Acute mannitol and saline volume expansion in the rat: effect on transepithelial potential difference in proximal tubules.

1. Transepithelial potential difference (PDte) of proximal tubules was measured in rats under control conditions (C), and mannitol-saline and saline extracellular fluid volume expansion (MVE, SVE, respectively) under conditions of normal net lumen to basal sodium transport. 2. PDte was measured in kidneys bathed with Hartmann's solution or covered with mineral oil under both volume-expanded conditions together with their controls. 3. PDte was significantly lower in kidneys bathed with Hartmann's solution than those covered with oil. 4. In MVE rats, with mineral oil covering the kidneys, PDte (expressed as mean and s.e.m.) was for the control 2.20 +/- 0.05 (n = 45) mV and MVE 1.97 +/- 0.04 (n = 36) mV, lumen positive, a significant reduction of 10% (P less than 0.001). In SVE rats, with mineral oil covering the kidneys, PDte was for C = 2.42 +/- 0.05 (n = 74) mV and SVE = 1.93 +/- 0.03 (n = 67) mV, a significant reduction (P less than 0.001) of 20%. 5. According to thermodynamic considerations, neither of these changes is sufficient to explain the 50% inhibition of Na transport measured previously during MVE and SVE with autologous tubular fluid. The present results offer further evidence supporting the idea that the inhibition of Na transport during MVE and SVE is largely due to inhibition of the active Na transporting step.