Effect of glyburide on glycogen metabolism in cultured rat hepatocytes

Abstract

Sulfonylurea agents decrease hepatic glucose production and fasting glucose levels in type II diabetic patients without changing fasting insulin concentrations. This raises the possibility that these drugs may act directly on hepatic carbohydrate metabolism. Cultured rat hepatocytes were used to test this hypothesis. To ascertain whether this in vitro system was suitable to demonstrate an effect of sulfonylurea agents (eg, the well-documented insulin-potentiating action), we initially measured the effect of glyburide (2 μg/mL) on insulin-stimulated net glucose- 14C incorporation into glycogen. Glyburide increased sensitivity to insulin (ie, shifted the dose-response curve to the left) without affecting either responsiveness or insulin binding. Thus, the ED 50 was significantly lowered (8.4 v 15.2 ng/mL), whereas the percent increase (181% v 170%) over the basal level, specific tracer insulin binding (5.3% v 5.1% per mg protein), and the Scatchard plots were similar. Since an effect of sulfonylurea agents could be demonstrated in this system, and the glycogen pathways supply 75% of hepatic glucose production after an overnight fast, we next measured the direct effect of glyburide (2 μg/mL) on glycogen storage and breakdown. Glycogenolysis was assessed by measuring the breakdown of prelabeled glycogen (from galactose- 14C) and glycogen synthesis by the incorporation of glucose-C 14 into glycogen. Glyburide significantly inhibited glycogenolysis and stimulated glycogen synthesis. Furthermore, glyburide significantly stimulated glycogen synthase while glycogen phosphorylase was unaffected. In conclusion, glyburide directly inhibited glycogenolysis, stimulated glycogen synthesis and glycogen synthase, and potentiated the action of insulin on glycogen synthesis at a postbinding site in cultured rat hepatocytes. These effects, if expressed in vivo, could account for the decreased hepatic glucose production and fasting glucose concentrations in type II diabetic patients.