Stimulation of anterior pituitary prolactin release by melittin, an activator of phospholipase A2.

Melittin, a 26-amino acid polypeptide contained in bee venom and an activator of phospholipase A2, stimulated PRL secretion from bovine anterior pituitary cells in vitro. Over the dose range 0.25-2 micrograms/ml, melittin stimulation of PRL release was dose-related, reversible, and calcium dependent. Within this same dose range melittin did not deplete cell PRL stores, nor did it alter [3H]leucine uptake or trypan blue exclusion, indicators of cell viability. The phospholipase A2 inhibitors quinacrine and dibromoacetophenone blocked stimulation of PRL release by melittin and by themselves inhibited spontaneous PRL secretion. Addition of phospholipase A2 to pituitary cell cultures was associated with increased PRL secretion. A possible product of phospholipase A2 action, arachidonic acid, also stimulated PRL release. Indomethacin, an inhibitor of arachidonic acid conversion to prostaglandins, did not block melittin-induced PRL release but instead enhanced it. These data suggest that phospholipase A2 may participate in controlling PRL secretion by causing release of arachidonic acid from membrane phospholipids. Arachidonic acid or its noncyclooxygenase metabolite may serve as an intracellular regulator of secretion in the lactotroph.