Cyclic AMP-dependent phosphorylation of the inositol-1,4,5-trisphosphate receptor inhibits Ca2+ release from platelet membranes.

Purified internal platelet membranes were treated with the catalytic subunit of protein kinase A to determine its effect on inositol-1,4,5-trisphosphate (IP3)-mediated Ca2+ release. Release kinetics were monitored using rhod-2, a Ca(2+)-specific fluorophore. Protein kinase A maximally inhibited the rate of IP3-mediated Ca2+ release by approximately 30% at saturating IP3 (10 microM). This inhibition was eliminated by pretreatment with a specific kinase inhibitor peptide. Partial purification of the platelet IP3 receptor showed that both endogenous kinases and added A kinase directly phosphorylate the receptor. Since the IP3 receptor is phosphorylated in the absence of added kinase, the observed inhibition (30%) by protein kinase A does not represent the maximal effect of phosphorylation.