Effects of crude and highly purified bovine inhibin (Mr 32 000 form) on gonadotrophin production by ovine pituitary cells in vitro: inhibin enhances gonadotrophin-releasing hormone-induced release of LH

Abstract

Primary cultures of ovine pituitary cells (from adult ewes) were used to investigate the actions of steroid-free bovine follicular fluid (bFF) and highly-purified Mr 32,000 bovine inhibin on basal and gonadotrophin-releasing hormone (GnRH)-induced release of FSH and LH. Residual cellular contents of each hormone were also determined allowing total gonadotrophin content/well to be calculated. As in rats, both crude and highly purified inhibin preparations promoted a dose (P less than 0.001)- and time (P less than 0.001)-dependent suppression of basal and GnRH-induced release of FSH as well as an inhibition of FSH synthesis, reflected by a fall in total FSH content/well. However, while neither inhibin preparation affected basal release of LH or total LH content/well, GnRH-induced LH release was significantly (P less than 0.001) increased by the presence of either bFF (+75%) or highly-purified inhibin (+64%) in a dose- and time-dependent manner. This unexpected action of bFF on GnRH-induced LH release was abolished in the presence of 5 microliters specific anti-inhibin serum, confirming that the response was indeed mediated by inhibin. Furthermore, neither oestradiol-17 beta (1 pmol/1-10 nmol/l) nor monomeric alpha-subunit of bovine inhibin (2.5-40 ng/ml) significantly affected basal or GnRH-induced release of LH. These in-vitro findings for the ewe lend support to a number of recent in-vivo observations and indicate that, in addition to its well-documented suppressive effect on the synthesis and secretion of FSH, inhibin may actually facilitate LH release in this species, in marked contrast to its action in the rat.