Consistent with previously reported values for L-type Ca2+ channels (e.g., [19], [44] [50]), the peak mean open probability of channel openings during 5 s test pulses averaged 0.23±0.03 (N = 13) and the slope conductance determined from channel currents measured at three different test potentials averaged 19.8±3.9 pS (N = 6).
Peak ensemble average current (Ipeak) in old tg CaV1.2 mice was enhanced (−56±14 fA vs. −23±7 fA, p<0.05) due to an increased fraction of active sweeps, mean open time, and mean open probability, and a decrease of mean closed time (tclosed).
From this analysis, we hypothesize that ranolazine acts as an open pore blocker, reducing the mean open probability at 30 μM to 63% of initial values, from 0.6 to 0.38, without lowering the number of active channels, while veratridine 1 μM increases the number of active channels in late gating mode 2.5-fold, without influencing their mean open probability.
Using fluctuation analysis, we observed that ranolazine 30 μM decreased mean open probability p from 0.6 to 0.38 without modifying the number of active channels n, while veratridine 1 μM increased n 2.5-fold without changing p. In human iPSC-derived cardiomyocytes, veratridine 1 μM reversibly increased APD90 2.12 ± 0.41-fold (mean ± SEM, n = 6).
In doing so, we could evaluate the mean open probability p and further the average number of Na + channels n open in the late mode in selected traces, over the last 100 ms of activity during the 300-ms depolarizing pulses.
GluR1-S845 phosphorylatincreasesases the mean open probability of the channel [42], hence could explain the increase in mEPSC amplitude in GluR1-S831A mutants.
Idealization of the currents was performed at a bandwith of 10 kHz using the segmentation k-means hidden Markov algorithm (Qin 2004) with a C↔O model (both rate constants = 100 s−1) or by a half-amplitude threshold-crossing algorithm after additional low-pass filtering to 3 kHz to obtain single channel open amplitude, open probability, and mean open and close times.
It has been proposed that RyR3 may be preferentially targeted to the SR within these junctions because, compared with RyR1 and RyR2, RyR3 exhibits a relatively high EC50 for CICR, a marked Ca2+-dependent increase in gain (open probability, Po + mean open time) and a greater resistance to inactivation by high (mM) [Ca2+] (for detailed discussion see Ref. 48).
The intraburst open time distributions obtained from both α1β2γ2- and α3β2γ2-mediated single-channel currents were best fitted by two exponential probability density functions and, interestingly, the time constants together with respective percentages and mean open time found for α1β2γ2 receptors were indistinguishable from those observed for α3β2γ2 receptors (Fig. 6C).
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