Receptor-induced heterologous desensitization of receptor-regulated phospholipase C

Activation of the P_2Y purinoceptor on turkey erythrocytes results in a G₁₁-mediated activation of a phospholipase C-β isoenzyme and hydrolysis of polyphosphoinositides. The role of the protein kinase C and Ca²⁺-mobilizing arms of the inositol lipid signalling cascade in P_2Y purinoceptor-induced desensitization of phospholipase C has been examined using erythrocytes as a model system. Preincubation of intact erythrocytes with either the P_2Y purinoceptor agonist, ADPβS, or the 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 β-adrenoceptor-stimulated phospholipase C activities in membranes isolated from these cells. The extent of heterologous desensitization induced by ADPβS and PMA were additive suggesting that they did not share a common mechanism. A lack of involvement of activation of protein kinase C in P_2Y purinoceptor-induced heterologous desensitization was further supported by the observation that although protein kinase C inhibitors or down-regulation of protein kinase C resulted in a loss of PMA-induced desensitization, neither treatment affected the extent of P_2Y purinoceptor-induced desensitization. In addition, elevation of intracellular Ca²⁺ or prevention of its elevation did not induce heterologous desensitization and had no effect on the desensitization induced by ADPβS. Thus, neither the protein kinase C nor Ca²⁺ mobilizing arms of the inositol lipid signaling pathway appear to be involved in P_2Y purinoceptor 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 ADPβS or the β-adrenoceptor agonist, isoproterenol, followed by rechallenge with each of the receptor agonist revealed that receptor-specific desentization occurs in addition to heterologous desensitization. Thus, multiple mechanisms account for agonist-induced desentization of the inositol lipid signalling system of turkey erythrocytes.