In contrast with previous reports, it was found that membrane-protein phosphorylation by the catalytic subunit (CS) of cyclic AMP-dependent protein kinase had no effect on Ca2+ uptake into platelet membrane vesicles or on subsequent Ca2+ release by inositol 1,4,5-trisphosphate (IP3). Furthermore, IP-20, a highly potent synthetic peptide inhibitor of CS, which totally abolished membrane protein phosphorylation by endogenous or exogenous CS, also had no effect on either Ca2+ uptake or release by IP3. Commercial preparations of protein kinase inhibitor protein (PKI) usually had no effect, but one preparation partially inhibited Ca2+ uptake, which is attributable to the gross impurity of the commercial PKI preparation. IP3-induced release of Ca2+ was also unaffected by the absence of ATP from the medium, supporting the conclusion that Ca2+ release by IP3 does not require the phosphorylation of membrane protein.

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