Metabolic Requirement for Inorganic Phosphate by the Rabbit Proximal Tubule

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Abstract
These studies examine the effects of acute changes in the availability of inorganic phosphate on the function of isolated proximal renal tubules from rabbit kidney. We removed phosphate from the extracellular fluids and measured fluid absorption rates in isolated perfused tubules and oxygen consumption rates in suspensions of cortical tubules. In proximal convoluted tubules, the selective removal of phosphate from the luminal fluid reduced fluid absorption rates from 1.11±0.12 to −0.01±0.08 nl/mm · min. This effect on fluid absorption was dependent on the presence of glucose transport and metabolism. The addition of phlorizin to the phosphate-free luminal fluid preserved fluid absorption rates (1.12±0.12 nl/mm · min) as did the substitution of nonmetabolized α-methyl d-glucopyranoside for glucose (1.05±0.21 nl/mm · min) or the addition of 2-deoxyglucose, an inhibitor of glycolysis, to the bathing medium (1.01±0.15 nl/mm · min). There was no effect on fluid absorption if phosphate was removed from the bath only. Additionally, removal of phosphate from the luminal fluid of proximal straight rather than convoluted tubules had no effect on fluid absorption rates. Oxygen consumption rates in suspensions of cortical tubules were reduced from 18.9±0.6 to 10.6±0.6 nmol O2/mg tubular protein · min by the removal of phosphate from the medium. This inhibition was prevented by the substitution of α-methyl d-glucopyranoside for glucose in the phosphate-free medium. The data indicate that under certain conditions, proximal convoluted tubules require the presence of phosphate in the luminal fluid to preserve tubular function. In the absence of intraluminal phosphate, glucose metabolism causes a reduction in both oxidative metabolism and fluid absorption. This response is analogous to the Crabtree effect and suggests limitations on the intracellular availability of inorganic phosphate.