Temporal patterns of serum luteinizing hormone and testosterone and endocrine response to luteinizing hormone releasing hormone in aging male rats.

Temporal patterns of serum testosterone and luteinizing hormone (LH) concentration and responsiveness to luteinizing hormone releasing hormone (LHRH) were measured in young (2 to 3 months), middle-aged (12 to 13 months), and aged (22 to 26 months) male rats. Average testosterone and LH concentrations, as well as individual sample variations, were higher in young than in middle-aged and aged rats and higher in middle-aged than in aged groups. In addition, the young group showed a diurnal pattern of serum testosterone concentration and greater ranges of LH and testosterone concentrations in individual blood samples. Young male rats showed greater increases in serum LH and testosterone than middle-aged and aged groups after intravenous LHRH, 1 and 5 ng/g of body weight. These data indicate that LH and testosterone secretion are progressively reduced in the aged male rat and suggest alterations in neuroendocrine control of pituitary gonadotropin secretion as the most important alteration in the gonadal control system in these animals.