The aim of the present study was to characterize the role of glucagon in countering the prolonged hypoglycemia resulting from insulin infusion and to determine whether its effect is manifest through glycogenolysis and/or gluconeogenesis. Two groups of 18-h fasted somatostatin-treated dogs were given intraportal insulin at 5 mU.kg-1.min-1. In one group (SimGGN; n = 6), glucagon was infused intraportally so as to mimic the normal response to hypoglycemia. In a second group (BasGGN; n = 6), glucagon was infused at a basal rate. Glucose turnover and gluconeogenesis were assessed by combining tracer and hepatic balance techniques. Exogenous glucose was infused as needed to maintain equivalent hypoglycemia at approximately 45 mg/dl in the two groups. Although glucagon concentrations were significantly different, the levels of other counterregulatory hormones were equivalent in both experimental protocols. Endogenous glucose production (EGP) in SimGGN doubled from 2.4 +/- 0.2 to 5.4 +/- 0.8 mg.kg-1.min-1 by 1 h before dropping to 4.5 +/- 0.2 mg.kg-1.min-1 in the 3rd h of insulin infusion. EGP in BasGGN was initially 2.5 +/- 0.1 mg.kg-1.min-1, unchanged by 1 h, and increased to 3.9 +/- 0.2 mg.kg-1.min-1 by the 3rd h of insulin infusion. In the 1st h of insulin infusion, the rise in gluconeogenesis in both groups was equal and represented only a small part of total EGP. By the 3rd h, gluconeogenesis was the major contributor to total EGP, and gluconeogenic efficiency increased significantly more in SimGGN than BasGGN (261 vs. 140%, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)