1-Alpha-hydroxycholecalciferol-induced changes in the renal heandling of phosphate and the serum parathyroid hormone level.

The effect of 1-alpha-hydroxycholecalciferol (1alpha-OH-D3) on the renal handling of phosphate and the immunoreactive parathyroid hormone in serum (i-PTH) has been 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 mug 1alpha-OH-D3 per 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 calcium 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 less than 0.01) during the treatment, while i-PTH decreased significantly (mean 37.0%. p less than 0.01). An inverse significant correlation was demonstrated between TmP/GFR index and i-PTH both before (r = -0.87, p less than 0.001) and after (r = -0.79, p less than 0.01) the administration o alpha-OH-D3, while none of the other factors investigated were correlated to the index. It is concluded that 1alpha-OH-D3 increases the TmP/GFR index and reduces i-PTH in a parallel manner and it is therefore suggested that the 1alpha-OH-D3-induced changes in the renal handling of phosphate may be explained as being mediated solely via the suppression of i-PTH.