Requirement for a functional proximal C-terminal EF-hand for the effects of the distal C terminus on Ca_V1.2 current. Effect of coexpression of distal_1801–2171 on mean normalized Na⁺ currents resulting from expression of (A) Ca_V1.2Δ1800 or (B) Ca_V1.2Δ1800 containing mutation D1546R in the EF-hand (Ca_V1.2Δ1800(D1546R)). n = 15, 17, 10, and 15 for Ca_V1.2Δ1800(D1546R), Ca_V1.2Δ1800(D1546R) plus distal_1801–2171, Ca_V1.2Δ1800, and Ca_V1.2Δ1800 plus distal_1801–2171, respectively. Mg_i was 0.26 mM. (C) Mean coupling ratios measured as the ratio of peak tail current at −40 mV (nA) after a depolarization to the reversal potential and gating charge movement measured at the reversal potential (pC). Gating charge movement was measured by applying a series of test pulses from the HP of −80 mV to potentials between +60 and 80 mV in 2-mV increments and integrating the gating charge movement at the reversal potential for the ionic current (Hulme et al., 2006). n = 10, 9, 11, and 8. (D) Normalized conductance–voltage relationships. Peak tail currents were measured upon repolarization to −40 mV after 20-ms depolarization to potentials between −40 and 200 mV. Ba²⁺ was used as charge carrier, and Mg_i was 0.8 mM for the experiments in C and D.