Characteristics of the paracellular pathway of rabbit cortical collecting duct

The nature of the paracellular pathway of the rabbit cortical collecting duct (CCD) was examined under conditions designed to eliminate all cellular ion transport. Transcellular conductive pathways were blocked by addition of amiloride and Ba²⁺ to the perfusate. Cl self-exchange was eliminated by removing Cl mfrom the bath solution, and HCO₃ transport was eliminated by omitting HCO₃ and CO₂ from the solutions. The residual transepithelial conductance (G(T)) and the radioisotopic tracer flux under these conditions probably occur via the paracellular pathway. The G(T) measured in nontransporting CCD bathed in NaCl solutions was 1.1-1.2 mS/cm². When Na or Cl was replaced by a less mobile ion, the G(T) decreased by an amount commensurate with the decrease in solution conductivity. The Na-to-Cl permeablity ratio determined by NaCl dilution voltages ranged from 0.55 go 0.82. An independent estimate of paracellular selectivity was obtained for comparing the lumen-to-bath tracer rate coefficients for Na (κ(Na)) and Cl (κ(Cl)). The ration κ(Na):κ(Cl) was 0.75. These observations suggest that the paracellular pathway displays a Na:Cl permselectivity not substantially different from the ratio of their mobolities in water (0.65). Additional experiments demonstrated that the summed partial ionic conductances of Na and Cl calculated from the tracer fluxes were in close agreement with the measured G(T). We conclude hat the paracellular conductance of CCD is nonselective in character and ~1-2 mS/cm² in magnitude.