Influence of pH on the prostacyclin (PGI2) mediated inhibition of platelet function

Prostacyclin (PGI2) stimulates platelet adenylate cyclase, elevates intracellular levels of cyclic adenosine monophosphate and blocks the response to aggregating agents. It is rapidly hydrolyzed (T 1/2-5 min) to 6-keto prostaglandin F1 alpha at acid or neutral pH. As a result, platelets incubated with PGI2 will recover spontaneously and respond to aggregating agents within 15--60 min, depending on the initial PGI2 concentration. In the present study we have evaluated the influence of temperature and pH on the stability of PGI2 and its effects on platelet function. PGI1 in Tris buffer was stabilized at several pH levels and stored at 37 degrees C, 23 degrees C, and 4 degrees C. Inhibitory influence in platelet function was lost rapidly at pH 7.2--7.4, lasted several hours at pH 7.8 and was retained indefinitely at pH 7.8 and was retained indefinitely at pH 8 or above. PGI2 (2.8 nM) completely inhibited the response to arachidonic acid for 15 min. at pH 7.4, for at least 1 hour at pH 7.8 and showed no reversal of inhibition after 48 hours at pH 8. However, PGI2 inhibited samples at pH 8 completely recovered their sensitivity to arachidonic acid when the pH was reduced to 7.4. These findings indicate that the biological activity of PGI2, though labile at neutral pH, is stable at pH 8 and can inhibit, cAMP mediated platelet functions for at least 48 hours. Because of its pH dependence, PGI2 may be a useful agent for prolonging the sensitivity of stored platelets.