Fluid transfer by Necturus gall bladder epithelium as a function of osmolarity

Abstract

When perfused with a saline medium whose tonicity ranged from 400 mosmolar down to near 1 mosmolar Necturus gall bladder trans­ferred volumes of fluid which were quasi-isotonic. At the very low pressures pertaining during an experiment, the flow rates could not be explained by simple mechanical filtration and were inhibited by pre-­treatment with ouabain, indicating that the fluid transfer was an active secretion by the epithelial cells. Electron micrographs of the cells show that at the lower osmolarities used the epithelial cells were swollen but intact. Dimensions of the lateral interspaces obtained from both trans­mission and scanning electron micrographs have been used to calculate the osmolarity of the secretion as a function of bathing osmolarity, according to standing-gradient osmotic theory. The results show that the theory can­not possibly provide an acceptable description of the fluid production, and that the water is almost certainly crossing the epithelium by an extracellular route through the intercellular junctions.