Age-related changes in reproductive hormones and in Leydig cell responsivity in the male Fischer 344 rat.

Age-related changes in testicular responsiveness to pituitary hormone stimulation were measured in Fischer rats. Suspensions of dispersed Leydig cells were prepared from the testes of groups which were 5, 12 and 29 months old. The suspended cells were challenged with 250, 750 and 2000ng Monkey-LH (LER 1909-2) for two hours and the medium subsequently assayed for testosterone. Concentrations of the hormone were determined per million cells in suspension. Besides measurement of testosterone production by each age group, pituitary and plasma LH, FSH and prolactin, and plasma testosterone were also assayed by radioimmuno-technique. Total testis weight increased in the oldest animals as the result of a high incidence of gonadal pathology. If tumorous tissue was excluded, however, testis weight fell for this group. Also, significant decreases in Leydig cell number were apparent in 12 and 29-month-old rats. Basal testosterone production, on a per million cell basis, however, was not significantly differnet in young and middle-aged rats, nor was the change in the amount of steroid produced by LH stimulation of the isolated Leydig cells. Seminal vesicle weight, prostate weight and serum testosterone decreased with age. Similarly, pituitary LH and FSH concentration decreased, but pituitary prolactin increased. Histological changes and pituitary hypertrophy reflected this increase in prolactin concentration. It is proposed that decreased testicular function in old age may not totally be the result of a decrement in Leydig cell steroid producing capacity, but that it may also occur due to Leydig cell loss, possibly as the result of age associated changes in the brain and pituitary.