Inhibitory effect of 2,3-butanedione monoxime (BDM) on Na(+)/Ca(2+) exchange current in guinea-pig cardiac ventricular myocytes

A novel method to extend viability and functionality of living heart slices

Living heart slices have recently emerged as a powerful experimental model for fundamental cardiac research. By retaining the structure and function of the native myocardium while maintaining the simplicity of cell culture models, heart slices can be easily employed in electrophysiological, pharmacological, biochemical, and structural investigations. One single heart yields many slices (>20 slices for rodents, >100 slices for porcine or human hearts), however due to the low throughput of most assays and rapid slice degeneration within 24 h of preparation, many slices remain unused and are discarded at the end of the preparation day. Here we present a novel method to extend viability and functionality of living heart slices, enabling their use in experiments over several consecutive days following preparation. By combining hypothermic conditions with inhibition of myosin II ATPase using 2,3-butanedione monoxime (BDM), slices prepared from the left ventricle of porcine hearts remain viable and exhibit preserved contractile function and morphology for up to 6 days. Electrophysiological function was also confirmed over the 6 days by extracellular field potentials recordings. This simple method not only maximizes the use of slices prepared from one single heart, thus reducing the number of animals required, but also increases data reproducibility by allowing multiple electrophysiological, pharmacological, biochemical, and structural studies to be performed from the same heart.

Control of Ca2+ and metabolic homeostasis by the Na+/Ca2+ exchangers (NCXs) in health and disease

Spatial and temporal control of calcium (Ca²⁺) levels is essential for the background rhythms and responses of living cells to environmental stimuli. Whatever other regulators a given cellular activity may have, localized and wider scale Ca²⁺ events (sparks, transients, and waves) are hierarchical determinants of fundamental processes such as cell contraction, excitability, growth, metabolism and survival. Different cell types express specific channels, pumps and exchangers to efficiently generate and adapt Ca²⁺ patterns to cell requirements. The Na⁺/Ca²⁺ exchangers (NCXs) in particular contribute to Ca²⁺ homeostasis by buffering intracellular Ca²⁺ loads according to the electrochemical gradients of substrate ions - i.e., Ca²⁺ and sodium (Na⁺) - and under a dynamic control of redundant regulatory processes. An interesting feature of NCX emerges from the strict relationship that connects transporter activity with cell metabolism: on the one hand NCX operates under constant control of ATP-dependent regulatory processes, on the other hand the ion fluxes generated through NCX provide mechanistic support for the Na⁺-driven uptake of glutamate and Ca²⁺ influx to fuel mitochondrial respiration. Proof of concept evidence highlights therapeutic potential of preserving a timed and balanced NCX activity in a growing rate of diseases (including excitability, neurodegenerative, and proliferative disorders) because of an improved ability of stressed cells to safely maintain ion gradients and mitochondrial bioenergetics. Here, we will summarize and review recent works that have focused on the pathophysiological roles of NCXs in balancing the two-way relationship between Ca²⁺ signals and metabolism.

The effective use of blebbistatin to study the action potential of cardiac pacemaker cells of zebrafish (Danio rerio) during incremental warming

Blebbistatin potently inhibits actin-myosin interaction, preventing contractile activity of excitable cells including cardiac myocytes, despite electrical excitation of an action potential (AP). We collected intracellular microelectrode recordings of pacemaker cells located in the sinoatrial region (SAR) of the zebrafish heart at room temperature and during acute warming to investigate whether or not blebbistatin inhibition of contraction significantly alters pacemaker cell electrophysiology. Changes were evaluated based on 16 variables that characterized the AP waveform. None of these AP variables nor the spontaneous heart rate were significantly modified with the application of 10 μM blebbistatin when recordings were made at room temperature. Compared with the control group, the blebbistatin-treated group showed minor changes in the rate of spontaneous diastolic depolarization (P = 0.027) and the 50% and 80% repolarization (P = 0.008 and 0.010, respectively) in the 26°C–29°C temperature bin, but not at higher temperatures. These findings suggest that blebbistatin is an effective excitation-contraction uncoupler that does not appreciably affect APs generated in pacemaking cells of the SAR and can, therefore, be used in zebrafish cardiac studies.

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Blebbistatin uncouples excitation-contraction in zebrafish cardiomyocytes.

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Blebbistatin does not modify the pacemaker action potential variables.

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Temperature does not modify the effect of blebbistatin.

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First validation of the use of blebbistatin in adult fish.

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Methodology of intracellular microelectrode recording of zebrafish pacemaker cells.

Effects of extended pharmacological disruption of zebrafish embryonic heart biomechanical environment on cardiac function, morphology, and gene expression

BACKGROUND

Biomechanical stimuli are known to be important to cardiac development, but the mechanisms are not fully understood. Here, we pharmacologically disrupted the biomechanical environment of wild-type zebrafish embryonic hearts for an extended duration and investigated the consequent effects on cardiac function, morphological development, and gene expression.

RESULTS

Myocardial contractility was significantly diminished or abolished in zebrafish embryonic hearts treated for 72 hours from 2 dpf with 2,3-butanedione monoxime (BDM). Image-based flow simulations showed that flow wall shear stresses were abolished or significantly reduced with high oscillatory shear indices. At 5 dpf, after removal of BDM, treated embryonic hearts were maldeveloped, having disrupted cardiac looping, smaller ventricles, and poor cardiac function (lower ejected flow, bulboventricular regurgitation, lower contractility, and slower heart rate). RNA sequencing of cardiomyocytes of treated hearts revealed 922 significantly up-regulated genes and 1,698 significantly down-regulated genes. RNA analysis and subsequent qPCR and histology validation suggested that biomechanical disruption led to an up-regulation of inflammatory and apoptotic genes and down-regulation of ECM remodeling and ECM-receptor interaction genes. Biomechanics disruption also prevented the formation of ventricular trabeculation along with notch1 and erbb4a down-regulation.

CONCLUSIONS

Extended disruption of biomechanical stimuli caused maldevelopment, and potential genes responsible for this are identified.

Early changes in myocyte contractility and cardiac function in streptozotocin-induced type 1 diabetes in rats

Diabetes can elicit direct deleterious effects on the myocardium, independent of coronary artery disease or hypertension. These cardiac disturbances are termed diabetic cardiomyopathy showing increased risk of heart failure with or without reduced ejection fraction. Presently, there is no specific treatment for this type of cardiomyopathy and in the case of type I diabetes, it may start in early childhood independent of glycemic control. We hypothesized that alterations in isolated myocyte contractility and cardiac function are present in the early stages of experimental diabetes in rats before overt changes in myocardium structure occur. Diabetes was induced by single-dose injection of streptozotocin (STZ) in rats with data collected from control and diabetic animals 3 weeks after injection. Left ventricle myocyte contractility was measured by single-cell length variation under electrical stimulation. Cardiac function and morphology were studied by high-resolution echocardiography with pulsed-wave tissue Doppler imaging (TDI) measurements and three-lead surface electrocardiogram. Triglycerides, cholesterol and liver enzyme levels were measured from plasma samples obtained from both groups. Myocardial collagen content and perivascular fibrosis of atria and ventricle were studied by histological analysis after picrosirius red staining. Diabetes resulted in altered contractility of isolated cardiac myocytes with increased contraction and relaxation time intervals. Echocardiography showed left atrium dilation, increased end-diastolic LV and posterior wall thickness, with reduced longitudinal systolic peak velocity (S’) of the septum mitral annulus at the apical four-chamber view obtained by TDI. Triglycerides, aspartate aminotransferase and alkaline phosphatase were elevated in diabetic animals. Intertitial collagen content was higher in atria of both groups and did not differ among control and diabetic animals. Perivascular intramyocardial arterioles collagen did not differ between groups. These results suggest that alterations in cardiac function are present in the early phase in this model of diabetes type 1 and occur before overt changes in myocardium structure appear as evaluated by intersticial collagen deposition and perivascular fibrosis of intramyocardial arterioles.

Acetylcholine Delays Atrial Activation to Facilitate Atrial Fibrillation

Background

Acetylcholine (ACh) shortens action potential duration (APD) in human atria. APD shortening facilitates atrial fibrillation (AF) by reducing the wavelength for reentry. However, the influence of ACh on electrical conduction in human atria and its contribution to AF are unclear, particularly when combined with impaired conduction from interstitial fibrosis.

Objective

To investigate the effect of ACh on human atrial conduction and its role in AF with computational, experimental, and clinical approaches.

Methods

S1S2 pacing (S1 = 600 ms and S2 = variable cycle lengths) was applied to the following human AF computer models: a left atrial appendage (LAA) myocyte to quantify the effects of ACh on APD, maximum upstroke velocity (V _max ), and resting membrane potential (RMP); a monolayer of LAA myocytes to quantify the effects of ACh on conduction; and 3) an intact left atrium (LA) to determine the effects of ACh on arrhythmogenicity. Heterogeneous ACh and interstitial fibrosis were applied to the monolayer and LA models. To corroborate the simulations, APD and RMP from isolated human atrial myocytes were recorded before and after 0.1 μM ACh. At the tissue level, LAAs from AF patients were optically mapped ex vivo using Di-4-ANEPPS. The difference in total activation time (AT) was determined between AT initially recorded with S1 pacing, and AT recorded during subsequent S1 pacing without (n = 6) or with (n = 7) 100 μM ACh.

Results

In LAA myocyte simulations, S1 pacing with 0.1 μM ACh shortened APD by 41 ms, hyperpolarized RMP by 7 mV, and increased V _max by 27 mV/ms. In human atrial myocytes, 0.1 μM ACh shortened APD by 48 ms, hyperpolarized RMP by 3 mV, and increased V _max by 6 mV/ms. In LAA monolayer simulations, S1 pacing with ACh hyperpolarized RMP to delay total AT by 32 ms without and 35 ms with fibrosis. This led to unidirectional conduction block and sustained reentry in fibrotic LA with heterogeneous ACh during S2 pacing. In AF patient LAAs, S1 pacing with ACh increased total AT from 39.3 ± 26 ms to 71.4 ± 31.2 ms (p = 0.036) compared to no change without ACh (56.7 ± 29.3 ms to 50.0 ± 21.9 ms, p = 0.140).

Conclusion

In fibrotic atria with heterogeneous parasympathetic activation, ACh facilitates AF by shortening APD and slowing conduction to promote unidirectional conduction block and reentry.

Effect of pralidoxime on coronary perfusion pressure during cardiopulmonary resuscitation in a pig model

Objective

Pralidoxime is widely used for the treatment of organophosphate poisoning. Multiple studies have reported its vasoconstrictive property, which may facilitate the restoration of spontaneous circulation (ROSC) after cardiac arrest by increasing the coronary perfusion pressure (CPP). 2,3-Butanedione monoxime, which belongs to the same oxime family, has been shown to facilitate ROSC by reducing left ventricular ischemic contracture. Because pralidoxime and 2,3-butanedione monoxime have several common mechanisms of action, both drugs may have similar effects on ischemic contracture. Thus, we investigated the effects of pralidoxime administration during cardiopulmonary resuscitation in a pig model with a focus on ischemic contracture and CPP.

Methods

After 14 minutes of untreated ventricular fibrillation, followed by 8 minutes of basic life support, 16 pigs randomly received either 80 mg/kg of pralidoxime (pralidoxime group) or an equivalent volume of saline (control group) during advanced cardiovascular life support (ACLS).

Results

Mixed-model analyses of left ventricular wall thickness and chamber area during ACLS revealed no significant group effects or group-time interactions, whereas a mixed-model analysis of the CPP during ACLS revealed a significant group effect (P=0.038) and group-time interaction (P<0.001). Post-hoc analyses revealed significant increases in CPP in the pralidoxime group, starting at 5 minutes after pralidoxime administration. No animal, except one in the pralidoxime group, achieved ROSC; thus, the rate of ROSC did not differ between the two groups.

Conclusion

In a pig model of cardiac arrest, pralidoxime administered during cardiopulmonary resuscitation did not reduce ischemic contracture; however, it significantly improved CPP.

Eliminating contraction during culture maintains global and local Ca2+ dynamics in cultured rabbit pacemaker cells

Pacemaker cells residing in the sinoatrial node generate the regular heartbeat. Ca²⁺ signaling controls the heartbeat rate-directly, through the effect on membrane molecules (NCX exchange, K⁺ channel), and indirectly, through activation of calmodulin-AC-cAMP-PKA signaling. Thus, the physiological role of signaling in pacemaker cells can only be assessed if the Ca²⁺ dynamics are in the physiological range. Cultured cells that can be genetically manipulated and/or virally infected with probes are required for this purpose. Because rabbit pacemaker cells in culture experience a decrease in their spontaneous action potential (AP) firing rate below the physiological range, Ca²⁺ dynamics are expected to be affected. However, Ca²⁺ dynamics in cultured pacemaker cells have not been reported before. We aim to a develop a modified culture method that sustains the global and local Ca²⁺ kinetics along with the AP firing rate of rabbit pacemaker cells. We used experimental and computational tools to test the viability of rabbit pacemaker cells in culture under various conditions. We tested the effect of culture dish coating, pH, phosphorylation, and energy balance on cultured rabbit pacemaker cells function. The cells were maintained in culture for 48 h in two types of culture media: one without the addition of a contraction uncoupler and one enriched with either 10 mM BDM (2,3-Butanedione 2-monoxime) or 25 μM blebbistatin. The uncoupler was washed out from the medium prior to the experiments. Cells were successfully infected with a GFP adenovirus cultured with either BDM or blebbistatin. Using either uncoupler during culture led to the cell surface area being maintained at the same level as fresh cells. Moreover, the phospholamban and ryanodine receptor densities and their phosphorylation level remained intact in culture when either blebbistatin or BDM were present. Spontaneous AP firing rate, spontaneous Ca²⁺ kinetics, and spontaneous local Ca²⁺ release parameters were similar in the cultured cells with blebbistatin as in fresh cells. However, BDM affects these parameters. Using experimental and a computational model, we showed that by eliminating contraction, phosphorylation activity is preserved and energy is reduced. However, the side-effects of BDM render it less effective than blebbistatin.

Cardiac metabolic effects of KNa1.2 channel deletion and evidence for its mitochondrial localization

Controversy surrounds the molecular identity of mitochondrial K+ channels that are important for protection against cardiac ischemia-reperfusion injury. Although KNa1.2 (sodium-activated potassium channel encoded by Kcn2) is necessary for cardioprotection by volatile anesthetics, electrophysiological evidence for a channel of this type in mitochondria is lacking. The endogenous physiological role of a potential mito-KNa1.2 channel is also unclear. In this study, single channel patch-clamp of 27 independent cardiac mitochondrial inner membrane (mitoplast) preparations from wild-type (WT) mice yielded 6 channels matching the known ion sensitivity, ion selectivity, pharmacology, and conductance properties of KNa1.2 (slope conductance, 138 ± 1 pS). However, similar experiments on 40 preparations from Kcnt2-/- mice yielded no such channels. The KNa opener bithionol uncoupled respiration in WT but not Kcnt2-/- cardiomyocytes. Furthermore, when oxidizing only fat as substrate, Kcnt2-/- cardiomyocytes and hearts were less responsive to increases in energetic demand. Kcnt2-/- mice also had elevated body fat, but no baseline differences in the cardiac metabolome. These data support the existence of a cardiac mitochondrial KNa1.2 channel, and a role for cardiac KNa1.2 in regulating metabolism under conditions of high energetic demand.-Smith, C. O., Wang, Y. T., Nadtochiy, S. M., Miller, J. H., Jonas, E. A., Dirksen, R. T., Nehrke, K., Brookes, P. S. Cardiac metabolic effects of KNa1.2 channel deletion and evidence for its mitochondrial localization.

Following Endocardial Tissue Movements via Cell Photoconversion in the Zebrafish Embryo

During embryogenesis, cells undergo dynamic changes in cell behavior, and deciphering the cellular logic behind these changes is a fundamental goal in the field of developmental biology. The discovery and development of photoconvertible proteins have greatly aided our understanding of these dynamic changes by providing a method to optically highlight cells and tissues. However, while photoconversion, time-lapse microscopy, and subsequent image analysis have proven to be very successful in uncovering cellular dynamics in organs such as the brain or the eye, this approach is generally not used in the developing heart due to challenges posed by the rapid movement of the heart during the cardiac cycle. This protocol consists of two parts. The first part describes a method for photoconverting and subsequently tracking endocardial cells (EdCs) during zebrafish atrioventricular canal (AVC) and atrioventricular heart valve development. The method involves temporally stopping the heart with a drug in order for accurate photoconversion to take place. Hearts are allowed to resume beating upon removal of the drug and embryonic development continues normally until the heart is stopped again for high-resolution imaging of photoconverted EdCs at a later developmental time point. The second part of the protocol describes an image analysis method to quantify the length of a photoconverted or non-photoconverted region in the AVC in young embryos by mapping the fluorescent signal from the three-dimensional structure onto a two-dimensional map. Together, the two parts of the protocol allows one to examine the origin and behavior of cells that make up the zebrafish AVC and atrioventricular heart valve, and can potentially be applied for studying mutants, morphants, or embryos that have been treated with reagents that disrupt AVC and/or valve development.

Murine Short Axis Ventricular Heart Slices for Electrophysiological Studies

Murine cardiomyocytes have been extensively used for in vitro studies of cardiac physiology and new therapeutic strategies. However, multicellular preparations of dissociated cardiomyocytes are not representative of the complex in vivo structure of cardiomyocytes, non-myocytes and extracellular matrix, which influences both mechanical and electrophysiological properties of the heart. Here we describe a technique to prepare viable ventricular slices of adult mouse hearts with a preserved in vivo like tissue structure, and demonstrate their suitability for electrophysiological recordings. After excision of the heart, ventricles are separated from the atria, perfused with Ca²⁺-free solution containing 2,3-butanedione monoxime and embedded in a 4% low-melt agarose block. The block is placed on a microtome with a vibrating blade, and tissue slices with a thickness of 150-400 µm are prepared keeping the vibration frequency of the blade at 60-70 Hz and moving the blade forward as slowly as possible. Thickness of the slices depends on the further application. Slices are stored in ice cold Tyrode's solution with 0.9 mM Ca²⁺ and 2,3-butanedione monoxime (BDM) for 30 min. Afterwards, slices are transferred to 37 °C DMEM for 30 min to wash out the BDM. Slices can be used for electrophysiological studies with sharp electrodes or micro electrode arrays, for force measurements to analyze contractile function or to investigate the interaction of transplanted stem cell-derived cardiomyocytes and host tissue. For sharp electrode recordings, a slice is placed into a 3 cm cell culture dish on the heating plate of an inverted microscope. The slice is stimulated with a unipolar electrode, and intracellular action potentials of cardiomyocytes within the slice are recorded with a sharp glass electrode.

In Vivo Cannulation Methods for Cardiomyocytes Isolation from Heart Disease Models

Isolation of high quality cardiomyocytes is critically important for achieving successful experiments in many cellular and molecular cardiology studies. Methods for isolating cardiomyocytes from the murine heart generally are time-sensitive and experience-dependent, and often fail to produce high quality cells. Major technical difficulties can be related to the surgical procedures needed to explant the heart and to cannulate the vessel to mount onto the Langendorff system before in vitro reperfusion can begin. During this period, transient hypoxia and ischemia may damage the heart, resulting in low yield and poor quality of cells, especially for heart disease models that have fragile cells. We have developed novel in vivo cannulation methods to minimize hypoxia and ischemia, and fine-tuned the entire protocol to produce high quality ventricular myocytes. The high cell quality has been confirmed using important structural and functional criteria such as morphology, t-tubule structure, action potential morphology, Ca²⁺ signaling, responsiveness to beta-adrenergic agonist, and ability to have robust contraction under mechanically loaded condition. Together these assessments show the preservation of the cardiac excitation–contraction machinery in cells isolated using this technique. The in vivo cannulation method enables consistent isolation of high-quality cardiomyocytes, even from heart disease models that were notoriously difficult for cell isolation using traditional methods.

Inhibitory effect of YM-244769, a novel Na+/Ca2+ exchanger inhibitor on Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

Recently, YM-244769 (N-(3-aminobenzyl)-6-{4-[(3-fluorobenzyl)oxy]phenoxy} nicotinamide) has been reported as a new potent and selective Na⁺/Ca²⁺ exchange (NCX) inhibitor by using various cells transfected with NCX using the ⁴⁵Ca²⁺ fluorescent technique. However, the electrophysiological study of YM-244769 on NCX had not been performed in the mammalian heart. We examined the effects of YM-244769 on NCX current (I_NCX) in single cardiac ventricular myocytes of guinea pigs by using the whole-cell voltage clamp technique. YM-244769 suppressed the bidirectional I_NCX in a concentration-dependent manner. The IC₅₀ values of YM-244769 for the bidirectional outward and inward I_NCX were both about 0.1 μM. YM-244769 suppressed the unidirectional outward I_NCX (Ca²⁺ entry mode) with an IC₅₀ value of 0.05 μM. The effect on the unidirectional inward I_NCX (Ca²⁺ exit mode) was less potent, with 10 μM of YM-244769 resulting in the inhibition of only about 50 %. At 5 mM intracellular Na⁺ concentration, YM-244769 suppressed I_NCX more potently than it did at 0 mM [Na⁺]_i. Intracellular application of trypsin via the pipette solution did not change the blocking effect of YM-244769. In conclusion, YM-244769 inhibits the Ca²⁺ entry mode of NCX more potently than the Ca²⁺ exit mode, and inhibition by YM-244769 is [Na⁺]_i-dependent and trypsin-insensitive. These characteristics are similar to those of other benzyloxyphenyl derivative NCX inhibitors such as KB-R7943, SEA0400, and SN-6. The potency of YM-244769 as an NCX1 inhibitor is higher than those of KB-R7943 and SN-6 and is similar to that of SEA0400.

Nicorandil stimulates a Na⁺/Ca²⁺ exchanger by activating guanylate cyclase in guinea pig cardiac myocytes

Nicorandil, a hybrid of an ATP-sensitive K(+) (KATP) channel opener and a nitrate generator, is used clinically for the treatment of angina pectoris. This agent has been reported to exert antiarrhythmic actions by abolishing both triggered activity and spontaneous automaticity in an in vitro study. It is well known that delayed afterdepolarizations (DADs) are caused by the Na(+)/Ca(2+) exchange current (I NCX). In this study, we investigated the effect of nicorandil on the cardiac Na(+)/Ca(2+) exchanger (NCX1). We used the whole-cell patch clamp technique and the Fura-2/AM (Ca(2+) indicator) method to investigate the effect of nicorandil on I NCX in isolated guinea pig ventricular myocytes and CCL39 fibroblast cells transfected with dog heart NCX1. Nicorandil enhanced I NCX in a concentration-dependent manner. The EC50 (half-maximum concentration for enhancement of the drug) values were 15.0 and 8.7 μM for the outward and inward components of I NCX, respectively. 8-Bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), a membrane-permeable analog of guanosine 3',5'-cyclic monophosphate (cGMP), enhanced I NCX. 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a guanylate cyclase inhibitor (10 μM), completely abolished the nicorandil-induced I NCX increase. Nicorandil increased I NCX in CCL39 cells expressing wild-type NCX1 but did not affect mutant NCX1 without a long intracellular loop between transmembrane segments (TMSs) 5 and 6. Nicorandil at 100 μM abolished DADs induced by electrical stimulation with ouabain. Nicorandil enhanced the function of NCX1 via guanylate cyclase and thus may accelerate Ca(2+) exit via NCX1. This may partially contribute to the cardioprotection by nicorandil in addition to shortening action potential duration (APD) by activating KATP channels.

Neuroprotection Against Diisopropylfluorophosphate in Acute Hippocampal Slices

Diisopropylfluorophosphate (DFP) is an irreversible inhibitor of acetylcholine esterase and a surrogate of the organophosphorus (OP) nerve agent sarin. The neurotoxicity of DFP was assessed as a reduction of population spike (PS) area elicited by synaptic stimulation in acute hippocampal slices. Two classical antidotes, atropine, and pralidoxime, and two novel antidotes, 4R-cembranotriene-diol (4R) and a caspase nine inhibitor, were tested. Atropine, pralidoxime, and 4R significantly protected when applied 30 min after DFP. The caspase inhibitor was neuroprotective when applied 5-10 min before or after DFP, suggesting that early synaptic apoptosis is responsible for the loss of PSs. It is likely that apoptosis starts at the synapses and, if antidotes are not applied, descends to the cell bodies, causing death. The acute slice is a reliable tool for mechanistic studies, and the assessment of neurotoxicity and neuroprotection with PS areas is, in general, pharmacologically congruent with in vivo results and predicts the effect of drugs in vivo. 4R was first found to be neuroprotective in slices and later we demonstrated that 4R is neuroprotective in vivo. The mechanism of neurotoxicity of OPs is not well understood, and there is a need for novel antidotes that could be discovered using acute slices.

Methods for isolating atrial cells from large mammals and humans

The identification of disturbances in the cellular structure, electrophysiology and calcium handling of atrial cardiomyocytes is crucial to the understanding of common pathologies such as atrial fibrillation. Human right atrial specimens can be obtained during routine cardiac surgery and may be used for isolation of atrial myocytes. These samples provide the unique opportunity to directly investigate the effects of human disease on atrial myocytes. However, atrial myocytes vary greatly between patients, there is little if any access to truly healthy controls and the challenges associated with assessing the in vivo effects of drugs or devices in man are considerable. These issues highlight the need for animal models. Large mammalian models are particularly suitable for this purpose as their cardiac structure and electrophysiology are comparable with humans. Here, we review techniques for obtaining atrial cardiomyocytes. We start with background information on solution composition. Agents shown to increase viable cell yield will then be explored followed by a discussion of the use of tissue-dissociating enzymes. Protocols are detailed for the perfusion method of cell isolation in large mammals and the chunk digest methods of cell isolation in humans.

The role of Na+/Ca2+ exchanger subtypes in neuronal ischemic injury

The Na(+)/Ca(2+) exchanger (NCX) plays an important role in the maintenance of Na(+) and Ca(2+) homeostasis in most cells including neurons under physiological and pathological conditions. It exists in three subtypes (NCX1-3) with different tissue distributions but all of them are present in the brain. NCX transports Na(+) and Ca(2+) in either Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, depending on membrane potential and transmembrane ion gradients. During neuronal ischemia, Na(+) and Ca(2+) ionic disturbances favor NCX to work in reverse mode, giving rise to increased intracellular Ca(2+) levels, while it may regain its forward mode activity on reperfusion. The exact significance of NCX in neuronal ischemic and reperfusion states remains unclear. The differential role of NCX subtypes in ischemic neuronal injury has been extensively investigated using various pharmacological tools as well as genetic models. This review discusses the mode of action of NCX in ischemic and reperfusion states, the differential roles played by NCX subtypes in these states as well as the role of NCX in pre- and postconditioning. NCX subtypes carry variable roles in ischemic injury. Furthermore, the mode of action of each subtype varies in ischemia and reperfusion states. Thus, therapeutic targeting of NCX in stroke should be based on appropriate timing of the administration of NCX subtype-specific strategies.

Analysis of tubular membrane networks in cardiac myocytes from atria and ventricles

In cardiac myocytes a complex network of membrane tubules - the transverse-axial tubule system (TATS) - controls deep intracellular signaling functions. While the outer surface membrane and associated TATS membrane components appear to be continuous, there are substantial differences in lipid and protein content. In ventricular myocytes (VMs), certain TATS components are highly abundant contributing to rectilinear tubule networks and regular branching 3D architectures. It is thought that peripheral TATS components propagate action potentials from the cell surface to thousands of remote intracellular sarcoendoplasmic reticulum (SER) membrane contact domains, thereby activating intracellular Ca²⁺ release units (CRUs). In contrast to VMs, the organization and functional role of TATS membranes in atrial myocytes (AMs) is significantly different and much less understood. Taken together, quantitative structural characterization of TATS membrane networks in healthy and diseased myocytes is an essential prerequisite towards better understanding of functional plasticity and pathophysiological reorganization. Here, we present a strategic combination of protocols for direct quantitative analysis of TATS membrane networks in living VMs and AMs. For this, we accompany primary cell isolations of mouse VMs and/or AMs with critical quality control steps and direct membrane staining protocols for fluorescence imaging of TATS membranes. Using an optimized workflow for confocal or superresolution TATS image processing, binarized and skeletonized data are generated for quantitative analysis of the TATS network and its components. Unlike previously published indirect regional aggregate image analysis strategies, our protocols enable direct characterization of specific components and derive complex physiological properties of TATS membrane networks in living myocytes with high throughput and open access software tools. In summary, the combined protocol strategy can be readily applied for quantitative TATS network studies during physiological myocyte adaptation or disease changes, comparison of different cardiac or skeletal muscle cell types, phenotyping of transgenic models, and pharmacological or therapeutic interventions.

The mechanical uncoupler blebbistatin is associated with significant electrophysiological effects in the isolated rabbit heart

Blebbistatin (BS) is a recently discovered inhibitor of the myosin II isoform and has been adopted as the mechanical uncoupler of choice for optical mapping, because previous studies suggest that BS has no significant cardiac electrophysiological effects in a number of species. The aim of this study was to determine whether BS affects cardiac electrophysiology in isolated New Zealand White rabbit hearts. Langendorff-perfused hearts (n= 39) in constant-flow mode had left ventricular monophasic action potential duration (MAPD) measured at apical and basal regions during constant pacing (300 ms cycle length). Standard action potential duration restitution was obtained using the single extrastimulus method with measurement of the maximal restitution slope. Ventricular fibrillation threshold was measured as the minimal current inducing sustained ventricular fibrillation with burst pacing (30 stimuli, at 30 ms intervals). Optical action potentials were recorded using the voltage-sensitive dye di-4-ANEPPS. Measurements were taken at baseline and after 60 min perfusion with BS (5 μm). Blebbistatin significantly prolonged left ventricular apical (mean ± SEM; from 129.9 ± 2.9 to 170.7 ± 4.1 ms, P < 0.001, n= 8) and basal MAPD (from 135.0 ± 2.3 to 163.3 ± 5.6 ms, P < 0.001) and effective refractory period (from 141.3 ± 4.8 to 175.6 ± 3.7 ms, P < 0.001) whilst increasing the maximal slope of restitution (apex, from 0.79 ± 0.09 to 1.57 ± 0.16, P < 0.001; and base, from 0.71 ± 0.06 to 1.44 ± 0.24, P < 0.001) and ventricular fibrillation threshold (from 5.3 ± 1.1 to 17.0 ± 2.9 mA, P < 0.001). In other hearts, blebbistatin significantly prolonged optically recorded action potentials (from 136.5 ± 6.3 to 173.0 ± 7.9 ms, P < 0.05, n= 4). In control experiments, the increase of MAPD with blebbistatin was present whether the hearts were perfused in constant-pressure mode (n= 5) or in unloaded conditions (n= 5). These data show that blebbistatin significantly affects cardiac electrophysiology. Its use in optical mapping studies should be treated with caution.

Modifications of mechanoelectric feedback induced by 2,3-butanedione monoxime and Blebbistatin in Langendorff-perfused rabbit hearts

AIM

Myocardial stretching is an arrhythmogenic factor. Optical techniques and mechanical uncouplers are used to study the mechanoelectric feedback. The aim of this study is to determine whether the mechanical uncouplers 2,3-butanedione monoxime and Blebbistatin hinder or modify the electrophysiological effects of acute mechanical stretch.

METHODS

The ventricular fibrillation (VF) modifications induced by acute mechanical stretch were studied in 27 Langendorff-perfused rabbit hearts using epicardial multiple electrodes and mapping techniques under control conditions (n = 9) and during the perfusion of 2,3-butanedione monoxime (15 mM) (n = 9) or Blebbistatin (10 μm) (n = 9).

RESULTS

In the control series, myocardial stretch increased the complexity of the activation maps and the dominant frequency (DF) of VF from 13.1 ± 2.0 Hz to 19.1 ± 3.1 Hz (P < 0.001, 46% increment). At baseline, the activation maps showed less complexity in both the 2,3-butanedione monoxime and Blebbistatin series, and the DF was lower in the 2,3-butanedione monoxime series (11.4 ± 1.2 Hz; P < 0.05). The accelerating effect of mechanical stretch was abolished under 2,3-butanedione monoxime (maximum DF = 11.7 ± 2.4 Hz, 5% increment, ns vs baseline, P < 0.0001 vs. control series) and reduced under Blebbistatin (maximum DF = 12.9 ± 0.7 Hz, 8% increment, P < 0.01 vs. baseline, P < 0.0001 vs. control series). The variations in complexity of the activation maps under stretch were not significant in the 2,3-butanedione monoxime series and were significantly attenuated under Blebbistatin.

CONCLUSION

The accelerating effect and increased complexity of myocardial activation during VF induced by acute mechanical stretch are abolished under the action of 2,3-butanedione monoxime and reduced under the action of Blebbistatin.

The role of dynamic instability and wavelength in arrhythmia maintenance as revealed by panoramic imaging with blebbistatin vs. 2,3-butanedione monoxime

Unlike other excitation-contraction uncouplers, blebbistatin has few electrophysiological side effects and has gained increasing acceptance as an excitation-contraction uncoupler in optical mapping experiments. However, the possible role of blebbistatin in ventricular arrhythmia has hitherto been unknown. Furthermore, experiments with blebbistatin and 2,3-butanedione monoxime (BDM) offer an opportunity to assess the contribution of dynamic instability and wavelength of impulse propagation to the induction and maintenance of ventricular arrhythmia. Recordings of monophasic action potentials were used to assess effects of blebbistatin in Langendorff-perfused rabbit hearts (n = 5). Additionally, panoramic optical mapping experiments were conducted in rabbit hearts (n = 7) that were sequentially perfused with BDM, then washed out, and subsequently perfused with blebbistatin. The susceptibility to arrhythmia was investigated using a shock-on-T protocol. We found that 1) application of blebbistatin did not change action potential duration (APD) restitution; 2) in contrast to blebbistatin, BDM flattened APD restitution curve and reduced the wavelength; and 3) incidence of sustained arrhythmia was much lower under blebbistatin than under BDM (2/123 vs. 23/99). While arrhythmias under BDM were able to stabilize, the arrhythmias under blebbistatin were unstable and terminated spontaneously. In conclusion, the lower susceptibility to arrhythmia under blebbistatin than under BDM indicates that blebbistatin has less effects on arrhythmia dynamics. A steep restitution slope under blebbistatin is associated with higher dynamic instability, manifested by the higher incidence of not only wave breaks but also wave extinctions. This relatively high dynamic instability leads to the self-termination of arrhythmia because of the sufficiently long wavelength under blebbistatin.

Methods in cardiomyocyte isolation, culture, and gene transfer

Since techniques for cardiomyocyte isolation were first developed 35 years ago, experiments on single myocytes have yielded great insight into their cellular and sub-cellular physiology. These studies have employed a broad range of techniques including electrophysiology, calcium imaging, cell mechanics, immunohistochemistry and protein biochemistry. More recently, techniques for cardiomyocyte culture have gained additional importance with the advent of gene transfer technology. While such studies require a high quality cardiomyocyte population, successful cell isolation and maintenance during culture remain challenging. In this review, we describe methods for the isolation of adult and neonatal ventricular myocytes from rat and mouse heart. This discussion outlines general principles for the beginner, but also provides detailed specific protocols and advice for common caveats. We additionally review methods for short-term myocyte culture, with particular attention given to the importance of substrate and media selection, and describe time-dependent alterations in myocyte physiology that should be anticipated. Gene transfer techniques for neonatal and adult cardiomyocytes are also reviewed, including methods for transfection (liposome, electroporation) and viral-based gene delivery.

Regional increase of extracellular potassium leads to electrical instability and reentry occurrence through the spatial heterogeneity of APD restitution

The heterogeneities of electrophysiological properties of cardiac tissue are the main factors that control both arrhythmia induction and maintenance. Although the local increase of extracellular potassium ([K(+)](o)) due to coronary occlusion is a well-established metabolic response to acute ischemia, the role of local [K(+)](o) heterogeneity in phase 1a arrhythmias has yet to be determined. In this work, we created local [K(+)](o) heterogeneity and investigated its role in fast pacing response and arrhythmia induction. The left marginal vein of a Langendorff-perfused rabbit heart was cannulated and perfused separately with solutions containing 4, 6, 8, 10, and 12 mM of K(+). The fluorescence dye was utilized to map the voltage distribution. We tested stimulation rates, starting from 400 ms down to 120 ms, with steps of 5-50 ms. We found that local [K(+)](o) heterogeneity causes action potential (AP) alternans, 2:1 conduction block, and wave breaks. The effect of [K(+)](o) heterogeneity on electrical stability and vulnerability to arrhythmia induction was largest during regional perfusion with 10 mM of K(+). We detected three concurrent dynamics: normally propagating activation when excitation waves spread over tissue perfused with normal K(+), alternating 2:2 rhythm near the border of [K(+)](o) heterogeneity, and 2:1 aperiodicity when propagation was within the high [K(+)](o) area. [K(+)](o) elevation changed the AP duration (APD) restitution and shifted the restitution curve toward longer diastolic intervals and shorter APD. We conclude that spatial heterogeneity of the APD restitution, created with regional elevation of [K(+)](o), can lead to AP instability, 2:1 block, and reentry induction.

Inhibitory effect of azimilide on Na+/Ca2+ exchange current in guinea-pig cardiac myocytes

We examined the effect of azimilide, a class III antiarrhythmic drug, on Na(+)/Ca(2+) exchange current (I(NCX)) in guinea-pig cardiac single ventricular cells. External application of azimilide suppressed bi-directional I(NCX) in a concentration-dependent manner. IC(50) values for outward and inward I(NCX) were 45 and 40 µM, respectively, with Hill coefficients of 1. Azimilide attenuated I(NCX) in the presence of trypsin in the patch pipette, indicating that azimilide is a trypsin-insensitive NCX inhibitor. Delayed afterdepolarization induced by electrical stimulation with ouabain disappeared in the presence of 30 µM azimilide. We conclude that azimilide inhibits NCX at supratherapeutic concentrations.

Cardiac basal metabolism: energetic cost of calcium withdrawal in the adult rat heart

AIM

Cardiac basal metabolism upon extracellular calcium removal and its relationship with intracellular sodium and calcium homeostasis was evaluated.

METHODS

A mechano-calorimetric technique was used that allowed the simultaneous and continuous measurement of both heat rate and resting pressure in arterially perfused quiescent adult rat hearts. Using pharmacological tools, the possible underlying mechanisms related to sodium and calcium movements were investigated.

RESULTS

Resting heat rate (expressed in mW g(-1)(dry wt)) increased upon calcium withdrawal (+4.4 +/- 0.2). This response was: (1) unaffected by the presence of tetrodotoxin (+4.3 +/- 0.6), (2) fully blocked by both, the decrease in extracellular sodium concentration and the increase in extracellular magnesium concentration, (3) partially blocked by the presence of either nifedipine (+2.8 +/- 0.4), KB-R7943 (KBR; +2.5 +/- 0.2), clonazepam (CLO; +3.1 +/- 0.3) or EGTA (+1.9 +/- 0.3). The steady heat rate under Ca(2+)-free conditions was partially reduced by the addition of Ru360 (-1.1 +/- 0.2) but not CLO in the presence of EGTA, KBR or Ru360.

CONCLUSION

Energy expenditure for resting state maintenance upon calcium withdrawal depends on the intracellular rise in both sodium and calcium. Our data are consistent with a mitochondrial Ca(2+) cycling, not detectable under normal calcium diastolic levels. The experimental condition here analysed, partially simulates findings reported under certain pathological situations including heart failure in which mildly increased levels of both diastolic sodium and calcium have also been found. Therefore, under such pathological conditions, hearts should distract chemical energy to fuel processes associated with sodium and calcium handling, making more expensive the maintenance of their functions.

Structure and dynamics of an Arp2/3 complex-independent component of the lamellipodial actin network

Sea urchin coelomocytes contain an unusually broad lamellipodial region and have served as a useful model experimental system for studying the process of actin-based retrograde/centripetal flow. In the current study the small molecule drug 2,3-butanedione monoxime (BDM) was employed as a means of delocalizing the Arp2/3 complex from the cell edge in an effort to investigate the Arp2/3 complex-independent aspects of retrograde flow. Digitally-enhanced phase contrast, fluorescence and polarization light microscopy, along with rotary shadow transmission electron microscopy methods demonstrated that BDM treatment resulted in the centripetal displacement of the Arp2/3 complex and the associated dendritic lamellipodial (LP) actin network from the cell edge. In its wake there remained an array of elongate actin filaments organized into concave arcs that displayed retrograde flow at approximately one quarter the normal rate. Actin polymerization inhibitor experiments indicated that these arcs were generated by polymerization at the cell edge, while active myosin-based contraction in BDM treated cells was demonstrated by localization with antiphospho-myosin regulatory light chain (MRLC) antibody, the retraction of the cytoskeleton in the presence of BDM, and the response of the BDM arcs to laser-based severing. The results suggest that BDM treatment reveals an Arp2/3 complex-independent actin structure in coelomocytes consisting of elongate filaments integrated into the LP network and that these filaments represent a potential connection between the LP network and the central cytoskeleton.

Acute inhibitory effect of dronedarone, a noniodinated benzofuran analogue of amiodarone, on Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

Using the whole-cell voltage-clamp method, we examined an acute effect of dronedarone, a noniodinated benzofuran analogue of amiodarone, on Na+/Ca2+ exchange current (INCX) in guinea pig cardiac ventricular cells. The INCX was recorded by ramp pulses with a holding potential of -60 mV using a pipette solution containing 226 nM free Ca2+ (20 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and 10 mM Ca2+) and 20 mM Na+. The external solution contained 140 mM Na+, 1 mM Ca2+, and blockers of other currents and pumps such as Cs+, nifedipine, ryanodine, and ouabain. A selective potent NCX inhibitor, KB-R7943 (100 microM), was used to completely inhibit INCX. Dronedarone inhibited INCX in a concentration-dependent manner. The IC50 values for the outward and inward INCX inhibition were about 33 and 28 microM, respectively, with the Hill coefficient of 1 for both. The inhibitory effect of dronedarone at 50 microM on INCX did not change in the presence of trypsin in the pipette solution. Therefore, dronedarone is classified as a trypsin-insensitive NCX inhibitor and distinct from amiodarone which is a trypsin sensitive. We conclude that dronedarone inhibits INCX but the potency is tenfold less than that of amiodarone. Dronedarone may modestly inhibit INCX in a therapeutic concentration range.

Blebbistatin extends culture life of adult mouse cardiac myocytes and allows efficient and stable transgene expression

The characterization of cellular phenotypes of heart disorders can be achieved by isolating cardiac myocytes from mouse models or genetically modifying wild-type cells in culture. However, adult mouse cardiac myocytes show extremely low tolerance to isolation and primary culture conditions. Previous studies indicate that 2,3-butanedione monoximine (BDM), a nonspecific excitation-contraction coupling inhibitor, can improve the viability of isolated adult mouse cardiac myocytes. The mechanisms of the beneficial and unwanted nonspecific actions of BDM on cardiac myocytes are not understood. To understand what contributes to murine adult cardiac myocyte stability in primary culture and improve this model system for experimental use, the specific myosin II inhibitor blebbistatin was explored as a media supplement to inhibit mouse myocyte contraction. Enzymatically isolated adult mouse cardiac myocytes were cultured with blebbistatin or BDM as a media supplement. Micromolar concentrations of blebbistatin significantly increased the viability, membrane integrity, and morphology of adult cardiac myocytes compared with cells treated with previously described 10 mM BDM. Cells treated with blebbistatin also showed efficient adenovirus gene transfer and stable transgene expression, and unlike BDM, blebbistatin does not appear to interfere with cell adhesion. Higher concentrations of BDM actually worsened myocyte membrane integrity and transgene expression. Therefore, the specific inhibition of myosin II activity by blebbistatin has significant beneficial effects on the long-term viability of adult mouse cardiac myocytes. Furthermore, the unwanted effects of BDM on adult mouse cardiac myocytes, perhaps due to its nonspecific activities or action as a chemical phosphatase, can be avoided by using blebbistatin.

A simple technique for isolating healthy heart cells from mouse models

Single heart cells of mouse models provide powerful tools for heart research. However, their isolation is not easy, and it imposes a significant bottleneck on their use in cellular studies of the heart. Aiming to overcome this problem, this report introduces a novel technique that reproducibly isolates healthy heart cells from mouse models. Using simple devices that ensure easy handling and the rapid aortic cannulation of a small mouse heart, cell isolation was done under physiological conditions without using the "KB" medium or 2,3-butanedione monoxime (BDM). The isolated cells consistently had a healthy appearance and a high viability of 75 +/- 5% (mean +/- SD) in Tyrode solution containing 1.8 mM Ca2+. After 8 h of storage at 37 degrees C, they still had a viability of 45 +/- 12%. The cells showed normal contraction properties when field-stimulated, and they generated normal action potentials and membrane currents under the whole-cell clamp condition. The beta-adrenergic signal transduction of the cells was also normal when it was examined with the isoproterenol enhancement of the L-type Ca2+ current.

Characterization of SN-6, a novel Na+/Ca2+ exchange inhibitor in guinea pig cardiac ventricular myocytes

We examined the effect of SN-6, a new benzyloxyphenyl Na(+)/Ca(2+) exchange (NCX) inhibitor on the Na(+)/Ca(2+) exchange current (I(NCX)) and other membrane currents in isolated guinea pig ventricular myocytes using the whole-cell voltage-clamp technique. SN-6 suppressed I(NCX) in a concentration-dependent manner. The IC(50) values of SN-6 were 2.3 microM and 1.9 microM for the outward and inward components of the bi-directional I(NCX), respectively. On the other hand, SN-6 suppressed the outward uni-directional I(NCX) more potently (IC(50) value of 0.6 microM) than the inward uni-directional I(NCX). SN-6 at 10 microM inhibited the uni-directional inward I(NCX) by only 22.4+/-3.1%. SN-6 and KB-R7943 suppressed I(NCX) more potently when intracellular Na(+) concentration was higher. Thus, both drugs inhibit NCX in an intracellular Na(+) concentration-dependent manner. Intracellular application of trypsin via a pipette solution did not change the blocking effect of SN-6 on I(NCX). Therefore, SN-6 is categorized as an intracellular-trypsin-insensitive NCX inhibitor. SN-6 at 10 microM inhibited I(Na), I(Ca), I(K) and I(K1) by about 13%, 34%, 33% and 13%, respectively. SN-6 at 10 microM shortened the action potential duration at 50% repolarization (APD(50)) by about 34%, and that at 90% repolarization (APD(90)) by about 25%. These results indicate that SN-6 inhibits NCX in a similar manner to that of KB-R7943. However, SN-6 at 10 microM affected other membrane currents less potently than KB-R7943.

Electrophysiological effects of SN-6, a novel Na+/Ca2+ exchange inhibitor on membrane currents in guinea pig ventricular myocytes

We examined the effect of SN-6 on the Na+/Ca2+ exchanger (NCX) current (I(NCX)) and other membrane currents in isolated guinea pig ventricular myocytes using the whole-cell voltage clamp technique. SN-6 suppressed the bidirectional I(NCX) in a concentration-dependent manner. The IC50 values of SN-6 were 2.3 microM and 1.9 microM for the outward and inward components of the bidirectional I(NCX), respectively. On the other hand, SN-6 suppressed the unidirectional outward I(NCX) more potently than the inward I(NCX), with an IC(50) value of 0.6 microM. SN-6 at 10 microM inhibited the unidirectional inward I(NCX) by only 22.4 +/- 3.1%. SN-6 suppressed I(NCX) more potentially when intracellular Na+ concentration became higher. SN-6 inhibited I(Na), I(Ca), I(Kr), I(Ks), and I(K1) by about 13%, 34%, 33%, 18%, and 13%, respectively. SN-6 shortened the action potential duration (APD) by about 34% and 25% at APD(50) and APD(90), respectively. These results indicate that SN-6 inhibits NCX in a similar manner to that of KB-R7943. SN-6 and KB-R7943 inhibit the unidirectional outward I(NCX) more potently than the unidirectional inward I(NCX). Both drugs inhibit NCX in an intracellular Na+ concentration-dependent manner. However, SN-6 affected other membrane currents less potently than KB-R7943.

2,3-butanedione monoxime affects cystic fibrosis transmembrane conductance regulator channel function through phosphorylation-dependent and phosphorylation-independent mechanisms: the role of bilayer material properties

2,3-Butanedione monoxime (BDM) is widely believed to act as a chemical phosphatase. We therefore examined the effects of BDM on the cystic fibrosis transmembrane regulator (CFTR) Cl(-) channel, which is regulated by phosphorylation in a complex manner. In guinea pig ventricular myocytes, forskolin-activated whole-cell CFTR currents responded biphasically to external 20 mM BDM: a rapid approximately 2-fold current activation was followed by a slower (tau approximately 20 s) inhibition (to approximately 20% of control). The inhibitory response was abolished by intracellular dialysis with the phosphatase inhibitor microcystin, suggesting involvement of endogenous phosphatases. The BDM-induced activation was studied further in Xenopus laevis oocytes expressing human epithelial CFTR. The concentration for half-maximal BDM activation (K(0.5)) was state-dependent, approximately 2 mM for highly and approximately 20 mM for partially phosphorylated channels, suggesting a modulated receptor mechanism. Because BDM modulates many different membrane proteins with similar K(0.5) values, we tested whether BDM could alter protein function by altering lipid bilayer properties rather than by direct BDM-protein interactions. Using gramicidin channels of different lengths (different channel-bilayer hydrophobic mismatch) as reporters of bilayer stiffness, we found that BDM increases channel appearance rates and lifetimes (reduces bilayer stiffness). At 20 mM BDM, the appearance rates increase approximately 4-fold (for the longer, 15 residues/monomer, channels) to approximately 10-fold (for the shorter, 13 residues/monomer channels); the lifetimes increase approximately 50% independently of channel length. BDM thus reduces the energetic cost of bilayer deformation, an effect that may underlie the effects of BDM on CFTR and other membrane proteins; the state-dependent changes in K(0.5) are consistent with such a bilayer-mediated mechanism.

Role of Mechanical Stress in the Form of Cardiomyocyte Death During the Early Phase of Reperfusion

BACKGROUND

The hypothesis that mechanical stress during reperfusion produces myocyte oncosis and inhibits apoptosis was tested in the present study.

METHODS AND RESULTS

Isolated and perfused rat hearts were subjected to 30 min ischemia followed by 150 min reperfusion. In the control-reperfusion heart, the form of myocyte death was a mixture of apoptosis only, oncosis only, and both apoptosis and oncosis. Apoptotic myocytes contained mitochondria that maintained membrane potential (Deltapsim), whereas oncotic myocytes contained only Deltapsim-collapsed mitochondria. Treatment with the contractile blocker 2,3-butanedione monoxime (BDM) during reperfusion increased caspase-3 activity and produced predominantly apoptosis. However, withdrawal of BDM provoked oncosis in terminal deoxynucleotide nick-end labeling (TUNEL)-positive myocytes. Myocardial stretch by inflating an intraventricular balloon at the time of reperfusion with BDM increased only oncotic myocytes, whereas the same mechanical stress 120 min after reperfusion increased oncotic myocytes positive for TUNEL. Increased mechanical stress at the time of reperfusion by treatment with isoproterenol or hyposmotic buffer inhibited caspase-3 activity and increased only oncotic myocytes. Co-treatment with the caspase-3 inhibitor, Ac-DEVD-CHO, and BDM during reperfusion inhibited myocyte apoptosis and oncosis but did not inhibit oncosis after withdrawal of BDM.

CONCLUSIONS

These results suggest that mechanical stress is a critical determinant of the form of myocyte death during the early phase of reperfusion.

Topics on the Na+/Ca2+ Exchanger: Pharmacological Characterization of Na+/Ca2+ Exchanger Inhibitors

Using the whole-cell voltage clamp, we examined acute effects of various agents on Na(+)/Ca(2+) exchange current (I(NCX)) in guinea-pig cardiac ventricular cells and transfected cells. Among the antiarrhythmic drugs, amiodarone, bepridil, dronedarone, cibenzoline, azimilide, and aprindine inhibited I(NCX) in a concentration-dependent manner. We also investigated the effects on NCX of 2,3-buanedione monoxim (BDM) and selective NCX inhibitors such as KB-R7943, SEA0400, and SN-6. The presence of trypsin in the pipette solution attenuated the inhibitory effects on NCX of amiodarone, bepridil, and BDM, suggesting that these drugs inhibit NCX from the cytosolic side. In contrast, the trypsin-insensitive NCX inhibitors were aprindine, azimilide, dronedarone, cibenzoline, KB-R7943, SEA0400, and SN-6. KB-R7943, SEA0400, and SN-6 suppressed the uni-directional outward I(NCX) more potently than the uni-directional inward I(NCX). The mechanism of this mode-dependency is unknown, but is suggested to be related to intracellular Na(+) concentration.

Examination of stimulation mechanism and strength-interval curve in cardiac tissue

Understanding the basic mechanisms of excitability through the cardiac cycle is critical to both the development of new implantable cardiac stimulators and improvement of the pacing protocol. Although numerous works have examined excitability in different phases of the cardiac cycle, no systematic experimental research has been conducted to elucidate the correlation among the virtual electrode polarization pattern, stimulation mechanism, and excitability under unipolar cathodal and anodal stimulation. We used a high-resolution imaging system to study the spatial and temporal stimulation patterns in 20 Langendorff-perfused rabbit hearts. The potential-sensitive dye di-4-ANEPPS was utilized to record the electrical activity using epifluorescence. We delivered S1-S2 unipolar point stimuli with durations of 2-20 ms. The anodal S-I curves displayed a more complex shape in comparison with the cathodal curves. The descent from refractoriness for anodal stimulation was extremely steep, and a local minimum was clearly observed. The subsequent ascending limb had either a dome-shaped maximum or was flattened, appearing as a plateau. The cathodal S-I curves were smoother, closer to a hyperbolic shape. The transition of the stimulation mechanism from break to make always coincided with the final descending phase of both anodal and cathodal S-I curves. The transition is attributed to the bidomain properties of cardiac tissue. The effective refractory period was longer when negative stimuli were delivered than for positive stimulation. Our spatial and temporal analyses of the stimulation patterns near refractoriness show always an excitation mechanism mediated by damped wave propagation after S2 termination.

Effects of amiodarone on mutant Na+/Ca2+ exchangers expressed in CCL 39 cells

Using the whole cell voltage clamp, we reported previously that amiodarone acutely inhibits Na+/Ca2+ exchange current (INCX) in guinea pig cardiac ventricular myocytes. Intracellular application of trypsin via the patch pipette attenuated the blocking effect of amiodarone, suggesting that amiodarone affects the Na+/Ca2+ exchanger (NCX) from the cytoplasmic side. Here, we attempted to detect the site of amiodarone inhibition using wild type NCX1, mutants, and NCX3 expressed in CCL39 fibroblasts. INCX was recorded by ramp pulses. Amiodarone at 30 microM inhibited INCX by 80% in cells expressing wild type NCX1. However, 30 microM amiodarone inhibited INCX by about 55% in cells expressing mutant NCX1 with amino acids 217-671 (DeltaXIP) or 247-671 (Delta247-671) deleted in the long intracellular loop between the transmembrane segments (TM) 5 and 6. INCXs from NCX mutants deleted of cytoplasmic TM1-2, TM3-4 or the C-terminus were inhibited by amiodarone to a similar extent as the wild type. Amiodarone also inhibited INCX of NCX3 by 76%. These results suggest that a long intracellular loop may be involved in the inhibition of NCX1 by amiodarone, but that other intracellular loops, XIP region or C terminus are not involved in the amiodarone inhibition of NCX1.

Effects of 2,3-Butanedione Monoxime on Induction of Action Potential Bursts in Central Snail Neurons: Direct and Indirect Modulations of Ionic Currents

The effects of 2,3-butanedione monoxime (BDM) on induction of action potential bursts were studied pharmacologically on the RP4 central neuron of giant African snail (Achatina fulica Ferussac). The effect of okadaic acid on the neuron was also tested. The RP4 neuron showed a spontaneous firing of action potential. Okadaic acid (1 micromol/l) did not alter the frequency of spontaneous action potential while BDM (3 mmol/l) reversibly elicited bursts of potential (BoP) of the RP4 neuron. The BoP elicited by BDM (3 mmol/l) were reversed 20 min after incubation with diazoxide (500 micromol/l) while the BoP were not altered in preparations treated with okadaic acid and BDM. The BDM-elicited BoP were not inhibited after administration with (a) hexamethonium (100 micromol/l), (b) atropine (1 mmol/l), (c) d-tubocurarine (100 micromol/l), (d) prazosin (100 micromol/l), (e) propranolol (100 micromol/l), (f) calcium-free solution, (g) high K(+) (12 mmol/l) or (h) with high Mg(2+) (30 mmol/l) solutions. The BDM-elicited BoP were inhibited by pretreatment with KT-5720 (10 micromol/l) or H89 (10 micromol/l), the protein kinase A inhibitors. However, the BoP were not affected after application of chelerythrine (10 micromol/l) or Ro 31-8220 (10 micromol/l), the protein kinase C inhibitors. Voltage-clamped studies revealed that BDM elicited a negative slope resistance (NSR) at membrane potentials between -50 and -10 mV. The NSR was not detectable at the same membrane potential in control RP4 neuron. It is suggested that the BoP elicited by BDM were not due to (1) the synaptic effects of neurotransmitters; (2) the activation of cholinergic, adrenergic receptors, or (3) phosphatase activity of the neuron. The BDM-elicited BoP were dependent on the protein kinase A related cAMP in the neuron and the delayed outward K(+) current may contribute to the BDM-elicited BoP.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Ischemic preconditioning-mediated restoration of membrane dystrophin during reperfusion correlates with protection against contraction-induced myocardial injury

Dystrophin is an integral membrane protein involved in the stabilization of the sarcolemmal membrane in cardiac muscle. We hypothesized that the loss of membrane dystrophin during ischemia and reperfusion is responsible for contractile force-induced myocardial injury and that cardioprotection afforded by ischemic preconditioning (IPC) is related to the preservation of membrane dystrophin. Isolated and perfused rat hearts were subjected to 30 min of global ischemia, followed by reperfusion with or without the contractile blocker 2,3-butanedione monoxime (BDM). IPC was introduced by three cycles of 5-min ischemia and 5-min reperfusion before the global ischemia. Dystrophin was distributed exclusively in the membrane of myocytes in the normally perfused heart but was redistributed to the myofibril fraction after 30 min of ischemia and was lost from both of these compartments during reperfusion in the presence or absence of BDM. The loss of dystrophin preceded uptake of the membrane-impermeable Evans blue dye by myocytes that occurred after the withdrawal of BDM and was associated with creatine kinase release and the development of contracture. Although IPC did not alter the redistribution of membrane dystrophin induced by 30 min of ischemia, it facilitated the restoration of membrane dystrophin during reperfusion. Also, myocyte necrosis was not observed when BDM was withdrawn after complete restoration of membrane dystrophin. These results demonstrate that IPC-mediated restoration of membrane dystrophin during reperfusion correlates with protection against contractile force-induced myocardial injury and suggest that the cardioprotection conferred by IPC can be enhanced by the temporary blockade of contractile activity until restoration of membrane dystrophin during reperfusion.

The electrophysiological and mechanical effects of 2,3-butane-dione monoxime and cytochalasin-D in the Langendorff perfused rabbit heart

Procedures that reduce contraction are used to facilitate optical measurements of membrane potential, but it is unclear to what extent they affect the excitability of the heart. This study has examined the electrophysiological consequences of a range of extracellular [Ca2+] (0.7-2.5 mmol l(-1)), 2,3-butane-dione monoxime (BDM; 1-20 mmol l(-1)) and cytochalasin-D (Cyto-D; 1-5 micromol l(-1)).

METHODS

Monophasic action potentials (MAPs) were recorded from the basal epicardial surface of the left ventricle of isolated rabbit hearts. Conduction delay (CD) and time to 90% repolarisation of the monophasic action potential (MAPD90) were measured. The effects of BDM and Cyto-D on restitution were studied at a [Ca2+] of 1.9 mmol l(-1). Restitution curves for MAPD90 were generated using a standard S1-S2 protocol.

RESULTS

All manoeuvres decreased left ventricular developed pressure (LVDP): 0.7 mmol l(-1) Ca2+ to 74.0 +/- 6.1%, 20 mmol l(-1) BDM to 4.5 +/- 1.0%, and 5 micromol l(-1) Cyto-D to 12.8 +/- 3.5% of control value. CD decreased from a control value (33.3 +/- 1.0 ms, n= 16) to 93.0 +/- 2.2% in 0.7 mmol l(-1) Ca2+, but increased to 133.7 +/- 10.5% in 20 mmol l(-1) BDM and 127.4 +/- 10.6% in 5 micromol l(-1) Cyto-D. At 350 ms pacing cycle length, MAPD90 (control = 119.6 +/- 1.7 ms n= 16) was prolonged by reduced extracellular [Ca2+]. BDM had no effects on MAPD90 at control pacing rates. Cyto-D caused a significant prolongation (to 115.0 +/- 3.0% of control, n= 6) at the highest concentration studied (5 micromol l(-1)). Both BDM (20 mmol l(-1)) and Cyto-D (3 micromol l(-1)) flattened the restitution curves but neither agent altered maximum MAPD90.

CONCLUSIONS

Extracellular [Ca2+] of 1.9 mmol l(-1) in conjunction with a moderate dose of Cyto-D (3 micromol l(-1)) reduced contractility with minimal effects on action potential duration and conduction at a fixed pacing cycle length. However, both BDM and Cyto-D had pronounced effects on electrical restitution.

Pharmacology of Na+/Ca2+ exchanger

KB-R7943 inhibits the Na(+)/Ca(2+) exchanger in an independent manner or in a manner dependent on the direction of the current. This effect may be due to the experimental protocols bawed on the competition between the drug and external substrate ions. Some antiarrhythmic drugs inhibit NCX. A new column of NCX was added in Sicilian Gambit.

Myosin light chain phosphorylation and growth cone motility

According to the treadmill hypothesis, the rate of growth cone advance depends upon the difference between the rates of protrusion (powered by actin polymerization at the leading edge) and retrograde F-actin flow, powered by activated myosin. Myosin II, a strong candidate for powering the retrograde flow, is activated by myosin light chain (MLC) phosphorylation. Earlier results showing that pharmacological inhibition of myosin light chain kinase (MLCK) causes growth cone collapse with loss of F-actin-based structures are seemingly inconsistent with the treadmill hypothesis, which predicts faster growth cone advance. These experiments re-examine this issue using an inhibitory pseudosubstrate peptide taken from the MLCK sequence and coupled to the fatty acid stearate to allow it to cross the membrane. At 5-25 microM, the peptide completely collapsed growth cones from goldfish retina with a progressive loss of lamellipodia and then filopodia, as seen with pharmacological inhibitors, but fully reversible. Lower concentrations (2.5 microM) both simplified the growth cone (fewer filopodia) and caused faster advance, doubling growth rates for many axons (51-102 microm/h; p <.025). Rhodamine-phalloidin staining showed reduced F-actin content in the faster growing growth cones, and marked reductions in collapsed ones. At higher concentrations, there was a transient advance of individual filopodia before collapse (also seen with the general myosin inhibitor, butanedione monoxime, which did not accelerate growth). The rho/rho kinase pathway modulates MLC dephosphorylation by myosin-bound protein phosphatase 1 (MPP1), and manipulations of MPP1 also altered motility. Lysophosphatidic acid (10 microM), which causes inhibition of MPP1 to accumulate activated myosin II, caused a contracted collapse (vs. that due to loss of F-actin) but was ineffective after treatment with low doses of peptide, demonstrating that the peptide acts via MLC phosphorylation. Inhibiting rho kinase with Y27632 (100 microM) to disinhibit the phosphatase increased the growth rate like the MLCK peptide, as expected. These results suggest that: varying the level of MLCK activity inversely affects the rate of growth cone advance, consistent with the treadmill hypothesis and myosin II powering of retrograde F-actin flow; MLCK activity in growth cones, as in fibroblasts, contributes strongly to controlling the amount of F-actin; and the phosphatase is already highly active in these cultures, because rho kinase inhibition produces much smaller effects on growth than does MLCK inhibition.

Chronic administration of amiodarone does not affect Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

We investigated chronic effects of amiodarone on Na+/Ca2+ exchange current (INCX) and on the level of Na+/Ca2+ exchanger (NCX1) mRNA in guinea pig ventricular myocytes using the whole-cell clamp technique and RT-PCR analysis, respectively. Guinea pigs were intraperitoneally injected with 80 mg/kg per day of amiodarone or the vehicle (saline) for 1 or 4 weeks. Single ventricular cells were isolated from the hearts of both groups of animals. Action potential duration at 90% repolarization level was prolonged to 143% and 165% of the control values by treatment with amiodarone for 1 and 4 weeks, respectively. INCX density and the level of NCX1 mRNA were not significantly changed by chronic treatment with amiodarone. The level of thyroid hormone (T4) within the blood was not changed by the treatments. These results suggest that chronic treatment with amiodarone does not affect the Na+/Ca2+ exchanger, with respect to the level of its mRNA and current density in guinea pig ventricular myocytes.

Mechanisms of shock-induced arrhythmogenesis during acute global ischemia

Little is known about the mechanisms of vulnerability and defibrillation under ischemic conditions. We investigated these mechanisms in 18 Langendorff-perfused rabbit hearts during 75% reduced-flow ischemia. Electrical activity was optically mapped from the anterior epicardium during right ventricular shocks applied at various phases of the cardiac cycle while the excitation-contraction decoupler 2,3-butanedione monoxime (BDM; 15 mM) was used to suppress motion artifacts caused by contraction of the heart. During ischemia, vulnerable window width increased [from 30-90% of the action potential duration (APD) in the control to -10 to 100% of the APD in ischemia]. Moreover, arrhythmia severity increased along with the reduction of APD (176 +/- 9 ms in control and 129 +/- 26 ms in ischemia, P < 0.01) and increased dispersion of repolarization (45 +/- 17 ms in control and 73 +/- 28 ms in ischemia, P < 0.01). Shock-induced virtual electrode polarization was preserved. Depolarizing (contrary to hyperpolarizing) response time constants increased. Virtual electrode-induced wavefronts of excitation had much more tortuous pathways leading to wavefront fractionation. Defibrillation failure at all shock strengths was observed in four hearts. Optical mapping revealed that the shock extinguished the arrhythmia; however, the arrhythmia self-originated after an isoelectric window of 339 +/- 189 ms. In conclusion, in most cases, virtual electrode-induced phase singularity (VEIPS) was responsible for shock-induced arrhythmogenesis during acute global ischemia. Enhancement of arrhythmogenesis was associated with an increased dispersion of repolarization and altered deexcitation. In four hearts, arrhythmogenesis could not be explained by VEIPS.

Inhibitory effect of aprindine on Na+/Ca2+ exchange current in guinea-pig cardiac ventricular myocytes

1. Using the whole-cell voltage clamp technique, the effect of aprindine on Na+/Ca2+ exchange current (I(NCX)) was examined in guinea-pig single cardiac ventricular myocytes and CCL39 fibroblasts expressing a dog cardiac Na+/Ca2+ exchanger (NCX1). 2. I(NCX) was recorded by ramp pulses from the holding potential of -60 mV with the external solution containing 140 mM Na+ and 1 mM Ca2+, and the pipette solution containing 20 mM Na+, 20 mM BAPTA and 13 mM Ca2+ (433 nM free Ca2+). 3. External application of aprindine suppressed I(NCX) in a concentration-dependent manner. The IC50 values of outward (measured at 50 mV) and inward (measured at -100 mV) I(NCX) components were 48.8 and 51.8 microM with Hill coefficients of 1.3 and 1, respectively. 4. Intracellular application of trypsin via the pipette solution did not change the blocking effect of aprindine, suggesting that aprindine does not affect the exchanger from the cytoplasmic side. 5. Aprindine inhibited I(NCX) of a mutant NCX1 with a deletion of amino acids 247 - 671 in the large intracellular domain between the transmembrane segments 5 and 6 in a similar manner to that of the wild-type, suggesting that the site of aprindine inhibition is not in the large intracellular domain of NCX1. 6. A kinetic study indicated that aprindine was cooperatively competitive with KB-R7943, another inhibitor of NCX and that aprindine was a competitive inhibitor with respect to external Ca2+. 7. We conclude that aprindine may modestly inhibit I(NCX) in a therapeutic range of concentrations (around 2.5 approximately 6.9 microM) possibly at an external or intra-membranous site of the exchanger.

Role of the Na(+)-Ca(2+) exchanger as an alternative trigger of CICR in mammalian cardiac myocytes

Ca(2+) influx through the L-type Ca(2+) channels is the primary pathway for triggering the Ca(2+) release from the sarcoplasmic reticulum (SR). However, several observations have shown that Ca(2+) influx via the reverse mode of the Na(+)-Ca(2+) exchanger current (I(Na-Ca)) could also trigger the Ca(2+) release. The aim of the present study was to quantitate the role of this alternative pathway of Ca(2+) influx using a mathematical model. In our model 20% of the fast sodium channels and the Na(+)-Ca(2+) exchanger molecules are located in the restricted subspace between the sarcolemma and the SR where triggering of the calcium-induced calcium release (CICR) takes place. After determining the strengths of the alternative triggers with simulated voltage-clamps in varied membrane voltages and resting [Na](i) values, we studied the CICR in simulated action potentials, where fast sodium channel current contributes [Na](i) of the subspace. In low initial [Na](i) the Ca(2+) influx via the L-type Ca(2+) channels is the major trigger for Ca(2+) release from the SR, and the Ca(2+) influx via the reverse mode of the Na(+)-Ca(2+) exchanger cannot trigger the CICR. However, depending on the initial [Na](i), the contribution of the Ca(2+) entry via the exchanger may account for 25% (at [Na](i) = 10 mM) to nearly 100% ([Na](i) = 30 mM) of the trigger Ca(2+). The shift of the main trigger from L-type calcium channels to the exchanger reduced the delay between the action potential upstroke and the intracellular calcium transient. This may contribute to the function of the myocyte in physiological situations where [Na](i) is elevated. These main results remain the same when using different estimates for the most crucial parameters in the modeling or different models for the exchanger.