Desensitization of adenylate cyclase and cyclic AMP flux during the early stages of liver regeneration

Abstract

Liver regeneration is controlled by a complex network of interactions between hormones, growth factors, and a variety of hepatotrophic factors. Transient increases in cAMP in the early stages of liver regeneration that are necessary for DNA synthesis and subsequent mitosis have been reported; however, studies on the mechanisms that control cellular cAMP levels during liver regeneration, namely adenylate cyclase activity, cAMP‐dependent phosphodiesterase activity, and cAMP efflux from the cell, have been generally incomplete. In this study we have shown that although there are three peaks in intracellular cAMP levels in the first 24 hours after partial hepatectomy, the adenylate cyclase activity stimulated by glucagon, prostaglandin E₂, adrenaline, and fluoride in vitro decreases with time. K_d and B_MAX of hepatocyte glucagon and β receptors were similar to the sham controls. Our results are consistent with a mixed homologous/heterologous desensitization of the adenylate cyclase system. There was also a loss of cAMP‐dependent phosphodiesterase activity after partial hepatectomy. We speculate that even though the hormone‐stimulated adenylate cyclase system has been desensitized, the system retains the ability to respond to the transient pulses of the variety of hormones secreted after partial hepatectomy and thus raise the intracellular concentration of cAMP. The decrease in cAMP‐dependent phosphodiesterase may be necessary to prevent rapid breakdown of cAMP.