Osmotic pressure regulates αβγ-rENaC expressed inXenopus oocytes

Abstract

The hypothesis that amiloride-sensitive Na+channels (ENaC) are involved in cell volume regulation was tested. Anisosmotic ND-20 media (ranging from 70 to 450 mosM) were used to superfuse Xenopus oocytes expressing αβγ-rat ENaC (αβγ-rENaC). Whole cell currents were reversibly dependent on external osmolarity. Under conditions of swelling (70 mosM) or shrinkage (450 mosM), current amplitude decreased and increased, respectively. In contrast, there was no change in current amplitude of H2O-injected oocytes to the above osmotic insults. Currents recorded from αβγ-rENaC-injected oocytes were not sensitive to external Cl− concentration or to the K+ channel inhibitor BaCl2. They were sensitive to amiloride. The concentration of amiloride necessary to inhibit one-half of the maximal rENaC current expressed in oocytes ( K i; apparent dissociation constant) decreased in swollen cells and increased in shrunken oocytes. The osmotic pressure-induced Na+ currents showed properties similar to those of stretch-activated channels, including inhibition by Gd3+ and La3+, and decreased selectivity for Na+. αβγ-rENaC-expressing oocytes maintained a nearly constant cell volume in hypertonic ND-20. The present study is the first demonstration that αβγ-rENaC heterologously expressed in Xenopus oocytes may contribute to oocyte volume regulation following shrinkage.