beta-Endorphin-induced release of 5-hydroxytryptamine from human platelets.

Platelets of healthy human subjects were incubated with 5-hydroxytryptamine (5-HT) followed by a second incubation with either beta-endorphin (beta-end) or the combination of beta-end and naloxone. beta-End (300 pg/ml) reduced the levels of 5-HT to 50% of initial values within 15-40 min. After 40-80 min incubation, the levels of 5-HT decreased to approximately zero. Addition of increasing amounts of beta-end (up to 300 pg/ml) produced increasing releases of 5-HT with increasing doses of beta-end. The response was dose-related, however variable, across subjects. Addition of either 28.7 or 57.4 pg naloxone to 300 pg beta-end did not antagonize the effects of beta-end on 5-HT.