Arachidonic acid metabolism by platelets of differing size

S ummary . The relationship between mean platelet volume (MPV) and platelet arachidonic acid metabolism was examined by studying the ability of human platelets of different size to incorporate and metabolize tritiated arachidonic acid ([³HIAA). Platelet phospholipids were labelled with [jH]AA and the platelets were then fractionated into size-dependent subpopulations by counterflow centrifugation. The incorporation of [jH]AA increased through the fractions proportional to the MPV. After thrombin stimulation the per cent of total jH-radioactivity released from the platelets decreased as the MPV increased. However, fractionation of the released %-radioactivity by HPLC (high performance liquid chromatography) demonstrated that MPV had no significant influence on the per cent of total platelet 3H-radioacti-vity released as cyclooxygenase products or as HETE (12-hydroxyeicosatetraenoic acid) but that the release of unmetabolized [3H]AA decreased as MPV increased. In separate experiments using unlabelled platelets the absolute release of thromboxane BL (TXB2) after collagen- and thrombin-induced aggregation was measured by radioimmunoassay and was found to increase in proportion to the MPV. These results demonstrate that the release of arachidonic acid metabolites is qualitatively similar in platelets of different size. However, the absolute ability of platelets to incorporate arachidonic acid, convert it to active metabolites and release them is proportional to their volume. The ability of platelets to release unmetabolized arachidonic acid varies inversely with their MPV.     

The incorporation of acetate and palmitate into lipids by human platelets

The optimal conditions for the incorporation of acetate-1-(14)C and palmitic acid-1-(14)C into platelet lipids have been described. In buffer incubations with acetate there was a sharp pH optimum at 6.8; in plasma incubations, there was a broad pH optimum between 6.8-7.4. Maximal incorporation of acetate occurred at a final concentration of 1.5 mmoles/liter. In buffer, no labeled lipids were released from platelets into the medium. In plasma, 40% of newly formed lipids was recovered in the plasma. 75% of the incorporated acetate could be recovered in ceramide, lecithin, and free fatty acids. Platelet fatty acids were formed both by de novo synthesis and chain elongation. The fatty acids formed by de novo synthesis exchanged with plasma free fatty acids. In buffer incubations no turnover of newly labeled lipids occurred, but in the plasma incubations exchange of newly labeled lecithin with plasma lipids was demonstrable. Palmitic acid-1-(14)C added to plasma was incorporated into platelet lipids. The distribution among the lipid classes of palmitate taken up from plasma was the same as that of palmitate formed intracellularly by de novo synthesis.