Prostaglandin E1 action on adenylate cyclase activity and membrane lipids in platelets

The effect of prostaglandin E1 (PGE1) on temperature-induced membrane lipid phase transitions of human blood platelets was investigated using excimer forming 1-pyrenedecanoic acid as fluorescent probe. PGE1 and PGF2 alpha at 10 microM concentrations had a small but reproducible effect on the melting curves between 20 degrees and 30 degrees C and lowered the temperature for the onset of the phase transition from 26 degrees to 21 degrees C. The adenylate cyclase of the platelet membrane was half maximally stimulated by PGE1 at 0.8 microM and maximally activated at 10 microM concentrations. PGF2 alpha at 100 microM concentrations produced only 40% of the maximal stimulation achieved with PGE1. Stimulation of adenylate cyclase by 10 microM PGE1 increased steeply with temperature between 20 degrees to 30 degrees C. Binding experiments with [3H]PGE1 indicated a high affinity and a low affinity site with KD 13nM and 2 microM, respectively. Non-radioactive PGE1 inhibited binding of [3H]PGE1 to both sites at concentrations corresponding to their respective dissociation constants. PGF2 alpha inhibited binding of PGE1 only at greater than or equal to 1 microM concentrations. It is assumed that only the high affinity binding sites represent specific PGE1 receptors. Binding of PGE1 to the low affinity sites and the observed effects on membrane structure may result from interactions of PGE1 with membrane lipids. Since these latter effects and also stimulation of adenylate cyclase required similar concentrations, it is proposed that local interactions of PGE1 with membrane lipids are a prerequisite for specific receptor-mediated adenylate cyclase activation.