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

Amiloride-sensitive Na+ channels play an important role in transducing Nasalt taste. Previous studies revealed that in rodent taste cells, the chann el shares electrophysiological and pharmacological properties with the epit helial Na+ channel, ENaC. Using subunit-specific antibodies directed agains t alpha, beta, and gamma subunits of rat ENaC (rENaC), we observed cytoplas mic immunoreactivity for all three subunits in nearly all taste cells of fu ngiform papillae, and in about half of the taste cells in foliate and valla te papillae. The intensity of labeling in cells of vallate papillae was sig nificantly lower than that of fungiform papillae, especially for beta and g amma subunits. Dual localization experiments showed that immunoreactivity f or the taste cell-specific G protein, gustducin, occurs in a subset of rENa C positive taste cells. Aldosterone is known to increase the amiloride sensitivity of the NaCl tast e response. In our study, increases in blood aldosterone levels enhanced th e intensity of apical immunoreactivity for beta and gamma rENaC in taste ce lls 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 v allate taste cells, which are normally unresponsive to amiloride, aldostero ne treatment induced an amiloride sensitive current in about half of the ce lls. Immunoreactivity for rENaC subunits also was present in nonsensory epitheli al cells, especially in the anterior portion of the tongue. In addition, im munoreactivity fur all subunits, but especially beta and gamma, was associa ted with some nerve fibers innervating taste papillae. These extragustatory sites of rENaC expression may indicate a role for this channel in paracell ular transduction of sodium ions. (C) Wiley-Liss, Inc.