Osmotically induced electrical changes in isolated bullfrog small intestine.

1. Steady state values of cell water, intracellular Na+ and K+ concentrations, and the electrical parameters ETr, Em, and Isc in the mucosa of isolated bullfrog small intestine were determined following immersion in sodium sulfate Ringer solutions with identical ionic composition but different osmolalities. 2. Cell water and intracellular K+ concentration were inversely related to the osmolality of the bathing medium. During 1 h immersion, intracellular Na+ concentration was not significantly affected by an increase or decrease in external osmolality. 3. Replacement of a hypotonic or an approximately isotonic (normal) medium by a medium of greater osmolality caused statistically significant decreases in ETr, Isc and the (inside negative) magnitude of Em. Conversely, when a hypertonic or a normal medium was replaced by one of lower osmolality, significant increases in the magnitude of these parameters were observed. 4. An equivalent circuit model for the epithelial cell layer, in which the resistance of a relatively highly conducting extracellular shunt pathway is assumed to be the major determinant of the electrical responses of the small intestine to external osmolality, has been shown to account satisfactorily for the observed changes in ETr and Em. In terms of this model, the experimentally observed dependence of Isc on external osmolality requires that, even when both the mucosal and the serosal sides of the tissue are bathed by identical media, isolated bullfrog small intestine maintains a finite diffusion potential across the shunt pathway. This is consistent with current views concerning transepithelial ionic transfer mechanisms.