Transient transfection and electrophysiological recordings
Culturing of HEK293T cells was performed as described (Scholl et al., 2013). Cells were transfected with 3 µg of CACNA1HWT or CACNA1HM1549V expression plasmids. For each construct, two clones were functionally tested. Whole cell patch clamp recordings were performed on a HEKA EPC10 amplifier (HEKA Elektronik, Ludwigshafen, Germany) as described previously (Scholl et al., 2013). The extracellular solution contained: 5 mM CaCl2, 125 mM TEA-Cl, 10 mM HEPES, 15 mM Mannitol, pH 7.4. Pipette solution contained: 100 mM CsCl, 5 mM TEA-Cl, 3.6 mM PCr-Na2, 10 mM EGTA, 5 mM Mg-ATP, 0.2 mM Na-GTP, 10 mM HEPES, pH 7.4 (titration with CsOH).
Voltage dependences of activation were determined from the peak current–voltage relation and fit by a Boltzmann function as described (Marcantoni et al., 2010; Scholl et al., 2013). The fraction of non-inactivated channels was determined by dividing the peak amplitude at −20 mV before and after 5 s long pulses to voltages between −90 and −20 mV. Time courses of activation or inactivation were analyzed by fitting a mono-exponential function (Scholl et al., 2013). The recovery from inactivation was measured using envelope protocols consisting of an inactivation of channels during a 5 s pulse to −20 mV followed by holding the membrane potential at −90 for increasing durations (Coulter et al., 1989). Afterwards, peak currents at −20 mV were measured and divided by the previous peak current. A plot of these ratios vs the duration of the pulse to −90 mV was fit with a mono-exponential function to obtain time constants for the recovery from inactivation.
Non-stationary noise analysis was performed as described (Hebeisen and Fahlke, 2005) using a voltage protocol that activates channels at −20 mV followed by the analysis of the decay of currents and variance at −90 mV. The initial variance at the holding potential of −90 mV before activation was regarded as background variance and subtracted from the recordings. The Lorentzian noise produced by channel opening and closing depends on the unitary current amplitude (i), the number of channels (N), and the absolute open probability (P):(1) σ2=N·i2·p·(1−p).
Since the macroscopic current amplitude is given by(2) I=N·p·i,the variance-current relationship results in a quadratic distribution:(3) σ2=i·〈I〉−(〈I〉2N).
The single channel amplitude (i) was derived from the initial slope of a plot of the variance against the mean isochronal current results. Due to a low open probability (p < 0.5) at 5 mM of external Ca2+, the recorded data points only described a small part of the usual parabola and did not allow for determination of the number of channels and open probabilities.
Data were analyzed in FitMaster (HEKA Elektronik), SigmaPlot (Jandel Scientific, San Rafael, CA) and Python. Statistical comparisons were performed using Student's t-test or Mann–Whitney rank sum test.