Resensitization of hepatocyte glucagon-stimulated adenylate cyclase can be inhibited when cyclic AMP phosphodiesterase inhibitors are used to elevate intracellular cyclic AMP concentrations to supraphysiological values

Abstract

Treatment of intact hepatocytes with glucagon led to the rapid desensitization of adenylate cyclase, which reached a maximum around 5 min after application of glucagon, after which resensitization ensued. Complete resensitization occurred some 20 min after the addition of glucagon. In hepatocytes which had been preincubated with the cyclic AMP phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine (IBMX), glucagon elicited a stable desensitized state where resensitization failed to occur even 20 min after exposure of hepatocytes to glucagon. Treatment with IBMX alone did not elicit desensitization. The action of IBMX in stabilizing the glucagon-mediated desensitized state was mimicked by the non-methylxanthine cyclic AMP phosphodiesterase inhibitor Ro-20-1724 [4-(3-butoxy-4-methoxylbenzyl)-2-imidazolidinone]. IBMX inhibited the resensitization process in a dose-dependent fashion with an EC50(concn. giving 50 % ofmaximal effect) of 26 + 5 ,M, which was similar to the EC50 value of 22 + 6,M observed for the ability of IBMX to augment the glucagon-stimulated rise in intracellular cyclic AMP concentrations. Pre-treatment of hepatocytes with IBMX did not alter the ability of either angiotensin or the glucagon analogue TH-glucagon, ligands which did not increase intracellular cyclic AMP concentrations, to cause the rapid desensitization and subsequent resensitization of adenylate cyclase. It is suggested that, although desensitization of glucagon-stimulated adenylate cyclase is elicited by a cyclic AMP-independent process, the resensitization of adenylate cyclase can be inhibited by a process which is dependent on elevated cyclic AMP concentrations. This action can be detected by attenuating the degradation of cyclic AMP by using inhibitors of cyclic AMP phosphodiesterase.

Glucagon‐Induced Refractoriness of Hepatocyte Adenylate Cyclase: Comparison of Homologous and Heterologous Components and Evidence Against a Role of cAMP

“…In accordance with our findings, an analogue of glucagon that does not increase the intracellular CAMP, was recently found to be equally effective as glucagon in eliciting adenylate cyclase refractoriness, and it was suggested that a distinct receptor population acting via the inositol phospholipid metabolism mediates the glucagoninduced desensitization of the adenylate cyclase (Murphy et al 1987). Other data indicate that the resensitization of the glucagon-stimulated adenylate cyclase could be inhibited by a process which is dependent on elevated cAMP concentrations (Murphy & Houslay 1988).…”

Insulin and vasopressin elicit inhibition of cholera-toxin-stimulated adenylate cyclase activity in both hepatocytes and the P9 immortalized hepatocyte cell line through an action involving protein kinase C

“…It is suggested that the PKC-mediated phosphorylation of a membrane protein attenuates cholera-toxin-stimulated adenylate cyclase activity in hepatocytes and P9 cells. The cellular selectivity of such an action may be due to the target for this inhibitory action of PKC being a particular isoform of adenylate cyclase which provides the major activity in hepatocytes and P9 cells, but not in either CHO or in intracellular cyclic AMP concentrations [15][16][17][18]. This effect is in itself transient, with resensitization attaining near-completion some 20 min after hormone addition.…”

Glucagon desensitization of adenylate cyclase and stimulation of inositol phospholipid metabolism does not involve the inhibitory guanine nucleotide regulatory protein Gi, which is inactivated upon challenge of hepatocytes with glucagon

“…That desensitization can be mimicked by the tumour-promoting agent TPA suggests that activation of protein kinase C may play a pivotal role in mediating the desensitization process (Heyworth et al, 1984b(Heyworth et al, , 1985a. Any role of increased cyclic AMP appears to be restricted to attenuating the resensitization process (Murphy & Houslay, 1988).…”