Serum testosterone and testicular response to HCG in young and aged male rats

Serum testosterone and testicular response to HCG was measured in young (3 - 6 mo) and aged (20 - 30 mo) male Long-Evans rats. Serum testosterone was measured by radioimmunoassays on serial blood samples taken before and after injections of HCG. Control group serum testosterone concentrations were lower in aged compared to young groups. Intravenous injection of 1, 5, and 20 IU of HCG increased serum testosterone in both age groups and testosterone remained elevated throughout the 150-min sampling interval. The increase in serum testosterone concentrations following acute HCG stimulation was greater in young than in aged rats. Serum testosterone concentrations and testicular response to intravenous HCG injections were increased to similar levels in young and aged male rats following 7 days of daily subcutaneous injections of 5 IU of HCG/100 gm bw. These data suggest that while testicular secretion and responsiveness is reduced with age, the aged male rat retains the capacity to secrete more testosterone than it normally maintains.