5.3 µmol · kg
1 · min
1).
To the extent that the kidneys are responsible for a portion of TDGP,
this would be an overestimate. In our earlier study (20), the kidney
produced ~2.0
µmol · kg
1 · min
1
of glucose after an overnight fast in the conscious dog. A study by
Cersosimo et al. (7) demonstrated that a selective
increase of 51 pmol/l in renal artery insulin could decrease renal
glucose output by ~73%. Therefore, because the rise in arterial
insulin in the Combo group was 102 pmol/l, renal glucose release would be expected to be virtually zero and thus TDGP would be a reflection of
glucose release by the liver. The net hepatic
[3-3H]glucose balance
data confirm that hepatic glucose uptake was increased during
hyperinsulinemia (1.2 basal to 4.2 µmol · kg
1 · min
1
by the end of the experiment). Both estimates of hepatic glucose uptake
suggest that it was 4-5
µmol · kg
1 · min
1
during the increase in insulin.
If glycogenolysis is shut off by the rise in portal insulin as
suspected, the carbon produced by the liver as glucose (5.3 µmol · kg1 · min
1)
and lactate (0.8 µmol · kg
1 · min
1
glucose equivalents), as well as the amount theoretically oxidized within the liver (~2.0
µmol · kg
1 · min
1),
must be derived from the gluconeogenic precursors being taken up (2.9 µmol · kg
1 · min
1
glucose equivalents) and exogenous glucose extracted by the liver. This
suggests that glucose uptake by the liver was ~5.2
µmol · kg
1 · min
1,
again in tune with the above estimates. To the extent that
glycogenolysis continued, the need for hepatic glucose uptake would be
reduced. Regardless of its absolute value, it is clear that, when
insulin was raised in the peripheral circulation and portal vein
concurrently, hepatic glucose uptake occurred. Thus the effects of
equal 102-pmol/l increases in peripheral and portal vein insulin
brought about simultaneously are additive in suppression of NHGO in
that they cause a significant movement of glucose into the hepatocyte,
which neither causes alone. As noted previously, the latter could be explained by an interaction of the direct and indirect actions of
insulin on the liver or by the fact that the simultaneous increase in
arterial and portal insulin created a greater rise in the liver sinusoids than when either was increased alone.
During a selective increase in peripheral insulin, net hepatic lactate
output increased significantly, whereas NHGO fell (27). Glycogenolytically derived carbon was redirected through glycolysis to
lactate without a net decrease in glycogen breakdown. These changes
correlated with a temporal fall in the NEFA level. In fact, when NEFA
levels were maintained, the increase in net hepatic lactate output did
not occur and the fall in NHGO was significantly blunted (28). A
selective increase in portal insulin, on the other hand, suppressed
glycogenolysis, failed to alter NEFA levels, and suppressed net hepatic
lactate output (27). In the current studies, when portal and peripheral
insulin levels were increased simultaneously, NEFA levels again fell.
As a result, net hepatic lactate output again increased significantly
60 min into the experiment and was still increased by 1.8 µmol · kg1 · min
1
at the end of the study. In the control group, net hepatic lactate output decreased progressively, eventually falling by 5.1 µmol · kg
1 · min
1.
If the difference between the two groups is converted to an amount of
glucose that was broken down to create this amount of lactate, it would
equal 3.5 µmol · kg
1 · min
1
of glucose equivalents. Interestingly, the significant increase in
lactate output correlated temporally with the increase in glucose uptake by the liver. Thus glycolysis was again increased, even though
glycogenolysis was inhibited, but in this case the glucose carbon being
converted to lactate seems to have been derived from the circulation.
The exact biochemical mechanism by which the fall in NEFA levels
influences the increase in net hepatic lactate output and net hepatic
uptake of glucose is unknown. The fall in NEFA may bring about multiple
effects, including potential changes in substrate levels and the redox
state of the liver. A fall in citrate levels would activate
phosphofructokinase, whereas a change in the mitochondial redox state
could activate pyruvate dehydrogenase, both of which could enhance
glycolysis and in turn cause lactate dehydrogenase to convert pyruvate
to lactate. The difference in activation time of the enzymes may cause
the peak in net hepatic lactate output at 1 h and the eventual decrease in lactate output by the end of the experiment.
Other investigators have shown (8, 9) that, in euglycemic humans,
insulin levels >600 pmol/l can increase splanchnic glucose uptake
slightly (3.9 µmol · kg1 · min
1). Similarly, in the
dog (21) net hepatic glucose uptake was 3.3 µmol · kg
1 · min
1
during euglycemic hyperinsulinemia of 720 pmol/l. No experiments to
date have clearly shown that hepatic glucose uptake is increased in
response to small changes in insulin (102 pmol/l) under euglycemic conditions. It has been shown, however, that relatively small changes
in arterial insulin (144 pmol/l) can result in net hepatic uptake of
glucose under hyperglycemic conditions (24). The results from the
current study may be interpreted to indicate that peripheral insulin,
working through the suppression of NEFA levels, could play a role in
regulating the uptake of glucose by the liver. However, the effect can
only occur when hepatic sinusoidal insulin has increased enough to
suppress glycogenolysis. The consequences of NEFA suppression under
hyperinsulinemic conditions on the movement of glucose into the liver
remain to be investigated.
These findings may indicate why other investigators (1, 3, 6) have concluded that insulin's ability to suppress HGP appears relatively slow (~60 min). Insulin appears to have three different effects on the liver, which occur at different times. One action of insulin is an immediate and direct effect on the liver to suppress glycogenolysis. We have previously demonstrated that a selective increase of 84 pmol/l in portal vein insulin rapidly (~15 min) and completely suppresses glycogenolysis (27). A secondary effect appears dependent on an increase in glucose uptake by the liver, which is maximal at 1 h. In a previous study, Pagliassotti et al. (25) showed that it required 45 min for an effect of insulin on net hepatic glucose uptake to become manifest under hyperglycemic conditions. A third effect appears to be a time-dependent suppression of the gluconeogenic precursor supply to the liver. The sum of these three effects is to create a rapid first-phase suppression of HGP in combination with a slower secondary phase. Therefore, the overall response of HGP to insulin appears to be slow (~60 min). It is important to note that the first phase, which represents a suppression of glycogenolysis, is rapid (~15 min).
In summary, we found that simultaneous equal increases in peripheral and portal vein insulin of 102 pmol/l were not additive in their ability to suppress TDGP. They were, however, additive in their ability to decrease NHGO. The latter occurred because the effects of the changes in insulin were synergistic in increasing the uptake of glucose by the liver. Whether this was due to the greater increase in sinusoidal insulin or the result of a unique response to the combined direct (liver) and indirect (fat) effects of insulin remains to be determined.
![]() |
ACKNOWLEDGEMENTS |
---|
The authors thank Jon Hastings, Pam Venson, Wanda Snead, Paul Flakoll, and Annapurna Venkatakrishnan for excellent technical assistance.
![]() |
FOOTNOTES |
---|
This research was supported in part by Grants 2RO1 DK-18243 and 5P60 DK-2059 from the National Institute of Diabetes and Digestive and Kidney Diseases.
This work was presented in part at the 56th Annual Meeting of the American Diabetes Association, San Francisco, CA, June 8-11, 1996.
Present address of D. K. Sindelar: Metabolism (151), Dept. of Veterans Affairs, 1660 South Columbian Way, Seattle, WA 98108-1597.
Address for reprint requests: D. K. Sindelar, Dept. of Molecular Physiology and Biophysics, 702 Light Hall, Vanderbilt Univ. School of Medicine, 21st Ave. South and Garland, Nashville, TN 37232-0615.
Received 4 April 1997; accepted in final form 5 August 1997.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Ader, M.,
and
R. N. Bergman.
Peripheral effects of insulin dominate suppression of fasting hepatic glucose production.
Am. J. Physiol.
258 (Endocrinol. Metab. 21):
E1020-E1032,
1990
2.
Adkins, B. A.,
S. R. Myers,
G. K. Hendrick,
R. W. Stevenson,
P. E. Williams,
and
A. D. Cherrington.
Importance of the route of intravenous glucose delivery on hepatic glucose balance in the conscious dog.
J. Clin. Invest.
79:
557-565,
1987[Medline].
3.
Bergman, R. N.,
D. C. Bradley,
and
M. Ader.
On insulin action in vivo.
In: New Concepts in the Pathogenesis of NIDDM, edited by S. Efendic,
C. G. Ostenson,
and M. Vranic. New York: Plenum, 1993, p. 181-198.
4.
Bevilacqua, S.,
R. Bonadonna,
G. Buzzigoli,
C. Boni,
D. Ciociaro,
F. Maccari,
M. Giorico,
and
E. Ferrannini.
Acute elevation of free fatty acid levels leads to hepatic insulin resistance in obese subjects.
Metabolism
36:
502-506,
1987[Medline].
5.
Boden, G.,
X. Chen,
J. Ruiz,
J. White,
and
L. Rossetti.
Mechanism of fatty acid-induced inhibition of glucose uptake.
J. Clin. Invest.
93:
2438-2446,
1994[Medline].
6.
Bradley, D. C.,
R. A. Poulin,
and
R. N. Bergman.
Dynamics of hepatic and peripheral insulin effects suggest common rate-limiting step in vivo.
Diabetes
42:
296-306,
1993[Abstract].
7.
Cersosimo, E.,
R. L. Judd,
and
J. M. Miles.
Insulin regulation of renal glucose metabolism in conscious dogs.
J. Clin. Invest.
93:
2584-2589,
1994[Medline].
8.
DeFronzo, R. A.,
and
E. Ferrannini.
Regulation of hepatic glucose metabolism in humans.
Diabetes Metab. Rev.
3:
415-459,
1987[Medline].
9.
DeFronzo, R. A.,
E. Ferrannini,
R. Hendler,
P. Felig,
and
J. Wahren.
Regulation of splanchnic and peripheral glucose uptake by insulin and hyperglycemia in man.
Diabetes
32:
35-45,
1983[Medline].
10.
Dobbins, R.,
S. Davis,
D. Neal,
C. Cobelli,
J. Jaspan,
and
A. Cherrington.
Compartmental modeling of glucagon kinetics in the conscious dog.
Metabolism
44:
452-459,
1995[Medline].
11.
Dobbins, R. L.,
S. N. Davis,
D. W. Neal,
C. Cobelli,
and
A. D. Cherrington.
Pulsatility does not alter the response to a physiological increment in glucagon in the conscious dog.
Am. J. Physiol.
266 (Endocrinol. Metab. 29):
E467-E478,
1994
12.
Giacca, A.,
S. Fisher,
Z. Q. Shi,
R. Gupta,
H. Lavina,
A. Lickley,
and
M. Vranic.
Importance of peripheral insulin levels for insulin-induced suppression of glucose production in depancreatized dogs.
J. Clin. Invest.
90:
1769-1777,
1992[Medline].
13.
Giacca, A.,
S. J. Fisher,
R. H. McCall,
Z. Q. Shi,
and
M. Vranic.
Direct and indirect effects of insulin in suppressing glucose production in depancreatized dogs: role of glucagon.
Endocrinology
138:
999-1007,
1997
14.
Goresky, C. A.,
C. G. Bach,
and
B. E. Nadeau.
Red cell carriage of label: its limiting effect on the exchange of materials in the liver.
Circ. Res.
36:
328-351,
1975[Abstract].
15.
Hother-Nielsen, O.,
J. E. Henriksen,
J. J. Holst,
and
H. Beck-Nielsen.
Effects of insulin on glucose turnover rates in vivo: isotope dilution versus constant specific activity technique.
Metabolism
45:
82-91,
1996[Medline].
16.
Katz, H.,
P. Butler,
M. Homan,
A. Zerman,
A. Caumo,
C. Cobelli,
and
R. Rizza.
Hepatic and extrahepatic insulin action in humans: measurement in the absence of non-steady-state error.
Am. J. Physiol.
264 (Endocrinol. Metab. 27):
E561-E566,
1993
17.
Leevy, C. M.,
C. L. Mendenhall,
W. Lesko,
and
M. M. Howard.
Estimation of hepatic blood flow with indocyanine green.
J. Clin. Invest.
41:
1169-1179,
1962.
18.
Lloyd, B.,
J. Burrin,
P. Smythe,
and
K. G. M. M. Alberti.
Enzymatic fluorometric continuous-flow assays for blood glucose, lactate, pyruvate, alanine, glycerol, and 3-hydroxybutyrate.
Clin. Chem.
24:
1724-1729,
1978
19.
Mari, A.
Estimation of the rate of appearance in the non-steady state with a two-compartment model.
Am. J. Physiol.
263 (Endocrinol. Metab. 26):
E400-E415,
1992
20.
McGuinness, O. P.,
T. Fugiwara,
S. Murrell,
D. Bracy,
D. Neal,
D. O'Connor,
and
A. D. Cherrington.
Impact of chronic stress hormone infusion on hepatic carbohydrate metabolism in the conscious dog.
Am. J. Physiol.
265 (Endocrinol. Metab. 28):
E314-E322,
1993
21.
McGuinness, O. P.,
S. R. Myers,
D. Neal,
and
A. D. Cherrington.
Chronic hyperinsulinemia decreases insulin action but not insulin sensitivity.
Metabolism
39:
931-937,
1989.
22.
Moore, M. C.,
A. D. Cherrington,
G. Cline,
E. M. Jones,
D. W. Neal,
C. Badet,
and
G. I. Shulman.
Sources of carbon for hepatic glycogen synthesis in the conscious dog.
J. Clin. Invest.
88:
578-587,
1991[Medline].
23.
Morgan, C. R.,
and
A. L. Lazarow.
Immunoassay of insulin: two antibody system. Plasma insulin of normal, subdiabetic, and diabetic rats.
Am. J. Med. Sci.
257:
415-419,
1963.
24.
Myers, S. R.,
O. P. McGuinness,
D. W. Neal,
and
A. D. Cherrington.
Intraportal glucose delivery alters the relationship between net hepatic glucose uptake and the insulin concentration.
J. Clin. Invest.
87:
930-939,
1991[Medline].
25.
Pagliassotti, M. J.,
L. C. Holste,
M. C. Moore,
D. W. Neal,
and
A. D. Cherrington.
Comparison of the time course of insulin and the portal signal on hepatic glucose and glycogen metabolism in the conscious dog.
J. Clin. Invest.
97:
81-91,
1996
26.
Price, C. P.,
B. Lloyd,
and
K. G. Alberti.
A kinetic spectrophotometric assay for rapid determination of acetoacetate in blood.
Clin. Chem.
23:
1893-1897,
1977
27.
Sindelar, D. K.,
J. H. Balcom,
C. A. Chu,
D. W. Neal,
and
A. D. Cherrington.
A comparison of the effects of a selective increase in peripheral or portal insulin on hepatic glucose production.
Diabetes
45:
1594-1604,
1996[Abstract].
28.
Sindelar, D. K.,
C. A. Chu,
M. Rohlie,
D. W. Neal,
L. L. Swift,
and
A. D. Cherrington.
The role of fatty acids in mediating the effects of peripheral insulin on hepatic glucose production in the conscious dog.
Diabetes
46:
187-196,
1997[Abstract].
29.
Venkatakrishnan, A.,
M. J. Abel,
R. A. Campbell,
E. P. Donahue,
T. C. Uselton,
and
P. J. Flakoll.
Whole blood analysis of gluconeogenic amino acids of de novo gluconeogenesis using pre-column o-phthalaldehyde derivatization and high performance liquid chromatography.
J. Chromatogr. B Biomed. Appl.
676:
1-6,
1996[Medline].
30.
Wasserman, D. H.,
R. J. Geer,
P. E. Williams,
D. B. Lacy,
and
N. N. Abumrad.
Interaction of gut and liver in nitrogen metabolism during exercise.
Metabolism
40:
307-314,
1991[Medline].