Effect of hypophysectomy on the occupied and unoccupied binding sites for 1,25-dihydroxyvitamin D 3 in rat intestine

Abstract

It is not known why the intestinal active transport of calcium per unit of mucosal mass is not affected by hypophysectomy (HX) even though serum 1,25-dihydroxyvitamin D 3 [1,25(OH) 2D 3 and intestinal calcium-binding protein are decreased. In order to study the effect of HX on the quantity of intestinal receptor of 1,25(OH) 2D 3 and its binding characteristics, the intestinal total occupied and unoccupied binding sites for 1,25(OH) 2D 3 were measured, by the use of the mercurial reagent mersalyl, in the intestine of HX and age-matched control rats. In addition, the effect of bovine growth hormone (bGH) replacement on the quantity of both binding states was examined in the HX rats. Results of Scatchard analysis and sucrose density gradients showed that the 3.5S receptor of the HX rat intestine was not distinguishable from that seen in the intact rat intestinal cytosol. Under vitamin D-supplemented conditions, HX was shown to reduce the levels of occupied receptors when the data were expressed on the basis of cytosolic protein. The reduced occupied sites could, in fact, have resulted from the reduction in plasma 1,25(OH) 2D 3 levels. No synthesis rates were determined. The unoccupied and total binding sites for 1,25(OH) 2D 3 per length of intestine were lower in the HX group than in the intact group. Administration of bGH resulted in an increase of endogenously occupied binding sites without affecting the total binding activity. Under vitamin D-depleted (-D) conditions, the total binding activity (intestinal) for 1,25(OH) 2D 3 was increased in the intact but not in the -DHX rats. Administration of bGH to the -DHX rat resulted in no effect on the binding levels of 1,25(OH) 2D 3 receptor. The current study demonstrates that HX does not affect the binding affinity and the concentration of intestinal total binding receptor 1,25(OH) 2D 3. The decrease in endogenously occupied binding sites for 1,25(OH) 2D 3 without an affect on the intestinal active transport of calcium could be interpreted to mean that HX affects other factors controlling calcium absorption than 1,25(OH) 2D 3.