Differential energy requirements for platelet responses. A simultaneous study of aggregation, three secretory processes, arachidonate liberation, phosphatidylinositol breakdown and phosphatidate production

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Abstract
Previous studies have indicated different energy requirements for some platelet responses; these differences could, however, be due to inadequate methodology and differences in platelet preparation. The present study describes the effect of decreasing ATP availability on seven platelet responses measured in gel-filtered human platelets. The cells, prelabelled with 5-hydroxy[3H]tryptamine, [3H]- or [14C]adenine, [32P]Pi or [3H]arachidonate, were incubated with antimycin A and 2-deoxy-d-glucose. Platelet responses induced by thrombin and collagen (secretion only), level of metabolic ATP and the adenylate energy charge (AEC) were determined at various times during incubation. Platelet aggregation was rapidly inhibited after a lag of 5–15 min and with 50% inhibition at AEC = 0.55–0.60. Secretion of 5-hydroxy[14C]tryptamine and ATP + ADP from dense granules and of fibrinogen and β-thromboglobin from α-granules were inhibited in parallel, without a lag and with 50% inhibition at AEC = 0.65–0.70. The inhibition of secretion of platelet factor 4 from the α-granules followed another pattern with 50% inhibition at AEC = 0.70–0.80. Breakdown of [3H]-phosphatidylinositol, formation of [3H]- and [32P]-phosphatidate, liberation of [3H]arachidonate and secretion of acid hydrolases were inhibited in parallel and inhibition was present at the start of incubation with 50% inhibition at AEC = 0.80–0.87. These results suggest that the responses have different energy requirements, increasing in the order: aggregation < dense granule and α-granule secretion < acid hydrolase secretion, phosphatidylinositol breakdown, phosphatidate formation and arachidonate liberation. The powerful inhibition of phosphatidylinositol breakdown by metabolic inhibitors suggests that energy-requiring steps are involved in the activation of phospholipase C.

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