The direct effects of catecholamines on hepatic glucose
production occur via
1- and
2-receptors
in the dog
Chang An
Chu,
Dana K.
Sindelar,
Kayano
Igawa,
Stephanie
Sherck,
Doss W.
Neal,
Maya
Emshwiller, and
Alan D.
Cherrington
Department of Molecular Physiology and Biophysics, Vanderbilt
University School of Medicine, Nashville, Tennessee 37232
 |
ABSTRACT |
The role of
- and
-adrenergic receptor subtypes in
mediating the actions of catecholamines on hepatic glucose production (HGP) was determined in sixteen 18-h-fasted conscious dogs maintained on a pancreatic clamp with basal insulin and glucagon. The experiment consisted of a 100-min equilibration, a 40-min basal, and two 90-min
test periods in groups 1 and 2, plus a 60-min
third test period in groups 3 and 4. In
group 1 [
-blockade with norepinephrine (
-blo+NE)],
phentolamine (2 µg · kg
1 · min
1) was
infused portally during both test periods, and NE (50 ng · kg
1 · min
1) was
infused portally at the start of test period 2. In
group 2,
-blockade with epinephrine (
-blo+EPI),
propranolol (1 µg · kg
1 · min
1) was
infused portally during both test periods, and EPI (8 ng · kg
1 · min
1) was
infused portally during test period 2. In group
3 (
1-blo+NE), prazosin (4 µg · kg
1 · min
1) was
infused portally during all test periods, and NE (50 and 100 ng · kg
1 · min
1) was
infused portally during test periods 2 and 3,
respectively. In group 4 (
2-blo+EPI), butoxamine (40 µg · kg
1 · min
1) was
infused portally during all test periods, and EPI (8 and 40 ng · kg
1 · min
1) was
infused portally during test periods 2 and 3,
respectively. In the presence of
- or
1-adrenergic
blockade, a selective rise in hepatic sinusoidal NE failed to increase
net hepatic glucose output (NHGO). In a previous study, the same rate
of portal NE infusion had increased NHGO by 1.6 ± 0.3 mg · kg
1 · min
1. In the
presence of
- or
2-adrenergic blockade, the selective rise in hepatic sinusoidal EPI caused by EPI infusion at 8 ng · kg
1 · min
1 also failed
to increase NHGO. In a previous study, the same rate of EPI
infusion had increased NHGO by 1.6 ± 0.4 mg · kg
1 · min
1. In
conclusion, in the conscious dog, the direct effects of NE and
EPI on HGP are predominantly mediated through
1- and
2-adrenergic receptors, respectively.
adrenergic receptor; hepatic glucose production; glycogenolytic
rate
 |
INTRODUCTION |
IN STRESSFUL
CONDITIONS and pathophysiological states (e.g., exercise,
hypoglycemia, and shock), the rise in circulating catecholamines plays
an important role in stimulating liver glucose output (2, 5, 7, 21, 22,
26). The stimulatory effects of the catecholamines
on hepatic glucose production arise from their actions on extrahepatic
tissues (muscle and adipose tissue) and on the liver. Previous studies
in humans (21, 22) and other animals
(7, 26) have shown that plasma catecholamines
can stimulate glycogenolysis in muscle and lipolysis in adipose tissue and thereby move lactate, alanine, glycerol, and free fatty acids (FFA)
to the liver. This in turn increases hepatic gluconeogenesis. Our
recent studies (2, 3, 5) have
shown that the direct hepatic effects of the catecholamines
[norepinephrine (NE) and epinephrine (EPI)] are attributable to their
stimulation of glycogenolysis. Taken together, the above studies have
shown that the direct and indirect effects of the catecholamines on the
liver relate to their glycogenolytic and gluconeogenic actions,
respectively. Furthermore, in vitro work (10,
25, 27) has suggested that EPI works on the
liver primarily via
-receptors, whereas NE works through
-receptors.
Recent studies in dog hepatocytes (14, 15)
showed that the distribution of adrenergic receptors in canine liver is
similar to the distribution in human liver (13), that is,
predominantly the
1- and
2-subtypes.
Because the intracellular signaling pathways of
1- and
2-adrenergic receptor subtypes are mediated through the
G protein isoform Gq as well as Ca2+, and the isoform Gs as
well as cAMP, respectively (10, 27), it becomes of interest to determine whether EPI and NE bring about the
same hepatic action (glycogenolysis) in the dog through different adrenergic mechanisms. This is all the more important in light of our
recent finding (5) that the patterns of the stimulatory effects of the two catecholamines on hepatic glycogenolysis are quite
different. The direct effect of EPI is similar to that of glucagon, in
that it is quick but wanes with time. The action of NE, although on a
molar basis less potent than that of EPI, is sustained over time.
The aim of the present study, therefore, was to determine whether EPI
and NE exert their action on glucose production through different
hepatic adrenergic receptor subtypes in the conscious dog. To focus on
their hepatic actions, the catecholamines were infused portally to
avoid their effects on muscle and adipose tissue, and a pancreatic
clamp was used to eliminate their effects on the pancreas. Similarly,
the adrenergic blockers were infused portally to avoid their effects on
the cardiovascular system.
 |
METHODS AND MATERIALS |
Experiments were carried out on sixteen 18-h-fasted conscious
mongrel dogs (20-30 kg) of either sex that had been fed a standard diet of meat and chow described elsewhere (2,
3). The animals were housed in a facility that met the
guidelines of the American Association for the Accreditation of
Laboratory Animal Care, 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 ultrasonic (Transonic Systems, Ithaca, NY) flow
probes into or around appropriate blood vessels, as described elsewhere
(2, 3). Each dog was used for only one
experiment. All dogs studied had 1) a leukocyte count
<18,000/mm3, 2) a hematocrit >35%,
3) a good appetite, and 4) normal stools.
The 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 two 90-min test periods (0-90 and 90-180 min) in
groups 1 and 2, plus a 60-min third test period
in groups 3 and 4 (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 (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 242 µU · kg
1 · min
1) remained
unchanged thereafter. The study included four groups. In the first
group [
-blockade + NE (
-blo+NE); n = 4],
phentolamine (2 µg · kg
1 · min
1) in a
solution of 0.07% ascorbic acid was infused during both test periods
via the splenic and jejunal vein catheters. NE (50 ng · kg
1 · min
1) in 0.07%
ascorbic acid was then infused during the second test period via the
same catheters. In the second protocol (
1-blo+NE; n = 4), prazosin (4 µg · kg
1 · min
1) was
infused into the splenic and jejunal catheters during all test periods,
and NE (50 and 100 ng · kg
1 · min
1) was
infused through the same catheters during test periods 2 and
3, respectively. In the third protocol (
-blo+EPI;
n = 4), propranolol (1 µg · kg
1 · min
1) and EPI
(8 ng · kg
1 · min
1) were
infused in place of phentolamine and NE, respectively. In the
fourth protocol (
2-blo+EPI; n = 4),
butoxamine (40 µg · kg
1 · min
1) was
infused into the splenic and jejunal catheters during all test periods,
and EPI (8 and 40 ng · kg
1 · min
1) was
infused through the same catheters during the second and third test
periods, respectively. The infusion rates of EPI and NE used in
test period 2 of the present study were the same as those
used in our previous studies (2, 5), in which
EPI and NE alone had significant stimulatory effects on hepatic glucose production through an effect on glycogenolysis. The infusion rates of
EPI and NE used in test period 3 of the present study were the same as those used in our previous study (3), in which the plasma levels of the catecholamines were increased to the extent
seen in extremely stressful situations (i.e., severe hypoglycemia, exhaustive exercise, or hemorrhagic shock). The doses of phentolamine and propranolol infused in the current study were the same as those
used in our previous study (3), in which the two together completely blocked the hepatic glycogenolytic effect of high levels of
NE and EPI. The doses of prazosin and butoxamine were chosen from a
dose-response study of the effect of the blockers on the actions of NE
and EPI, respectively (data not shown). Any direct effects of the
adrenergic blockers were presumed not to change between the
adrenergic-blockade-alone period and the blockade-plus-catecholamine infusion period.
Blood pressure and heart rate were measured using methods described
elsewhere (2, 3). Plasma and blood glucose,
plasma [3H]- and [14C]glucose, blood
lactate, glycerol,
-hydroxybutyrate (BOHB), alanine, glutamine,
glutamate, glycine, serine, threonine, and plasma FFA were determined
with previously described methods (2, 3). The
levels of insulin, glucagon, cortisol, EPI, and NE were also determined
as described elsewhere (2, 3).
Doppler flow probes and indocyanine green dye (ICG) were used to
estimate total hepatic blood flow (2, 3). The
hepatic blood flow did not change significantly in response to any
treatment throughout the study. Because in our studies hepatic blood
flows measured by the Doppler method were more stable than those
determined by the ICG method, and they did not require an assumption as
to the relative contribution of the hepatic artery and portal vein, the
data in the figures and tables 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 were calculated with the use
of arteriovenous difference methods described elsewhere
(2, 3). Hepatic sinusoidal plasma NE and EPI
levels were calculated by means of an equation described previously
(2, 5). It should be noted that, to the
extent that there was hepatic glucose uptake (HGU), total hepatic
glucose release [net hepatic glucose output (NHGO) + HGU] would
be slightly higher (
0.2
mg · kg
1 · min
1) than NHGO
(2, 19).
Total glucose production (Ra) and utilization
(Rd) were determined by use of both one- and
two-compartment models, as previously described (2,
3). 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 the figures and tables 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 · kg
1 · min
1)
overestimates total hepatic glucose release (18). This
overestimate, however, should be equal in the four groups and would not
be expected to change appreciably during the test periods in any group.
Gluconeogenic efficiency was assessed with a double isotope technique
described elsewhere (2, 3). Because the
conversion of [14C]alanine to [14C]glucose
by the kidney is minimal (15), [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 by use of methods described
previously (2, 3).
Statistical analysis.
All statistical comparisons were made with repeated-measures ANOVA with
post hoc analysis by use of 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.
 |
RESULTS |
Hormone levels.
The arterial and portal plasma levels of insulin and glucagon remained
at basal values in all groups throughout the study (Fig.
2). Similarly, the arterial plasma
cortisol levels did not change significantly (data not shown). The
arterial, portal, and hepatic sinusoidal plasma levels of NE
and EPI remained unchanged during the basal and first test
periods in all protocols (Fig. 3,
4, and Table
1). During the second test period of the
-blo+NE group, the arterial, portal, and hepatic sinusoidal plasma
levels of NE increased from 154 ± 63 to 300 ± 51, 137 ± 31 to 3,351 ± 338, and 136 ± 14 to 2,868 ± 275 pg/ml (P < 0.05 vs. basal for the portal and
sinusoidal levels, Fig. 3), respectively. During the second and third
test periods of the
1-blo+NE group, the plasma levels of
NE increased from 227 ± 42 to 320 ± 40 and 404 ± 58 pg/ml in the artery, 218 ± 76 to 3,765 ± 371 and 7,422 ± 926 pg/ml in the portal vein, and 215 ± 57 to 2,875 ± 283 and 5,721 ± 739 pg/ml in the hepatic sinusoid, respectively
(P < 0.05 vs. basal for the portal and sinusoidal
levels, Fig. 3).

View larger version (29K):
[in this window]
[in a new window]
|
Fig. 2.
Arterial and portal plasma levels of insulin and glucagon
during control and test periods 1, 2, and
3 in presence of a pancreatic clamp in conscious 18-h-fasted
dogs. Values are means ± SE.
|
|

View larger version (25K):
[in this window]
[in a new window]
|
Fig. 3.
Arterial, portal, and hepatic sinusoidal plasma levels of
NE during control and test periods 1, 2, and
3 in presence of a pancreatic clamp in conscious 18-h-fasted
dogs. Values are means ± SE. * P < 0.05 vs.
corresponding basal period.
|
|

View larger version (23K):
[in this window]
[in a new window]
|
Fig. 4.
Arterial, portal, and hepatic sinusoidal plasma levels of
EPI during control and test periods 1, 2, and
3 in presence of a pancreatic clamp in conscious 18-h-fasted
dogs. Values are means ± SE. * P < 0.05 vs.
corresponding basal period.
|
|
View this table:
[in this window]
[in a new window]
|
Table 1.
Arterial plasma levels of norepinephrine and epinephrine during the
basal, portal blockade, portal blockade +
catecholamine, and portal blockade + high
catecholamine periods of four groups in the presence of a pancreatic
clamp in conscious 18-h-fasted dogs
|
|
During the second test period of the
-blo+EPI group, the arterial,
portal, and hepatic sinusoidal plasma levels of EPI increased from
53 ± 9 to 66 ± 11, 29 ± 9 to 746 ± 68, and
32 ± 8 to 668 ± 60 pg/ml (P < 0.05 vs.
basal for the portal and sinusoidal levels; Fig. 4), respectively.
During the second and third test periods of the
2-blo+EPI group, the plasma levels of EPI increased from 104 ± 36 to 94 ± 23 and 117 ± 50 pg/ml in the artery,
51 ± 10 to 633 ± 110 and 3,221 ± 345 pg/ml in the
portal vein, and 68 ± 11 to 508 ± 80 and 2,514 ± 322 pg/ml in the hepatic sinusoid, respectively (P < 0.05 vs. basal for the portal and sinusoidal levels; Fig. 4).
Hepatic blood flow, arterial blood pressure, and heart rate.
Neither hepatic blood flow, mean arterial blood pressure, nor heart
rate changed in any protocol (Table 2).
View this table:
[in this window]
[in a new window]
|
Table 2.
Hepatic blood flow, mean arterial blood pressure, and heart rate during
the basal, portal blockade, portal blockade +
catecholamine, and portal blockade + high
catecholamine periods of four groups in the presence of a pancreatic
clamp in conscious 18-h-fasted dogs
|
|
Glucose levels and kinetics.
The arterial blood glucose level did not change significantly in the
-blo+NE,
1-blo+NE, or
2-blo+EPI groups
throughout the study (Fig. 5,
A and B). NHGO did not change in response to either form of
-adrenergic blockade (Fig. 5A). Similarly,
-adrenergic blockade prevented the increase in NHGO and
tracer-determined glucose production that normally results from NE
infusion.

View larger version (34K):
[in this window]
[in a new window]
|
Fig. 5.
Arterial blood glucose, net hepatic glucose output, and
tracer-determined glucose production during control and test
periods 1, 2, and 3 in presence of a
pancreatic clamp in conscious 18-h-fasted dogs. Values are means ± SE. * P < 0.05 vs. corresponding basal period.
|
|
Portal infusion of the
-blocker propranolol increased the arterial
blood glucose level from 83 ± 4 to 114 ± 8 by the end of
the first test period (P < 0.05; Fig. 5B).
This was the result of an increase in NHGO from 2.0 ± 0.5 to
2.8 ± 0.7 mg · kg
1 · min
1, which
occurred within 30 min of blocker infusion. In the presence of
-blockade, portal EPI failed to increase the arterial glucose level
or NHGO (Fig. 5B). Portal infusion of the
2-blocker butoxamine did not change the arterial blood
glucose level or NHGO (Fig. 5B). In the presence of the
2-blocker, neither portal EPI infusion failed to
increase the arterial glucose level or NHGO significantly (Fig.
5B). The glucose production data obtained by the tracer method confirmed a small stimulatory effect of propranolol (Fig. 5,
A and B).
Tracer-determined Rd did not change significantly during
- or
1-blockade (P < 0.05) and was
not affected by portal NE infusion (Table
3). Rd did not change
significantly during
-blockade but increased slightly during the
portal EPI infusion in the presence of propranolol (Table 3;
P < 0.05). Rd did not change significantly in the
2-blo+EPI group (Table 3). Glucose clearance did
not change significantly during
- or
1-blockade but
decreased slightly in response to high-dose portal NE infusion (Table
3). Glucose clearance did not change significantly in the
-blo+EPI
and
2-blo+EPI groups (Table 3).
View this table:
[in this window]
[in a new window]
|
Table 3.
TDGU and TDCL during the basal, portal blockade, portal blockade
+ catecholamine, and portal blockade + high catecholamine periods of four groups in the
presence of a pancreatic clamp in conscious 18-h-fasted dogs
|
|
Blood levels and net hepatic balance of lactate.
Neither the arterial level of lactate nor the net hepatic lactate
balance changed significantly in the
-blo+NE and
1-blo+NE groups (Fig.
6A). The arterial lactate
level increased from 515 ± 108 to 677 ± 189 and to 846 ± 262 µmol/l (P < 0.05) during the first and second
test periods, respectively, in the
-blo+EPI protocol (Fig.
6B). Net hepatic lactate balance switched from net uptake to
net output (
1.4 ± 1.5 to 4.5 ± 1.8 µmol · kg
1 · min
1,
P < 0.05) in response to
-blockade and remained in
output during EPI infusion. Neither the arterial level of lactate nor
the net hepatic lactate balance changed significantly in the
2 blo+EPI group (Fig. 6B).

View larger version (31K):
[in this window]
[in a new window]
|
Fig. 6.
Arterial blood levels and net hepatic balances of lactate
during control and test periods 1, 2, and
3 in presence of a pancreatic clamp in conscious 18-h-fasted
dogs. Values are means ± SE. * P < 0.05 vs.
corresponding basal period.
|
|
Glycerol, FFA, BOHB, and alanine.
Neither the arterial levels nor the net hepatic balances of glycerol
changed significantly in the
-blo+NE,
1-blo+NE, and
-blo+EPI groups. The arterial level and net hepatic balance of glycerol decreased slightly in the
2-blo+EPI group
during high-dose portal EPI infusion. Neither the arterial plasma
levels nor the net hepatic balances of FFA changed significantly in
the
-blo+NE,
1-blo+NE, and
-blo+EPI groups
throughout the study. The arterial plasma level of FFA decreased
gradually from 749 ± 93 to 440 ± 90, and to 320 ± 68 µmol/l (both P < 0.05), and net hepatic uptake decreased from 2.3 ± 0.8 to 0.8 ± 0.5, and to 0.9 ± 0.4 µmol · kg
1 · min
1
(both P < 0.05), respectively, during the second and
third test periods in the
2-blo+EPI group. Neither the
arterial levels nor the net hepatic balances of BOHB changed
significantly in the
-blo+NE,
1-blo+NE, and
-blo+EPI groups throughout the study. The arterial level and net
hepatic output of BOHB decreased gradually from 25 ± 4 to 17 ± 3 and to 16 ± 2 µmol/l (both P < 0.05), as well as from 0.7 ± 0.1 to 0.4 ± 0.1 and to 0.4 ± 0.1 µmol · kg
1 · min
1 (both
P < 0.05), respectively, during the second and third
test periods in the
2-blo+EPI group.
The blood level and net hepatic balance of alanine did not change
significantly in the
-blo+NE and
1-blo+NE groups
(Table 4). The blood level (283 ± 21 to 399 ± 62 µmol/l, P < 0.05) and net
hepatic balance (
1.6 ± 0.4 to
2.6 ± 0.8 µmol · kg
1 · min
1,
P < 0.05) of alanine increased slightly in the
blo+EPI group during the portal EPI infusion. The blood level did not
change significantly, and the net hepatic balance of alanine increased slightly in the
2-blo+EPI group from
2.0 ± 0.2 to
3.8 ± 1.0 µmol · kg
1 · min
1 during
the high dose portal EPI infusion.
View this table:
[in this window]
[in a new window]
|
Table 4.
Arterial blood or plasma levels and net hepatic balance of glycerol,
FFA, BOHB, and alanine during the basal, portal blockade, and portal
blockade + catecholamine periods of four groups in
the presence of a pancreatic clamp in conscious 18-h-fasted dogs
|
|
Gluconeogenic amino acids.
The arterial levels and net hepatic balances of glutamate, glutamine,
glycine, serine, and threonine did not change significantly in response
to any treatment (Table 5).
View this table:
[in this window]
[in a new window]
|
Table 5.
Arterial blood levels and net hepatic balances of glutamate, glutamine,
glycine, serine, and threonine during the basal, portal blockade, and
portal catecholamine + blockade periods of the
-adrenergic blocker + norepinephrine and
-adrenergic blocker + epinephrine groups in the
presence of a pancreatic clamp in conscious 18-h-fasted dogs
|
|
Hepatic gluconeogenic and glycogenolytic rates.
The gluconeogenic rate did not change significantly in
response to any treatment (Fig.
7, A and B).
Because no significant change was seen in gluconeogenic rate, the
increase in NHGO observed in response to
-blocker infusion alone
must have resulted from an increase in hepatic glycogenolysis (1.8 ± 0.3 to 2.8 ± 0.3 mg · kg
1 · min
1;
P < 0.05; Fig. 7B). Because neither NHGO
nor gluconeogenesis changed significantly in response to NE infusion,
it is clear that in the presence of
- or
1-blockade,
NE was unable to increase hepatic glycogenolysis significantly (Fig. 7,
A and B and Fig. 8). Likewise, in the presence of
- or
2-blockade, EPI was unable to increase hepatic
glycogenolysis significantly (Fig. 7, A and B and
Fig. 8).

View larger version (31K):
[in this window]
[in a new window]
|
Fig. 7.
Net hepatic gluconeogenic and glycogenolytic rates during
control and test periods 1, 2, and 3 in presence of a pancreatic clamp in conscious 18-h-fasted dogs. Values
are means ± SE. * P < 0.05 vs. corresponding
basal period.
|
|

View larger version (18K):
[in this window]
[in a new window]
|
Fig. 8.
Summary of the change in net hepatic glucose output and
tracer-determined endogenous glucose production over the first 30 min
of each test period in presence of a pancreatic clamp in conscious
18-h-fasted dogs. Values are means ± SE. * P < 0.05 vs. corresponding NE or EPI treatment alone.
|
|
 |
DISCUSSION |
The aim of the present study was to determine whether epinephrine
and norepinephrine exert their actions on hepatic glucose production in
the conscious dog through different adrenergic receptor subtypes. The
arterial and portal levels of insulin and glucagon were clamped at
basal values in all groups (Fig. 2), thereby eliminating any effect of
the catecholamines on the pancreas. By bringing about a selective
increase in hepatic sinusoidal norepinephrine or epinephrine, we were
also able to eliminate the peripheral (muscle and fat) effects of the
catecholamines. As a result, there were no significant changes in the
blood metabolite levels or any cardiovascular parameters throughout the
study (Figs. 6 and 7, Tables 2 and 4). Similarly, by infusing all
adrenergic blockers intraportally, we were able to eliminate the
effects of these blockers on the cardiovascular system, because most of
them were metabolized by the liver through a first pass effect (Table
2). We were thus able to address directly the effect of the
catecholamines on the liver per se. Our results showed that the direct
effect of norepinephrine on hepatic glucose production was almost
completely abolished by
- (phentolamine) or
1-
(prazosin) blockers. Likewise, the direct effect of epinephrine on
hepatic glucose production was markedly inhibited by
- (propranolol)
or
2- (butoxamine) blockers.
In one of our previous studies using the pancreatic clamp to fix
insulin and glucagon at basal levels (5), the same rate of
portal norepinephrine infusion as the one used in the present study
increased NHGO from 1.9 ± 0.2 to 3.5 ± 0.4 mg · kg
1 · min
1 (Fig. 8).
It also increased the arterial blood glucose level from 79 ± 5 to
89 ± 6 mg/dl within 30 min. In a control study (5),
that degree of hyperglycemia alone decreased NHGO from 2.1 ± 0.2 to 1.8 ± 0.4 mg · kg
1 · min
1, so the net
effect of norepinephrine on hepatic glucose production was a rise of
1.9 mg · kg
1 · min
1. In the
current study, in the presence of
- or
1-adrenergic blockers, a physiological rise in hepatic sinusoidal norepinephrine had
no significant effect on NHGO (1.6 ± 0.5 to 1.9 ± 0.7 or
1.6 ± 0.1 to 1.6 ± 0.2 mg · kg
1 · min
1 over the
first 30 min; Fig. 8). Furthermore, in the presence of an
1-adrenergic blocker, even high sinusoidal plasma
norepinephrine levels corresponding to those seen in the synaptic
clefts in extremely stressful conditions (
5,700 pg/ml) failed to
increase NHGO significantly (1.7 ± 0.3 to 2.0 ± 0.3 mg · kg
1 · min
1; Fig. 8).
The results obtained with tracer-determined glucose production
paralleled those seen with NHGO. Because our earlier study
(5) was performed recently using the same methodology, similar insulin (mean of 245 µU · kg
1 · min
1), and the
same glucagon infusion rates, the usual caveats regarding the use of
historical data for comparison should not apply. Taken together, the
data from our current and earlier studies (5) indicate
that the direct effect of norepinephrine on hepatic glucose production
is markedly inhibited by
-adrenergic blockade and, furthermore, that
the effects of the catecholamine are predominantly attributable to
1-receptors. In our previous study (5), the direct effect of norepinephrine on hepatic glucose production was
attributable to its effect on hepatic glycogenolysis. Because no
significant changes were seen in any gluconeogenic parameter or in NHGO
in the current study, one can conclude that the effect of
norepinephrine on glycogenolysis was predominantly mediated by
1-adrenergic receptors.
Garceau et al. (11) reported that, in the anesthetized
dog, the increase in hepatic venous glucose concentration caused by
hepatic arterial norepinephrine injection was partially inhibited by
either phentolamine or propranolol delivered peripherally. In a human
study, Meguid et al. (17) showed that in the absence of a
pancreatic clamp, the norepinephrine-induced rise in blood glucose
concentration was blocked by 60% when phentolamine was infused.
Interpretation of the data from those studies is complicated by the
fact that effects of norepinephrine on the pancreas and/or muscle and
adipose tissues, as well as on the cardiovascular system and liver,
were all present. It has been shown in vitro (6, 12) that norepinephrine has very low affinity for
2-receptors (7% that of epinephrine) and that it is
this
-subtype that predominates in canine liver (14,
17). This is consistent with our data in which, in the
presence of the
1-blocker, a slight glycemic (15-20%) effect (Fig. 8) was seen only in response to extremely high norepinephrine infusion (test period 3). This may also
explain the failure of others to completely block the effects of
norepinephrine. Likewise, in vitro work has shown that norepinephrine
has a high affinity for
1-receptors, and it is this
-adrenergic receptor subtype that is found in the canine liver
(6, 12, 14). Thus these in vitro
data are in line with our findings and suggest that, in the dog,
norepinephrine stimulates hepatic glucose production predominantly by
interaction with
1-receptors.
In another of our previous studies using the pancreatic clamp
(2), portal epinephrine infusion at the same rate as in
the present study increased NHGO from 2.1 ± 0.3 to 3.7 ± 0.5 mg · kg
1 · min
1 (Fig.
8) and arterial blood glucose level from 76 ± 2 to 92 ± 3 mg/dl within 30 min. In the control protocol (2), that
degree of hyperglycemia alone decreased NHGO from 2.1 ± 0.2 to
1.4 ± 0.3 mg · kg
1 · min
1, so the net
effect of epinephrine on hepatic glucose production was an increase of
2.3 mg · kg
1 · min
1. In the
present study, the effect of the same rise in hepatic sinusoidal
epinephrine on NHGO was completely inhibited by portal
- or
2-adrenergic blockers (2.4 ± 0.9 to 2.2 ± 0.9 or 1.2 ± 0.1 to 1.4 ± 0.2 mg · kg
1 · min
1 over the
first 30 min; Fig. 8). Once again, the similarity of our earlier study
(2) to the current one and its recent date should allow
direct comparison between the two groups. In that study, the direct
effect of epinephrine on hepatic glucose production was solely
attributable to its effect on hepatic glycogenolysis. Because no
significant changes were seen in any gluconeogenic parameter or in NHGO
in the current study, one can conclude that the effects of epinephrine
on glycogenolysis must be inhibited by
-adrenergic blockade.
It should be noted that, in the presence of the
2-adrenergic blocker butoxamine, a sinusoidal plasma
epinephrine level seen only during extreme stress (
2,500 pg/ml)
increased both NHGO and tracer-determined glucose production by only
0.6 mg · kg
1 · min
1 over
the first 30 min [
75% inhibition of the response expected based on
our earlier data (3)]. There are two possible
explanations for the incomplete blockade. First, because the effect of
this high level of plasma epinephrine on glucose production was
inhibited only 15% by a low-dose butoxamine infusion (4 µg · kg
1 · min
1; data not
shown), it is possible that the dose of butoxamine (40 µg · kg
1 · min
1) used in
the present study was not high enough to completely abolish the effect
of the high level of epinephrine on
2-adrenergic receptors. Second, it could be that the effect was attributable to a
small
-adrenergic action of the catecholamine on hepatic glucose production.
In an earlier study, Steiner et al. (25) showed that
preincubation of canine hepatocytes with propranolol (200 nmol/l)
caused a 77% inhibition of the glucose output caused by epinephrine. Phentolamine (200 nmol/l), on the other hand, caused a 27% inhibition of the glucose output caused by epinephrine. This suggested that the
glycogenolytic effect of epinephrine on the canine hepatocyte is
mediated primarily by a
-adrenergic mechanism but with a small
-component. Rizza et al. (20) reported that, in the
absence of a pancreatic hormone clamp, epinephrine can stimulate
glucose production in humans via both
- and
-adrenergic
mechanisms. Because both insulin and glucagon increased in their study,
it was not possible to determine which adrenergic mechanism was
involved in the direct effect of epinephrine on hepatic glucose production.
As noted above, our results suggest that epinephrine exerts little of
its effects through
-stimulation. In agreement with our data,
Deibert and DeFronzo (9) reported that, in the presence of
a euglycemic-hyperinsulinemic clamp, all of the effects of epinephrine
on glucose production in the human could be accounted for by a
-adrenergic mechanism. Similarly, Best et al. (1), in
another human study, showed a lack of a direct
-adrenergic effect of
epinephrine on glucose production in the presence of a pancreatic
clamp. Likewise, Rizza et al. (20) reported that, in the
presence of pancreatic hormone clamp in the human, peripherally delivered phentolamine failed to alter the effects of peripherally delivered epinephrine on glucose production. On the other hand, peripherally delivered propranolol (
-adrenergic blocker) inhibited the effects of epinephrine by 80%. As in our study, the failure to
completely inhibit the effect of epinephrine on hepatic glucose production may have resulted from incomplete blockade or a small
-component (20). Taking all of the findings together,
one can conclude that the effect of epinephrine on NHGO in the
conscious dog is predominantly mediated by
2-adrenergic receptors.
Our previous studies (3, 4) showed that
combined
- +
-adrenergic blockade per se increased arterial
glucose (77 ± 3 to 92 ± 7 mg/dl), NHGO (2.0 ± 0.2 to
3.3 ± 0.3 mg · kg
1 · min
1), and net
hepatic lactate output (2.8 ± 2.7 to 9.1 ± 4.8 µmol · kg
1 · min
1). In
the current study,
-adrenergic blockade with propranolol alone
increased arterial glucose (86 ± 4 to 114 ± 12 mg/dl), as well as NHGO (1.7 ± 0.5 to 2.5 ± 1.0 mg · kg
1 · min
1), and
switched the liver from net lactate uptake to output (
2.4 ± 2.6 to 3.7 ± 2.2 µmol · kg
1 · min
1). No
such change was seen during the portal infusion of the
-adrenergic blockers or the
2-blocker. The present data thus
indicate that the effects of the combined
- +
-adrenergic
blockade seen in our previous study (3, 4)
were attributable to the
-adrenergic blocker propranolol. Also,
because no such change was seen during the portal infusion of the
2-adrenergic blocker butoxamine in the present study,
the effect must be attributable to propranolol itself or to
1-stimulation and not to
2-adrenergic
receptor stimulation. In agreement with our data, Shaw and Wolfe
(24) reported that propranolol alone increased glucose
production either in the presence or in the absence of a pancreatic
hormone clamp in conscious dogs. The explanation for the effect of
propranolol on hepatic glucose production is not clear. One possibility
is that propranolol may have an intrinsic (partial agonist) effect on
-adrenergic receptors and thereby increase glucose production. Another is that propranolol may inhibit glucose oxidation and energy
expenditure (7, 23, 24) and thus
indirectly increase hepatic glucose release. Regardless, the
explanation for this interesting finding remains to be elucidated.
In conclusion, 1) the direct effect of norepinephrine on
hepatic glucose production (glycogenolysis) is predominantly mediated through
1-adrenergic receptors; 2) the direct
effect of epinephrine on hepatic glucose production (glycogenolysis) is
predominantly mediated through
2-adrenergic receptors.
 |
ACKNOWLEDGEMENTS |
We are grateful to Dr. Mary Courtney Moore for valuable comments
and careful review of this manuscript. We specially appreciate assistance from Margaret Salvino, Jon Hastings, Eric J. Allen, Pam
Venson, Wanda Snead, Paul Flakoll, E. Patrick Donahue, and Yang Ying.
 |
FOOTNOTES |
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).
Address for reprint requests and other correspondence: C. A. Chu, Dept. of Molecular Physiology and Biophysics, 702 Light Hall,
Vanderbilt Univ. School of Medicine, Nashville, TN
37232-0615.
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. §1734 solely to indicate this fact.
Received 22 October 1999; accepted in final form 13 March 2000.
 |
REFERENCES |
1.
Best, JD,
Ward WK,
Pfeifer MA,
and
Halter JB.
Lack of a direct
-adrenergic effect of epinephrine on glucose production in human subjects.
Am J Physiol Endocrinol Metab
246:
E271-E276,
1984[Abstract/Free Full Text].
2.
Chu, CA,
Sindelar DK,
Neal DW,
Allen EJ,
Donahue EP,
and
Cherrington AD.
Comparison of the direct and indirect effects of epinephrine on hepatic glucose production.
J Clin Invest
99:
1044-1056,
1997[Abstract/Free Full Text].
3.
Chu, CA,
Sindelar DK,
Neal DW,
and
Cherrington AD.
Direct effects of catecholamines on hepatic glucose production in conscious dogs are due to glycogenolysis.
Am J Physiol Endocrinol Metab
271:
E127-E137,
1996[Abstract/Free Full Text].
4.
Chu, CA,
Sindelar DK,
Neal DW,
and
Cherrington AD.
Portal adrenergic blockade does not inhibit the gluconeogenic effects of circulating catecholamines on the liver.
Metabolism
46:
458-465,
1997[ISI][Medline].
5.
Chu, CA,
Sindelar DK,
Neal DW,
and
Cherrington AD.
The direct effect of norepinephrine on HGP in the conscious dog.
Am J Physiol Endocrinol Metab
274:
E162-E171,
1998[Abstract/Free Full Text].
6.
Clutter, WE,
Rizza RA,
and
Gerich JE.
Regulation of glucose metabolism by sympathochromaffin catecholamines.
Diabetes Metab Rev
4:
1-15,
1988[ISI][Medline].
7.
Connolly, CC,
Steiner KE,
Stevenson RW,
Neal DW,
Williams PE,
Alberti KGMM,
and
Cherrington AD.
Regulation of glucose metabolism by norepinephrine in conscious dogs.
Am J Physiol Endocrinol Metab
261:
E764-E772,
1991[Abstract/Free Full Text].
8.
DeFronzo, RA,
Thorin D,
Felber JP,
Simonson DC,
Thiebaud D,
Jequier E,
and
Golay A.
Effect of beta and alpha adrenergic blockade on glucose-induced thermogenesis in man.
J Clin Invest
73:
633-639,
1984[ISI][Medline].
9.
Deibert, DC,
and
DeFronzo RA.
Epinephrine-induced insulin resistance in man.
J Clin Invest
65:
717-721,
1980[ISI][Medline].
10.
Exton, JH.
Mechanisms of hormonal regulation of hepatic glucose metabolism.
Diabetes Metab Rev
3:
163-183,
1987[Medline].
11.
Garceau, D,
Yamaguchi N,
and
Goyer R.
Hepatic adrenoceptors involved in the glycogenolytic response to exogenous (-)-norepinephrine in the dog liver in vivo.
Life Sci
37:
1963-1970,
1985[ISI][Medline].
12.
Hieble, JP,
Bondinell WE,
and
Ruffolo RR.
- and
-Adrenoceptors: from the gene to the clinic. 1. Molecular biology and adrenoceptor subclassification.
J Med Chem
38:
3415-3444,
1995[ISI][Medline].
13.
Kawai, Y,
Powell A,
and
Arinze IJ.
Adrenergic receptors in human liver plasma membranes: predominance of
2- and
1-receptor subtypes.
J Clin Endocrinol Metab
62:
827-832,
1986[Abstract].
14.
Lefkowitz, RJ.
Heterogeneity of adenylate cyclase-coupled
-adrenergic receptors.
Biochem Pharmacol
24:
583-590,
1975[ISI][Medline].
15.
Liu, MS,
and
Ghosh S.
Changes in
-adrenergic receptors in dog livers during endotoxic shock.
Am J Physiol Regulatory Integrative Comp Physiol
244:
R718-R723,
1983[ISI][Medline].
16.
McGuinness, OP,
Fujiwara T,
Murrell S,
Bracy D,
Neal DW,
O'Connor D,
and
Cherrington AD.
Impact of chronic stress hormone infusion on hepatic carbohydrate metabolism in the conscious dog.
Am J Physiol Endocrinol Metab
265:
E314-E322,
1993[Abstract/Free Full Text].
17.
Meguid, MM,
Moore MC,
Fitzpatrick G,
and
Moore FD.
Norepinephrine-induced insulin and substrate changes in normal man: incomplete reversal by phentolamine.
J Surg Res
18:
365-369,
1975[ISI][Medline].
18.
Meyer, C,
Stumvoll M,
Chintalapudl U,
Gutierrez O,
Kreider M,
Perriello G,
Welle S,
and
Gerich J.
Alanine and glutamine: selective markers for hepatic and renal gluconeogenesis in humans.
Diabetes
45, Suppl1:
945,
1996[ISI].
19.
Pagliassotti, MJ,
Holste LC,
Moore MC,
Neal DW,
and
Cherrington AD.
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[Abstract/Free Full Text].
20.
Rizza, RA,
Cryer PE,
Haymond MW,
and
Gerich JE.
Adrenergic mechanisms for the effect of epinephrine on glucose production and clearance in man.
J Clin Invest
65:
682-689,
1980[ISI][Medline].
21.
Rizza, RA,
Haymond MW,
Cryer PE,
and
Gerich JE.
Differential effects of epinephrine on glucose production and disposal in man.
Am J Physiol Endocrinol Metab Gastrointest Physiol
237:
E356-E362,
1979[Abstract/Free Full Text].
22.
Sacca, L,
Morrone G,
Cicala M,
Corso G,
and
Ungaro B.
Influence of epinephrine, norepinephrine, and isoproterenol on glucose homeostasis in normal man.
J Clin Endocrinol Metab
50:
680-684,
1980[ISI][Medline].
23.
Schimmel, RJ.
Roles of alpha and beta adrenergic receptors in control of glucose oxidation in hamster epididymal adipocytes.
Biochem Biophys Acta
428:
379-387,
1976[ISI][Medline].
24.
Shaw, JH,
and
Wolfe RR.
The integrated effect of adrenergic blockade on glucose, fatty acid, and glycerol kinetics: responses in the basal state and during hormone control with somatostatin-hormonal infusion.
J Surg Res
42:
257-272,
1987[ISI][Medline].
25.
Steiner, KE,
Stevenson RW,
Green DR,
and
Cherrington AD.
Mechanism of epinephrine's glycogenolytic effect in isolated canine hepatocytes.
Metabolism
34:
1020-1023,
1985[ISI][Medline].
26.
Stevenson, RW,
Steiner KE,
Connolly CC,
Fuchs H,
George K,
Alberti MM,
Williams PE,
and
Cherrington AD.
Dose-related effects of epinephrine on glucose production in conscious dogs.
Am J Physiol Endocrinol Metab
260:
E363-E370,
1991[Abstract/Free Full Text].
27.
Summers, RJ,
and
McMartin LR.
Adrenoceptors and their second messenger systems.
J Neurochem
60:
10-23,
1993[ISI][Medline].
Am J Physiol Endocrinol Metab 279(2):E463-E473
0193-1849/00 $5.00
Copyright © 2000 the American Physiological Society