W. Jonathan Lederer

Ca2+‐dependent components of inactivation of unitary cardiac L‐type Ca2+ channels

A Ca2+ ion‐dependent inactivation (CDI) of L‐type Ca2+ channels (LCC) is vital in limiting and shaping local Ca2+ ion signalling in a variety of excitable cell types. However, under physiological conditions the unitary LCC properties that underlie macroscopic inactivation are unclear. Towards this end, we have probed the gating kinetics of individual cardiac LCCs recorded with a physiological Ca2+ ion concentration (2 mm) permeating the channel, and in the absence of channel agonists. Upon depolarization the ensemble‐averaged LCC current decayed with a fast and a slow exponential component. We analysed the unitary behaviour responsible for this biphasic decay by means of a novel kinetic dissection of LCC gating parameters. We found that inactivation was caused by a rapid decrease in the frequency of LCC reopening, and a slower decline in mean open time of the LCC. In contrast, with barium ions permeating the channel ensemble‐averaged currents displayed only a single, slow exponential decay and little time dependence of the LCC open time. Our results demonstrate that the fast and slow phases of macroscopic inactivation reflect the distinct time courses for the decline in the frequency of LCC reopening and the open dwell time, both of which are modulated by Ca2+ influx. Analysis of the evolution of CDI in individual LCC episodes was employed to examine the stochastic nature of the underlying molecular switch, and revealed that influx on the order of a thousand Ca2+ ions may be sufficient to trigger CDI. This is the first study to characterize both the unitary kinetics and the stoichiometry of CDI of LCCs with a physiological Ca2+ concentration. These novel findings may provide a basis for understanding the mechanisms regulating unitary LCC gating, which is a pivotal element in the local control of Ca2+‐dependent signalling processes.

Physiologic gating properties of unitary cardiac L-type Ca2+ channels

The contraction of adult mammalian ventricular cardiomyocytes is triggered by the influx of Ca(2+) ions through sarcolemmal L-type Ca(2+) channels (LCCs). However, the gating properties of unitary LCCs under physiologic conditions have remained elusive. Towards this end, we investigated the voltage-dependence of the gating kinetics of unitary LCCs, with a physiologic concentration of Ca(2+) ions permeating the channel. Unitary LCC currents were recorded with 2mM external Ca(2+) ions (in the absence of LCC agonists), using cell-attached patches on K-depolarized adult rat ventricular myocytes. The voltage-dependence of the peak probability of channel opening (Po vs. Vm) displayed a maximum value of 0.3, a midpoint of -12 mV, and a slope factor of 8.5. The maximum value for Po of the unitary LCC was significantly higher than previously assumed, under physiologic conditions. We also found that the mean open dwell time of the unitary LCC increased twofold with depolarization, ranging from 0.53+/-0.02 ms at -30 mV to 1.08+/-0.03 ms at 0 mV. The increase in mean LCC open time with depolarization counterbalanced the decrease in the single LCC current amplitude; the latter due to the decrease in driving force for Ca(2+) ion entry. Thus, the average amount of Ca(2+) ions entering through an individual LCC opening ( approximately 300-400 ions) remained relatively constant over this range of potentials. These novel results establish the voltage-dependence of unitary LCC gating kinetics using a physiologic Ca(2+) ion concentration. Moreover, they provide insight into local Ca(2+)-induced Ca(2+) release and a more accurate basis for mathematical modeling of excitation-contraction coupling in cardiac myocytes.

Letter Regarding Article by Darbar et al, “Unmasking of Brugada Syndrome by Lithium”

To the Editor: It was with great interest that we read the study by Darbar and colleagues in a recent issue of Circulation .1 The authors reported on 2 patients who developed the Brugada ECG pattern after administration of lithium, a commonly used drug not previously reported to block cardiac Na+ channels. Surprisingly, Darbar and colleagues found that LiCl caused a concentration-dependent block of peak INa, with an IC50 of 6.8±0.4 μmol/L, a level 100 times below the therapeutic range (&1 mmol/L). They concluded that lithium is a potent blocker of cardiac Na+ channels and may unmask patients with the Brugada syndrome. It is well known from numerous studies (conducted during the last 3 decades) that Li+ ions permeate Na+ channels, as well as do Na+ ions, in a variety of excitable cell …