Effects of Ca_Vβ subunits and Mg_i on VDI of Ca_V1.2 with Na⁺ as charge carrier. (A and B) Comparison of VDI of mouse ventricular myocyte Ca_V1.2 current (I_Ca,L) and current due to Ca_V1.2 expressed in tsA-201 cells I_CaV1.2. (A) Normalized and averaged current traces with Na⁺ as charge carrier. (B) Mean r₃₀₀ for I_Ca,L (n = 4) and I_CaV1.2 (n = 7). Mg_i was 0.26 mM. (C and D) Ca_V1.2 and the indicated Ca_Vβ subunits expressed in tsA-201 cells and studied with high (2.4 mM) Mg_i. (C) Mean normalized current traces in response to 1,000-ms depolarizations to 0 mV. (D) Mean r₁₀₀₀ for Ca_V1.2 expressed with Ca_Vβ_1b (n = 15) and Ca_Vβ_2a (n = 6). (E–H) Ca_V1.2 and the indicated Ca_Vβ subunits expressed in tsA-201 cells at low (0.26 mM) Mg_i with pulse durations of 300 ms (E and F) and 1,000 ms (G and H). (E and G) Mean normalized currents in response to depolarizations to 0 mV. (E) Mean r₃₀₀ for Ca_V1.2 expressed with the indicated β subunits. Ca_Vβ_2a: n = 16; Ca_Vβ_2b: n = 10. Data from ventricular myocytes I_Ca,L(Na) are replotted for comparison. (H) Mean r₁₀₀₀ for Ca_V1.2 expressed with Ca_Vβ_1b, Ca_Vβ_2a, or Ca_Vβ_2b.