EFFECTS OF GONADAL-STEROIDS ON TONIC LUTEINIZING-HORMONE (LH) RELEASE AND LUTEINIZING-HORMONE-RELEASING HORMONE-INDUCED LH-RELEASE FROM BOVINE PITUITARY-CELLS CULTURED IN-VITRO

The objective of this study was to investigate the direct effect of estradiol-17 beta (E(2)) and progesterone (P-4) on LH release by pituitary cells of midluteal phase cows in vitro. Pituitaries were collected at the slaughterhouse; cells were dissociated with collagenase and hyaluronidase and maintained in a static culture system in Medium 199 (M199). Various concentrations of E(2) (0.1-100 nM) and P-4 (0.1 and 10 nM) were used to stimulate the cells for 2, 4, 6, 15, 24, 48, or 72 h. In addition, the synergistic action of E(2) and P-4 was investigated by exposure of the cells to a combination of the two hormones. At the end of each incubation, the cells were challenged with LHRH (1 nM) for 2 h. The medium was collected for LH analysis at the end of each incubation period and after the LHRH challenge; furthermore, intracellular LH content was quantified at the end of each experiment. The results indicate a positive action of E(2) on basal release of LH beginning after 15 h of exposure (p < 0.01). LHRH-induced LH release was modulated by E(2) in a time-dependent manner with an effect at first inhibitory, then stimulatory, and finally inhibitory again (p < 0.04). P-4 alone did not affect LH release, but it negatively influenced LHRH-induced LH release. P-4 also exerted a positive action on intracellular LH after 6 h of incubation. A substantial inhibitory effect (p < 0.001) on both tonic LH release and LHRH-induced LH release was observed in cells exposed for 16 h to P-4 after a priming with E(2) for 4 h. These data clearly demonstrate that both ovarian steroids are effective in influencing LH secretion by acting directly at the pituitary level.