Simultaneous minute by minute determination of unidirectional and net water fluxes in frog urinary bladder. A reexamination of the two barriers in series hypothesis.

Unidirectional and net water fluxes were simultaneously estimated in frog urinary bladder. The minute by minute tritiated water (3HOH) transepithelial flux and the net volume of fluid traversing the tissue were employed. It was observed that: (1) the time course of the increase in the 3HOH flux induced by antidiuretic hormone had a very similar pattern to that reported for the increase in the net movement. (2) Unstirred layers strongly affected the magnitude of the antidiuretic hormone-induced increase in 3HOH fluxes while the time course of the response was almost non-affected. In non-stimulated bladders 3HOH fluxes were poorly modified by medium stirring. New steady-state conditions for 3HOH fluxes were established 1 min after stirring rate modifications. (3) The simultaneously determined net water flux was not affected by a modification in the unstirred layers, indicating that the variations in the measured net water fluxes are a good estimation of the changes in the mucosal border permeability. (4) The presence of an osmotic gradient during hormonal challenge (implying net water fluxes, cell swelling and dilation of the intracellular spaces) did not modify the time course of 3HOH movements. These results suggest that the time course of the increase in water permeability is an intrinsic characteristic of the experimental system that could result from the addition of permeability units that increase in number during the development of the hormonal action.