RAPID INVERSE CHANGES IN ALPHA-1B-ADRENERGIC AND BETA-2-ADRENERGIC RECEPTORS AND GENE TRANSCRIPTS IN ACUTELY ISOLATED RAT-LIVER CELLS

In vitro incubation of hepatocytes acutely isolated from adult male rats leads to a rapid conversion of the adrenergic activation of glycogenolysis from an alpha-1-receptor (alpha-1AR) to a beta-2-receptor (beta-2AR) mediated response within 4 h. In order to understand the underlying mechanism, we examined time-dependent changes in alpha-1,- and beta-2-adrenergic activation of glycogenolysis and second messenger systems, the cellular density and affinity of alpha-1AR and beta-2AR, and the steady state levels of alpha-(1B)AR and beta-2AR mRNAs. Incubation of hepatocytes for 4 h resulted in a decrease in phosphorylase activation and inositol 1,4,5 trisphosphate accumulation in response to phenylephrine, a 40% decrease in alpha-1AR density, and a 70% decrease in alpha(1B)AR mRNA levels. Incubation of hepatocytes for 4 h also resulted in the emergence of a phosphorylase response to isoproterenol, an increase in isoproterenol-induced but not in glucagon- or forskolin-induced cAMP accumulation, no significant change in beta-2AR density, and a twofold increase in beta-2AR mRNA levels. Exposure of cells to cycloheximide, 2-mu-M throughout the 4 h incubation, prevented the emergence of the phosphorylase response to isoproterenol and reduced beta-2AR densities, while the decrease in alpha-1AR 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 alpha(1B)AR mRNA and increase in beta-2AR mRNA levels and corresponding inverse changes in the synthesis of alpha(1B)AR and beta-2AR which account, at least in part, for the rapid conversion from alpha-1- to beta-2-adrenergic glycogenolysis.