A QUANTITATIVE STUDY OF DUAL-ACTION OF NICKEL IONS ON THE TASTE RESPONSE TO CALCIUM-IONS OF SINGLE FIBERS OF THE FROG GLOSSOPHARYNGEAL NERVE - INHIBITION AND ENHANCEMENT BY NICKEL IONS

Unitary discharges from single water fibers of the frog glossopharynge al nerve, caused by stimulation with 0.02-5 mM CaSO4, were recorded fr om fungiform papillae with a suction electrode. NiSO4 at concentration s of 0.2-2 mM, namely, at concentrations that are barely effective in producing impulses, had a dual action on the Ca2+ response: NiSO4 caus ed both inhibition and enhancement of the Ca2+ response. In the presen t study, this dual action of Ni2+ ions on the Ca2+ response was invest igated in detail. Single water fibers yielded a saturation type of con centration-response curve for CaSO4, which suggested that sulfate ions do not affect the Ca2+ response. Thus, sulfates were used as test sal ts in the present study. At low concentrations of Ca2+ ions, Ni2+ ions inhibited the Ca2+ response, but at higher concentrations of Ca2+ ion s they enhanced it. The results can be explained quantitatively by the hypothesis that Ni2+ ions inhibit the Ca2+ response by competing with Ca2+ ions for the Ca2+ receptor (X(Ca)) that is responsible for the C a2+ response and that Ni2+ ions enhance the Ca2+ response by acting on a membrane element that interacts with X(Ca). Double-reciprocal plots of the data indicate that the enhancing action of Ni2+ ions is satura ted at 1-2 mM Ni2+ ions and that Ni2+ ions at these concentrations inc rease the maximal response of the Ca2+ response by 182%. Dissociation constants for the Ca-X(Ca) complex and the Ni-X(Ca) complex were 4.2 x 10(-5) M and 7.6 x 10(-5) M, respectively. The analysis suggests that Ni2+ ions enhance the Ca(2+)response by affecting the Ca-X(Ca) comple x without altering the affinity of X(Ca) for Ca2+ ions.