Stéphanie C.M. Choisy

Atrial Remodeling and the Substrate for Atrial Fibrillation in Rat Hearts with Elevated Afterload

Background— Although arterial hypertension and left ventricular hypertrophy are considered good epidemiological indicators of the risk of atrial fibrillation (AF) in patients, the link between elevated afterload and AF remains unclear. We investigated atrial remodeling and the substrate for arrhythmia in a surgical model of elevated afterload in rats. Methods and Results— Male Wistar rats (aged 3–4 weeks) were anesthetized and subjected to either partial stenosis of the ascending aorta (AoB) or sham operation (Sham). Experiments were performed on excised hearts 8, 14, and 20 weeks after surgery. Unipolar electrograms were recorded from the left atrial epicardial surface of perfused hearts using a 5×5 electrode array. Cryosections of left atrial tissue were retained for histological and immunocytochemical analyses. Compared to Sham, AoB hearts showed marked left atrial hypertrophy and fibrosis at 14 and 20 weeks postsurgery. The incidence and duration of pacing-induced AF was increased in hearts from AoB rats at 20 weeks postsurgery. The substrate for arrhythmia was associated with reduced vectorial conduction velocity and greater inhomogeneity in conduction but without changes in effective refractory period. Left atrial expression of the gap junction protein, connexin43, was markedly reduced in AoB compared with Sham hearts. Conclusions— Using a small-animal model, we demonstrate that elevated afterload in the absence of systemic hypertension results in increased inducibility of AF and left atrial remodeling involving fibrosis, altered atrial connexin43 expression, and marked conduction abnormalities.

Selective activation of heteromeric SK channels contributes to action potential repolarization in mouse atrial myocytes

BACKGROUND Activation of small conductance calcium-activated potassium (SK) channels is proposed to contribute to repolarization of the action potential in atrial myocytes. This role is controversial, as these cardiac SK channels appear to exhibit an uncharacteristic pharmacology. OBJECTIVES The objectives of this study were to resolve whether activation of SK channels contributes to atrial action potential repolarization and to determine the likely subunit composition of the channel. METHODS The effect of 2 SK channel inhibitors was assessed on outward current evoked in voltage clamp and on action potential duration in perforated patch and whole-cell current clamp recording from acutely isolated mouse atrial myocytes. The presence of SK channel subunits was assessed using immunocytochemistry. RESULTS A significant component of outward current was reduced by the SK channel blockers apamin and UCL1684. Block by apamin displayed a sensitivity indicating that this current was carried by homomeric SK2 channels. Action potential duration was significantly prolonged by UCL1684, but not by apamin. This effect was accompanied by an increase in beat-to-beat variability and action potential triangulation. This pharmacology was matched by that of expressed heteromeric SK2-SK3 channels in HEK293 cells. Immunocytochemistry showed that atrial myocytes express both SK2 and SK3 channels with an overlapping expression pattern. CONCLUSION Only proposed heteromeric SK2-SK3 channels are physiologically activated to contribute to action potential repolarization, which is indicated by the difference in pharmacology of evoked outward current and prolongation of atrial action potential duration. The effect of blocking this channel on the action potential suggests that SK channel inhibition during cardiac function has the potential to be proarrhythmic.

Activation of Glibenclamide-Sensitive ATP-Sensitive K + Channels During β-Adrenergically Induced Metabolic Stress Produces a Substrate for Atrial Tachyarrhythmia

Background— Cardiac ATP-sensitive K+ channels have been suggested to contribute to the adaptive physiological response to metabolic challenge after &bgr;-adrenoceptor stimulation. However, an increased atrial K+-conductance might be expected to be proarrhythmic. We investigated the effect of ATP-sensitive K+ channel blockade on the electrophysiological responses to &bgr;-adrenoceptor-induced metabolic challenge in intact atria. Methods and Results— Atrial electrograms were recorded from the left atrial epicardial surface of Langendorff-perfused rat hearts using a 5×5 electrode array. Atrial effective refractory period and conduction velocity were measured using an S1–S2 protocol. The proportion of hearts in which atrial tachyarrhythmia was produced by burst-pacing was used as an index of atrial tachyarrhythmia-inducibility. Atrial nucleotide concentrations were measured by high performance liquid chromatography. Perfusion with ≥10–9 mol/L of the &bgr;-adrenoceptor agonist, isoproterenol (ISO), resulted in a concentration-dependent reduction of atrial effective refractory period and conduction velocity. The ISO-induced changes produced a proarrhythmic substrate such that atrial tachyarrhythmia could be induced by burst-pacing. Atrial [ATP] was significantly reduced by ISO (10–6 mol/L). Perfusion with either of the ATP-sensitive K+ channel blockers, glibenclamide (10–5 mol/L) or tolbutamide (10–3 mol/L), in the absence of ISO had no effect on basal atrial electrophysiology. On the other hand, the proarrhythmic substrate induced by 10–6 mol/L ISO was abolished by either of the sulfonylureas, which prevented induction of atrial tachyarrhythmia. Conclusions— Atrial ATP-sensitive K+ channels activate in response to &bgr;-adrenergic metabolic stress in Langendorff-perfused rat hearts, resulting in a proarrhythmic substrate.

Inhibition of a TREK-like K+ channel current by noradrenaline requires both β1- and β2-adrenoceptors in rat atrial myocytes

Aims Noradrenaline plays an important role in the modulation of atrial electrophysiology. However, the identity of the modulated channels, their mechanisms of modulation, and their role in the action potential remain unclear. This study aimed to investigate the noradrenergic modulation of an atrial steady-state outward current (IKss). Methods and results Rat atrial myocyte whole-cell currents were recorded at 36°C. Noradrenaline potently inhibited IKss (IC50 = 0.90 nM, 42.1 ± 4.3% at 1 µM, n = 7) and potentiated the L-type Ca2+ current (ICaL, EC50 = 136 nM, 205 ± 40% at 1 µM, n = 6). Noradrenaline-sensitive IKss was weakly voltage-dependent, time-independent, and potentiated by the arachidonic acid analogue, 5,8,11,14-eicosatetraynoic acid (EYTA; 10 µM), or by osmotically induced membrane stretch. Noise analysis revealed a unitary conductance of 8.4 ± 0.42 pS (n = 8). The biophysical/pharmacological properties of IKss indicate a TREK-like K+ channel. The effect of noradrenaline on IKss was abolished by combined β1-/β2-adrenoceptor antagonism (1 µM propranolol or 10 µM β1-selective atenolol and 100 nM β2-selective ICI-118,551 in combination), but not by β1- or β2-antagonist alone. The action of noradrenaline could be mimicked by β2-agonists (zinterol and fenoterol) in the presence of β1-antagonist. The action of noradrenaline on IKss, but not on ICaL, was abolished by pertussis toxin (PTX) treatment. The action of noradrenaline on ICaL was mediated by β1-adrenoceptors via a PTX-insensitive pathway. Noradrenaline prolonged APD30 by 52 ± 19% (n = 5; P < 0.05), and this effect was abolished by combined β1-/β2-antagonism, but not by atenolol alone. Conclusion Noradrenaline inhibits a rat atrial TREK-like K+ channel current via a PTX-sensitive mechanism involving co-operativity of β1-/β2-adrenoceptors that contributes to atrial APD prolongation.

Activation of Glibenclamide-Sensitive KATP Channels during β-Adrenergically-Induced Metabolic Stress Produces a Substrate for Atrial Tachyarrhythmia

Background— Cardiac ATP-sensitive K+ channels have been suggested to contribute to the adaptive physiological response to metabolic challenge after &bgr;-adrenoceptor stimulation. However, an increased atrial K+-conductance might be expected to be proarrhythmic. We investigated the effect of ATP-sensitive K+ channel blockade on the electrophysiological responses to &bgr;-adrenoceptor-induced metabolic challenge in intact atria. Methods and Results— Atrial electrograms were recorded from the left atrial epicardial surface of Langendorff-perfused rat hearts using a 5×5 electrode array. Atrial effective refractory period and conduction velocity were measured using an S1–S2 protocol. The proportion of hearts in which atrial tachyarrhythmia was produced by burst-pacing was used as an index of atrial tachyarrhythmia-inducibility. Atrial nucleotide concentrations were measured by high performance liquid chromatography. Perfusion with ≥10–9 mol/L of the &bgr;-adrenoceptor agonist, isoproterenol (ISO), resulted in a concentration-dependent reduction of atrial effective refractory period and conduction velocity. The ISO-induced changes produced a proarrhythmic substrate such that atrial tachyarrhythmia could be induced by burst-pacing. Atrial [ATP] was significantly reduced by ISO (10–6 mol/L). Perfusion with either of the ATP-sensitive K+ channel blockers, glibenclamide (10–5 mol/L) or tolbutamide (10–3 mol/L), in the absence of ISO had no effect on basal atrial electrophysiology. On the other hand, the proarrhythmic substrate induced by 10–6 mol/L ISO was abolished by either of the sulfonylureas, which prevented induction of atrial tachyarrhythmia. Conclusions— Atrial ATP-sensitive K+ channels activate in response to &bgr;-adrenergic metabolic stress in Langendorff-perfused rat hearts, resulting in a proarrhythmic substrate.

Short QT interval linked to androgen misuse: wider significance and possible basis.

To the Editor: We read with great interest the recent report by Dr. Bigi and colleagues1 showing significantly shortened rate-corrected QT (QTC) intervals in professional male bodybuilders taking anabolic androgenic steroids (AAS) compared to drug-free bodybuilders and age-matched sedentary controls. Not only are the findings of this study important in terms of providing a potential predictive measure of AAS misuse in strength athletes (QTC interval values of ≤380 ms in a bodybuilder may provide 83% sensitivity and 88% specificity in predicting AAS abuse),1 but they also contribute toward growing evidence (reviewed in reference 2) for an important role for androgens in cardiac repolarization. Women exhibit QTC intervals that are significantly longer than those from men, with malefemale differences commencing at puberty.2 Data from animal studies indicate that orchiectomized male rabbits exhibit abbreviated ventricular repolarization that is more reminiscent of females and that is restored by androgen administration, implicating testosterone in observed repolarization differences.2,3 Studies comparing women with virilization syndrome (due to polycystic ovary syndrome or adrenal hyperplasia) with healthy agematched controls report markedly shortened repolarization phases of the electrocardiogram (ECG) from the virilized group, with the duration of repolarization in virilized subjects being inversely associated with testosterone level.4,5 Thus, it appears that raised androgen levels can impact significantly

Modulation by endothelin-1 of spontaneous activity and membrane currents of atrioventricular node myocytes from the rabbit heart

Background The atrioventricular node (AVN) is a key component of the cardiac pacemaker-conduction system. Although it is known that receptors for the peptide hormone endothelin-1 (ET-1) are expressed in the AVN, there is very little information available on the modulatory effects of ET-1 on AVN electrophysiology. This study characterises for the first time acute modulatory effects of ET-1 on AVN cellular electrophysiology. Methods Electrophysiological experiments were conducted in which recordings were made from rabbit isolated AVN cells at 35–37°C using the whole-cell patch clamp recording technique. Results Application of ET-1 (10 nM) to spontaneously active AVN cells led rapidly (within ∼13 s) to membrane potential hyperpolarisation and cessation of spontaneous action potentials (APs). This effect was prevented by pre-application of the ETA receptor inhibitor BQ-123 (1 µM) and was not mimicked by the ETB receptor agonist IRL-1620 (300 nM). In whole-cell voltage-clamp experiments, ET-1 partially inhibited L-type calcium current (ICa,L) and rapid delayed rectifier K+ current (IKr), whilst it transiently activated the hyperpolarisation-activated current (If) at voltages negative to the pacemaking range, and activated an inwardly rectifying current that was inhibited by both tertiapin-Q (300 nM) and Ba2+ ions (2 mM); each of these effects was sensitive to ETA receptor inhibition. In cells exposed to tertiapin-Q, ET-1 application did not produce membrane potential hyperpolarisation or immediate cessation of spontaneous activity; instead, there was a progressive decline in AP amplitude and depolarisation of maximum diastolic potential. Conclusions Acutely applied ET-1 exerts a direct modulatory effect on AVN cell electrophysiology. The dominant effect of ET-1 in this study was activation of a tertiapin-Q sensitive inwardly rectifying K+ current via ETA receptors, which led rapidly to cell quiescence.

Electrophysiological properties of myocytes isolated from the mouse atrioventricular node: L‐type ICa, IKr, If, and Na‐Ca exchange

The atrioventricular node (AVN) is a key component of the cardiac pacemaker‐conduction system. This study investigated the electrophysiology of cells isolated from the AVN region of adult mouse hearts, and compared murine ionic current magnitude with that of cells from the more extensively studied rabbit AVN. Whole‐cell patch‐clamp recordings of ionic currents, and perforated‐patch recordings of action potentials (APs), were made at 35–37°C. Hyperpolarizing voltage commands from −40 mV elicited a Ba2+‐sensitive inward rectifier current that was small at diastolic potentials. Some cells (Type 1; 33.4 ± 2.2 pF; n = 19) lacked the pacemaker current, If, whilst others (Type 2; 34.2 ± 1.5 pF; n = 21) exhibited a clear If, which was larger than in rabbit AVN cells. On depolarization from −40 mV L‐type Ca2+ current, ICa,L, was elicited with a half maximal activation voltage (V0.5) of −7.6 ± 1.2 mV (n = 24). ICa,L density was smaller than in rabbit AVN cells. Rapid delayed rectifier (IKr) tail currents sensitive to E‐4031 (5 μmol/L) were observed on repolarization to −40 mV, with an activation V0.5 of −10.7 ± 4.7 mV (n = 8). The IKr magnitude was similar in mouse and rabbit AVN. Under Na‐Ca exchange selective conditions, mouse AVN cells exhibited 5 mmol/L Ni‐sensitive exchange current that was inwardly directed negative to the holding potential (−40 mV). Spontaneous APs (5.2 ± 0.5 sec−1; n = 6) exhibited an upstroke velocity of 37.7 ± 16.2 V/s and ceased following inhibition of sarcoplasmic reticulum Ca2+ release by 1 μmol/L ryanodine, implicating intracellular Ca2+ cycling in murine AVN cell electrogenesis.

Effects of candesartan, an angiotensin II receptor type I blocker, on atrial remodeling in spontaneously hypertensive rats.

Hypertension‐induced structural remodeling of the left atrium (LA) has been suggested to involve the renin–angiotensin system. This study investigated whether treatment with an angiotensin receptor blocker, candesartan, regresses atrial remodeling in spontaneously hypertensive rats (SHR). Effects of treatment with candesartan were compared to treatment with a nonspecific vasodilatator, hydralazine. Thirty to 32‐week‐old adult male SHR were either untreated (n = 15) or received one of either candesartan cilexetil (n = 9; 3 mg/kg/day) or hydralazine (n = 10; 14 mg/kg/day) via their drinking water for 14 weeks prior to experiments. Untreated age‐ and sex‐matched Wistar‐Kyoto rats (WKY; n = 13) represented a normotensive control group. Untreated SHR were hypertensive, with left ventricular hypertrophy (LVH) compared to WKY, but there were no differences in systolic pressures in excised, perfused hearts. LA from SHR were hypertrophied and showed increased fibrosis compared to those from WKY, but there was no change in connexin‐43 expression or phosphorylation. Treatment with candesartan reduced systolic tail artery pressures of conscious SHR below those of normotensive WKY and caused regression of both LVH and LA hypertrophy. Although hydralazine reduced SHR arterial pressures to those of WKY and led to regression of LA hypertrophy, it had no significant effect on LVH. Notably, LA fibrosis was unaffected by treatment with either agent. These data show that candesartan, at a dose sufficient to reduce blood pressure and LVH, did not cause regression of LA fibrosis in hypertensive rats. On the other hand, the data also suggest that normalization of arterial pressure can lead to the regression of LA hypertrophy.

Acute desensitization of acetylcholine and endothelin-1 activated inward rectifier K+ current in myocytes from the cardiac atrioventricular node.

Highlights ► ACh and ET-1 activate a K+ current in cardiac atrioventricular nodal cells. ► Tertiapin-Q sensitive IKACh activated via M2 receptors shows bi-exponential ‘fade’. ► ET-1 activates a similar current that also fades. ► The fade reflects desensitization rather than altered K+ ion driving force. ► Acetylcholine is able to cross-desensitize the AVN cell response to endothelin-1.