title: Interaction of Ca2+ and protein phosphorylation in the rabbit platelet release reaction.
abstract: Ca2+ flux and protein phosphorylation have been implicated as playing an important role in the induction of the platelet release reaction. However, the interactions between Ca2+, protein phosphorylation, and the release reaction have been difficult to study because secretion in human platelets is independent of extracellular Ca2+. Thus, we studied rabbit platelets, which, unlike human platelets, require extracellular Ca2+ for serotonin release to occur. Thrombin, basophil platelet-activating factor (PAF), or ionophore A23187 treatment of intact 32PO43--loaded rabbit platelets resulted in a 200-400% increase in phosphorylation of P7P and P9P, respectively. These peptides were similar in all respects to the peptides phosphorylated in thrombin-treated human platelets. When Ca2+ was replaced in the medium by EGTA, (a) thrombin- and PAF-induced rabbit platelet [3H]serotonin release was inhibited by 60-75%, whereas ionophore-induced release was blocked completely; (b) thrombin-, PAF-, or ionophore-induced P9P phosphorylation was inhibited by 60%; and (c) ionophore-induced P7P phosphorylation was decreased by 60%, whereas that caused by thrombin or PAF was decreased by only 20%. At 0.25-0.5 U/ml of thrombin, phosphorylation preceded [3H]serotonin release with the time for half-maximal release being 26.0 +/- 1.3 s SE (n = 3) and the time for half-maximal phosphorylation being 12.3 +/- 1.3 s SE (n = 3) for P7P and 3.7 +/- 0.17 s SE (n = 3) for P9P. P9P phosphorylation was significantly inhibited (P less than 0.015) by removal by Ca2+ from the medium at a time point before any thrombin- or ionophore-induced serotonin release was detectable. Thus, our data suggest that Ca2+ flux precedes the onset of serotonin secretion and that the rabbit platelet is an appropriate model in which to study the effects of Ca2+ on protein phosphorylation during the platelet release reaction.
mesh_terms: D000818:Animals; D001779:Blood Coagulation Factors / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology*; D001798:Blood Proteins / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000097:blood / Q000494:pharmacology*; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D010766:Phosphorylation; D011817:Rabbits; D012701:Serotonin / Q000378:metabolism*; D013045:Species Specificity; D013917:Thrombin / Q000494:pharmacology
chemical_list: D001779:Blood Coagulation Factors; D001798:Blood Proteins; D012701:Serotonin; D000001:Calcimycin; D013917:Thrombin; D002118:Calcium