Enhancing effects of transition metals on the salt taste responses of single fibers of the frog glossopharyngeal nerve: specificity of and similarities among Ca2+, Mg2+ and Na+ taste responses.

Fibers of the frog glossopharyngeal nerve (water fibers) that are sensitive to water also respond to CaCl2, MgCl2 and NaCl. In the present study, interaction among cations (Ca2+, Mg2+ and Na+) on taste cell membrane in frogs was studied using transition metals (NiCl2, CoCl2 and MnCl2), which themselves are barely effective in producing neural response at concentrations below 5 mM. Unitary discharges from single water fibers were recorded from fungiform papillae with suction electrode. Transition metal ions (0.05-5.0 mM) had exclusively enhancing effects on the responses to 50 mM Ca2+, 100 mM Mg2+ and 500 mM Na+. The effects of transition metal ions were always reversible. The rank order of effectiveness of transition metals at 1 mM in the enhancement of the responses to 50 mM CaCl2, 100 mM MgCl2 and 500 mM NaCl was NiCl2 > CoCl2 > MnCl2. The concentration of transition metal ions effective to enhance salt response was almost the same among Ca2+, Mg2+ and Na+ responses. The results suggest that a common mechanism is involved in the enhancement of Ca2+, Mg2+ and Na+ taste responses. The enhanced Mg2+ response and the enhanced Na+ response were greatly inhibited by the addition of Ca2+ ions, and the enhanced Ca2+ response was inhibited by the addition of Mg2+ or Na+ ions, suggesting that competitive antagonism occurs between Ca2+ and Mg2+ ions and between Ca2+ and Na+ ions in the presence of Ni2+ ions. Ni2+ ions had a dual effect on the Ca2+ response induced by low concentration (0.1 mM) of CaCl2: enhancement at lower concentrations (0.02-0.1 mM) of NiCl2 and inhibition at higher concentrations (0.5-5 mM) of NiCl2. The present results suggest that transition metal ions do not affect the receptor-antagonist complex, but affect only the receptor-agonist complex.