Secretion of 5-hydroxytryptamine from electropermeabilised human platelets. Effects of GTP and cyclic 3',5'-AMP.

Enhancement by thrombin of Ca2+-dependent 5HT secretion in the absence of added GTP decreases as the time between electropermeabilisation and addition of thrombin is increased. No decrease occurs if thrombin is added with GTP. Observation of apparent GTP-independent receptor/phospholipase C coupling may result from the presence of bound GTP in the preparation. Enhancement by GTP of Ca2+-dependent 5HT secretion occurs with a significant lag indicating an agonist-independent effect. Cyclic 3'5'-AMP inhibits enhancement by GTP of Ca2+-dependent 5HT secretion while having no effect on enhancement induced by GTP gamma S. Hence cyclic AMP may impair receptor/phospholipase C coupling by enhancing Np GTPase activity.