Direct Evidence of a Glucagon-Dependent Regulation of the Concentration of Glucagon Receptors in the Liver

Chronic treatment with exogenous glucagon in the rat results in a reduced concentration of glucagon receptors in the liver. To determine if this is a direct effect of the hormone on its own receptors, the following experimental model in vilro was designed. Cultured rat hepatocytes were exposed to glucagon at 37°C, washed and incubated with mono[125I]iodoglucagon (125I-glucagon), resulting in reduced binding of 125I-glucagon. The magnitude of the diminution in binding was dependent on the concentration of glucagon present as well as on the duration of the exposure. Analysis of the data indicated that the decrease in binding of 125I-glucagon was due to a loss of receptor numbers per cell rather than any change in receptor affinity for the hormone. Degradation of 125I-glucagon during the incubation of cells in the presence or absence of glucagon was studied and did not account for the differences observed in binding. The binding of mono[125I]iodoinsulin of these cells was not affected by glucagon, suggesting specificity of the phenomenon. Although the cells incubated with glucagon incorporate less L-[14C]valine into hepatocyte proteins, this effect was smaller than the reduction in glucagon receptors. Exposure of the cells to cycloheximide (1 μg/ml) produced a progressive loss of glucagon receptors, and this effect was additive to the glucagon-induced receptor loss. This suggests that cycloheximide inhibited receptor synthesis while glucagon accelerated receptor loss. Glucagon-dependent reduction of glucagon receptors closely correlated with the decreased glucagon-stimulated production of CAMP. Furthermore, epinephrine (0.5 μM) stimulation of cyclic AMP production was similar in hepatocytes previously incubated with or without glucagon, indicating a specific role for glucagon in modifying target-cell sensitivity.