Conductive properties of papillary surface epithelium

The surface epithelium of rabbit renal papilla was dissected free from its supporting tissue and mounted in an Ussing chamber. The conductive properties of the epithelium and of the apical and basolateral cell membranes were examined with KCl-filled microelectrodes. The transepithelial voltage was 0.07 ± 0.15 mV, and the transepithelial resistance was 107 ± 15 Ω·cm² (n = 29). The fractional resistance of the apical membrane (fR_a) was 0.93 ± 0.01 (n = 103 cells, 29 tissues). The apical membrane was not conductive to Na⁺, K₊, or Cl^-. An increase in the K⁺ concentration of the basolateral solution from 5 to 50 mM depolarized the basolateral membrane voltage (V_b) from -59 ± 1.6 to -31.2 ± 2.2 mV (n = 28 cells) and increased fR_a from 0.935 ± 0.01 to 0.962 ± 0.01 (P < 0.001, n = 21 cells). Likewise, 5 mM barium in the basolateral solution depolarized V_b from -57.7 ± 2.0 to -29.8 ± 2.2 mV (n = 21 cells). A tenfold decrease in the Cl^- concentration of the basolateral solution caused an 8.3 ± 1.9 mV depolarization in V_b. Thus the basolateral cell membrane is conductive to K⁺ and Cl^-. Exposure of the apical membrane to amphotericin B demonstrated that the transepithelial resistance is determined primarily by the paracellular pathway.