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

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Abstract
When perfused with a saline medium whose tonicity ranged from 400 mOsm down to near 1 mOsm, 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 pretreatment with ouabain, indicating that the fluid transfer was an active secretion by the epithelial cells. Electron micrographs of the cells showed that at the lower osmolarities used the epithelial cells were swollen but intact. Dimensions of the lateral interspaces obtained from transmission and scanning electron micrographs were used to calculate the osmolarity of the secretion as a function of bathing osmolarity, according to standing-gradient osmotic theory. The theory cannot possibly provide an acceptable description of the fluid production, and the water is almost certainly crossing the epithelium by an extracellular route through the intercellular junctions.

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