Electrical potential differences across distal renal tubules of Amphiuma

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Abstract
Electrical potential differences were measured across peritubular and luminal cell membranes of distal renal tubules of the urodele amphibian, Amphiuma. In the blood-perfused kidney the peritubular membrane potential was 70 mv, cell negative. The transtubular potential was 45 mv, lumen negative. In kidneys perfused with solutions containing 2. 5 mM K, peritubular and transtubular potentials were 72 and 43 mv, respectively. Elevation of perfusion fluid K depolarized both cell membranes. The extent of this depolarization suggested that the peritubular membrane is highly selective in its permeability to K, but that the luminal membrane is permeable to some other ion species as well. Replacement of perfusion fluid Na by choline led to a marked hyperpolarization of the luminal cell membrane. This was judged consistent with diffusion of Na from cell to luminal fluid, across a membrane whose permeability to Na and K were approximately equal. Comparison of potential differences with intracellular concentrations of K and Na lends support to these conclusions regarding the passive permeabilities of these membranes. In addition, they imply the operation at the peritubular membrane of an active process transporting Na out of, and K into, the distal tubular cell.