EFFECT OF VITAMIN-D DEPLETION ON CALCIUM-TRANSPORT BY THE LUMINAL AND BASOLATERAL MEMBRANES OF THE PROXIMAL AND DISTAL NEPHRONS

To study the effect of vitamin D on calcium (Ca2+) reabsorption by the kidney, we measured Ca2+ uptake by the basolateral and luminal membranes of proximal and distal tubules obtained from rabbits fed a vitamin D-deficient diet for 3 weeks. Results were compared to those obtained with a group of control animals fed a normal diet. Serum Ca2+ concentrations were comparable in both groups. In the control group, serum PTH, 25-hydroxyvitamin D3, and 1,25-dihydroxyvitamin D3 remained relatively stable. In the vitamin D-deficient animals, serum PTH levels slightly, but not significantly, increased, and the levels of vitamin D metabolites abruptly fell. Vitamin D depletion produced a 40% decrease in ATP-dependent Ca2+ uptake by the basolateral membrane of the distal tubule. There was no change in the activity of the Na+/Ca2+ exchanger. A very significant effect was also observed in the luminal membrane of the distal tubule, where a 50% decrease in Ca2+ uptake was observed after the third week of vitamin depletion. Administration of 0.1 mug 1,25-dihydroxyvitamin D3 16 and 2 h before death partially reestablished normal uptake. In contrast, no change in Ca2+ uptake could be detected in the basolateral or luminal membranes of the proximal tubule. These observations provide the first evidence of an effect of vitamin D on Ca2+ transport at both the basolateral and luminal membranes of the distal segment of the nephron.