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 Ca2+-specific fluorophore. Protein kinase A maximally inhibited the rate of IP3-mediated Ca2+release by approximately 30% at saturating IP3(10 μM). 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. © 1992.
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