Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0615
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ABSTRACT |
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To determine the
effect of a selective rise in liver sinusoidal norepinephrine (NE) on
hepatic glucose production (HGP), norepinephrine (50 ng · kg1 · min
1)
was infused intraportally (Po-NE) for 3 h into five 18-h-fasted conscious dogs with a pancreatic clamp. In the control protocol, NE
(0.2 ng · kg
1 · min
1) and glucose were
infused peripherally to match the arterial NE and blood glucose levels
in the Po-NE group. Hepatic sinusoidal NE levels rose ~30-fold in the
Po-NE group but did not change in the control group. The arterial NE
levels did not change significantly in either group. During the portal
NE infusion, HGP increased from 1.9 ± 0.2 to 3.5 ± 0.4 mg · kg
1 · min
1
(15 min; P < 0.05) and then
gradually fell to 2.4 ± 0.4 mg · kg
1 · min
1
by 3 h. HGP in the control group did not change (2.0 ± 0.2 to 2.0 ± 0.2 mg · kg
1 · min
1)
for 15 min but then gradually fell to 1.1 ± 0.2 mg · kg
1 · min
1
by the end of the study. Because the fall in HGP from 15 min on was
parallel in the two groups, the effect of NE on HGP (the difference
between HGP in the two groups) did not decline over time.
Gluconeogenesis did not change significantly in either group. In
conclusion, elevation in hepatic sinusoidal NE significantly increases
HGP by selectively stimulating glycogenolysis. Compared with the
previously determined effects of epinephrine or glucagon on HGP, the
effect of NE is, on a molar basis, less potent but nore sustained over
time.
gluconeogenesis; adrenergic receptors
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INTRODUCTION |
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IT IS WELL KNOWN that norepinephrine, acting as a neurotransmitter or circulating hormone, can modify pancreatic hormone secretion and, as a result, indirectly regulate amino acid, fat, and glucose metabolism (40, 41). It is also known that norepinephrine can directly stimulate adipose tissue lipolysis, muscle glycogenolysis, and proteolysis, as well as hepatic glucose production (8, 9, 32, 35). The stimulatory effect of norepinephrine on hepatic glucose production is thought to arise both from its direct action on the liver per se and from its indirect effects arising from an increase in gluconeogenic precursor release from extrahepatic tissues (muscle and adipose tissue).
Previous studies in humans (20, 32, 35) suggested that an increase in
circulating norepinephrine can stimulate gluconeogenesis by mobilizing
alanine, as well as lactate, from muscle, and glycerol from adipose
tissue. Connolly et al. (8, 9) showed in dogs that an increase in
arterial norepinephrine (100 ± 24 to 3,244 ± 807 pg/ml) had a pronounced effect on adipose tissue lipolysis (plasma
glycerol rose fourfold) and muscle glycogenolysis (lactate release by
nonhepatic tissues increased by 11 µmol · kg1 · min
1).
They also showed that hepatic glucose production increased from 2.8 ± 0.2 to 3.4 ± 0.4 mg · kg
1 · min
1.
At the same time gluconeogenic efficiency rose by 212% and the maximal
gluconeogenic rate rose threefold. These data indicate that circulatory
norepinephrine increased hepatic glucose production by stimulating
gluconeogenesis as a result of an increase in gluconeogenic precursor
release from muscle and adipose tissue. The greater increase in
gluconeogenesis than in hepatic glucose production in that study
suggests that gluconeogenesis (indirect effect of norepinephrine) may
in fact have suppressed hepatic glycogenolysis. This is further
supported by our recent finding (5) that the gluconeogenic effects of
epinephrine, which also occur because of an increase in gluconeogenic
precursor release from extrahepatic tissues, decrease the
catecholamine's glycogenolytic effect on the liver. Thus the effect of
the elevation in hepatic sinusoidal norepinephrine per se (direct
effect of norepinephrine) was probably masked by the actions of the
rise in arterial norepinephrine on muscle and adipose tissue. Because
in all studies that have examined the effects of norepinephrine on
glucose production in vivo the catecholamine has been administered
peripherally, it has not been possible to evaluate the direct effect of
the catecholamine on the liver in vivo in the absence of its
overwhelming peripheral effects on glucose utilization, muscle
glycogenolysis, and adipose tissue lipolysis.
The question thus arises as to whether, in the absence of its peripheral gluconeogenic action, norepinephrine would increase hepatic glycogenolysis. This possibility is supported by earlier studies in different animal species (11, 15, 18, 22, 33), which showed that electrical stimulation of the distal cut end of the splanchnic sympathetic nerves rapidly increases norepinephrine release, glucose output, and the activity of glycogenolytic enzymes in the liver. This question becomes all the more important because norepinephrine released from sympathetic nerve terminals within the liver would selectively alter liver metabolism. The liver removes most (>95%) of the norepinephrine delivered to it (5), so very little norepinephrine released from hepatic nerve terminals reaches the peripheral tissues. Thus an increase in neural input to the liver would result in selective hepatic activation. Infusion of the catecholamine directly into the hepatic portal vein would allow us to assess the direct effect of norepinephrine on the liver in the absence of its effects on extrahepatic tissues. In view of the difficulties in delivering norepinephrine via the portal vein and in directly assessing hepatic gluconeogenesis in humans, we decided to address this question in the conscious dog.
A second reason to assess the direct effect of norepinephrine on the
liver arises from the finding that the distribution of adrenergic
receptors in the canine liver is, like those in humans, predominantly
1 and
2 (19, 23, 25). A previous
study (36) showed that epinephrine exerts its effect on hepatocytes
predominantly via
-adrenergic receptors. Because the
2-adrenergic receptor has a
much lower affinity for norepinephrine than epinephrine, the direct
effects of norepinephrine on the liver are presumably mediated through
1-receptors. Additionally, our
recent unpublished data showed that the effect of norepinephrine on the
liver was 85% inhibited by phentolamine (an
-adrenergic blocker).
Furthermore, the intracellular signal pathways of
1- and
2-adrenergic receptors are
quite different; they are mediated through the
Gq protein as well as
Ca2+, and through the
Gs protein as well as adenosine
3',5'-cyclic monophosphate (cAMP), respectively. Thus,
although we know that in the absence of its indirect effects on muscle
and fat epinephrine's action on the liver is mediated via an effect on
glycogenolysis, it is not clear whether such would also be the case for
norepinephrine.
The first aim of the present study, therefore, was to determine the direct effects of norepinephrine on hepatic glycogenolysis and gluconeogenesis in vivo in the absence of its action on muscle and adipose tissue (i.e., supply of gluconeogenic precursors reaching the liver), as well as in the absence of its pancreatic effects on insulin and glucagon secretion. The second aim was to test whether there are any differences between the direct effects of norepinephrine and epinephrine on the liver, given their differing affinity for the various subtypes of adrenergic receptors.
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MATERIALS AND METHODS |
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Experiments were carried out on ten 18-h-fasted conscious mongrel dogs (20-28 kg) of either sex that had been fed a standard diet of meat and chow described elsewhere (5, 6). The animals were housed in a facility that met American Association for the Accreditation of Laboratory Animal Care guidelines, and the protocols were approved by the Vanderbilt University Medical Center Animal Care Committee.
A laparotomy was performed 16-18 days before each experiment to implant catheters and Doppler flow probes into or around appropriate blood vessels, as described elsewhere (5, 6). Each dog was used for only one experiment. All dogs studied had 1) leukocyte count <18,000/mm3, 2) a hematocrit >35%, 3) a good appetite, and 4) normal stools.
Each experiment consisted of a 100-min tracer equilibration and hormone
adjustment period (140 to
40 min), a 40-min basal period
(
40 to 0 min), and a 180-min test period (0 to 180 min; Fig.
1). In all studies, a priming dose of
purified [3-3H]glucose
(42 µCi) was given at
140 min, followed by a constant infusion
of [3-3H]glucose (0.35 µCi/min),
[U-14C]alanine (0.35 µCi/min), and indocyanine green dye (ICG; 0.1 mg · m
2 · min
1).
An infusion of somatostatin (0.8 µg · kg
1 · min
1) was started at
130 min to inhibit endogenous insulin and glucagon secretion.
Concurrently, intraportal replacement infusions of insulin (300 µU · kg
1 · min
1)
and glucagon (0.65 ng · kg
1 · min
1)
were started. The plasma glucose level was monitored every 5 min, and
euglycemia was maintained by adjusting the rate of insulin infusion.
The final alteration in the insulin infusion rate was made
30 min
before the start of the basal period, and the rate of insulin infusion
(mean of 245 µU · kg
1 · min
1)
remained unchanged thereafter. The study included two groups. In the
first group (Po-NE), norepinephrine (50 ng · kg
1 · min
1),
in a solution of 0.07% ascorbic acid, was infused during the test
period via the splenic and jejunal vein catheters. In the second group
(Con), norepinephrine (2 ng · kg
1 · min
1)
was infused via the right cephalic vein catheter to match the arterial
norepinephrine level in the Po-NE group. The peripheral NE infusion was
based on our previous studies (5, 6), which showed that the liver
extracts almost all (>98%) of the catecholamine delivered
intraportally. Arterial glucose levels in the control group were
clamped to the level in the Po-NE group by an infusion of exogenous
glucose (20% dextrose) via the right cephalic vein. Blood pressure and
heart rate were measured using methods described elsewhere (5, 6).
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Plasma and blood glucose, plasma
[3H]- and
[14C]glucose, blood
lactate, glycerol, -hydroxybutyrate (BOHB), alanine, glutamine, glutamate, glycine, serine, threonine, and plasma free fatty acid (FFA)
were determined using previously described methods (5, 6). The levels
of insulin, glucagon, cortisol, epinephrine, and norepinephrine were
also determined as described elsewhere (5, 6).
Doppler flow probes and ICG were used to estimate total hepatic blood
flow (5, 6). The total hepatic blood flows in the test periods of the
two groups were 27 ± 2 or 29 ± 3 ml · kg1 · min
1
when measured with Doppler flow probes and 24 ± 3 or 26 ± 2 ml · kg
1 · min
1
when measured with ICG (Po-NE and Con groups, respectively). Because in
our studies hepatic blood flows measured using the Doppler method were
more stable than those determined with the ICG method, data in Figs.
1-5 and Tables 1-4 are those calculated with Doppler-measured
flows. The net hepatic balance and fractional extraction of blood
glucose, lactate, glycerol, BOHB, alanine, other gluconeogenic amino
acids, and plasma FFA in the present study were calculated using
arteriovenous difference (a-v) methods described elsewhere (5, 6).
Hepatic sinusoidal plasma norepinephrine levels were
calculated using Doppler-determined plasma flow
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It should be noted, to the extent that there is hepatic glucose uptake
(HGU), net hepatic glucose output (NHGO) slightly underestimates total
hepatic glucose release (NHGO+HGU). On the basis of an earlier study
(27), HGU in the control period of the present protocols was ~0.2
mg · kg1 · min
1.
However, because the increment in hepatic sinusoidal norepinephrine level increased hepatic glucose production, it is likely that HGU fell
slightly during norepinephrine infusion, so that total hepatic glucose
release during the test period was probably within
0.1
mg · kg
1 · min
1
of net hepatic glucose output. In the control group, hyperglycemia (
16 mg/dl in blood glucose) would be expected to increase HGU slightly (0.1 mg · kg
1 · min
1),
as indicated by the data of Pagliassotti et al. (30). This would cause
NHGO to underestimate total hepatic glucose release by
0.3
mg · kg
1 · min
1.
Total glucose production (Ra)
and utilization (Rd) were
determined using both one- and two-compartment models, as previously described (5, 6). The results were similar regardless of which approach
was employed because the deviations from steady state were minimal. The
Ra and
Rd data shown in Fig. 3 and Table 2, respectivley, are those calculated with the two-compartment method.
It should also be noted, because the kidneys produce a small amount of
glucose, that the rate of endogenous glucose production determined by
the tracer method slightly (0.3 mg · kg1 · min
1)
overestimates total hepatic glucose release (27). This overestimate, however, should have been equal in the two groups and would not have
been expected to change during the test period in either group.
Gluconeogenic efficiency was assessed using a double-isotope technique
described elsewhere (5, 6). Because the conversion of
[14C]alanine to
[14C]glucose by the
kidney is minimal (28),
[14C]glucose
production in our study was almost exclusively attributable to the
liver. Maximal and minimal rates of gluconeogenesis from circulating
gluconeogenic precursors were calculated using the methods described
previously (5, 6). Once the maximal and minimal gluconeogenic rates
were obtained, hepatic glycogenolysis was estimated by subtracting
either the maximal or minimal gluconeogenic rate from either NHGO or
total endogenous glucose production. Because a recent study in the dog
(14) showed that the maximal gluconeogenic rate is closer to the
gluconeogenic rate determined using the biopsy and high-performance
liquid chromatography method of Giaccari and Rossetti (13), hepatic
glycogenolysis presented (see Fig. 4) was calculated by subtracting the
maximal gluconeogenic rate from NHGO or tracer-determined glucose
production.
Statistical analysis. All statistical comparisons were made using repeated-measures analysis of variance with post hoc analysis by univariate F tests or the paired Student's t-test where appropriate. Statistical significance was accepted at P < 0.05. Data are expressed as means ± SE.
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RESULTS |
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Hormone levels. The arterial and portal plasma levels of insulin and glucagon remained at basal values in both groups throughout the study (Table 1). The arterial plasma levels of epinephrine and cortisol also remained unchanged in both groups (Table 1). During the test period the arterial plasma levels of norepinephrine increased slightly in both the Po-NE group [188 ± 24 to 231 ± 28 pg/ml; not significant (NS)] and the control group (150 ± 10 to 206 ± 13 pg/ml; NS) (Fig. 2). The portal vein and hepatic sinusoidal plasma levels of norepinephrine increased from 126 ± 16 to 4,951 ± 666 pg/ml (P < 0.05) and from 139 ± 16 to 4,154 ± 559 pg/ml (P < 0.05), respectively, in the Po-NE group, but did not change (108 ± 8 to 102 ± 7 pg/ml and 121 ± 11 to 123 ± 8 pg/ml, respectively) in the control group (Fig. 2).
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Hepatic blood flow, arterial blood pressure, and heart rate. Hepatic blood flow remained stable in both groups. The systolic, diastolic, and mean arterial blood pressures, as well as heart rate, did not change significantly in either group.
Glucose levels and kinetics.
During portal norepinephrine infusion, the arterial blood glucose level
gradually rose from 79 ± 5 to 93 ± 7 mg/dl
(P < 0.05) by 60 min and remained
constant thereafter (Fig. 3). The arterial glucose level in the control group (76 ± 3 to 94 ± 4 mg/dl by 60 min, P < 0.05) was clamped to
that seen in the Po-NE group. In response to the portal infusion of
norepinephrine, NHGO increased from 1.9 ± 0.2 to 3.5 ± 0.4 mg · kg1 · min
1
by 15 min (P < 0.05) and then
gradually fell back to 2.4 ± 0.4 mg · kg
1 · min
1
by the end of the study (Fig. 3). In the presence of the hyperglycemic clamp alone (control group), NHGO did not change initially,
but gradually fell to 1.1 ± 0.2 mg · kg
1 · min
1
by the end of the study (P < 0.05;
Fig. 3). Because the fall in NHGO from 15 min on was parallel in the
two groups, the effect of norepinephrine on net hepatic glucose
production (the difference between the Po-NE and control groups) was
sustained over time [
1.4 and
1.3
mg · kg
1 · min
1
initially (15-30 min) and during the last 30 min of the test period, respectively; Fig. 3]. The changes in tracer-determined endogenous glucose production paralleled those in NHGO in both groups
and also indicated that the effect of norepinephrine did not wane with
time (
1.1 and
1.1
mg · kg
1 · min
1
initially and during the last 30 min of the test period, respectively; Fig. 3). Tracer-determined glucose utilization remained essentially unchanged in each group (Table 2). Glucose
clearance fell from 2.2 ± 0.2 to 1.8 ± 0.2 ml · kg
1 · min
1
(P < 0.05) and from 2.4 ± 0.2 to
1.9 ± 0.1 ml · kg
1 · min
1
(P < 0.05) in the Po-NE and control
groups, respectively (Table 2).
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Arterial blood level, net hepatic uptake, and fractional extraction of alanine. The arterial level, net hepatic uptake, and fractional extraction of alanine did not change significantly in both groups throughout (Table 3).
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Arterial blood level and net hepatic balance of lactate. The arterial blood levels and net hepatic uptake of lactate did not change significantly in either group (Table 3).
Arterial blood level, net hepatic uptake, and fractional extraction of glycerol, FFA, and BOHB. Neither the blood level nor the net uptake or fractional extraction of glycerol by the liver changed in either group (Table 3). Similarly, there were no changes in FFA or BOHB metabolism (Table 3).
Arterial blood levels and net hepatic balances of gluconeogenic
amino acids.
The arterial blood glutamine levels remained essentially unchanged in
the Po-NE and control groups (Table 4). Net
hepatic glutamine balance, on the other hand, switched from output to uptake in both groups (0.43 ± 0.67 to 0.75 ± 0.41 µmol · kg
1 · min
1
in Po-NE and from 0.71 ± 0.74 to
0.51 ± 1.15 µmol · kg
1 · min
1
in control), but there once again was no effect of portal
norepinephrine infusion (Table 4). The arterial blood levels and net
hepatic balances of the other gluconeogenic amino acids were not
significantly altered by saline or norepinephrine infusion (Table 4).
In the control group the net fractional extraction of the gluconeogenic amino acids by the liver rose slightly, but not significantly, by the
end of the study (Table 4). In the Po-NE group the changes were
slightly greater than those in the control group so that they achieved
significance relative to the control period but not to the saline group
(Table 4). Taken together, the amino acid data indicate that the
intraportal infusion of norepinephrine had no detectable effect on
gluconeogenic amino acid metabolism.
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Gluconeogenic parameters.
Hepatic gluconeogenic efficiency (24 ± 8 to 28 ± 7% and 24 ± 9 to 28 ± 10%), as well as the maximal (0.6 ± 0.2 to 0.6 ± 0.2 mg · kg1 · min
1 and 0.5 ± 0.2 to
0.6 ± 0.2 mg · kg
1 · min
1)
and minimal (0.1 ± 0.0 to 0.2 ± 0.1 mg · kg
1 · min
1
and 0.1 ± 0.0 to 0.2 ± 0.1 mg · kg
1 · min
1)
gluconeogenic rates, remained unchanged in the Po-NE and control groups, respectively (Fig. 4).
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Hepatic glycogenolytic rate.
In response to norepinephrine infusion, hepatic glycogenolysis,
calculated using the a-v difference data (Fig.
5), increased by 1.5 ± 0.5 mg · kg1 · min
1
(P < 0.05) within 15 min. In the
control group hepatic glycogenolysis gradually fell, reaching 1.2 ± 0.4 mg · kg
1 · min
1
(P < 0.05) by the end of the study
(Fig. 5). The effect of norepinephrine on hepatic glycogenolysis (the
difference between the glycogenolytic rate in the two groups) was,
therefore, sustained over time (
1.5 and
1.3
mg · kg
1 · min
1
initially and during the last 30 min of the test period, respectively; Fig. 5). When glycogenolysis was estimated using the tracer data, the
pattern and magnitude of change were similar to those seen when the a-v
difference data were used (
1.1 and
1.1
mg · kg
1 · min
1
initially and during the last 30 min of the test period, respectively; Fig. 5).
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DISCUSSION |
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The aim of the present study was to determine the effects of a selective increase in hepatic sinusoidal norepinephrine on hepatic glucose production. Because the arterial and portal levels of insulin and glucagon were clamped at basal values in both groups, the epinephrine and cortisol levels were unchanged in both groups, and the changes in the arterial glucose concentrations were matched in both groups, we were able to assess the effects of a selective increase in hepatic sinusoidal norepinephrine (from 139 ± 16 to 4,154 ± 559 pg/ml) on hepatic glucose metabolism. Given that the norepinephrine level was selectively increased within the hepatic sinusoids, we were able to separate the direct effects of the catecholamine on hepatic glucose production from the indirect effects that come about by virtue of its ability to increase the flow of gluconeogenic precursors and FFA from muscle and adipose tissue to the liver.
In response to the rise in hepatic sinusoidal norepinephrine, hepatic
glucose production increased from 1.9 ± 0.2 to 3.5 ± 0.4 mg · kg1 · min
1
by 15 min (P < 0.05) and then
gradually fell over time. The efficient clearance of norepinephrine by
the liver (>98%) prevented a significant increase in the arterial
norepinephrine level (188 ± 24 to 231 ± 28 pg/ml). This in turn
explains the absence of any lipolytic or glycogenolytic effects of
norepinephrine on adipose tissue or muscle, respectively. It should be
pointed out that portal infusion of norepinephrine may have increased
the net hepatic fractional extraction of alanine and several other
gluconeogenic amino acids slightly by the end of the study (even though
the change did not reach significance). Because the arterial blood levels of those amino acids fell, any effect of this increase in
fractional extraction on gluconeogenesis was offset by the decreasing
plasma amino acid levels. Neither gluconeogenic efficiency nor the
estimated gluconeogenic rate (maximal or minimal) changed during the 3 h of norepinephrine infusion. It seems unlikely that higher levels of
norepinephrine would produce a direct gluconeogenic effect on the
liver. In a recent study (6), we simultaneously infused both
norepinephrine and epinephrine intraportally at very high rates for 90 min in the presence of pancreatic clamp, and even together they failed
to have a direct gluconeogenic effect on the liver. Whether a longer
elevation (in excess of 3 h) in the hepatic sinusoidal norepinephrine
concentration would produce a meaningful gluconeogenic effect on the
liver remains to be determined. Taken together, the above data suggest
that the increase in glucose production caused by the selective release
of norepinephrine from sympathetic nerve terminals within the liver
would be solely attributable to an increase in hepatic
glycogenolysis.
Because our data do not support a direct gluconeogenic action of norepinephrine on the liver, the question arises as to whether a direct gluconeogenic effect of the catecholamine has been observed in vitro. Indeed, norepinephrine increases hepatic glucose output from the perfused liver of the 24-h-fasted (glycogen-depleted) rat (34). The reason that the catecholamine can stimulate gluconeogenesis directly at the liver in vitro but not in vivo is unclear, but several possibilities must be considered. First, in the present study all of the basal neural and hormonal signals impacting on the liver were present, whereas in the in vitro studies they were absent. Second, the distribution of adrenergic receptor subtypes in rat liver is different from that in dog liver (2, 12, 25, 36). Additionally, a 24-h-fasted rat liver is devoid of glycogen, whereas the liver of the overnight-fasted dog is not (5, 6, 34, 37, 38). Because the gluconeogenic effect of the catecholamine is seen in the perfused fasted rat liver but not in the perfused fed rat liver (1, 2, 31, 34), it is possible that the glycogenolytic effect of norepinephrine may have a suppressive effect on its gluconeogenic action. Finally, to see the gluconeogenic effect of norepinephrine in the perfused rat liver, very high levels (5-10 mM) of gluconeogenic precursors (i.e., lactate, alanine, glycerol) had to be included in the perfusate (34). Because in the present study norepinephrine was administered intraportally, the load of gluconeogenic precursors reaching the liver remained at basal values. This may mean that a direct effect of the catecholamine on gluconeogenesis within the liver can only be manifest if there is an elevated load of gluconeogenic precursors reaching the liver. Each or all of the above suggestions could explain why there was no increase in gluconeogenesis during portal norepinephrine infusion in the current study, whereas such has been reported to occur in vitro. Whether a direct gluconeogenic effect of norepinephrine on the liver would become apparent in the presence of an increase in the load of gluconeogenic precursors or FFA reaching the liver remains to be determined.
Previous studies in the human (32, 35) and the dog (8, 9) showed that norepinephrine given via a limb vein increased hepatic glucose production mainly by stimulating gluconeogenesis. Because norepinephrine was delivered peripherally in those studies, it dramatically increased the supply of alanine, lactate, and glycerol reaching the liver as a result of its glycogenolytic and lipolytic effects on muscle and adipose tissue, respectively. The data of Connolly et al. (8) suggested, in fact, that hepatic glycogenolysis was reduced by norepinephrine when its gluconeogenic effects were manifest. In our recent study (5) we showed that the glycogenolytic effect of epinephrine on the liver was significantly suppressed when its effect on gluconeogenic precursor supply was present. This suggests that the glycogenolytic suppression observed by Connolly et al. was due to a suppressive effect resulting from the peripheral actions of norepinephrine to increase the supply of gluconeogenic precursors and FFA reaching the liver. Indeed, in the present study, in the absence of its peripheral effects norepinephrine had a significant glycogenolytic effect on the liver.
The infusion rate of norepinephrine used in the current study elevated hepatic sinusoidal levels of the catecholamine to 4,154 ± 559 pg/ml. This level was chosen in an attempt to mimic synaptic cleft norepinephrine levels seen during moderate stress (i.e., mild hypoglycemia or heavy exercise). Estimates of norepinephrine levels occurring within synapses have been made (21) by blocking reuptake and degradation of norepinephrine at the same time as measuring the plasma level of norepinephrine required to cause a 20-mmHg pressor response. That work suggested that plasma norepinephrine levels of 3,500-4,000 pg/ml must be achieved to elevate synaptic cleft norepinephrine levels to those present during hypotensive stress. One should bear in mind, however, that in response to more extreme stress (i.e., deep hypoglycemia, hemorrhagic shock) the norepinephrine levels at nerve terminals within the liver could reach levels in excess of 10,000 pg/ml (35). Furthermore, a recent study in the human (17) showed that norepinephrine can reach levels as much as 7,000 pg/ml in plasma during exhaustive exercise, meaning that even higher levels must exist at nerve terminals within the liver. Clearly, the norepinephrine levels used in the present study are consistent with those that must occur around hepatocytes during certain stressful situations.
In the present study, a 30-fold elevation in the hepatic sinusoidal
plasma norepinephrine increased NHGO (the difference between the two
groups) by 1.6 mg · kg1 · min
1
within 15 min. This is similar to the increment caused by a 10-fold increase in hepatic sinusoidal plasma epinephrine (1.9 mg · kg
1 · min
1)
(5). Because the basal norepinephrine level is approximately threefold
that of the basal epinephrine level, the potency of epinephrine is, on
a molar basis, ~10-fold that of norepinephrine. Because the direct
effects of the two catecholamines on the liver both reflect alterations
in glycogenolysis, one has to explain their different potencies by some
other mechanism. One possible explanation is that binding affinities of
the two hormones for the adrenergic receptor are different. Another
possibility is that the two catecholamines stimulate hepatic
glycogenolysis via different adrenergic receptors. It has been shown in
vitro that epinephrine (10, 23) and norepinephrine (10, 12) stimulate hepatic glucose production primarily through
2-adrenergic receptors, which
increase the level of cAMP, and through
1-adrenergic receptors, which
increase the level of Ca2+ in the
cytosol, respectively. Early in vitro studies (7, 12, 29) showed that
the stimulation of hepatic glucose production by norepinephrine, acting
through
1-adrenergic receptors,
is smaller in magnitude than that caused by equimolar epinephrine acting through
2-adrenergic
receptors. An vitro study using dog hepatocytes (36) showed that the
effect of epinephrine on glucose output was inhibited by 77 and 27% in
the presence of propranolol and phentolamine, respectively. These data
suggest that, in the dog, epinephrine acts primarily via
-adrenergic
receptors. Because the major type of adrenergic receptor in dog liver
is
2 (23, 25), and because
2-receptors are much less
sensitive to norepinephrine than to epinephrine (7, 10, 16), the effect
of norepinephrine on hepatic glucose production in the current study is
probably mediated through
-adrenergic receptors.
In the present study the effect of norepinephrine on hepatic glucose
production, as indicated by the difference between the changes in
tracer-determined glucose production in the two groups, remained
essentially unchanged over time whether the tracer or a-v difference
data are examined (Fig. 3). It should be noted that the use of the
tracer method to estimate the magnitude of the norepinephrine effect
results in a slight overestimate of hepatic glucose release because
glucose produced by the kidney is included (see
MATERIALS AND METHODS). The error
was probably equal in the two groups and is unlikely to have changed
over time, so that it would have no impact on the calculated effect of
norepinephrine (i.e., the difference between glucose production in the
two groups). The use of a-v difference data would, on the other hand,
lead to a small underestimate of hepatic glucose release in both
groups, but the error would be slightly greater (0.2 to 0.3 mg · kg
1 · min
1; see
MATERIALS AND METHODS) in the
control group because of the effect of hyperglycemia per se on hepatic
glucose uptake. This would exaggerate the difference between the two
groups and would explain why there was a slightly greater effect of
norepinephrine on hepatic glucose production when the latter was
calculated using a-v data. The sustained effect (Figs. 3 and 5) of
norepinephrine on hepatic glucose production (glycogenolysis) is in
contrast to the waning effect of epinephrine or glucagon reported in
earlier publications (6, 26, 38, 39). It has been shown in vitro (4,
10, 24) that the effects of the latter two hormones on hepatic glucose
production are transient, reflecting a spike decline pattern of
intracellular cAMP production. An early study in the dog (3) showed
that a selective fourfold increase in glucagon (insulin was kept
constant at basal) increased tracer-determined hepatic glucose
production by 4.8 mg · kg
1 · min
1
within 30 min but that the magnitude of the increase fell to 2.8 mg · kg
1 · min
1
by 3 h. Similarly, in a recent study (5) we showed that a selective
10-fold increase in liver sinusoidal epinephrine (i.e., insulin and
glucagon were kept constant and basal) increased
tracer-determined hepatic glucose production by 1.9 mg · kg
1 · min
1
within 30 min but that the effect fell to 0.8 mg · kg
1 · min
1
by 3 h. The time dependence of glucagon's and epinephrine's action was also clearly evident when a-v difference data were examined. The
persistent effect of norepinephrine on hepatic glucose production and
glycogenolysis in the present study could reflect the fact that the
initial rise in glucose production caused by norepinephrine was
somewhat less than that seen with epinephrine and glucagon. Alternatively, it could be taken to further support the concept that
the action of norepinephrine on the liver is mediated by a different
intracellular mechanism than epinephrine or glucagon.
In conclusion, the direct effect of norepinephrine on hepatic glucose production is attributable to a stimulation of glycogenolysis. The catecholamine has little, if any, direct gluconeogenic effect on the liver in the absence of its ability to increase the gluconeogenic precursor load to the liver. Compared with the previously determined effects of epinephrine or glucagon on hepatic glucose production, the effect of norepinephrine is less potent but more sustained.
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ACKNOWLEDGEMENTS |
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We are grateful to Dr. Mary Courtney Moore, Dr. Masakazu Shiota, Dr. David Wasserman, Dr. Owen P. McGuinness, and Maya Emshwiller for their valuable comments and careful review of this manuscript. We especially appreciate assistance from Jon Hastings, Melanie Scott, Tricia Jackson, Tommy Monohan, Pam Venson, Wanda Snead, Paul Flakoll, and Yang Ying.
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FOOTNOTES |
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This research was supported in part by National Institute of Diabetes and Digestive and Kidney Diseases Grants 2RO1 DK-18243 and 5P60 DK-20593 (Diabetes Research and Training Center). Part of this work was presented at the 57th Annual Meeting of the American Diabetes Association, Boston, MA, in June, 1997.
Address for reprint requests: C. A. Chu, Dept. of Molecular Physiology and Biophysics, 702 Light Hall, Vanderbilt Univ. School of Medicine, Nashville, TN 37232-0615.
Received 30 June 1997; accepted in final form 1 October 1997.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Blackmore, P. F.,
F. T. Brumley,
J. L. Marks,
and
J. H. Exton.
Studies on -adrenergic activation of hepatic glucose output. Relationship between
-adrenergic stimulation of calcium efflux and activation of phosphorylase in isolated rat liver parenchymal cells.
J. Biol. Chem.
253:
4851-4858,
1978[Medline].
2.
Chan, T. M.,
and
J. H. Exton.
Studies on an alpha-adrenergic activition of hepatic glucose output.
J. Biol. Chem.
253:
6393-6400,
1978[Medline].
3.
Cherrington, A. D.,
M. P. Diamond,
D. R. Green,
and
P. E. Williams.
Evidence for an intrahepatic contribution to the waning effect of glucagon on glucose production in the conscious dog.
Diabetes
31:
917-922,
1982[Abstract].
4.
Cherrington, A. D., and J. H. Exton.
Studies on the role of cAMP-dependent protein kinase in the
actions of glucagon and catecholamines on liver glycogen metabolism.
Metabolism 25, Suppl. 1: 1351-1354,
1976.
5.
Chu, C. A.,
D. K. Sindelar,
D. W. Neal,
E. J. Allen,
E. P. Donahue,
and
A. D. Cherrington.
Comparison of the direct and indirect effects of epinephrine on hepatic glucose production.
J. Clin. Invest.
99:
1044-1056,
1997
6.
Chu, C. A.,
D. K. Sindelar,
D. W. Neal,
and
A. D. Cherrington.
Direct effects of catecholamines on hepatic glucose production in conscious dog are due to glycogenolysis.
Am. J. Physiol.
271 (Endocrinol. Metab. 34):
E127-E137,
1996
7.
Clutter, W. E.,
R. A. Rizza,
J. E. Gerich,
and
P. E. Cryer.
Regulation of glucose metabolism by sympathochromaffin catecholamines.
Diabetes Metab. Rev.
4:
1-15,
1988[Medline].
8.
Connolly, C. C.,
K. E. Steiner,
R. W. Stevenson,
D. W. Neal,
P. E. Williams,
K. G. M. M. Alberti,
and
A. D. Cherrington.
Regulation of glucose metabolism by norepinephrine in conscious dogs.
Am. J. Physiol.
261 (Endocrinol. Metab. 24):
E764-E772,
1991
9.
Connolly, C. C.,
K. E. Steiner,
R. W. Stevenson,
D. W. Neal,
P. E. Williams,
K. G. M. M. Alberti,
and
A. D. Cherrington.
Regulation of lipolysis and ketogenesis by norepinephrine in conscious dogs.
Am. J. Physiol.
261 (Endocrinol. Metab. 24):
E466-E472,
1991
10.
Exton, J. H.
Mechanism of hormonal regulation of hepatic glucose metabolism.
Diabetes Metab. Rev.
3:
163-183,
1987[Medline].
11.
Gardemann, A.,
G. P. Puschel,
and
K. Jungermann.
Nervous control of liver metabolism and hemodynamics.
Eur. J. Biochem.
207:
399-411,
1992[Medline].
12.
Garrison, J. C.,
and
M. K. Borland.
Regulation of mitochondrial pyruvate carboxylation and gluconeogenesis in rat hepatocytes via an 1-adrenergic, adenosine 3': 5'-monophosphate-independent mechanism.
J. Biol. Chem.
254:
1129-1133,
1979[Abstract].
13.
Giaccari, A.,
and
L. Rossetti.
Predominant role of gluconeogenesis in the hepatic glycogen repletion of diabetic rats.
J. Clin. Invest.
89:
36-45,
1992[Medline].
14.
Goldstein, R., B. Palmer, R. Liu, and D. Massillon. The
effects of chronic hypercortisolemia on gluconeogenesis assessed using
two independent methods in vivo. Diabetes 44, Suppl. 1: 201, 1995.
15.
Hartmann, H.,
K. Beckh,
and
K. Jungermann.
Direct control of glycogen metabolism in the perfused rat liver by the sympathetic innervation.
Eur. J. Biochem.
123:
521-526,
1982[Abstract].
16.
Hieble, J. P.,
W. E. Bondinell,
and
R. R. Ruffolo, Jr.
- and
-Adrenoceptors: from the gene to the clinic. 1. Molecular biology and adrenoceptor subclassification.
J. Med. Chem.
38:
3415-3444,
1995[Medline].
17.
Jackman, M.,
P. Wendling,
D. Friars,
and
T. E. Graham.
Metabolic, catecholamine, and endurance responses to caffeine during intense exercise.
J. Appl. Physiol.
81:
1658-1663,
1996
18.
Jungermann, K.,
A. Gardemann,
U. Beuers,
C. Balle,
J. Sannemann,
K. Beckh,
and
H. Hartmann.
Regulation of liver metabolism by the hepatic nerves.
Adv. Enzyme Regul.
26:
63-88,
1987[Medline].
19.
Kawai, Y.,
A. Powell,
and
I. J. Arinze.
Adrenergic receptors in human liver plasma membranees: predominance of 2- and
1-receptor subtypes.
J. Clin. Endocrinol. Metab.
62:
827-832,
1986[Abstract].
20.
Keller, U.,
P. P. G. Gerber,
and
W. Stauffacher.
Stimulatory effect of norepinephrine on ketogenesis in normal and insulin-deficient humans.
Am. J. Physiol.
247 (Endocrinol. Metab. 10):
E732-E739,
1984
21.
Kopin, I. J.,
Z. Z. Grojec,
M. A. Bayorh,
and
D. S. Goldstein.
Estimation of intrasynaptic norepinephrine concentrations at vascular neuroeffector junctions in vivo.
Nauny Schmiedeberg's Arch. Pharmacol.
325:
298-305,
1984[Medline].
22.
Lautt, W. W.,
and
C. Wong.
Hepatic glucose balance in response to direct stimulation of sympathetic nerves in the intact liver of cats.
Can. J. Physiol. Pharmacol.
56:
1022-1028,
1978[Medline].
23.
Lefkowitz, R. J.
Heterogeneity of adenylate cyclase-coupled -adrenergic receptors.
Biochem. Pharmacol.
24:
583-590,
1975[Medline].
24.
Liljenquist, J. E.,
J. D. Bomboy,
S. B. Lewis,
B. C. Sinclair-Smith,
P. W. Felts,
W. W. Lacy,
O. B. Crofford,
and
G. W. Liddle.
Effect of glucagon on net splanchnic cyclic AMP production in normal and diabetic men.
J. Clin. Invest.
53:
198-204,
1974[Medline].
25.
Liu, M. S.,
and
S. Ghosh.
Changes in -adrenergic receptors in dog livers during endotoxic shock.
Am. J. Physiol.
244 (Regulatory Integrative Comp. Physiol. 13):
R718-R723,
1983[Medline].
26.
Magnusson, I.,
D. L. Rothman,
D. P. Gerard,
L. D. Katz,
and
G. I. Shulman.
Contribution of hepatic glycogenolysis to glucose production in humans in response to a physiological increase in plasma glucagon concentration.
Diabetes
44:
185-189,
1995[Abstract].
27.
McGuinness, O. P.,
T. Fujiwara,
S. Murrell,
D. Bracy,
D. W. 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
28.
Meyer, C., M. Stumvoll, U. Chintalapudl, O. Gutierrez, M. Kreider, G. Perriello, S. Welle, and J. Gerich. Alanine and
glutamine: selective markers for hepatic and renal gluconeogenesis in
humans. Diabetes 45, Suppl. 1: 945, 1996.
29.
Morgan, N. G.,
R. Charest,
P. F. Blackmore,
and
J. H. Exton.
Potentiation of 1-adrenergic responses in rat liver by a cAMP-independent mechanism.
Proc. Natl. Acad. Sci. USA
81:
4208-4212,
1984[Abstract].
30.
Pagliassotti, M. J.,
L. C. Holste,
M. C. Moore,
D. W. Neal,
and
A. D. Cherrington.
Comparison of the time courses of insulin and the portal signal on hepatic glucose and glycogen metabolism in the conscious dog.
J. Clin. Invest.
97:
81-91,
1996
31.
Saitoh, Y,
and
M. Ui.
Stimulation of glycogenolysis and gluconeogenesis by epinephrine independent of its beta-adrenergic function in perfused rat liver.
Biochem. Pharmacol.
25:
841-845,
1976[Medline].
32.
Schade, D. S.,
and
R. P. Eaton.
The regulation of plasma ketone body concentration by counterregulatory hormones in man. III. Effects of norepinephrine in normal man.
Diabetes
28:
5-10,
1979[Medline].
33.
Shimazu, T,
and
A. Fukuda.
Increased activities of glycogenolytic enzymes in liver after splanchnic nerve stimulation.
Science
150:
1607-1608,
1965[Medline].
34.
Shiota, M.,
T. Tanaka,
and
T. Sugano.
Effect of norepinephrine on gluconeogenesis in perfused livers of cold-exposed rats.
Am. J. Physiol.
249 (Endocrinol. Metab. 12):
E281-E286,
1985
35.
Silverberg, A. B.,
S. D. Shah,
M. W. Haymond,
and
P. E. Cryer.
Norepinephrine: hormone and neurotransmitter in man.
Am. J. Physiol.
234 (Endocrinol. Metab. Gastrointest. Physiol. 3):
E252-E256,
1978[Medline].
36.
Steiner, K. E.,
R. W. Stevenson,
D. R. Green,
and
A. D. Cherrington.
Mechanism of epinephrine's glycogenolytic effect in isolated canine hepatocytes.
Metabolism
34:
1020-1023,
1985[Medline].
37.
Steiner, K. E.,
R. W. Stevenson,
B. A. Marshall,
and
A. D. Cherrington.
The effects of epinephrine on ketogenesis in the dog after a prolonged fast.
Metabolism
40:
1057-1062,
1991[Medline].
38.
Stevenson, R. W.,
K. E. Steiner,
C. C. Connolly,
H. Fuchs,
K. G. M. M. Alberti,
P. E. Williams,
and
A. D. Cherrington.
Dose-related effects of epinephrine on glucose production in conscious dogs.
Am. J. Physiol.
260 (Endocrinol. Metab. 23):
E363-E370,
1991
39.
Stevenson, R. W.,
K. E. Steiner,
M. A. Davis,
and
A. D. Cherrington.
Similar dose responsiveness of hepatic glycogenolysis and gluconeogenesis to glucagon in vivo.
Diabetes
36:
382-389,
1987[Abstract].
40.
Struthers, A. D.,
D. C. Brown,
M. J. Brown,
B. Schumer,
and
S. R. Bloom.
Selective 2 receptor blockade facilitates the insulin response to adrenaline but not to glucose in man.
Clin. Endocrinol.
23:
530-546,
1985.
41.
Waern, A. U., C. Berne, L. Wibell, and H. Lithell.
Short term influence of a postsynaptic -adrenoceptor blocking
drug (prazosin) on carbohydrate metabolism. Acta Med.
Scand. 665, Suppl.:
75-77, 1982.