Salt-water coupling in leaky epithelia

Abstract

The theory of quasi-isotonic transport by cellular osmosis (the standing-gradient theory) has been challenged on the grounds that the osmotic permeabilities of the mucosal and interspace membranes are too low; if they were as high as the theory requires then the osmotic permeability of the whole epithelium would be 2-3 orders of magnitude higher than observed. This objection has basically been accepted for it is now claimed that these enormous permeabilities do exist, but are masked by unstirred-layer effects; I show that this is incorrect because unstirred-layer corrections are small and that the situation has not changed since 1975. The view that the route of fluid transport is junctional is replacing the cellular theory, and trans-junctional water flows seem to account for major fractions of the flow in various epithelia. I argue on grounds of general theory that these are unlikely to be osmotic flows because the junctional pores cannot satisfy both the osmotic and diffusive properties required of them, but the basic osmotic theory is also rather vague here. Non-osmotic theories, if junctional flow is accepted, must be either electro-kinetic or peristaltic.

The roles of paracellular and transcellular pathways and submucosal space in isotonic water absorption by rabbit ileum.

“…Hill (1975Hill ( , 1980 and Sackin & Boulpaep (1975) (Diamond, 1979;Barry & Diamond, 1984) that unstirred layer effects mask the high mucosal Lp is not supported by the experimental results and analysis in this paper, which show that the unstirred layer effect external to rabbit ileal mucosa is insubstantial. However, because the volume of the lateral intercellular space is small (20-25 1 cm-2 serosal area), and the rate of solute pumping is rapid (1-2 nmol s-1 cm-2 serosal area), a step change in the tonicity of the mucosal solution is followed by a rapid adjustment in the tonicity of the lateral intercellular space, so that transient change in flow into the lateral intercellular space, even with 422 WATER ABSORPTION IN RABBIT ILEUM the relatively low Lp (1 x 10-8 cm s-1 cmH2O'-), is complete within a few seconds.…”

Effect of amiloride on sodium and water reabsorption in the rabbit gall‐bladder.

“…Diamond & Bossert, 1967). Recent studies of epithelial geometry (Rostgaard & Frederiksen, 1981), water permeability (Hill, 1980), and lateral space hypertonicity (Simon, Curci, Gebler & Fr6mter, 1981), however, have led to a critical re-evaluation of this hypothesis. Previous results from rabbit gall-bladder suggest, in fact, that the route for most if not all of the net salt and water transport is through the epithelial cells (Frederiksen et al 1979).…”

Paracellular non‐electrolyte permeation during fluid transport across rabbit gall‐bladder epithelium

“…This interpretation depends rather heavily on fluxes determined at very low osmolality (20 m-osmole kg-1) and it is possible that an elevated shunt permeability, such as is observed in the rabbit gall-bladder under less severe conditions, is a complicating factor which needs to be eliminated. If, on the other hand, the analysis of Whittembury et al proves to be correct, their results suggest that the guinea-pig gall-bladder epithelium is an unusually leaky mammalian epithelium, and this observation, like that of Hill & Hill (1978a) in Necturus, will inevitably impose considerable constraints on the possible driving forces envisaged for junctional water flow (Hill, 1980).…”