Epithelial Na+ channel subunits in rat taste cells: localization and regulation by aldosterone

Amiloride-sensitive Na+ channels play an important role in transducing Na+ salt taste. Previous studies revealed that in rodent taste cells, the channel shares electrophysiological and pharmacological properties with the epithelial Na+ channel, ENaC. Using subunit-specific antibodies directed against alpha, beta, and gamma subunits of rat ENaC (rENaC), we observed cytoplasmic immunoreactivity for all three subunits in nearly all taste cells of fungiform papillae, and in about half of the taste cells in foliate and vallate papillae. The intensity of labeling in cells of vallate papillae was significantly lower than that of fungiform papillae, especially for beta and gamma subunits. Dual localization experiments showed that immunoreactivity for the taste cell-specific G protein, gustducin, occurs in a subset ofrENaC positive taste cells. Aldosterone is known to increase the amiloride sensitivity of the NaCl taste response. In our study, increases in blood aldosterone levels enhanced the intensity of apical immunoreactivity for beta and gamma rENaC in taste cells of all papillae. In addition, whole cell recordings from isolated taste cells showed that in fungiform papillae, aldosterone increased the number of amiloride-sensitive taste cells and enhanced the current amplitude. In vallate taste cells, which are normally unresponsive to amiloride, aldosterone treatment induced an amiloride sensitive current in about half of the cells. Immunoreactivity for rENaC subunits also was present in nonsensory epithelial cells, especially in the anterior portion of the tongue. In addition, immunoreactivity for all subunits, but especially beta and gamma, was associated with some nerve fibers innervating taste papillae. These extragustatory sites of rENaC expression may indicate a role for this channel in paracellular transduction of sodium ions.