Synergistic effect of progesterone, testosterone, and estradiol in the stimulation of chick renal 25-hydroxyvitamin D3-1alpha-hydroxylase.

Castrate male chickens have low levels of renal 25-hydroxyvitamin D3-1 alpha-hydroxylase (13 pmol 1,25-dihydroxyvitamin D3/200 mg tissue . 10 min). This activity is markedly increased by the administration of 5 mg/kg estradiol and 5-20 mg/kg testosterone 24 h before in vitro measurement of the enzyme. Both hormones are required for this stimulation. The testosterone, but not estradiol, requirement for the stimulation can also be satisfied by progesterone. Progesterone (12.5 mg/kg) given in addition to estradiol and testosterone stimulates even further the 25-hydroxyvitamin D3-1 alpha-hydroxylase activity to very high levels (388 pmol 1,25-dihydroxyvitamin D3/200 MG tissue . 10 min). This is accompanied by a 3-fold increase in plasma 1,25-dihydroxyvitamin D3 (from 55 to 170 pg/ml), which provides in vivo evidence for the stimulation of the 1 alpha-hydroxylase by sex hormones in birds.