Perturbation of renal amino acid transport by brush border membrane vesicles in the vitamin D-deficient rat

Abstract

Vitamin D deficiency is characterized by secondary hyperparathyroidism, phosphaturia, bicarbonaturia, and generalized amino aciduria. While the site at which the phosphaturia ensues has been described to occur at the apical membrane of the renal proximal tubule, no studies are available for amino aciduria. Thus, weanling rats were fed five vitamin D-deficient diets for 4-6 weeks: (i) VLC, 0.02% Ca, 0.3% P; (ii) VLC + 1,25[OH]2D, same + 500 pmole ip for 2 days; (iii) LC, 0.45% Ca, 0.3% P; (iv) HC, 2.5% Ca, 0.3% P; and (v) VLP, 1.2% cA, 0.1% P. The normal diet contained 1.2% Ca, 0.7% P, and 2.5 micrograms% vitamin D. Amino acids, serum 25[OH]D, 1,25[OH]2D, and PTH, using a specific anti-rat PTH antibody, were measured. There were 4.65 +/- 1.1- and 10 +/- 1.39-fold increases in the urinary excretion of taurine and proline, respectively, irrespective of diet. Hypocalcemia, secondary hyperparathyroidism, and increased concentrations of urinary cAMP were demonstrated in all diets, except VLP. Taurinuria and prolinuria manifested at the renal brush border membrane. There was 21-25% and 26-39% attenuation in the peak of the overshoot of Na(+)-dependent uptake of taurine and proline, respectively, that was statistically significant as compared to that of normal diets (P less than 0.01). VLC resulted in a reduction in the Vmax of taurine (VLC, 78.26 +/- 6.88 vs normal, 115.4 +/- 6.26 pmole/mg protein/min, P less than 0.01) and proline (VLC, 402.06 +/- 31.26 vs normal, 589.49 +/- 37.42 pmole/mg protein/15 sec, P less than 0.01) uptake. Acute supplementation with pharmacological doses of 1,25[OH]2D normalized the Vmax of taurine and proline uptake, without affecting their renal excretion. The VLP diet induced and increase in the Km of taurine (VLP, 58.95 +/- 1.88 microM vs normal, 39.75 +/- 2.75 microM P less than 0.01) and proline (VLP, 116.75 +/- 8.87 microM vs normal, 76.82 +/- 7.27 microM P less than 0.01) uptake, without an associated perturbation in the Vmax of uptake. We conclude that the amino aciduria of vitamin D deficiency manifests at the apical membrane of the proximal tubule by an attenuation in the Na(+)-dependent uptake of amino acids. This is associated with a reduction in the initial rate of uptake or number of active transporters in the presence of secondary hyperparathyroidism and hypocalcemia, or a decrease in the affinity of the symport in the presence of P depletion. The data suggest the interplay of multiple factors in the causation of amino aciduria.