Release of Choline Metabolites from Human Platelets: Evidence for Activation of Phospholipase D and of Phosphatidylcholine-specific Phospholipase C

In aspirin-treated platelets labelled by preincubation with [3H]-choline, enhanced release of both [3H]-choline and [3H]-choline phosphate resulted from stimulation by collagen or thrombin. No such release accompanied stimulation by ADP, platelet-activating factor or adrenaline. Release of [3H]-choline phosphate was entirely dependent on aggregate formation whereas release of [3H]-choline was reduced but not eliminated, if aggregation was prevented. The properties of [3H]-choline and [3H]-choline phosphate release indicated that both collagen and thrombin induced activation of phospholipase D in the absence of aggregate formation. Such activation was augmented if aggregate formation occurred. Aggregation induced by these two agonists also caused activation of phosphatidylcholine-specific phospholipase C. These effects were prevented in the presence of staurosporine and could also be induced by addition of a synthetic 1,2-diacylglycerol indicating a role for protein kinase C.