Although sulfonylurea agents have been used in the clinical management of type II diabetes (non-insulin-dependent diabetes mellitus, NIDDM) for over two decades, the mechanisms responsible for their hypoglycemie action remain controversial. We have quantitated glycemie control, endogenous insulin secretion in response to mixed meals, adipocyte insulin binding, insulin-mediated peripheral glucose disposal, and basal hepatic glucose output in 17 type II diabetic subjects before and after 3 mo of therapy with the second-generation, sulfonylurea compound glyburide in an attempt to identify the factors responsible for the clinical response to the drug. In addition, 9 subjects were treated for an additional 15 mo to see if the response to the drug changed with time.

The mean fasting serum glucose level fell from an initial value of 264 ± 17 mg/dl to 178 ± 16 mg/dl after 3 mo of drug therapy. Endogenous insulin secretion increased in all subjects, but the increase was most marked in those subjects who continued to exhibit fasting hyperglycemie (fasting serum glucose > 175 mg/dl) after 3 mo of therapy. Adipocyte insulin binding was unchanged after 3 mo of therapy, while the maximal rate of peripheral glucose disposal was increased by 23%, indicating enhancement of peripheral insulin action at a postreceptor site(s). Basal hepatic glucose output showed a significant correlation with the fasting serum glucose level both before and after therapy (r = 0.86, P < 0.001) and fell from 141 ±12 mg/m2/min before therapy to 107 ± 11 mg/m2/min after 3 mo of therapy. A significant correlation was also found between the decrease in the fasting glucose level and both the reduction in basal hepatic glucose output (r = 0.81, P < 0.001) and the enhancement of postreceptor function in peripheral tissues (r = 0.68, P < 0.005). After 18 mo of therapy, those subjects exhibiting an initial good response to the drug demonstrated a slight decrease in endogenous insulin secretion compared with the levels seen at 3 mo, adipocyte insulin binding had increased to the normal range, postreceptor function was further enhanced, and basal hepatic glucose output remained unchanged from the levels observed after 3 mo of therapy.

We conclude that (1) glyburide therapy increases endogenous insulin secretion, increases adipocyte insulin binding after 18, but not 3, mo of therapy, enhances peripheral insulin action by acting primarily at a post-receptor site, and reduces basal hepatic glucose output; (2) the increase in postreceptor function and the reduction of basal hepatic glucose output appear to be the crucial determinants of the clinical response to the sulfonylurea agent; and (3) the response pattern to sulfonylurea compounds in terms of these various parameters can vary as a function of the duration of treatment.

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