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

We have previously demonstrated that for mate and oxalate stimulate volume absorption in the rat proximal tubule, consistent with Cl-/formate and Cl-/oxalate exchange processes across the apical membrane. To sustain Cl- absorption by these processes requires mechanisms far 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 mu M. Addition of 5, 50, and 500 mu M formate to both luminal and capillary perfusates significantly increased net Cl- absorption (J(Cl)) by 26, 26, and 46%, respectively. J(Cl) was stimulated 38% by 1 mu M oxalate added to the perfusates. Removal of sulfate completely prevented the stimulation of J(Cl) by 1 mu M oxalate but had no effect on the stimulation of J(Cl) by formate. Luminal addition of the Na+/H+ exchange inhibitor ethylisopropylamiloride completely blocked the stimulation of J(Cl) by 50 mu M formate but had no effect on stimulation by oxalate. We conclude that pbysiological 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.