Defibrillator-type shocks may cause electric and contractile dysfunction. In this study, we determined the relationship between probability of lethal injury and electric field intensity (E in isolated rat ventricular myocytes, with emphasis on field orientation and stimulus waveform. This relationship was sigmoidal with irreversible injury for E > 50 V/cm . During both threshold and lethal stimulation, cells were twofold more sensitive to the field when it was applied longitudinally (versus transversally) to the cell major axis. For a given E, the estimated maximum variation of transmembrane potential (Delta V(max)) was greater for longitudinal stimuli, which might account for the greater sensitivity to the field. Cell death, however, occurred at lower maximum Delta V(max) values for transversal shocks. This might be explained by a less steep spatial decay of transmembrane potential predicted for transversal stimulation, which would possibly result in occurrence of electroporation in a larger membrane area. For the same stimulus duration, cells were less sensitive to field-induced injury when shocks were biphasic (versus monophasic). Ours results indicate that, although significant myocyte death may occur in the E range expected during clinical defibrillation, biphasic shocks are less likely to produce irreversible cell injury.

The electrophysiological properties of the myogenic cardiac cells of insects have been analyzed, but the mechanisms that regulate the pacemaker activity have not been elucidated yet. In mammalian pacemaker cells, different types of membrane ion channels seem to be sequentially activated, perhaps in a cooperative fashion with the current generated by Ca(2+) extrusion mediated by the electrogenic Na(+)/Ca(2+) exchanger, which is sustained by the diastolic sarcoplasmic reticulum (SR) Ca(2+) release. The objective of the present work was to investigate the role of the SR function on the basal beating rate (BR), and BR modulation by extracellular Ca(2+) concentration ([Ca(2+)](o)) and neurotransmitters in the in situ dorsal vessel (heart) of the mealworm beetle Tenebrio molitor. The main observations were as follows: 1) basal BR was reduced by 50% by inhibition of SR function, but not affected by perfusion with CsCl or ZD7288; 2) spontaneous activity was abolished by Cd(2+); 3) a robust positive chronotropic response could be elicited to serotonin (5-HT), but not to norepinephrine or carbamylcholine; 4) SR inhibition abolished the sustained chronotropic stimulation by [Ca(2+)](o) elevation and by 5-HT, while the latter was unaffected by CsCl. It is concluded that, in T. molitor heart, BR is markedly, but not exclusively, dependent on the SR function, and that BR control and modulation by both [Ca(2+)](o) and 5-HT requires a functional SR.

Electric field (E) stimulation is widely used in experiments with myocardial preparations and in the clinical setting (e.g., defibrillation). As a rule, stimuli are applied in a single direction, which limits excitatory cell recruitment because myocytes are disposed in different directions and their sensitivity to E depends on the stimulus orientation with respect to the cell major axis. Here, we propose a stimulatory approach, namely rapidly switching multidirectional stimulation (RSMS), in which stimuli are delivered in three directions within the electric refractory period. In populations of randomly oriented isolated rat cardiomyocytes, RSMS doubled the percentage of cells excited by near-threshold E (P < 0.001), which was more than the increase in recruitment in a single direction achieved by doubling E intensity. This effect was similar for monophasic and biphasic pulses, but for the latter, a given percent recruitment was obtained with 20-30% lower E intensity ( P < 0.01), so that RSMS with biphasic pulses allowed at least 60% reduction of E intensity for recruitment of >70% of the cells. RSMS can be applied to improve stimulation efficiency in experiments with isolated cardiac myocytes, and may be a promising alternative for decreasing shock intensity requirements for cardioversion and defibrillation.

Although high-intensity electric fields (HEF) application is currently the only effective therapy available to terminate ventricular fibrillation, it may cause injury to cardiac cells. In this study we determined the relation between HEF pulse length and cardiomyocyte lethal injury. We obtained lethality curves by survival analysis, which were used to determine the value of HEF necessary to kill 50% of cells (E50) and plotted a strength-duration (SxD) curve for lethality with 10 different durations: 0.1, 0.2, 0.5, 1, 3, 5, 10, 20, 35 and 70 ms. For the same durations we also obtained an SxD curve for excitation and established an indicator for stimulatory safeness (stimulation safety factor - SSF) as the ratio between the SxD curve for lethality and one for excitation. We found that the lower the pulse duration, the higher the HEF intensity required to cell death. Contrary to expectations, the highest SSF value does not correspond to the lowest pulse duration but to the one of 0.5 ms. As defibrillation threshold has been described as duration-dependent, our results imply that the use of shorter stimulus duration - instead of the one typically used in the clinic (10 ms) - might increase defibrillation safeness.

Application of high intensity electric fields (HIEF) to the myocardium is commonly used for cardiac defibrillation/cardioversion. Although effective at reversing life-threatening arrhythmias, HIEF may cause myocyte damage due to membrane electropermeabilization. In this study, the influence of cell length and width on HIEF-induced lethal injury was analyzed in isolated rat cardiomyocytes in parallel alignment with the field. The field-induced maximum variation of membrane potential (∆V max ) was estimated with the Klee-Plonsey model. The studied myocyte population was arranged in two group pairs for comparison: the longest vs. the shortest cells, and the widest vs. narrowest cells. Threshold field intensity was significantly lower in the longest vs. shortest myocytes, whereas cell width influence was not significant. The threshold ∆V max was comparable in all groups. Likewise, a significant leftward shift of the lethality curve (i.e., relationship of the probability of lethality vs. field intensity) of the longest cells was observed, evidencing greater sensitivity to HIEF-induced damage. However, the lethality curve as a function of ∆V max was similar in all groups, confirming a prediction of the Klee-Plonsey model. The similar results for excitation and injury at threshold and HIEF stimulation, respectively, indicate that: a) the effect of cell length on the sensitivity to the field would be attributable to differences in field-induced membrane polarization that lead to excitation or lethal electroporation; b) the Klee-Plonsey model seems to be reliable for analysis of cell interaction with HIEF; c) it is possible that increased cell length in hypertrophied hearts enhances myocyte fragility upon defibrillation/cardioversion.

Electric field stimulation is widely used for heart pacing and arrhythmia reversion. In this study, we analysed the influence of waveform and direction of external stimulating electric field on the excitation threshold of isolated ventricular myocytes. The threshold field (E(T)) was lower when the field was applied longitudinally (E(T,L)) rather than transversally (E(T,T)) to the cell major axis. Rheobase was greater for transversal stimulation, but chronaxie and estimated membrane polarization were similar for both directions. The calculated maximal variation in membrane potential at the threshold (DeltaV(T) approximately 15 mV) was insensitive to field direction. As DeltaV(T) values were similar, we assumed that the E(T,T)/E(T,L) ratio might be described solely as the ratio of the major and minor cell semi-axes. Accordingly, the ratio thus estimated was comparable to that determined experimentally. Stimulus waveform significantly affected both E(T) and DeltaV(T), which were greater for monophasic versus biphasic stimuli. Direction and waveform effects were independent. We conclude that (a) direction affects E(T) by its influence on the ability of a given field intensity to cause threshold membrane polarization and (b) threshold-lowering effects of longitudinal stimulation and biphasic waveforms apparently depend on different mechanisms, are additive and thus may be combined to decrease the energy requirement for myocardial stimulation.

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