Prostaglandin endoperoxides promote calcium release from a platelet membrane fraction in vitro

A calcium sequestering platelet membrane fraction was prepared and the effect of arachidonic acid, PGG2 and PGH2 on calcium content evaluated. At 4 degrees C, 6.7--16.7 micrometers arachidonic acid caused significant release of calcium from preloaded vesicles. Such release was completely inhibited by aspirin pretreating the platelets from which the membrane fraction was prepared. Gamma-linolenic acid, not a substrate for prostaglandin synthesis, did not cause calcium release. At 37 degrees C, after a 5 minute calcium loading of the membrane vesicles, arachidonic acid, PGG2, and PGH2 caused release of calcium. Calcium release by the PGG2 and PGH2 was only slightly inhibited by aspirin. Imidazole, which prevented conversion of the prostaglandin endoperoxides to thromboxanes, also only slightly inhibited calcium release. Other prostaglandins including PGD2, PGE1, PGE2 and PGD2 had no effect on the calcium content of the vesicles. These studies suggest that PGG2 and PGH2 may exert their effects on platelets by mobilizing calcium from an internal membrane store to make it available to promote platelet activation.