The Acute Effect of 25-Hydroxycholecalciferol on Renal Handling of Phosphorus

Abstract

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.