Ernest M. Wright

The mechanism of cation permeation in rabbit gallbladder : Conductances, the current-voltage relation, the concentration dependence of anion-cation discrimination, and the calcium competition effect.

SummaryThe questions underlying ion permeation mechanisms, the types of experiments available to answer these questions, and the properties of some likely permeation models are examined, as background to experiments designed to characterize the mechanism of alkali cation permeation across rabbit gallbladder epithelium. Conductance is found to increase linearly with bathing-solution salt concentrations up to at least 400mm. In symmetrical solutions of single alkali chloride salts, the conductance sequence is K+>Rb+>Na+>Cs+∼Li+. The current-voltage relation is linear in symmetrical solutions and in the presence of a single-salt concentration gradient up to at least 800 mV. The anion/cation permeability ratio shows little change with concentration up to at least 300mm. Ca++ reduces alkali chloride single-salt dilution potentials, the magnitude of the effect being interpreted as an inverse measure of cation equilibrium constants. The equilibrium-constant sequence deduced on this basis is K+>Rb+>Na+∼Cs+∼Li+. These results suggest (1) that the mechanism of cation permeation in the gallbladder is not the same as that in a macroscopic ion-exchange membrane; (2) that cation mobility ratios are closer to one than are equilibrium-constant ratios; (3) that the rate-limiting step for cation permeation is in the membrane interior rather than at the membrane-solution interface; and (4) that the rate-controlling membrane is one which is sufficiently thick that it obeys microscopic electroneutrality.

Models for Isotonic Transport Across Apical Membranes of Epithelial Cells

The mechanisms and routes by which water is transported across epithelia are debated. It is generally held, and probably true, that the water transport is coupled in a secondary fashion to the transport of ions but the underlying mechanism is not understood at all. So far, most models for transepithelial water transport have been based upon osmotic mechanisms, in which water transport by the cellular route arises from transmembrane osmotic differences. Such models, however, have been unable to explain a number of well-established epithelial properties, for a review see Zeuthen (1992). With the finding that certain membrane proteins function as molecular water pumps, it has been possible to suggest a more general model for transepithelial water transport which would explain, for example, uphill and isotonic transport of water (Zeuthen, 1996andZeuthen, 2000).