Glucagon is a primary controller of hepatic glycogenolysis and gluconeogenesis during muscular work

Abstract

The effects of the exercise-induced rise in glucagon were studied during 2.5 h of treadmill exercise in 18-h fasted dogs. Five dogs were studied during paired experiments in which pancreatic hormones were clamped at basal levels during a control period (using somatostatin and intraportal hormone replacement), then altered during exercise to stimulate the normal exercise-induced fall in insulin, while glucagon was 1) increased to mimic its normal exercise-induced rise (SG) and 2) maintained at a basal level (BG). Six additional dogs were studied as described with saline infusion alone (C). Gluconeogenesis (GNG) and glucose production (Ra) were measured using tracers [( 3-3H]glucose and [U-14C]alanine) and arteriovenous differences. Glucose fell slightly during exercise in C and was infused in SG and BG so as to mimic the response in C. Glucagon rose from 60 +/- 3 and 74 +/- 5 pg/ml to 118 +/- 14 and 122 +/- 17 pg/ml with exercise in C and SG and was unchanged from basal in BG (67 +/- 6 pg/ml). In C, SG, and BG, insulin fell during exercise by 5 +/- 1, 6 +/- 1, and 6 +/- 1 microU/ml. Ra rose from 3.3 +/- 0.2 and 3.0 +/- 0.2 mg.kg-1.min-1 to 8.6 +/- 0.8 and 9.5 +/- 1.5 mg.kg-1.min-1 with exercise in C and SG, but from only 3.0 +/- 0.2 to 5.5 +/- 0.8 mg.kg-1.min-1 in BG. GNG increased by 248 +/- 38 and 183 +/- 75% with exercise in C and SG but by only 56 +/- 21% in BG. Intrahepatic gluconeogenic efficiency was also enhanced by the rise in glucagon increasing by 338 +/- 55 and 198 +/- 52% in C and SG but by only 54 +/- 46% in BG. The rise in hepatic fractional alanine extraction was 0.38 +/- 0.04 and 0.33 +/- 0.04 during exercise in C and SG and only 0.08 +/- 0.06 in BG. Ra was increased beyond that which could be explained by effects on GNG alone, hence hepatic glycogenolysis must have also been enhanced by the rise in glucagon. In conclusion, in the dog, the exercise-induced rise in glucagon 1) controls approximately 65% of the increase in Ra, 2) increases hepatic glycogenolysis and GNG, and 3) enhances GNG by stimulating precursor extraction by the liver and precursor conversion to glucose within the liver.