The role of Ca2+ uptake in the response of human platelets to adrenaline and to 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor).

Thrombin induces Ca2+ uptake into both stirred and unstirred human platelets in the presence or absence of acetylsalicylate. This Ca2+ uptake is closely correlated with adenine nucleotide secretion in accord with previous observations [Massini, P. and Lüscher, E.F. (1974) Biochim. Biophys. Acta 372, 109-121] but a low level of secretion is observed in the absence of significant Ca2+ uptake. 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-O-alkylAcGEPC) also induces Ca2+ uptake into both stirred and unstirred human platelets in the presence and absence of acetylsalicylate. Aggregation and adenine nucleotide secretion induced by 1-O-alkylAcGEPC can be observed in the absence of added fibrinogen, but addition of fibrinogen causes a very marked shift to the left in the dose/response curves for aggregation and Ca2+ uptake induced by 1-O-alkylAcGEPC. In the absence of added fibrinogen a close correlation is observed between Ca2+ uptake and adenine nucleotide secretion induced by 1-O-alkylAcGEPC. In the presence of added fibrinogen significant aggregation can be observed in the absence of detectable Ca2+ uptake. Adrenaline induces Ca2+ uptake only into stirred human platelets in the presence of added fibrinogen. Blockade of secretion, e.g. by addition of acetylsalicylate, also prevents Ca2+ uptake. Addition of adrenaline fails to cause breakdown of phosphatidylinositol or phosphatidylcholine in acetylsalicylate-treated platelets under conditions where such a response is observed on addition of thrombin. We conclude that Ca2+ uptake into human platelets induced by thrombin, 1-O-alkylAcGEPC and adrenaline is closely associated with the secretory response and in some circumstances, e.g. stimulation by thrombin, is clearly a consequence of this latter response. Previous reports of Ca2+ uptake as an initiating event in the response of human platelets to adrenaline [Owen, N.E., Feinberg, H. and Le Breton, G.C. (1981) Am J. Physiol. 239, H483-488] have not been confirmed in this study.