Recapitulation of an Ion Channel IV Curve Using Frequency Components

INTRODUCTION: Presently, there are no established methods to measure multiple ion channel types simultaneously and decompose the measured current into portions attributable to each channel type. This study demonstrates how impedance spectroscopy may be used to identify specific frequencies that highly correlate with the steady state current amplitude measured during voltage clamp experiments. The method involves inserting a noise function containing specific frequencies into the voltage step protocol. In the work presented, a model cell is used to demonstrate that no high correlations are introduced by the voltage clamp circuitry, and also that the noise function itself does not introduce any high correlations when no ion channels are present. This validation is necessary before the technique can be applied to preparations containing ion channels. The purpose of the protocol presented is to demonstrate how to characterize the frequency response of a single ion channel type to a noise function. Once specific frequencies have been identified in an individual channel type, they can be used to reproduce the steady state current voltage (IV) curve. Frequencies that highly correlate with one channel type and minimally correlate with other channel types may then be used to estimate the current contribution of multiple channel types measured simultaneously

Old cogs, new tricks: A scaffolding role for connexin43 and a junctional role for sodium channels?

AbstractCardiac conduction is the process by which electrical excitation is communicated from cell to cell within the heart, triggering synchronous contraction of the myocardium. The role of conduction defects in precipitating life-threatening arrhythmias in various disease states has spurred scientific interest in the phenomenon. While the understanding of conduction has evolved greatly over the last century, the process has largely been thought to occur via movement of charge between cells via gap junctions. However, it has long been hypothesized that electrical coupling between cardiac myocytes could also occur ephaptically, without direct transfer of ions between cells. This review will focus on recent insights into cardiac myocyte intercalated disk ultrastructure and their implications for conduction research, particularly the ephaptic coupling hypothesis

Electrophysiologic effects of the IK1 inhibitor PA-6 are modulated by extracellular potassium in isolated guinea pig hearts

The pentamidine analog PA‐6 was developed as a specific inward rectifier potassium current (I(K) (1)) antagonist, because established inhibitors either lack specificity or have side effects that prohibit their use in vivo. We previously demonstrated that BaCl(2), an established I(K) (1) inhibitor, could prolong action potential duration (APD) and increase cardiac conduction velocity (CV). However, few studies have addressed whether targeted I(K) (1) inhibition similarly affects ventricular electrophysiology. The aim of this study was to determine the effects of PA‐6 on cardiac repolarization and conduction in Langendorff‐perfused guinea pig hearts. PA‐6 (200 nm) or vehicle was perfused into ex‐vivo guinea pig hearts for 60 min. Hearts were optically mapped with di‐4‐ANEPPS to quantify CV and APD at 90% repolarization (APD (90)). Ventricular APD (90) was significantly prolonged in hearts treated with PA‐6 (115 ± 2% of baseline; P < 0.05), but not vehicle (105 ± 2% of baseline). PA‐6 slightly, but significantly, increased transverse CV by 7%. PA‐6 significantly prolonged APD (90) during hypokalemia (2 mmol/L [K+](o)), although to a lesser degree than observed at 4.56 mmol/L [K+](o). In contrast, the effect of PA‐6 on CV was more pronounced during hypokalemia, where transverse CV with PA‐6 (24 ± 2 cm/sec) was significantly faster than with vehicle (13 ± 3 cm/sec, P < 0.05). These results show that under normokalemic conditions, PA‐6 significantly prolonged APD (90), whereas its effect on CV was modest. During hypokalemia, PA‐6 prolonged APD (90) to a lesser degree, but profoundly increased CV. Thus, in intact guinea pig hearts, the electrophysiologic effects of the I(K) (1) inhibitor, PA‐6, are [K+](o)‐dependent

A Novel Frequency Analysis Method for Assessing Kir2.1 and Nav1.5 Currents

Voltage clamping is an important tool for measuring individual currents from an electrically active cell. However, it is difficult to isolate individual currents without pharmacological or voltage inhibition. Herein, we present a technique that involves inserting a noise function into a standard voltage step protocol, which allows one to characterize the unique frequency response of an ion channel at different step potentials. Specifically, we compute the fast Fourier transform for a family of current traces at different step potentials for the inward rectifying potassium channel, Kir2.1, and the channel encoding the cardiac fast sodium current, Nav1.5. Each individual frequency magnitude, as a function of voltage step, is correlated to the peak current produced by each channel. The correlation coefficient vs. frequency relationship reveals that these two channels are associated with some unique frequencies with high absolute correlation. The individual IV relationship can then be recreated using only the unique frequencies with magnitudes of high absolute correlation. Thus, this study demonstrates that ion channels may exhibit unique frequency responses

Cardiac dynamics: Alternans and arrhythmogenesis

published_or_final_versio

Cable properties and propagation velocity in a long single chain of simulated myocardial cells

<p>Abstract</p> <p>Background</p> <p>Propagation of simulated action potentials (APs) was previously studied in short single chains and in two-dimensional sheets of myocardial cells <abbrgrp><abbr bid="B1">1</abbr><abbr bid="B2">2</abbr><abbr bid="B3">3</abbr></abbrgrp>. The present study was undertaken to examine propagation in a long single chain of cells of various lengths, and with varying numbers of gap-junction (g-j) channels, and to compare propagation velocity with the cable properties such as the length constant (<it>λ</it>).</p> <p>Methods and Results</p> <p>Simulations were carried out using the PSpice program as previously described. When the electric field (EF) mechanism was dominant (0, 1, and 10 gj-channels), the longer the chain length, the faster the overall velocity (<it>θ</it>_ov). There seems to be no simple explanation for this phenomenon. In contrast, when the local-circuit current mechanism was dominant (100 gj-channels or more), <it>θ</it>_ovwas slightly slowed with lengthening of the chain. Increasing the number of gj-channels produced an increase in <it>θ</it>_ovand caused the firing order to become more uniform. The end-effect was more pronounced at longer chain lengths and at greater number of gj-channels.</p> <p>When there were no or only few gj-channels (namely, 0, 10, or 30), the voltage change (ΔV_m) in the two contiguous cells (#50 & #52) to the cell injected with current (#51) was nearly zero, i.e., there was a sharp discontinuity in voltage between the adjacent cells. When there were many gj-channels (e.g., 300, 1000, 3000), there was an exponential decay of voltage on either side of the injected cell, with the length constant (<it>λ</it>) increasing at higher numbers of gj-channels. The effect of increasing the number of gj-channels on increasing <it>λ </it>was relatively small compared to the larger effect on <it>θ</it>_ov. <it>θ</it>_ovbecame very non-physiological at 300 gj-channels or higher.</p> <p>Conclusion</p> <p>Thus, when there were only 0, 1, or 10 gj-channels, <it>θ</it>_ovincreased with increase in chain length, whereas at 100 gj-channels or higher, <it>θ</it>_ovdid not increase with chain length. When there were only 0, 10, or 30 gj-channels, there was a very sharp decrease in ΔV_min the two contiguous cells on either side of the injected cell, whereas at 300, 1000, or 3000 gj-channels, the voltage decay was exponential along the length of the chain. The effect of increasing the number of gj-channels on spread of current was relatively small compared to the large effect on <it>θ</it>_ov.</p