Anterior pituitary luteinizing hormone secretion during continuous perifusion in aging male rats.

The possibility that the gonadotrope population of the anterior pituitary experiences an age-related alteration in function was investigated by in vivo and in vitro methods in young (4- to 6-month) and old (18- to 20-month) male Sprague-Dawley rats. Plasma concentrations of testosterone were 50% lower in the old rats, but resting concentrations of luteinizing hormone (LH) were similar in the two age groups. After leg-restraint and blood-withdrawal stress, plasma LH levels were significantly elevated in both young and old males; however, LH levels achieved by aged males were 39% less than those achieved by young males. During perifusion of anterior pituitary, release of LH (ng/ml per 10 min) was stable for 7 h; old anterior pituitary released only 52% as much LH as young anterior pituitaries. The anterior pituitary LH content after perifusion was not altered with age. Castration 2 weeks prior to perifusion caused elevations in plasma LH and in LH released from anterior pituitary during perifusion that were similar in the two age groups. Implantation of testosterone-filled Silastic capsules 2 weeks prior to perifusion elevated plasma testosterone and reduced plasma LH levels in both age groups. The in vitro release of LH from anterior pituitaries was similarly reduced in both age groups. Administration of varying doses of LH-releasing hormone (LHRH) during perifusion caused similar releases of LH above baseline levels in young and old rats. These in vitro results show that aged male rat anterior pituitaries release less LH than anterior pituitaries from young males. However, the magnitude of the LH response of anterior pituitaries to LHRH is not reduced with aging. These findings suggest that the decline in androgen status in old rats is not attributable to a deficit in pituitary responsiveness to LHRH. The effects of manipulating testosterone levels failed to implicate a change in anterior pituitary sensitivity to feedback as a cause for the hormonal status of aged male rats.