ExtractIn order to determine the effect of vitamin D on tubular reabsorption of phosphate, micropuncture studies were performed in the proximal convolution of normal rats, of parathyroidectomized rats, and of rats with rickets. The studies showed that vitamin D exerted a direct effect on transtubular reabsorption of phosphate that was independent of parathyroid function. The glomerular filtration rate was not affected by vitamin D. The reabsorption of phosphate of normal rats was 13.51 yumoles/min per kg. This corresponds to a fractional reabsorption of 73%. With vitamin D the fractional reabsorption increased to 86%. Despite continuous infusion of vitamin D, the phosphate reabsorption decreased to 13.95 /^moles/min/kg (fractional reabsorption 74%) after 180-240 min. Parathyroidectomized rats also showed a decreased transtubular reabsorption of phosphate during vitamin D infusion. In animals under a phosphate load who suffered from rickets, it was possible to normalize the decreased rate of reabsorption of phosphate (fractional reabsorption 59%) by the administration of vitamin D (fractional reabsorption 78%). SpeculationWe were unable to determine why, despite continuous infusion of vitamin D and independent of the dosage, a decrease occurred in the increased reabsorption after 90-120 min and why after 180-240 min this function returned to the starting value. Since at this latter time toxic renal damage is unlikely and renal function was controlled during the whole experiment, the question arises whether inadequate activation, reactive breakdown of vitamin D, or development of resistance to the physiological activity of the vitamin is involved.

No abstract

No abstract

No abstract

No abstract