Effect of testosterone on the rate of myofibrillar protein breakdown in castrated and adrenalectomized male rats measured by the urinary excretion of 3-methylhistidine

Urinary output in N^τ-methylhistidine (3-MeHis) was used to evaluate changes in myofibrillar protein breakdown rate in the skeletal muscles of castrated and adrenalectomized-castrated young male rats, and the responses of breakdown to testosterone and corticosterone replacement. The castrated rats grew less well than intact control rats and excreted significantly less 3-MeHis. Administration of low and high doses of testosterone restored the weight of the levator ani of the castrated animals but failed to affect growth rate or 3-MeHis output. Although this might suggest that testosterone has an indirect effect on muscle turnover through changes in growth rate, this parallelism was not confirmed by studies on adrenalectomized-castrated rats. Adrenalectomy of castrated rats caused a further reduction in growth rate and in output of 3-MeHis, but administration of testosterone now reduced 3-MeHis output still further while stimulating growth. Treatment of the adrenalectomized-castrated rats with low doses of corticosterone increased growth modestly, while depressing 3-MeHis output, and again output of this metabolite was further reduced when testosterone was added to the treatment, although growth rate was unaffected. Finally, large doses of testosterone given to adrenalectomized-castrated rats receiving excessive amounts of corticosterone did not prevent the growth failure or the increased output of 3-MeHis due to the corticosteroid, though it appeared to reduce the magnitude of the response to corticosterone alone. It is concluded that the effects of testosterone on growth and on 3-MeHis output are not necessarily associated and that the functional status of the adrenal cortex modifies the response of myofibrillar breakdown rate to testosterone treatment.