Sites of action of testosterone and estradiol on longitudinal bone growth

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Abstract
The mechanisms by which sex steroids influence body growth were investigated. The effect of different doses of testosterone propionate on longitudinal bone growth and body weight gain was studied in gonadectomized male rats, gonadohypophysectomized male rats and gonadohypophysectomized male rats given replacement therapy with bovine growth hormone (bGH). The effect of different doses of estradiol benzoate on the same growth parameters was studied in female rats divided into the same experimental group as the males. Accumulated longitudinal bone growth was determined using oxytetracycline as an intravital marker. Testosterone caused a dose-dependent increase in longitudinal bone growth in gonadectomized male rats. In contrast, testosterone exerted no significant increase in longitudinal bone growth in gonadohypophysectomized male rats with and without bGH replacement therapy. Treatment with estrogen inhibited longitudinal bone growth and body weight gain. The inhibitory effect of estradiol was approximately the same in gonadohypophysectomized female rats given bGH replacement therapy as in gonadectomized female rats. Apparently, testosterone exerts its stimulatory effect on body growth mainly modulating hypothalamopituitary functions, e.g., by altering the secretory pattern of GH. It seems that changes in the hypothalamopituitary functions are less significant for the inhibitory effect of estradiol on body growth. The sites of action for estrogen and testosterone in modulating body growth in the rat are different.