OR26-07 Mild Physiologic Hyperglycemia Does Not Affect Glucose Mediated but Impairs Insulin-Mediated Suppression of Plasma Glucagon in Healthy Normal Glucose Tolerant Subjects

Abstract

Plasma glucagon levels are regulated by plasma glucose concentrations as well as intra-islet and circulating insulin concentrations. Hyperglycemia adversely affects skeletal muscle and hepatic insulin sensitivity (glucotoxicity). However, the effect of physiologic hyperglycemia on glucose-mediated and insulin-mediated suppression of glucagon is not known. The aim of the present study was to evaluate effect of chronic (48 or 72 hours) physiologic increase (+45 mg/dl) in plasma glucose concentration on the suppression of plasma glucagon concentration in healthy NGT individuals: 12 without family history of T2DM (FH-) (9M/3F, age = 50± 4 yrs, BMI = 27 ± 1 kg/m2) and 8 with FH of T2DM (FH+) (4M/4F, age = 48±2, BMI = 26±1 kg/m2). Subjects received an OGTT and 2-step hyperglycemic (+125 and +300 mg/dl) clamp (duration of each step = 80 minutes) before and after 72 hour glucose infusion. On another occasion subjects participated in a 3-step hyperinsulinemic (10, 20, 40 mU/m2·min) euglycemic clamp before and after a 48 hour glucose infusion. Plasma insulin and C-peptide concentrations were obtained every 2-5 minutes during each hyperglycemic clamp step and plasma glucagon concentrations were measured every 10 minutes. The ratio of insulin/glucagon was measured and used as an index of insulin-medicated suppression of plasma glucagon. FPG concentration increased from 97±4 to 140±4 mg/dl during the 72 hour glucose infusion. Following chronic glucose infusion, plasma insulin levels were significantly higher during the basal state and during each hyperglycemic clamp step (by 59% and 78%). There was no difference in plasma glucagon levels following chronic glucose infusion and the degree of suppression of glucagon during 2-step hyperglycemic (+125 and +300 mg/dl) were similar. However, the plasma insulin/glucagon ratio was significantly higher during the fasting state (by 76%) and during the first (by 128%) and second (by 178%) hyperglycemic clamp steps. Similarly during the three step euglycemic clamp (10, 20, 40 mU/m2·min) studies following 48 hr glucose infusion, despite similar plasma glucose concentrations during each clamp step, plasma insulin and glucagon concentrations were higher following chronic glucose infusion. These results demonstrate that sustained physiologic hyperglycemia for 48 hrs or 72 hours (i.e. glucotoxicity) does not affect the glucose mediated suppression of glucagon, but impairs insulin-mediated suppression of glucagon, and could contribute to fasting and post-prandial hyperglycemia in T2DM patients.