RECEPTOR-INDUCED HETEROLOGOUS DESENSITIZATION OF RECEPTOR-REGULATED PHOSPHOLIPASE-C

Activation of the P-2Y purinoceptor on turkey erythrocytes results in a G(11)-mediated activation of a phospholipase C-beta isoenzyme and hy drolysis of polyphosphoinositides. The role of the protein kinase C an d Ca2+-mobilizing arms of the inositol lipid signalling cascade in P-2 Y purinoceptor-induced desensitization of phospholipase C has been exa mined using erythrocytes as a model system. Preincubation of intact er ythrocytes with either the P-2Y purinoceptor agonist, ADP beta S, or t he protein kinase C-activating phorbol ester, phorbol 12-myristate, 13 acetate (PMA), resulted in a time of preincubation-dependent decrease in guanine nucleotide-, P-2Y purinoceptor-, and beta-adrenoceptor-sti mulated phospholipase C activities in membranes isolated from these ce lls. The extent of heterologous desensitization induced by ADP beta S and PMA were additive suggesting that they did not share a common mech anism. A lack of involvement of activation of protein kinase C in P-2Y purinoceptor-induced heterologous desensitization was further support ed by the observation that although protein kinase C inhibitors or dow n-regulation of protein kinase C resulted in a loss of PMA-induced des ensitization, neither treatment affected the extent of P-2Y purinocept or-induced desensitization. In addition, elevation of intracellular Ca 2+ or prevention of its elevation did not induce heterologous desensit ization and had no effect on the desensitization induced by ADP beta S . Thus, neither the protein kinase C nor Ca2+ mobilizing arms of the i nositol lipid signalling pathway appear to be involved in P-2Y purinoc eptor promoted heterologous desensitization of phospholipase C. These results are consistent with the existence of a novel feedback pathway for agonist-induced heterologous desensitization of a second messenger generating enzyme. Preincubation of cells with ADP beta S or the beta -adrenoceptor agonist, isoproterenol, followed by rechallenge with eac h of the receptor agonists revealed that receptor-specific desensitiza tion occurs in addition to heterologous desensitization. Thus, multipl e mechanisms account for agonist-induced desensitization of the inosit ol lipid signalling system of turkey erythrocytes.