title: Has vitamin D a direct renal effect on the tubular reabsorption of phosphate? A study in parathyroidectomized (PTX) and non-PTX man.
abstract: 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 microgram 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). A significant inverse correlation was demonstrated between the 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 of 1alpha-OH-D3, while none of the other factors investigated were correlated to the index. This may suggest that the stimulating effect of biologically active vitamin D on the tubular reabsorption of phosphate is mediated via the parallel suppression of PTH, but does not exclude that biologically active vitamin D exerts a direct effect on the human renal tubule. Therefore, the effect of 1alpha-OH-D3 was studied in 5 totally parathyroidectomized patients, in whom concomitant suppression of PTH would not occur. Estimation of TmP/GFR was performed 1) when the patients were vitamin D depleted and hypocalcemic, and 2) after 14-27 days of treatment with 1alpha-OH-D3 to obtain stable normocalcemia. In patients with absent parathyroid function, no increasing effect of 1alpha-OH-D3 on TmP/GFR could be demonstrated. It is therefore concluded 1) that 1alpha-OH-D3 exhibits no antiphosphaturic effect in the absence of PTH and 2) that the previously demonstrated antiphosphaturic effect of 1alpha-OH-D3 in man is mediated via a concomitant suppression of PTH.
mesh_terms: D000042:Absorption; D000328:Adult; D002118:Calcium / Q000097:blood; D005260:Female; D005919:Glomerular Filtration Rate / Q000187:drug effects; D006801:Humans; D006943:Hyperglycemia / Q000378:metabolism; D006980:Hyperthyroidism / Q000378:metabolism; D007684:Kidney Tubules / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D010280:Parathyroid Glands / Q000502:physiology*; D010281:Parathyroid Hormone / Q000097:blood; D010710:Phosphates / Q000378:metabolism*; D014807:Vitamin D / Q000494:pharmacology*
chemical_list: D010281:Parathyroid Hormone; D010710:Phosphates; D014807:Vitamin D; D002118:Calcium