Pathways of ion movement in the canine tracheal epithelium

The pathways of ion movement across canine tracheal epithelium, a Cl-secreting tissue, were examined by three techniques. First, the measurement of simultaneous, unidirectional fluxes of Na or Cl and mannitol, a large hydrophilic molecule that serves as a marker of the paracellular pathway, indicated that a significant fraction of both the Na flux from submucosa to mucosa (J Na sm) and the flux of Cl from mucosa to submucosa (J Cl ms) traverse the cellular pathway. The ratio of the Na-to-Cl diffusion coefficients through the paracellular pathway was 0.23, in contrast to the free solution ratio of 0.63. Second, in voltage-clamp experiments we examined the effect of transepithelial voltage differences on the unidirectional fluxes of Na and Cl. The results agree with the previous findings, suggesting that there are voltage-independent, or transcellular, backfluxes of Na and Cl, and that the relative permeability of Na to Cl through the voltage-dependent (presumably paracellular) pathway was 0.28. Third, measurement of transepithelial diffusion potentials gave a Na-to-Cl permeability ratio of 0.31 +/- 0.02 (mean +/- SE). These results suggest that there are significant transcellular backfluxes of Na and Cl and that the paracellular pathway in the canine trachea is anion selective. An anion-selective pathway would tend to shunt the secreted Cl back through the paracellular pathway, thus minimizing the net ion and fluid movement across the tissue in the open-circuit condition.