OSMOTIC-PRESSURE REGULATES ALPHA-BETA-GAMMA-RENAC EXPRESSED IN XENOPUS OOCYTES

The hypothesis that amiloride-sensitive Nat channels (ENaC) are involv ed in cell volume regulation was tested. Anisosmotic ND-20 media (rang ing from 70 to 450 mosM) were used to superfuse Xenopus oocytes expres sing alpha beta gamma-rat ENaC (alpha beta gamma-rENaC). Whole cell cu rrents were reversibly dependent on external osmolarity. Under conditi ons of swelling (70 mosM) or shrinkage (450 mosM), current amplitude d ecreased and increased, respectively. In contrast, there was no change in current amplitude of H2O-injected oocytes to the above osmotic ins ults. Currents recorded from alpha beta gamma-rENaC-injected oocytes w ere not sensitive to external Cl- concentration or to the K+ channel i nhibitor 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 i n swollen cells and increased in shrunken oocytes. The osmotic pressur e-induced Na+ currents showed properties similar to those of stretch-a ctivated channels, including inhibition by Gd3+ and La3+, and decrease d selectivity for Na+. alpha beta gamma-rENaC-expressing oocytes maint ained a nearly constant cell volume in hypertonic ND-20. The present s tudy is the first demonstration that alpha beta gamma-rENaC heterologo usly expressed in Xenopus oocytes may contribute to oocyte volume regu lation following shrinkage.