Sex differences following gonadectomy in basal gonadotropin secretion rate of rat pituitary fragments in vitro

Sex differences in the acute response of circulating luteinizing hormone (LH) and follicle-stimulating hormone (FSH) to withdrawal from gonadal negative feedback in the rat are well established. To investigate postgonadectomy changes at the anterior pituitary level that may underlie dramatic in vivo sex differences, we used a computer-controlled pituitary perifusion system to measure in vitro basal secretion rates (BSRs) of LH and FSH following gonadectomy in the absence of exogenous gonadotropin-releasing hormone (GnRH). We compared BSRs of pituitaries removed from intact rats and from males and females 2 and 6 days post-gonadectomy. Glands were cut into quarters, placed into individual chambers, and perifused in Medium 199 at 10 ml/h for 4 h. In females (n = 12/gp), BSR of LH was not significantly elevated above intact levels by 2 days but had tripled by 6 days post-ovariectomy, while BSR of FSH had already doubled by 2 days and doubled again by 6 days. These changes in BSR in females paralleled changes in serum levels of both hormones. In males (n = 14/gp), although serum LH and FSH had increased 7-fold by 2 days post-orchidectomy, BSRs of LH and FSH had decreased to 75% and 64% of intact levels, respectively, by 6 days. These findings suggest important sex differences at the pituitary level in the responses to withdrawal from gonadal feedback that persist in culture in the absence of direct hypothalamic (GnRH) input.