β1 increases the Ca²⁺ dependence of Popen at a constant voltage. (A and B) Outward K⁺ currents recorded at 0 mV and filtered at 2 kHz from macropatches containing ΔE channels (A) or ΔE + β1 channels (B) exposed to the indicated [Ca²⁺]. These currents demonstrate that Popen increases in a Ca²⁺-dependent manner when voltage is constant. A comparison of the dose–response relations for the effect of Ca²⁺ on Popen at 0 mV in the absence or presence of β1 is show. Each point represents the average of between 7 and 30 patches at each [Ca²⁺] tested. Error bars represent SEM. The Popen curves are fitted with Eq. 1 yielding values of: ΔE, K_C1 = 4.3 µM, K_O1 = 1.1 µM, K_C2 = 7.9 µM, K_O2 = 1.3 µM, M = 4.7 × 10⁻⁶; ΔE + β1, K_C1 = 2.2 µM, K_O1 = 0.5 µM, K_C2 = 32 µM, K_O2 = 0.6 µM, M = 2.0 × 10⁻⁸. Additionally, ΔE was fitted assuming that the two types of binding sites are equivalent. The fit yield values of K_C = 5.9 µM, K_O = 1.2 µM, and M = 4.2 × 10⁻⁶.