The influence of extracellular volume expansion on renal phosphate reabsorption in the dog.

Extracellular volume expansion (ECVE) was produced, by normal saline infusion, in five normal and six thyroparathyroidectomized anesthetized dogs while glomerular filtration rate was reduced by the inflation of an intra-aortic balloon located above the renal arteries. The effect of ECVE on the maximum renal tubular reabsorptive capacity of phosphate (phosphate Tm) was also evaluated in five additional dogs. During ECVE, phosphate excretion increased both in normal and thyroparathyroidectomized dogs, and a direct and significant correlation was found between the fractional excretion of phosphate and sodium. Despite a substantial decrease in filtered phosphate which is produced by the acute reduction in glomerular filtration rate, phosphate excretion, during ECVE, exceeded control values. ECVE was associated with a reduction in phosphate Tm. The results demonstrate that ECVE increases phosphate excretion independent of changes in glomerular filtration rate and parathyroid gland activity. The data indicate that ECVE produced by saline infusion decreases the renal tubular reabsorption of phosphate.