1‐Alpha‐hydroxycholecalciferol‐induced Changes in the Renal Handling of Phosphate and the Serum Parathyroid Hormone Level

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Abstract
The effect of 1-.alpha.-hydroxycholecalciferol (1.alpha.-OH-D3) on the renal handling of phosphate and the immunoreactive parathyroid hormone in serum (i-PTH) was studied in 10 patients with a wide range of glomerular filtration rate (GFR), maximal tubular reabsorption of phosphate (TmP) and i-PTH. The patients were treated with 2 .mu.g 1.alpha.-OH-D3/day for approximately 80 days. Before and after this period of treatment, the TmP, i-PTH, 51Cr EDTA clearance, extracellular volume, standard bicarbonate, and serum Ca were measured in each patient. The TmP/GFR ratio was used as an index of the renal handling of phosphate. The index increased significantly (mean 26.5%, P < 0.01) during the treatment, while i-PTH decreased significantly (mean 37.0%, P < 0.01). An inverse significant correlation was demonstrated between TmP/GFR index and i-PTH both before (r = -0.87, P < 0.001) and after (r = 0.79, P < 0.01) the administration of .alpha.-OH-D3, while none of the other factors investigated were correlated to the index. 1.alpha.-OH-D3 probably increases the TmP/GFR index and reduces i-PTH in a parallel manner and it is suggested that the 1.alpha.-OH-D3-induced changes in the renal handling of phosphate may be explained as being mediated solely via the suppression of i-PTH.