Identification of Gating Modes in Single Native Na ⁺ Channels From Human Atrium and Ventricle

Abstract

Abstract-The aim of the present study was to investigate the single-channel properties of different gating modes in the native human cardiac Na ϩ channel. Patch-clamp experiments were performed at low noise using ultrathick-walled pipettes. In 17 cell-attached patches containing only one channel, fast back and forth switching between five different Na ϩ -channel gating modes (F-mode, M1-mode, M2-mode, S-mode, and P-mode) was identified, but no difference in the gating properties was found between normal and diseased cardiomyocytes from atrium or ventricle, respectively. Hodgkin-Huxley fits to the ensemble-averaged currents yielded the activation-time ( m ) and inactivation-time ( h ) constants. m was comparably fast in the F-mode, M1-mode, M2-mode, and S-mode (0.15 ms) and slow in the P-mode (0.3 ms). h ranged from 0.35 ms (F-mode) to 4.5 ms (S-mode and P-mode). The mean open-channel lifetime ( o ) was shortest in the F-mode and P-mode (0.15 ms) and longest in the S-mode (1.25 ms). The time before which half of the first channel openings occurred (t 0.5 ) was comparably short in the F-mode, M1-mode, M2-mode, and S-mode (0.3 ms) and long in the P-mode (0.9 ms). It is concluded that (1) a single native human cardiac Na ϩ channel can be recorded at low noise, (2) this channel can change its gating properties at a time scale of milliseconds, (3) lifetimes of the observed gating modes are short ranging from milliseconds to seconds only, and (4) Key Words: patch clamp Ⅲ human myocardial sodium channels Ⅲ modal gating Ⅲ channel activation Ⅲ channel inactivation A lthough Na ϩ channels have been subject to intense electrophysiological investigation in various tissues, 1 little is known about the native human cardiac Na ϩ channel. Previously published studies have been conducted in heterologous expression systems, where channel proteins are removed from their physiological environment. [2][3][4] In these experiments, the Na ϩ channel ␣-subunit (hH1) was expressed alone, or coexpression with the ␤ 1 -subunit was performed. Other methods used the reconstituted Na ϩ -channel protein fused into planar lipid bilayers. 5 A major disadvantage of these methods is the artificial environment, in which electrophysiological experiments on the channel were performed. In the present study, results on single native human cardiac Na ϩ channels are presented. Our approach may lead to important insights into the physiological function of this channel protein.Using this approach, arrhythmias may be understood in more detail. It has been shown that the LQT3 syndrome and the Brugada syndrome are based on different mutations in the gene (SCN5A, 3p21) encoding for hH1. 6 -8 In heterologous expression systems, a distinct mutation (⌬KPQ) underlying the LQT3 syndrome is associated with late Na ϩ currents. 9 From fits of the results of these experiments to different kinetic models, it was postulated that modal gating of the channel protein may be the reason for the late currents. 9,10 Hence, genetic defects may underlie different rates of switch...

Omega-3 Fatty Acids and Cardiac Arrhythmias: Prior Studies and Recommendations for Future Research

“…63,156 Biophysical and pharmacological properties of the Na ϩ current may differ in the atria versus ventricles of the same mammalian heart, and computational studies have suggested that Na ϩ current inhibition may be able to reduce AF. [157][158][159] In future studies, it will be important to do the following:…”

Late sodium current in failing heart: Friend or foe?

“…Our results suggest that Na + channels can function in several different 'modes,' each with a different inactivation rate." As I NaL lasts hundreds of milliseconds, the numerous short-lived modes (a few milliseconds of membrane depolarization) identified by further patch clamp studies in myocardial cells of a variety of species (mice (Bohle and Benndorf, 1995), guinea pig (Nilius, 1988) and humans (Bohle et al, 2002)) contribute to I NaT rather than I NaL . More specifically, all 5 gating modes of Na + channel (F, M1, M2, S, and P-mode) identified by Böhle et al (Bohle et al, 2002) in human VCs operate within 5 ms of membrane depolarization and thus have no contribution to I NaL .…”

Letter Regarding Article by Michels et al, “Single-Channel Properties Support a Potential Contribution of Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels and I f to Cardiac Arrhythmias”

“…Thus, it remains unclear whether f-channels could exhibit 2 distinct conductances (ie, long, low conductance openings and short, higher conductance openings), a hypothesis we are investigating using low-noise recording techniques. 6 Although we did not obtain any shift of the baseline during our 3-s pulses, our experiments cannot rule out a small current component contributed by low conductance channels. Interestingly, using recording techniques similar to ours, an insect homolog of HCN channels (HvCNG) recently displayed a similar conductance of 30 pS.…”