Field stimulation of isolated chick heart cells: comparison of experimental and theoretical activation thresholds

Noisy stimulation effect in calcium dynamics on cardiac cells

Noise is present in nature, and it affects the nervous and cardiovascular system. Noise added to stimuli may change the performance of excitable cells. In this paper, we study the effect of noise on the two main heart cell types: pacemaker and myocardial cells. This study investigates whether noise can induce changes in calcium dynamics on the two main heart cell types: pacemaker and myocardial cells, when stimuli with periodic electrical signals are disturbed by Gaussian white noise. Calcium dynamic parameters were obtained using imaging signals. Our results show that low intensities of noise favor amplitude and raise rate calcium dynamics, although our results show that the pacemaker cells are not affected by a noisy stimulus. Altogether, these findings suggest that noise plays a key role in calcium dynamics.

Modulation of action potentials using PEDOT:PSS conducting polymer microwires

We describe the use of PEDOT:PSS conducting polymer microwires to modulate action potentials in single cells.

PEDOT

PSS conducting polymer microwires are electrochemically synthesized with diameters ranging from 860 nm to 4.5 μm and conductivities of ~30 S/cm. The length of the microwires is controlled by the spacing of the electrodes used for the electrochemical polymerization. We demonstrate the use of these microwires to control the action potentials of cardiomyocytes, showing that the cellular contractions match the frequency of the applied voltage. Membrane integrity assays confirm that the voltage delivered by the wires does not damage cells. We expect the conducting polymer microwires will be useful as minimally invasive devices to control the electrical properties of cells with high spatial precision.

Paradoxical loss of excitation with high intensity pulses during electric field stimulation of single cardiac cells

Transmembrane potential responses of single cardiac cells stimulated at rest were studied with uniform rectangular field pulses having durations of 0.5-10 ms. Cells were enzymatically isolated from guinea pig ventricles, stained with voltage sensitive dye di-8-ANEPPS, and stimulated along their long axes. Fluorescence signals were recorded with spatial resolution of 17 microm for up to 11 sites along the cell. With 5 and 10 ms pulses, all cells (n = 10) fired an action potential over a broad range of field amplitudes (approximately 3-65 V/cm). With 0.5 and 1 ms pulses, all cells (n = 7) fired an action potential for field amplitudes ranging from the threshold value (approximately 4-8 V/cm) to 50-60 V/cm. However, when the field amplitude was further increased, five of seven cells failed to fire an action potential. We postulated that this paradoxical loss of excitation for higher amplitude field pulses is the result of nonuniform polarization of the cell membrane under conditions of electric field stimulation, and a counterbalancing interplay between sodium current and inwardly rectifying potassium current with increasing field strength. This hypothesis was verified using computer simulations of a field-stimulated guinea pig ventricular cell. In conclusion, we show that for stimulation with short-duration pulses, cells can be excited for fields ranging between a low amplitude excitation threshold and a high amplitude threshold above which the excitation is suppressed. These results can have implications for the mechanistic understanding of defibrillation outcome, especially in the setting of diseased myocardium.

The effects of electrode position on the excitability of rat atria during postnatal development

Cardiac excitability is determined by the direction of the electric field, which is defined by the positioning of electrodes. However, important morphological and physiological modifications that happen during the postnatal development of the heart may affect the cardiac threshold. In this work we have evaluated the effect of electrode positioning on the excitability threshold of isolated Wistar rat atria (left auricles) during postnatal development. This was performed by determining the parameters of strength-duration curves for stimuli (rheobase, chronaxie and normalized minimum pulse energy) of atria from rats at ages (days) 5, 15, 30, 60, 90 and 120. These parameters were determined using electric field stimulation in four different orientations (apex-base, base-apex, left-right and right-left). Atrial rheobase decreased by 1.5- to 4-fold with animal age and was altered by electric field orientation in a diversified way, whereas atrial chronaxie increased only with animal age. The minimum pulse energy decreased two- to nine-fold with ageing. This was mainly due to rheobase dependence with electric field direction. We showed that the appropriate cardiac stimulation depends on the effects of three combined factors (pulse parameters, electrode position and animal age) on the atrial tissue excitability.

Sinusoidal stimulation of myocardial tissue: effects on single cells

INTRODUCTION

Cardiac tissue subjected to sinusoidal stimulus is characterized by action potentials (APs) that have extended plateau phases, sustained for the duration of the stimulus. Extended action potential durations (APDs) are beneficial because they disrupt wandering wavelets in the fibrillating heart. To investigate the mechanisms by which periodic stimulus affects cardiac tissue, particularly the development of sustained depolarization, computer simulations of single cardiac cells exposed to alternating current (AC) are performed.

METHODS AND RESULTS

Two modes of stimulation of the cell are examined: external field stimulation and transmembrane current injection. Several membrane models, including Luo-Rudy I and II, are used in the simulations. External AC field stimuli increase the APD of the single cell. The extended plateau of the cellular AP is characterized by periodic oscillations that are 1:2 phase locked with the applied stimulus. This specific behavior is due to the variations in stimulus magnitude and polarity along the cell border, which elicit opposite electrical responses from the cell sides. These pointwise responses are averaged in the macroscopic cellular response and result in sustained oscillatory depolarization that lasts for the duration of the stimulus. In contrast, the cell undergoing current injection does not develop an extended APD.

CONCLUSION

The simulations demonstrate that variation of membrane potential within a cell is of paramount importance to the formation of an extended AP plateau in response to AC stimulation.