Alterations in daily rhythms of testosterone and progesterone in old male rats.

Studies were undertaken to characterize the daily patterns of secretion of testosterone (T), progesterone (P) and dihydrotestosterone (DHT) in young (3-4 months old) and old (19-20 months old) male Sprague-Dawley rats. In young rats, plasma T concentrations show dramatic daily variations with peak and nadir levels at 1130 and 2330 h, respectively. In old male rats, no such rhythmic pattern in plasma T was observed. Plasma P concentrations varied significantly in both young and old male rats, with peak levels occurring 8 h (1930 h) after the plasma T peak in young rats. Additionally, in old male rats, the daily increase in plasma P was significantly larger and was sustained for a longer period than in young rats. Plasma DHT concentrations were similar and changed little during the day in both young and old animals. In view of the known P influence on gonadal secretion, the possible significance of these age-related changes in daily patterns in plasma P and T are discussed.