Interactions between prostaglandin E2 and inhibitors of platelet aggregation which act through cyclic AMP

Prostaglandin (PG) E2 potentiates platelet aggregation at low concentrations (10(-8)-10(-6) M). It also inhibits aggregation at a higher concentration (10(-5) M), probably by acting through cyclic adenosine 3',5'-monophosphate (cyclic AMP). The mechanism of this biphasic effect of PGE2 and its implications for thrombosis are not clearly understood. Using a sensitive cyclic AMP assay, in conjunction with platelet aggregation studies, we have examined the interactions between PGE2 and other inhibitors of platelet aggregation which act through cyclic AMP. Low concentrations of PGE2 reversed the inhibition of platelet aggregation and increase in cyclic AMP levels induced by PGI2, PGD2 and adenosine (which stimulate adenylate cyclase (AC) through separate and specific platelet receptors). In contrast, low concentrations of PGE2 added to the inhibition of platelet aggregation and increase in cyclic AMP levels induced by forskolin (which stimulates AC directly) and AH-P 719 and DN-9693 (which inhibit cyclic AMP phosphodiesterase (PDE]. These results suggest that the biphasic effect of PGE2 may be mediated by interaction with two separate platelet receptors. Low concentrations appear to potentiate aggregation by acting at a receptor which is directly coupled to an inhibitory guanine nucleotide-binding protein (Gi), possibly the putative PG endoperoxide receptor. High concentrations of PGE2 appear to inhibit aggregation by acting at an additional receptor, probably the PGI2 receptor. The ease with which PGE2 reverses the effects of PGI2, PGD2 and adenosine, but adds to the effects of AH-P 719 and DN-9693, suggests that PDE inhibitors might offer greater potential than these AC stimulators as an anti-thrombotic strategy.(ABSTRACT TRUNCATED AT 250 WORDS)