Enhancing effects of binary mixtures of acid with salt on the gustatory neural activity in the clawed toad, Xenopus...

Binary mixtures of acid with salt were observed to produce enhancing effects on oral chemoreceptor responses by recording the activity of the whole glossopharyngeal nerve of the clawed toad (Xenopus laevis). The mixtures of HCl with various inorganic salts elicited responses larger than the sum of the responses to individual component chemicals of each mixture. The results indicated that the mixture response was composed of a large HCI response and a negligibly small salt response. The concentration-response curve for HCl shifted toward lower concentrations due to the presence of NaCl without affecting the maximal response, and the Hill constant for the curve was unchanged, suggesting that the binding affinity of protons increases with an increase in the concentration of the mixed NaCl. The enhancing effects of the mixtures of HCl and various monovalent sodium salts having equal ionic strength showed distinct differences among the anion species of the salts. The responses to the mixtures of HCl with typical chloride salts having cations of various valencies depended on the Cl- ion concentration up to 0.1 M, whereas they depended on ionic strength as well, suggesting a possible role of salt cations in the enhancing effects of the mixtures. Thus, the enhancing effects of salts on the acid responses are interpreted mostly in terms of the interaction of the salt anions on the receptor membrane around the receptor sites for acid. The additive role of the cationic activity of the mixed salts, however, is also involved.