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

Abstract

Summary. Transmural fluxes of 3H-mannitol and 22Na or a6CI were measured simultaneously in portions of isolated turtle colon stripped of serosal musculature. The relationships between mannitol flux and the flux of Na or C1 are characteristic of simple diffusion and suggest that transmural mannitol flow is largely confined to a paracellular pathway where Na, C1 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 macosa 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 HCO 3 secretion. Mucosa to serosa and serosa to mucosa fluxes of C1 appear to be largely restricted to the paracellular shunt path and there is no evidence for any net flow of C1 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 C1 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.The simplest model which can account for the transport properties of an epithelial cell layer is one which allows for two parallel paths for transmural solute and water flow, one through the cells and another between

Tight junction properties of the immortalized human bronchial epithelial cell lines Calu‐3 and 16HBE14o‐

“…This concept has been supported experimentally in growtharrested cells which show no decrease in TER as monolayers age despite the properties of the TJs being similar to those in dividing cell monolayers [27]. Consistent with the development of effective occluding junctions, monolayers of 16HBE14o-or Calu-3 cells were found to develop a low permeability to the cell impermeant sugar, mannitol [28].…”

Na+ transport in cystic fibrosis respiratory epithelia. Abnormal basal rate and response to adenylate cyclase activation.

“…The partial ionic conductances for Na' and C1-(JQs', J's) accounted for -70% of G. We do not know whether the greater disparity between the sum of the partial ionic conductances for Na' and Ct-and G in CF tissues reflects the contribution ofan active component to G, e.g., associated with the greater Na+ transport rate, or an unmeasured ion. Mannitol permeability coefficients, a marker of paracellular permeability (19,20), were not different for any of the three groups.…”

Epithelial transport pathways of rat colon determined in vivo by impulse response analysis.

“…(Frizzell et al 1976) and in isolated toad colon (Dawson, 1977) have also indicated that Na and Cl movements from blood to lumen are consistent with diffusion through a nondiscriminating barrier. In our experiments, the entire flux of these ions into the lumen appeared due to simple diffusion as in the dialysis membrane model and no appearances in the impulse response functions suggested that any other process was involved.…”

“…The observed unidirectional fluxes across the epithelium result from the total movement through both routes and various methods have been used in vitro in an attempt to dissect the total flux into its components. Electrical effects, the changes in ionic movements occurring with applied currents and the use of labelled molecules, such as urea or mannitol, as markers for the paracellular pathway, have all been explored (Mandel & Curran, 1972;Frizzell, Koch & Schultz, 1976;Dawson, 1977). The method described in the present paper was devised to enable studies of the transport pathways to be made in vivo and in such a way that would allow future investigations of the effects of various modifying factors such as drugs,…”