Figure 4. Ca²⁺ sensitivities of different splice variants.

A, voltage activation of current (I) of 4:61 α-subunit channels derived from tail current measurements at -80 mV. Each set of points, corresponding to a different Ca²⁺ concentration, was fitted with a Boltzmann relationship: I/I_max= 1/(1 + exp(V_½ - V)/V_ε), where I_max= 3.7 nA. Ca²⁺ concentrations (in μM) for each symbol are indicated. B, V_½ values derived from results similar to those in A plotted against Ca²⁺ concentration for different α-subunit isoforms. C, voltage activation of 4:-26 α-subunit and 4:-26 α+β-subunit (0.1 β/α mole ratio) channels derived from tail current measurements in 5 μM Ca²⁺; results were fitted with Boltzmann relationships as in A. D, effects of β-subunit on the V_½versus Ca²⁺ concentration plot for the 4:-26 and 31:3 variants.