In vitro incubation of hepatocytes isolated from adult male rats leads to a rapid conversion of the adrenergic activation of glycogenolysis from an alpha1-receptor to a beta2-receptor mediated response within 4 hr. In order to understand the underlying mechanism, we examined time-dependent changes in alpha1 and beta2-adrenergic activation of glycogenolysis and second messenger systems, the cellular density and affinity of these receptors, and the steady state levels of their mRNAs. Incubation of hepatocytes for 4 hr resulted in a decrease in phosphorylase activation and IP3 accumulation in response to phenylephrine, a 40% decrease in alpha1-receptor density, and a 70% decrease in alpha1B-receptor mRNA levels. A 4 hr incubation also resulted in the emergence of a phosphorylase response to isoproterenol, an increase in isoproterenol-, but not in glucagon- or forskolin-induced cAMP accumulation, no change in beta2-receptor density, and a doubling of beta2-receptor densities, while the decrease in alpha1-receptor density was not affected and the decrease in phosphorylase activation by phenylephrine was attenuated. The results indicate that dissociation of rat liver cells triggers a rapidly developing decrease in alpha1B-receptor mRNA and increase in beta2- receptor mRNA levels and corresponding inverse changes in the synthesis of the respective receptor proteins, which account, at least in part, for the rapid conversion from alpha1- to beta-adrenergic glycogenolysis.
