Noninvasive and Invasive Study of the Pumping Function of Rat Heart in Myocardial Infarction.

The cardiohemodynamics was studied 2 and 4 weeks after myocardial infarction modeling in Wistar rats. We compared the data obtained by echocardiography (echoCG) and catheterization of the left ventricle. The myocardial infarction was modeled by ligation of the left anterior descending coronary artery. EchoCG and the left ventricle catheterization were performed before and after myocardial infarction modeling. Similar results were obtained by both methods, namely the left ventricle dilatation, bradycardia, a reduced ejection fraction and delayed relaxation. According to echoCG, the end-diastolic left ventricle volume increased by 2 times, and initial diastolic left ventricle volume - by more than 5 times. The left ventricle catheterization showed lower rise, by 32 and 69%, respectively. The overestimated volume of the left ventricle in myocardial infarction according to echoCG data in comparison with catheterization can be explained by changes in the geometry of the ventricle (bulging of a part of the ventricular wall).

The Role of Arterial Elasticity in Determining the Degree of Chronic Heart Failure in Myocardial Infarction.

Aim      To study the left ventricular (LV) contractile and pumping function during the recovery phase following ligation of the anterior descending coronary artery (CA).Material and methods  Cardiodynamic parameters were studied in Wistar rats 2-4 weeks after experimental myocardial infarction (MI). MI was induced by ligation of the anterior descending CA under zoletil anesthesia. LV catheterization was performed with a standard FTH-1912B-8018 PV catheter inserted into the LV through the right carotid artery.Results After the induction of MI, the mortality rate of animals was 50%. Survived animals developed significant LV dilatation and a decrease in ejection fraction (EF) by an average of 31%. However, major indexes of the pumping function, including minute volume, heart work, and maximum ejection velocity, were within a normal range whereas the maximum filling velocity was almost doubled. Approximately 50% of hearts with dilated LV had normal EF, delayed relaxation, and increased LV diastolic pressure, which qualified this group as a diastolic dysfunction group. The systolic dysfunction group with EF less than 50% of normal had similar values of myocardial contractility and relaxation but differed from the diastolic dysfunction group in more than 50% reduced maximum LV ejection velocity and 1.7 times increased elasticity of the arterial wall. A close inverse correlation was found between these values (r= -0.91).Conclusion      The study results showed that, with a similar myocardial contractile function, the cardiac pumping function is determined by the elasticity of the aortic wall. Therefore, restriction of reactive fibrosis during MI is an important task of modern cardiology.

HEMODYNAMICS AND CARDIAC CONTRACTILE FUNCTION IN TYPE 1 DIABETES.

The cardiohemodynamics was studied 1 week after the administration of streptozotocin (60 mg / kg) or 2 weeks after a dose of 30 mg / kg. All rats had a significantly elevated level of glucose in the blood (up to 27-31 mM). In an echocardiographic study, about 1/3 of diabetic animals exhibited systolic dysfunction, and the remaining 2/3 - diastolic dysfunction with an increase in isovolumic relaxation time by 1.5 times. The catheterization of the left ventricle (LV) with a sensor that allows simultaneous measuring LV pressure and volume in both groups revealed decreased cardiac output by 25-31% and maximal ejection rate by 34-50%. However, LV developed pressure, the maximal rate of its development and the level of blood pressure remained within the control values, thus reduced LV ejection rate was probably due to increased arterial stiffness - a negative correlation was found between these indicators (r = - 0.70). The diastolic dysfunction group differed from systolic dysfunction by a significantly smaller end diastolic volume by 22%. Thus, in type 1 diabetes, LV remodeling with reduced end diastolic volume allows to maintain a normal ejection fraction in the presence of distinct heart failure.

Normalisation of diabetic heart pump function at decreased functional load.

Aim      To study left ventricular (LV) hemodynamics in presence of decreased blood inflow to the heart as well as changes in myocardial content of energy metabolites in diabetic rats.Material and methods  Diabetic cardiomyopathy is characterized by impaired heart contractility and by transition of cardiomyocyte energy metabolism fatty acids exclusively as a source of energy. This reduces the efficiency of energy utilization and increases the heart vulnerability to hypoxia. This study was performed on rats with type 1 diabetes mellitus induced by administration of streptozotocin (60 mg/kg). The LV pump function was studied with a catheter that allows simultaneous measurement of LV pressure and volume in each cardiac cycle.Results Blood glucose was approximately sixfold increased at 2 weeks. Heart failure was detected with decreases in ejection fraction by 27%, minute volume by 39%, and stroke work by 41%. Systolic dysfunction was based on a decrease in LV peak ejection velocity by more than 50%. Furthermore, the LV developed pressure and contractility index were within the normal range, while 1.5 times increased arterial stiffness was the factor that hampered ejection. The sum of adenine nucleotides was decreased by 21%, the ATP content was decreased by 29%, and also creatine phosphate formation was reduced in the myocardium of diabetic rats. Lactate content in the diabetic myocardium was increased almost threefold, which indicated mobilization of aerobic glycolysis. With the reduced preload, equal diastolic volume (0.3 ml), and equal blood pressure (60 mm Hg), the diabetic heart pump function did not differ from the control.Conclusion      In type 1 diabetes mellitus, decreases in functional load and oxygen consumption normalize the myocardial pump function with disturbed energy metabolism.

Systolic Dysfunction of the Heart in Type 1 Diabetes Mellitus.

Impaired insulin synthesis is accompanied by hyperglycemia and the development of diabetic cardiomyopathy. Echocardiography and left-ventricular catheterization were employed for studying the contractile function of the left ventricle in 2 weeks after administration of streptozotocin (60 mg/kg). The results obtained by both methods were similar and indicated the development of systolic dysfunction with a 27% decrease in cardiac output. The invasive study showed that the maximum rate of left-ventricular pressure development, the contractility index, and systolic left-ventricular pressure were within the normal range, but the peak ejection rate was reduced by 28%. BP was normal, but the vascular stiffness index was increased by about 1.5 times and inversely correlated with the peak ejection rate (r=-0.69). The results showed that systolic dysfunction in type 1 diabetes model was due to reduced ejection from the left ventricle at normal rate of left-ventricular pressure development.

Gaseous Nitric Oxide and Dinitrosyl Iron Complexes with Thiol-Containing Ligands as Potential Medicines that Can Relieve COVID-19.

It is shown that the inhalation of gaseous nitric oxide (gNO) or sprayed aqueous solutions of binuclear dinitrosyl iron complexes with glutathione or N-acetyl-L-cysteine by animals or humans provokes no perceptible hypotensive effects. Potentially, these procedures may be useful in COVID-19 treatment. The NO level in complexes with hemoglobin in blood decreases as the gNO concentration in the gas flow produced by the Plazon system increases from 100 to 2100 ppm, so that at 2000 ppm more than one-half of the gas can be incorporated into dinitrosyl complexes formed in tissues of the lungs and respiratory tract. Thus, the effect of gNO inhalation may be similar to that observed after administration of solutions of dinitrosyl iron complexes, namely, to the presence of dinitrosyl iron complexes with thiol-containing ligands in lung and airway tissues. With regard to the hypothesis posited earlier that these complexes can suppress coronavirus replication as donors of nitrosonium cations (Biophysics 65, 818, 2020), it is not inconceivable that administration of gNO or chemically synthesized dinitrosyl iron complexes with thiol-containing ligands may help treat COVID-19. In tests on the authors of this paper as volunteers, the tolerance concentration of gNO inhaled within 15 min was approximately 2000 ppm. In tests on rats that inhaled sprayed aqueous solutions of dinitrosyl iron complexes, their tolerance dose was approximately 0.4 mmol/kg body weight.

Compensatory Changes of the Diastole under Conditions of Inflow Restriction to the Heart.

This work was designed to study changes in the mechanical properties of rat myocardium during short-term (2-3 sec) compression of the lower vena cava. A catheter was inserted into the left ventricle, allowing simultaneous measurement of left-ventricular volume and pressure. The decrease in the left-ventricular end-diastolic volume caused by inflow restriction was accompanied by less pronounced decrease in the left-ventricular stroke volume and maximum rate of left-ventricular pressure development. This was coincided with accelerated relaxation and deeper fall of the minimum left-ventricular diastolic pressure. The lower was left-ventricular end-systolic volume, the greater was the degree of these changes. It is assumed that the "restoring force" that naturally appears under conditions of low filling of the left ventricle is determined by elastic N2B part of the titin molecule that is compressed during strong shortening of myofibrils and accelerates their return to the previous length during relaxation. As a result of better filling of the left ventricle, the heart can maintain left-ventricular stroke volume at the appropriate level.

[Prevention of Diastolic Dysfunction Caused by Doxorubicin by Mitochondrial Antioxidant Plastomitin].

Aim      An attempt to prevent the development of diastolic dysfunction (DD) with the mitochondrial antioxidant plastomitin on a model of doxorubicin-induced cardiomyopathy. DD is a type of chronic heart failure. Due to the increasing number of patients with this condition and the absence of effective therapy, development of means for DD correction is a relevant objective.Material and methods  Cardiomyopathy was modeled in 17 rats by two subcutaneous injections of doxorubicin 2 mg/kg/week. The other group (n=17), also administered with doxorubicin, received plastomicin 0.32 mg/kg daily subcutaneously. Left ventricular function was evaluated with echocardiography (EchoCG) and cardiac catheterization with simultaneous pressure and volume monitoring.Results According to EchoCG data the ejection fraction remained unchanged in the experimental groups. Cardiac catheterization showed disorders of both myocardial contractility and relaxability only in the doxorubicin group.Conclusion      A course of plastomitin in combination with the doxorubicin treatment can maintain normal heart contractility and thereby, prevent the known doxorubicin cardiotoxicity.

Reaction of the Heart to High Frequency Stimulation against the Background of Acute Doxorubicin Treatment.

Short-term high frequency electrostimulation (8-10 Hz) of the isolated isovolumic rat heart rapidly increased the rate of pressure rise and drop and the diastolic pressure. At the same time, the relaxation rate constant (RRC), being independent of the developed pressure, remained unaltered. These findings suggested that diastolic pressure rise was not caused by incomplete myocardial relaxation. Doxorubicin (3 µM) moderately reduced the developed pressure, but the relaxation rate constant remained unchanged. The dynamics and degree of changes in all indicators of the cardiac contractile function in high-frequency stimulation were the same as in control. It can be hypothesized that the initial effect of doxorubicin was not related to ionic transport system disturbances in cardiomyocytes.

[Early changes of energy metabolism, isoformic content and level of titin phosphorylation at diastolic dysfunction].

RELEVANCE:  Diastolic dysfunction occurring at hypertension, obesity, diabetes, or treatment with doxorubicin tends to prevail in all patterns of chronic heart failure. Lack of effective therapy forces to look more into the metabolic processes in cardiomyocytes. OBJECTIVE:  Assess energy metabolism in cardiomyocytes and changes in titin, a giant myofibril protein that responsible for their elasticity. MATERIAL AND METHODS:  The study model was cardiomyopathy occurring after the 4-week administration of doxorubicin (2 mg/kg weekly). Diastolic dysfunction was identified by echocardiography and catheterization with the simultaneous measurement of pressure and volume of the left ventricle (LV). RESULTS:  The levels of adenine nucleotides and phosphocreatine in the heart of animals treated with doxorubicin differed little from the normal values, but lactate levels were increased manifold. A 50% increase in the level of titin phosphorylation was detected, which correlated (r = 0,94) with a nearly twofold increase in the share of a more elastic N2BA-isoform of this protein. CONCLUSION: This form of diastolic dysfunction involves the activation of anaerobic metabolism and increased stretching of myofibrils facilitating LV filling.

[Why Myocardial Relaxation Always Slows at Cardiac Pathology?]

Chronic heart failure (CHF) in most cases is due to a decrease in myocardial contractility. In particular, this results in a reduction in the maximum rate of the pressure development in the left ventricle. At the same time the maximal rate of pressure fall at relaxation is also reduced. This is not surprising, since both depend on Ca ++ myoplasmic concentration. But most of cardiac pathologies have been associated with the impairement of myocardial relaxation to a greater extent than the contraction. In the review a new view has been proposed according to which this phenomenon is attributable to restructuring of titin, the sarcomeric protein that connects the ends of myosin filaments with the sarcomeric board, lines Z. A spring-like molecule of titin shrinks at sarcomeric contraction and straightens in parallel with removing of Ca ++ from myofibrils. A reduction of its stiffness, facilitating the filling of the left ventricle, can reduce restoring force of titin and thereby slow relaxation. The survey provides information about the functions of the calcium transport system and titin in the normal heart and in CHF observed both in experimental models and in patients.

[Mitochondrial Antioxidant Plastomitin Improves Cardiac Function in Doxorubicin-Induced Cardiomyopathy].

The aim of the study was to ascertain whether the use of plastomitin, the mitochondrial antioxidant, can affect the development of systolic dysfunction that occurs in rats after 4 weeks of doxorubicin treatment (2 mg/kg weekly). Materials and methods. Male Wistar rats weighing 320-380 g were used in this work. Echocardiographic study was carried out using Vevo 1100 with linear probe 13-24 MHz frequency. Results. Echocardiographic study of rats through 8 weeks from the beginning of doxorubicin treatment showed the presence of systolic dysfunction with decrease of ejection fraction of the left ventricle (LV) by 32%. Hearts of rats, to which plastomitin (0.32 mg/kg daily) was administered simultaneously with doxorubicin, showed significantly increased ejection fraction and shortening fraction as compared with doxorubicin group, and these values were close to the control. In experiments with simultaneous registration of LV pressure and volume, it was found that the hearts of all rats treated with doxorubicin showed reduced contractility index and stroke work, while maintaining normal cardiac output. Such compensation in experiments with treatment with doxorubicin alone was achieved through significant reduction in the peripheral resistance, slowing of myocardial relaxation, and facilitation of LV diastolic filling during prolonged diastolic pause (the heart rate was slowed by 23%). In experiments with simultaneous application of doxorubicin and plastomitin, the compensation was achieved through preservation of myocardial contractility and relaxability, the heart rate and peripheral resistance. This method of compensation is more beneficial for the body, because it does not restrict the supply of organs and tissues with oxygen, and has significant advantage over doxorubicin group at equal heart rate. Conclusion. The results allow to conclude that the use of plastomitin together with doxorubicin prevents the development of doxorubicin-induced systolic dysfunction.

[Pressure and Volume Characteristics of the Left Ventriclе in Its Diastolic and Systolic Dysfunction].

The Aim of the study was a detailed investigation of pressure volume-loop (PV-loop) curves in the rat heart during development of doxorubicin cardiomyopathy. MATERIALS AND METHODS: Cardiomyopathy in rats has been developed after 4 weeks doxorubicin administration (2 mg / kg weekly). RESULTS: Echocardiographic study of rats in 8 weeks from onset of doxorubicin administration showed preponderance of systolic dysfunction (67 %) with decrease of left ventricular (LV) ejection fraction (EF) by 30 %. Simultaneous registration of LV pressure and volume showed that diastolic LV volume was preserved in doxorubicin-treated rats due to considerable lengthening of the diastole, the heart rate was reduced by 22 %. These hearts also showed slowing of relaxation, reduced maximal rate of pressure development and stroke work, as well as significant reduction in peripheral arterial resistance. Diastolic dysfunction differed from the systolic one by normal systolic EF and preserved LV contractility index as well as lower diastolic LV pressure throughout the diastole. CONCLUSIONS: Based on these data, four compensatory mechanisms associated with cardiomyopathy were distinguished - 1) slowing of myocardial relaxation, prolonging myofibrillar active state, 2) reduction of peripheral arterial resistance for easier LV ejection, 3) heart rate reduction, prolonging diastolic pause and thus facilitating better LV filling and 4) increased pressure in the small circle, also contributing to the LV rapid filling.

[Metilin Improves Сardiac Function in Rabbits With Chronic Heart Failure].

AIM: To study effects of intravenous infusion of a cardioprotective drug metilin, developed at the "National Medical Research Center of Cardiology" on indices of cardiac function in rabbits in vivo after prolonged administration of doxorubicin. MATERIALS AND METHODS: Animals of the experimental group were intravenously injected with doxorubicin (2 mg / kg once a week) for 8 weeks, animals of the control group received the same volume of saline. Myocardial damage was characterized by an increase in concentration of malondialdehyde (MDA), troponin (TnI) and MB-fraction of creatine kinase (CK-MB) in venous blood and by disturbances in the left ventricle (LV) structure at morphological examination. Metilin effects on cardiac function were assessed by echocardiography and LV catheterization by the Millar catheter tip pressure transducer. RESULTS: Doxorubicin administration led to a decrease of the body mass of animals, an increase of the plasma concentration of cardiac markers CK-MB and TnI, lipid peroxidation (LPO) product MDA in venous blood, and pronounced disturbances in the structure of LV fibers and microvessels. At the same time, a significant decrease of myocardial contractility indices was observed. Manifestations of this decrease were increase of the end-diastolic and end-systolic dimensions (EDD and ESD, respectively), and decreases in the shortening fraction and ejection fraction (SF and EF, respectively) compared to baseline values. These changes indicated development of chronic heart failure (CHF) in animals of the experimental group. Against this background, intravenous infusion of metilin significantly increased SF and EF, but did not affect the heart rate. Beneficial effects of metilin on the indices of cardiac contractility and relaxation were maintained after the infusion was stopped. Noteworthy, metilin exerted greater influence on cardiac function of rabbits with CHF compared to control animals that did not receive doxorubicin. CONCLUSION: The obtained results indicate the potential of metilin to reduce LV dysfunction during chemotherapy with doxorubicin.

[Contractile Function of Isolated Hearts With Preserved and Reduced Ejection Fraction In Vivo].

The aim of the study was comparison of contractile function of isolated hearts of rats with doxorubicin-induced myocardial injury which were tentatively divided according to the level of ejection fraction determined by echocardiography in vivo. After 4 weeks of doxorubicin administration (2 mg/kg subcutaneously once a week) about half of animals had normal (86±1 %) and the other half reduced (61±4 %) ejection fraction. The first group was defined as diastolic heart failure (DHF) and the other as systolic (SHF). The maximal pressure developed by the isolated heart in isovolumic mode was reduced in DHF by 13 %, and in SHF by 34 %. The relaxation index in both groups was lowed by 22-24 %. Both groups were characterized by a decline in the ability to raise developed pressure while increasing coronary perfusion pressure, as well as by the loss of the ability of coronary vessels to maintain a stable flow rate while increasing perfusion pressure. The hearts of control animals were able to raise the cardiac work index (the product of developed pressure and heart rate) during prolonged high frequency (7-9 Hz) stimulation, while the two groups treated with doxorubicin reduced the level of this index. The content of basic energy metabolites (ATP, phosphocreatine, creatine) in both groups was reduced by 20-38 %. The results showed that the hearts in DHF and SHF groups showed approximately the same level of reduction of the contractile function and energy metabolism, and hence their difference in vivo was probably due to varying degrees of mobilization of compensatory mechanisms.

Contractile Function of Isolated Hearts With Preserved and Reduced Ejection Fraction In Vivo.

The aim of the study was comparison of contractile function of isolated hearts of rats with doxorubicin-induced myocardial injury which were tentatively divided according to the level of ejection fraction determined by echocardiography in vivo. After 4 weeks of doxorubicin administration (2 mg/kg subcutaneously once a week) about half of animals had normal (86±1%) and the other half reduced (61±4%) ejection fraction. The first group was defined as diastolic heart failure (DHF) and the other as systolic (SHF). The maximal pressure developed by the isolated heart in isovolumic mode was reduced in DHF by 13%, and in SHF by 34%. The relaxation index in both groups was lowed by 22-24%. Both groups were characterized by a decline in the ability to raise developed pressure while increasing coronary perfusion pressure, as well as by the loss of the ability of coronary vessels to maintain a stable flow rate while increasing perfusion pressure. The hearts of control animals were able to raise the cardiac work index (the product of developed pressure and heart rate) during prolonged high frequency (7-9 Hz) stimulation, while the two groups treated with doxorubicin reduced the level of this index. The content of basic energy metabolites (ATP, phosphocreatine, creatine) in both groups was reduced by 20-38%. The results showed that the hearts in DHF and SHF groups showed approximately the same level of reduction of the contractile function and energy metabolism, and hence their difference in vivo was probably due to varying degrees of mobilization of compensatory mechanisms.

[The Time Course of Formation Of Systolic Dysfunction of the Heart in Doxorubicin Cardiomyopathy].

The anthracycline-induced cardiomyopathy is a frequent and menacing complication of antitumor therapy leading to chronic heart failure. A study of the formation of heart failure can reveal early signs of the development of systolic dysfunction of the heart. In this work in rats we studied cardiac function at different duration of doxorubicin treatment, the most effective anthracycline antibiotic. Cumulative doses of doxorubicin were 8-20 mg/kg, and the term of study lasted from 6 to 20 weeks. The echocardiography and catheterization of the left ventricle (LV) have been use. The ejection fraction and other indicators of LV contractility decreased steadily with increasing dose and duration of the study, in parallel with rat survival. However, the cardiac output related to the unit of body weight, as well as diastolic LV size, remained at a level close to control within 8-10 weeks. Only after 20 weeks when the ejection fraction decreased from 81+/-1 to 49+/-4%, diastolic LV volume increased by 59%. Invasive indicators of myocardial contractility and relaxability significantly decreased by 11 and 19% after doxorubicin dose of 8 mg/kg, while time of preejection and time of systole increased by 18 and 10%. These changes progressed with increasing doses of doxorubicin. At each dose, the relaxation constant declined relatively deeper than contractility index by 8-25%. The results show that: 1) the gradual formation of cardiac insufficiency mobilizes a variety of compensatory mechanisms that retard cardiac dilatation; 2) the development of systolic dysfunction takes place with a predominantly violation of relaxation process; 3) an elongation of the preejection period and duration of systole may serve as noninvasive criteria for the formation of the systolic dysfunction.

[The Action of Apelin-12 and Its Analog on Hemodynamics and Cardiac Contractile Function of Rats With Isoproterenol-Induced Myocardial Lesion].

Introduction of isoproterenol (beta-adrenoreceptor agonist) into rats is one of the widespread experimental models of heart failure. It is caused by diffuse ischemic damage of cardiomyocytes, followed by development of substitutive fibrosis. Apelin is a natural regulator of the myocardial contractility. The effects of apelin molecule fragment, apelin-12 and its more stable synthetic analogue, apelin-12-2 on cardiac contractile function of rats with isoproterenol-induced myocardial lesion (IML) and control animals has been studied in this work using invasive (catheterization of the left ventricle) and non-invasive (echocardiography and impedansometry) methods. Infusion of both peptides was made by sequentially increasing rate from 0.5 to 50 µg/kg/min. In the control group, efficacy of apelin-12 was low while apelin-12-2 moderately but significantly increased indices of myocardial contractility and relaxability. These changes were more pronounced in rats with IML and, in addition, the heart rate and LV systolic pressure increased in this group. These results correlate well with echocardiographic studies which showed increases of LV end diastolic volume, stroke volume and ejection fraction by 17-38%. These alterations are probably due to improved Ca2+ transport in cardiomyocytes, as in experiments on isolated cardiomyocytes both apelins have facilitated and improved Ca2+ removal from myoplasma. The results allow to conclude that apelin-12-2 seems to be a promising candidate for further development as a therapeutic agent in heart failure.

[Cardiotropic action of oxacom in experimental heart failure].

Prolonged hypotensive effect of oxacom (dinitrosyl iron complexes with ligand glutathione) has been well documented in animals and healthy volunteers, but effect on the heart is not defined. In this work, the blood pressure (BP), and the pressure in the left ventricle (LV) were recorded in rats. Intravenous bolus injection of oxacom (10 mg/kg) caused an immediate average BP decrease by 20-30 mm Hg followed by a slow recovery. The LV systolic and diastolic pressures did not change, but the maximal speed of pressure development, and contractility index at peak action of oxacom increased by 26-33%, while isovolumic relaxation constant fell by 30%. These changes gradually normalized within 10-15 minutes. Effect of oxacom on hearts with isoproterenol-induced diffuse ischemic myocardial lesions and diastolic heart failure was similar to that observed in the control group except the isovolumic relaxation constant rose 1.5-fold while elevated LV diastolic pressure fell. Results suggest that oxacom exerts the positive cardiotropic action especially at heart failure.

Redox control of cardiac rhythm.

The rhythm of cardiac beats is generated by pacemaker cells differing from other cardiomyocytes by the presence of slow diastolic depolarization. Consistently activated transmembrane ionic currents provide cyclic excitation of pacemakers, forming the original "membrane clocks". A new concept has been forwarded in the last decade according to which periodic fluctuations in myoplasmic Ca2+ level ("calcium clocks") not only influence a course of "membrane clocks", but they also can serve as independent generators of the rhythm. Transport of Ca2+ in cells is under constant influence of active forms of oxygen and nitrogen. Both superoxide and NO in moderate doses facilitate Ca2+ output from the sarcoplasmic reticulum, accelerating the course of "calcium clocks", but in higher doses they have opposite effect that may be neutralized mainly by reduced glutathione. The control of cardiac rhythm by active forms of oxygen and nitrogen represents a feedback mechanism by which mitochondria and NO-synthases support Ca2+ homeostasis in cells that can be temporarily disturbed under mechanical loads or hypoxia.