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The development of interatrial septum (IAS) is a complicated process, which continues during postnatal life. The hypertrophic signals in developing heart are mediated among others by α-adrenergic pathways. These facts suggest the presence of specific electrophysiological features in developing IAS. This study was aimed to investigate the electrical activity in the tissue preparations of IAS from rat heart in normal conditions and under stimulation of adrenoreceptors. Intracellular recording of electrical activity revealed less negative level of resting membrane potential in IAS if compared to myocardium of left atrium. In normal conditions, non-paced IAS preparations were quiescent, but noradrenaline (10 -5  M) and phenylephrine (10 -5  M) induced spontaneous action potentials, which could be abolished by α1-blocker prazosin (10 -5  M), but not β1-blocker atenolol (10 -5  M). Optical mapping showed drastic phenylephrine-induced slowing of conduction in adult rat IAS. The α1-dependent ectopic automaticity of IAS myocardium might be explained by immunohistochemical data indicating the presence of transcription factor GATA4 and abundant α1A-adrenoreceptors in myocytes from adult rat IAS. An elevated sensitivity to adrenergic stimulation due to involvement of α1-adrenergic pathways may underlie increased proarrhythmic potential of adult IAS at least in rats., (© 2022. The Author(s) under exclusive licence to University of Navarra.)

We identified a single nucleotide variation (SNV) (c.1264A > G) in the KCNQ1 gene in a 5-year-old boy who presented with a prolonged QT interval. His elder brother and mother, but not sister and father, also had this mutation. This missense mutation leads to a p.Lys422Glu (K422E) substitution in the Kv7.1 protein that has never been mentioned before. We inserted this substitution in an expression plasmid containing Kv7.1 cDNA and studied the electrophysiological characteristics of the mutated channel expressed in CHO-K1, using the whole-cell configuration of the patch-clamp technique. Expression of the mutant Kv7.1 channel in both homo- and heterozygous conditions in the presence of auxiliary subunit KCNE1 results in a significant decrease in tail current densities compared to the expression of wild-type (WT) Kv7.1 and KCNE1. This study also indicates that K422E point mutation causes a dominant negative effect. The mutation was not associated with a trafficking defect; the mutant channel protein was confirmed to localize at the cell membrane. This mutation disrupts the poly-Lys strip in the proximal part of the highly conserved cytoplasmic A−B linker of Kv7.1 that was not shown before to be crucial for channel functioning.

The orderly contraction of the vertebrate heart is determined by generation and propagation of cardiac action potentials (APs). APs are generated by the integrated activity of time- and voltage-dependent ionic channels which carry inward Na + and Ca 2+ currents, and outward K + currents. This review compares atrial and ventricular APs and underlying ion currents between different taxa of vertebrates. We have collected literature data and attempted to find common electrophysiological features for two or more vertebrate groups, show differences between taxa and cardiac chambers, and indicate gaps in the existing data. Although electrical excitability of the heart in all vertebrates is based on the same superfamily of channels, there is a vast variability of AP waveforms between atrial and ventricular myocytes, between different species of the same vertebrate class and between endothermic and ectothermic animals. The wide variability of AP shapes is related to species-specific differences in animal size, heart rate, stage of ontogenetic development, excitation-contraction coupling, temperature and oxygen availability. Some of the differences between taxa are related to evolutionary development of genomes, which appear e.g. in the expression of different Na + and K + channel orthologues in cardiomyocytes of vertebrates. There is a wonderful variability of AP shapes and underlying ion currents with which electrical excitability of vertebrate heart can be generated depending on the intrinsic and extrinsic conditions of animal body. This multitude of ionic mechanisms provides excellent material for studying how the function of the vertebrate heart can adapt or acclimate to prevailing physiological and environmental conditions., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Catecholamines mediate the 'fight or flight' response in a wide variety of vertebrates. The endogenous catecholamine adrenaline increases heart rate and contractile strength to raise cardiac output. The increase in contractile force is driven in large part by an increase in myocyte Ca 2+ influx on the L-type Ca current (I CaL ) during the cardiac action potential (AP). Here, we report a K + - based mechanism that prolongs AP duration (APD) in fish hearts following adrenergic stimulation. We show that adrenergic stimulation inhibits the delayed rectifier K + current (I Kr ) in rainbow trout (Oncorhynchus mykiss) cardiomyocytes. This slows repolarization and prolongs APD which may contribute to positive inotropy following adrenergic stimulation in fish hearts. The endogenous ligand, adrenaline (1 μM), which activates both α- and β-ARs reduced maximal I Kr tail current to 61.4 ± 3.9% of control in atrial and ventricular myocytes resulting in an APD prolongation of ~20% at both 50 and 90% repolarization. This effect was reproduced by the α-specific adrenergic agonist, phenylephrine (1 μM), but not the β-specific adrenergic agonist isoproterenol (1 μM). Adrenaline (1 μM) in the presence of β 1 and β 2 -blockers (1 μM atenolol and 1 μM ICI-118551, respectively) also inhibited I Kr . Thus, I Kr suppression following α-adrenergic stimulation leads to APD prolongation in the rainbow trout heart. This is the first time this mechanism has been identified in fish and may act in unison with the well-known enhancement of I CaL following adrenergic stimulation to prolong APD and increase cardiac inotropy., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Cardiac-specific microRNA miR-133a-3p modulates adrenergic signaling. Adrenergic receptors and their intracellular pathways are the key players in proarrhythmic ectopy derived from the myocardial sleeves of the pulmonary veins. We studied the effect of miR-133a-3p on ectopy induced by norepinephrine in myocardial tissue of rat pulmonary veins. Using microelectrode technique, we revealed facilitation of proarrhythmic pattern of spontaneous bursts of action potentials induced by norepinephrine in tissue preparations of the pulmonary veins isolated from rats in 24 h after injection of a transfection mixture containing miR-133a-3p (1 mg/kg) in vivo. According to ELISA data, the cAMP level in the pulmonary vein myocardium of rats receiving miR-133a-3p was 2-fold higher than in control animals. Bioinformatic analysis showed that mRNA of protein phosphatases and some phosphodiesterases are most probable targets of miR-133a-3p. The proarrhythmic effect of miR-133a-3p can be related to inhibition of the expression of phosphodiesterases accompanied by cAMP accumulation and increased intracellular β-adrenergic signaling., (© 2022. Springer Science+Business Media, LLC, part of Springer Nature.)

Mir-133a-3p is the most abundant myocardial microRNA. The impact of mir-133a-3p on cardiac electrophysiology is poorly explored. In this study, we investigated the effects of mir-133a-3p on the main ionic currents critical for action potential (AP) generation and electrical activity of the heart. We used conventional ECG, sharp microelectrodes and patch-clamp to clarify a role of mir-133a-3p in normal cardiac electrophysiology in rats after in vivo and in vitro transfection. Mir-133a-3p caused no changes to pacemaker APs and automaticity in the sinoatrial node. No significant changes in heart rate (HR) were observed in vivo; however, miR transfection facilitated HR increase in response to β-adrenergic stimulation. Mir-133a-3p induced repolarization abnormalities in the atrial working myocardium and the L-type calcium current (I Ca,L ) was significantly increased. The main repolarization currents, including the transient outward (I to ), ultra-rapid (I K,ur ), and inward rectifier (I K1 ) remained unaffected in atrial cardiomyocytes. Mir-133a-3p affected both I Ca,L and I to in ventricular cardiomyocytes. Systemic administration of mir-133a-3p induced QT-interval prolongation. Bioinformatic analysis revealed protein phosphatase 2 (PPP2CA/B) and Kcnd3 (encoding K v 4.3 channels generating I to ) as the main miR-133a-3p targets in the heart. No changes in mRNA expression of Cacna1c (encoding Ca v 1.2 channels generating I Ca,L ) and Kcnd3 were seen in mir-133a-3p treated rats. However, the expression of Ppp2cA, encoding PPP2CA, and Kcnip2 encoding KChIP2, a K v 4.3 regulatory protein, were significantly decreased. The accumulation of mir-133a-3p in cardiac myocytes causes chamber-specific electrophysiological changes. The suppression of PPP2CA, involved in adrenergic signal transduction, and Kchip2 may indirectly mediate mir-133a-3p-induced augmentation of I Ca,L and attenuation of I to ., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Birds developed endothermy and four-chambered high-performance heart independently from mammals. Though avian embryos are extensively studied and widely used as various models for heart research, little is known about cardiac physiology of adult birds. Meanwhile, cardiac electrophysiology is in search for easily accessible and relevant model objects which resemble human myocardium in the pattern of repolarizing currents (I Kr , I Ks , I Kur and I to ). This study focuses on the configuration of electrical activity and electrophysiological phenotype of working myocardium in adult Japanese quails (Coturnix japonica). The resting membrane potential and action potential (AP) waveform in quail atrial myocardium were similar to that in working myocardium of rodents. Using whole-cell patch clamp and sharp glass microelectrodes, we demonstrated that the repolarization of quail atrial and ventricular myocardium is determined by voltage-dependent potassium currents I Kr , I Ks and I to - the latter was previously considered as an exclusive evolutionary feature of mammals. The specific blockers of these currents, dofetilide (3 μmol l -1 ), HMR 1556 (30 μmol l -1 ) and 4-aminopyridine (3 mmol l -1 ), prolonged AP in both ventricular and atrial myocardial preparations. The expression of the corresponding channels responsible for these currents in quail myocardium was investigated with quantitative RT-PCR and western blotting. In conclusion, the described pattern of repolarizing ionic currents and channels in quail myocardium makes this species a novel and suitable experimental model for translational cardiac research and reveals new information related to the evolution of cardiac electrophysiology in vertebrates., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Aim: This study is aimed at investigation of electrophysiological effects of α1-adrenoreceptor (α1-AR) stimulation in the rat superior vena cava (SVC) myocardium, which is one of the sources of proarrhythmic activity., Methods: α1-ARs agonists (phenylephrine-PHE or norepinephrine in presence of atenolol-NE + ATL) were applied to SVC and atrial tissue preparations or isolated cardiomyocytes, which were examined using optical mapping, glass microelectrodes or whole-cell patch clamp. α1-ARs distribution was evaluated using immunofluorescence. Kir2.X mRNA and protein level were estimated using RT-PCR and Western blotting., Results: PHE or NE + ATL application caused a significant suppression of the conduction velocity (CV) of excitation and inexcitability in SVC, an increase in the duration of electrically evoked action potentials (APs), a decrease in the maximum upstroke velocity (dV/dt max ) and depolarization of the resting membrane potential (RMP) in SVC to a greater extent than in atria. The effects induced by α1-ARs activation in SVC were attenuated by protein kinase C inhibition (PKC). The whole-cell patch clamp revealed PHE-induced suppression of outward component of I K1 inward rectifier current in isolated SVC, but not atrial myocytes. These effects can be mediated by α1A subtype of α-ARs found in abundance in rat SVC. The basal I K1 level in SVC was much lower than in atria as a result of the weaker expression of Kir2.2 channels., Conclusion: Therefore, the reduced density of I K1 in rat SVC cardiomyocytes and sensitivity of this current to α1A-AR stimulation via PKC-dependent pathways might lead to proarrhythmic conduction in SVC myocardium by inducing RMP depolarization, AP prolongation, CV and dV/dt max decrease., (© 2020 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd.)

Small G-proteins of Rho family modulate the activity of several classes of ion channels, including K + channels Kv1.2, Kir2.1, and ERG; Ca 2+ channels; and epithelial Na + channels. The present study was aimed to check the RhoA potential regulatory effects on Na + current (I Na ) transferred by Na + channel cardiac isoform Na V 1.5 in heterologous expression system and in native rat cardiomyocytes. Whole-cell patch-clamp experiments showed that coexpression of Na V 1.5 with the wild-type RhoA in CHO-K1 cell line caused 2.7-fold decrease of I Na density with minimal influence on steady-state activation and inactivation. This effect was reproduced by the coexpression with a constitutively active RhoA, but not with a dominant negative RhoA. In isolated ventricular rat cardiomyocytes, a 5-h incubation with the RhoA activator narciclasine (5 × 10 -6  M) reduced the maximal I Na density by 38.8%. The RhoA-selective inhibitor rhosin (10 -5  M) increased the maximal I Na density by 25.3%. Experiments with sharp microelectrode recordings in isolated right ventricular wall preparations showed that 5 × 10 -6  M narciclasine induced a significant reduction of action potential upstroke velocity after 2 h of incubation. Thus, RhoA might be considered as a potential negative regulator of sodium channels cardiac isoform Na V 1.5.

Background The sinus node (SN) is the primary pacemaker of the heart. SN myocytes possess distinctive action potential morphology with spontaneous diastolic depolarization because of a unique expression of ion channels and Ca 2+ -handling proteins. MicroRNAs (miRs) inhibit gene expression. The role of miRs in controlling the expression of genes responsible for human SN pacemaking and conduction has not been explored. The aim of this study was to determine miR expression profile of the human SN as compared with that of non-pacemaker atrial muscle. Methods and Results SN and atrial muscle biopsies were obtained from donor or post-mortem hearts (n=10), histology/immunolabeling were used to characterize the tissues, TaqMan Human MicroRNA Arrays were used to measure 754 miRs, Ingenuity Pathway Analysis was used to identify miRs controlling SN pacemaker gene expression. Eighteen miRs were significantly more and 48 significantly less abundant in the SN than atrial muscle. The most interesting miR was miR-486-3p predicted to inhibit expression of pacemaking channels: HCN1 (hyperpolarization-activated cyclic nucleotide-gated 1), HCN4, voltage-gated calcium channel (Ca v )1.3, and Ca v 3.1. A luciferase reporter gene assay confirmed that miR-486-3p can control HCN4 expression via its 3' untranslated region. In ex vivo SN preparations, transfection with miR-486-3p reduced the beating rate by ≈35±5% ( P <0.05) and HCN4 expression ( P <0.05). Conclusions The human SN possesses a unique pattern of expression of miRs predicted to target functionally important genes. miR-486-3p has an important role in SN pacemaker activity by targeting HCN4, making it a potential target for therapeutic treatment of SN disease such as sinus tachycardia.

The effect of small G-proteins of the Rho family on sodium current conducted by cardiac isoform Na V 1.5 of voltage-gated sodium channels was studied in heterologous expression system, CHO-K1 cell line transfected with a plasmid containing the Na V 1.5 gene. The influence of cotransfection with genes of wild-type, constitutively-active, and dominant-negative small G-proteins RhoA, Rac1, and Cdc2 on the parameters of sodium current and its noninactivating component (I Na,late ) was estimated. Among three studied small G-proteins, only RhoA (wild-type and constitutively-active type) strongly affected sodium current reducing its peak amplitude, but not the value of I Na,late . Cotransfection with wild-type Rac1 resulted in a minor decrease in sodium current. Thus, small G-protein RhoA has potential capability for suppression of sodium current, although physiological relevance of this property has to be verified.

Extracellular ATP and nicotinamide adenine dinucleotide (β-NAD) demonstrate properties of neurotransmitters and neuromodulators in peripheral and central nervous system. It has been shown previously that ATP and β-NAD affect cardiac functioning in adult mammals. Nevertheless, the modulation of cardiac activity by purine compounds in the early postnatal development is still not elucidated. Also, the potential influence of ATP and β-NAD on cholinergic neurotransmission in the heart has not been investigated previously. Age-dependence of electrophysiological effects produced by extracellular ATP and β-NAD was studied in the rat myocardium using sharp microelectrode technique. ATP and β-NAD could affect ventricular and supraventricular myocardium independent from autonomic influences. Both purines induced reduction of action potentials (APs) duration in tissue preparations of atrial, ventricular myocardium, and myocardial sleeves of pulmonary veins from early postnatal rats similarly to myocardium of adult animals. Both purine compounds demonstrated weak age-dependence of the effect. We have estimated the ability of ATP and β-NAD to alter cholinergic effects in the heart. Both purines suppressed inhibitory effects produced by stimulation of intracardiac parasympathetic nerve in right atria from adult animals, but not in preparations from neonates. Also, ATP and β-NAD suppressed rest and evoked release of acetylcholine (ACh) in adult animals. β-NAD suppressed effects of parasympathetic stimulation and ACh release stronger than ATP. In conclusion, ATP and β-NAD control the heart at the postsynaptic and presynaptic levels via affecting the cardiac myocytes APs and ACh release. Postsynaptic and presynaptic effects of purines may be antagonistic and the latter demonstrates age-dependence.

The effects of sympathetic cotransmitter NAD+ (10 μM) on bioelectric activity of the heart under conditions of adrenergic stimulation were studied on isolated spontaneously contracting preparations (without stimulation) of the right atrium from 2-7-day-old rats. Action potentials were recorded in the working myocardium using standard microelectrode technique. Perfusion of the right atrium with norepinephrine solution (1 μM) altered the configuration and significantly lengthened the action potentials. NAD + against the background of norepinephrine stimulation significantly decreased the duration of action potentials, in particular, at 25% repolarization. The effect of purine compounds NAD + , ATP, and adenosine on bioelectrical activity of the heart of newborn rats was studied under basal conditions (without norepinephrine stimulation). The effect of NAD + against the background of adrenergic stimulation was more pronounced than under basal conditions and was probably determined by suppression of I CaL , which can be the main mechanism of NAD + action on rat heart.

The modulatory influence of diadenosine tetraphosphate (Ap4A) and diadenosine pentaphosphate (Ap5A) on the effect of intramural autonomic nerve stimulation in isolated rabbit sinoatrial node were examined. Electrical activity of the sinoatrial node was recorded intracellularly. Against the background of blockade of adrenergic effects with propranolol (3×10 -6 M) or in preparations isolated 2 h after injection of reserpine (2 mg/kg), nerve stimulation induced short-term membrane hyperpolarization and diminished the sinus node firing rate. These phenomena were not affected by Ap4A or Ap5A (10 -5 M). Under the action of atropine (3×10 -6 M) that completely eliminated the cholinergic influences, nerve stimulation enhanced the sinus node firing rate by 17.30±3.45% from the initial rate. Both Ap4A and Ap5A moderated the stimulation-induced elevation of firing rate to 9.9±2.8 and 10.5±2.9%, respectively. The data suggest that diadenosine polyphosphates significantly modulate the sympathetic influences on the heart rhythm, but have no effect on the parasympathetic control over activity of sinoatrial node.

Discrete mechanical stretch of isolated spontaneously contracting cardiac myocytes was employed to examine the kinetics of NO production in these cells. NO oscillations were detected with fluorescent dye 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate. The mechanisms underlying stretch-induced changes in NO concentration remain unclear and further studies are needed to evaluate the role of NO oscillation in the regulation of cardiomyocyte function.

We studied the effect of extracellular purine nucleotides (NAD + and ATP) on spontaneous arrhythmogenic activity caused by norepinephrine in myocardial sleeves of pulmonary veins. In pulmonary veins, NAD + and ATP reduced the frequency of action potentials and their duration at regular type of spontaneous activity caused by norepinephrine. NAD + and ATP lengthened the intervals between spike bursts at periodic (burst) type of spontaneous activity. In addition, ATP shortened the duration of spike bursts and the number of action potentials in the "bursts" caused by norepinephrine in the pulmonary veins. It was hypothesized that NAD + and ATP attenuate the effects of sympathetic stimulation and when released together with norepinephrine from sympathetic endings in vivo, probably, reduce arrhythmogenic activity in myocardial sleeves of pulmonary veins.

Background: Recently, NAD+ has been considered as an essential factor, participating in nerve control of physiological functions and intercellular communication. NAD+ also has been supposed as endogenous activator of P1 and P2 purinoreceptors. Effects of extracellular NAD+ remain poorly investigated in cardiac tissue. This study aims to investigate the effects of extracellular NAD+ in different types of supraventricular and ventricular working myocardium from rat and their potential mechanisms., Methods: The standard technique of sharp microelectrode action potential recording in cardiac multicellular preparations was used to study the effects of NAD+., Results: Extracellular NAD+ induced significant changes in bioelectrical activity of left auricle (LA), right auricle (RA), pulmonary veins (PV) and right ventricular wall (RV) myocardial preparations. 10-100 μM NAD+ produced two opposite effects in LA and RA - quickly developing and transient prolongation of action potentials (AP) and delayed sustained AP shortening, which follows the initial positive effect. In PV and RV only AP shortening was observed in response to NAD+ application. In PV preparations AP shortening induced by NAD+ may be considered as a potential proarrhythmic effect. Revealed cardiotropic effects of NAD+ are likely to be mediated by P2 purine receptors, since P1 blocker DPCPX failed to affect them and P2 antagonist suramin abolished NAD + -induced alterations of electrical activity. P2X receptors may be responsible for NAD + -induced short-lasting AP prolongation, while P2Y receptors mediate persistent AP shortening. The latter effect is partially removed by PLC inhibitor U73122 showing the potential involvement of phosphoinositide signaling pathway in mediation of NAD+ cardiotropic effects., Conclusions: Extracellular NAD+ is supposed to be a novel regulator of cardiac electrical activity. P2 receptors represent the main target of NAD+ at least in the rat heart.

Effects of nucleotide polyphosphate compounds (nicotinamide adenine dinucleotide, NAD(+); diadenosine tetraphosphate, Ap4A) on the confi guration of action potentials were studied in isolated preparations of guinea pig sinoatrial node and right atrial appendage (auricle). In the working myocardium, NAD(+) and Ap4A in concentrations of 10(-5) and 10(-4) M had no effect on resting potential, but significantly reduced the duration of action potentials; the most pronounced decrease was found at 25% repolarization. In the primary pacemaker of the sinoatrial node, both concentrations of NAD(+) and Ap4A induced hyperpolarization and reduction in the rate of slow diastolic depolarization, but significant slowing of the sinus rhythm was produced by these substances only in the concentration of 10(-4) M. Moreover, AP shortening and marked acceleration of AP upstroke were observed in the pacemaker myocardium after application of polyphosphates. Comparative analysis of the effects of NAD(+) and Ap4A in the working and pacemaker myocardium drove us to a hypothesis on inhibitory effects of these substances on L-type calcium current accompanied by stimulation of one or several potassium currents, which induce enhancement of repolarization and hyperpolarization of membranes probably mediated by the activation of purine receptors.

Diadenosine polyphosphates (Ap(n)As) are endogenously produced molecules which have been identified in various tissues of mammalian organism, including myocardium. Ap(n)As contribute to the blood clotting and are also widely accepted as regulators of blood vascular tone. Physiological role of Ap(n)As in cardiac muscle has not been completely elucidated. The present study aimed to investigate the effects of diadenosine tetra- (Ap4A) and penta- (Ap5A) polyphosphates on contractile function and action potential (AP) waveform in rat supraventricular and ventricular myocardium. We have also demonstrated the effects of A4pA and Ap5A in myocardial sleeves of pulmonary veins (PVs), which play a crucial role in genesis of atrial fibrillation. APs were recorded with glass microelectrodes in multicellular myocardial preparations. Contractile activity was measured in isolated Langendorff-perfused rat hearts. Both Ap4A and Ap5A significantly reduced contractility of isolated Langendorff-perfused heart and produced significant reduction of AP duration in left and right auricle, interatrial septum, and especially in right ventricular wall myocardium. Ap(n)As also shortened APs in rat pulmonary veins and therefore may be considered as potential proarrhythmic factors. Cardiotropic effects of Ap4A and Ap5A were strongly antagonized by selective blockers of P2 purine receptors suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), while P1 blocker DPCPX was not effective. We conclude that Ap(n)As may be considered as new class of endogenous cardioinhibitory compounds. P2 purine receptors play the central role in mediation of Ap4A and Ap5A inhibitory effects on electrical and contractile activity in different regions of the rat heart.

In rat sinoatrial node, NAD(+) (10 μM) reduced the rate of spontaneous action potentials, duration of action potentials, and the velocity of slow diastolic depolarization, but the rate of action potential front propagation increases. In passed rabbit Purkinje fibers, NAD(+) (10 μM) reduced the duration of action potentials. Under conditions of spontaneous activity of Purkinje fibers, NAD(+) reduced the fi ring rate and the rate of slow diastolic depolarization. The effects of extracellular NAD(+) on bioelectric activity of the pacemaker (sinoatrial node) and conduction system of the heart (Purkinje fibers) are probably related to activation of P1 and P2 purinoceptors.

This study is aimed to the investigation of the nicotinamide adenine dinucleotide (NAD+) effects and mechanisms of action in a heart. NAD+ (mcM) induces multiphase alternation of contractile activity of isolated rat heart: short positive inotropic action is followed by a negative inotropic phase. NAD+ (1-100 mcM) induces decreasing of action potential duration (APD) in rat atrial myocardium (from 45 +/- 0.82 ms in control experiments to 39 +/- 1.05 (n = 8) and 32 +/- 2 (n = 8) during application of 10 and 100 mcM of NAD+, respectively). Significant APD increase (from 45 +/- 0.82 ms to 74 +/- 1.89 (n = 8) ms) was observed during washing out of NAD+ (100 mcM). ATP or adenosine was unable to increase APD both during application or washing out. NAD+ induced APD decrease was not suppressed by P1-antagonist theophylline. P1-purinoreceptor and metabolite independent direct action of NAD+ in rat heart is suggested. Activation of P2X or P2Y receptors, cyclic ADP-ribose accumulation in cardiomyocytes is proposed as a main mechanism of NAD(+)-induced effects in the heart.