The mechanism of the action of growth hormone on vitamin D metabolism in the rat.

GH has been shown to stimulate intestinal calcium absorption in rats and humans. We have investigated in rats whether GH might affect intestinal calcium absorption by stimulating the production of 1,25-dihydroxyvitamin D3 [1,25-(OH2D3], the active metabolite of vitamin D3. The tissue distribution of [3H]1,25-(OH2D3 8-40 h after iv injection of [3H]25-hydroxyvitamin D3 ([3H]25OHD3) was measured in sham controls, hypophysectomized, and GH-treated hypophysectomized rats. Since the plasma disappearance rate of iv [3H]1,25-(OH)2D3 was not significantly affected by hypophysectomy, the recovery of [3H]1,25-(OH)2D3 after [3H]25OHD3 administration was taken to be an indirect measure of renal 25-OHD3-1-hydroxylase. Hypophysectomy was found to reduce the recovery of [3H]1,25-(OH)2D3 from serum and intestinal mucosa by 70 +/- 2% (range). A 6-day course of GH treatment of hypophysectomized rats restored the formation of 1,25-(OH)2D3 to normal, and a significant effect was noted within 2 days, before any increase in renal weight was detectable. No other pituitary hormones appeared to be necessary. The markedly atrophic intestinal mucosa of hypophysectomized rats incorporated iv [3H]1,25-(OH)2D3 normally. However, it remains to be determined whether 1,25-(OH)2D3 alone can correct the decreased calcium transport in hypophysectomized rats.