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Abstract

To explain the origin of the free-flow potential profile along rat kidney proximal tubule we have measured the transepithelial potential difference in early and late proximal tubular loops under well defined perfusion conditions. During elimination of all transeptithelial concentration differences through luminal and peritubular perfusion with simple HCO_2- Ringer's solution a lumen-positive potential difference was observed. It averaged +1.0±0.39 mV in early loops and +0.2±0.31 mV in late loops. It disappeared upon poisoning with cyanide and appears to be generated by active H⁺ secretion/HCO⁻₃ absorption. Plasma-like concentrations of glucose or amino acids shifted the potential difference into lumennegative direction by up to −1.3 mV in early loops and −0.3 mV in late loops. This potential shift is caused by induction of active Na⁺ absorption through cotransport with the respective organic solute. Besides active transport potentials the tubule is able to generate membrane diffusion potentials from transepithelial ion concentration gradients and streaming potentials from osmotic concentration gradients. Whereas membrane diffusion potentials account for about 80% of the free-flow potential difference in intermediate and late tubular loops, streaming potentials are too small to play a role under free-flow conditions.