Three different potassium channels in human atrium. Contribution to the basal potassium conductance

The Properties of the Transient Outward, Inward Rectifier and Acetylcholine-Sensitive Potassium Currents in Atrial Myocytes from Dogs in Sinus Rhythm and Experimentally Induced Atrial Fibrillation Dog Models

AIMS

Atrial fibrillation (AF) is the most common chronic/recurrent arrhythmia, which significantly impairs quality of life and increases cardiovascular morbidity and mortality. Therefore, the aim of the present study was to investigate the properties of three repolarizing potassium currents which were shown to contribute to AF-induced electrical remodeling, i.e., the transient outward (Ito), inward rectifier (IK1) and acetylcholine-sensitive (IK,ACh) potassium currents in isolated atrial myocytes obtained from dogs either with sinus rhythm (SR) or following chronic atrial tachypacing (400/min)-induced AF.

METHODS

Atrial remodeling and AF were induced by chronic (4-6 weeks of) right atrial tachypacing (400/min) in dogs. Transmembrane ionic currents were measured by applying the whole-cell patch-clamp technique at 37 °C.

RESULTS

The Ito current was slightly downregulated in AF cells when compared with that recorded in SR cells. This downregulation was also associated with slowed inactivation kinetics. The IK1 current was found to be larger in AF cells; however, this upregulation was not statistically significant in the voltage range corresponding with atrial action potential (-80 mV to 0 mV). IK,ACh was activated by the cholinergic agonist carbachol (CCh; 2 µM). In SR, CCh activated a large current either in inward or outward directions. The selective IK,ACh inhibitor tertiapin (10 nM) blocked the outward CCh-induced current by 61%. In atrial cardiomyocytes isolated from dogs with AF, the presence of a constitutively active IK,ACh was observed, blocked by 59% with 10 nM tertiapin. However, in "AF atrial myocytes", CCh activated an additional, significant ligand-dependent and tertiapin-sensitive IK,ACh current.

CONCLUSIONS

In our dog AF model, Ito unlike in humans was downregulated only in a slight manner. Due to its slow inactivation kinetics, it seems that Ito may play a more significant role in atrial repolarization than in ventricular working muscle myocytes. The presence of the constitutively active IK,ACh in atrial myocytes from AF dogs shows that electrical remodeling truly developed in this model. The IK,ACh current (both ligand-dependent and constitutively active) seems to play a significant role in canine atrial electrical remodeling and may be a promising atrial selective drug target for suppressing AF.

Cardioprotective effect of succinate dehydrogenase inhibition in rat hearts and human myocardium with and without diabetes mellitus

Ischemia reperfusion (IR) injury may be attenuated through succinate dehydrogenase (SDH) inhibition by dimethyl malonate (DiMAL). Whether SDH inhibition yields protection in diabetic individuals and translates into human cardiac tissue remain unknown. In isolated perfused hearts from 24 weeks old male Zucker diabetic fatty (ZDF) and age matched non-diabetic control rats and atrial trabeculae from patients with and without diabetes, we compared infarct size, contractile force recovery and mitochondrial function. The cardioprotective effect of a 10 minutes DiMAL administration prior to global ischemia and ischemic preconditioning (IPC) was evaluated. In non-diabetic hearts exposed to IR, DiMAL 0.1 mM reduced infarct size compared to IR (55 ± 7% vs. 69 ± 6%, p < 0.05). Mitochondrial respiration was reduced by DiMAL 0.6 mM compared to sham and DiMAL 0.1 mM (p < 0.05). In diabetic hearts an increased concentration of DiMAL (0.6 mM) was required for protection compared to IR (64 ± 13% vs. 79 ± 8%, p < 0.05). Mitochondrial function remained unchanged. In trabeculae from humans without diabetes, IPC and DiMAL improved contractile force recovery compared to IR (43 ± 12% and 43 ± 13% vs. 23 ± 13%, p < 0.05) but in patients with diabetes only IPC provided protection compared to IR (51 ± 15% vs. 21 ± 8%, p < 0.05). Neither IPC nor DiMAL modulated mitochondrial respiration in patients. Cardioprotection by SDH inhibition is possible in human tissue, but depends on diabetes status. The narrow therapeutic range and discrepancy in respiration between experimental and human studies may limit clinical translation.

Vernakalant hydrochloride for the treatment of atrial fibrillation: evaluation of its place in clinical practice

Vernakalant is an intravenous anti-arrhythmic drug available in Europe, Canada and some countries in Asia for the restoration of sinus rhythm in acute onset atrial fibrillation. Currently, it is not available in USA because the US FDA have ongoing concerns about its safety. Vernakalant has a unique pharmacological profile of multi-ion channel activity and atrial-specificity that distinguishes it from other anti-arrhythmic drugs. This is thought to enhance efficacy but there are concerns of adverse events stemming from its diverse pharmacology. This ambiguity has prompted a review of the available clinical evidence on efficacy and safety to help re-evaluate its place in clinical practice.

Towards chamber specific heart-on-a-chip for drug testing applications

Modeling of human organs has long been a task for scientists in order to lower the costs of therapeutic development and understand the pathological onset of human disease. For decades, despite marked differences in genetics and etiology, animal models remained the norm for drug discovery and disease modeling. Innovative biofabrication techniques have facilitated the development of organ-on-a-chip technology that has great potential to complement conventional animal models. However, human organ as a whole, more specifically the human heart, is difficult to regenerate in vitro, in terms of its chamber specific orientation and its electrical functional complexity. Recent progress with the development of induced pluripotent stem cell differentiation protocols, made recapitulating the complexity of the human heart possible through the generation of cells representative of atrial & ventricular tissue, the sinoatrial node, atrioventricular node and Purkinje fibers. Current heart-on-a-chip approaches incorporate biological, electrical, mechanical, and topographical cues to facilitate tissue maturation, therefore improving the predictive power for the chamber-specific therapeutic effects targeting adult human. In this review, we will give a summary of current advances in heart-on-a-chip technology and provide a comprehensive outlook on the challenges involved in the development of human physiologically relevant heart-on-a-chip.

Vernakalant in Atrial Fibrillation: A Relatively New Weapon in the Armamentarium Against an Old Enemy

Atrial fibrillation is the most common sustained cardiac arrhythmia, and its prevalence is increasing with age; also it is associated with significant morbidity and mortality. Rhythm control is advised in recent-onset atrial fibrillation, and in highly symptomatic patients, also in young and active individuals. Moreover, rhythm control is associated with lower incidence of progression to permanent atrial fibrillation. Vernakalant is a relatively new anti-arrhythmic drug that showed efficacy and safety in recent-onset atrial fibrillation. Vernakalant is indicated in atrial fibrillation (⩽7 days) in patients with no heart disease (class I, level A) or in patients with mild or moderate structural heart disease (class IIb, level B). Moreover, Vernakalant may be considered for recent-onset atrial fibrillation (⩽3 days) post cardiac surgery (class IIb, level B). Although it is mainly indicated in patients with recent-onset atrial fibrillation and with no structural heart disease, it can be given in moderate stable cardiac disease as alternative to Amiodarone. Similarly to electrical cardioversion, pharmacological cardioversion requires a minimal evaluation and cardioversion should be included in a comprehensive management strategy for better outcome.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Isolation of human atrial myocytes for simultaneous measurements of Ca2+ transients and membrane currents

The study of electrophysiological properties of cardiac ion channels with the patch-clamp technique and the exploration of cardiac cellular Ca(2+) handling abnormalities requires isolated cardiomyocytes. In addition, the possibility to investigate myocytes from patients using these techniques is an invaluable requirement to elucidate the molecular basis of cardiac diseases such as atrial fibrillation (AF).(1) Here we describe a method for isolation of human atrial myocytes which are suitable for both patch-clamp studies and simultaneous measurements of intracellular Ca(2+) concentrations. First, right atrial appendages obtained from patients undergoing open heart surgery are chopped into small tissue chunks ("chunk method") and washed in Ca(2+)-free solution. Then the tissue chunks are digested in collagenase and protease containing solutions with 20 μM Ca(2+). Thereafter, the isolated myocytes are harvested by filtration and centrifugation of the tissue suspension. Finally, the Ca(2+) concentration in the cell storage solution is adjusted stepwise to 0.2 mM. We briefly discuss the meaning of Ca(2+) and Ca(2+) buffering during the isolation process and also provide representative recordings of action potentials and membrane currents, both together with simultaneous Ca(2+) transient measurements, performed in these isolated myocytes.

Tissue-specific effects of acetylcholine in the canine heart

Acetylcholine (ACh) release from the vagus nerve slows heart rate and atrioventricular conduction. ACh stimulates a variety of receptors and channels, including an inward rectifying current [ACh-dependent K⁺ current (IK,ACh)]. The effect of ACh in the ventricle is still debated. We compared the effect of ACh on action potentials in canine atria, Purkinje, and ventricular tissue as well as on ionic currents in isolated cells. Action potentials were recorded from ventricular slices, Purkinje fibers, and arterially perfused atrial preparations. Whole cell currents were recorded under voltage-clamp conditions, and unloaded cell shortening was determined on isolated cells. The effect of ACh (1-10 μM) as well as ACh plus tertiapin, an IK,ACh-specific toxin, was tested. In atrial tissue, ACh hyperpolarized the membrane potential and shortened the action potential duration (APD). In Purkinje and ventricular tissues, no significant effect of ACh was observed. Addition of ACh to atrial cells activated a large inward rectifying current (from -3.5 ± 0.7 to -23.7 ± 4.7 pA/pF) that was abolished by tertiapin. This current was not observed in other cell types. A small inhibition of Ca²⁺ current (ICa) was observed in the atria, endocardium, and epicardium after ACh. ICa inhibition increased at faster pacing rates. At a basic cycle length of 400 ms, ACh (1 μM) reduced ICa to 68% of control. In conclusion, IK,ACh is highly expressed in atria and is negligible/absent in Purkinje, endocardial, and epicardial cells. In all cardiac tissues, ACh caused rate-dependent inhibition of ICa.

Impact of O-GlcNAc on cardioprotection by remote ischaemic preconditioning in non-diabetic and diabetic patients

AIMS

Post-translational modification of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) is cardioprotective but its role in cardioprotection by remote ischaemic preconditioning (rIPC) and the reduced efficacy of rIPC in type 2 diabetes mellitus is unknown. In this study we achieved mechanistic insight into the remote stimulus mediating and the target organ response eliciting the cardioprotective effect by rIPC in non-diabetic and diabetic myocardium and the influence of O-GlcNAcylation.

METHODS AND RESULTS

The cardioprotective capacity and the influence on myocardial O-GlcNAc levels of plasma dialysate from eight healthy volunteers and eight type 2 diabetic patients drawn before and after subjection to an rIPC stimulus were tested on human isolated atrial trabeculae subjected to ischaemia/reperfusion injury. Dialysate from healthy volunteers exposed to rIPC improved post-ischaemic haemodynamic recovery (40 ± 6 vs. 16 ± 2%; P < 0.01) and increased myocardial O-GlcNAc levels. Similar observations were made with dialysate from diabetic patients before exposure to rIPC (43 ± 3 vs. 16 ± 2%; P < 0.001) but no additional cardioprotection or further increase in O-GlcNAc levels was achieved by perfusion with dialysate after exposure to rIPC (44 ± 4 and 42 ± 5 vs. 43 ± 3%; P = 0.7). The glutamine:fructose-6-phosphate amidotransferase (GFAT) inhibitor azaserine abolished the cardioprotective effects and the increment in myocardial O-GlcNAc levels afforded by plasma from diabetic patients and healthy volunteers treated with rIPC.

CONCLUSIONS

rIPC and diabetes mellitus per se influence myocardial O-GlcNAc levels through circulating humoral factors. O-GlcNAc signalling participates in mediating rIPC-induced cardioprotection and maintaining a state of inherent chronic activation of cardioprotection in diabetic myocardium, restricting it from further protection by rIPC.

Vernakalant: A novel agent for the termination of atrial fibrillation

PURPOSE

The pharmacology, pharmacokinetics, safety, clinical efficacy, and role of intravenous vernakalant hydrochloride for the rapid conversion of atrial fibrillation (AF) to normal sinus rhythm are reviewed.

SUMMARY

Vernakalant, currently being evaluated by the Food and Drug Administration (FDA), for the termination of atrial fibrillation, differs in pharmacology from other antiarrhythmics; it achieves action potential interference through blockade of sodium and potassium currents. Vernakalant's actions appear to be directed at relatively atrial-selective potassium currents, which result in lengthening of the atrial action potential and prolongation of the atrial action potential plateau, while not significantly affecting the Q-T interval or the ventricular effective refractory period. As a result, the proarrhythmic effects observed with all other agents approved by FDA for the treatment of AF are eliminated. In clinical trials of vernakalant versus placebo, a statistically significant number of patients converted to normal sinus rhythm after receiving vernakalant. For patients with atrial fibrillation continuing for 3-72 hours, the median time to conversion was between 8 and 14 minutes, with 79% of those who converted remaining in sinus rhythm at 24 hours.

CONCLUSION

Intravenous vernakalant, a novel, relatively atrial-selective antiarrhythmic agent, appears to offer an effective and safe approach to the rapid conversion of recent-onset AF to normal sinus rhythm.

Voltage-clamp-based methods for the detection of constitutively active acetylcholine-gated I(K,ACh) channels in the diseased heart

Vagal nerve stimulation can promote atrial fibrillation (AF) that requires activation of the acetylcholine (ACh)-gated potassium current I(K,ACh). In chronic AF (cAF), I(K,ACh) shows strong activity despite the absence of ACh or analogous pharmacological stimulation. This receptor-independent, constitutive I(K,ACh) activity is suggested to represent an atrial-selective anti-AF therapeutic target, but the underlying molecular mechanisms are unknown. This chapter provides an overview of the voltage-clamp techniques that can be used to study constitutive I(K,ACh) activity in atrial myocytes and summarizes briefly the current knowledge about the potential underlying mechanism(s) of constitutive I(K,ACh) activity in diseased heart.

Specificities of atrial electrophysiology: Clues to a better understanding of cardiac function and the mechanisms of arrhythmias

The electrical properties of the atria and ventricles differ in several aspects reflecting the distinct role of the atria in cardiac physiology. The study of atrial electrophysiology had greatly contributed to the understanding of the mechanisms of atrial fibrillation (AF). Only the atrial L-type calcium current is regulated by serotonine or, under basal condition, by phosphodiesterases. These distinct regulations can contribute to I(Ca) down-regulation observed during AF, which is an important determinant of action potential refractory period shortening. The voltage-gated potassium current, I(Kur), has a prominent role in the repolarization of the atrial but not ventricular AP. In many species, this current is based on the functional expression of K(V)1.5 channels, which might represent a specific therapeutic target for AF. Mechanisms regulating the trafficking of K(V)1.5 channels to the plasma membrane are being actively investigated. The resting potential of atrial myocytes is maintained by various inward rectifier currents which differ with ventricle currents by a reduced density of I(K1), the presence of a constitutively active I(KACh) and distinct regulation of I(KATP). Stretch-sensitive or mechanosensitive ion channels are particularly active in atrial myocytes and are involved in the secretion of the natriuretic peptide. Integration of knowledge on electrical properties of atrial myocytes in comprehensive schemas is now necessary for a better understanding of the physiology of atria and the mechanisms of AF.

Age-related attenuation of isoflurane preconditioning in human atrial cardiomyocytes: roles for mitochondrial respiration and sarcolemmal adenosine triphosphate-sensitive potassium channel activity

BACKGROUND

Clinical trials suggest that anesthetic-induced preconditioning (APC) produces cardioprotection in humans, but the mechanisms of APC and significance of aging for APC in humans are not well understood. Here, the impact of age on the role of two major effectors of APC, mitochondria and sarcolemmal adenosine triphosphate-sensitive potassium (sarcKATP) channels, in preconditioning of the human atrial myocardium were investigated.

METHODS

Right atrial appendages were obtained from adult patients undergoing cardiac surgery and assigned to mid-aged (MA) and old-aged (OA) groups. APC was induced by isoflurane in isolated myocardium and isolated cardiomyocytes. Mitochondrial oxygen consumption measurements, myocyte survival testing, and patch clamp techniques were used to investigate mitochondrial respiratory function and sarcKATP channel activity.

RESULTS

After in vitro APC with isoflurane, the respiratory function of isolated mitochondria was better preserved after hypoxia-reoxygenation stress in MA than in OA. In isolated intact myocytes, APC significantly decreased oxidative stress-induced cell death in MA but not in OA, and isoflurane protection from cell death was attenuated by the sarcKATP channel inhibitor HMR-1098. Further, the properties of single sarcKATP channels were similar in MA and OA, and isoflurane sensitivity of pinacidil-activated whole cell KATP current was no different between MA and OA myocytes.

CONCLUSION

Anesthetic-induced preconditioning with isoflurane decreases stress-induced cell death and preserves mitochondrial respiratory function to a greater degree in MA than in OA myocytes; however, sarcKATP channel activity is not differentially affected by isoflurane. Therefore, effectiveness of APC in humans may decrease with advancing age partly because of altered mitochondrial function of myocardial cells.

Vernakalant (RSD1235): a novel, atrial-selective antifibrillatory agent

This review summarizes the mechanistic properties and the recent experience in the development of a new antiarrhythmic agent, RSD1235 (recently named vernakalant), for the acute conversion of atrial fibrillation to sinus rhythm. Atrial fibrillation is the most common sustained cardiac arrhythmia that is observed in clinical practice and is associated with increased morbidity and mortality, resulting from stroke and exacerbation of heart failure. At present, there is a lack of pharmacologic agents that are able to safely and effectively convert the arrhythmia back to sinus rhythm. Vernakalant has the electrophysiologic properties of a multiple ion channel blocker, developed using a novel approach to target potassium channels that are selectively present in human atria rather than ventricles, and using a rate-dependent blocking strategy for its additional sodium channel block. This paper reviews the mechanism of action of this drug, its performance in preclinical models of efficacy and human disease, and its actions on patients in the completed and published preregistration clinical trials for vernakalant. Overall, vernakalant converted 51.5% of patients who had < 7 days duration of atrial fibrillation and it did this without significantly more cardiovascular adverse events than placebo. Therefore, it must be considered as an important new agent for the treatment of this growing health problem.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Effect of the degree of ischaemic injury and reoxygenation time on the type of myocardial cell death in man: role of caspases

Background

The importance of apoptosis in the injury sustained by the human myocardium during ischaemia and reoxygenation and the underlying mechanisms remain unclear. To quantify apoptosis and necrosis induced by simulated ischaemia/reoxygenation in the human atrial myocardium, free-hand sections of right atrial appendage (n = 8/group) were subjected to 90 minutes simulated ischaemia followed by 2, 8 and 24 hours reoxygenation.

Results

Apoptosis, as assessed by TUNEL, was greater than necrosis after 90 minutes simulated ischaemia and 2 hours reoxygenation (35.32 ± 3.22% vs 13.55 ± 1.3%; p < 0.05) but necrosis was greater than apoptosis by 24 hours reoxygenation (45.20 ± 2.75% vs 4.82 ± 0.79%; p < 0.05). Total caspase activation was similar after 90 minutes simulated ischaemia followed by 2 hours and 24 hours reoxygenation (515270 ± 99570 U vs 542940 ± 95216 U; p = NS). However, caspase-3 like activation was higher at 2 hours than at 24 hours reoxygenation (135900 ± 42200 U vs 54970 ± 19100 U; p < 0.05). Inhibition of caspase-3 by z.DEVD.fmk (70 nM) almost completely abolished apoptosis from 23.26 ± 2.854% to 0.73 ± 0.28 % (p < 0.05), without affecting necrosis.

Conclusion

Cell death by apoptosis and necrosis in the human myocardium subjected to simulated ischaemia/reoxygenation depends on the degree of the ischaemic insult and have a different time-course with apoptosis happening early during reoxygenation and necrosis becoming more important later. Importantly, the apoptosis induced by simulated ischaemia/reoxygenation is mainly mediated by activation of caspase-3 but it does not affect necrosis.

Coupling to Gs and G(q/11) of histamine H2 receptors heterologously expressed in adult rat atrial myocytes

The predominant histamine receptor subtype in the supraventricular and ventricular tissue of various mammalian species is the H2 receptor (H2-R) subtype, which is known to couple to stimulatory G proteins (Gs), i.e. the major effects of this autacoid are an increase in sinus rate and in force of contraction. To investigate histamine effects in H2-R-transfected rat atrial myocytes, endogenous GIRK currents and L-type Ca2+ currents were used as functional assays. In H2-R-transfected myocytes, exposure to His resulted in a reversible augmentation of L-type Ca2+ currents, consistent with the established coupling of this receptor to the Gs-cAMP-PKA signalling pathway. Mammalian K+ channels composed of GIRK (Kir3.x) subunits are directly controlled by interaction with betagamma subunits released from G proteins, which couple to seven-helix receptors. In mock-transfected atrial cardiomyocytes, activation of muscarinic K+ channels (IK(ACh)) was limited to Gi-coupled receptors (M2R, A1R). In H2-R-overexpressing cells, histamine activated IK(ACh) via Gs-derived betagamma subunits since the histamine-induced current was insensitive to pertussis toxin. These data indicate that overexpression of Gs-coupled H2-R results in a loss of target specificity due to an increased agonist-induced release of Gs-derived betagamma subunits. When IK(ACh) was maximally activated by GTP-gamma-S, histamine induced an irreversible inhibition of the inward current in a fraction of H2-R-transfected cells. This inhibition is supposed to be mediated via a G(q/11)-PLC-mediated depletion of PIP2, suggesting a partial coupling of overexpressed H2-R to G(q/11). Dual coupling of H2-Rs to Gs and Gq is demonstrated for the first time in cardiac myocytes. It represents a novel mechanism to augment positive inotropic effects by activating two different signalling pathways via one type of histamine receptor. Activation of the Gs-cAMP-PKA pathway promotes Ca2+ influx through phosphorylation of L-type Ca2+ channels. Simultaneous activation of Gq-signalling pathways might result in phosphoinositide turnover and Ca2+ release from intracellular stores, thereby augmenting H2-induced increases in [Ca2+]i.

Protection of the human heart with ischemic preconditioning during cardiac surgery: role of cardiopulmonary bypass

OBJECTIVE

Studies on the effects of ischemic preconditioning in the human heart have yielded conflicting results and therefore remain controversial. This study investigated whether ischemic preconditioning was able to protect against myocardial tissue damage in patients undergoing coronary artery surgery with cardiopulmonary bypass and on the beating heart.

METHODS

A total of 120 patients were studied and divided into 3 groups: group I: cardiopulmonary bypass with intermittent crossclamp fibrillation; group II: cardiopulmonary bypass with cardioplegic arrest using cold blood cardioplegia; group III: surgery on the beating heart. In each group (n = 40), patients were randomly subdivided (n = 20/subgroup) into control and preconditioning groups (1 cycle of 5 minutes of ischemia/5 minutes reperfusion before intervention). Ischemic preconditioning was induced by clamping the ascending aorta in groups I and II or by clamping the coronary artery in group III. Serial venous blood levels of troponin T were analyzed before surgery and at 1, 4, 8, 24, and 48 hours after termination of ischemia. In addition, in vitro studies using right atrial specimens obtained before the institution of cardiopulmonary bypass, and then again 10 minutes after initiation of bypass, were performed. The specimens were equilibrated for 30 minutes before being allocated to 1 of the following 2 groups (n = 6 per group): (1) ischemia alone (90 minutes of ischemia followed by 120 minutes of reoxygenation) or (2) preconditioning with 5 minutes of ischemia and 5 minutes of reoxygenation before the long ischemic insult. Creatine kinase leakage (U/g wet weight) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction (mmol/l per gram wet weight), an index of cell viability, were assessed at the end of the experiment.

RESULTS

There were no perioperative myocardial infarctions or deaths in any of the groups studied. The total release of troponin T was similar in groups I and II (patients undergoing surgery with cardiopulmonary bypass) and in the release profile; they were unaffected by ischemic preconditioning. In contrast, the total troponin T release for the first 48 hours was significantly reduced by ischemic preconditioning in group III (patients undergoing surgery without cardiopulmonary bypass) from 3.1 +/- 0.1 to 2.1 +/- 0.2 ng. h. mL. Furthermore, the release profile that peaked at 8 hours in the control group shifted to the left at 1 hour. In the in vitro studies, the atrial muscles obtained before cardiopulmonary bypass were protected by ischemic preconditioning (creatine kinase = 2.6 +/- 0.2 and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide reduction = 152 +/- 24 vs creatine kinase = 5.4 +/- 0.6 and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide reduction = 87 +/- 16 in controls; P <.05); however, the muscles obtained 10 minutes after initiation of cardiopulmonary bypass were already protected (creatine kinase = 0.8 +/- 0.1 and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide reduction = 316 +/- 38), and ischemic preconditioning did not result in further improvements.

CONCLUSIONS

Ischemic preconditioning is protective in patients undergoing coronary artery surgery on the beating heart without the use of cardiopulmonary bypass, but it offers no additional benefit when associated with bypass regardless of the mode of cardioprotection used, because cardiopulmonary bypass per se induces preconditioning.

NADH supplementation decreases pinacidil-primed I K ATP in ventricular cardiomyocytes by increasing intracellular ATP

1 The aim of this study was to investigate the effect of nicotinamide-adenine dinucleotide (NADH) supplementation on the metabolic condition of isolated guinea-pig ventricular cardiomyocytes. The pinacidil-primed ATP-dependent potassium current I(K(ATP)) was used as an indicator of subsarcolemmal ATP concentration and intracellular adenine nucleotide contents were measured. 2 Membrane currents were studied using the patch-clamp technique in the whole-cell recording mode at 36-37 degrees C. Adenine nucleotides were determined by HPLC. 3 Under physiological conditions (4.3 mM ATP in the pipette solution, ATP(i)) I(K(ATP)) did not contribute to basal electrical activity. 4 The ATP-dependent potassium (K((ATP))) channel opener pinacidil activated I(K(ATP)) dependent on [ATP](i) showing a significantly more pronounced activation at lower (1 mM) [ATP](i). 5 Supplementation of cardiomyocytes with 300 micro g ml(-1) NADH (4-6 h) resulted in a significantly reduced I(K(ATP)) activation by pinacidil compared to control cells. The current density was 13.8+/-3.78 (n=6) versus 28.9+/-3.38 pA pF(-1) (n=19; P<0.05). 6 Equimolar amounts of the related compounds nicotinamide and NAD(+) did not achieve a similar effect like NADH. 7 Measurement of adenine nucleotides by HPLC revealed a significant increase in intracellular ATP (NADH supplementation: 45.6+/-1.88 nmol mg(-1) protein versus control: 35.4+/-2.57 nmol mg(-1) protein, P<0.000005). 8 These data show that supplementation of guinea-pig ventricular cardiomyocytes with NADH results in a decreased activation of I(K(ATP)) by pinacidil compared to control myocytes, indicating a higher subsarcolemmal ATP concentration. 9 Analysis of intracellular adenine nucleotides by HPLC confirmed the significant increase in ATP.

Distribution of the muscarinic K+ channel proteins Kir3.1 and Kir3.4 in the ventricle, atrium, and sinoatrial node of heart

The functionally important effects on the heart of ACh released from vagal nerves are principally mediated by the muscarinic K+ channel. The aim of this study was to determine the abundance and cellular location of the muscarinic K+ channel subunits Kir3.1 and Kir3.4 in different regions of heart. Western blotting showed a very low abundance of Kir3.1 in rat ventricle, although Kir3.1 was undetectable in guinea pig and ferret ventricle. Although immunofluorescence on tissue sections showed no labeling of Kir3.1 in rat, guinea pig, and ferret ventricle and Kir3.4 in rat ventricle, immunofluorescence on single ventricular cells from rat showed labeling in t-tubules of both Kir3.1 and Kir3.4. Kir3.1 was abundant in the atrium of the three species, as shown by Western blotting and immunofluorescence, and Kir3.4 was abundant in the atrium of rat, as shown by immunofluorescence. Immunofluorescence showed Kir3.1 expression in SA node from the three species and Kir3.4 expression in the SA node from rat. The muscarinic K+ channel is activated by ACh via the m2 muscarinic receptor and, in atrium and SA node from ferret, Kir3.1 labeling was co-localized with m2 muscarinic receptor labeling throughout the outer cell membrane.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Failure to precondition pathological human myocardium

OBJECTIVES

We investigated the effects of ischemic preconditioning (PC) on diabetic and failing human myocardium and the role of mitochondrial KATP channels on the response in these diseased tissues.

BACKGROUND

There is conflicting evidence to suggest that PC is a healthy heart phenomenon.

METHODS

Right atrial appendages were obtained from seven different groups of patients: nondiabetics, diet-controlled diabetics, noninsulin-dependent diabetics (NIDD) receiving KATP channel blockers, insulin-dependent diabetics (IDD), and patients with left ventricular ejection fraction (LVEF) >50%, LVEF between 30% and 50% and LVEF <30%. After stabilization, the muscle slices were randomized into five experimental groups (n = 6/group): 1) aerobic control-incubated in oxygenated buffer for 210 min, 2) ischemia alone-90 min ischemia followed by 120 min reoxygenation, 3) preconditioning by 5 min ischemia/5 min reoxygenation before 90 min ischemia/120 min reoxygenation, 4) diazoxide (Mito KATP opener, 0.1 mm)-for 10 min before the 90 min ischemia/120 min reoxygenation and 5) glibenclamide (10 microm)-10 min exposure prior to PC (only in the diabetic patient groups). Creatine kinase leakage into the medium (CK, U/g wet wt) and MTT reduction (OD/mg wet wt), an index of cell viability, were assessed at the end of the experiment.

RESULTS

Ischemia caused similar injury in both normal and diseased tissue. Preconditioning prevented the effects of ischemia in all groups except NIDD, IDD and poor cardiac function (<30%). In the diazoxide-treated groups, protection was mimicked in all groups except the NIDD and IDD groups. Interestingly, glybenclamide abolished protection in nondiabetic and diet-controlled NIDD groups and did not affect NIDD groups receiving KATP channel blockers or IDD groups.

CONCLUSIONS

These results show that failure to precondition the diabetic heart is due to dysfunction of the mitochondrial KATP channels and that the mechanism of failure in the diabetic heart lies in elements of the signal transduction pathway different from the mitochondrial KATP channels.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

The sulphonylurea glibenclamide inhibits voltage dependent potassium currents in human atrial and ventricular myocytes

1 It was the aim of our study to investigate the effects of the sulphonylurea glibenclamide on voltage dependent potassium currents in human atrial myocytes. 2 The drug blocked a fraction of the quasi steady state current (ramp response) which was activated positive to -20 mV, was sensitive to 4-aminopyridine (500 microM) and was different from the ATP dependent potassium current IK(ATP). 3 Glibenclamide dose dependently inhibited both, the peak as well as the late current elicited by step depolarization positive to -20 mV. The IC50 for reduction in charge area of total outward current was 76 microM. 4 The double-exponential inactivation time-course of the total outward current was accelerated in the presence of glibenclamide with a tau(fast) of 12.7+/-1.5 ms and a tau(slow) of 213+/-25 ms in control and 5.8+/-1.9 ms (P<0.001) and 101+/-20 ms (P<0.05) under glibenclamide (100 microM). 5 Our data suggest, that both repolarizing currents in human atrial myocytes, the transient outward current (Ito1) and the ultrarapid delayed rectifier current (IKur) were inhibited by glibenclamide. 6 In human ventricular myocytes glibenclamide inhibited Ito1 without affecting the late current. 7 Our data suggest that glibenclamide inhibits human voltage dependent cardiac potassium currents at concentrations above 10 microM.

Protection of human myocardium in vitro by K(ATP) activation with low concentrations of bimakalim

We investigated whether the adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channel activation by bimakalim, at concentrations devoid of both negative inotropic and action-potential duration (APD) shortening effects, might exhibit myocardial protection after hypoxia and reoxygenation in human atrial myocardium by using 112 preparations. The recovery of contractility of human atrial trabeculae, subjected either to short-duration (5 min) or to long-duration (60 min) and severe (high pacing rate) hypoxia followed by reoxygenation, was assessed by challenging with dobutamine. Treated preparations were exposed to 10 or 100 nM bimakalim, 1 microM glibenclamide, or both before hypoxia. Variations of isometric developed tension (%DT) or APD90 were studied. At concentrations <100 nM, bimakalim showed no negative inotropic effects and did not modify significantly APD90 either in normoxia or in hypoxic conditions. In the short-duration hypoxia protocol, preparations treated with bimakalim showed a dobutamine-induced %DT increase significantly higher (p < 0.001) than in controls and similar to that observed in the absence of hypoxia. This bimakalim effect was blocked by glibenclamide. In the long-duration hypoxia protocol, %DT after dobutamine was 50% of that observed in normoxic preparations. Preparations treated with bimakalim showed after dobutamine %DT more than twofold above controls (p < 0.001), whereas in the glibenclamide group, recovery of DT with dobutamine remained 50% of what observed in normoxia (p < 0.001). In conclusion, exposure to hypoxia (either short- or long-lasting) and reoxygenation affects contractility of human atrial myocardium with pronounced reduction of the positive inotropic action of dobutamine. Pretreatment with bimakalim restores the response expected in the absence of hypoxia, and glibenclamide blocks the effect of bimakalim or further impairs the response to dobutamine when used alone before long-duration hypoxia. Evidence is provided for protective effects of the K(ATP) opener bimakalim on the human myocardial contractile function in conditions of hypoxia/reoxygenation, at concentrations at which negative inotropism and APD90 shortening are not contributory.

Activation of muscarinic K+ current by beta-adrenergic receptors in cultured atrial myocytes transfected with beta1 subunit of heterotrimeric G proteins

Muscarinic K+ channels (IK(ACh)) in native atrial myocytes are activated by betagamma subunits of pertussis toxin (Ptx)-sensitive heterotrimeric G proteins coupled to different receptors. betagamma subunits of Ptx-insensitive Gs, coupled to beta-adrenergic receptors, do not activate native IK(ACh). In atrial myocytes from adult rats transfected with rat brain beta1 subunit IK(ACh) can be activated by stimulation of beta-adrenergic receptors using isoprenaline. This effect is insensitive to Ptx. These findings demonstrate for the first time promiscuous (Ptx-insensitive) coupling of Gsbetagamma to GIRK channels in their native environment.

Electrophysiological mechanisms for the antiarrhythmic activities of naloxone on cardiac tissues

It has been reported that naloxone, an opioid antagonist, has antiarrhythmic activity in vivo. In Langendorff perfused rat hearts, we found that ischemia-reperfusion-induced ventricular tachyarrhythmia reverted to normal sinus rhythm after the treatment with naloxone (3 approximately 10 microM). The method of voltage and current clamp were used to study the underlying mechanism of its antiarrhythmic activity on isolated cardiac myocytes. In isolated rat ventricular and in guinea-pig and human atrial myocytes, naloxone prolonged the action potential duration reversibly. In rat ventricular myocytes, naloxone (1 approximately 30 microM) inhibited sodium current (I(Na)), transient outward potassium current (I(to)), and calcium current (I(Ca)). On the contrary, the addition of naloxone significantly increased inward rectifier potassium current (I(K1)). For the effect on I(Na), naloxone did not shift the inactivation curve of I(Na) but retarded the I(Na) recovery rate from inactivation state. Naloxone suppressed I(to) with a significant left-shift of the inactivation curve, however, the time course of I(to) recovery from inactivation was not affected. In guinea pig atrial myocytes, naloxone (10 microM) decreased the delayed rectifier K+ current (IK). These results show that naloxone exert various extent of inhibition on I(Na), I(to), IK and I(Ca). The prolongation of cardiac action potential is related to the inhibition of I(to) and IK. The antiarrhythmic activity of naloxone is more closely related to the inhibition of Na+ and K+ currents rather than the blockade of myocardial opioid receptors.

Function, regulation, pharmacology, and molecular structure of ATP-sensitive K+ channels in the cardiovascular system

ATP-sensitive K+ (K[ATP]) channels are inhibited by intracellular ATP and activated by intracellular nucleoside diphosphates, and thus provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective as well as proarrhythmic effects. In the vascular smooth muscle, the K(ATP) channel is believed to mediate the relaxation of vascular tone. Thus, K(ATP) channels play important regulatory roles in the cardiovascular system. Furthermore, K(ATP) channels are the targets of two important classes of drugs, i.e., the antidiabetic sulfonylureas, which block the channels, and a series of vasorelaxants called "K+ channel openers," which tend to maintain the channels in an open conformation. Recently, the molecular structure of K(ATP) channels has been clarified. The K(ATP) channel in pancreatic beta-cells is a complex composed of at least two subunits, a member of inwardly rectifying K+ channels and a sulfonylurea receptor. Subsequently, two additional homologs of the sulfonylurea receptor, which form cardiac and smooth muscle type K(ATP) channels, respectively, have been reported. Further works are now in progress to understand the molecular mechanisms of K(ATP) channel function.

Antiarrhythmic therapy--future trends and forecast for the 21st century

This article discusses recent changes in antiarrhythmic therapy, with a focus on nonpharmacologic therapy (electrode catheter ablation, implantable cardioverter-defibrillators [ICDs]), and puts them into perspective for the coming years. The treatment of supraventricular tachycardias and tachycardia involving accessory pathways is likely to remain the domain of catheter ablation. With promising new techniques under investigation, the spectrum of arrhythmias that can be cured will probably be expanded. Treatment of life-threatening ventricular arrhythmias is likely to remain the domain of the ICD in the foreseeable future. With the safety net of the ICD in place, new antiarrhythmic drugs or other forms of antiarrhythmic therapy can be developed and tested.

Ischaemic preconditioning may abolish the protection afforded by ATP-sensitive potassium channel openers in isolated human atrial muscle

The ATP-sensitive potassium channel (KATP channel) has been implicated in the mechanism underlying ischaemic preconditioning protection. This study based on human atrium compared the protective effects of ischaemic preconditioning with pre-operative nicorandil (a KATP channel opener with nitrate actions). We also examined the added effect of ischaemic preconditioning to that of nicorandil on ischaemic protection. The protective effects of other KATP channel openers devoid of nitrate actions were also examined. Atrial trabeculae harvested from patients undergoing routine myocardial revascularisation were divided on the basis of whether patients had been ingesting nicorandil orally preoperatively. Trabeculae were superfused with oxygenated Tyrode's solution and following stabilisation underwent 90 minutes simulated ischaemia followed by 120 minutes reoxygenation (n = 6 per group). Atrial trabeculae exposed to nicorandil underwent either no treatment (N), or ischaemic preconditioning (N + PC) using 3 minutes simulated ischaemia and 7 minutes reoxygenation prior to the 90 minutes simulated ischaemia. Similarly trabeculae not exposed to nicorandil underwent either no treatment, controls (C), or ischaemic preconditioning (PC). The experimental endpoint was recovery of contractile function presented as percentage baseline function. Further groups were examined using other KATP channels openers with and without ischaemic preconditioning. In the control group, following 120 minutes reoxygentation the recovery of function reached 28.8 +/- 3.5%. In contrast, exposure to nicorandil alone improved recovery of function (55.5% +/- 5.3) to a similar extent as PC (55.3% +/- 2.5) when compared to controls (p < 0.05, ANOVA). The addition of ischaemic preconditioning to nicorandil exposure abolished protection (29.7% +/- 3.1 ). Findings were confirmed using the other KATP channels openers. Clinically available nicorandil appears to afford ischaemic protection to isolated human atrial muscle. The addition of a short ischaemic episode to nicorandil exposure seems to completely abolish this protection. Although the mechanism underlying this effect remains unknown, we believe that this observation may have clinical implications.

Activation of inwardly rectifying potassium channels by muscarinic receptor-linked G protein in isolated human ventricular myocytes

Muscarinic receptor-linked G protein, Gi, can directly activate the specific K+ channel (IK(ACh)) in the atrium and in pacemaker tissues in the heart. Coupling of Gi to the K+ channel in the ventricle has not been well defined. G protein regulation of K+ channels in isolated human ventricular myocytes was examined using the patch-clamp technique. Bath application of 1 microM acetylcholine (ACh) reversibly shortened the action potential duration to 74.4 +/- 12.1% of control (at 90% repolarization, mean +/- SD, n = 8) and increased the whole-cell membrane current conductance without prior beta-adrenergic stimulation in human ventricular myocytes. The ACh effect was reversed by atropine (1 microM). In excised inside-out patch configurations, application of GTPgammaS (100 microM) to the bath solution (internal surface) caused activation of IK(ACh) and/or the background inwardly-rectifying K+ channel (IK1) in ventricular cell membranes. IK(ACh) exhibited rapid gating behavior with a slope conductance of 44 +/- 2 pS (n = 25) and a mean open lifetime of 1.8 +/- 0.3 msec (n = 21). Single channel activity of GTPgammaS-activated IK1 demonstrated long-lasting bursts with a slope conductance of 30 +/- 2 pS (n = 16) and a mean open lifetime of 36.4 +/- 4.1 msec (n = 12). Unlike IK(ACh), G protein-activated IK1 did not require GTP to maintain channel activity, suggesting that these two channels may be controlled by G proteins with different underlying mechanisms. The concentration of GTP at half-maximal channel activation was 0.22 microM in IK(ACh) and 1.2 microM in IK1. Myocytes pretreated with pertussis toxin (PTX) prevented GTP from activating these channels, indicating that muscarinic receptor-linked PTX-sensitive G protein, Gi, is essential for activation of both channels. G protein-activated channel characteristics from patients with terminal heart failure did not differ from those without heart failure or guinea pig. These results suggest that ACh can shorten the action potential by activating IK(ACh) and IK1 via muscarinic receptor-linked Gi proteins in human ventricular myocytes.

Chronotropic and inotropic effects of terikalant on isolated, blood-perfused atrial and ventricular preparations of dogs

We investigated the effects of terikalant, which blocks inward rectifier K+ current, on the sinus rate, atrial and ventricular contractile force in the isolated, blood-perfused right atrial and left ventricular preparations of dogs, and the effects of terikalant on the negative cardiac responses to acetylcholine, adenosine or pinacidil (an ATP-sensitive K+ channel opener) and on the positive cardiac responses to norepinephrine. Terikalant (1-100 nmol) decreased sinus rate and briefly and slightly increased atrial contractile force in isolated atria. However, terikalant did not increase ventricular contractile force in isolated ventricles. Neither propranolol nor atropine inhibited the positive inotropic and negative chronotropic responses to terikalant, respectively. Terikalant (10 or 30 nmol) did not significantly affect the negative cardiac responses to acetylcholine, adenosine nor pinacidil and the positive responses to norepinephrine. These results suggest that terikalant decreases sinus rate with a small changes in myocardial contractile force and does not affect the cardiac responses to muscarinic and adenosine receptor agonists, ATP-sensitive K+ channel openers nor beta-adrenoceptor agonists in the dog heart.

Characterization of an ultrarapid delayed rectifier potassium channel involved in canine atrial repolarization

1. Depolarizing pulses positive to 0 mV elicit a transient outward current (Ito) and a sustained 'pedestal' current in canine atrial myocytes. The pedestal current was highly sensitive to 4-aminopyridine (4-AP) and TEA, with 50% inhibitory concentrations (EC50) of 5.3 +/- 0.7 and 307 +/- 25 microM, respectively. When the pedestal current was separated from Ito with prepulses or by studying current sensitive to 10 mM TEA, it showed very rapid activation and deactivation. We therefore designated the current IKur,d, for 'ultrarapid delayed rectifier, dog'. IKur,d inactivation was bi-exponential, with mean time constants of 609 +/- 91 and 5563 +/- 676 ms during a 20 s pulse to +40 mV. 2. The reversal potential of IKur,d tail currents are dependent on extracellular potassium concentration ([K+]o; slope, 54.7 mV decade-1). The envelope of tails test was satisfied and the current inwardly rectified at > or = +40 mV. The current was insensitive to E-4031, dendrotoxin and chloride substitution, but was inhibited by barium, with an EC50 of 1.65 mM. Lanthanum ions caused a positive shift in voltage dependence without producing direct inhibition. 3. Single-channel activity was observed in cell-attached, inside-out and outside-out patches. Upon depolarization from -50 to +30 mV, single channels had similar time constants and [K+]o dependence to whole-cell current. Channel open probability (Po) increased with depolarization in a saturable fashion and the Po-voltage relation had a half-activation voltage and slope factor similar to whole-cell IKur,d. 4. Unitary channel current was linearly related to depolarization potential to +40 mV; at more positive potentials, inward rectification occurred. The unitary conductance was 20.3 and 35.5 pS for an [K+]o of 5.4 and 130 mM, respectively. Single-channel activity was strongly inhibited by 50 microM 4-AP or 10 mM TEA. Both 4-AP and TEA decreased open time, suggesting open-channel block. 5. Selective inhibition of IKur,d with 50 microM 4-AP or 0.3-5 mM TEA prolonged canine atrial action potentials, indicating that IKur,d contributes to canine atrial repolarization. The single-channel and macroscopic properties of IKur,d have many similarities to those of currents carried by Kv3.1 cloned channels and our findings thus suggest a possible role for Kv3.1 channels in cardiac repolarization.

Effects of potassium channel blockers on the action potentials and contractility of the rat right ventricle

1. The effects of several potassium channel blockers on the action potentials and contractile force of the electrically driven rat right ventricle have been determined. 2. Glibenclamide, which blocks the ATP-sensitive potassium channels, had no effect on the ventricular action potentials or contractile force responses. 3. 4-Aminopyridine, which blocks the Na(+)-activated potassium channels in ventricles, at 0.3-3 mM increased the amplitude and prolonged the action potentials, and also augmented the force responses to cardiac stimulation and to isoprenaline. 4. Clofilium, a selective blocker of the delayed outward rectifying potassium channel, at 0.1 and 0.3 microM prolonged the action potentials. At 0.1 microM, clofilium augmented the cardiac stimulation responses and, at 0.3 microM, clofilium augmented the maximal responses to isoprenaline. At 1 and 3 microM, clofilium had a lesser ability to prolong action potentials and did not alter force responses. 5. Procaine blocks the Na(+)-activated and the delayed outward rectifying potassium channels and, at higher concentrations, sodium channels. Procaine, at 30 microM, prolonged the action potentials and augmented the force responses to isoprenaline, presumably by blocking potassium channels. Procaine, at 1 mM, had no effect on action potentials but reduced the maximal force responses to isoprenaline, probably by blocking sodium channels. 6. Tetraethylammonium blocks the inward rectifying and delayed outward rectifying potassium channels. Tetraethylammonium, at 1 and 3 mM, prolonged the action potentials and augmented all of the force responses; these effects are likely to be predominantly due to blocking the outward rectifying potassium channel. Thus, in the presence of procaine, the effects of tetraethylammonium are predominantly due to the additional blockade of the inward rectifying potassium channel and there were no effects. 7. None of the potassium channel blockers at any of the concentrations tested had arrhythmogenic effects alone or in the presence of isoprenaline. 8. In summary, this study has shown that blockade of the Na(+)-activated and the delayed outward rectifying, but not the ATP-sensitive or inward rectifying, potassium channel is associated with prolongation of the action potentials, augments the contractile force responses, and is not arrhythmogenic on the rat right ventricle. New drugs that block the Na(+)-activated or delayed outward rectifying potassium channel may have potential as positive inotropes in the treatment of heart failure.

beta-Adrenergic modulation of the inwardly rectifying potassium channel in isolated human ventricular myocytes. Alteration in channel response to beta-adrenergic stimulation in failing human hearts

The beta-adrenergic modulation of the inwardly-rectifying K+ channel (IK1) was examined in isolated human ventricular myocytes using patch-clamp techniques. Isoproterenol (ISO) reversibly depolarized the resting membrane potential and prolonged the action potential duration. Under the whole-cell C1- -free condition, ISO applied via the bath solution reversibly inhibited macroscopic IdK1. The reversal potential of the ISO-sensitive current was shifted by approximately 60 mV per 10-fold change in the external K+ concentration and was sensitive to Ba2+. The ISO-induced inhibition of IK1 was mimicked by forskolin and dibutyrl cAMP, and was prevented by including a cAMP-dependent protein kinase (PKA) inhibitor (PKI) in the pipette solution. In single-channel recordings from cell-attached patches, bath applied ISO could suppress IK1 channels by decreasing open state probability. Bath application of the purified catalytic sub-unit of PKA to inside-out patches also inhibited IK1 and the inhibition could be antagonized by alkaline phosphatase. When beta-adrenergic modulation of IK1 was compared between ventricular myocytes isolated from the failing and the nonfailing heart, channel response to ISO and PKA was significantly reduced in myocytes from the failing heart. Although ISO inhibited IK1 in a concentration-dependent fashion in both groups, a half-maximal concentration was greater in failing (0.12 microM) than in nonfailing hearts (0.023 microM). These results suggest that IK1 in human ventricular myocytes can be inhibited by a PKA-mediated phosphorylation and the modulation is significantly reduced in ventricular myocytes from the failing heart compared to the nonfailing heart.

A novel K+ channel beta-subunit (hKv beta 1.3) is produced via alternative mRNA splicing

Voltage-gated K+ channels can form multimeric complexes with accessory beta-subunits. We report here a novel K+ channel beta-subunit cloned from human heart, hKv beta 1.3, that has 74-83% overall identity with previously cloned beta-subunits. Comparison of hKv beta 1.3 with the previously cloned hKv beta 3 and rKv beta 1 proteins indicates that the carboxyl-terminal 328 amino acids are identical, while unique variable length amino termini exist. Analysis of human beta-subunit cDNA and genomic nucleotide sequences confirm that these three beta-subunits are alternatively spliced from a common beta-subunit gene. Co-expression of hKv beta 1.3 in Xenopus oocytes with the delayed rectifier hKv1.5 indicated that hKv beta 1.3 has unique functional effects. This novel beta-subunit induced a time-dependent inactivation during membrane voltage steps to positive potentials, induced a 13-mV hyperpolarizing shift in the activation curve, and slowed deactivation (tau = 13 +/- 0.5 ms versus 35 +/- 1.7 ms at -40 mV). Most notably, hKv beta 1.3 converted the Kv1.5 outwardly rectifying current voltage relationship to one showing strong inward rectification. These data suggest that Kv channel current diversity may arise from association with alternatively spliced Kv beta-subunits. A simplified nomenclature for the K+ channel beta-subunit subfamilies is suggested.

Electrophysiological basis for antiarrhythmic efficacy, positive inotropy and low proarrhythmic potential of (-)-caryachine

1. (-)-Caryachine, isolated from the plant (Cryptocarya chinensis), increased the contractility of atrial and right ventricular strips and significantly suppressed the reperfusion arrhythmias in adult rabbit heart (ED50 = 1.27 microM). 2. Data obtained by the whole-cell voltage clamp technique has shown that (-)-caryachine causes a negative shift of the steady-state Na channel inactivation and a slower rate of recovery from inactivation. The maximal Na current amplitude decreased to 67 +/- 7%, 29 +/- 8% and 12 +/- 5% after 0.5, 1.5 and 4.5 microM (-)-caryachine, respectively. 3. This agent also had effects on the time- and voltage-dependent K currents. (-)-Caryachine markedly suppressed the 4-AP-sensitive transient outward current (I10). However, it produced very little voltage-dependent shift in inactivation. After 0.5, 1.5 and 4.5 microM of the compound, the respective value of I10 elicited at +60 mV was 80 +/- 7%, 45 +/- 8% and 15 +/- 3%. At higher concentrations, the inward rectifier K current (IK1) was also inhibited but to a much smaller extent. Its slope conductance after 0.5, 1.5 and 4.5 microM (-)-caryachine was reduced to 71 +/- 9%, 51 +/- 12% and 42 +/- 11%, respectively. The outward hump of inward rectification was not changed. 4. In contrast, the L-type Ca current was not significantly changed by (-)-caryachine. 5. Electrophysiological studies in perfused whole heart preparations revealed that (-)-caryachine increased the intra-atrial conduction interval and also prolonged the atrial refractory period. No proarrhythmic effects were induced during the infusion of this compound (up to 13.5 microM). 6. We conclude that (-)-caryachine predominantly blocks the Na and I10 currents. These changes alter the electrophysiological properties of the heart and terminate the induced ventricular arrhythmias. The relatively selective I10 inhibition, safety margin of Ik1 suppression and lack of effect on Ica-L will provide an opportunity to develop an effective antiarrhythmic agent with positive inotropy as well as low proarrhythmic potential.

Characterization of a voltage-gated K+ channel beta subunit expressed in human heart

Voltage-gated K+ channels are important modulators of the cardiac action potential. However, the correlation of endogenous myocyte currents with K+ channels cloned from human heart is complicated by the possibility that heterotetrameric alpha-subunit combinations and function-altering beta subunits exist in native tissue. Therefore, a variety of subunit interactions may generate cardiac K+ channel diversity. We report here the cloning of a voltage-gated K+ channel beta subunit, hKv beta 3, from adult human left ventricle that shows 84% and 74% amino acid sequence identity with the previously cloned rat Kv beta 1 and Kv beta 2 subunits, respectively. Together these three Kv beta subunits share > 82% identity in the carboxyl-terminal 329 aa and show low identity in the amino-terminal 79 aa. RNA analysis indicated that hKv beta 3 message is 2-fold more abundant in human ventricle than in atrium and is expressed in both healthy and diseased human hearts. Coinjection of hKv beta 3 with a human cardiac delayed rectifier, hKv1.5, in Xenopus oocytes increased inactivation, induced an 18-mV hyperpolarizing shift in the activation curve, and slowed deactivation (tau = 8.0 msec vs. 35.4 msec at -50 mV). hKv beta 3 was localized to human chromosome 3 by using a human/rodent cell hybrid mapping panel. These data confirm the presence of functionally important K+ channel beta subunits in human heart and indicate that beta-subunit composition must be accounted for when comparing cloned channels with endogenous cardiac currents.

Characterization of the acetylcholine-sensitive muscarinic K+ channel in isolated feline atrial and ventricular myocytes

M2-cholinergic receptor activation by acetylcholine (ACh) is known to cause a negative inotropic and chronotropic action in atrial tissues. This effect is still controversial in ventricular tissues. The ACh-sensitive muscarinic K+ channel (IK(ACh)) activity was characterized in isolated feline atrial and ventricular myocytes using the patch-clamp technique. Bath application of ACh (1 microM) caused shortening of action potential duration without prior stimulation with catecholamines in atrial and ventricular myocytes. Resting membrane potential was slightly hyperpolarized in both tissues. These effects of ACh were greater in atrium than in ventricle. ACh increased whole-cell membrane current in atrial and ventricular myocytes. The current-voltage (I-V) relationship of the ACh-induced current in ventricle exhibited inward-rectification whose slope conductance was smaller than that in atrium. In single channel recording from cell-attached patches, IK(ACh) activity was observed when ACh was induced in the pipette solution in both tissues. The channel exhibited a slope conductance of 47 +/- 1 pS (mean +/- SD, n = 14) in atrium and 47 +/- 2 pS (n = 10) in ventricle (not different statistically; NS). The open times were distributed according to a single exponential function with mean open lifetime of 2.0 +/- 0.3 msec (n = 14) in atrium and 1.9 +/- 0.3 msec (n = 10) in ventricle (NS); these conductance and kinetic properties were similar between the two tissues. However, the relationship between the concentration of ACh and single channel activity showed a higher sensitivity to ACh in atrium (IC50 = 0.03 microM) than in ventricle (IC50 = 0.15 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in muscarinic K+ channel response to acetylcholine and to G protein-mediated activation in atrial myocytes isolated from failing human hearts

BACKGROUND

A variety of previous studies have demonstrated reduced diastolic potential and electrical activity in atrial specimens from patients with heart disease. Although K+ channels play a major role in determining resting membrane potential and repolarization of the action potential, little is known about the effects of preexisting heart disease on human atrial K+ channel activity.

METHODS AND RESULTS

We characterized the inwardly rectifying K+ channel (IKI) and the muscarinic K+ channel [IK(ACh)] in atrial myocytes isolated from patients with heart failure (HF) and compared electrophysiological characteristics with those from donors (control) by the patch-clamp technique. Resting membrane potentials of isolated atrial myocytes from HF were more depolarized (-51.1 +/- 9.7 mV, mean +/- SD, n = 30 patients) than those from donors (-73.0 +/- 7.2 mV, n = 4 patients, P < .001). The action potential duration in HF was longer than that in donors. Although acetylcholine (ACh) shortened the action potential, reduced the overshoot, and hyperpolarized the atrial cell membrane in HF, these effects were attenuated compared with those observed in donors. The whole-cell membrane current slope conductance in HF was small, the reversal potential was more positive, and the sensitivity to ACh was less compared with donors. In single-channel recordings from cell-attached patches, IK1 channel conductance and gating characteristics were the same in HF and donor atria. When ACh was included in the pipette solution, IK(ACh) was activated in both groups. Single-channel slope conductance of IK(ACh) averaged 42 +/- 3 pS (n = 28) in HF and 44 +/- 2 pS (n = 4) in donors, and mean open lifetime was 1.3 +/- 0.3 milliseconds (n = 24) in HF and 1.5 +/- 0.4 milliseconds (n = 4) in donors. These values were virtually identical in the two groups (not significantly different, NS), although both single IK1 and IK(ACh) channel densities were less in HF. Channel open probability of IK(ACh) was also less in HF (4.0 +/- 1.2%, n = 24) than in donors (6.8 +/- 1.1%, n = 3, P < .01). The concentration of ACh at half-maximal activation was 0.11 mumol/L in HF and 0.03 mumol/L in donors. In excised inside-out patches, IK(ACh) from HF required higher concentrations of GTP and GTP gamma S to activate the channel compared with donors. These results suggest a reduced IK(ACh) channel sensitivity to M2 cholinergic receptor-linked G protein (Gi) in HF compared with donors.

CONCLUSIONS

Atrial myocytes isolated from failing human hearts exhibited a lower resting membrane potential and reduced sensitivity to ACh compared with donor atria. Whole-cell and single-channel measurements suggest that these alterations are caused by reduced IK1 and IK(ACh) channel density and reduced IK(ACh) channel sensitivity to Gi-mediated channel activation in HF.

Ion channel modulators as potential positive inotropic compound for treatment of heart failure

1. Current positive inotropy therapy of heart failure is associated with major problems: digoxin and the phosphodiesterase inhibitors can cause life-threatening toxicity while beta-adrenoceptor agonists become less effective inotropic compounds as heart failure progresses. A new approach to positive inotropy is ion channel modulation. 2. An increased influx of Na+ during the cardiac action potential, as measured with DPI 201-106 and BDF 9148 which increase the probability of the open state of the Na+ channel, will increase force of contraction. 3. Activation of L-type Ca2+ channels with Bay K 8644 will increase influx of Ca2+ and increase the force of contraction. However the Ca2+ channel activators developed to date have little potential for the treatment of heart failure as they are vasoconstrictors. 4. Blocking cardiac K+ channels is a possible mechanism of positive inotropy. Terikalant inhibits the inward rectifying K+ channel, tedisamil inhibits the transient outward K+ channel and dofetilide is one of the newly developed inhibitors of the slow delayed outward rectifying K+ channel. All these drugs prolong the cardiac action potential to increase Ca2+ entry and force of contraction. 5. Thus drugs which increase Na+ influx or block K+ channels represent exciting possibilities for positive inotropy and the potential of these compounds for the treatment of heart failure needs to be fully evaluated.

Sustained depolarization-induced outward current in human atrial myocytes. Evidence for a novel delayed rectifier K+ current similar to Kv1.5 cloned channel currents

Depolarization of human atrial myocytes activates a transient outward current that rapidly inactivates, leaving a sustained outward current after continued depolarization. To evaluate the ionic mechanism underlying this sustained current (Isus), we applied whole-cell voltage-clamp techniques to single myocytes isolated from right atrial specimens obtained from patients undergoing coronary bypass surgery. The magnitude of Isus was constant for up to 10 seconds at +30 mV and was unaffected by 40 mmol/L tetraethylammonium, 100 nmol/L dendrotoxin, 1 mmol/L Ba2+, 0.1 mumol/L atropine, or removal of Cl- in the superfusate. Isus could be distinguished from the 4-aminopyridine (4AP)-sensitive transient outward current (Ito1) by differences in voltage-dependent inactivation (1000-millisecond prepulse to -20 mV reduced Ito1 by 91.7 +/- 0.1% [mean +/- SEM], P < .001, versus 9.4 +/- 0.4% reduction of Isus) and 4AP sensitivity (IC50 for block of Ito1, 1.96 mmol/L; for Isus, 49 mumol/L). Isus activation had a voltage threshold near -30 mV, a half-activation voltage of -4.3 mV, and a slope factor of 8.0 mV. Isus was not inactivated by 1000-millisecond prepulses but was reduced by 16 +/- 8% (P < .05) at a holding potential of -20 mV relative to values at a holding potential of -80 mV. Isus activated very rapidly, with time constants (tau) at 25 degrees C ranging from 18.2 +/- 1.8 to 2.1 +/- 0.2 milliseconds at -10 to +50 mV, two orders of magnitude faster than previously described kinetics of the rapid component of the delayed rectifier K+ current. At 16 degrees C, Isus activation was greatly slowed (tau at +10 mV, 46.7 +/- 4.1 milliseconds; tau at 25 degrees C, 7.1 +/- 0.8 milliseconds; P < .01), and the envelope of tails test was satisfied. The reversal potential of Isus tail currents changed linearly with log [K+]o (slope, 55.3 +/- 2.9 mV per decade), and the fully activated current-voltage relation showed substantial outward rectification. Selective inhibition of Isus with 50 mumol/L 4AP increased human atrial action potential duration by 66 +/- 11% (P < .01). In conclusion, Isus in human atrial myocytes is due to a very rapidly activating delayed rectifier K+ current, which shows limited slow inactivation, is insensitive to tetraethylammonium, Ba2+, and dendrotoxin, and is highly sensitive to 4AP. These properties resemble the characteristics of channels encoded by the Kv1.5 group of cardiac cDNAs and may represent a physiologically significant manifestation of such channels in human atrium.

Potassium currents in isolated human atrial and ventricular cardiocytes

The whole-cell configuration of the patch-clamp technique was applied to study and compare ion currents in single ventricular and atrial cardiocytes isolated from human myocardium. In ventricular cardiocytes the K+ inward rectifier current (IK1) was three times larger than in atrial cardiocytes, while its inactivation kinetics were twice as slow when measured at -140 mV. The magnitude of these variables depended on the test potential but was independent of changes in holding potential. A transient outward current (I(to)) was observed in both ventricular and atrial cardiocytes. The amplitude of the inactivating component of Ito was not significantly different in atrial and ventricular cells, but the time course of inactivation was significantly longer in atrial than in ventricular cardiocytes. Steady-state inactivation of Ito in atrial cells was well described by a two-state Boltzmann function having a midpoint potential of -41.4 mV and a slope factor of 6.9 mV-1. No discernible K+ delayed rectifier current (IK) was observed in either cell type. In four of the 12 atrial cells studied, a time dependent inward current was observed at negative test potentials having a 240 +/- 21 ms time constant for activation and an amplitude of 101 +/- 28 pA. This current, which resembled the pacemaker current (I(f)), was not observed in any of the ventricular cells examined.

Molecular and functional diversity of cloned cardiac potassium channels

Action potential duration is an important determinant of refractoriness in cardiac tissue and thus of the ability to propagate electrical impulses. Action potential duration is controlled in part by activation of K+ currents. Block of K+ channels and the resultant prolongation of action potential duration has become an increasingly attractive mode of anti-arrhythmic intervention. Detailed investigation of individual cardiac K+ channels has been hampered by the presence of multiple types of K+ channels in cardiac cells and the difficulty of isolating individual currents. We have approached this problem by employing a combination molecular cloning technology, heterologous channel expression systems, and biophysical analysis of expressed channels. We have focused on six different channels cloned from the rat and human cardiovascular systems. Each channel has unique functional and pharmacological characteristics, and as a group they comprise a series of mammalian K+ channel isoforms that can account for some of the diversity of channels in the mammalian heart. Each channel appears to be encoded by a different gene with little or no evidence for alternate splicing of RNA transcripts to account for the differences in primary amino acid sequence. In addition to the unique kinetic properties of these channel isoforms when expressed as homotetrameric assemblies, the formation of heterotetrameric K+ channels is also observed. The formation of heterotetrameric channels from the different gene products to create new channels with unique kinetic and pharmacological properties might further account for cardiac K+ channel diversity.

Characteristics of transient outward current in human ventricular myocytes from patients with terminal heart failure

A variety of outward currents exists in ventricular myocardium of different species influencing action potential duration and electrical activity. Transient outward currents have been reported in ventricular tissue of some animals but are small or absent in others. This study was conducted to investigate whether a transient outward current exists in human ventricular myocardium and to characterize its basic electrophysiological properties. Currents were recorded from enzymatically isolated human ventricular myocytes obtained from explanted hearts of 22 patients with terminal heart failure. In almost all cells studied, a transient outward current could be recorded on depolarization to between -20 and +80 mV. The size of the transient outward current was usually large enough to mask the Ca2+ current. It could be recorded under conditions in which Ca2+ influx and intracellular Ca2+ transients were suppressed. Basic current characteristics were similar to transient outward currents observed in other species. Inactivation of the transient outward current was monoexponential, with a time constant of 54.8 +/- 3.7 milliseconds at +40 mV. Half-maximal activation occurred at 16.7 +/- 1.6 mV; half-maximal steady-state inactivation occurred at -34.5 +/- 2.3 mV. Frequency-dependent reduction of peak transient outward current was 29.8 +/- 1.4% at 2 Hz compared with resting conditions. Recovery from inactivation was voltage dependent and had a biexponential time course; the faster time constant (41.0 +/- 6.5 milliseconds at -80 mV) accounted for 86.0 +/- 5.2% of total current. The transient outward current was sensitive to 4-aminopyridine (IC50, 1.15 mM). These results indicate that a large Ca(2+)-independent transient outward K+ current is present in human ventricular myocytes that might be regulated by physiological or pathological events and is a potential site for pharmacological intervention.