Thrombin stimulation of human platelets:Phosphoinositides as the only source of the diacylglycerol moiety in phosphatidic acid

Abstract

Recent reports on various cell types including platelets have concluded in the heterogenous production of phosphatidic acid (PA) during the stimulus-response coupling. Human platelets were pulse-labelled simultaneously with [32P] P. and [32H]glycerol. Extracts were analyzed for masses and radioactivities of ATP and phosphoinositides. When the cells were stimulated with low concentrations of thrombin, the production of [32P] PA was evident without measurable production of [3H] PA. At higher doses of the agonist,[3H] PA was formed, but distinctly later than [32P] PA.This suggested a heterogenous production of PA upon thrombin stimulation of platelets. The specific H-radioactivity of PA in unstimulated cells was about 50% of that of the phosphoinositides. Upon l80 sec of stimulation with 0.5 U/ml of thrombin, the specific [3H] PA radioactivity increased to the level of the phosphoinositides which remained constant during platelet activation. Since other phospholipids incorporate [3H] glycerol much slower than the phosphoinositides, these latter remain the only possible source of the diacylglycerol moiety of PA. The specific 32P-radioactivity of PA in unstimulated cells was only 4% of that of α-ATP and similar to the specific 32P-radioactivity of phosphatidylinositol in unstimulated platelets. After Jyijin of stimulation with 0.5 U/ml of thrombin, specific [32P] PA was similar to that of γ-ATP. The descrepancy in [32P] Pi. and [3H] glycerol incorporation into PA upon thrombin stimulation of platelets is therefore mainly due to a thrombin-induced shift inspecific 32P-radioactivity in PA.