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 Ni(2+)-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 Ni(2+)-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 Ni(2+)-binding element that is affected by Ni2+ ions and, thus, Ni2+ ions can induce an enhancement of the Mg2+ response.