Arachidonate synthesis and uptake in isolated guinea-pig megakaryocytes and platelets.

Arachidonic acid (20:4) synthesis and uptake were studied in guinea-pig megakaryocytes and platelets. Isolated megakaryocytes and platelets were incubated with [3H]20:4, 8,11,14-[14C]eicosatrienoic acid (gamma-20:3) and [14C]linoleic acid (18:2) and their lipids were analyzed for radioactivity. The study showed that there was 0.153 microM of unesterified 20:3 and 0.237 microM of 20:4 in guinea-pig plasma in nonfasting animals. At these concentrations, 42.6 pmol of gamma-20:3 and 53.3 pmol of 20:4 were taken up per h per 10(5) megakaryocytes in vitro. Megakaryocytes desaturated 27% of the gamma-20:3 to 20:4 but could not desaturate 18:2. Platelets could not desaturate gamma-20:3 or 18:2. In megakaryocytes, the distribution of 20:4 synthesized by the desaturation of gamma-20:3 and 20:4 taken up reflected the endogenous distribution of 20:4 in megakaryocyte phospholipids and 20:4 was predominantly incorporated into phosphatidylethanolamine (PE). In contrast, the distribution of [3H]20:4 taken up into platelets did not reflect the endogenous distribution. 20:4 was primarily incorporated into platelet phosphatidylcholine and phosphatidylinositol. The study showed that megakaryocytes but not platelets possess a delta 5 desaturase and can synthesize 20:4 from gamma-20:3. Neither cell was shown to have a delta 6 desaturase. Megakaryocytes appear to have the capacity to determine the composition of all pools of platelet 20:4 either by uptake or synthesis of 20:4. Platelets, most likely, have a limited capacity to alter structural pools of 20:4 contained primarily in PE and phosphatidylserine (PS).