Summary. The effect of sulfonylureas on long-term regulation of glucose uptake by insulin and insulin-like growth factors has been studied in the L6 line of cultured skeletal muscle cells. These cells have previously been shown to possess many characteristics of differentiated skeletal muscle and to bind and respond to physiological concentrations of insulin and insulin-like growth factors I and II. Tolazamide (halfmaximal at 0.2 mg/ml) augments the effects of insulin, insulin-like growth factor I, and insulin-like growth factor II on glucose uptake, increasing both sensitivity and maximal efficacy of the hormones. In the absence of added hormone, tolazamide has no effect on glucose uptake. A similar increase in insulin-stimulated glucose uptake with unaltered basal uptake occurs with glybufide (half-maximal at 0.5 ~g/ml). The action of tolazamide requires long-term exposure to the sulfonylurea (22 h) and is inhibited by cycloheximide, suggesting a process that involves new protein synthesis. In contrast to glucose uptake, amino acid uptake in L6 cells is increased by tolazamide in the absence of hormones. Insulin and the insulin-like growth factors also stimulate amino acid uptake, but this effect is not further augmented by tolazamide. Thus, sulfonylureas appear to directly modulate amino acid uptake, but to indirectly augment glucose uptake through an effect on insulin and insulin-like growth factor stimulated pathways. Neither insulin binding nor insulin degradation is altered by tolazamide, indicating a post-binding mechanism of action. The L6 cultured skeletal muscle cell line should be useful in future studies on the mechanism of the extrapancreatic actions of sulfonylureas.

Insulin-like growth factor I (IGF I) plays a key role in the regulation of cell proliferation. Progression of the cell cycle is regulated by stimulatory and inhibitory pathways. In order to understand the mechanisms through which IGF I regulates cardiac muscle growth, we have studied the effects of IGF I on inhibitory pathways involving p53 and WAF1 in cultured cardiac muscle cell line H9C2. The onset of DNA synthesis in response to IGF I stimulation was preceded by activation of p53 expression. In addition, IGF I increased p53-dependent and p53-independent induction of WAF1 in H9C2 cells. Dose-response studies showed that IGF I effects on p53-dependent and p53-independent induction of WAF1 occur at physiological concentrations of IGF I. These data indicate that IGF I coordinately regulates inhibitory pathways of cell cycle progression, and that p53-dependent and p53-independent induction of WAF1 may provide negative control mechanisms to regulate stimulatory pathways of cell cycle progression activated by IGF I.