Contribution of arachidonate metabolites to basal and thyrotropin releasing-hormone-stimulated release of prolactin from purified lactotrophs in primary culture.

Among the different biochemical pathways which have been suggested to play a role in the control of prolactin (Prl) release from anterior pituitaries, arachidonate and its metabolites have been proposed to be involved in the process of Prl release. In this study we investigated the contribution of arachidonate metabolites to both basal and TRH-stimulated Prl release from perifused lactotrophs in culture (derived from pituitary glands of lactating female rats), which exhibit a high sustained release of Prl in absence of inhibitory input. Inhibition of the general oxidative metabolism of arachidonate by 10(-5) M ETYA or of the arachidonate lipoxygenase metabolism by 10(-5) M NDGA decreased basal Prl release to 45 +/- 10% (n = 3) and 36 +/- 4% (n = 6) of the control release, respectively. Indomethacin, an inhibitor of the cyclooxygenase pathway, was without effect. Of the lipoxygenase metabolites tested at 10(-6) M only 15-HPETE and 15-HETE induced Prl release. 15-HETE elicited prolactin release in a concentration dependent manner with a maximal effect at 10(-6) M (10.72 +/- 3 ng/ml vs control 5.1 +/- 0.8 ng/ml, n = 3). The quantity of Prl release induced by TRH was markedly decreased in the presence of NDGA. However, the fraction of Prl release elicited by TRH, calculated as a percentage of the amount of Prl released prior to TRH application, was similar under control conditions, and in the presence of NDGA. In contrast, inhibition of the protein kinases A and G by H8 (10(-5) M) failed to alter basal Prl release but inhibited the effect of TRH by 58 +/- 6% (n = 3). These data suggest that in absence of inhibitory inputs the high sustained release of Prl observed in cultures of lactotrophs derived from lactating female rats depends on the availability of lipoxygenase metabolites, and that the blockade of lipoxygenase reduces the absolute amount of Prl released by TRH without suppressing the ability of TRH to stimulate Prl release.