Growth and bone haemodynamic responses to castration in male rats. Reversibility by testosterone.

Orchidectomy in postpubertal 55 day old rats, compared to sham-operated controls, led beyond 2 months to a decrease in body weight (87% of controls by 120 d), tibial length (97% of controls) and in tibial calcium content (85% of controls). Bone plasma flow increased three times to reach a peak at 31 days; it was decreased but no significantly at 86 and 120 days. The number of osteoclasts was maximal at 51 days (X 2.3) and was still elevated at 120 days. The calcium accretion rate increased briefly at 31 days (110% of controls) and was diminished at 86 and 120 days (78% of controls). The initial 'physiological' changes in the tibia occurred before any weight change and might be directly due to the lack of androgens. They can be interpreted as inducing the conditions for enhanced bone resorption. Testosterone replacement therapy, initiated after the initial haemodynamic response, inhibited the negative effect of castration on bone growth.