Effect of glyburide on beta cell responsiveness to glucose in non-insulin-dependent diabetes mellitus

Since the introduction of glyburide in 1984, many studies have evaluated the effects of this oral hypoglycemic agent on beta cell function in patients with non-insulin-dependent diabetes mellitus. The early studies, which were performed in patients receiving concomitant insulin therapy, may have underestimated the true effect of glyburide on insulin secretion. The more recent studies demonstrate that both short- and long-term glyburide therapy increase C-peptide levels in diabetic as well as nondiabetic subjects and that the effects of glyburide are comparable to those of the other second-generation sulfonylurea, glipizide. The effects of glyburide on insulin secretory rates calculated from plasma C-peptide levels were recently evaluated using individually derived C-peptide kinetic parameters and a validated open two-compartment model of peripheral C-peptide kinetics. Glyburide did not influence fasting insulin secretion (196 +/- 34 versus 216 +/- 23 pmol/min) but did cause an increase in the total amount of insulin secreted over a 24-hour period (447 +/- 58 versus 561 +/- 55 nmol). This increase in the production of insulin was generated by an increase in amplitude of secretory pulses occurring after lunch and dinner rather than by a greater number of pulses. The full effect of glyburide on the beta cell became evident when glucose concentrations were clamped at the hyperglycemic level of 300 mg/dL both before and during treatment for a 3-hour period. During that time, insulin secretion rates increased by 221 percent in response to glyburide. Glyburide did not, however, completely reverse the beta cell secretory defect characteristic of non-insulin-dependent diabetes mellitus. In the patients receiving glyburide, the sluggish insulin secretory response to breakfast persisted, and the insulin secretory response during the hyperglycemic clamping was less than the response normally seen in nondiabetic subjects. These experiments suggest that the primary effect of glyburide on the beta cell is to increase its responsiveness to glucose. Although the precise mechanism of action of glyburide at the cellular level is unclear, in vitro studies suggest that its effect is mediated through binding with specific receptors on the beta cell membrane, which in turn leads to alterations in the cellular efflux of potassium ions and influx of calcium ions.