Further studies on ion permeation in proximal tubule of necturus kidney.

In the perfused Necturus kidney, the ratio of transepithelial sodium-to-chloride permeabilities (PNa/PCl) was estimated from dilution potentials at 1:7. The ratio of transepithelial permeabilities PHCO3/cl was estimated from bi-ionic potentials at 1:4 and that of Plact/PCl at 1:12. The permeability sequence, therefore, is PCl greater than PHCO3 greater than than PNa or PnaCl greater than PNaHCO3. The latter is similar to the situation in the rat. However, because of the low Na permeability, passive NaCl absorption by solvent drag is probably less important in Necturus than in the rat. The measurement of transepithelial input conductance during replacement of extracellular chloride by either bicarbonate or lactate shows that the former reduces the transmural conductance to 31% of control values as compared with 40% for the latter. Such discrepancies between permeability and conductance sequences are consistent either with an interference of the test anion with chloride permeation or with the presence of positive fixed changes within the shunt pathway.