Signaling mechanisms that mediate nitric oxide production induced by acetylcholine exposure and withdrawal in cat atrial myocytes

Transcriptome Analysis Reveals Enhancement of Cardiogenesis-Related Signaling Pathways by S-Nitroso- N -Pivaloyl- d -Penicillamine: Implications for Improved Diastolic Function and Cardiac Performance

ABSTRACT

We previously reported a novel compound called S-nitroso- N -pivaloyl- d -penicillamine (SNPiP), which was screened from a group of nitric oxide donor compounds with a basic chemical structure of S-nitroso- N -acetylpenicillamine, to activate the nonneuronal acetylcholine system. SNPiP-treated mice exhibited improved cardiac output and enhanced diastolic function, without an increase in heart rate. The nonneuronal acetylcholine-activating effects included increased resilience to ischemia, modulation of energy metabolism preference, and activation of angiogenesis. Here, we performed transcriptome analysis of SNPiP-treated mice ventricles to elucidate how SNPiP exerts beneficial effects on cardiac function. A time-course study (24 and 48 hours after SNPiP administration) revealed that SNPiP initially induced Wnt and cyclic guanosine monophosphate-protein kinase G signaling pathways, along with upregulation of genes involved in cardiac muscle tissue development and oxytocin signaling pathway. We also observed enrichment of glycolysis-related genes in response to SNPiP treatment, resulting in a metabolic shift from oxidative phosphorylation to glycolysis, which was suggested by reduced cardiac glucose contents while maintaining adenosine tri-phosphate levels. In addition, SNPiP significantly upregulated atrial natriuretic peptide and sarcolipin, which play crucial roles in calcium handling and cardiac performance. These findings suggest that SNPiP may have therapeutic potential based on the pleiotropic mechanisms elucidated in this study.

Peculiarities of the Acetylcholine Action on the Contractile Function of Cardiomyocytes from the Left and Right Atria in Rats

Acetylcholine (ACh) is the neurotransmitter of the parasympathetic nervous system that modulates cardiac function, and its high concentrations may induce atrial fibrillation. We compared the ACh action on the mechanical function of single cardiomyocytes from the left atria (LA) and the right atria (RA). We exposed single rat LA and RA cardiomyocytes to 1, 10, and 100 µM ACh for 10-15 min and measured the parameters of sarcomere shortening-relengthening and cytosolic calcium ([Ca²⁺]_i) transients during cell contractions. We also studied the effects of ACh on cardiac myosin function using an in vitro motility assay and analyzed the phosphorylation level of sarcomeric proteins. In LA cardiomyocytes, ACh decreased the time to peak sarcomere shortening, time to 50% relengthening, and time to peak [Ca²⁺]_i transients. In RA cardiomyocytes, ACh affected the time of shortening and relengthening only at 10 µM. In the in vitro motility assay, ACh reduced to a greater extent the sliding velocity of F-actin over myosin from LA cardiomyocytes, which was accompanied by a more pronounced decrease in phosphorylation of the myosin regulatory light chain (RLC) in LA cardiomyocytes than in RA cardiomyocytes. Our findings indicate that ACh plays an important role in modulating the contractile function of LA and RA, provoking more pronounced changes in the time course of sarcomere shortening-relengthening and the kinetics of actin-myosin interaction in LA cardiomyocytes.

Asymmetry and Heterogeneity: Part and Parcel in Cardiac Autonomic Innervation and Function

The cardiac autonomic nervous system (cANS) regulates cardiac adaptation to different demands. The heart is an asymmetrical organ, and in the selection of adequate treatment of cardiac diseases it may be relevant to take into account that the cANS also has sidedness as well as regional differences in anatomical, functional, and molecular characteristics. The left and right ventricles respond differently to adrenergic stimulation. Isoforms of nitric oxide synthase, which plays an important role in parasympathetic function, are also distributed asymmetrically across the heart. Treatment of cardiac disease heavily relies on affecting left-sided heart targets which are thought to apply to the right ventricle as well. Functional studies of the right ventricle have often been neglected. In addition, many principles have only been investigated in animals and not in humans. Anatomical and functional heterogeneity of the cANS in human tissue or subjects is highly valuable for understanding left- and right-sided cardiac pathology and for identifying novel treatment targets and modalities. Within this perspective, we aim to provide an overview and synthesis of anatomical and functional heterogeneity of the cANS in tissue or subjects, focusing on the human heart.

Deregulation of Ca2+-Signaling Systems in White Adipocytes, Manifested as the Loss of Rhythmic Activity, Underlies the Development of Multiple Hormonal Resistance at Obesity and Type 2 Diabetes

Various types of cells demonstrate ubiquitous rhythmicity registered as simple and complex Ca²⁺-oscillations, spikes, waves, and triggering phenomena mediated by G-protein and tyrosine kinase coupled receptors. Phospholipase C/IP₃-receptors (PLC/IP₃R) and endothelial NO-synthase/Ryanodine receptors (NOS/RyR)–dependent Ca²⁺ signaling systems, organized as multivariate positive feedback generators (PLC-G and NOS-G), underlie this rhythmicity. Loss of rhythmicity at obesity may indicate deregulation of these signaling systems. To issue the impact of cell size, receptors’ interplay, and obesity on the regulation of PLC-G and NOS-G, we applied fluorescent microscopy, immunochemical staining, and inhibitory analysis using cultured adipocytes of epididumal white adipose tissue of mice. Acetylcholine, norepinephrine, atrial natriuretic peptide, bradykinin, cholecystokinin, angiotensin II, and insulin evoked complex [Ca²⁺]_i responses in adipocytes, implicating NOS-G or PLC-G. At low sub-threshold concentrations, acetylcholine and norepinephrine or acetylcholine and peptide hormones (in paired combinations) recruited NOS-G, based on G proteins subunits interplay and signaling amplification. Rhythmicity was cell size- dependent and disappeared in hypertrophied cells filled with lipids. Contrary to control cells, adipocytes of obese hyperglycemic and hypertensive mice, growing on glucose, did not accumulate lipids and demonstrated hormonal resistance being non responsive to any hormone applied. Preincubation of preadipocytes with palmitoyl-L-carnitine (100 nM) provided accumulation of lipids, increased expression and clustering of IP₃R and RyR proteins, and partially restored hormonal sensitivity and rhythmicity (5–15% vs. 30–80% in control cells), while adipocytes of diabetic mice were not responsive at all. Here, we presented a detailed kinetic model of NOS-G and discussed its control. Collectively, we may suggest that universal mechanisms underlie loss of rhythmicity, Ca²⁺-signaling systems deregulation, and development of general hormonal resistance to obesity.

Activation of the cardiac non-neuronal cholinergic system prevents the development of diabetes-associated cardiovascular complications

BACKGROUND

Acetylcholine (ACh) plays a crucial role in the function of the heart. Recent evidence suggests that cardiomyocytes possess a non-neuronal cholinergic system (NNCS) that comprises of choline acetyltransferase (ChAT), choline transporter 1 (CHT1), vesicular acetylcholine transporter (VAChT), acetylcholinesterase (AChE) and type-2 muscarinic ACh receptors (M2AChR) to synthesize, release, degrade ACh as well as for ACh to transduce a signal. NNCS is linked to cardiac cell survival, angiogenesis and glucose metabolism. Impairment of these functions are hallmarks of diabetic heart disease (DHD). The role of the NNCS in DHD is unknown. The aim of this study was to examine the effect of diabetes on cardiac NNCS and determine if activation of cardiac NNCS is beneficial to the diabetic heart.

METHODS

Ventricular samples from type-2 diabetic humans and db/db mice were used to measure the expression pattern of NNCS components (ChAT, CHT1, VAChT, AChE and M2AChR) and glucose transporter-4 (GLUT-4) by western blot analysis. To determine the function of the cardiac NNCS in the diabetic heart, a db/db mouse model with cardiac-specific overexpression of ChAT gene was generated (db/db-ChAT-tg). Animals were followed up serially and samples collected at different time points for molecular and histological analysis of cardiac NNCS components and prosurvival and proangiogenic signaling pathways.

RESULTS

Immunoblot analysis revealed alterations in the components of cardiac NNCS and GLUT-4 in the type-2 diabetic human and db/db mouse hearts. Interestingly, the dysregulation of cardiac NNCS was followed by the downregulation of GLUT-4 in the db/db mouse heart. Db/db-ChAT-tg mice exhibited preserved cardiac and vascular function in comparison to db/db mice. The improved function was associated with increased cardiac ACh and glucose content, sustained angiogenesis and reduced fibrosis. These beneficial effects were associated with upregulation of the PI3K/Akt/HIF1α signaling pathway, and increased expression of its downstream targets-GLUT-4 and VEGF-A.

CONCLUSION

We provide the first evidence for dysregulation of the cardiac NNCS in DHD. Increased cardiac ACh is beneficial and a potential new therapeutic strategy to prevent or delay the development of DHD.

Non-neuronal cardiac acetylcholine system playing indispensable roles in cardiac homeostasis confers resiliency to the heart

BACKGROUND

We previously established that the non-neuronal cardiac cholinergic system (NNCCS) is equipped with cardiomyocytes synthesizes acetylcholine (ACh), which is an indispensable endogenous system, sustaining cardiac homeostasis and regulating an inflammatory status, by transgenic mice overexpressing choline acetyltransferase (ChAT) gene in the heart. However, whole body biological significances of NNCCS remain to be fully elucidated.

METHODS AND RESULTS

To consolidate the features, we developed heart-specific ChAT knockdown (ChATKD) mice using 3 ChAT-specific siRNAs. The mice developed cardiac dysfunction. Factors causing it included the downregulation of cardiac glucose metabolism along with decreased signal transduction of Akt/HIF-1alpha/GLUT4, leading to poor glucose utilization, impairment of glycolytic metabolites entering the tricarboxylic (TCA) cycle, the upregulation of reactive oxygen species (ROS) production with an attenuated scavenging potency, and the downregulated nitric oxide (NO) production via NOS1. ChATKD mice revealed a decreased vagus nerve activity, accelerated aggression, more accentuated blood basal corticosterone levels with depression-like phenotypes, several features of which were accompanied by cardiac dysfunction.

CONCLUSION

The NNCCS plays a crucial role in cardiac homeostasis by regulating the glucose metabolism, ROS synthesis, NO levels, and the cardiac vagus nerve activity. Thus, the NNCCS is suggested a fundamentally crucial system of the heart.

Characteristic Effects of the Cardiac Non-Neuronal Acetylcholine System Augmentation on Brain Functions

Since the discovery of non-neuronal acetylcholine in the heart, this specific system has drawn scientific interest from many research fields, including cardiology, immunology, and pharmacology. This system, acquired by cardiomyocytes independent of the parasympathetic nervous system of the autonomic nervous system, helps us to understand unsolved issues in cardiac physiology and to realize that the system may be more pivotal for cardiac homeostasis than expected. However, it has been shown that the effects of this system may not be restricted to the heart, but rather extended to cover extra-cardiac organs. To this end, this system intriguingly influences brain function, specifically potentiating blood brain barrier function. Although the results reported appear to be unusual, this novel characteristic can provide us with another research interest and therapeutic application mode for central nervous system diseases. In this review, we discuss our recent studies and raise the possibility of application of this system as an adjunctive therapeutic modality.

S-Nitroso-N-Pivaloyl-D-Penicillamine, a novel non-neuronal ACh system activator, modulates cardiac diastolic function to increase cardiac performance under pathophysiological conditions

We have previously reported the development of a novel chemical compound, S-Nitroso-N-Pivaloyl-D-Penicillamine (SNPiP), for the upregulation of the non-neuronal cardiac cholinergic system (NNCCS), a cardiac acetylcholine (ACh) synthesis system, which is different from the vagus nerve releasing of ACh as a neurotransmitter. However, it remains unclear how SNPiP could influence cardiac function positively, and whether SNPiP could improve cardiac function under various pathological conditions. SNPiP-injected control mice demonstrated a gradual upregulation in diastolic function without changes in heart rate. In contrast to some parameters in cardiac function that were influenced by SNPiP 24 h or 48 h after a single intraperitoneal (IP) injection, 72 h later, end-systolic pressure, cardiac output, end-diastolic volume, stroke volume, and ejection fraction increased. IP SNPiP injection also improved impaired cardiac function, which is a characteristic feature of the db/db heart, in a delayed fashion, including diastolic and systolic function, following either several consecutive injections or a single injection. SNPiP, a novel NNCCS activator, could be applied as a therapeutic agent for the upregulation of NNCCS and as a unique tool for modulating cardiac function via improvement in diastolic function.

Treatment of hypertension by increasing impaired endothelial TRPV4-KCa2.3 interaction

The currently available antihypertensive agents have undesirable adverse effects due to systemically altering target activity including receptors, channels, and enzymes. These effects, such as loss of potassium ions induced by diuretics, bronchospasm by beta‐blockers, constipation by Ca²⁺ channel blockers, and dry cough by ACEI, lead to non‐compliance with therapies (Moser, 1990). Here, based on new hypertension mechanisms, we explored a new antihypertensive approach. We report that transient receptor potential vanilloid 4 (TRPV4) interacts with Ca²⁺‐activated potassium channel 3 (KCa2.3) in endothelial cells (ECs) from small resistance arteries of normotensive humans, while ECs from hypertensive patients show a reduced interaction between TRPV4 and KCa2.3. Murine hypertension models, induced by high‐salt diet, N(G)‐nitro‐l‐arginine intake, or angiotensin II delivery, showed decreased TRPV4‐KCa2.3 interaction in ECs. Perturbation of the TRPV4‐KCa2.3 interaction in mouse ECs by overexpressing full‐length KCa2.3 or defective KCa2.3 had hypotensive or hypertensive effects, respectively. Next, we developed a small‐molecule drug, JNc‐440, which showed affinity for both TRPV4 and KCa2.3. JNc‐440 significantly strengthened the TRPV4‐KCa2.3 interaction in ECs, enhanced vasodilation, and exerted antihypertensive effects in mice. Importantly, JNc‐440 specifically targeted the impaired TRPV4‐KCa2.3 interaction in ECs but did not systemically activate TRPV4 and KCa2.3. Together, our data highlight the importance of impaired endothelial TRPV4‐KCa2.3 coupling in the progression of hypertension and suggest a novel approach for antihypertensive drug development.

Non-neuronal cardiac cholinergic system influences CNS via the vagus nerve to acquire a stress-refractory propensity

We previously developed cardiac ventricle-specific choline acetyltransferase (ChAT) gene-overexpressing transgenic mice (ChAT tgm), i.e. an in vivo model of the cardiac non-neuronal acetylcholine (NNA) system or non-neuronal cardiac cholinergic system (NNCCS). By using this murine model, we determined that this system was responsible for characteristics of resistance to ischaemia, or hypoxia, via the modulation of cellular energy metabolism and angiogenesis. In line with our previous study, neuronal ChAT-immunoreactivity in the ChAT tgm brains was not altered from that in the wild-type (WT) mice brains; in contrast, the ChAT tgm hearts were the organs with the highest expression of the ChAT transgene. ChAT tgm showed specific traits in a central nervous system (CNS) phenotype, including decreased response to restraint stress, less depressive-like and anxiety-like behaviours and anti-convulsive effects, all of which may benefit the heart. These phenotypes, induced by the activation of cardiac NNCCS, were dependent on the vagus nerve, because vagus nerve stimulation (VS) in WT mice also evoked phenotypes similar to those of ChAT tgm, which display higher vagus nerve discharge frequency; in contrast, lateral vagotomy attenuated these traits in ChAT tgm to levels observed in WT mice. Furthermore, ChAT tgm induced several biomarkers of VS responsible for anti-convulsive and anti-depressive-like effects. These results suggest that the augmentation of the NNCCS transduces an effective and beneficial signal to the afferent pathway, which mimics VS. Therefore, the present study supports our hypothesis that activation of the NNCCS modifies CNS to a more stress-resistant state through vagus nerve activity.

Cholesterol regulates contractility and inotropic response to β2-adrenoceptor agonist in the mouse atria: Involvement of Gi-protein-Akt-NO-pathway

Majority of cardiac β2-adrenoceptors is located in cholesterol-rich microdomains. Here, we have investigated the underlying mechanisms by which a slight to moderate cholesterol depletion with methyl-β-cyclodextrin (MβCD, 1 and 5mM) interferes with contractility and inotropic effect of β2-adrenergic agonist (fenoterol, 50μM) in the mouse atria. Treatment with MβCD itself increased amplitude of Ca²⁺ transient but did not change the contraction amplitude due to a clamping action of elevated NO. Cholesterol depletion significantly attenuated the positive inotropic response to fenoterol which is accompanied by increase in NO generation and decrease in Ca²⁺ transient. Influence of 1mM MβCD on the fenoterol-driven changes in both contractility and NO level was strongly attenuated by inhibition of G_i-protein (pertussis toxin), Akt (Akt 1/2 kinase inhibitor) or NO-synthase (L-NAME). After exposure to 5mM MβCD, pertussis toxin or Akt inhibitor could recover the β2-agonist effects on contractility, NO production and Ca²⁺ transient, while L-NAME only reduced NO level. An adenylyl cyclase activator (forskolin, 50nM) had no influence on the MβCD-induced changes in the β2-agonist effects. Obtained results suggest that slight cholesterol depletion upregulates G_i-protein/Akt/NO-synthase signaling that attenuates the positive inotropic response to β2-adrenergic stimulation without altering the Ca²⁺ transient. Whilst moderate cholesterol depletion additionally could suppress the enhancement of the Ca²⁺ transient amplitude caused by the β2-adrenergic agonist administration in G_i-protein/Akt-dependent but NO-independent manner.

Reduction of Mitochondria–Endoplasmic Reticulum Interactions by Acetylcholine Protects Human Umbilical Vein Endothelial Cells From Hypoxia/Reoxygenation Injury

Objective—

We explored the role of endoplasmic reticulum (ER)–mitochondria Ca 2+ cross talk involving voltage-dependent anion channel-1 (VDAC1)/glucose-regulated protein 75/inositol 1,4,5-trisphosphate receptor 1 complex and mitofusin 2 in endothelial cells during hypoxia/reoxygenation (H/R), and investigated the protective effects of acetylcholine.

Approach and Results—

Acetylcholine treatment during reoxygenation prevented intracellular and mitochondrial Ca 2+ increases and alleviated ER Ca 2+ depletion during H/R in human umbilical vein endothelial cells. Consequently, acetylcholine enhanced mitochondrial membrane potential and inhibited proapoptotic cascades, thereby reducing cell death and preserving endothelial ultrastructure. This effect was likely mediated by the type-3 muscarinic acetylcholine receptor and the phosphatidylinositol 3-kinase/Akt pathway. In addition, interactions among members of the VDAC1/glucose-regulated protein 75/inositol 1,4,5-trisphosphate receptor 1 complex were increased after H/R and were associated with mitochondrial Ca 2+ overload and cell death. Inhibition of the partner of the Ca 2+ channeling complex (VDAC1 siRNA) or a reduction in ER–mitochondria tethering (mitofusin 2 siRNA) prevented the increased protein interaction within the complex and reduced mitochondrial Ca 2+ accumulation and subsequent endothelial cell death after H/R. Intriguingly, acetylcholine could modulate ER–mitochondria Ca 2+ cross talk by inhibiting the VDAC1/glucose-regulated protein 75/inositol 1,4,5-trisphosphate receptor 1 complex and mitofusin 2 expression. Phosphatidylinositol 3-kinase siRNA diminished acetylcholine-mediated inhibition of mitochondrial Ca 2+ overload and VDAC1/glucose-regulated protein 75/inositol 1,4,5-trisphosphate receptor 1 complex formation induced by H/R.

Conclusions—

Our data suggest that ER–mitochondria interplay plays an important role in reperfusion injury in the endothelium and may be a novel molecular target for endothelial protection. Acetylcholine attenuates both intracellular and mitochondrial Ca 2+ overload and protects endothelial cells from H/R injury, presumably by disrupting the ER–mitochondria interaction.

Inhibition of atrial fibrillation by low-level vagus nerve stimulation: the role of the nitric oxide signaling pathway

PURPOSE

We examined the role of the phosphatidylinositol-3 kinase (PI3K)/nitric oxide (NO) signaling pathway in low-level vagus nerve stimulation (LLVNS)-mediated inhibition of atrial fibrillation (AF).

METHODS

In 17 pentobarbital anesthetized dogs, bilateral thoracotomies allowed the attachment of electrode catheters to the superior and inferior pulmonary veins and atrial appendages. Rapid atrial pacing (RAP) was maintained for 6 h. Each hour, programmed stimulation was used to determine the window of vulnerability (WOV), a measure of AF inducibility, at all sites. During the last 3 h, RAP was overlapped with right LLVNS (50 % below that which slows the sinus rate). In group 1 (n = 7), LLVNS was the only intervention, whereas in groups 2 (n = 6) and 3 (n = 4), the NO synthase inhibitor N (G)-nitro-L-arginine methyl ester (L-NAME) and the PI3K inhibitor wortmannin, respectively, were injected in the right-sided ganglionated plexi (GP) during the last 3 h. The duration of acetylcholine-induced AF was determined at baseline and at 6 h. Voltage-sinus rate curves were constructed to assess GP function.

RESULTS

LLVNS significantly decreased the acetylcholine-induced AF duration by 8.2 ± 0.9 min (p < 0.0001). Both L-NAME and wortmannin abrogated this effect. The cumulative WOV (the sum of the individual WOVs) decreased toward baseline with LLVNS (p < 0.0001). L-NAME and wortmannin blunted this effect during the fifth (L-NAME only, p < 0.05) and the sixth hour (L-NAME and wortmannin, p < 0.05). LLVNS suppressed the ability of GP stimulation to slow the sinus rate, whereas L-NAME and wortmannin abolished this effect.

CONCLUSION

The anti-arrhythmic effects of LLVNS involve the PI3K/NO signaling pathway.

Caveolae create local signalling domains through their distinct protein content, lipid profile and morphology

Compartmentation of signalling allows multiple stimuli to achieve diverse cellular responses with only a limited pool of second messengers. This spatial control of signalling is achieved, in part, by cellular structures which bring together elements of a particular cascade. One such structure is the caveola, a flask-shaped lipid raft. Caveolae are well-recognised as signalosomes, platforms for assembly of signalling complexes of receptors, effectors and their targets, which can facilitate efficient and specific cellular responses. Here we extend this simple model and present evidence to show how the protein and lipid profiles of caveolae, as well as their characteristic morphology, define their roles in creating local signalling domains in the cardiac myocyte. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

Regulation of heart function by endogenous gaseous mediators-crosstalk between nitric oxide and hydrogen sulfide

Both nitric oxide (NO) and hydrogen sulfide (H(2)S) are two important gaseous mediators regulating heart function. The present study examined the interaction between these two biological gases and its role in the heart. We found that l-arginine, a substrate of NO synthase, decreased the amplitudes of myocyte contraction and electrically induced calcium transients. Sodium hydrogen sulfide (an H(2)S donor), which alone had minor effect, reversed the negative inotropic effects of l-arginine. The effect of l-arginine + sodium hydrogen sulfide was abolished by three thiols (l-cysteine, N-acetyl-cysteine, and glutathione), suggesting that the effect of H(2)S + NO is thiol sensitive. The stimulatory effect on heart contractility was also induced by GYY4137, a slow-releasing H(2)S donor, when used together with sodium nitroprusside, an NO-releasing donor. More importantly, enzymatic generation of H(2)S from recombinant cystathionine-γ-lyase protein also interacted with endogenous NO generated from l-arginine to stimulate heart contraction. In summary, our data suggest that endogenous NO may interact with H(2)S to produce a new biological mediator that produces positive inotropic effect. The crosstalk between H(2)S and NO also suggests an intriguing potential for the endogenous formation of a thiol-sensitive molecule, which may be of physiological significance in the heart.

Pharmacology of the internal anal sphincter and its relevance to faecal incontinence

1 The internal anal sphincter (IAS) has a spontaneous tone and is the main contributor to the maintenance of faecal continence. The spontaneous resting tone exhibited by the sphincter can be modified by neurotransmitters from the autonomic and enteric nervous systems. 2 In this review, the influence of the sympathetic and parasympathetic nervous systems on IAS tone are discussed and the putative roles of nitric oxide, carbon monoxide, vasoactive intestinal peptide and adenosine triphosphate in non-adrenergic non-cholinergic transmission are considered. 3 Faecal incontinence is a common condition that places a heavy financial burden on the health service and severely affects patients' quality of life. Resting anal pressure is reduced in patients with faecal incontinence and agents that increase sphincter tone tend to relieve symptoms. The results of clinical studies of the use of phenylephrine to treat faecal incontinence are reviewed. 4 It is concluded that the IAS is a potential target for drug development for the treatment of faecal incontinence.

Extract from leaf of Psidium guajava L depresses the guinea pig atrial contractility by interfering with potassium and calcium channels

The negative inotropic effect of aqueous fraction (AqF) obtained from the acetic extract of Psidium guajava L leaf was investigated on the guinea pig left atrium. Myocardial force was measured isometrically (27 ± 0.1 ºC, 2 Hz). AqF (100 μg/ml) reduced contractility of about 85 ± 9.4 % (n = 4, p < 0.001, Fcalc = 51.70, F(0.01; 4; 21) = 5.09, EC50 = 14.28 ± 3 μg/mL) in a concentration-dependent fashion. This effect was reduced by 20 mM of tetraethylammonium (TEA), increasing EC50 to 50 ± 7 μg/ml (n = 4, p < 0.001, Fcalc = 282.13; F(0.01; 21; 66) = 2.36). AqF (100 μg/ml) shifted to the right the CaCl2 concentration-effect curve, increasing the EC50 from 2170 ± 112 to 2690 ± 132 μM (n = 3, p < 0.001, Fcalc = 220.80 ; F(0.01; 29; 60) = 2.19). L-NAME (100 μM) did not modify the AqF inotropic effect (n = 3, p > 0.05) sugesting that the oxide nitric pathway did not participate of the action mechanism of AqF. We can conclude that AqF depresses the atrial contractile by reducing the calcium entry in myocardial cells and also by openenig potassium channels of cardiac tissue.

Targeted G-protein inhibition as a novel approach to decrease vagal atrial fibrillation by selective parasympathetic attenuation

AIMS

The parasympathetic nervous system is thought to play a key role in atrial fibrillation (AF). Since parasympathetic signalling is primarily mediated by the heterotrimeric G-protein, Galpha(i)betagamma, we hypothesized that targeted inhibition of Galpha(i) interactions in the posterior left atrium (PLA) would modify the substrate for vagal AF.

METHODS AND RESULTS

Cell-penetrating(cp)-Galpha(i)1/2 and cp-Galpha(i)3 C-terminal peptides were assessed for their ability to attenuate cholinergic-parasympathetic signalling in isolated feline atrial myocytes and in canine left atrium (LA). Confocal fluorescence microscopy indicated that cp-Galpha(i)1/2 and/or cp-Galpha(i)3 peptides moderated carbachol attenuation of cellular Ca(2+) transients in isolated atrial myocytes. High-density epicardial mapping of dog PLA, left atrial pulmonary veins (PVs), and left atrial appendage (LAA) indicated that the delivery of cp-Galpha(i)1/2 peptide or cp-Galpha(i)3 peptide into the PLA prolonged effective refractory periods at baseline and during vagal stimulation in the PLA and to varying extents also in the LAA and PV regions. After delivery of cp-Galpha(i) peptides into the PLA, AF inducibility during vagal stimulation was significantly diminished.

CONCLUSION

These results demonstrate the feasibility of using specific G(i)-protein inhibition to achieve selective parasympathetic denervation in the PLA, with a resulting change in vagal responsiveness across the entire LA.

Characteristics and function of cardiac mitochondrial nitric oxide synthase

We used laser scanning confocal microscopy in combination with the nitric oxide (NO)-sensitive fluorescent dye DAF-2 and the reactive oxygen species (ROS)-sensitive dyes CM-H(2)DCF and MitoSOX Red to characterize NO and ROS production by mitochondrial NO synthase (mtNOS) in permeabilized cat ventricular myocytes. Stimulation of mitochondrial Ca(2+) uptake by exposure to different cytoplasmic Ca(2+) concentrations ([Ca(2+)](i) = 1, 2 and 5 microm) resulted in a dose-dependent increase of NO production by mitochondria when L-arginine, a substrate for mtNOS, was present. Collapsing the mitochondrial membrane potential with the protonophore FCCP or blocking the mitochondrial Ca(2+) uniporter with Ru360 as well as blocking the respiratory chain with rotenone or antimycin A in combination with oligomycin inhibited mitochondrial NO production. In the absence of L-arginine, mitochondrial NO production during stimulation of Ca(2+) uptake was significantly decreased, but accompanied by increase in mitochondrial ROS production. Inhibition of mitochondrial arginase to limit L-arginine availability resulted in 50% inhibition of Ca(2+)-induced ROS production. Both mitochondrial NO and ROS production were blocked by the nNOS inhibitor (4S)-N-(4-amino-5[aminoethyl]aminopentyl)-N'-nitroguanidine and the calmodulin antagonist W-7, while the eNOS inhibitor L-N(5)-(1-iminoethyl)ornithine (L-NIO) or iNOS inhibitor N-(3-aminomethyl)benzylacetamidine, 2HCl (1400W) had no effect. The superoxide dismutase mimetic and peroxynitrite scavenger MnTBAP abolished Ca(2+)-induced ROS generation and increased NO production threefold, suggesting that in the absence of MnTBAP either formation of superoxide radicals suppressed NO production or part of the formed NO was transformed quickly to peroxynitrite. In the absence of L-arginine, mitochondrial Ca(2+) uptake induced opening of the mitochondrial permeability transition pore (PTP), which was blocked by the PTP inhibitor cyclosporin A and MnTBAP, and reversed by L-arginine supplementation. In the presence of the mtNOS cofactor (6R)-5,6,7,8,-tetrahydrobiopterin (BH(4); 100 microm) mitochondrial ROS generation and PTP opening decreased while mitochondrial NO generation slightly increased. These data demonstrate that mitochondrial Ca(2+) uptake activates mtNOS and leads to NO-mediated protection against opening of the mitochondrial PTP, provided sufficient availability of l-arginine and BH(4). In conclusion, our data show the importance of L-arginine and BH(4) for cardioprotection via regulation of mitochondrial oxidative stress and modulation of PTP opening by mtNOS.

Cardiac regulation by phosphoinositide 3-kinases and PTEN

The diverse effects mediated by PI3K/PTEN (phosphoinositide 3-kinase/phosphatase and tensin homologue deleted on chromosome 10) signalling in the heart clearly support an important biological and pathophysiological role for this signalling cascade. PI3Ks are a family of evolutionarily conserved lipid kinases that mediate many cellular responses to physiological and pathophysiological stimuli. Class I PI3K can be activated by either receptor tyrosine kinase/cytokine receptor activation (class IA) or G-protein-coupled receptors (class IB), leading to the generation of phosphatidyl inositol (3,4,5)P3 and recruitment and activation of Akt/protein kinase B, 3'-phosphoinositide-dependent kinase-1 (PDK1), or monomeric G-proteins, and phosphorylation of a wide range of downstream targets including glycogen synthase kinase 3beta (GSK3beta), mTOR (mammalian target of rapamycin), p70S6 kinase, endothelial nitric oxide synthase, and several anti-apoptotic effectors. Class IA (PI3Kalpha, beta, and delta) and class IB (PI3Kgamma) PI3Ks mediate distinct phenotypes in the heart under negative control by the 3'-lipid phosphatase PTEN, which dephosphorylates PtdIns(3,4,5)P3 to generate PtdIns(4,5)P2. PI3Kalpha, PI3Kgamma, and PTEN are expressed in cardiomyocytes, fibroblasts, endothelial cells, and vascular smooth muscle cells, where they modulate cell survival, hypertrophy, contractility, metabolism, and mechanotransduction. The PI3K/PTEN signalling pathways are involved in a wide variety of diseases including myocardial hypertrophy and contractility, heart failure, and preconditioning. In this review, we discuss the signalling pathways mediated by PI3K class I isoforms and PTEN and their roles in cardiac structure and function.

Laminin acts via focal adhesion kinase/phosphatidylinositol-3' kinase/protein kinase B to down-regulate beta1-adrenergic receptor signalling in cat atrial myocytes

We previously reported that short-term (2 h) plating of cat atrial myocytes on the extracellular matrix protein, laminin (LMN) decreases adenylate cyclase activity and beta(1)-adrenergic receptor (beta(1)-AR) stimulation of L-type Ca(2+) current (I(Ca,L)). The present study sought to determine whether LMN-mediated down-regulation of beta(1) signalling is due to down-regulation of adenylate cyclase and to gain insight into the signalling mechanisms responsible. beta(1)-AR stimulation was achieved by 0.01 microm isoproterenol (isoprenaline) plus 0.1 microm ICI 118551, a selective beta(2)-AR antagonist. Atrial myocytes were plated for at least 2 h on uncoated cover-slips (-LMN) or cover-slips coated with LMN (+LMN). As previously reported, beta(1)-AR stimulation of I(Ca,L) was significantly smaller in +LMN compared to -LMN atrial myocytes. In -LMN myocytes, 10 microm LY294002 (LY), a specific inhibitor of PI-(3)K, had no effect on beta(1)-AR stimulation of I(Ca,L). In +LMN myocytes, however, LY significantly increased beta(1)-AR stimulation of I(Ca,L). Western blots revealed that compared with -LMN myocytes, +LMN myocytes showed a significant increase in Akt phosphorylation at Ser-473, which was prevented by LY. In another approach, +LMN myocytes were infected (multiplicity of infection (MOI), 100; 24 h) with replication-defective adenoviruses (Adv) expressing dominant-negative inhibitors of focal adhesion kinase (FAK) (Adv-FRNK or Adv-Y397F-FAK) or Akt (Adv-dnAkt). Compared with control cells infected with Adv-beta-galactosidase, cells infected with Adv-FRNK, Adv-Y397F-FAK or Adv-dnAkt each exhibited a significantly greater beta(1)-AR stimulation of I(Ca,L). In -LMN myocytes LY had no effect on forskolin (FSK)-stimulated I(Ca,L). However, in +LMN myocytes LY significantly increased FSK-stimulated I(Ca,L). Similar results were obtained in +LMN atrial myocytes infected with Adv-FRNK. We conclude that LMN binding to beta(1)-integrin receptors acts via FAK/PI-(3)K/Akt to inhibit adenylate cyclase activity and thereby down-regulates beta(1)-AR-mediated stimulation of I(Ca,L). These findings provide new insight into the cellular mechanisms by which the extracellular matrix can modulate atrial beta-AR signalling.

Efferent vagal nerve stimulation induces tissue inhibitor of metalloproteinase-1 in myocardial ischemia-reperfusion injury in rabbit

Vagal nerve stimulation has been suggested to ameliorate left ventricular (LV) remodeling in heart failure. However, it is not known whether and to what degree vagal nerve stimulation affects matrix metalloproteinase (MMP) and tissue inhibitor of MMP (TIMP) in myocardium, which are known to play crucial roles in LV remodeling. We therefore investigated the effects of electrical stimulation of efferent vagal nerve on myocardial expression and activation of MMPs and TIMPs in a rabbit model of myocardial ischemia-reperfusion (I/R) injury. Anesthetized rabbits were subjected to 60 min of left coronary artery occlusion and 180 min of reperfusion with (I/R-VS, n = 8) or without vagal nerve stimulation (I/R, n = 7). Rabbits not subjected to coronary occlusion with (VS, n = 7) or without vagal stimulation (sham, n = 7) were used as controls. Total MMP-9 protein increased significantly after left coronary artery occlusion in I/R-VS and I/R to a similar degree compared with VS and sham values. Endogenous active MMP-9 protein level was significantly lower in I/R-VS compared with I/R. TIMP-1 mRNA expression was significantly increased in I/R-VS compared with the I/R, VS, and sham groups. TIMP-1 protein was significantly increased in I/R-VS and VS compared with the I/R and sham groups. Cardiac microdialysis technique demonstrated that topical perfusion of acetylcholine increased dialysate TIMP-1 protein level, which was suppressed by coperfusion of atropine. Immunohistochemistry demonstrated a strong expression of TIMP-1 protein in cardiomyocytes around the dialysis probe used to perfuse acetylcholine. In conclusion, in a rabbit model of myocardial I/R injury, vagal nerve stimulation induced TIMP-1 expression in cardiomyocytes and reduced active MMP-9.

Signalling mechanisms in contraction-mediated stimulation of intracellular NO production in cat ventricular myocytes

In this study we sought to determine whether contractile activity has a role as a signalling mechanism in the activation of intracellular nitric oxide (NO(i)) production induced by electrical stimulation of cat ventricular myocytes. Field stimulation (FS) of single ventricular myocytes elicited frequency-dependent increases in NO(i) that were blocked by the calmodulin (CaM) inhibitor 10 microM W-7 and partially inhibited by the phosphatidylinositol 3'-kinase (PI-(3)K) inhibitor 10 microMm LY294002. Increasing extracellular [Ca(2+)] caused a concentration-dependent increase in FS-induced NO(i) that was partially inhibited by LY294002. The negative inotropic agents BDM (5 mm) or blebbistatin (10 microM) decreased cell shortening and NO(i) production without concomitant changes in L-type Ca(2+) current (I(Ca,L)) or [Ca(2+)](i) transients. The positive inotropic agents EMD 57033 or CGP 48506 (1 microM) increased cell shortening and NO(i) production without concomitant changes in I(Ca,L) or [Ca(2+)](i) transients. FS-induced NO(i) production was decreased in myocytes infected (100 multiplicity of viral infection (MOI); 24 h) with a replication-deficient adenovirus expressing a dominant-negative mutant of protein kinase B (Akt) compared with cells infected with a control adenovirus expressing beta-galactosidase. FS-induced NO(i) was partially inhibited by either endothelial (eNOS) or neuronal nitric oxide synthase (nNOS) inhibitors and completely blocked by simultaneous exposure to both. FS-induced [Ca(2+)](i) transients were increased by the nNOS inhibitor nNOS-I (0.24 microM), decreased by the eNOS inhibitor L-NIO (1 microM) and unchanged by exposure to both inhibitors. We conclude that in cat ventricular myocytes, FS-induced NO(i) production requires both Ca(2+)-dependent CaM signalling and Ca(2+)-independent PI-(3)K-Akt signalling activated by contractile activity. FS activates NO(i) production from both eNOS and nNOS, and each source of NO(i) exerts opposing effects on [Ca(2+)](i) transient amplitude. These findings are important for understanding the regulation of NO(i) signalling in the normal and mechanically failing heart.

Role of Endothelial Nitric Oxide Synthase in the Regulation of SREBP Activation by Oxidized Phospholipids

Oxidized-1-palmitoyl-2-arachidonyl- sn -glycero-3-phosphorylcholine (Ox-PAPC), found in atherosclerotic lesions and other sites of chronic inflammation, activates endothelial cells (EC) to synthesize chemotactic factors, such as interleukin (IL)-8. Previously, we demonstrated that the sustained induction of IL-8 transcription by Ox-PAPC was mediated through the activation of sterol regulatory element–binding protein (SREBP). We now present evidence for the role of endothelial nitric oxide synthase (eNOS) in the activation of SREBP by Ox-PAPC. Ox-PAPC treatment of EC induced a dose- and time-dependent activation of eNOS, as measured by phosphorylation of serine 1177, dephosphorylation of threonine 495, and the conversion of l -arginine to l -citrulline. Activation of eNOS by Ox-PAPC was regulated through a phosphatidylinositol-3-kinase/Akt-mediated mechanism. These studies also demonstrated that pretreatment of EC with NOS inhibitor, N ω -nitro- l -arginine-methyl ester (L-NAME), significantly inhibited Ox-PAPC–induced IL-8 synthesis. Because SREBP activation had been previously shown to regulate IL-8 transcription by Ox-PAPC, we examined the effects of L-NAME on Ox-PAPC–induced SREBP activation. Our data demonstrated that Ox-PAPC–induced SREBP activation and expression of SREBP target genes were significantly reduced by pretreatment with L-NAME. Interestingly, treatment of EC with NO donor, S -nitroso- N -acetylpenicillamine, did not activate SREBP, suggesting that NO alone was not sufficient for SREBP activation. Rather, our findings indicated that superoxide (O 2 ·− ), in combination with NO, regulated SREBP activation by Ox-PAPC. We found that Ox-PAPC treatment generated O 2 ·− through an eNOS-mediated mechanism and that mercaptoethylguanidine, a peroxynitrite scavenger, reduced SREBP activation by Ox-PAPC. Taken together, these findings propose a novel role for eNOS in the activation of SREBP and SREBP-mediated inflammatory processes.

The compartmentalization of prolactin signaling in the mouse mammary gland

In mammary epithelial cells, prolactin (PRL) activates at least two signaling pathways: Jak/Stat and nitric oxide (NO). The former induces differentiation as measured by alpha-lactalbumin accumulation, while experiments with sodium nitroprusside (SNP) show that NO inhibits differentiation. In order to resolve this apparent contradiction, the kinetics, inducibility, and cellular localization of NO production and sensitivity in mammary cells were examined. First, mammary cells remained responsive to PRL throughout the incubation with respect to NO production. Second, although desensitization occurred with continuous PRL exposure, recovery began as quickly as 30 min after PRL withdrawal. Since PRL is secreted in pulses in vivo, complete desensitization was not a likely explanation for the cells' escape from NO inhibition. Finally, the cellular site of transduction was examined using the caveolar disrupting agent, methyl-beta-cyclodextrin (MBCD). MBCD inhibited the accumulation of PRL-induced NO but not alpha-lactalbumin. This finding was confirmed by membrane fractionation studies where the PRL-induced NO production occurred primarily in caveolae and PRL-stimulated tyrosine phosphorylation of Stat5, which transcribes the alpha-lactalbumin gene, occurred predominantly in noncaveolar membranes. Finally, endogenous elevations of NO by arginine did not inhibit differentiation. As such, the inhibition seen with SNP appeared to be an artifact of the ubiquitous generation of NO from SNP. Physiologically, PRL induces NO only in caveolae and this restricted distribution does not interfere with differentiation.

Phenylephrine acts via IP3-dependent intracellular NO release to stimulate L-type Ca2+ current in cat atrial myocytes

This study determined the effects of alpha1-adrenergic receptor (alpha1-AR) stimulation by phenylephrine (PE) on L-type Ca2+ current (I(Ca,L)) in cat atrial myocytes. PE (10 microm) reversibly increased I(Ca,L) (51.3%; n = 40) and shifted peak I(Ca,L) activation voltage by -10 mV. PE-induced stimulation of I(Ca,L) was blocked by each of 1 microm prazocin, 10 microm L-NIO, 10 microm W-7, 10 microm ODQ, 2 microm H-89 or 10 microm LY294002, and was unaffected by 10 microm chelerythrine or incubating cells in pertussis toxin (PTX). PE-induced stimulation of I(Ca,L) also was inhibited by each of 10 microm ryanodine or 5 microm thapsigargin, by blocking IP3 receptors with 2 microm 2-APB or 10 microm xestospongin C or by intracellular dialysis of heparin. In field-stimulated cells, PE increased intracellular NO (NOi) production. PE-induced NOi release was inhibited by each of 1 microm prazocin, 10 microm L-NIO, 10 microm W-7, 10 microm LY294002, 2 microm H-89, 10 microm ryanodine, 5 microm thapsigargin, 2 microm 2-APB or 10 microm xestospongin C, and unchanged by PTX. PE (10 microm) increased phosphorylation of Akt, which was inhibited by LY294002. Confocal microscopy showed that PE stimulated NOi release from subsarcolemmal sites and this was prevented by 2 mm methyl-beta-cyclodextrin, an agent that disrupts caveolae formation. PE also increased local, subsarcolemmal SR Ca2+ release via IP3-dependent signalling. Electron micrographs of atrial myocytes show peripheral SR cisternae in close proximity to clusters of caveolae. We conclude that in cat atrial myocytes PE acts via alpha1-ARs coupled to PTX-insensitive G-protein to release NOi, which in turn stimulates I(Ca,L). PE-induced NOi release requires stimulation of both PI-3K/Akt and IP3-dependent Ca2+ signalling. NO stimulates I(Ca,L) via cGMP-mediated cAMP-dependent PKA signalling. IP3-dependent Ca2+ signalling may enhance local SR Ca2+ release required to activate Ca2+-dependent eNOS/NOi production from subsarcolemmal caveolae sites.

The Cytosolic Phospholipase A2 Pathway, a Safeguard of β2-Adrenergic Cardiac Effects in Rat

We have recently demonstrated that in human heart, beta2-adrenergic receptors (beta2-ARs) are biochemically coupled not only to the classical adenylyl cyclase (AC) pathway but also to the cytosolic phospholipase A2 (cPLA2) pathway (Pavoine, C., Behforouz, N., Gauthier, C., Le Gouvello, S., Roudot-Thoraval, F., Martin, C. R., Pawlak, A., Feral, C., Defer, N., Houel, R., Magne, S., Amadou, A., Loisance, D., Duvaldestin, P., and Pecker, F. (2003) Mol. Pharmacol. 64, 1117-1125). In this study, using Fura-2-loaded cardiomyocytes isolated from adult rats, we showed that stimulation of beta2-ARs triggered an increase in the amplitude of electrically stimulated [Ca2+]i transients and contractions. This effect was abolished with the PKA inhibitor, H89, but greatly enhanced upon addition of the selective cPLA2 inhibitor, AACOCF3. The beta2-AR/cPLA2 inhibitory pathway involved G(i) and MSK1. Potentiation of beta2-AR/AC/PKA-induced Ca2+ responses by AACOCF3 did not rely on the enhancement of AC activity but was associated with eNOS phosphorylation (Ser1177) and L-NAME-sensitive NO production. This was correlated with PKA-dependent phosphorylation of PLB (Ser16). The constraint exerted by the beta2-AR/cPLA2 pathway on the beta2-AR/AC/PKA-induced Ca2+ responses required integrity of caveolar structures and was impaired by Filipin III treatment. Immunoblot analyses demonstrated zinterol-induced translocation of cPLA and its cosedimentation with MSK1, eNOS, PLB, and sarcoplasmic reticulum Ca2+ pump (SERCA) 2a in a low density caveolin-3-enriched membrane fraction. This inferred the gathering of beta2-AR signaling effectors around caveolae/sarcoplasmic reticulum (SR) functional platforms. Taken together, these data highlight cPLA as a cardiac beta2-AR signaling pathway that limits beta2-AR/AC/PKA-induced Ca2+ responses in adult rat cardiomyocytes through the impairment of eNOS activation and PLB phosphorylation.

A role for heterotrimeric GTP-binding proteins and ERK1/2 in insulin-mediated, nitric-oxide-dependent, cyclic GMP production in human umbilical vein endothelial cells

AIMS/HYPOTHESIS

Insulin is known to stimulate endothelial nitric oxide synthesis, although much remains unknown about the intracellular mechanisms involved. This study aims to examine, in human endothelial cells, the specific contribution of heterotrimeric Gi proteins and extracellular signal-regulated protein kinases 1/2 (ERK1/2) in insulin signalling upstream of nitric-oxide-dependent cyclic GMP production.

METHODS

Human umbilical vein endothelial cells were treated with 1 nmol/l insulin in the presence or absence of inhibitors of tyrosine kinases (erbstatin), Gi proteins (pertussis toxin) or ERK1/2 (PD098059 or U0126), and nitric oxide production was examined by quantification of intracellular cyclic GMP. Activation/phosphorylation of ERK1/2 by insulin was examined by immunoblotting with specific antibodies, and direct association of the insulin receptor with Gi proteins was examined by immunoprecipitation.

RESULTS

Treatment of cells with a physiological concentration of insulin (1 nmol/l) for 5 min increased nitric-oxide-dependent cyclic GMP accumulation by 3.3-fold, and this was significantly inhibited by erbstatin. Insulin-stimulated cyclic GMP production was significantly reduced by pertussis toxin and by the inhibitors of ERK1/2, PD098059 and U0126. Immunoblotting indicated that insulin stimulated the phosphorylation of ERK1/2 after 5 min and 1 h, and that this was completely abolished by pertussis toxin, but insensitive to the nitric oxide synthase inhibitor L-NAME. No direct interaction of the insulin receptor beta with Gialpha2 could be demonstrated by immunoprecipitation.

CONCLUSIONS/INTERPRETATION

This study demonstrates, for the first time, that nitric oxide production induced by physiologically relevant concentrations of insulin, is mediated by the post-receptor activation of a pertussis-sensitive GTP-binding protein and subsequent downstream activation of the ERK1/2 cascade.

Heterogeneity of action potential durations in isolated mouse left and right atria recorded using voltage-sensitive dye mapping

An imaging system for di-4-ANEPPS (4-[beta-[2-(di-n-butylamino)-6-naphthylvinyl]pyridinium]) voltage-sensitive dye recordings has been adapted for recording from an in vitro mouse heart preparation that consists of both atria in isolation. This approach has been used to study inter- and intra-atrial activation and conduction and to monitor action potential durations (APDs) in the left and right atrium. The findings from this study confirm some of our previous findings in isolated mouse atrial myocytes and demonstrate that many electrophysiological properties of mouse atria closely resemble those of larger mammals. Specifically, we made the following observations: 1) Activation in mouse atria originates in the sinoatrial node and spreads into the right atrium and, after a delay, into the left atrium. 2) APD in the left atrium is shorter than in the right atrium. 3) Sites in the posterior walls have longer APDs than sites in the atrial appendages. 4) Superfusion of this preparation with 4-aminopyridine and tetraethylammonium resulted in increases in APD, consistent with their inhibitory effects on the K+ currents known to be expressed in mouse atria. 5) The muscarinic agonist carbachol shortened APD in all areas of the preparation, except the left atrial appendage, in which carbachol had no statistically significant effect on APD. These results validate a new approach for monitoring activation, conduction, and repolarization in mouse atria and demonstrate that the physiological and pharmacological properties of mouse atria are sufficiently similar to those of larger animals to warrant further studies using this preparation.