Electrophysiological properties of the tongue epithelium of the toad Bufo marinus

Abstract

SUMMARY The dorsal lingual epithelium from the tongue of the toad Bufo marinus was mounted in an Ussing-type chamber, and the short-circuit current (Isc) was measured using a low-noise voltage clamp. With NaCl Ringer bathing the mucosal and serosal surfaces of the isolated tissue, an outwardly directed (mucosa-positive) Isc was measured that averaged -10.71±0.82 μA cm-2 (mean ± S.E.M., N=24) with a resistance of 615±152 Ω cm2 (mean ± S.E.M., N=10). Substitution of chloride with sulfate as the anion produced no significant change in Isc. Fluctuation analysis with either NaCl or Na2SO4 Ringer bathing both sides of the tissue revealed a spontaneous Lorentzian component, suggesting that the Isc was the result of K+ secretion through spontaneously fluctuating channels in the apical membrane of the epithelium. This hypothesis was supported by the reversible inhibition of Isc by Ba2+ added to the mucosal Ringer. Analysis of the kinetics of Ba2+ inhibition of Isc indicates that there might be more than one type of K+ channel carrying the Isc. This hypothesis was supported by power spectra obtained with a serosa-to-mucosa K+gradient, which could be fitted to two Lorentzian components. At present, the K+ secretory current cannot be localized to taste cells or other cells that might be associated with the secretion of saliva or mucus. Nonetheless, the resulting increase in [K+] in fluid bathing the mucosal surface of the tongue could presumably affect the sensitivity of the taste cells. These results contrast with those from the mammalian tongue, in which a mucosa-negative Isc results from amiloride-sensitive Na+ transport.