The effects of exogenously administered testosterone on spermatogenesis in intact and hypophysectomized rats

The effects of exogenously administered testosterone on the maintenance of spermatogenesis in intact and hypophysectomized rats were examined. Adult male rats were given Silastic implants containing testosterone in lengths ranging from 0.5-20 cm, and their effects on daily sperm production (DSP); serum FSH, LH, and testosterone; and interstitial fluid testosterone were determined in intact and hypophysectomized rats over a 13-week period. In intact rats, DSP levels were suppressed to 4-30% of control values at lower testosterone doses (2- to 6-cm implants), while DSP levels were partially maintained (65-93%) at higher doses (greater than or equal to 8 cm implants). Under these conditions LH levels were suppressed while FSH levels were reduced to 30-60% of control values. A steep testosterone-induced change in DSP levels was observed in both intact (2- to 3-fold) and hypophysectomized (18-fold) animals over a narrow testosterone dose range (implant lengths, 6-8 cm). Interstitial fluid testosterone levels associated with this change in DSP levels range from 6-8% of control values, while serum testosterone levels were elevated 1- to 2-fold above control values. A comparison of the testosterone implant lengths that caused a 50% change in DSP levels after 7 weeks of treatment was similar in intact and hypophysectomized rats. At high testosterone doses (greater than or equal to 10-cm implants), maximal DSP levels decreased 10% by 7 weeks and 35% by 13 weeks. DSP and serum testosterone levels were highly correlated (r = 0.54-0.83), while DSP and interstitial fluid testosterone showed a less correlation (r = 0.36-0.70) under the various experimental conditions examined. In conclusion, the testosterone-induced maintenance of DSP in rats is associated with several dose-related responses or events that appear to be differentially regulated. At low testosterone concentrations, DSP levels are partially maintained in intact rats but are totally suppressed in hypophysectomised rats, suggesting that a pituitary factor, probably FSH, has a potentiating effect at these testosterone concentrations. At intermediate testosterone doses resulting in the testosterone-induced maintenance of DSP, the similarity of dose-response curves (ED50 and maximum response) in intact and hypophysectomised rats would suggest that pituitary hormones are not required for this aspect of the process. The observation that the maintenance of DSP by testosterone under some experimental conditions is poorly correlated with interstitial fluid testosterone levels raises questions as to the mechanisms by which testosterone acts to stimulate spermatogenesis.