Kurt E. Steiner

Similar Dose Responsiveness of Hepatic Glycogenolysis and Gluconeogenesis to Glucagon In Vivo

This study was undertaken to determine whether the dose-dependent effect of glucagon on gluconeogenesis parallels its effect on hepatic glycogenolysis in conscious overnight-fasted dogs. Endogenous insulin and glucagon secretion were inhibited by somatostatin (0.8 μg · kg−1 · min−1), and intraportal replacement infusions of insulin (213 ± 28 (αU kg−1 · min−1) and glucagon (0.65 ng · kg−1 · min−1) were given to maintain basal hormone concentrations for 2 h (12 ± 2 and 108 ± 23 pg/ml, respectively). The glucagon infusion was then increased 2-, 4-, 8-, or 12-fold for 3 h, whereas the rate of insulin infusion was left unchanged. Glucose production (GP) was determined with 3-[3H]glucose, and gluconeogenesis (GNG) was assessed with tracer (U-[14C]alanine conversion to [14C]glucose) and arteriovenous difference (hepatic fractional extraction of alanine, FEA) techniques. Increases in plasma glucagon of 53 ± 8, 199 ± 48, 402 ± 28, and 697 ±149 pg/ml resulted in initial (15- 30 min) increases in GP of 1.1 ± 0.4 (N = 4), 4.9 ± 0.5 (N = 4), 6.5 ± 0.6 (N = 6), and 7.7 ± 1.4 (N = 4) mg kg−1 · min−1, respectively; increases in GNG (∼3 h) of 48 ± 19, 151 ± 50, 161 ± 25, and 157 ± 7%, respectively; and increases in FEA (3 h) of 0.14 ± 0.07, 0.37 ± 0.05, 0.42 ± 0.04, and 0.40 ± 0.17, respectively. In conclusion, GNG and glycogenolysis were similarly sensitive to stimulation by glucagon in vivo, and the dose-response curves were markedly parallel.

The Relative Importance of First- and Second-Phase Insulin Secretion in Countering the Action of Glucagon on Glucose Turnover in the Conscious Dog

While it is known that glucagon induces a biphasic release of insulin when infused into a normal animal, it is not known whether the resultant pattern of insulin secretion has important metabolic consequences. To ascertain whether such is the case, glucagon was elevated fourfold in the presence of first-, second-, or combined first- and second-phase insulin release to determine ability of the latter hormone to antagonize the effect of glucagon on glucose turnover in the conscious dog. To separate the effects of the different phases of insulin release the “pancreatic clamp” technique, in which somatostatin is given to inhibit the endocrine pancreas and replacement amounts of insulin and glucagon are given intraportally, was used. In this way, a rise in glucagon (∼220 pg/ml) was brought about in the presence of simulated first-phase (peak IRI 25 μU/ml at 5 min; basal by 30 min), secondphase (peak IRI 19 μU/ml at 30 min and sustained elevation thereafter), or first- plus second-phase (peak IRI 33 μU/ml at 5 min; 17 μU/ml at 30 min and sustained elevation thereafter) insulin release. Optimal glycemic control required both first- and second-phase insulin release. A selective deficiency of first-phase release resulted in a transient (2 h) worsening of the glucagon-induced hyperglycemia (twofold the normal increment). This defect was attributable to a larger initial rise in glucose production (3.6 ± 0.6 mg/kg · min) than that observed when both phases of insulin release were present (0.9 ± 0.4 mg/kg · min). First-phase insulin release had no significant effect on glucose clearance. A selective deficiency of second-phase release resulted in marked (sixfold) and prolonged worsening of the glucagon-induced hyperglycemia. In this case, however, the hyperglycemia was primarily the result of a defect in glucose clearance. Glucose clearance fell by 29 ± 7% instead of rising by 30 ± 4% as it did when both firstand second-phase release were present. Glucose production was mildly elevated between 15 and 75 min when second-phase insulin release was deficient relative to that apparent when both the first- and secondphases of release were present; this also contributed to the abnormal hyperglycemia. We conclude that both phases of insulin release are vital to full counterregulation of the action of glucagon on glucose metabolism. First-phase insulin release is important to counter the quick effect of glucagon on glucose production, while second-phase insulin release is important to sustain that inhibition and to augment glucose utilization. Absence of first-phase release results in a transient (2 h) and moderate (20–30 mg/dl) glycemic defect while an absence of second-phase release results in a prolonged and dramatic (70–100 mg/dl) defect.

Effects of morphine on glucose homeostasis in the conscious dog.

This study was designed to assess the effects of morphine sulfate on glucose kinetics and on glucoregulatory hormones in conscious overnight fasted dogs. One group of experiments established a dose-response range. We studied the mechanisms of morphine-induced hyperglycemia in a second group. We also examined the effect of low dose morphine on glucose kinetics independent of changes in the endocrine pancreas by the use of somatostatin plus intraportal replacement of basal insulin and glucagon. In the dose-response group, morphine at 2 mg/h did not change plasma glucose, while morphine at 8 and 16 mg/h caused a hyperglycemic response. In the second group of experiments, morphine (16 mg/h) caused an increase in plasma glucose from a basal 99 +/- 3 to 154 +/- 13 mg/dl (P less than 0.05). Glucose production peaked at 3.9 +/- 0.7 vs. 2.5 +/- 0.2 mg/kg per min basally, while glucose clearance declined to 1.7 +/- 0.2 from 2.5 +/- 0.1 ml/kg per min (both P less than 0.05). Morphine increased epinephrine (1400 +/- 300 vs. 62 +/- 8 pg/ml), norepinephrine (335 +/- 66 vs. 113 +/- 10 pg/ml), glucagon (242 +/- 53 vs. 74 +/- 14 pg/ml), insulin (30 +/- 9 vs. 10 +/- 2 microU/ml), cortisol (11.1 +/- 3.3 vs. 0.9 +/- 0.2 micrograms/dl), and plasma beta-endorphin (88 +/- 27 vs. 23 +/- 6 pg/ml); all values P less than 0.05 compared with basal. These results show that morphine-induced hyperglycemia results from both stimulation of glucose production as well as inhibition of glucose clearance. These changes can be explained by rises in epinephrine, glucagon, and cortisol. These in turn are part of a widespread catabolic response initiated by high dose morphine that involves activation of the sympathetic nervous system, the endocrine pancreas, and the pituitary-adrenal axis. Also, we report the effect of a 2 mg/h infusion of morphine on glucose kinetics when the endocrine pancreas is clamped at basal levels. Under these conditions, morphine exerts a hypoglycemic effect (25% fall in plasma glucose, P less than 0.05) that is due to inhibition of glucose production (by 25-43%, P less than 0.05). The hypoglycemia was independent of detectable changes in insulin, glucagon, epinephrine and cortisol, and was not reversed by concurrent infusion of a slight molar excess of naloxone. Therefore, we postulate that the hypoglycemic effect of morphine results from the interaction of the opiate with non-mu receptors either in the liver or the central nervous system.

Effects of insulin on glucagon-stimulated glucose production in the conscious dog

The relative importance of insulin and glucagon as primary regulators of glucose metabolism in vivo was assessed in 18-hour fasted conscious dogs. Glucose turnover was determined using [3-3H]glucose and gluconeogenesis was assessed using tracer ([14C]alanine) and A-V difference techniques during a 40-minute control period and a 3-hour period during which various hormonal perturbations were brought about. During the infusion of somatostatin and basal intraportal replacement amounts of insulin and glucagon for the entire study, the plasma glucose concentration (109 +/- 5 mg/dL), glucose production (3.24 +/- 0.30 mg/kg/min), and glucose utilization (3.17 +/- 0.32 mg/kg/min) remained unchanged. When the glucagon infusion rate was increased fourfold at the end of the control period, the plasma glucose level increased from 107 +/- 4 to 225 +/- 23 mg/dL by 1 hour and remained elevated. Glucose production increased from 3.14 +/- 0.29 to 7.66 +/- 0.51 mg/kg/min by 15 minutes and decreased to 4.23 +/- 0.35 mg/kg/min by 3 hours. Glucose utilization rose from a basal value of 3.20 +/- 0.26 to 5.46 +/- 0.27 mg/kg/min by 3 hours. When a fourfold increase in the insulin infusion rate was brought about at the end of the control period, glucose production decreased from 2.83 +/- 0.20 to 1.16 +/- 0.57 mg/kg/min by 1 hour, after which it increased slightly (1.62 +/- 0.81 mg/kg/min). Glucose utilization increased from 2.92 +/- 0.30 to 8.12 +/- 1.12 mg/kg/min by 3 hours. Euglycemia was maintained by glucose infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of alanine concentration independent of changes in insulin and glucagon on alanine and glucose homeostasis in the conscious dog

The effect of an alanine load per se on hepatic alanine balance and hepatic glucose production is unclear. To examine this question, alanine was infused into six postabsorptive dogs at a rate of 6 mumol/kg-min, while maintaining insulin and glucagon levels using the pancreatic clamp technique. The arterial alanine concentration rose from a basal level of 227 +/- 16 mumol/L to 497 +/- 40 mumol/L during alanine infusion (P less than .01). The net hepatic fractional extraction of alanine remained unchanged, while hepatic alanine uptake increased from 3.0 +/- 0.3 to 6.0 +/- 0.4 mumol/kg-min (P less than .01). Conversion of alanine into glucose increased 87% to 2.7 +/- 0.3 mumol/kg-min during alanine infusion (P less than .01) while gluconeogenic efficiency remained essentially unchanged. Despite the increased gluconeogenic rate, the total rate of glucose production was unchanged. These data suggest that an increase in the alanine load to the liver causes a proportional increase in net hepatic alanine uptake and the gluconeogenic rate, but that in an overnight fasted animal this increase is insufficient to significantly increase glucose production.

Relative Importance of First- and Second-Phase Insulin Secretion in Glucose Homeostasis in Conscious Dog: II. Effects on Gluconeogenesis

The normal pancreatic response to an exogenous glucagon infusion is a biphasic release of insulin. In our study the ability of each component of insulin release to counter the effects of the glucagon on gluconeogenesis and alanine metabolism was assessed by mimicking first- and/or second-phase insulin release with infusions of somatostatin and intraportal insulin. When a fourfold increase in glucagon was brought about in the presence of fixed basal insulin release, there was a large increase in overall glucose production and gluconeogenesis. The increase in the conversion of [14C]alanine into [14C]glucose (169 ± 42%, P < .05) was accompanied by an increase in the fractional extraction of alanine by the liver (FEA 0.32 ± 0.06 to 0.66 ± 0.10, P < .05) and net hepatic alanine uptake (NHAU 2.97 ± 0.45 to 4.61 ± 0.48 μmol kg1 · min1 P < .05). Simulated first-phase insulin release had no effect on the ability of glucagon to increase FEA (0.32 ± 0.03 to 0.66 ± 0.03, P < .05) or NHAU (3.69 ± 0.80 to 5.10 ± 0.69 μmol · kg1 · min−1 P < .05) but did limit the increase in overall gluconeogenic conversion (114 ± 37%). Second-phase insulin release had no effect on either the glucagon-induced increase in FEA (0.35 ± 0.08 to 0.73 ± 0.04) or NHAU (3.35 ± 0.92 to 5.13 ± 0.85 μmol · kg−1 · min−1) but completely inhibited the increase in overall gluconeogenic conversion. Combined first- and second-phase insulin release was also unable to prevent the glucagon-induced increase in FEA (0.35 ± 0.09 to 0.65 ± 0.06, P < .05) and-NHAU (2.59 ± 0.56 to 3.50 ± 0.37 μmol · kg−1 · min−1) but completely inhibited the glucagon-induced rise in gluconeogenic conversion. These data show that the glucagon-induced increase in gluconeogenic conversion was remarkably sensitive to relatively small (≃8 μU/ml) changes in circulating insulin. Even a brief (5-min) pulse of insulin markedly reduced the effect of glucagon on the overall gluconeogenic process for a prolonged period. Furthermore, the inhibitory action of insulin appeared to occur within the hepatocyte rather than at the cell membrane because the increase in the fractional extraction of alanine by the liver and indeed the rise in hepatic alanine uptake caused by glucagon were unaffected by the increase in insulin.

Mechanism of epinephrine's glycogenolytic effect in isolated canine hepatocytes☆

Epinephrine (10(-7) mol/L) addition to isolated canine hepatocytes activates glycogen phosphorylase from 12.3 +/- 0.4 to 28.6 +/- 2.6 U/g and glucose output from 42 +/- 3 to 170 +/- 24 nmol/mg/h. Preincubation of hepatocytes with propranolol (2 X 10(-5) mol/L) caused a 73% inhibition of phosphorylase activation and a 77% inhibition of the stimulation of glucose output by epinephrine. Phentolamine (2 X 10(-5) mol/L) on the other hand, caused a 16% inhibition of phosphorylase activation and a 27% inhibition of the stimulation of glucose output by epinephrine. These results were unaffected by the sex of the animal. In the dog the glycogenolytic effects of epinephrine appear to be mediated primarily by a beta-adrenergic mechanism.

Regulation of Ketogenesis by Epinephrine and Norepinephrine in the Overnight-fasted, Conscious Dog

The effects on ketogenesis and lipolysis of a norepinephrine (0.04 μg/kg-min), epinephrine (0.04 μg/kg-min), or saline infusion were examined in the overnight-fasted, conscious dog. Plasma insulin and glucagon levels were maintained constant by means of a somatostatin infusion (0.8 μg/kg-min) and intraportal replacement infusions of insulin and glucagon. In saline-infused dogs, plasma epinephrine (62 ± 8 pg/ml), norepinephrine (92 ± 29 pg/ml), blood glycerol (87 ± 10μM), and plasma nonesterified fatty acid (NEFA) (0.82 ± 0.17 mM) levels did not change. Total blood ketone body levels tended to rise (62 ± 10 to 83 ± 11 μM) by 3 h as did total ketone body production (1.5 ± 0.4 to 2.2 ± 0.4 (μmol/kg-min) over the same time interval. Norepinephrine infusion to produce plasma levels of 447 ± 86 pg/ml caused a sustained 50% rise in glycerol levels (66 ± 17 to 99 ± 15 μmol/L, P < 0.05) and 53% rise in nonesterified fatty acids (0.53 ± 0.07 to 0.81 ± 0.15 μmol/L, P < 0.05). Total ketone body levels rose by 43% (51 ± 8 to 73 ± 10 μmol/L) and ketone body production rose by a similar proportion (1.5 ± 0.2 to 2.2 ± 0.3 μmol/kg-min), changes that did not differ significantly from control animals. A similar increment in plasma epinephrine levels (75 ± 15 to 475 ± 60 pg/ml) caused glycerol levels to rise by 82% (105 ± 23 to 191 ± 26 (μmol/L) in 30 min, but this rise was not sustained and the level fell to 146 ± 14 (μmol/L by 120 min. Plasma nonesterified fatty acids rose from a basal value of 0.89 ± 0.19 mM to 1.25 ± 0.29 mM during the first 30 min, but fell to 0.60 ± 0.12 mM by 2 h. Ketone body levels remained unchanged (66 ± 10 μmol/L) and ketone body production declined (1.5 ± 0.3 to 1.0 ± 0.2 μmol/kgmin). These data indicate that (1) although the sustained increase in lipolysis caused by norepinephrine was greater than that caused by fasting alone, the ketogenic responses were not different, and (2) epinephrine has a transient lipolytic effect and an antiketogenic effect compared with controls.

The role of phosphorylation in the α-adrenergic-mediated inhibition of rat hepatic pyruvate kinase

Phenylephrine in the presence of 1-methyl-3-isobutylxanthine and propanolol caused a 40-50% inhibition of pyruvate kinase (type L) activity in isolated hepatocytes, which was accompanied by a 2-3-fold increase in the phosphate content of the enzyme. These changes were blocked by the alpha-adrenergic antagonist dihydroergocryptine and could not be accounted for by the slight increase in cyclic AMP-dependent protein kinase activity generated by the alpha-adrenergic agonist. It is concluded that a significant component of the inhibition of hepatic pyruvate kinase mediated by alpha-adrenergic agonists can be attributed to a cyclic AMP-independent alteration in the phosphorylation state of the enzyme.

The Effects of Epinephrine on Ketogenesis in the Dog After a Prolonged Fast

The effects of a selective increase in epinephrine on ketogenesis and lipolysis were determined in the conscious dog following a prolonged fast (7 days). Plasma insulin and glucagon were fixed at basal levels by infusion of somatostatin (0.8 micrograms/kg/min) and basal intraportal replacement amounts of insulin (210 +/- 20 microU/kg/min) and glucagon (0.65 ng/kg/min). Following a 40-minute control period, saline or epinephrine (0.04 microgram/kg/min) was infused for 3 hours. Plasma insulin, glucagon, and norepinephrine levels did not change during saline (6 +/- 1 microU/mL, 83 +/- 17 pg/mL, and 137 +/- 38 pg/mL, respectively) or epinephrine (10 +/- 1 microU/mL, 73 +/- 18 pg/ml, 98 +/- 13 pg/mL, respectively) infusion. Plasma epinephrine levels increased from 80 +/- 26 to 440 +/- 47 pg/mL in response to infusion of the catecholamine, but remained unchanged during saline infusion. Glycerol levels (93 +/- 10 mumol/L) remained unchanged during saline infusion, but increased in response to epinephrine (108 +/- 9 to 170 +/- 18 mumol/L by 30 minutes). The glycerol level had returned to baseline and to the value apparent in saline controls by 60 minutes. The nonesterified fatty acid (NEFA) level declined slowly during the 3-hour saline infusion, but was elevated in response to epinephrine infusion (1.27 +/- 0.16 to 1.97 +/- 0.25 mmol/L at 30 minutes). After the initial epinephrine-induced increase, the NEFA level declined so that by 3 hours it was not significantly different from the basal or saline values.(ABSTRACT TRUNCATED AT 250 WORDS)