Substances that increase the cyclic AMP content prevent platelet aggregation and the concurrent release of pharmacologically active substances evoked by arachidonic acid

Abstract

Arachidonic acid-induced platelet aggregation was inhibited by prostaglandins E₁ and F_2α (PGE₁ and PGF_2α), papaverine and dibutyryl cyclic AMP. Prostaglandin E₂ displayed a biphasic effect, as concentrations below 2 μM potentiated aggregation, whereas concentrations above it were inhibitory. Isoproterenol (up to 10 mM) failed to block aggregation but inhibition was uncovered in presence of adrenergic α-blocking agents. Isoproterenol potentiated aggregation due to sub-threshold amounts of arachidonic acid, and this effect, but not that due to PGE₂, was suppressed by the α-blocking agents. Isoproterenol and PGE₂ appear thus to enhance arachidonic acid-induced platelet aggregation after interacting with different receptor sites. The yield of rabbit aorta contracting activity formed during AA-induced aggregation was markedly reduced by PGE₁, dibutyryl cyclic AMP and high concentrations of PGE₂, and was increased by low concentrations of the latter. PG-like activity was not significantly reduced when aggregation and generation of rabbit aorta contracting activity were inhibited by bibutyryl cyclic AMP. It is hypothesized that interaction of human platelets and arachidonic acid results in formation of different pharmacologically active materials, possibly bearing similar lipoperoxide structures. Generation of one portion of these materials is controlled by the adenyl cyclase-cyclic AMP system, whereas another portion, that comprises the natural PG, is cyclic AMP-independent. Prostaglandins formed during platelet aggregation have a regulatory role and modulate the platelet response, rather than constitute a trigger stimulus for aggregation.