Relationship between insulin binding and glycogenesis in cultured fetal hepatocytes.

Binding of 125I-insulin and the stimulatory effect of insulin on 14C-glucose incorporation into glycogen have been studied in cultured fetal rat hepatocytes. Measurement of both variables was possible at 37 degrees C because of the slow rate of insulin degradation in the medium. 125I-insulin binding approached maximum after 10 min, thus largely preceding the insulin glycogenic effect which became significant after 45 min. Maximal effect was observed after 3 h with 10 nmol/l insulin when 16,000 specific sites per hepatocyte were occupied and half-maximal response was achieved with 0.3 nmol/l insulin (2,900 sites/hepatocyte). Dissociation of bound insulin was rapid (t 1/2 = 3 min) and accelerated by the availability of native insulin. In hepatocytes preincubated with insulin, binding was measured after 30 min incubation in the absence of hormone which allowed the liberation of most (95%) of the bound insulin. No modification of insulin binding was observed over extended periods (2-24 h) of exposure to 10 nmol/l insulin, when the glycogenic effect of insulin showed striking variations, notably a cessation of the effect between 4 and 12 h. Thus the time-dependence of the glycogenic effect of insulin cannot be related to a defect in insulin binding in cultured fetal hepatocytes.