Disrupted Calcium Release as a Mechanism for Atrial Alternans Associated with Human Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, but our knowledge of the arrhythmogenic substrate is incomplete. Alternans, the beat-to-beat alternation in the shape of cardiac electrical signals, typically occurs at fast heart rates and leads to arrhythmia. However, atrial alternans have been observed at slower pacing rates in AF patients than in controls, suggesting that increased vulnerability to arrhythmia in AF patients may be due to the proarrythmic influence of alternans at these slower rates. As such, alternans may present a useful therapeutic target for the treatment and prevention of AF, but the mechanism underlying alternans occurrence in AF patients at heart rates near rest is unknown. The goal of this study was to determine how cellular changes that occur in human AF affect the appearance of alternans at heart rates near rest. To achieve this, we developed a computational model of human atrial tissue incorporating electrophysiological remodeling associated with chronic AF (cAF) and performed parameter sensitivity analysis of ionic model parameters to determine which cellular changes led to alternans. Of the 20 parameters tested, only decreasing the ryanodine receptor (RyR) inactivation rate constant (ki_Ca) produced action potential duration (APD) alternans seen clinically at slower pacing rates. Using single-cell clamps of voltage, fluxes, and state variables, we determined that alternans onset was Ca²⁺-driven rather than voltage-driven and occurred as a result of decreased RyR inactivation which led to increased steepness of the sarcoplasmic reticulum (SR) Ca²⁺ release slope. Iterated map analysis revealed that because SR Ca²⁺ uptake efficiency was much higher in control atrial cells than in cAF cells, drastic reductions in ki_Ca were required to produce alternans at comparable pacing rates in control atrial cells. These findings suggest that RyR kinetics may play a critical role in altered Ca²⁺ homeostasis which drives proarrhythmic APD alternans in patients with AF.     

窦性节律下家兔心率与在位心室肌纤维有效不应期的关系

在自然呼吸和窦性节律下,用浮置式玻璃微电极引导在体单个左心室肌纤维动作电位,作为兴奋的指标,以其 0相触发产生期前的试验刺激,测定有效不应期(ERP)。38只家兔的测定结果表明,在R-R间期为205—330ms的范围内,随着心率加快(R-R间期缩短),ERP减小,而 ERP/RR 间期增大,说明 ERP 与心率直接有关。并且,在较快的心率时,ERP 相对延长。通过相关与回归分析,制出了能够删除心率影响的校正公式。静脉注射酒石酸锑钾(50mg/kg)发现,在窦性和起搏节律下,酒石酸锑钾均能轻度延长 ERP(P<0.001)。窦性节律下的校正后值与起搏节律的测定结果完全一致。证明校正后值能够用来比较处理前后不同心率条件下,各种药物、离子及其它因素对ERP 的影响。 本文的校正公式虽然只适用于同种动物和方法,但此校正公式的制作原理也可以广泛应用到其它多种动物。     

A rabbit ventricular action potential model replicating cardiac dynamics at rapid heart rates

Mathematical modeling of the cardiac action potential has proven to be a powerful tool for illuminating various aspects of cardiac function, including cardiac arrhythmias. However, no currently available detailed action potential model accurately reproduces the dynamics of the cardiac action potential and intracellular calcium (Ca_i) cycling at rapid heart rates relevant to ventricular tachycardia and fibrillation. The aim of this study was to develop such a model. Using an existing rabbit ventricular action potential model, we modified the L-type calcium (Ca) current (I_Ca,L) and Ca_i cycling formulations based on new experimental patch-clamp data obtained in isolated rabbit ventricular myocytes, using the perforated patch configuration at 35-37°C. Incorporating a minimal seven-state Markovian model of I_Ca,L that reproduced Ca- and voltage-dependent kinetics in combination with our previously published dynamic Ca_i cycling model, the new model replicates experimentally observed action potential duration and Ca_i transient alternans at rapid heart rates, and accurately reproduces experimental action potential duration restitution curves obtained by either dynamic or S1S2 pacing.     

Multistability property in cardiac ionic models of mammalian and human ventricular cells

The underlying mechanisms of irregular cardiac rhythms are still poorly understood. Many experimental and modeling studies are aimed at identifying factors which cause cardiac arrhythmias. However, a lack of understanding of heart rhythm dynamical properties makes it difficult to uncover precise mechanisms of electrical instabilities, and hence to predict the onset of heart rhythm disorders. We review and compare the existing methods of studying cardiac dynamics, including restitution protocol (S1-S2), dynamic restitution protocol and multistability test protocol (S1-CI-S2). We focus on cardiac cell dynamics to elucidate regularities of heart rhythm. We demonstrate the advantages of our newly proposed systematic approach of analysis of cardiac cell dynamics using mammalian Luo Rudy 1991 and human ventricular Ten Tusscher 2006 single cell models under healthy and diseased conditions such as altered K⁺ or Ca²⁺ related currents. We investigate the role of ionic properties and the shape of an action potential on the nonlinear dynamics of electrical processes in periodically stimulated cardiac cells. We show the existence of multistability property for human ventricular cells. Moreover, the multistability is proposed to be an intrinsic property of cardiac cells, and is also suggested to be one of the mechanisms which could underlie the sudden triggering of life-threatening ventricular arrhythmias in the human heart.     

Contractile parameters and occurrence of alternans in isolated rat myocardium at supra-physiological stimulation frequency

The cardiac refractory period prevents the heart from tetanic activation that is typically used in noncardiac striated muscle tissue. To what extent the refractory period prevents successive action potentials to activate the excitation-contraction coupling process and contractile machinery at supra-physiological rates, such as those present during ventricular fibrillation, is unknown. Using multicellular trabeculae isolated from rat hearts, we studied amplitude and kinetics of contraction at rates well above the normal in vivo rat heart range. We show that even at twice the maximal heart rate of the rat, little or no mechanical instability is observed; twitch contractions are at steady state, albeit with an elevated active diastolic force. Although the amplitude of contraction increased within in vivo heart rates (positive force-frequency response), at frequencies beyond the maximal heart rate (10-30 Hz) a steady decline of contractile amplitude is observed. Not until 30 Hz do the majority of the isolated muscle preparations show mechanical alternans, where strong and weak beats alternate. Interestingly, unlike striated limb skeletal muscle, fusing of twitch contractions did not cause a continuous increase in peak force: at frequencies of 10 Hz and above, systolic force declines with relatively little elevation in diastolic force. Contractile kinetics continued to accelerate, from 1 Hz up to 30 Hz, whereas the relative speed of contraction and relaxation remained closely coupled, reflected by a singular linear relationship between the maximal and minimal derivative of force (dF/dt). We conclude that cardiac muscle can produce mechanically stable steady-state contractions at supra-physiological pacing rates, while these contractions continue to decline in amplitude and increase in diastolic force past maximal heart rate.     

Voltage-mediated mechanism for calcium wave synchronization and arrhythmogenesis in atrial tissue

A wide range of atrial arrythmias are caused by molecular defects in proteins that regulate calcium (Ca) cycling. In many cases, these defects promote the propagation of subcellular Ca waves in the cell, which can perturb the voltage time course and induce dangerous perturbations of the action potential (AP). However, subcellular Ca waves occur randomly in cells and, therefore, electrical coupling between cells substantially decreases their effect on the AP. In this study, we present evidence that Ca waves in atrial tissue can synchronize in-phase owing to an order-disorder phase transition. In particular, we show that, below a critical pacing rate, Ca waves are desynchronized and therefore do not induce substantial AP fluctuations in tissue. However, above this critical pacing rate, Ca waves gradually synchronize over millions of cells, which leads to a dramatic amplification of AP fluctuations. We exploit an underlying Ising symmetry of paced cardiac tissue to show that this transition exhibits universal properties common to a wide range of physical systems in nature. Finally, we show that in the heart, phase synchronization induces spatially out-of-phase AP duration alternans which drives wave break and reentry. These results suggest that cardiac tissue exhibits a phase transition that is required for subcellular Ca cycling defects to induce a life-threatening arrhythmia.     

Changes in heart rate and circadian cardiac rhythm as physiological biomarkers for estimation of functional state of crayfish <Emphasis Type="Italic">Pontastacus leptodactylus</Emphasis> Esch. upon acidification of the environment

The changes in heart rate and circadian cardiac rhythm of crayfish Pontastacus leptodactylus Esch. kept in a lightning regime that is close to natural under optimal or low pH values were studied. The heart rate was registered in real time using an original noninvasive fiberoptic method. Upon acidification, disorders in circadian cardiac rhythm and organism reaction (by heart rate) in the suspension test were detected. The characteristics of cardiac activity are considered criteria for estimating the crayfish’s functional state at normal and stress conditions caused by the changes in the quality of the environment.     

Mapping cardiac surface mechanics with structured light imaging

Cardiovascular disease often manifests as a combination of pathological electrical and structural heart remodeling. The relationship between mechanics and electrophysiology is crucial to our understanding of mechanisms of cardiac arrhythmias and the treatment of cardiac disease. While several technologies exist for describing whole heart electrophysiology, studies of cardiac mechanics are often limited to rhythmic patterns or small sections of tissue. Here, we present a comprehensive system based on ultrafast three-dimensional (3-D) structured light imaging to map surface dynamics of whole heart cardiac motion. Additionally, we introduce a novel nonrigid motion-tracking algorithm based on an isometry-maximizing optimization framework that forms correspondences between consecutive 3-D frames without the use of any fiducial markers. By combining our 3-D imaging system with nonrigid surface registration, we are able to measure cardiac surface mechanics at unprecedented spatial and temporal resolution. In conclusion, we demonstrate accurate cardiac deformation at over 200,000 surface points of a rabbit heart recorded at 200 frames/s and validate our results on highly contrasting heart motions during normal sinus rhythm, ventricular pacing, and ventricular fibrillation.     

Reduction in number of sarcolemmal KATP channels slows cardiac action potential duration shortening under hypoxia

The cardiovascular system operates under demands ranging from conditions of rest to extreme stress. One mechanism of cardiac stress tolerance is action potential duration shortening driven by ATP-sensitive potassium (K_ATP) channels. K_ATP channel expression has a significant physiologic impact on action potential duration shortening and myocardial energy consumption in response to physiologic heart rate acceleration. However, the effect of reduced channel expression on action potential duration shortening in response to severe metabolic stress is yet to be established. Here, transgenic mice with myocardium-specific expression of a dominant negative K_ATP channel subunit were compared with littermate controls. Evaluation of K_ATP channel whole cell current and channel number/patch was assessed by patch clamp in isolated ventricular cardiomyocytes. Monophasic action potentials were monitored in retrogradely perfused, isolated hearts during the transition to hypoxic perfusate. An 80–85% reduction in cardiac K_ATP channel current density results in a similar magnitude, but significantly slower rate, of shortening of the ventricular action potential duration in response to severe hypoxia, despite no significant difference in coronary flow. Therefore, the number of functional cardiac sarcolemmal K_ATP channels is a critical determinant of the rate of adaptation of myocardial membrane excitability, with implications for optimization of cardiac energy consumption and consequent cardioprotection under conditions of severe metabolic stress.     

Antidiabetic sulfonylureas control action potential properties in heart cells via high affinity receptors that are linked to ATP-dependent K+ channels

Both avian and mammalian heart cells have high affinity receptors for antidiabetic sulfonylureas. The biochemical identification of these receptors has been carried out with [3H]glibenclamide. The Kd values for the most potent sulfonylureas, such as glibenclamide itself, are in the nanomolar range. Comparative studies of structure-function relationships indicate high similarities of binding properties between the sulfonylurea receptors in cardiac cells and insulinoma cells, respectively. The duration of the action potential of guinea pig cardiac cells was drastically reduced by decreasing intracellular ATP concentrations by perfusion or by blockade of oxidative phosphorylation. Glibenclamide was found to restore normal or nearly normal action potential properties in [ATP]in-depleted cardiac cells. Single channel recording using the patch-clamp technique has shown that this effect is associated with high affinity blockade of ATP-sensitive K+ channels by sulfonylureas.     

The Effect of Aging on the Specialized Conducting System: A Telemetry ECG Study in Rats over a 6 Month Period

Advanced age alone appears to be a risk factor for increased susceptibility to cardiac arrhythmias. We previously observed in the aged rat heart that sinus rhythm ventricular activation is delayed and characterized by abnormal epicardial patterns although conduction velocity is normal. While these findings relate to an advanced stage of aging, it is not yet known when and how ventricular electrical impairment originates and which is the underlying substrate. To address these points, we performed continuous telemetry ECG recordings in freely moving rats over a six-month period to monitor ECG waveform changes, heart rate variability and the incidence of cardiac arrhythmias. At the end of the study, we performed in-vivo multiple lead epicardial recordings and histopathology of cardiac tissue. We found that the duration of ECG waves and intervals gradually increased and heart rate variability gradually decreased with age. Moreover, the incidence of cardiac arrhythmias gradually increased, with atrial arrhythmias exceeding ventricular arrhythmias. Epicardial multiple lead recordings confirmed abnormalities in ventricular activation patterns, likely attributable to distal conducting system dysfunctions. Microscopic analysis of aged heart specimens revealed multifocal connective tissue deposition and perinuclear myocytolysis in the atria. Our results demonstrate that aging gradually modifies the terminal part of the specialized cardiac conducting system, creating a substrate for increased arrhythmogenesis. These findings may open new therapeutic options in the management of cardiac arrhythmias in the elderly population.     

Mechano-electric feedback and arrhythmias

The mechanical state of the heart feeds back to modify cardiac rate and rhythm. Mechanical stretch of myocardial tissue causes immediate and chronic responses that lead to the common end point of arrhythmia. This review provides a brief summary of the author's personal choice of contributions that she considers have fostered our understanding of the role of mechano-electric feedback in arrhythmogenesis.

Acute mechanical stretch reversibly depolarises the cell membrane and shortens the action potential duration. These electrophysiological changes are related to the activation of mechano-sensitive ion channels. Several different ion channels are involved in the sensing of stretch, among them K⁺-selective, Cl⁻-selective, non-selective, and ATP-sensitive K⁺ channels. Sodium and Ca²⁺ entering the cells via non-selective ion channels are thought to contribute to the genesis of stretch-induced arrhythmia. Mechano-sensitive channels have been cloned from non-vertebrate and vertebrate species.

Chronic stress on the heart activates gene expression in cardiomyocytes and non-myocytes. The signal transduction involves atrial natriuretic peptides and growth factors that initiate remodelling processes leading to hypertrophy which in turn may contribute to the electrical instability of the heart by increasing the responsiveness of mechano-sensitive channels. Selective block of these channels could provide some new form of treatment of mechanically induced arrhythmias, although at present there are no drugs available with sufficient selectivity. Detailed understanding of how mechanical strain on myocardial cells is translated into channel activation will allow to identify new targets for putative antiarrhythmic drugs.     

L-type Ca2+ channel mutations and T-wave alternans: a model study

A number of mutations have been linked to diseases for which the underlying mechanisms are poorly understood. An example is Timothy Syndrome (TS), a multisystem disorder that includes severe cardiac arrhythmias. Here we employ theoretical simulations to examine the effects of a TS mutation in the L-type Ca(2+) channel on cardiac dynamics over multiple scales, from a gene mutation to protein, cell, tissue, and finally the ECG, to connect a defective Ca(2+) channel to arrhythmia susceptibility. Our results indicate that 1) the TS mutation disrupts the rate-dependent dynamics in a single cardiac cell and promotes the development of alternans; 2) in coupled tissue, concordant alternans is observed at slower heart rates in mutant tissue than in normal tissue and, once initiated, rapidly degenerates into discordant alternans and conduction block; and 3) the ECG computed from mutant-simulated tissue exhibits prolonged QT intervals at physiological rates and with small increases in pacing rate, T-wave alternans, and alternating T-wave inversion. At the cellular level, enhanced Ca(2+) influx due to the TS mutation causes electrical instabilities. In tissue, the interplay between faulty Ca(2+) influx and steep action potential duration restitution causes arrhythmogenic discordant alternans. The prolongation of action potentials causes spatial dispersion of the Na(+) channel excitability, leading to inhomogeneous conduction velocity and large action potential spatial gradients. Our model simulations are consistent with the ECG patterns from TS patients, which suggest that the TS mutation is sufficient to cause the clinical phenotype and allows for the revelation of the complex interactions of currents underlying it.     

迷走神经对家兔在体心脏心室肌细胞跨膜电位的影响

本研究观察了电刺激迷走神经对家兔在体心脏心室肌细胞跨膜电位的作用及钾通道阻滞剂氯化四乙基铵对这一作用的影响。结果表明,在自然心率条件下,迷走神经刺激可使静息电位(RP)、动作电位振幅(APA)和0相最大上升速率(dv/dt)_(max)增加,动作电位时程(APD)缩短。冠脉注射氯化四乙基铵使心室肌细胞复极过程明显延长,迷走神经刺激不再引起 RP、APA 增大,动作电位时程不再缩短,(dv/dt)_(max)反而减小。这些结果提示,迷走神经刺激对正常心室肌细胞跨膜电位的影响可能是通过外向 K~ 流增加引起的。     

Function of myocardial alpha-adrenoceptors

Myocardial α-adrenoceptors are similar to vascular α-adrenoceptors; therefore the drugs which are used to study myocardial α-adrenoceptors can affect the heart indirectly by acting on vascular α-adrenoceptors. High concentrations of α-adrenoceptor stimulant and α-adrenoceptor blocking drugs can exert cardiac effects that do not result from action on α-adrenoceptors; therefore relatively low concentrations of these drugs must be used to obtain specific effects.An α-adrenoceptor mediated positive inotropic effect has been observed in relatively slow beating isolated heart preparations; it is not associated with shortening of the duration of systole or an increase in myocardial cyclic AMP concentration and is probably accompanied by an increase in Ca²⁺ influx. Usually α-adrenoceptor stimulation has no effect on heart rate.Myocardial α-adrenoceptor mediated ventricular arrhythmias have been caused in animals by high concentrations of catecholamines, and a transient increase in α-adrenoceptor concentration has been found in ischemic myocardium. We do not know how myocardial α-adrenoceptor stimulation causes arrhythmias. In isolated heart preparations low concentrations of epinephrine and norepinephrine prolong refractory period and action potential duration by α-adrenoceptor stimulation, and higher concentrations of the catecholamines shorten refractory period and action potential duration by α-adrenoceptor stimulation. In isolated specialized conducting fibers low concentrations of catecholamines reduce automaticity by α-adrenoceptor stimulation and higher concentrations increase automaticity by β-adrenoceptor stimulation. In partially depolorized ventricle preparations α-adrenoceptor stimulation has been reported both to depress and to restore electrical and mechanical activity. Clearly, much remains to be done before we understand α-adrenoceptor mediated cardiac effects.     

心肌电兴奋传导速度对心律失常影响的仿真研究

心电场是由心肌的电活动产生的。心肌细胞的电特性及心肌细胞间的传导关系决定了体表电位的分布及心电图的变化。心肌电兴奋传导速度则是影响心肌间兴奋传导关系的重要参数之一。由于很难通过实验方法来人为改变电兴奋传导速度,因而临床上有关该参数对心律影响的定量知识相当缺乏。本文采用真实三雏躯干模型及心脏模型,对心肌电兴奋传导速度与心律变化的关系进行定量仿真研究。结果表明,兴奋传导速度决定了整个心电图的变化,而局部普通心肌的传导速度在相当范围内变化似乎对心电图影响不明显,但传导速度超过一定范围后可能产生突变。     

Control of cardiac rhythm by ORK1, a Drosophila two-pore domain potassium channel

Unravelling the mechanisms controlling cardiac automatism is critical to our comprehension of heart development and cardiac physiopathology. Despite the extensive characterization of the ionic currents at work in cardiac pacemakers, the precise mechanisms initiating spontaneous rhythmic activity and, particularly, those responsible for the specific control of the pacemaker frequency are still matters of debate and have not been entirely elucidated. By using Drosophila as a model animal to analyze automatic cardiac activity, we have investigated the function of a K+ channel, ORK1 (outwardly rectifying K+ channel-1) in cardiac automatic activity. ORK1 is a two-pore domain K+ (K2P) channel, which belongs to a diverse and highly regulated superfamily of potassium-selective leak channels thought to provide baseline regulation of membrane excitability. Cardiac-specific inactivation of Ork1 led to an increase in heart rhythm. By contrast, when overexpressed, ORK1 completely prevented heart beating. In addition, by recording action potentials, we showed that the level of Ork1 activity sets the cardiac rhythm by controlling the duration of the slow diastolic depolarization phase. Our observations identify a new mechanism for cardiac rhythm control and provide the first demonstration that K2P channels regulate the automatic cardiac activity.     

Nonlinear dynamics of two-dimensional cardiac action potential duration mapping model with memory

The aim of this work is the analysis of the nonlinear dynamics of two-dimensional mapping model of cardiac action potential duration (2D-map APD) with memory derived from one dimensional map (1D-map). Action potential duration (APD) restitution, which relates APD to the preceding diastolic interval (DI), is a useful tool for predicting cardiac arrhythmias. For a constant rate of stimulation the short action potential during alternans is followed by a longer DI and inversely. It has been suggested that these differences in DI are responsible for the occurrence and maintenance of APD alternans. We focus our attention on the observed bifurcations produced by a change in the stimulation period and a fixed value of a particular parameter in the model. This parameter provides new information about the dynamics of the APD with memory, such as the occurrence of bistabilities not previously described in the literature, as well as the fact that synchronization rhythms occur in different ways and in a new fashion as the stimulation frequency increases. Moreover, we show that this model is flexible enough as to accurately reflect the chaotic dynamics properties of the APD: we have highlighted the fractal structure of the strange attractor of the 2D-map APD, and we have characterized chaos by tools such as the calculation of the Lyapunov exponents, the fractal dimension and the Kolmogorov entropy, with the next objective of refining the study of the nonlinear dynamics of the duration of the action potential and to apply methods of controlling chaos.

     

家兔急性缺血早期心室肌细胞跨膜电位的变化及迷走神经的保护作用

以在体家兔的心脏为对象,应用浮置微电极技术研究了急性缺血早期心室肌细胞跨膜电位的变化及迷走神经的保护作用。 冠状动脉的一个分支阻断后 1—5min,静息电位(RP)、动作电位振幅(APA)和 0相最大上升速率(dv/dt)_(max)均减小(P<0.01)。动作电位时程APD_(30)、APD_(50)和APD_(90)均缩短(P<0.01)。反映 2相平台时程的 APD_(30)和 APD_(50)缩短较总时程 APD_(90)的缩短更明显。 在自然心率的条件下,左颈迷走神经电刺激,可使急性缺血的心肌电位的变化有所恢复:RP、APA、(dv/dt)_(max)均增加,APD_(30)、APD_(50)、APD_(90)延长(P_均<0.01)。刺激迷走神经时缺血的心室肌细胞跨膜电位有所恢复,对防止缺血早期的室性心律失常可能具有重要作用。     

Peculiarities of Ca2+ regulation of functional activity of myocardium of frog Rana temporaria

To elucidate role of intra- and extracellular Ca2+ in regulation of rhythm and strength of frog heart contractions, there were studied ECC and isometric contraction of myocardium preparations in response to verapamil, adrenaline, and blockers of alpha- and beta-adrenoreceptors. It has been shown that after an intramuscular injection of verapamil (6 mg/kg), bradycardia develops, the heart rate (HR) decreasing by 50-70 %. Further, the cardiac arrest occurred; however, administration to the animals of adrenaline (100 mg/kg) restored the cardiac rhythm for a short while. After an intramuscular injection of adrenaline at doses of 0.1-10 mg/kg, no essential changes were observed in the potential action amplitude and HR; an increase of the administered adrenalin concentration to 100 mg/kg was not accompanied by the cardiac rhythm stimulation, as this takes place in homoiothermal animals and human; on the contrary, an essential HR deceleration was revealed. Phentolamine (5 mg/kg) gradually decelerated HR rhythm by 32-45 %. The potential amplitude changed insignificantly. A subsequent intracardiac injection of adrenaline (100 mg/kg) on the background of block of alpha-adrenoreceptors produced acceleration of the rhythm (by 13-21%) and fall of the electrogram amplitude. These results can indicate that in the frog heart, phentolamine interacts predominanty with alpha-adrenoreceptors. An intracardiac administration of propranolol (1 mg/kg) to frogs promoted inhibition of beta-adrenergic receptors and produced a gradual cardiac rhythm deceleration. In experiments on assessment of verapamil effect on the character of contractions this preparation at a concentration of 150 microM was established to produce a significant dosedependent decrease of the contraction strength. A rise of verapamil concentration in the sample to 200 microM led to a decrease of the amplitude, on average, by 68-70 % and in individual preparations -- by 80-85 %; however, administration into the sample of adrenaline (10 microM) restored the cardiac contraction strength. Adrenaline (1 nM--100 microM) increased markedly the contraction amplitude. Phentolamine (10 microM) did not inhibit transmission of contractile signal to cardiomyocytes; this was manifested in that the contraction amplitude after addition of adrenaline (10 microM) into the sample was approximately the same as in the sample containing no phentolamine. Propranolol (10 microM) eliminated the stimulatory action of adrenaline (10 microM). The results of these experiments indicate that in the frog ventricular cardiomyocytes the main adrenaline acceptors are beta-adrenoreceptors.