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 glossopharyngeal nerve, caused by stimulation with 0.02-5 mM CaSO4, were recorded from fungiform papillae with a suction electrode. NiSO4 at concentrations of 0.2-2 mM, namely, at concentrations that are barely effective in producing impulses, had a dual action on the Ca2+ response: NiSO4 caused both inhibition and enhancement of the Ca2+ response. In the present study, this dual action of Ni2+ ions on the Ca2+ response was investigated in detail. Single water fibers yielded a saturation type of concentration-response curve for CaSO4, which suggested that sulfate ions do not affect the Ca2+ response. Thus, sulfates were used as test salts in the present study. At low concentrations of Ca2+ ions, Ni2+ ions inhibited the Ca2+ response, but at higher concentrations of Ca2+ ions 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 (XCa) that is responsible for the Ca2+ response and that Ni2+ ions enhance the Ca2+ response by acting on a membrane element that interacts with XCa. Double-reciprocal plots of the data indicate that the enhancing action of Ni2+ ions is saturated at 1-2 mM Ni2+ ions and that Ni2+ ions at these concentrations increase the maximal response of the Ca2+ response by 182%. Dissociation constants for the Ca-XCa complex and the Ni-XCa complex were 4.2 x 10(-5) M and 7.6 x 10(-5) M, respectively. The analysis suggests that Ni2+ ions enhance the Ca2+ response by affecting the Ca-XCa complex without altering the affinity of XCa for Ca2+ ions.