Abstract: | Unitary discharges from single water fibers of the frog glossopharyngealnerve, caused by stimulation with 0.025 mM CaSO4, wererecorded from fungiform papillae with a suction electrode. NiSO4at concentrations of 0.22 mM, namely, at concentrationsthat are barely effective in producing impulses, had a dualaction on the Ca2+ response: NiSO4 caused both inhibition andenhancement of the Ca2+ response. In the present study, thisdual action of Ni2+ ions on the Ca2+ response was investigatedin detail. Single water fibers yielded a saturation type ofconcentration-response curve for CaSO4, which suggested thatsulfateions do not affect the Ca2+ response. Thus, sulfateswere used as test salts in the present study. At low concentrationsof Ca2+ ions, Ni2+ ions inhibited the Ca2+ response, but athigher concentrations of Co2+ ions they enhanced it. The resultscan be explained quantitatively by the hypothesis that Ni2+ions inhibit the Ca2+ response by competing with Ca2+ ions forthe Ca2+ receptor (Xca) that is responsible for the Ca2+ responseand that Ni2+ ions enhance the Ca2+ response by acting on amembrane element that interacts with Xca. Double-reciprocalplots of the data indicate that the enhancing action of Ni2+ions is saturated at 12 mM Ni2+ ions and that Ni2+ ionsat these concentrations increase the maximal response of theCa2+ response by 182%. Dissociation constants for the Ca-Xcacomplex and the Ni-Xca, complex were 4.2 x 105 M and7.6 x 105 M, respectively. The analysis suggests thatNi2+ ions enhance the Ca2+ response by affecting the Ca-Xcacomplex without altering the affinity of Xca, for Ca2+ ions. |