Triene prostaglandins: Prostaglandin D 3 and icosapentaenoic acid as potential antithrombotic substances

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Abstract
Addition of the 3-series fatty acid precursor (icosapentaenoic acid, IPA), its endoperoxide [prostaglandin (PG) H3] or thromboxane A3 to human platelet-rich plasma (PRP) did not result in aggregation of the platelets. Preincubation of human PRP with exogenous PGH3 actually inhibited aggregation by increasing platelet cyclic[c]AMP concentrations. PGH3 undergoes rapid spontaneous degradation to PGD3 in human PRP. The PGD3 so formed was adequate to account for the increase of platelet cAMP and inhibition of aggregation. Addition of PGD-specific antisera to human PRP blocked the platelet inhibitory activity of exogenous PGH3. PGD3 had considerable potential as a circulating antithrombotic agent. Pretreatment of human PRP with the adenylate cyclase inhibitor 2'',5''-dideoxyadenosine blocked the increase of platelet cAMP and the inhibition of aggregation normally produced by PGI2, PGE1, PGD2, PGH3 and PGD3. The dideoxyadenosine unmasked a direct but moderate reversible aggregatory effect in response to the subsequent addition of PGH3. The dideoxyadenosine markedly enhanced the aggregation produced by exogenous PGH2. IPA was readily incorporated into tissue lipids but was a poor substrate for kidney, blood vessel or heart cyclooxygenase. IPA was a poor substrate for platelet cyclooxygenase. IPA was readily deacylated from the renal phospholipid pool in response to bradykinin, a substance that also stimulated the release of arachidonic acid. A diet that relied primarily on cold-water fish, as in the case of the Greenland Eskimos, lowers endogenous arachidonic acid and markedly increases IPA content of tissue lipids. Because IPA had the potential to act as an antagonist with arachidonic acid for platelet cyclooxygenase and lipoxygenase, the simultaneous release of IPA could suppress any residual arachidonic acid conversion to its aggregatory metabolites.