Inhibition Of Platelet Release Reaction By Cytidine Monophosphate (CMP): Correlation With Inhibition Of Membrane Sialyltransferase Activity

To provide further evidence for the contention that platelet surface sialyltransferase plays a major role in platelet release reaction, we evaluated the effect of CMP, a sialyltransferase inhibitor, on platelet aggregation and release. CMP exerted a dose-related inhibitory effect (IC₅₀50=1.5mM) on the basal enzyme activity of intact washed platelets or membrane preparation. It blocks the enzyme stimulation by collagen, thrombin, ADP and U46619, a thromboxane A₂ agonist. Serial experiments were then carried out in a lumiaggregometer to determine its effect on platelet release and aggregation. Pretreatment of platelet rich plasma or washed platelet suspension with CMP resulted in a significant reduction of ATP release induced by thrombin, collagen, ADP, U46619 and arachidonate. The dose-response curve of release inhibition was parallel to the enzyme inhibition. Platelet aggregation induced by ADP, collagen and sodium arachidonate was significantly inhibited by CMP but the dose-response curve shifted to the right. Platelet aggregation induced by thrombin or U46619 was unaffected. To determine whether the inhibitory effect of CMP might be mediated through stimulation of adenylate cyclase, platelet cyclic AMP content was determined by radioimmunoassay. There was no significant difference in the cyclic AMP content between CMP-treated and control platelets. CMP did not cause liberation of lactic dehydrogenase from platelets. These findings clearly indicate that inhibition of release reaction by CMP is linked to the inhibition of surface sialyltransferase and is independent of cyclic AMP. This study confirms the notion that platelet membrane sialyltransferase is involved in the initiation of platelet release reaction.