Fig. 5.

The effect of calcium on voltage gating. Average (n > 10) half-activation voltages (V_1/2) were estimated from Boltzmann fits to G–V relationships for different cytoplasmic calcium concentrations (Figs. 3, 4). Standard error bars are comparable to the size of the symbols used. The line is a least squares fit to (V_1/2 = A * log[Ca²⁺] + B). α₀β showed a higher steady-state affinity and a greater sensitivity to calcium than did α₀. B,G/G_max (as measured in Figs.3, 4) for α₀ and α₀β at −50 mV is plotted as a function of the calcium concentration. The results are from a single experiment in which all calcium conditions were used. Six different concentrations of calcium were used (200, 1, 5, 25, 50, and 200 μm). The points were fit with a Hill equation (Eq. 5) to obtain the concentration of calcium necessary to open half of the channels (K_D) and the Hill coefficient (n). C, Same analysis at +50 mV. The effect of β addition was greater at −50 mV than at +50 mV.