THE ROLE OF DIETARY CALCIUM IN THE PHYSIOLOGY OF VITAMIN-D TOXICITY -EXCESS DIETARY VITAMIN-D-3 BLUNTS PARATHYROID-HORMONE INDUCTION OF KIDNEY 1-HYDROXYLASE

We studied the effects of dietary calcium (Ca) restriction and excess vitamin D-3 On tissue 25-hydroxyvitamin D-1-hydroxylase (1-OHase) and 1,25(OH)(2)D/25-OH-D-24-hydroxylase (24-OHase) activities in rats. Eff ects were studied in four groups of rats, with each group receiving on e of the following diets: a control diet consisting of normal Ca and n ormal vitamin D-3 (NC), NC plus excess (75,000 IU/week) vitamin D-3 (N CT), low Ca and normal vitamin D-3 (LC), Or LC diet with excess vitami n D-3 (LCT), Rats fed the low-Ca diets (LC and LCT) had elevated plasm a parathyroid hormone (PTH) concentrations, increasing > 3-fold relati ve to rats fed the normal Ca diets. The elevated concentrations of PTH in LCT rats did not result in increased plasma 1,25-dihydroxycholecal ciferol [1,25(OH)(2)D-3] (NC = 115 +/- 7 pg/ml; LCT = 99 +/- 11 pg/ml) . Plasma 1,25(OH)(2)D in LC rats, however, was increased significantly (615 +/- 110, P = < 0.001), There were no differences in either plasm a Ca or phosphorus between the LC and LCT groups, Dietary Ca restricti on led to an 18-fold stimulation in renal 1-OHase activity in LC rats (P = < 0.01), while 1-OHase in the LCT rats was marginally but signifi cantly elevated 2.3-fold (P = < 0.05), The ability of PTH to downregul ate renal 24-OHase and the 1,25-dihydroxyvitamin D receptor (VDR) duri ng prolonged Ca restriction remained intact, irrespective of vitamin D status. Also, the metabolic clearance rate for 1,25(OH)(2)D-3 was enh anced by feeding excess vitamin Dst which was likely a result of the s ubstantial elevations in intestinal (25-fold) and renal (46-fold) 24-O Hase activities in the LCT and NCT groups, respectively. These data in dicate that calcium restriction accompanied by excess vitamin D-3 is a ttended by impaired responsiveness of renal 1-OHase to PTH and enhance d metabolic clearance of 1,25(OH)(2)D.