Abstract: | Single water fibers of the frog glossopharyngeal nerve respondto relatively high concentrations of NaCl (>80 mM). NiCl2at 1 mM enhanced the Na+ response and reduced the thresholdconcentration for NaCl to 20 mM. CaCl2 at 0.51 mM inducedan inhibition of the Ni2+-enhanced response to Na+ ions. A quantitativeexplanations for these results is provided by the hypothesisthat Ni2+ ions secondarily affect a sodium receptor or channel(designated XNa*) that is responsible for the Na+ response andthat Ca2+ ions inhibit the Ni2+-enhanced response to Na+ ionsby competing with Na+ ions for XNa*. Double-reciprocal plotsof the experimental data indicate that the affinity of XNa*for both Na+ ions (agonist) and Ca2+ ions (competitive antagonist)in the presence of 1 mM NiCl2 was five times higher than thepreviously reported values obtained in the absence of NiCl2(Kitada, 1991). Ni2+ ions at 1 mM enhanced the maximal responseto 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 theaffinity of the sodium receptor for the respective cations andan enhancement of the Na+ response. Chem Senses 21: 6573,1996. |