Pseudo-Streaming Potentials in Necturus Gallbladder Epithelium H. The Mechanism Is a Junctional Diffusion Potential

The mechanisms of apparent streaming potentials elicited across Necturus gallbladder epithelium by addition or removal of sucrose from the apical bathing solution were studied by assessing the time courses of: (a) the change in transepithelial voltage (Vm,), (b) the change in osmolality at the cell surface (estimated with a tetrabutylammonium [TBA+]-selective microelectrode, using TBA + as a tracer for sucrose), and (c) the change in cell impermeant solute concentration ([TMA+], measured with an intracellular double-barrel TMA+-selective microelectrode after loading the cells with TMA ÷ by transient permeabilization with nystatin). For both sucrose addition and removal, the time courses of V=, were the same as the time courses of the voltage signals produced by [TMA÷]~, while the time courses of the voltage signals produced by [TBA*]o were much faster. These results suggest that the apparent streaming potentials are caused by changes of [NaCI] in the lateral intercellular spaces, whose time course reflects the changes in cell water volume (and osmolality) elicited by the alterations in apical solution osmolality. Changes in cell osmolality are slow relative to those of the apical solution osmolality, whereas lateral space osmolality follows cell osmolality rapidly, due to the large surface area of lateral membranes and the small volume of the spaces. Analysis of a simple mathematical model of the epithelium yields an apical membrane Lp in good agreement with previous measurements and suggests that elevations of the apical solution osmolality elicit rapid reductions in junctional ionic selectivity, also in good agreement with experimental determinations. Elevations in apical solution [NaCI] cause biphasic transepithelial voltage changes: a rapid negative Vm, change of similar time course to that of a Na+/TBA + bi-ionic potential and a slow positive Vm~ change of similar time course to that of the sucrose-induced apparent streaming potential. We conclude that the Vm, changes elicited by addition of impermeant Address reprint requests to Dr. Luis Reuss, Department of Physiology and Biophysics, The University of Texas Medical Branch, Galveston, TX 77550-2779. Dr. Cotton's current address is Case Western Reserve University, Rainbow Babies and Children's Hospital, Pediatrics-Pulmonary Division, 2103 Adelbert, Cleveland, OH 44106. J. GEN. PHYSIOL. ~ The Rockefeller University Press • 0022-1295/92/03/0317/22 $2.00 317 Volume 99 March 1992 317-338 on A ril 9, 2017 D ow nladed fom Published March 1, 1992