Coupling block of closed and open states to voltage sensor movement improves fit of GV curves at both 4 and 300 µM Ca²⁺. (A) GV curves were generated from steady-state currents in a set of patches in which the blocking effects of 0, 1, 5, and 20 µM bbTBA were examined at both 4 and 300 µM. The full set of GV curves was then fit to Eq. 13, corresponding to a block with a completely state-independent blocking scheme (2a) for channels activated in accordance with a full allosteric activation scheme (Horrigan and Aldrich, 2002). Scheme 2a provides a poor fit (SSQ = 0.0417/pt) to the GV curves, particularly at 20 µM bbTBA and 300 µM Ca²⁺ (blue circle). K_bc = K_bo = 7.48 ± 0.43 µM (z_c = z_o = 0.17 ± 0.02 e). (B) GV curves were fit with the open-channel block model (Eq. 12; Scheme 1a). K_bo = 5.94 ± 0.39 µM (z_o = 0.13 ± 0.02 e); SSQ/pt = 0.0525. Deviations between the data and the fit are particularly apparent at 20 µM bbTBA and 4 µM Ca²⁺ (blue circle). (C) The GV curves were fit with Eq. 14 for Scheme 3a, in which block depends on movement of at least one voltage sensor. K_bo = K_bc = 6.36 ± 0.27 µM (z_o = z_c = 0.14 ± 0.01 e), with SSQ/pt = 0.0219. For D–I, fitted curves correspond to schemes in which K_bo ≠ K_bc. In D and E, both z_o and z_c varied independently, whereas in F and G, z_c = 0.0 e. (D) For Scheme 2′, the best fit to the GV curves yielded K_bo = 6.00 ± 0.3 µM, z_o = 0.095 ± 0.02 e, K_bc = 45.8 ± 35.9 µM, and z_c = 0.81 ± 0.21 e, with SSQ = 0.0140/pt. (E) For Scheme 3′, K_bo = 6.01 ± 0.27 µM, z_o = 0.10 ± 0.02 e, K_bc = 8.1 ± 6.7 µM, and z_c = 0.44 ± 0.23 e, with SSQ = 0.0131/pt. (F) For Scheme 2b′, with z_c constrained to 0 e, K_bo = 6.36 ± 0.45 µM, z_o = 0.15 ± 0.02 e, and K_bc = 21.4 ± 12.2, with SSQ = 0.0453/pt. The blue circles highlight the poorly fit region. (G) For Scheme 3b′, z_c = 0 e, K_bo = 6.48 ± 0.29 µM, z_o = 0.13 ± 0.01 e, and K_bc = 2.34 ± 0.38, with SSQ = 0.0192/pt.