Accurate evaluation of the unavoidable liquid junction potentials (LJP) involved whenever bath (B) and perfusate (P) solutions differ in ionic composition is of fundamental importance in the determination of proximal convoluted tubule (PCT) transepithelial potential, psi T, and its active and diffusion components, psi A and psi D. Therefore, a precise method of measuring these LJP, using Ag/AgCl electrodes, has been developed and successfully tested. The measured LJP, psi B-P, between artificial bath serum containing 6 g/100 ml albumin and ultrafiltrate-like perfusate is +2.6 mV. The average measured potential difference (PD) in rabbit PCT with these solutions was -3.0 mV, psi T thus being only -0.4 mV. Three maneuvers known to abolish psi A (hypothermia, 10(-4) ouabain, or K-free bath) brought the PD close to the reference potential, indicating that psi A congruent to PD and that psi D congruent to psi B-P. From dilution potentials, after correction for LJP (4.3 in the absence and 6.2 mV in the presence of proteins), PNa/PCl was calculated to be 0.9 for midcortical and 1.1 for juxtamedullary PCT in the absence of proteins and slightly lower in their presence. These values of LJP obtained with Ag/AgCl electrodes are between 30 and 100% larger than the ones measured using a saturated KCl bridge or the ones in previous studies, indicating that in these studies psi T as well as PNa/PCl has been overestimated.
