Chronic Chlorpropamide Therapy of Noninsulin-Dependent Diabetes Augments Basal and Stimulated Insulin Secretion by Increasing Islet Sensitivity to Glucose*

Abstract

To determine the effect of chronic sulfonylurea therapy on islet function in noninsulin-dependent diabetes mellitus (NIDDM), studies were performed in 18 untreated NIDDM patients before and after 12–16 weeks of chlorpropamide therapy. Fasting plasma glucose (FPG) fell with chlorpropamide therapy from 249 ± 16 to 157 ± 8 mg/dl (mean ± SEM; P < 0.001), and basal insulin increased from 17 ± 2 to 24 ± 3 μU/ml (P < 0.001). The percent change in basal insulin correlated with the pretreatment FPG (r = 0.62; P < 0.01) and inversely with the change in FPG during chlorpropamide (r = −0.57; P < 0.025). Thus, patients with the highest pretreatment FPG showed the largest relative increase in basal insulin and the largest fall of FPG with chlorpropamide therapy. In nine patients, arginine-stimulated acute insulin responses (AIR) were studied at each of three plasma glucose (PG) levels both before and during chlorpropamide treatment. AIR at FPG was not different before and during treatment. However, when PG during treatment was matched by glucose infusion to the pretreatment FPG, the AIR was clearly increased during chlorpropamide therapy (176 ± 65 vs. 49 ± 11 μU/ml; P < 0.02). When AIR is plotted against PG for each individual, the slope of the regression line generated (slope of glucose potentiation) is a measure of that patient's islet sensitivity to glucose. The logarithm of the slope of glucose potentiation correlated inversely with FPG (r = −0.92; P < 0.001). Chlorpropamide treatment increased the slopes of potentiation from 0.26 ± 0.11 to 1.47 ± 0.70 (P < 0.01). We conclude that chronic chlorpropamide therapy augments both basal and stimulated insulin secretion in NIDDM and that this may be an important mechanism of the drug's hypoglycemic effect. The data support the hypothesis that the hyperglycemia of NIDDM is related to islet insensitivity to glucose and that chlorpropamide treatment improves this impairment.