Electrophysiologic Mechanism Underlying Action Potential Prolongation by Sevoflurane in Rat Ventricular Myocytes

A comparative review on heart ion channels, action potentials and electrocardiogram in rodents and human: extrapolation of experimental insights to clinic

Electrocardiogram (ECG) is a non-invasive valuable diagnostic tool that is used in clinics for investigation and monitoring of heart electrical rhythm/conduction, ischemia/injury of heart, electrolyte disturbances and agents/drugs induced cardiac toxicity. Nowadays using animal models to study heart diseases such as electrical and mechanical disturbance is common. In addition, given to ethical consideration and availability, the use of small rodents has been a top priority for cardiovascular researchers. However, extrapolation of experimental findings from the lab to the clinic needs sufficient basic knowledge of similarities and differences between heart action potential and ECG of rodents and humans in normal and disease conditions. This review compares types of human action potentials, the dominant ion currents during action potential phases, alteration in ion channels activities in channelopathies-induced arrhythmias and the ECG appearance of mouse, rat, guinea pig, rabbit and human. Also, it briefly discusses the responsiveness and alterations in ECG following some interventions such as cardiac injury and arrhythmia induction. Overall, it provides a roadmap for researchers in selecting the best animal model/species whose studies results can be translated into clinical practice. In addition, this study will also be useful to biologists, physiologists, pharmacologists, veterinarians and physicians working in the fields of comparative physiology, pharmacology, toxicology and diseases.

Thrombospondin-4 induces prolongation of action potential duration in rat isolated ventricular myocytes

Expression of thrombospondin-4 (TSP-4), a matricellular protein, is increased in the heart tissue of various cardiac disease models. In dorsal root ganglion neurons, TSP-4 inhibits L-type Ca²⁺ channel (LTCC) activity. Although TSP-4 might be related to the electrophysiological properties in heart, it remains to be clarified. The present study aimed to clarify the effects of TSP-4 on action potential (AP), LTCC current (I_CaL) and voltage-dependent K⁺ (Kv) channel current (I_Kv) in rat isolated ventricular myocytes by a patch clamp technique. Ventricular myocytes were isolated from the heart of adult male Wistar rats. The ventricular myocytes were treated with TSP-4 (5 nM) or its vehicle for 4 hr. Then, whole-cell patch clamp technique was performed to measure AP (current-clamp mode) and I_CaL and I_Kv (voltage-clamp mode). The mRNA expression of Kv channels was examined by reverse transcription-polymerase chain reaction. TSP-4 had no effect on the resting membrane potential and peak amplitude of AP. On the other hand, TSP-4 significantly prolonged AP duration (APD) at 50% and 90% repolarization. TSP-4 significantly inhibited the peak amplitudes of I_CaL and I_Kv. TSP-4 had no effect on mRNA expression of Kv channels (Kcna4, Kcna5, Kcnb1, Kcnd2 and Kcnd3). The present study for the first time demonstrated that TSP-4 prolongs APD in rat ventricular myocytes, which is possibly mediated through the suppression of Kv channel activity.

Effects of central sympathetic activation on repolarization-dispersion during short-term myocardial ischemia in anesthetized rats

AIMS

Sympathetic activation during myocardial ischemia enhances arrhythmogenesis, but the underlying pathophysiologic mechanisms remain unclear. We investigated the central sympathetic effects on ventricular repolarization during the early-period post-coronary artery occlusion.

MAIN METHODS

We studied 12 Wistar rats (254±2 g) for 30 min following left coronary artery ligation, with (n=6) or without (n=6) pretreatment with the central sympatholytic agent clonidine. Mapping of left and right ventricular epicardial electrograms was performed with a 32-electrode array. As an index of sympathetic activation, heart rate variability in the frequency domain was calculated. Heart rate and repolarization duration were measured with a custom-made recording and analysis software, followed by calculation of intra- and inter-ventricular dispersion of repolarization.

KEY FINDINGS

Heart rate and heart rate variability indicated lower sympathetic activation in clonidine-treated rats during ischemia. Repolarization duration in the left ventricle prolonged after clonidine at baseline, independently of heart rate, but no differences were present 30 min post-ligation. Dispersion of repolarization in the right ventricle remained stable during ischemia, whereas it increased in the left ventricle, equally in both groups. A similar trend was observed for inter-ventricular dispersion, without differences between groups.

SIGNIFICANCE

In addition to intra-ventricular repolarization-dispersion, anterior-wall myocardial ischemia may also increase inter-ventricular repolarization-dispersion. Progressive central sympathetic activation occurs during myocardial ischemia, but it does not affect intra- or inter-ventricular dispersion of ventricular repolarization during the early phase. Further research is warranted on the potential effects during subsequent time-periods.

Unusual diagnosis of a persistent third-degree atrioventricular block during anesthesia in a "healthy" pediatric patient

We present an unusual diagnosis of a permanent third-degree atrioventricular block under general anesthesia in an otherwise healthy and asymptomatic child. This diagnosis of unclear causality represented a serious rare finding, requiring judicious management and resulting in the placement of a permanent pacemaker.

Ionic mechanisms of desflurane on prolongation of action potential duration in rat ventricular myocytes

PURPOSE

Despite the fact that desflurane prolongs the QTC interval in humans, little is known about the mechanisms that underlie these actions. We investigated the effects of desflurane on action potential (AP) duration and underlying electrophysiological mechanisms in rat ventricular myocytes.

MATERIALS AND METHODS

Rat ventricular myocytes were enzymatically isolated and studied at room temperature. AP was measured using a current clamp technique. The effects of 6% (0.78 mM) and 12% (1.23 mM) desflurane on transient outward K⁺ current (I(to)), sustained outward current (I(sus)), inward rectifier K⁺ current (I(KI)), and L-type Ca²⁺ current were determined using a whole cell voltage clamp.

RESULTS

Desflurane prolonged AP duration, while the amplitude and resting membrane potential remained unchanged. Desflurane at 0.78 mM and 1.23 mM significantly reduced the peak I(to) by 20 ± 8% and 32 ± 7%, respectively, at +60 mV. Desflurane (1.23 mM) shifted the steady-state inactivation curve in a hyperpolarizing direction and accelerated inactivation of the current. While desflurane (1.23 mM) had no effects on I(sus) and I(KI), it reduced the L-type Ca²⁺ current by 40 ± 6% (p<0.05).

CONCLUSION

Clinically relevant concentrations of desflurane appear to prolong AP duration by suppressing I(to) in rat ventricular myocytes.

Sevoflurane causes greater QTc interval prolongation in elderly patients than in younger patients

BACKGROUND

Sevoflurane and droperidol prolong the QT interval, and advancing age is not only associated with a prolongation of the QT interval but is also a risk factor for drug-induced QT interval prolongation. In this study, we compared the effect of sevoflurane and droperidol on the corrected QT (QTc) interval and the dispersion of ventricular repolarization (time interval from the peak to the end of the T wave [Tp-e]) in elderly patients with those in younger patients.

METHODS

Under sevoflurane anesthesia (1.5%-2.5%) with an antiemetic dose of droperidol (1.25 mg), the QT interval and the Tp-e interval, which indicates transmural dispersion of repolarization across the myocardial wall, were measured in 30 elderly patients (70 years and older) and in 30 younger patients (20-69 years) for 2 hours. The QT interval was normalized for heart rate (QTc) using 3 different formulas: Bazett, Matsunaga, and Van de Water. Data are presented as mean +/- sd.

RESULTS

The elderly group was 24.4 years older (P < 0.05) than the younger group. The QTc intervals in the 2 groups before anesthesia were not significantly different. Using all 3 formulas, the QTc interval in the elderly patient group was significantly prolonged by sevoflurane (the QTc intervals at preanesthesia and 60, 75, 90, and 120 minutes after sevoflurane exposure were 0.434 +/- 0.028 seconds, 0.450 +/- 0.037 seconds, 0.463 +/- 0.037 seconds, 0.461 +/- 0.037 seconds, and 0.461 +/- 0.038 seconds, respectively, with the Bazett formula). The sevoflurane-induced QTc interval prolongation in the elderly patient group was significantly greater than that in the younger patient group (0.450 +/- 0.037 seconds vs 0.432 +/- 0.034 seconds, 60 minutes after sevoflurane exposure; 0.463 +/- 0.037 seconds vs 0.441 +/- 0.037 seconds, 75 minutes after sevoflurane exposure; and 0.461 +/- 0.038 seconds vs 0.436 +/- 0.030 seconds, 120 minutes after sevoflurane exposure with the Bazett formula), but the sevoflurane-induced QTc interval prolongation was neither further enhanced with time nor by droperidol. The Tp-e interval was not affected in either group.

CONCLUSION

Sevoflurane causes greater QTc interval prolongation in elderly patients than in younger patients. Although sevoflurane does not affect the transmural dispersion of repolarization and sevoflurane-induced QTc prolongation does not advance with time and by droperidol administration, QT interval prolongation and its associated arrhythmias should be carefully monitored during sevoflurane anesthesia in elderly patients.

Differential effects of isoflurane on A-type and delayed rectifier K channels in rat substantia nigra

The authors previously demonstrated that isoflurane, a widely used volatile anesthetic, induced depolarization and increased the frequency of spontaneous action potentials in principal dopamine neurons in rat substantia nigra pars compacta. We studied the effects of isoflurane on voltage-dependent K channels to clarify the mechanisms of the increase in excitability in these neurons. Voltage-clamp whole-cell recordings were made in rat midbrain slices. We recorded the outward membrane currents in response to depolarizing voltage steps from -120 mV and -25 mV and isolated the transient outward current mediated through A-type K channels by subtraction. Isoflurane at clinically relevant concentrations accelerated the decay of the A-type K current and delayed the recovery from inactivation without changing the steady-state inactivation curves. Isoflurane did not affect the non-inactivating outward current. Addition of 4-aminopyridine partially occluded the excitatory effects of isoflurane in current-clamp recordings. These results demonstrate that isoflurane accelerated the inactivation and delayed the recovery from inactivation of A-type K channels in principal neurons in rat substantia nigra pars compacta without affecting delayed rectifier K channels. These effects may contribute in part to excitation of these neurons and the isoflurane-induced increases in dopamine release reported in vitro and in vivo.