Na and Cl transport across the isolated turtle colon: Parallel pathways for transmural ion movement

Summary

Transmural fluxes of³H-mannitol and²²Na or³⁶Cl were measured simultaneously in portions of isolated turtle colon stripped of serosal musculature. The relationships between mannitol flux and the flux of Na or Cl are characteristic of simple diffusion and suggest that transmural mannitol flow is largely confined to a paracellular pathway where Na, Cl and mannitol move much as in free solution. The contribution of “edge damage” to the transmural mannitol flow appears to be minimal. Mucosal hyperosmolarity causes “blisters” in epithelial tight junctions and increases the diffusional permeability to Na and mannitol, suggesting that the rate-limiting barrier in the shunt path is the tight junction. If the total mucosa to serosa flux of Na is corrected for the portion traversing the shunt pathway it is apparent that changes in the short-circuit current are completely accounted for by the mucosa to serosal movement of Na through a cellular path. In addition, the serosa to mucosa flux of Na appears to be restricted to the shunt. These observations suggest that there is no appreciable “backflux” of Na through the active, cellular path. In the presence of 10⁻⁴ m amiloride the short-circuit current is markedly reduced and the mucosa to serosa Na flux is restricted to the shunt, so that the net Na flux is abolished. The small amiloride-insensitive short-circuit current is consistent with HCO₃ secretion. Mucosa to serosa and serosa to mucosa fluxes of Cl appear to be largely restricted to the paracellular shunt path and there is no evidence for any net flow of Cl under short-circuit conditions. The total tissue conductance can be described as the sum of three components: a shunt conductance which is linearly related to the transmural mannitol flow, an “active” conductance which is linearly related to the short-circuit current and a small residual conductance. The shunt conductance is attributable to the diffusive movements of Na and Cl through the paracellular path. Variations in the active Na transport from tissue to tissue are largely attributable to variations in the apparent conductance of the active Na transport path. The driving force for active Na transport can be described as an apparent emf of approximately 130 mV. These results suggest that transmural mannitol flux provides a quantitative estimate of the ion permeability and electrical conductance of a paracellular shunt path across the isolated turtle colon and thereby facilitates the study of the transport characteristics and electrical properties of cellular paths for transepithelial solute movement.