Differential effects of trifluoperazine on arachidonate liberation, secretion and myosin phosphorylation in intact platelets.

Gel-filtered platelets prelabeled with [3H]-arachidonate and [14C]-adenine or [32P]-orthophosphate were stimulated with thrombin in the presence of various concentrations of trifluoperazine (TFP). Based on the presence of [14C]- or [32P]-labeled extracellular adenine nucleotides, TFP, above 50 microM, caused platelet lysis which reached 30-40% at 100 microM. In the non-lytic range (0-50 microM) TFP caused marked inhibition of [3H]-arachidonic acid liberation and [3H]-phosphatidylcholine breakdown which was complete at 25 microM. Breakdown of [3H]-phosphatidylinositol was partially (about 50%) inhibited at 25 microM TFP and little further inhibition occurred above this concentration. These results show that thrombin-induced liberation of [3H]-arachidonic acid occurs entirely by a TFP-sensitive mechanism, and suggest that the major portion of the arachidonate is liberated from phosphatidylcholine with a possible contribution from phosphatidylinositol. Dense granule secretion and acid hydrolase secretion were progressively inhibited by TFP, while the thiazine had only a small effect on phosphorylation of myosin. These results indicate that the inhibition of the secretory processes by TFP is not caused by action of TFP on myosin light chain kinase. It is suggested that the profound effect of TFP on arachidonic acid liberation but not myosin phosphorylation is due to different subcellular localization of these calmodulin-requiring enzymes: phospholipase A2 and myosin light chain kinase. The lipophilic TFP dissolves preferentially in the membranes where it has access to phospholipase A2 but not to myosin light chain kinase.