CALCIUM AND PHOSPHORUS METABOLISM IN OSTEOMALACIA X. FURTHER STUDIES ON VITAMIN D ACTION: EARLY SIGNS OF DEPLETION AND EFFECT OF MINIMAL DOSES

Moderator's view: Vitamin D deficiency treatment in advanced chronic kidney disease: a close look at the emperor's clothes

Two recent vitamin D supplementation (ergocalciferol) trials in stage G5D CKD patients with vitamin D insufficiency showed that this sterol effectively increases serum 25-hydroxyvitamin D [25(OH)D] but fails to modify serum PTH and other clinical outcomes. The Pro side of this polar view emphasizes that the duration of these studies was too short to allow sensible analyses based on a clinical endpoint. Furthermore, he notes that in the second study, the use of active forms of vitamin D, phosphate binders and cinacalcet could have hindered appreciation of the effect of ergocalciferol supplementation per se The Con side produces an updated meta-analysis showing that inactive vitamin D forms largely fail to reduce serum PTH and affect various relevant endpoints, including muscle strength, functional capacity, quality of life and hospitalization. Studies suggesting an effect of inactive vitamin D forms in advanced CKD are either very small and mainly based on sequential, uncontrolled observations or inherently weak, simple pre/post studies. No biological or clinical evidence exists that 25(OH)D may exert meaningful effects in CKD patients who are being treated with active forms of vitamin D. Careful a etiologic studies based on the omics sciences, i.e. precise pathophysiological profiling of individual CKD patients followed by consequential, well-targeted intervention(s) in the precision medicine scenario, will likely provide a definitive answer to the lingering question of whether inactive vitamin D forms may have biological effects beyond those produced by their proximate metabolite 1,25-dihydroxyvitamin D3.

Effect of metabolic acidosis and alkalosis on the renal response to parathyroid hormone infusion in normal man

Six normal volunteers were observed for 24 days on a constant metabolic diet in a study designed to test the hypothesis that the action of parathyroid hormone may be altered by renal acidosis.

The acute effect of 25-hydroxycholecalciferol on renal handling of phosphorus. Evidence for a parathyroid hormone-dependent mechanism

The acute effect of i.v. and direct intrarenal arterial infusion of 25-hydroxycholecalciferol (25HCC) and 1,25-dihydroxycholecalciferol (1,25-DHCC) on renal handling of phosphorus was evaluated in the following groups of rats: (a) intact animals, (b) parathyroidectomized (PTX) hypocalcemic rats, (c) PTX rats in which normocalcemia was maintained with calcium supplements and (d) PTX animals in which urinary phosphorus was augmented by (i) i.v. sodium phosphate, (ii) expansion of the extracellular fluid volume with normal saline, and (iii) i.v. parathyroid hormone (PTH). Clearances of inulin (C(In)), phosphorus (C(P)), and fractional clearances of phosphorus (C(P)/C(In)) of the experimental groups were compared with those of the corresponding control groups, and the clearances of the infused kidneys with those of the contralateral kidneys. In intact animals, i.v. 25HCC decreased C(P)/C(In) from 0.29+/-0.04 (mean +/-SE) to 0.19+/-0.04, and i.v. 1,25-DHCC decreased C(P)/C(In) from 0.25+/-0.04 to 0.15+/-0.02. The intrarenal infusion of both 25HCC and 1,25DHCC into intact animals failed to produce a unilateral change; however, it decreased C(P)/C(In) bilaterally. i.v. and intrarenal infusions of 25HCC or 1,25DHCC in PTX hypocalcemic and normocalcemic rats, and i.v. infusions of 25HCC in PTX rats receiving either sodium phosphate or normal saline, all failed to produce significant changes in C(P)/C(In). In contrast, 24HCC given i.v. to PTX animals receiving exogenous PTH was associated with a significant fall in C(P)/C(In), from 0.34+/-0.08 to 0.13+/-0.02. These results indicate that 25HCC enhances tubular reabsorption of phosphorus in rats, only in the presence of either endogenous or exogenous circulating PTH, but not in its absence and thus imply a PTH-dependent mechanism of 25HCC action on the kidney. This effect does not appear to be related to the conversion of 25HCC into 1,25DHCC, since the latter fails to affect tubular reabsorption of phosphorus in PTX rats.

Evidence for a direct action of cholecalciferol and 25-hydroxycholecalciferol on the renal transport of phosphate, sodium, and calcium

Acute clearance studies were performed in stable thyroparathyroidectomized dogs to evaluate the possibility of a direct renal action of vitamin D and its biologically active 25-hydroxylated metabolite. Alterations in renal hemodynamics and serum calcium concentration were minimized and attempts at vitamin D depletion were not undertaken. Steady-state volume expansion of modest degree was employed as the control experimental situation so that an effect of the vitamin to enhance phosphate reabsorption would not go undetected because of an already maximal phosphate reabsorptive rate secondary to parathyroidectomy. Under these experimental circumstances, 10,000 U of cholecalciferol and 25-120 U of 25-hydroxycholecalciferol (25-HCC) produced significant depressions in the percentage of filtered phosphate excreted (mean declines of 39 and 47%, respectively), which were not attributable to alterations in renal hemodynamics or to changes in the levels of serum calcium or phosphate. There was an accompanying decline in sodium and calcium excretion; mean percentage excretion rates for sodium fell by 38% with vitamin D and 26% with 25HCC, and for calcium this measurement declined by 46 and 23%, respectively. Furthermore, parathyroid hormone and 25HCC produced antagonistic effects on phosphate excretion. These observations provide the first conclusive evidence for a direct (proximal) tubular action of vitamin D to promote phosphate (as well as sodium and calcium) transport.

Isocyanates in industry: environmental control

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