Hypotonic regulation of mouse epithelial sodium channel in Xenopus laevis oocytes

The regulation of the epithelial Na⁺ channel (ENaC) during cell swelling is relevant in cellular processes in which cell volume changes occur, i.e., migration, proliferation and cell absorption. Its sensitivity to hypotonically induced swelling was investigated in the Xenopus oocyte expression system with the injection of the three subunits of mouse ENaC. We used voltage-clamp techniques to study the amiloride-sensitive Na⁺ currents (INa(amil)) and video microscopic methodologies to assess oocyte volume changes. Under conditions of mild swelling (25 % reduced hypotonicity) inward current amplitude decreased rapidly over 1.5 min. In contrast, there was no change in current amplitude of H₂O-injected oocytes to the osmotic insult. INa(amil) kinetics analysis revealed a decrease in the slower inactivation time constant during the hypotonic stimuli. Currents from ENaC-injected oocytes were not sensitive to external Cl⁻ reduction. Neither short- nor long-term cytochalasin D treatment affected the observed response. Oocytes expressing a DEG mutant β-ENaC subunit (β-S518K) with an open probability of 1 had reduced INa(amil) hypotonic response compared to oocytes injected with wild-type ENaC subunits. Finally, during the hypotonic response ENaC-injected oocytes did not show a cell volume difference compared with water-injected oocytes. On this basis we suggest that hypotonicity-dependent ENaC inhibition is principally mediated through an effect on open probability of channels in the membrane.