Prolactin Suppresses Luteinizing Hormone Secretion and Pituitary Responsiveness to Luteinizing Hormone-Releasing Hormone by a Direct Action at the Anterior Pituitary*

In pathological or experimental hyperprolactinemia, the elevated circulating levels of PRL are the usual cause of the impairment in gonadotropic function. The present study was undertaken to determine whether PRL could suppress basal LH secretion and LHRH-stimulated LH release by a direct action at the anterior pituitary. Anterior pituitaries from ovariectomized rats were incubated in medium 199 alone or in medium 199 containing ovine PRL, and basal and the LHRH-stimulated LH release were followed for 2 or 3 h in vitro. Ovine PRL at 40 and 80 micrograms/ml suppressed basal LH release by 41% and 72%, respectively, at 2 h of incubation. This suppressive effect of both concentrations of PRL continued to the third hour of incubation. LHRH at 5 ng/ml increased the release of LH from pituitaries incubated in medium alone by 57%, 61%, and 107% at 1, 2, and 3 h of incubation, respectively. However, in the pituitaries treated with 40 micrograms/ml ovine PRL, the stimulatory effects of LHRH were diminished at all time points measured. Pretreatment of anterior pituitaries with ovine PRL for 6 h significantly inhibited by 81% the LHRH (5 ng/ml) stimulation of LH release at 2 h of incubation. On the other hand, inhibition of endogenous PRL release by 10(-6) M bromocriptine enhanced the stimulatory effects of 5 ng/ml LHRH by 2.5-fold at 2 h of incubation. The inhibitory effects of PRL on basal and stimulated LH secretion appeared unique, since neither BSA nor vasopressin could elicit similar suppressive effects on LH. These results suggest that in anterior pituitaries exposed to elevated levels of PRL, LH secretion and pituitary responsiveness to LHRH could be impaired. This phenomenon may contribute in part to the antigonadotropic effects of PRL.