A quantitative study of the enhancing effect of nickel ions on the taste response to sodium ions of single fibers of the frog glossopharyngeal nerve: Competitive inhibition by calcium ions of the nickel-enhanced response to sodium ions

Single water fibers of the frog glossopharyngeal nerve respond to relatively high concentrations of NaCl (>80 mM). NiCl₂ at 1 mM enhanced the Na⁺ response and reduced the threshold concentration for NaCl to 20 mM. CaCl₂ at 0.5-1 mM induced an inhibition of the Ni²⁺-enhanced response to Na⁺ ions. A quantitative explanations for these results is provided by the hypothesis that Ni²⁺ ions secondarily affect a sodium receptor or channel (designated X_Na*) that is responsible for the Na⁺ response and that Ca²⁺ ions inhibit the Ni²⁺-enhanced response to Na⁺ ions by competing with Na⁺ ions for X_Na*. Double-reciprocal plots of the experimental data indicate that the affinity of X_Na* for both Na⁺ ions (agonist) and Ca²⁺ ions (competitive antagonist) in the presence of 1 mM NiCl₂ was five times higher than the previously reported values obtained in the absence of NiCl₂ (Kitada, 1991). Ni²⁺ ions at 1 mM enhanced the maximal response to Na⁺ ions by 190%. It appears that a sodium receptor (or channel) interacts with a Ni²⁺-binding element that is affected by Ni²⁺ ions and, thus, Ni²⁺ ions can induce both an increase in the affinity of the sodium receptor for the respective cations and an enhancement of the Na⁺ response.