Phorbol esters affect pituitary growth hormone (GH) and prolactin release: the interaction with GH releasing factor, somatostatin and bromocriptine.

Phorbol esters are tumor promotors that directly stimulate protein kinase C activity. We asked whether these agents affect basal or receptor initiated alterations in growth hormone (GH) and prolactin release. In 4 h incubations of anterior pituitary cells, phorbol esters enhanced basal and GH releasing factor (GRF)-induced GH release. Somatostatin reduced by 38% the 4-fold stimulation of GH release induced by phorbol ester. These tumor promoters also reversed the ability of bromocriptine, a dopamine agonist, to inhibit prolactin release, with no apparent effect on basal prolactin secretion. When these agents were applied for 24 h, an increase in the basal release of both GH and prolactin was apparent. These data lead us to suggest that an intact protein kinase C system may be necessary for the full expression of GRF-stimulated GH release and dopaminergic inhibition of prolactin release.