Stimulation of (3H)uridine incorporation into nuclear RNA of rat kidney by vitamin D metabolites

Cation-dependent vitamin D activation of human renal cortical guanylate cyclase

The objective of this investigation was to determine whether physiological levels of vitamin D and its metabolites have part of their mechanisms of action through stimulation of guanylate cyclase (EC 4.6.1.2). These sterols enhanced both soluble and particulate guanylate cyclase activities as well as cGMP levels two- to threefold in human and rat tissues. At a concentration of 1 nM, 1,25(OH)2D3 greater than 25(OH)D3 greater than vitamin D3 greater than 24,25(OH)2D3 = 25,26(OH)2D3 = vitamin D2. Dose-response curves revealed that maximal stimulation of guanylate cyclase by these sterols was at 1 nM and that there was no augmented guanylate cyclase activity at 0.01 nM. The precursors of vitamin D, cholesterol and 7-dehydrocholesterol, had no effect on guanylate cyclase activity. The activation of guanylate cyclase activity by the vitamin D sterols required the presence of manganese ion. Calcium was not as efficient as manganese in optimizing basal or hormone-stimulated guanylate cyclase activity. Vitamin D and its metabolites failed to stimulate adenylate cyclase (EC 4.6.1.1) activity. The data in this investigation suggest that guanylate cyclase may play a role in the mechanism of action of vitamin D at the cellular level.

In vitro stimulation of phosphate uptake in isolated chick renal cells by 1,25-dihydroxycholecalciferol

Renal cells isolated from vitamin D-deficient chicks had an increased Na+-dependent phosphate uptake when preincubated with 1,25-dihydroxycholecalciferol [1,25-(OH)2D3]. Phosphate uptake in the absence of Na+ and methyl alpha-glucoside uptake dependent on Na+ were not affected. Phosphate uptake was stimulated 15% by 0.010 pM 1,25-(OH)2D3. Maximal enhancement of 30% was obtained with 100 pM. The uptake when fully stimulated by preincubation in vitro approximated the uptake of cells isolated from chicks that were previously repleted with 1,25-(OH)2D3 in vivo. Cells from repleted chicks were not stimulated additionally when preincubated with 1,25-(OH)2D3 in vitro. The increase in phosphate uptake could be measured after a 1-hr preincubation period; full response required at least 2 hr. Phosphate uptake induced by 1,25-(OH)2D3 was blocked by cycloheximide and actinomycin D. Enhancement of phosphate uptake was relatively specific for the 1,25-(OH)2D3 analog of vitamin D3. The potency order was 1,25-(OH)2D3 greater than 25-(OH)D3 = 1-(OH)D3 greater than 24,25-(OH)2D3 greater than D3. Kinetically, 1,25-(OH)2D3 increased the Vmax of the phosphate uptake system; the affinity for phosphate was unaffected. 3H-Labeled 1,25-(OH)2D3 was taken up by the isolated renal cells. It was estimated that the stimulation of phosphate uptake might be initiated by very few molecules of 1,25-(OH)2D3 per cell. It is proposed that 1,25-(OH)2D3 contributes importantly to the mechanisms by which phosphate transport is regulated in the kidney.

Aminoaciduria--its relationship to vitamin D and parathyroid hormone

Early reports of patients with metabolic bone diseases such as nutritional osteomalacia, Fanconi syndrome, indicated an association with aminoaciduria. This association has since been described in osteomalacia of G. I. or hepatic origin, secondary to anticonvulsant therapy, tumors, and chronic renal failure. Aminoaciduria also occurs in primary hyperparathyroidism. In nutritional osteomalacia, vitamin D deficiency was thought to be responsible for the renal tubular abnormality, since it responded to treatment with vitamin D. However, since the description of aminoaciduria in hyperparathyroidism, the literature has been divided concerning the etiology of aminoacidura in conditions associated with abnormal vitamin D metabolism because secondary hyperparathyroidism often occurs in these conditions. Recently, some cases of Fanconi syndrome and a case of tumor-associated osteomalacia have been described with low or absent plasma 1,25-dihydroxycholecalciferol levels, normal serum PTH, and aminoaciduria. In one of these cases, and more recently in patients with chronic renal failure, it has been demonstrated that treatment with 1,25(OH)2D3 can improve amino acid transport independently from changes in serum PTH levels. 1,25(OH)2D3 therefore normally opposes the aminoaciduric effect of PTH. This is an agreement with observations which demonstrate that 1,25(OH)2D3 also opposes the phosphaturic action of parathyroid hormone.