Effect of hypophysectomy and 1,25-dihydroxyvitamin D on duodenal calcium absorption

Intestinal active and passive transport of calcium were studied in hypophysectomized (HX) and intact rats using the in vivo duodenal loop technique. In the vitamin D-supplemented condition, hypophysectomy resulted in a decrease in serum 1,25-dihydroxyvitamin D [1,25-(OH)2D]. Hypophysectomy prevented a gain in body weight and decreased intestinal mucosal weight and total calcium absorption. When the data were expressed per unit mucosal wet weight, duodenal active calcium transport was not different in the HX and intact groups, but passive transport was persistently decreased by hypophysectomy. Administration of bovine GH to the HX rats did not change the mucosal mass, but enhanced both active and passive duodenal transport to calcium. Vitamin D depletion for 6 weeks decreased serum 25-hydroxy-vitamin D and 1,25-(OH)2D levels in both intact and HX rats to about the same level. After bovine GH and 1,25-(OH)2D3 replacement, the calcium absorption studies suggest that 1) 1,25-(OH)2D3 enhances intestinal calcium passive transport as well as active transport in intact and HX rats; 2) GH enhances both active and passive transport of calcium in the presence of sufficient quantities of 1,25-(OH)2D; 3) this latter effect is independent of the metabolism of vitamin D; and 4) a decrease in mucosal mass is one of the factors that results in decreased calcium absorption after hypophysectomy.