The dose-response relationship between the hepatic sinusoidal insulin level and glucose production by the liver is such that a half-maximally effective concentration is at or slightly below the hormone levels seen basally after an overnight fast. In the normal individual, the direct effect of the hormone on the hepatocyte is far more important in restraining glucose production than its indirect effect mediated via a suppression of lipolysis. Because insulin regulates the liver in a direct fashion, its effect occurs within several minutes. Thus, the speed with which insulin works and the sensitivity of the liver to it predict that first-phase insulin release should have a significant effect in quickly suppressing hepatic glucose production. On the other hand, nonhepatic tissues are much less sensitive to insulin and respond slowly as a result of the need for insulin to cross the endothelial barrier. As a result, first-phase insulin is unlikely to significantly alter peripheral glucose disposal. Simulation studies in humans and dogs in which the effects of first-phase insulin were simulated confirmed the aforementioned predictions. In addition, they confirmed the ability of second-phase insulin release to have significant effects on both glucose production and utilization.

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 less than .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 less than .05) and net hepatic alanine uptake (NHAU 2.97 +/- 0.45 to 4.61 +/- 0.48 mumol . kg-1 . min-1, P less than .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 less than .05) or NHAU (3.69 +/- 0.80 to 5.10 +/- 0.69 mumol . kg-1 . min-1, P less than .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 mumol . kg-1 . min-1) but completely inhibited the increase in overall gluconeogenic conversion.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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 × 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 × 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 β-adrenergic mechanism.

The effects of somatostatin on epinephrine's ability to stimulate glucose output have been examined in hepatocytes isolated from dogs fasted overnight. Half-maximal stimulation of phosphorylase a activity and glucose output occurred at an epinephrine concentration of approx. 5·10 −9 M. Somatostatin at 10, 100 or 1000 ng/ml had no effect on the ability of a maximal (1·10 −7 M) and a submaximal (1·10 −8 M) dose of epinephrine to activate phosphorylase at 2 min, or to stimulate glucose output over 20 min. Since the doses of somatostatin used in the present study are up to 50-fold higher than the blood concentrations commonly found when somatostatin is used in vivo to inhibit pancreatic hormone secretion, it seems unlikely that use of somatostatin in this way would affect stimulation of hepatic glycogenolysis by epinephrine in vivo.

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.