Stimulatory effect of testosterone on renal 25-hydroxyvitamin D-3 1 alpha-hydroxylase in guinea pig.

Castration of male guinea pigs reduced the activity of the renal mitochondrial 25-hydroxyvitamin D-3 1 alpha-hydroxylase by about 50% without affecting the apparent Km for the reaction. Testosterone substitution for 10 days after castration increased the activity to about the same level as that of sham-operated animals. Ovariectomy of female guinea pigs had no effect on the 1 alpha-hydroxylase activity. Administration of testosterone to ovariectomized female guinea pigs increased the activity by about 50%, again without affecting the apparent Km. The different manipulations had no significant effect on circulating levels of 1,25-dihydroxyvitamin D or vitamin D-binding protein (DBP). It is concluded that testosterone may be a regulator of the 25-hydroxyvitamin D-3 1 alpha-hydroxylase in guinea pigs.