Enhancing effect of nickel ions on the response to magnesium ions of single fibers of the frog glossopharyngeal nerve: Competitive inhibition by calcium ions of the nickel-enhanced response to magnesium ions

Single fibers of the frog glossopharyngeal nerve respond to MgCl₂ at concentrations exceeding 10 mM. NiCl₂ at 1 mM enhanced the Mg²⁺ response. CaCl₂ at 0.5-2 mM induced an inhibition of the Ni²⁺-enhanced response to Mg²⁺ ions. A quantitative explanation for these results is provided by the hypothesis that Ni²⁺ ions secondarily affect a magnesium receptor (designated X(*)(Mg)) that is responsible for the Mg²⁺ response and that Ca²⁺ ions inhibit the Ni²⁺-enhanced response to Mg²⁺ ions by competing with Mg²⁺ ions for X(*)(Mg). Double-reciprocal plots of the experimental data indicate that Ni²⁺ ions do not affect the affinities of X(*)(Mg) for both Mg²⁺ ions (agonist) and Ca²⁺ ions (competitive antagonist) appreciably, and that Ni²⁺ ions at 1 mM enhanced the maximal response to Mg²⁺ ions by 270%. It appears that a magnesium receptor interacts with an Ni²⁺-binding element that is affected by Ni²⁺ ions and, thus, Ni²⁺ ions can induce an enhancement of the Mg²⁺ response.