CYCLIC AMP-INDUCED DESENSITIZATION OF G-PROTEIN-REGULATED PHOSPHOLIPASE-C IN TURKEY ERYTHROCYTE-MEMBRANES

The interaction of the cyclic AMP and inositol lipid signalling system s was studied in turkey erythrocytes. Elevation of intracellular cycli c AMP concentrations by pretreatment of the cells with forskolin or 8- Br-cAMP resulted in a marked decrease in responsiveness of phospholipa se C to G-protein activators in membranes prepared from treated cells. Decreases in responsiveness occurred with a t(1/2) of approximately 5 min and were reversible after transfer of desensitized cells to drug- fret medium. Pretreatment of the cells with forskolin inhibited inosit ol phosphate formation in a concentration-dependent manner and additio n of the phosphodiesterase inhibitor IBMX (3-isobutyl-1-methylxanthine ) during pretreatment increased the capacity of forskolin to desensiti ze phospholipase C activity. IBMX also produced a similar potentiation of forskolin-stimulated accumulation of cyclic AMP in turkey erythroc ytes. Isoproterenol pretreatment of the cells induced, like forskolin, partial inhibition of inositol phosphate generation in response to G- protein activators and to P-2Y purinoceptor and beta-adrenoceptor agon ists. The capacity of isoproterenol to induce desensitization of phosp holipase C activity also was increased by the presence of IBMX during pretreatment of the cells. H8 2-(methylamino)ethyl]-5-isoquinoline-sul fonamide), an inhibitor of cyclic AMP-regulated protein kinase, comple tely prevented forskolin-induced desensitization but only partially bl ocked isoproterenol-induced desensitization. These results indicate th at the cyclic AMP signalling cascade has a major inhibitory influence on receptor- and G-protein-activated inositol lipid signaling.