Department of Molecular Physiology and Biophysics and Diabetes Research and Training Center, Vanderbilt University, Nashville, Tennessee 37232
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ABSTRACT |
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Epinephrine
increases net hepatic glucose output (NHGO) mainly via increased
gluconeogenesis, whereas glucagon increases NHGO mainly via increased
glycogenolysis. The aim of the present study was to determine how the
two hormones interact in controlling glucose production. In 18-h-fasted
conscious dogs, a pancreatic clamp initially fixed insulin and glucagon
at basal levels, following which one of four protocols was instituted.
In G + E, glucagon (1.5 ng · kg1 · min
1;
portally) and epinephrine (50 ng · kg
1 · min
1;
peripherally) were increased; in G, glucagon was increased alone; in E,
epinephrine was increased alone; and in C, neither was increased. In G,
E, and C, glucose was infused to match the hyperglycemia seen in G + E
(~250 mg/dl). The areas under the curve for the increase in NHGO,
after the change in C was subtracted, were as follows: G = 661 ± 185, E = 424 ± 158, G + E = 1,178 ± 57 mg/kg. Therefore, the overall effects of the two hormones on NHGO
were additive. Additionally, glucagon exerted its full glycogenolytic effect, whereas epinephrine exerted its full gluconeogenic effect, such
that both processes increased significantly during concurrent hormone administration.
canine; gluconeogenesis; glycogenolysis; counterregulatory hormones
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INTRODUCTION |
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GLUCAGON AND EPINEPHRINE, the two primary counterregulatory hormones, are secreted in response to physiological stresses such as hypoglycemia, exercise, and infection. The individual actions of these two hormones on glucose production have been well defined, yet it remains unclear how they interact acutely in a physiological setting to stimulate glucose production. Glucagon has been shown to have rapid effects on hepatic glucose production, with half-maximal activation occurring in ~4.5 min (19). In conscious dogs, administration of glucagon at a fourfold basal rate in the presence of a pancreatic clamp and fixed basal insulin resulted in a rapid increase (180%) in glucose production that waned with time, such that after 3 h it was increased by only 41% (7). This effect of glucagon on glucose production has been shown to result primarily from a rapid, potent, time-dependent effect on glycogenolysis and to a lesser extent from a less potent, slower effect on gluconeogenesis (7). Studies in humans have also shown that glucagon can increase glucose production in a rapid, time-dependent manner primarily by increasing glycogenolysis (8, 41).
The mild effect of glucagon on gluconeogenesis is somewhat surprising when it is considered that the hormone is known to stimulate both transcription and activation of hepatic gluconeogenic enzymes (22, 39, 49, 50). In fact, glucagon has been shown to increase hepatic gluconeogenic efficiency in vivo both acutely (67) and chronically (43), yet the contribution of the rise in gluconeogenesis to the increase in glucose production was small. This paradox may be explained by the fact that glucagon has little effect on gluconeogenic substrate mobilization from muscle or fat. Thus any enhancement of gluconeogenic flux would initially increase gluconeogenesis, but then the gluconeogenic substrate levels in blood would fall and the gluconeogenic contribution to glucose production would return toward its basal rate.
Epinephrine has also been shown to increase glucose production in a rapid, time-dependent manner, albeit with a decreased sensitivity on a molar basis compared with glucagon (6, 56, 59, 66). The effect of epinephrine on glucose production results from a stimulation of both gluconeogenesis and glycogenolysis. Chu and colleagues (10-12) showed that the former is due to the indirect action of the hormone on peripheral substrate release, whereas the latter is due to the direct action of epinephrine on the liver. Chu et al. (10) also showed that when the hormone increased gluconeogenesis, it caused a compensatory decrease in its glycogenolytic action, implying a reciprocal relationship between the two processes. Support for a reciprocal relationship between gluconeogenesis and glycogenolysis can be found in several other previous studies in both humans (33, 34, 74) and dogs (15, 18). In those experiments, increasing the gluconeogenic precursor supply to the liver increased gluconeogenesis but did not increase total glucose production, thereby implying a decrease in glycogenolysis. On the other hand, inhibiting glycogen breakdown has not been uniformly shown to stimulate gluconeogenesis (23, 64), perhaps because gluconeogenic precursor supply was limiting.
The interaction of glucagon and epinephrine in regulating hepatic glucose production has not been extensively characterized. Two previous studies found that administration of glucagon and epinephrine concurrently resulted in an additive increase in glucose production in the dog (21) and human (62). However, insulin and glucose levels were not controlled in those studies, making interpretation of the data difficult. In addition, glucose production was not separated into its gluconeogenic and glycogenolytic components. Thus the aim of the present study was to analyze the interaction of glucagon and epinephrine in controlling hepatic glucose production at a time when plasma insulin was basal and fixed. Specifically, we wanted to determine whether glucagon, when elevated in the presence of an epinephrine-induced increase in gluconeogenic precursor supply to the liver, would have an increased effect on gluconeogenesis and as a result a decreased effect on glycogenolysis.
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RESEARCH DESIGN AND METHODS |
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Animals and surgical procedures. Studies were performed on 23 overnight-fasted, conscious mongrel dogs of either sex (19-26.9 kg, mean = 23.2 kg). Animals were fed once daily a diet of meat (Kal-Kan, Vernon, CA) and chow (Purina Lab Canine Diet no. 5006; Purina Mills, St. Louis, MO) comprised of 46% carbohydrate, 34% protein, 14% fat, and 6% fiber based on dry weight. 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.
Approximately 16 days before the study, a laparotomy was performed under general anesthesia (15 mg/kg body wt sodium pentothal before surgery; 1.0% isoflurane as an inhalation anesthetic during surgery). In all dogs, ultrasonic flow probes (Transonic Systems, Ithaca, NY) were positioned around the portal vein and a hepatic artery, as previously described (10). Silastic catheters (Dow Corning, Midline, MI) were inserted into a femoral artery, the portal vein, and the left common hepatic vein for blood sampling and into the splenic and jejunal veins for intraportal hormone delivery, as previously described (47). The catheters were filled with heparinized saline (200 U/ml; Abbott Laboratories, North Chicago, IL), and their free ends were knotted. The free ends of the catheters and the flow probe leads were placed in subcutaneous pockets until the study day. Animals were studied only if the following criteria were met before the study: 1) leukocyte count <18,000/mm3, 2) hematocrit >35%, 3) good appetite, and 4) normal stools. As a side note, in all dogs an ultrasonic flow probe was positioned around a renal artery, and a Silastic catheter was inserted into a renal vein. The renal glucose production data form the basis of a separate study. On the morning of a study, the Transonic leads and the catheters were exteriorized under local anesthesia (2% lidocaine; Abbott Laboratories). The dog was placed in a Pavlov harness, and the contents of the catheters were aspirated, after which the catheters were flushed with saline and subsequently used for blood sampling or infusion. Angiocaths (20 gauge; Becton Dickinson, Sandy, UT) were inserted into the right and left cephalic veins for infusion of [3-3H]glucose (New England Nuclear, Boston, MA) and glucose (20% dextrose, Baxter Healthcare, Deerfield, IL; or 50% dextrose, Abbott Laboratories) respectively. An angiocath was also placed in the left saphenous vein for indocyanine green dye (ICG; Sigma Chemical, St. Louis, MO) and somatostatin (Bachem, Torrance, CA) infusion. If required according to the protocol, an angiocath was placed in the right saphenous vein for peripheral epinephrine (Sigma Chemical) infusion.Experimental design.
Each experiment consisted of a 100-min tracer equilibration and hormone
adjustment period (140 to
40 min) followed by a 40-min control
period (
40 to 0 min). During these periods,
[3-3H]glucose (~50 µCi prime; ~0.50 µCi/min) and
ICG (0.07 mg/min) were infused. In addition, a pancreatic clamp was
performed. This involved infusion of somatostatin (0.8 µg · kg
1 · min
1)
through a peripheral vein to inhibit endogenous insulin and glucagon
secretion and replacement of insulin (~250
µU · kg
1 · min
1;
Eli Lilly, Indianapolis, IN) and glucagon (0.5 ng · kg
1 · min
1;
Bedford Laboratories, Bedford, OH) intraportally. The insulin infusion
rate was varied if necessary during the equilibration period to
maintain euglycemia. The control period was followed by a 4-h
experimental period (0-240 min) during which basal insulin was
maintained. Each dog underwent one of four experimental protocols. In
the G + E group (n = 6), glucagon (1.5 ng · kg
1 · min
1;
portally) and epinephrine (50 ng · kg
1 · min
1;
peripherally) were elevated; in the G group (n = 6),
glucagon (1.5 ng · kg
1 · min
1;
portally) alone was increased; in the E group (n = 6),
epinephrine (50 ng · kg
1 · min
1;
peripherally) alone was raised; and in the C group (n = 5), basal glucagon and epinephrine (no epinephrine infusion) were maintained. In the G, E, and C protocols, glucose was infused peripherally to match the plasma glucose seen in G + E (~250 mg/dl). The [3-3H]glucose infusion rate was also varied
throughout the experimental period to clamp the glucose specific
activity and thereby minimize errors in glucose turnover calculation.
In addition, to prevent a slow decline in glucagon levels, the glucagon
infusion rate was increased slightly each hour. In dogs receiving basal
glucagon, glucagon infusion was increased from 0.50 to 0.54, 0.58, and
0.62 ng · kg
1 · min
1
at times 60, 120, and 180 min, respectively. In dogs receiving threefold basal glucagon, glucagon infusion was increased from 1.5 to
1.62, 1.74, and 1.86 at times 60, 120, and 180 min, respectively. In
all dogs, mean arterial blood pressure and heart rate were determined
throughout the experiment at each sampling time point by use of either
a chart recorder with blood pressure transducer (Gould RS3200) or a
Digi-Med Blood Pressure Analyzer (Micro-Med, Louisville, KY).
Analytical procedures.
The immediate processing of the samples and the measurement of whole
blood glucose, glutamine, glutamate, acetoacetate, individual amino
acids (serine, threonine, glycine), and metabolites [lactate, alanine,
glycerol, -hydroxybutyrate (BOHB)] were described previously (10, 63). In addition, plasma levels of glucose,
[3-3H]glucose, ICG, catecholamines, insulin, glucagon,
cortisol, and nonesterified fatty acids (NEFA) were measured as
previously described (10, 63). C-peptide [in plasma to
which 500 kallikrein inhibitor units/ml Trasylol had been added (FBA
Pharmaceuticals, New York NY)] was determined via disequilibrium
double-antibody radioimmunoassay (Linco Research, St. Charles, MO) with
an interassay coefficient of variation of 5%.
Calculations.
Both ICG and Transonic flow probes were used to estimate total hepatic
blood flow in these studies. The net hepatic balances and net hepatic
fractional extractions of the measured substrates were calculated using
both Transonic-determined and ICG-determined flow. The data shown are
those calculated using Transonic-determined flow, as this flow does not
require an assumption about the distribution of arterial vs. portal
flow. Note that the same conclusions were drawn when ICG-determined
flow was used to calculate the data. Equations used were as follows
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Statistical analysis. Data are expressed as means ± SE. Statistical comparisons were made by one- and two-way analysis of variance (ANOVA) with repeated-measures design (except for the blood pressure and heart rate data: paired t-test) run on Sigma Stat (SPSS Science, Chicago, IL). Analysis of AUC data was made with one-way ANOVA. Post hoc analysis was performed with Tukey's test. Statistical significance was accepted at P < 0.05.
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RESULTS |
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Glucose and hormone levels.
In all four groups, plasma glucose levels rose from ~110 to ~250
mg/dl (Table 1). To achieve similar
glucose levels in all groups, different glucose infusion rates (GIR)
were required, as depicted in Table 1. The plasma insulin levels
remained essentially unchanged and basal and were not significantly
different from group to group (Table 1). Arterial plasma C-peptide
levels, measured as an index of endogenous insulin secretion, were low
and did not change in any group (data not shown), thereby confirming
continued inhibition of insulin release even in the presence of
hyperglycemia. Arterial and hepatic sinusoidal plasma glucagon levels
rose similarly in the protocols in which the glucagon infusion was
increased (G and G + E) but remained basal in the other protocols
(Table 1 and Fig. 1). Arterial and
hepatic sinusoidal plasma epinephrine levels rose similarly in the
protocols in which epinephrine was infused (E and G + E), but remained
basal when the catecholamine was not infused (Table 1 and Fig. 1).
Arterial cortisol levels as well as arterial and portal norepinephrine
levels remained basal in all groups throughout the studies (data not
shown).
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Arterial blood pressure and heart rate. Mean arterial blood pressure (mmHg) was initially similar in all groups (basal period: C = 106 ± 5, G = 121 ± 11, E = 112 ± 9, G + E = 129 ± 9) and remained stable in all but the E group, in which it fell modestly (average of experimental period: C = 106 ± 5, G = 121 ± 8, E = 96 ± 11, G + E = 130 ± 11, P < 0.05 for the change in E; paired t-test). As expected, heart rate rose modestly in both E and G + E (P < 0.05; paired t-test) as a result of epinephrine administration (C = 96 ± 12 to 94 ± 6, G = 92 ± 17 to 82 ± 11, E = 104 ± 13 to 134 ± 8, G + E = 70 ± 9 to 95 ± 9).
Glucose metabolism.
In all groups, basal NHGB
(mg · kg1 · min
1)
was similar (C = 1.2 ± 0.2, G = 1.7 ± 0.3, E = 1.8 ± 0.3, G + E = 1.4 ± 0.2; Fig. 2). In response to hyperglycemia (C),
NHGB changed from output to uptake (
2.5 ± 0.3 at 240 min). In
response to glucagon (G), NHGB rose to 4.6 ± 0.8 at 15 min and
waned with time (0.5 ± 0.8 at 240 min). The effect of glucagon
per se is represented in the inset to Fig. 2 as the
difference between the changes in G and C. In response to epinephrine
(E), NHGB rose (3.3 ± 0.9 at 15 min) but also waned with time,
falling to a rate significantly lower than basal (0.0 ± 1.0 at
240 min). The effect of epinephrine per se is represented in the
inset of Fig. 2 as the difference between the changes in E
and C. Finally, in the presence of both hormones (G + E),
NHGB rose to 7.3 ± 1.0 at 15 min, which was greater than with
either individual hormone. Once again, the response waned with time
(2.0 ± 0.5 at 240 min). The data in the inset of Fig.
2 indicate that the effects of glucagon and epinephrine on net hepatic
glucose production (
AUC) were additive. Changes in tracer-determined
endogenous glucose Ra paralleled the changes in NHGB (Table
2).
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Lactate: arterial levels and net hepatic balance.
In the control group, arterial blood lactate levels rose modestly due
to an increase in net hepatic lactate output during hyperglycemia (Fig.
3). When glucagon was increased, the
arterial blood lactate level rose as in the control group, also due to an increase in net hepatic lactate production. However, with glucagon, the rise in net hepatic lactate output and the lactate level occurred more quickly, presumably resulting from the hormone's effect on glycogenolysis. When epinephrine was increased, arterial lactate levels
rose to a markedly greater extent than in C or G despite the fact that
net hepatic output essentially ceased within 30 min. This indicates
that the catecholamine stimulated the net release of lactate from
nonhepatic tissues (most likely muscle). Finally, when both hormones
were increased concurrently, there was a brief increase in net hepatic
lactate output and a resulting rise in the blood lactate level,
followed by a fall in net hepatic lactate output to zero and a
continued rise in the lactate level to almost 2.5 mmol/l.
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Glycerol, NEFA, and ketones: arterial levels, net hepatic balance,
and net hepatic fractional extraction.
In both the hyperglycemic control protocol and the glucagon protocol,
arterial glycerol levels and net hepatic glycerol uptake drifted down
(significantly in G, nonsignificantly in C; Table 3). Epinephrine caused a rise in both
arterial glycerol levels and net hepatic glycerol uptake, both of which
waned with time. Finally, the combination of glucagon and epinephrine
resulted in changes that were similar to those seen with epinephrine
alone. Net hepatic glycerol fractional extraction did not change over time in any group and was not different among the groups (data not
shown).
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Alanine: arterial levels, net hepatic uptake, and net hepatic
fractional extraction.
In the hyperglycemic control group, arterial alanine levels rose, net
hepatic alanine uptake did not change, and net hepatic fractional
extraction of alanine tended to fall (Table
4). In the epinephrine infusion group,
both the arterial level and net hepatic uptake of alanine increased,
whereas net hepatic fractional extraction was sustained. In the two
groups involving glucagon infusion, the arterial alanine levels did not
change, but net hepatic alanine uptake increased and net hepatic
alanine fractional extraction tended to increase. Although only the
alanine data are portrayed, as it is the most important gluconeogenic
amino acid, the calculations to determine GNG and GLY flux incorporated the net hepatic balance of the other gluconeogenic amino acids as well
(serine, threonine, glycine, glutamine, and glutamate).
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Gluconeogenesis and glycogenolysis.
In response to hyperglycemia (Fig. 4),
hepatic GNG flux to G-6-P
(mg · kg1 · min
1)
did not change, whereas net hepatic GNG flux fell (
0.5 ± 0.2 to
1.3 ± 0.3 at 240 min, P < 0.05). Net hepatic
GLY flux
(mg · kg
1 · min
1)
also fell when hyperglycemia occurred (1.6 ± 0.3 to
1.4 ± 0.1 at 240 min, P < 0.05). In response to glucagon
(Fig. 4), GNG flux to G-6-P did not change significantly,
whereas net hepatic GNG flux fell quickly (by 15 min; P < 0.05) and remained modestly suppressed relative to its basal value.
Net hepatic GLY flux increased initially (1.9 ± 0.4 to 5.9 ± 1.0 at 15 min) and then waned with time (1.0 ± 0.7 at 240 min). In response to epinephrine (Fig. 5), hepatic GNG flux to G-6-P
almost tripled by 240 min (P < 0.05). Net hepatic GNG
flux also increased significantly (
0.8 ± 0.4 to 0.7 ± 0.6 at 240 min, P < 0.05). In contrast, there was a
nonsignificant rise in net hepatic GLY flux (2.5 ± 0.6 to
3.2 ± 1.2 at 15 min) that waned with time, eventually reaching a
rate significantly below basal (
1.0 ± 0.8 at 240 min). Finally,
in response to both hormones (Fig. 6),
hepatic GNG flux to G-6-P increased significantly, albeit to
a slightly lesser extent than with epinephrine alone. Net hepatic GNG
flux also increased in a similar manner (P < 0.05). In
contrast, net hepatic GLY flux increased significantly (1.7 ± 0.6 to 8.1 ± 1.6 at 15 min) and to a greater extent than with either
hormone alone, after which it waned with time (1.5 ± 0.6 at 240 min).
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DISCUSSION |
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The aim of the present study was to determine whether epinephrine could modify the action of glucagon on hepatic glucose production. We hypothesized that total hepatic production would be additive in the presence of both hormones but that glucagon's effect on gluconeogenesis would be augmented whereas its effect on glycogenolysis would be inhibited. The hormones were indeed found to have additive effects on hepatic glucose production regardless of the technique used to assess the process (NHGB or tracer-determined endogenous Ra). The present study confirmed previous findings that, over a 4-h period, glucagon's action is primarily glycogenolytic whereas epinephrine's action is primarily gluconeogenic. Contrary to our hypothesis, however, the results showed no synergistic effect of the two hormones on gluconeogenesis. Likewise, the glycogenolytic response to the two hormones was not less than the sum of their individual responses. In short, epinephrine did not alter the action of glucagon on hepatic glucose production; instead, the effects of the two hormones were additive, such that a simultaneous rise in both augmented both gluconeogenesis and glycogenolysis markedly.
These studies looked at the effects of physiological increments in glucagon and epinephrine on glucose production by the liver in the absence of changes in insulin and in the presence of matched hyperglycemia. The glucagon levels achieved were approximately one-half those needed for the hormone's maximal effect on glucose production (67). The epinephrine levels were such that they had a small but significant effect on glucose production and a marked effect on gluconeogenesis. In essence, we chose physiological levels of the two hormones that would produce large enough effects on glucose production to be significant alone but small enough to allow the detection of synergism if it occurred.
Our results confirm previous data that found additive effects of glucagon and epinephrine on tracer-determined glucose production (21, 62). However, in both previous studies the additive rise in the presence of both hormones was accompanied by an approximately twofold greater rise in peripheral insulin levels, in addition to an approximately twofold greater rise in the plasma glucose level, compared with the increments that occurred with either individual hormone (21). Therefore, it was possible that additive effects were observed in the previous studies only because hyperinsulinemia and hyperglycemia obscured the synergistic effects of the hormones. Unlike the previous studies, the present study controlled for insulin levels by use of a pancreatic clamp and glucose levels by use of a hyperglycemic clamp. Additionally, the present study separated glucose production into its gluconeogenic and glycogenolytic components. Despite the improved design, however, the conclusions remained the same.
Hepatic GNG flux to G-6-P changed as expected for the control group and for the individual-hormone treatment groups. Changes in net hepatic GNG flux closely resembled changes in GNG flux to G-6-P, even though absolute flux rates were lower. Glucagon treatment did not significantly increase either parameter, whereas epinephrine treatment increased both markedly. Combination of the two hormones did not result in a synergistic effect on gluconeogenesis. There are several possible reasons why synergism did not occur. First, both hyperglycemia and the increased glycogen breakdown that occurred when both hormones were coadministered would be expected to increase flux through the glycolytic pathway. This would, in turn, raise fructose-2,6-bisphosphate levels, making flux through G-6-P in the gluconeogenic direction less likely to occur (31, 50). Second, the gluconeogenic substrates lactate and alanine did not rise as high in the G + E group as in the E group (see below). The reduced availability of lactate and alanine may have limited the gluconeogenic response when the hormones were coadministered. Third, it is possible that there was a synergistic effect on GNG flux but it was too small to detect given the assumptions of the method used to estimate gluconeogenesis.
Net hepatic GLY flux also changed as expected in the control group and the individual hormone treatment groups. Net glycogen breakdown ceased in response to hyperglycemia; in fact, net glycogen synthesis occurred by the end of the study. The increase in glucagon resulted in a large increase in net glycogenolysis, whereas the increment in epinephrine did not increase net glycogenolysis significantly over the 4-h period. Net glycogenolysis increased to a similar extent during combined hormone infusion as during glucagon-alone administration. It is likely that the lack of synergism with regard to gluconeogenesis explains the lack of inhibition of glycogenolysis by the combination of the two hormones.
Glucose utilization increased in the control group due to hyperglycemia. However, glucose utilization tended to increase less in both the glucagon group [probably due to decreased glucose uptake by liver (7, 40, 51, 65)] and the epinephrine group [probably due to decreased glucose uptake by muscle (10)] than in the control group. When both hormones were combined, glucose utilization was significantly less than in the control (hyperglycemia alone) group. In fact, the increase in glucose utilization when both hormones were administered concurrently was not significant. This is important physiologically, because these hormones decrease glucose clearance individually by different mechanisms and thus together can increase glucose availability for the brain during times of stress.
Lactate levels rose in the hyperglycemic control group, as seen previously (65), likely due to increased glucose uptake by the liver and subsequent release of the carbon as lactate. Glucagon administration resulted in a small, quick rise in lactate production that waned with time, most likely the consequence of glucagon's rapid effect on glycogen breakdown, as reported previously (7, 8). This effect was short-lived, and after 1 h the glucagon group resembled the control group in both lactate levels and net hepatic balance. Lactate levels rose markedly with epinephrine treatment, as shown previously (6, 10, 14, 59), presumably due to increased lactate production from muscle glycogenolysis. Notably, lactate levels were significantly lower in the presence of both hormones than in the presence of epinephrine alone, even though both hormones stimulate lactate production by different organs. There are three possible explanations for this finding. The first relates to a known action of glucagon, which is to increase the efficiency of hepatic gluconeogenic precursor uptake (43, 67). In the combined-treatment group, the liver removed lactate at the same rate as in the epinephrine group, even though the arterial lactate level was much lower. Thus the liver was more efficient at removing lactate in the presence of both glucagon and epinephrine, probably because of stimulation of gluconeogenic enzyme activity by glucagon. This increased efficiency of uptake would likely allow steady state to be achieved earlier and thus result in a lower arterial lactate level. A second possible explanation for the lower lactate levels in the presence of both hormones is that lactate disappearance increased in response to glucagon at a site other than the liver. The third possible explanation is that glucagon decreased lactate appearance in the combined group. Because skeletal muscle has not been shown to possess glucagon receptors (4) and the kidney is not responsive to glucagon (25, 69), it seems unlikely that the effect on the lactate level was due to either of the latter possibilities.
Arterial alanine levels rose as expected in the control group due to hyperglycemia (65). Alanine levels remained unchanged in the presence of glucagon (67), the reason being that glucagon increases hepatic alanine fractional extraction by increasing alanine transport into the liver (36-38). As expected, alanine levels did not differ as a result of epinephrine treatment (10). However, when both hormones were administered together, the rise in alanine was less than with epinephrine alone. The possible explanations for this are the same as for lactate, with one additional possibility: the different lactate levels. Lactate administration increased alanine release from perfused rat skeletal muscle (58), and peripheral lactate infusion in the conscious dog increased the plasma alanine level (15). Thus alanine may have been lower in the combined group in part because lactate levels were lower.
Glycerol concentrations decreased in the control group, reflecting decreased lipolysis probably due to both hyperglycemia (16) and the infusion of somatostatin for an extended period of time (29). Glucagon is known to have little effect on lipolysis in vivo, and glucagon treatment had no demonstrable effect on glycerol levels in this study (2, 27). Epinephrine increased glycerol levels but only for a brief period, as expected (10, 16). Combined hormone treatment logically resembled epinephrine treatment, and glycerol levels increased and waned to similar values. For all groups, net hepatic glycerol uptake paralleled changes in arterial levels. In general, NEFA levels and net hepatic uptake tended to follow the same patterns as glycerol. Note that in both groups receiving epinephrine infusion, NEFA levels and uptake rates increased and waned, as expected. However, the elevations in both the level and net hepatic uptake in the combined hormone group were sustained for a longer period than in the epinephrine-alone group. This was perhaps due to the higher lactate level in the epinephrine group, as lactate has been shown to cause a fall in NEFA levels in vitro (3) and in vivo in dogs (15, 32, 44) and humans (1). Interestingly, NEFA increases gluconeogenesis in vivo (5, 9, 13, 52, 68, 73), and during the period (time 60-90 min) in which NEFA tended to be elevated in the combined group, there was a tendency for the GNG flux rate to be increased.
In summary, glucagon and epinephrine had additive effects on glucose production and perhaps glucose utilization. Furthermore, these hormones had additive effects on hepatic glycogenolysis. There was no synergism with regard to gluconeogenesis, probably due to the fact that glucagon increased the efficiency of hepatic gluconeogenesis without increasing the delivery of gluconeogenic precursors to the liver from muscle and adipose tissue. Regardless, it can be concluded that epinephrine did not modify glucagon's effect on either glycogenolysis or gluconeogenesis. When raised concurrently, glucagon and epinephrine do what neither can do alone, namely increase both components of hepatic glucose production. Under stress conditions, such changes in glucagon and epinephrine would undoubtedly be accompanied by changes in insulin, and it remains to be seen whether, in the presence of hyperinsulinemia, their interaction would be altered.
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ACKNOWLEDGEMENTS |
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We thank Margaret Converse, Wanda Snead, Eric Allen, Angela Penaloza, and Jon Hastings for excellent technical support. Additionally, we express gratitude to Dr. Mary Moore for careful reading of the manuscript.
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FOOTNOTES |
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This research was supported by a National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) grant for the Diabetes Research and Training Center at Vanderbilt University, P60-DK-20593, by an NIDDK grant for the Clinical Nutrition Research Unit at Vanderbilt University, P30-DK-26657, by an NIDDK grant for the Molecular Endocrinology Training Program, T32-DK-07563, and by another NIDDK grant, R37-DK-18243.
This work was presented in part at the 60th Annual Meeting of the American Diabetes Association, San Antonio, TX, June 2000.
Address for reprint requests and other correspondence: S. M. Gustavson, Vanderbilt Univ. Medical Center, Div. of Diabetes, Endocrinology, and Metabolism, 715 Preston Research Bldg., Nashville, TN 37232-6303 (E-mail: stephanie.m.gustavson{at}vanderbilt.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published December 27, 2002;10.1152/ajpendo.00308.2002
Received 11 July 2002; accepted in final form 19 December 2002.
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REFERENCES |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Ahlborg, G,
Hagenfeldt L,
and
Wahren J.
Influence of lactate infusion on glucose and FFA metabolism in man.
Scand J Clin Lab Invest
36:
193-201,
1976[ISI][Medline].
2.
Bertin, E,
Arner P,
Bolinder J,
and
Hagstrom-Toft E.
Action of glucagon and glucagon-like peptide-1-(7-36) amide on lipolysis in human subcutaneous adipose tissue and skeletal muscle in vivo.
J Clin Endocrinol Metab
86:
1229-1234,
2001
3.
Bjorntorp, P.
The effect of lactic acid on adipose tissue metabolism in vitro.
Acta Med Scand
178:
253-255,
1965[ISI][Medline].
4.
Burcelin, R,
Katz EB,
and
Charron MJ.
Molecular and cellular aspects of the glucagon receptor: role in diabetes and metabolism.
Diabetes Metab
22:
373-396,
1996[ISI][Medline].
5.
Chen, X,
Iqbal N,
and
Boden G.
The effects of free fatty acids on gluconeogenesis and glycogenolysis in normal subjects.
J Clin Invest
103:
365-372,
1999
6.
Cherrington, AD,
Fuchs H,
Stevenson RW,
Williams PE,
Alberti KG,
and
Steiner KE.
Effect of epinephrine on glycogenolysis and gluconeogenesis in conscious overnight-fasted dogs.
Am J Physiol Endocrinol Metab
247:
E137-E144,
1984
7.
Cherrington, AD,
Williams PE,
Shulman GI,
and
Lacy WW.
Differential time course of glucagon's effect on glycogenolysis and gluconeogenesis in the conscious dog.
Diabetes
30:
180-187,
1981[Abstract].
8.
Chhibber, VL,
Soriano C,
and
Tayek JA.
Effects of low-dose and high-dose glucagon on glucose production and gluconeogenesis in humans.
Metabolism
49:
39-46,
2000[ISI][Medline].
9.
Chu, CA,
Sherck SM,
Igawa K,
Sindelar DK,
Neal DW,
Emshwiller M,
and
Cherrington AD.
Effects of free fatty acids on hepatic glycogenolysis and gluconeogenesis in conscious dogs.
Am J Physiol Endocrinol Metab
282:
E402-E411,
2002
10.
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
11.
Chu, CA,
Sindelar DK,
Neal DW,
and
Cherrington AD.
Direct effects of catecholamines on hepatic glucose production in conscious dog are due to glycogenolysis.
Am J Physiol Endocrinol Metab
271:
E127-E137,
1996
12.
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].
13.
Clore, JN,
Glickman PS,
Nestler JE,
and
Blackard WG.
In vivo evidence for hepatic autoregulation during FFA-stimulated gluconeogenesis in normal humans.
Am J Physiol Endocrinol Metab
261:
E425-E429,
1991
14.
Clutter, WE,
Bier DM,
Shah SD,
and
Cryer PE.
Epinephrine plasma metabolic clearance rates and physiologic thresholds for metabolic and hemodynamic actions in man.
J Clin Invest
66:
94-101,
1980[ISI][Medline].
15.
Connolly, CC,
Stevenson RW,
Neal DW,
Wasserman DH,
and
Cherrington AD.
The effects of lactate loading on alanine and glucose metabolism in the conscious dog.
Metabolism
42:
154-161,
1993[ISI][Medline].
16.
Coppack, SW,
Jensen MD,
and
Miles JM.
In vivo regulation of lipolysis in humans.
J Lipid Res
35:
177-193,
1994[Abstract].
17.
DeBodo, RC,
Steele F,
Altszuler N,
Dunn A,
and
Bishop JS.
On the hormonal regulation of carbohydrate metabolism: studies with 14 C glucose.
Recent Prog Horm Res
19:
445-488,
1963[ISI].
18.
Diamond, MP,
Rollings RC,
Steiner KE,
Williams PE,
Lacy WW,
and
Cherrington AD.
Effect of alanine concentration independent of changes in insulin and glucagon on alanine and glucose homeostasis in the conscious dog.
Metabolism
37:
28-33,
1988[ISI][Medline].
19.
Dobbins, RL,
Davis SN,
Neal D,
Caumo A,
Cobelli C,
and
Cherrington AD.
Rates of glucagon activation and deactivation of hepatic glucose production in conscious dogs.
Metabolism
47:
135-142,
1998[ISI][Medline].
20.
Dobbins, RL,
Davis SN,
Neal DW,
Cobelli C,
and
Cherrington AD.
Pulsatility does not alter the response to a physiological increment in glucagon in the conscious dog.
Am J Physiol Endocrinol Metab
266:
E467-E478,
1994
21.
Eigler, N,
Sacca L,
and
Sherwin RS.
Synergistic interactions of physiologic increments of glucagon, epinephrine, and cortisol in the dog: a model for stress-induced hyperglycemia.
J Clin Invest
63:
114-123,
1979[ISI][Medline].
22.
Exton, JH.
Mechanisms of hormonal regulation of hepatic glucose metabolism.
Diabetes Metab Rev
3:
163-183,
1987[Medline].
23.
Fosgerau, K,
Mittelman SD,
Sunehag A,
Dea MK,
Lundgren K,
and
Bergman RN.
Lack of hepatic "interregulation" during inhibition of glycogenolysis in a canine model.
Am J Physiol Endocrinol Metab
281:
E375-E383,
2001
24.
Gerich, JE.
Control of glycaemia.
Baillieres Clin Endocrinol Metab
7:
551-586,
1993[ISI][Medline].
25.
Gerich, JE,
Meyer C,
and
Stumvoll MW.
Hormonal control of renal and systemic glutamine metabolism.
J Nutr
130:
995S-1001S,
2000[ISI][Medline].
26.
Goldstein, RE,
Rossetti L,
Palmer BA,
Liu R,
Massillon D,
Scott M,
Neal D,
Williams P,
Peeler B,
and
Cherrington AD.
Effects of fasting and glucocorticoids on hepatic gluconeogenesis assessed using two independent methods in vivo.
Am J Physiol Endocrinol Metab
283:
E946-E957,
2002
27.
Gravholt, CH,
Moller N,
Jensen MD,
Christiansen JS,
and
Schmitz O.
Physiological levels of glucagon do not influence lipolysis in abdominal adipose tissue as assessed by microdialysis.
J Clin Endocrinol Metab
86:
2085-2089,
2001
28.
Hamilton, KS,
Gibbons FK,
Bracy DP,
Lacy DB,
Cherrington AD,
and
Wasserman DH.
Effect of prior exercise on the partitioning of an intestinal glucose load between splanchnic bed and skeletal muscle.
J Clin Invest
98:
125-135,
1996
29.
Hendrick, GK,
Frizzell RT,
and
Cherrington AD.
Effect of somatostatin on nonesterified fatty acid levels modifies glucose homeostasis during fasting.
Am J Physiol Endocrinol Metab
253:
E443-E452,
1987
30.
Hopgood, MF,
Clark MG,
and
Ballard FJ.
Protein degradation in hepatocyte monolayers. Effects of glucagon, adenosine 3':5'-cyclic monophosphate and insulin.
Biochem J
186:
71-79,
1980[ISI][Medline].
31.
Hue, L.
Regulation of gluconeogenesis in the liver.
In: Handbook of Physiology: The Endocrine Pancreas and Regulation of Metabolism. Bethesda, MD: Am. Physiol. Soc, 2001, vol. II, sect. 7, chapt. 20, p. 649-657.
32.
Issekutz, B,
and
Miller H.
Plasma free fatty acids during exercise and the effect of lactic acid.
Proc Soc Exp Biol Med
110:
237-239,
1962.
33.
Jahoor, F,
Peters EJ,
and
Wolfe RR.
The relationship between gluconeogenic substrate supply and glucose production in humans.
Am J Physiol Endocrinol Metab
258:
E288-E296,
1990
34.
Jenssen, T,
Nurjhan N,
Consoli A,
and
Gerich JE.
Failure of substrate-induced gluconeogenesis to increase overall glucose appearance in normal humans. Demonstration of hepatic autoregulation without a change in plasma glucose concentration.
J Clin Invest
86:
489-497,
1990[ISI][Medline].
35.
Jungermann, K,
and
Katz N.
Functional specialization of different hepatocyte populations.
Physiol Rev
69:
708-764,
1989
36.
Kilberg, MS.
Amino acid transport in isolated rat hepatocytes.
J Membr Biol
69:
1-12,
1982[ISI][Medline].
37.
Kilberg, MS,
Barber EF,
and
Handlogten ME.
Characteristics and hormonal regulation of amino acid transport system A in isolated rat hepatocytes.
Curr Top Cell Regul
25:
133-163,
1985[ISI][Medline].
38.
Kilberg, MS,
Stevens BR,
and
Novak DA.
Recent advances in mammalian amino acid transport.
Annu Rev Nutr
13:
137-165,
1993[ISI][Medline].
39.
Kraus-Friedmann, N,
and
Feng L.
The role of intracellular Ca2+ in the regulation of gluconeogenesis.
Metabolism
45:
389-403,
1996[ISI][Medline].
40.
Liljenquist, JE,
Bomboy JD,
Lewis SB,
Sinclair-Smith BC,
Felts PW,
Lacy WW,
Crofford OB,
and
Liddle GW.
Effect of glucagon on net splanchnic cyclic AMP production in normal and diabetic men.
J Clin Invest
53:
198-204,
1974[ISI][Medline].
41.
Magnusson, I,
Rothman DL,
Gerard DP,
Katz LD,
and
Shulman GI.
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].
42.
Mari, A.
Estimation of the rate of appearance in the nonsteady state with a two-compartment model.
Am J Physiol Endocrinol Metab
263:
E400-E415,
1992
43.
McGuinness, OP,
Burgin K,
Moran C,
Bracy D,
and
Cherrington AD.
Role of glucagon in the metabolic response to stress hormone infusion in the conscious dog.
Am J Physiol Endocrinol Metab
266:
E438-E447,
1994
44.
Miller, H,
Issekutz B,
and
Rodahl K.
Effect of exercise on the metabolism of fatty acids in the dog.
Am J Physiol
205:
167-172,
1963
45.
Mortimore, GE,
Poso AR,
Kadowaki M,
and
Wert JJ, Jr.
Multiphasic control of hepatic protein degradation by regulatory amino acids. General features and hormonal modulation.
J Biol Chem
262:
16322-16327,
1987
46.
Mortimore, GE,
Poso AR,
and
Lardeux BR.
Mechanism and regulation of protein degradation in liver.
Diabetes Metab Rev
5:
49-70,
1989[ISI][Medline].
47.
Myers, SR,
Biggers DW,
Neal DW,
and
Cherrington AD.
Intraportal glucose delivery enhances the effects of hepatic glucose load on net hepatic glucose uptake in vivo.
J Clin Invest
88:
158-167,
1991[ISI][Medline].
48.
Parrilla, R,
Goodman MN,
and
Toews CJ.
Effect of glucagon: insulin ratios on hepatic metabolism.
Diabetes
23:
725-731,
1974[ISI][Medline].
49.
Pilkis, SJ,
Claus TH,
and
el-Maghrabi MR.
The role of cyclic AMP in rapid and long-term regulation of gluconeogenesis and glycolysis.
Adv Second Messenger Phosphoprotein Res
22:
175-191,
1988[ISI][Medline].
50.
Pilkis, SJ,
and
Granner DK.
Molecular physiology of the regulation of hepatic gluconeogenesis and glycolysis.
Annu Rev Physiol
54:
885-909,
1992[ISI][Medline].
51.
Pozefsky, T,
Tancredi RG,
Moxley RT,
Dupre J,
and
Tobin JD.
Metabolism of forearm tissues in man. Studies with glucagon.
Diabetes
25:
128-135,
1976[Abstract].
52.
Puhakainen, I,
and
Yki-Jarvinen H.
Inhibition of lipolysis decreases lipid oxidation and gluconeogenesis from lactate but not fasting hyperglycemia or total hepatic glucose production in NIDDM.
Diabetes
42:
1694-1699,
1993[Abstract].
53.
Radziuk, J,
and
Pye S.
Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis.
Diabetes Metab Res Rev
17:
250-272,
2001[ISI][Medline].
54.
Randle, PJ.
Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years.
Diabetes Metab Rev
14:
263-283,
1998[ISI][Medline].
55.
Randle, PJ,
Kerbey AL,
and
Espinal J.
Mechanisms decreasing glucose oxidation in diabetes and starvation: role of lipid fuels and hormones.
Diabetes Metab Rev
4:
623-638,
1988[ISI][Medline].
56.
Rizza, R,
Haymond M,
Cryer P,
and
Gerich J.
Differential effects of epinephrine on glucose production and disposal in man.
Am J Physiol Endocrinol Metab Gastrointest Physiol
237:
E356-E362,
1979
57.
Rosa, F.
Ultrastructural changes produced by glucagon, cyclic 3'5'-AMP and epinephrine on perfused rat livers.
J Ultrastruct Res
34:
205-213,
1971[ISI][Medline].
58.
Ruderman, NB,
and
Berger M.
The formation of glutamine and alanine in skeletal muscle.
J Biol Chem
249:
5500-5506,
1974
59.
Sacca, L,
Vigorito C,
Cicala M,
Corso G,
and
Sherwin RS.
Role of gluconeogenesis in epinephrine-stimulated hepatic glucose production in humans.
Am J Physiol Endocrinol Metab
245:
E294-E302,
1983
60.
Satake, S,
Moore MC,
Igawa K,
Converse M,
Farmer B,
Neal DW,
and
Cherrington AD.
Direct and indirect effects of insulin on glucose uptake and storage by the liver.
Diabetes
51:
1663-1671,
2002
61.
Schworer, CM,
and
Mortimore GE.
Glucagon-induced autophagy and proteolysis in rat liver: mediation by selective deprivation of intracellular amino acids.
Proc Natl Acad Sci USA
76:
3169-3173,
1979[Abstract].
62.
Shamoon, H,
Hendler R,
and
Sherwin RS.
Synergistic interactions among antiinsulin hormones in the pathogenesis of stress hyperglycemia in humans.
J Clin Endocrinol Metab
52:
1235-1241,
1981[Abstract].
63.
Sherck, SM,
Shiota M,
Saccomando J,
Cardin S,
Allen EJ,
Hastings JR,
Neal DW,
Williams PE,
and
Cherrington AD.
Pancreatic response to mild non-insulin-induced hypoglycemia does not involve extrinsic neural input.
Diabetes
50:
2487-2496,
2001
64.
Shiota, M,
Jackson PA,
Bischoff H,
McCaleb M,
Scott M,
Monohan M,
Neal DW,
and
Cherrington AD.
Inhibition of glycogenolysis enhances gluconeogenic precursor uptake by the liver of conscious dogs.
Am J Physiol Endocrinol Metab
273:
E868-E879,
1997
65.
Shulman, GI,
Lacy WW,
Liljenquist JE,
Keller U,
Williams PE,
and
Cherrington AD.
Effect of glucose, independent of changes in insulin and glucagon secretion, on alanine metabolism in the conscious dog.
J Clin Invest
65:
496-505,
1980[ISI][Medline].
66.
Stevenson, RW,
Steiner KE,
Connolly CC,
Fuchs H,
Alberti KG,
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
67.
Stevenson, RW,
Steiner KE,
Davis MA,
Hendrick GK,
Williams PE,
Lacy WW,
Brown L,
Donahue P,
Lacy DB,
and
Cherrington AD.
Similar dose responsiveness of hepatic glycogenolysis and gluconeogenesis to glucagon in vivo.
Diabetes
36:
382-389,
1987[Abstract].
68.
Stingl, H,
Krssak M,
Krebs M,
Bischof MG,
Nowotny P,
Furnsinn C,
Shulman GI,
Waldhausl W,
and
Roden M.
Lipid-dependent control of hepatic glycogen stores in healthy humans.
Diabetologia
44:
48-54,
2001[ISI][Medline].
69.
Stumvoll, M,
Meyer C,
Kreider M,
Perriello G,
and
Gerich J.
Effects of glucagon on renal and hepatic glutamine gluconeogenesis in normal postabsorptive humans.
Metabolism
47:
1227-1232,
1998[ISI][Medline].
70.
Szczepaniak, LS,
Babcock EE,
Schick F,
Dobbins RL,
Garg A,
Burns DK,
McGarry JD,
and
Stein DT.
Measurement of intracellular triglyceride stores by H spectroscopy: validation in vivo.
Am J Physiol Endocrinol Metab
276:
E977-E989,
1999
71.
Wahren, J,
Felig P,
Ahlborg G,
and
Jorfeldt L.
Glucose metabolism during leg exercise in man.
J Clin Invest
50:
2715-2725,
1971[ISI][Medline].
72.
Wall, JS,
Steele R,
DeBodo RC,
and
Altszuler N.
Effect of insulin in utilization and production of circulating glucose.
Am J Physiol
189:
43-50,
1957
73.
Williamson, JR,
Kreisberg RA,
and
Felts PW.
Mechanism for the stimulation of gluconeogenesis by fatty acids in perfused rat liver.
Proc Natl Acad Sci USA
56:
247-254,
1966[ISI][Medline].
74.
Wolfe, RR,
Jahoor F,
and
Shaw JH.
Effect of alanine infusion on glucose and urea production in man.
J Parenter Enteral Nutr
11:
109-111,
1987[Abstract].
75.
Woodside, KH,
Ward WF,
and
Mortimore GE.
Effects of glucagon on general protein degradation and synthesis in perfused rat liver.
J Biol Chem
249:
5458-5463,
1974