Anion permeation in the proximal tubule ofNecturus kidney: The shunt pathway

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Abstract
The effect of foreign anions on transepithelial potential difference and transepithelial input conductance was studied in the isolated perfusedNecturus kidney. Two microelectrodes (recording and current-injecting) were inserted into the lumen of single proximal tubules and the peritubular perfusate was shifted reversibly for 30–60 sec from a physiologic Ringer's solution to a test solution in which chloride was replaced isosmotically by a foreign anion. The permeability sequence, obtained by potential measurements, was: lactate < glutamate < gluconate < pyruvate < benzene sulfonate ≦ acetate ≦ F < propionate 33434 − and I− (both more permeable than chloride) produced an important decrease in transepithelial conductance, followed by incomplete reversibility when the perfusion was shifted again to chloride Ringer's. The results are best explained by the presence of weak positive fixed charges, governing anion permeation, at the shunt pathway of the proximal tubule. An analysis of the data allows tentative estimates of shape and size of the sites.