Mechanisms of stimulation of proximal tubule chloride transport by formate and oxalate

Abstract

We have previously demon-strated that formate and oxalate stimulate volume absorption in the rat proximal tubule, consistent with CWformate and Cl-/oxalate exchange processes across the apical membrane. To sustain Cl- absorption by these processes requires mechanisms for recycling formate and oxalate from lumen to cell. The aims of the present study were to characterize these mechanisms of formate and oxalate recycling. Proximal tubules and peritubular capillaries were simultaneously microperfused in the rat kidney in situ. Serum formate concentration was determined to be 56.5 ± 7.7 μM. Addition of 5, 50, and 500 uM formate to both luminal and capillary perfusates significantly increased net Cl-absorption (JCl) by 26, 26, and 46%, respectively. Jci was stimulated 38% by 1 μM oxalate added to the perfusates. Removal of sulfate completely prevented the stimulation of Jci by 1 7mu;M oxalate but had no effect on the stimulation of JCl by formate. Luminal addition of the Na+/H+ exchange inhibitor ethylisopropylamiloride completely blocked the stimulation of Jci by 50 uM formate but had no effect on stimulation by oxalate. We conclude that physiological concentrations of formate and oxalate markedly stimulate Cl- and fluid absorption in the rat proximal convoluted tubule. Whereas formate recycling most likely involves Na+/H+ exchange in parallel with H+-coupled formate entry, oxalate recycling involves sodium-sulfate cotransport in parallel with sulfate/oxalate exchange. Copyright ©1996 the American Physiological Society.