Primary cultures of rat fetal hepatocytes were used to study the effect of insulin on glycogenesis during 24 h. Measurements of both glycogen content and 14C-glucose incorporation into glycogen gave similar results. A clear stimulatory insulin effect was observed within one hour, reaching a maximum after three hours. However, from 4 to 12 h, the effect declined strongly and then disappeared. Thereafter, it reappeared but to a lesser extent. These striking variations were not caused by a lack of active insulin in the medium or by any amplification of time variations in hepatocyte basal glycogenic capacity. They were induced by the hormone itself as a function of time of insulin presence at the hepatocyte level. Therefore, the effect of insulin is time dependent.
The transient cessation in the stimulatory effect of insulin on net glycogenesis limits the total amount of glycogen stored and permits a more progressive glycogen accumulation. It is not yet clear whether this cessation is a result of a decrease in the insulin-stimulated glycogen synthesis, or an increase of glycogen breakdown, or both; but we showed that it does not require prior maximal stimulation of glycogenesis. After four hours of incubation with varying doses of insulin, a dosedependent decrease in the insulin glycogenic effect of a second maximal dose of hormone was observed, whereas glucagon-induced glycogenolysis was unchanged. The characteristics of the time dependence of the insulin effect suggest that it may play a regulatory role in fetal hepatocytes.
