Inhibitory effects of sevoflurane on pacemaking activity of sinoatrial node cells in guinea-pig heart

Electrophysiological and sick sinus syndrome effects of Remdesivir challenge in guinea-pig hearts

Remdesivir (RDV) is the first drug approved by the FDA for clinical treatment of hospitalized patients infected with COVID-19 because it has been shown to have good antiviral activity against a variety of viruses, including Arenaviridae and Coronaviridae viral families. However, it has been reported that its clinical treatment leads to the symptoms of sick sinus syndrome such as sinus bradycardia, conduction block, and sinus arrest, but the electrophysiological mechanism of its specific cardiac adverse events is still unclear. We report complementary, experimental, studies of its electrophysiological effects. In wireless cardiac telemetry experiments in vivo and electrocardiographic studies in ex vivo cardiac preparations, RDV significantly caused sinus bradycardia, sinus atrial block, and prolongation of the QT interval in guinea pigs. Dose-dependent effects of RDV on the electrical activities of sinoatrial node (SA node) preparations of guinea pigs were characterised by multielectrode, optical RH237 voltage mapping. These revealed reversibly reduced sinoatrial conduction time (SACT), increased AP durations (APDs), and decreased the pacemaking rate of the SA node. Patch-clamp experiments showed that RDV significantly inhibited the If current of HCN4 channels, resulting in a significant decrease in the spontaneous firing rate of SA node cells, which may underlie the development of sick sinus node syndrome. In addition, RDV significantly inhibits IKr currents in hERG channels, leading to prolongation of the QT interval and playing a role in bradycardia. Therefore, these findings provide insights into the understanding the bradycardia effect of RDV, which may be used as basic theoretical guidance for the intervention of its adverse events, and prompt safety investigations of RDV's cardiac safety in the future.

The virtual sinoatrial node: What did computational models tell us about cardiac pacemaking?

Since its discovery, the sinoatrial node (SAN) has represented a fascinating and complex matter of research. Despite over a century of discoveries, a full comprehension of pacemaking has still to be achieved. Experiments often produced conflicting evidence that was used either in support or against alternative theories, originating intense debates. In this context, mathematical descriptions of the phenomena underlying the heartbeat have grown in importance in the last decades since they helped in gaining insights where experimental evaluation could not reach. This review presents the most updated SAN computational models and discusses their contribution to our understanding of cardiac pacemaking. Electrophysiological, structural and pathological aspects - as well as the autonomic control over the SAN - are taken into consideration to reach a holistic view of SAN activity.

Probability that there is a mammalian counterpart of cardiac clock in insects

Whether or not the hyperpolarization-activated cyclic nucleotide-gated nonselective cation channel (HCN or funny current I_f ) is involved in pacemaking - recurrent heartbeat, it is attributed to electrical activities in all excitable cells, including those of invertebrates. In latter group of animals prevailingly the electrical signals and function of heart in terms of chrono- and inotropy are elucidated. Although in simpler models including insects experimental outcomes are reproducible and robust, involvement of "cardiac clock" mechanism in pacemaking is not conclusive. In this assay, the mechanisms of heartbeat are synthesized by focused comparisons between insect and mammalian hearts.

Dexmedetomidine Exerts a Negative Chronotropic Action on Sinoatrial Node Cells Through the Activation of Imidazoline Receptors

ABSTRACT

Dexmedetomidine (DEX), an α2-adrenoreceptor (α2-AR) and imidazoline receptor agonist, is most often used for the sedation of patients in the intensive care unit. Its administration is associated with an increased incidence of bradycardia; however, the precise mechanism of DEX-induced bradycardia has yet to be fully elucidated. This study was undertaken to examine whether DEX modifies pacemaker activity and the underlying ionic channel function through α2-AR and imidazoline receptors. The whole-cell patch-clamp techniques were used to record action potentials and related ionic currents of sinoatrial node cells in guinea pigs. DEX (≥10 nM) reduced sinoatrial node automaticity and the diastolic depolarization rate. DEX reduced the amplitude of hyperpolarization-activated cation current (If or Ih) the pacemaker current, even within the physiological pacemaker potential range. DEX slowed the If current activation kinetics and caused a significant shift in the voltage dependence of channel activation to negative potentials. In addition, efaroxan, an α2-AR and imidazoline I1 receptor antagonist, attenuated the inhibitory effects of DEX on sinoatrial node automaticity and If current activity, whereas yohimbine, an α2-AR-selective antagonist, did not. DEX did not affect the current activities of other channels, including rapidly and slowly activating delayed rectifier K+ currents (IKr and IKs), L-type Ca2+ current (ICa,L), Na+/Ca2+ exchange current (INCX), and muscarinic K+ current (IK,ACh). Our results indicate that DEX, at clinically relevant concentrations, induced a negative chronotropic effect on the sinoatrial node function through the downregulation of If current through an imidazoline I1 receptor other than the α2-AR in the clinical setting.

Characterization and functional role of rapid- and slow-activating delayed rectifier K+ currents in atrioventricular node cells of guinea pigs

The atrioventricular (AV) node is the only conduction pathway where electrical impulse can pass from atria to ventricles and exhibits spontaneous automaticity. This study examined the function of the rapid- and slow-activating delayed rectifier K⁺ currents (I_Kr and I_Ks) in the regulation of AV node automaticity. Isolated AV node cells from guinea pigs were current- and voltage-clamped to record the action potentials and the I_Kr and I_Ks current. The expression of I_Kr or I_Ks was confirmed in the AV node cells by immunocytochemistry, and the positive signals of both channels were localized mainly on the cell membrane. The basal spontaneous automaticity was equally reduced by E4031 and HMR-1556, selective blockers of I_Kr and I_Ks, respectively. The nonselective β-adrenoceptor agonist isoproterenol markedly increased the firing rate of action potentials. In the presence of isoproterenol, the firing rate of action potentials was more effectively reduced by the I_Ks inhibitor HMR-1556 than by the I_Kr inhibitor E4031. Both E4031 and HMR-1556 prolonged the action potential duration and depolarized the maximum diastolic potential under basal and β-adrenoceptor-stimulated conditions. I_Kr was not significantly influenced by β-adrenoceptor stimulation, but I_Ks was concentration-dependently enhanced by isoproterenol (EC₅₀: 15 nM), with a significant negative voltage shift in the channel activation. These findings suggest that both the I_Kr and I_Ks channels might exert similar effects on regulating the repolarization process of AV node action potentials under basal conditions; however, when the β-adrenoceptor is activated, I_Ks modulation may become more important.

Transient Receptor Potential Canonical Channel Blockers Improve Ventricular Contractile Functions After Ischemia/Reperfusion in a Langendorff-perfused Mouse Heart Model

Reperfusion of ischemic myocardium is accompanied by intracellular Ca overload, leading to cardiac dysfunction. However, the mechanisms underlying intracellular Ca overload have yet to be fully elucidated. The mechanism may involve the activation of store-operated Ca entry, which is primarily mediated through the transient receptor potential canonical (TRPC) channels. This study was undertaken to examine the possible involvement of TRPC channels in the development of contractile dysfunction associated with reperfusion of ischemic myocardium using a mouse heart model. The functional expression of TRPC channels was confirmed in mouse ventricular myocytes using immunocytochemistry, Western blotting, and patch-clamp experiments. The left ventricular functions were assessed by measuring left ventricular end-diastolic pressure, left ventricular developed pressure, and its first derivatives in a Langendorff-perfused mouse heart subjected to 30 minutes of normothermic (37°C) global ischemia followed by 60 minutes of reperfusion. Under control conditions, left ventricular functions were deteriorated during reperfusion, which was significantly ameliorated by administration of the TRPC channel blockers 2-aminoethoxydiphenyl borate and La during initial 5 minutes of reperfusion. Our findings suggest that TRPC channels are involved in mediating contractile dysfunction during reperfusion of ischemic myocardium and detect TRPC channels as a potential therapeutic target for preventing myocardial ischemia/reperfusion injury.

Postnatal developmental changes in the sensitivity of L-type Ca2+ channel to inhibition by verapamil in a mouse heart model

BackgroundIn the clinical setting, verapamil is contraindicated in neonates and infants, because of the perceived risk of hypotension or bradyarrhythmia. However, it remains unclear whether there is an age-dependent difference in the sensitivity of cardiac L-type Ca²⁺ channel current (I_Ca,L) to inhibition by verapamil.MethodsVentricular myocytes were enzymatically dissociated from the hearts of six different age groups (0, 7, 14, 21, 28 days, and 10-15 weeks) of mice, using a similar Langendorff-perfusion method. Whole-cell patch-clamp technique was applied to examine the sensitivity of I_Ca,L to inhibition, by three classes of structurally different L-type Ca²⁺ channel antagonists.ResultsVerapamil, nifedipine, and diltiazem concentration-dependently blocked the ventricular I_Ca,L in all six age groups. However, although nifedipine and diltiazem blocked ventricular I_Ca,L with a similar potency in all age groups, verapamil more potently blocked ventricular I_Ca,L in day 0, day 7, day 14, and day 21 mice, than in day 28, and 10-15-week mice.ConclusionIn a mouse heart model, ventricular I_Ca,L before the weaning age (~21 days of age) exhibited a higher sensitivity to inhibition by verapamil than that after the weaning age, which may explain one possible mechanism associated with the development of verapamil-induced hypotension in human neonates and infants.

Interaction of propofol with voltage-gated human Kv1.5 channel through specific amino acids within the pore region

The intravenous anesthetic propofol affects the function of a diversity of ligand-gated and voltage-gated ion channels. However, there is little information as to whether propofol directly interacts with voltage-gated ion channel proteins to modulate their functions. The Kv1.5 channel is activated by membrane depolarization during action potentials and contributes to atrial repolarization in the human heart. This study was undertaken to examine the effect of propofol on voltage-gated human Kv1.5 (hKv1.5) channel and to elucidate the underlying molecular determinants. Site-directed mutagenesis was carried out through six amino acids that reside within the pore domain of hKv1.5 channel. Whole-cell patch-clamp technique was used to record membrane currents through the wild type and mutant hKv1.5 channels heterologously expressed in Chinese hamster ovary cells. Propofol (≥5 μM) reversibly and concentration-dependently (IC50 of 49.3±9.4 μM; n=6) blocked hKv1.5 current. Propofol-induced block of hKv1.5 current gradually progressed during depolarizing voltage-clamp steps and was enhanced by higher frequency of activation, consistent with a preferential block of the channels in their open state. The degree of current block by propofol was significantly attenuated in T480A, I502A, I508A and V516A, but not in H463C and L510A mutants of hKv1.5 channel. Thus, several amino acids near the selectivity filter (Thr480) or within S6 (Ile502, Ile508 and Val516) are found to be critically involved in the blocking action of propofol. This study provides the first evidence suggesting that direct interaction with specific amino acids underlies the blocking action of propofol on voltage-gated hKv1.5 channel.

Ionic mechanisms underlying the negative chronotropic action of propofol on sinoatrial node automaticity in guinea pig heart

BACKGROUND AND PURPOSE

Propofol is a widely used intravenous anaesthetic agent, but has undesirable cardiac side effects, including bradyarrhythmia and its severe form asystole. This study examined the ionic and cellular mechanisms underlying propofol-induced bradycardia.

EXPERIMENTAL APPROACH

Sinoatrial node cells, isolated from guinea pig hearts, were current- and voltage-clamped to record action potentials and major ionic currents involved in their spontaneous activity, such as the hyperpolarization-activated cation current (If ), T-type and L-type Ca(2+) currents (ICa,T and ICa,L , respectively) and the rapidly and slowly activating delayed rectifier K(+) currents (IKr and IK s , respectively). ECGs were recorded from Langendorff-perfused, isolated guinea pig hearts.

KEY RESULTS

Propofol (≥5 μM) reversibly decreased the firing rate of spontaneous action potentials and their diastolic depolarization rate. Propofol impaired If activation by shifting the voltage-dependent activation to more hyperpolarized potentials (≥1 μM), slowing the activation kinetics (≥3 μM) and decreasing the maximal conductance (≥10 μM). Propofol decreased ICa,T (≥3 μM) and ICa,L (≥1 μM). Propofol suppressed IKs (≥3 μM), but had a minimal effect on IKr . Furthermore, propofol (≥5 μM) decreased heart rates in Langendorff-perfused hearts. The sinoatrial node cell model reasonably well reproduced the negative chronotropic action of propofol.

CONCLUSIONS AND IMPLICATIONS

Micromolar concentrations of propofol suppressed the slow diastolic depolarization and firing rate of sinoatrial node action potentials by impairing If activation and reducing ICa,T , ICa,L and IKs . These observations suggest that the direct inhibitory effect of propofol on sinoatrial node automaticity, mediated via multiple channel inhibition, underlies the propofol-induced bradycardia observed in clinical settings.

Computational modeling of inhibition of voltage-gated Ca channels: identification of different effects on uterine and cardiac action potentials

The uterus and heart share the important physiological feature whereby contractile activation of the muscle tissue is regulated by the generation of periodic, spontaneous electrical action potentials (APs). Preterm birth arising from premature uterine contractions is a major complication of pregnancy and there remains a need to pursue avenues of research that facilitate the use of drugs, tocolytics, to limit these inappropriate contractions without deleterious actions on cardiac electrical excitation. A novel approach is to make use of mathematical models of uterine and cardiac APs, which incorporate many ionic currents contributing to the AP forms, and test the cell-specific responses to interventions. We have used three such models-of uterine smooth muscle cells (USMC), cardiac sinoatrial node cells (SAN), and ventricular cells-to investigate the relative effects of reducing two important voltage-gated Ca currents-the L-type (ICaL) and T-type (ICaT) Ca currents. Reduction of ICaL (10%) alone, or ICaT (40%) alone, blunted USMC APs with little effect on ventricular APs and only mild effects on SAN activity. Larger reductions in either current further attenuated the USMC APs but with also greater effects on SAN APs. Encouragingly, a combination of ICaL and ICaT reduction did blunt USMC APs as intended with little detriment to APs of either cardiac cell type. Subsequent overlapping maps of ICaL and ICaT inhibition profiles from each model revealed a range of combined reductions of ICaL and ICaT over which an appreciable diminution of USMC APs could be achieved with no deleterious action on cardiac SAN or ventricular APs. This novel approach illustrates the potential for computational biology to inform us of possible uterine and cardiac cell-specific mechanisms. Incorporating such computational approaches in future studies directed at designing new, or repurposing existing, tocolytics will be beneficial for establishing a desired uterine specificity of action.

Direct negative chronotropic action of desflurane on sinoatrial node pacemaker activity in the guinea pig heart

BACKGROUND

Desflurane inhalation is associated with sympathetic activation and concomitant increase in heart rate in humans and experimental animals. There is, however, little information concerning the direct effects of desflurane on electrical activity of sinoatrial node pacemaker cells that determines the intrinsic heart rate.

METHODS

Whole-cell patch-clamp experiments were conducted on guinea pig sinoatrial node pacemaker cells to record spontaneous action potentials and ionic currents contributing to sinoatrial node automaticity, namely, hyperpolarization-activated cation current (If), T-type and L-type Ca currents (ICa,T and ICa,L, respectively), Na/Ca exchange current (INCX), and rapidly and slowly activating delayed rectifier K currents (IKr and IKs, respectively). Electrocardiograms were recorded from ex vivo Langendorff-perfused hearts and in vivo hearts.

RESULTS

Desflurane at 6 and 12% decreased spontaneous firing rate of sinoatrial node action potentials by 15.9% (n = 11) and 27.6% (n = 10), respectively, which was associated with 20.4% and 42.5% reductions in diastolic depolarization rate, respectively. Desflurane inhibited If, ICa,T, ICa,L, INCX, and IKs but had little effect on IKr. The negative chronotropic action of desflurane was reasonably well reproduced in sinoatrial node computer model. Desflurane reduced the heart rate in Langendorff-perfused hearts. High concentration (12%) of desflurane inhalation was associated with transient tachycardia, which was totally abolished by pretreatment with the β-adrenergic blocker propranolol.

CONCLUSIONS

Desflurane has a direct negative chronotropic action on sinoatrial node pacemaking activity, which is mediated by its inhibitory action on multiple ionic currents. This direct inhibitory action of desflurane on sinoatrial node automaticity seems to be counteracted by sympathetic activation associated with desflurane inhalation in vivo.

Remifentanil has a minimal direct effect on sinoatrial node pacemaker activity in the Guinea pig heart

BACKGROUND

Whereas remifentanil administration is associated with severe bradycardia, it has yet to be fully investigated whether the negative chronotropic action of remifentanil is mediated by its direct action on sinoatrial (SA) node pacemaker activity in the heart versus indirect results of enhanced vagal activity.

METHODS

We examined the effects of remifentanil and fentanyl on the spontaneous action potentials of guinea pig SA node cells at concentrations of 5, 10, 100, and 1000 nM using the amphotericin B-perforated whole-cell patch-clamp technique. Isolated guinea pig hearts were perfused in a Langendorff mode with 5, 10, 100, and 1000 nM remifentanil.

RESULTS

The spontaneous firing rate and diastolic depolarization rate (DDR) of the SA node action potentials were 189.1 ± 14.8 /min and 74.1 ± 2.9 mV/s (n = 8), respectively, under control conditions, and were not significantly affected by exposure to 5 nM (P = 1.0 for both spontaneous firing rate and DDR; n = 6), 10 nM (P = 0.62 for spontaneous firing rate, P = 0.99 for DDR; n = 6), or 100 nM (P = 0.23 for spontaneous firing rate, P = 0.38 for DDR; n = 6) remifentanil. However, 1000 nM remifentanil modestly but significantly decreased the spontaneous firing rate (P = 0.0087) and DDR (P = 0.0072, n = 6). Remifentanil did not affect the heart rate of isolated Langendorff-perfused guinea pig hearts at concentrations of 5 nM (P = 0.98), 10 nM (P = 0.35), or 100 nM (P = 0.24) but significantly reduced the heart rate at 1000 nM (P < 0.0001). Fentanyl did not affect the spontaneous firing rate and DDR at concentrations of 5 nM (P = 1.0 for both spontaneous firing rate and DDR) and 10 nM (P = 0.62 for spontaneous firing rate, P = 0.79 for DDR), but it significantly reduced both at 100 nM (P = 0.00038 for spontaneous firing rate, P = 0.0080 for DDR) and 1000 nM (P < 0.0001 for both spontaneous firing rate and DDR).

CONCLUSIONS

Clinically relevant concentrations (nanomolar order concentrations) of remifentanil do not produce significant direct effects on intrinsic cardiac automaticity; thus, suggesting that remifentanil-induced bradycardia in the clinical setting is independent of its direct cardiac effects.

The "funny" current (I(f)) inhibition by ivabradine at membrane potentials encompassing spontaneous depolarization in pacemaker cells

Recent clinical trials have shown that ivabradine (IVA), a drug that inhibits the funny current (I(f)) in isolated sinoatrial nodal cells (SANC), decreases heart rate and reduces morbidity and mortality in patients with cardiovascular diseases. While IVA inhibits I(f), this effect has been reported at essentially unphysiological voltages, i.e., those more negative than the spontaneous diastolic depolarization (DD) between action potentials (APs). We tested the relative potency of IVA to block I(f) over a wide range of membrane potentials, including those that encompass DD governing to the SANC spontaneous firing rate. A clinically relevant IVA concentration of 3 μM to single, isolated rabbit SANC slowed the spontaneous AP firing rate by 15%. During voltage clamp the maximal I(f) was 18 ± 3 pA/pF (at -120 mV) and the maximal I(f) reduction by IVA was 60 ± 8% observed at -92 ± 4 mV. At the maximal diastolic depolarization (~-60 mV) I(f) amplitude was only -2.9 ± 0.4 pA/pF, and was reduced by only 41 ± 6% by IVA. Thus, I(f) amplitude and its inhibition by IVA at physiologically relevant membrane potentials are substantially less than that at unphysiological (hyperpolarized) membrane potentials. This novel finding more accurately describes how IVA affects SANC function and is of direct relevance to numerical modeling of SANC automaticity.