Figure 6. Biophysical properties of Ca_v1.4^wt and Ca_v1.4^nob2 channels, coexpressed with β_2a and α₂–δ₁ subunits in tSA-201 cells.

A. Representative current waveforms for Ca_v1.4^wt (left) and Ca_v1.4^nob2 (right) recorded with 20 mM Ba²⁺ external saline. Horizontal scale bars denote 25 ms, and vertical scale bars 25 pA. B. Average activation (filled symbols) and inactivation (hollow) symbols for Ca_v1.4^wt (squares) and Ca_v1.4^nob2 (circles) recorded with 20 mM Ba²⁺ external saline. Average activation parameters from 11 Ca_v1.4^wt cells and 13 Ca_v1.4^nob2 cells are: V_{act, wt} = −3±4 mV, V_{act, nob2} = −1±4 mV (n = 13); G_{max, wt} = 4±3 nS and G_{max, nob2} = 3±1 nS; S_wt = 9±1 mV and S_nob2 = 8.2±0.8 mV. These values are stastically identical, and are summarized in Table 3. Average inactivation parameters from these cells are V_{inact, wt} = −18±11 mV and V_{inact, nob2} = −22±10 mV, with a large fraction of non-inactivating current for both channels. These values statistically identical and are summarized in Table 3. C. Average half-inactivation potentials for channels recorded with 2 mM Ca²⁺ as charge carrier. Currents were substantially smaller than with 20 mM Ba²⁺, but were distinguishable from background noise, and were obtained using a ramp protocol identical to that previously reported [31], obtained by ramping voltage from −100 mV to +100 mV over 500 ms. Values were V_{act, wt} = −17±8 mV (average peak current size −9±4 pA) and V_{act, nob2} = −17±6 mV (average peak current size −9±3 pA). The shift observed with switching from 20 mM Ba²⁺ to 2 mM Ca²⁺ as external charge carrier is similar to that we have previously reported for the human Ca_v1.4 channels [16].