Basolateral amiloride-sensitive Na⁺ transport pathway in rat tongue epithelium

1. Experiments were conducted to test for the presence of basolateral Na⁺ channels in the rat lingual epithelium. Researchers have proposed a model in which some lingual taste cells have Na⁺ channels in the basolateral membrane. That model is designed to account for the portion of the neural taste response and the portion of the transepithelial short-circuit current (I(sc)) in vitro that are insensitive to mucosal amiloride; some Na⁺ would diffuse across the tight junction into the cell via this lateral pathway, and would be transported out of the cell by Na⁺ pumps in the basal membrane. The model could also account for the differential effect of mucosal amiloride on Na⁺ salts of various anions, in which the neural taste responses to Na⁺ salts with anions larger than Cl^- are more sensitive to mucosal amiloride than is the taste response to NaCl. 2. Voltage-clamp data were obtained from an in vitro preparation of the anterior-dorsal rat tongue epithelium in which the connective tissue was removed by enzyme digestion. I(sc) in a modified Ussing chamber was reduced by amiloride in the submucosal solution. 3. The pattern of sensitivity to submucosal amiloride differed in several respects from the pattern for mucosal amiloride. The inhibition constant (K(i)) was 52 μM amiloride concentration, higher than for the apical amiloride-sensitive Na⁺ channel. The selectivity for Na⁺ over K⁺ was much less than for the response to mucosal amiloride; with 0.5 M NaCl or KCl on the mucosal side, the ratio of inhibition for the NaCl response to inhibition for the KCl response varied between 1 and 3. 4. As the concentration of NaCl in the mucosal solution was varied, submucosal amiloride caused little inhibition of I(sc) for mucosal NaCl below isosmotic concentration, with the percent inhibition increasing as mucosal salt concentration increased. With 0.5 M sodium gluconate in the mucosal solution, there was very little inhibition due to submucosal amiloride. 5. The results support the presence of amiloride-sensitive Na⁺ channels in the basolateral membranes of the dorsal tongue epithelium in rat, and are consistent with the proposed model in which these channels are present in taste cells.