Unimpaired postreceptor regulation of luteinizing hormone secretion by gonadotropin-releasing hormone and estrogen in aged rat anterior pituitary cells.

Abstract

Delayed, attenuated, or absence of the proestrous LH surge occurs in aging rats. To assess how aging affects the positive feedback action of 17 beta-estradiol (E2) on the pituitary, we determined the responsiveness of rat pituitary cells to GnRH and the secretagogues affecting intracellular signal transduction mechanisms in the presence or absence of E2. We also correlated the LH response to pituitary LH content. Anterior pituitaries excised from ovariectomized Sprague-Dawley rats, either young (3-4 months) or old (19-20 months), were enzymatically dispersed and then pretreated with or without E2 (0.6 nM) for 48 h, followed by incubation for 3 h with or without various secretagogues. The secretagogues included GnRH (1 and 10 nM), veratridine (increases Ca2+ influx; 5 and 10 microM), and phorbol 12-myristate 13-acetate (a protein kinase-C activator; 10 and 100 nM). LH in media and cells were measured by RIA and expressed on the basis of cellular DNA. GnRH, veratridine, and phorbol 12-myristate 13-acetate at all doses stimulated (P < 0.01) LH release in cells from both young and old rats. E2 stimulated (P < 0.05 to P < 0.01) all secretagogue-induced LH release in cells from both young and old rats, but only basal LH release (P < 0.05) in cells from young rats. The magnitude of both basal and secretagogue-induced LH release in either the presence or absence of E2 was smaller (P < 0.01) in cells from old than in those from young rats. The initial cellular LH was lower (P < 0.01) in cells from old than in those from young rats. The LH-releasing ability (expressed as a percentage of total cellular LH) of cells from old rats was identical (P > 0.05) to that of cells from young rats under all conditions studied. These results suggest that the reduced magnitude of LH release by cells from old rats may be attributed to reduced cellular LH, rather than to impaired estrogen feedback or impaired signal transduction mechanisms. It remains to be determined whether LH biosynthesis per cell and/or the number of gonadotropes decrease with age.