Relations between intracellular ion activities and extracellular osmolarity inNecturus gallbladder epithelium

Summary

The interactions between ion and water fluxes have an important bearing on osmoregulation and transepithelial water transport in epithelial cells. Some of these interactions were investigated using ion-selective microelectrodes in theNecturus gallbladder. The intracellular activities of K⁺ and Cl⁻ in epithelial cells change when the epithelium is adapted to transport in solutions of a low osmolarity. In order to achieve new steady states at low osmolarities, cells lost K⁺, Cl⁻ and some unidentified anions. Surprisingly, the apparent K⁺ concentration remained high: at an external osmolartity of 64 mOsm the intracellular K⁺ concentration averaged 95mm. This imbalance was sensitive to anoxia and ouabain. The effects of abrupt changes in the external osmolarities on the intracellular activities of Na⁺, K⁺ and Cl⁻ were also investigated. The gradients were effectuated by mannitol. The initial relative rates of change of the intracellular activities of Na⁺ and Cl⁻ were equal. The data were consistent with Na⁺ and Cl⁻ ions initially remaining inside the cell and a cell membraneL _p of 10⁻³ cm sec⁻¹ osm⁻¹, which is close to the values determine by Spring and co-workers (K.R. Spring, A. Hope & B.-E. Persson, 1981.In: Water Transport Across Epithelia. Alfred Benzon Symposium 15. pp. 190–200. Munskgaard, Copenhagen). The initial rate of change of the intracellular activity of K⁺ was only 0.1–0.2 times the change observed in Na⁺ and Cl⁻ activities, and suggests that K⁺ ions leave the cell during the osmotically induced H₂O efflux and enter with an induced H₂O influx. The coupling is between 98 and 102 mmoles liter⁻¹. Various explanations for the anomalous behavior of intracellular K⁺ ions are considered. A discussion of the apparent coupling between K⁺ and H₂O, observed in nonsteady states, and its effects on the distribution of K⁺ and H₂O across the cell membrane in the steady states, is presented.