To investigate whether exertion changes beta-cell reactivity to glucose stimulation and to characterize the beta-cell response to glucose in humans, we performed four sequential 90-min hyperglycemic clamps (7, 11, 20, and 35 mM). Concentrations of hormones and metabolites involved in glucoregulation were measured. Metabolic rate and substrate utilization were studied by indirect calorimetry. Studies were performed without prior exercise, as well as 2 and 48 h after 60 min of bicycle exercise at 150 W. We found 1) a progressive increase in insulin concentrations reaching 1,092 +/- 135 microU/ml with increasing glucose levels, 2) linear relationships between glucose concentrations and concentrations of C-peptide (r = 0.931 +/- 0.008) and proinsulin (r = 0.952 +/- 0.009),3) increased glucose oxidation with increasing glucose uptake, 4) increased plasma norepinephrine, O2 uptake, and beta-hydroxybutyrate at greater than or equal to 20 mM glucose, and 5) no change in beta-cell response or glucose-induced thermogenesis after one bout of exercise despite no compensating changes in plasma concentrations of hormones or metabolites. We conclude that the beta-cell has a very large secretory potential. Secretion of the beta-cell increases linearly with prolonged, graded hyperglycemia. The processing of proinsulin is unchanged during prolonged beta-cell stimulation. In addition, hyperglycemia and sympathetic nervous activity induced by hyperinsulinemia enhance metabolic rate and ketone body production. Finally, a single bout of exercise does not influence either the beta-cell response to intravenous glucose or glucose-induced thermogenesis.