Effect of down-regulation and return of insulin receptors on glycogen synthesis in cultured rat hepatocytes

Abstract

The dependence of the regulation of insulin receptors by insulin on the time hepatocytes were maintained in culture and the relationship between the return of down-regulated receptors and glycogen synthesis from labelled glucose were investigated in primary cultures of adult rat hepatocytes. Insulin receptor numbers, but not ligand affinity, decreased significantly within the first 24 h of culture, even in the absence of insulin, and then returned to the immediate ‘post-attachment’ level during 24–48 h. Therefore, down-regulation of insulin receptors by 10 nmol/l insulin was only minor during the 1st day in culture, but amounted to 50% of control levels after the 2nd day, whereas the rate of insulin degradation remained unaltered throughout the entire period of culture. When down-regulated monolayers were switched to insulin-free medium, receptors returned to control levels within 5–10 h. The reduced basal rate of glycogenesis as well as insulin-sensitivity and insulin responsiveness of this metabolic pathway also gradually increased to control levels. However, the time-dependent receptor return was dissociated from the increase in insulin-sensitivity, emphasising the importance of postbinding events. Since the changes both in basal rates and in insulin responsiveness of glycogenesis during the period of receptor return were inversely related to differences in the actual glycogen content between control and down-regulated cells, cellular glycogen content might participate in the regulation of glycogenesis as a ‘feedback inhibitor’.