Contractile and morphological impairment of cultured fetal mouse myocytes induced by oxygen radicals and oxidants. Correlation with intracellular Ca2+ concentration

Direct toxicity of cigarette smoke extract on cardiac function mediated by mitochondrial dysfunction in Sprague-Dawley rat ventricular myocytes and human induced pluripotent stem cell-derived cardiomyocytes

Cigarette smoke has been recognized as a major risk factor for cardiovascular disease. However, its direct effects on rodent and human cardiomyocytes and its cellular mechanisms are not fully understood. In this study, we examined the direct effects of cigarette smoke extract (CSE) on contractile functions, intracellular Ca2+ dynamics, and mitochondrial function using cultured or freshly isolated rat ventricular myocytes and human induced pluripotent stem cell (iPS)-derived cardiomyocytes. In rat cardiomyocytes, CSE (≥0.1%) resulted in a time- and concentration-dependent cessation of spontaneous beating of cultured cardiomyocytes, eventually leading to cell death, which indicates direct toxicity. In addition, 1% CSE reduced contractile function of freshly isolated ventricular myocytes. Similar contractile dysfunction (declined spontaneous beating rate and contractility) was also observed in human iPS-derived cardiomyocytes. Regarding intracellular Ca2+ dynamics, 1% CSE increased the Ca2+ transient amplitude by greatly increasing systolic Ca2+ levels and slightly increasing diastolic Ca2+ levels. CSE also accelerated the decay of Ca2+ transients, and triggered spike-shaped Ca2+ transients in some cells. These results indicate that CSE causes abnormal Ca2+ dynamics in cardiomyocytes. Furthermore, CSE induced a cascade of mitochondrial dysfunctions, including increased mitochondrial reactive oxygen species, opening of mitochondrial permeability transition pore, reduction of mitochondrial membrane potential, and release of cytochrome c from mitochondria. These results suggest that CSE-induced contractile dysfunction and myocardial cell death is caused by abnormal Ca2+ dynamics and subsequent mitochondrial dysregulation, which would result in reduced bioenergetics and activation of cell death pathways.

Blocking effect of ferritin on the ryanodine receptor-isoform 2

Iron, an essential element for most living organism, participates in a wide variety of physiological processes. Disturbance in iron homeostasis has been associated with numerous pathologies, particularly in the heart and brain, which are the most susceptible organs. Under iron-overload conditions, the generation of reactive oxygen species leads to impairment in Ca2+ signaling, fundamentally implicated in cardiac and neuronal physiology. Since iron excess is accompanied by increased expression of iron-storage protein, ferritin, we examined whether ferritin has an effect on the ryanodine receptor - isoform 2 (RYR2), which is one of the major components of Ca2+ signaling. Using the method of planar lipid membranes, we show that ferritin induced an abrupt, permanent blockage of the RYR2 channel. The ferritin effect was strongly voltage dependent and competitively antagonized by cytosolic TEA+, an impermeant RYR2 blocker. Our results collectively indicate that monomeric ferritin highly likely blocks the RYR2 channel by a direct electrostatic interaction within the wider region of the channel permeation pathway.

Plin5 deficiency exacerbates pressure overload-induced cardiac hypertrophy and heart failure by enhancing myocardial fatty acid oxidation and oxidative stress

While cardiac hypertrophy and heart failure are accompanied by significant alterations in energy metabolism, more than 50-70% of energy is obtained from fatty acid β-oxidation (FAO) in adult hearts under physiological conditions. Plin5 is involved in the metabolism of lipid droplets (LDs) and is highly abundant in oxidative tissues including heart, liver and skeletal muscle. Plin5 protects the storage of triglyceride (TG) in LDs by inhibiting lipolysis, thereby suppressing excess FAO and preventing excessive oxidative stress in the heart. In this study, we investigated the roles of Plin5 in cardiac hypertrophy and heart failure in mice treated with transverse aortic constriction (TAC). The results indicated that Plin5 deficiency aggravated myocardial hypertrophy in the TAC-treated mice and exacerbated the TAC-induced heart failure. We also found that Plin5 deficiency reduced the cardiac lipid accumulation and upregulated the levels of PPARα and PGC-1α, which stimulate mitochondrial proliferation. Moreover, Plin5 deficiency aggravated the TAC-induced oxidative stress. We consistently found that Plin5 knockdown disrupted TG storage and elevated FAO and lipolysis in H9C2 rat cardiomyocytes. In addition, Plin5 knockdown also provoked mitochondrial proliferation and lipotoxic injury in H9C2 cells. In conclusion, Plin5 deficiency increases myocardial lipolysis, elevates FAO and oxidative burden, and thereby exacerbates cardiac hypertrophy and heart failure in TAC-treated mice.

Functional and biochemical responses of skeletal muscle following a moderate degree of systemic iron loading in mice

Excessive iron loading may cause skeletal muscle atrophy and weakness because of its free radical generating properties. To determine whether a clinically relevant degree of iron loading impairs skeletal muscle function, young male mice received injections of iron dextran (4 mg iron/200 µl) or 2 mM d-glucose (control) 5 days/week for 2 weeks ( n = 10/group). Systemic iron loading induced an approximate fourfold increase in the skeletal muscle nonheme iron concentration. Soleus specific tension (1, 30-250 Hz) was lower among iron-loaded animals compared with controls despite similar body mass and muscle mass. Soleus lipid peroxidation (4-hydroxynonenal adducts) and protein oxidation (protein carbonyls) levels were similar between groups. In gastrocnemius muscle, reduced glutathione (GSH) and glutathione peroxidase activity were similar but glutathione disulfide (GSSG) and the GSSG/GSH ratio were greater in iron-loaded muscle. A greater protein expression level of endogenous thiol antioxidant thioredoxin (TRX) was observed among iron-loaded muscle whereas its endogenous inhibitor thioredoxin-interacting protein (TXNip) and the TRX/TXNip ratio were similar. Glutaredoxin2, a thiol-disulfide oxidoreductase activated by GSSG-induced destabilization of its iron-sulfur [2Fe-2S] cluster, was lower following iron loading. Additionally, protein levels of α-actinin and αII-spectrin at 240 kDa were lower in the iron-loaded group. Ryanodine receptor stabilizing subunit calstabin1 was also lower following iron loading. In summary, the contractile dysfunction that resulted from moderate iron loading may be mediated by a disturbance in the muscle redox balance and from changes arising from an increased proteolytic response and aberrant sarcoplasmic reticulum Ca²⁺ release. NEW & NOTEWORTHY Although severe iron loading is known to cause muscle oxidative stress and dysfunction, the effects of a moderate degree of systemic iron loading on muscle contractile function and biochemical responses remain unclear. This study demonstrates that a pathophysiological elevation in the skeletal muscle iron load leads to force deficits that coincide with impaired redox status, structural integrity, and lower ryanodine receptor-associated calstabin1 in the absence of muscle mass changes or oxidative damage.

Neuronal Ca2+ sensor-1 contributes to stress tolerance in cardiomyocytes via activation of mitochondrial detoxification pathways

Identification of the molecules involved in cell death/survival pathways is important for understanding the mechanisms of cell loss in cardiac disease, and thus is clinically relevant. Ca²⁺-dependent signals are often involved in these pathways. Here, we found that neuronal Ca²⁺-sensor-1 (NCS-1), a Ca²⁺-binding protein, has an important role in cardiac survival during stress. Cardiomyocytes derived from NCS-1-deficient (Ncs1^-/-) mice were more susceptible to oxidative and metabolic stress than wild-type (WT) myocytes. Cellular ATP levels and mitochondrial respiration rates, as well as the levels of mitochondrial marker proteins, were lower in Ncs1^-/- myocytes. Although oxidative stress elevated mitochondrial proton leak, which exerts a protective effect by inhibiting the production of reactive oxygen species in WT myocytes, this response was considerably diminished in Ncs1^-/- cardiomyocytes, and this would be a major reason for cell death. Consistently, H₂O₂-induced loss of mitochondrial membrane potential, a critical early event in cell death, was accelerated in Ncs1^-/- myocytes. Furthermore, NCS-1 was upregulated in hearts subjected to ischemia-reperfusion, and ischemia-reperfusion injury was more severe in Ncs1^-/- hearts. Activation of stress-induced Ca²⁺-dependent survival pathways, such as Akt and PGC-1α (which promotes mitochondrial biogenesis and function), was diminished in Ncs1^-/- hearts. Overall, these data demonstrate that NCS-1 contributes to stress tolerance in cardiomyocytes at least in part by activating certain Ca²⁺-dependent survival pathways that promote mitochondrial biosynthesis/function and detoxification pathways.

Modeling Doxorubicin-Induced Cardiotoxicity in Human Pluripotent Stem Cell Derived-Cardiomyocytes

Doxorubicin is a highly efficacious anti-cancer drug but causes cardiotoxicity in many patients. The mechanisms of doxorubicin-induced cardiotoxicity (DIC) remain incompletely understood. We investigated the characteristics and molecular mechanisms of DIC in human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs). We found that doxorubicin causes dose-dependent increases in apoptotic and necrotic cell death, reactive oxygen species production, mitochondrial dysfunction and increased intracellular calcium concentration. We characterized genome-wide changes in gene expression caused by doxorubicin using RNA-seq, as well as electrophysiological abnormalities caused by doxorubicin with multi-electrode array technology. Finally, we show that CRISPR-Cas9-mediated disruption of TOP2B, a gene implicated in DIC in mouse studies, significantly reduces the sensitivity of hPSC-CMs to doxorubicin-induced double stranded DNA breaks and cell death. These data establish a human cellular model of DIC that recapitulates many of the cardinal features of this adverse drug reaction and could enable screening for protective agents against DIC as well as assessment of genetic variants involved in doxorubicin response.

Myeloperoxidase impairs the contractile function in isolated human cardiomyocytes

We set out to characterize the mechanical effects of myeloperoxidase (MPO) in isolated left-ventricular human cardiomyocytes. Oxidative myofilament protein modifications (sulfhydryl (SH)-group oxidation and carbonylation) induced by the peroxidase and chlorinating activities of MPO were additionally identified. The specificity of the MPO-evoked functional alterations was tested with an MPO inhibitor (MPO-I) and the antioxidant amino acid Met. The combined application of MPO and its substrate, hydrogen peroxide (H2O2), largely reduced the active force (Factive), increased the passive force (Fpassive), and decreased the Ca(2+) sensitivity of force production (pCa50) in permeabilized cardiomyocytes. H2O2 alone had significantly smaller effects on Factive and Fpassive and did not alter pCa50. The MPO-I blocked both the peroxidase and the chlorinating activities, whereas Met selectively inhibited the chlorinating activity of MPO. All of the MPO-induced functional effects could be prevented by the MPO-I and Met. Both H2O2 alone and MPO + H2O2 reduced the SH content of actin and increased the carbonylation of actin and myosin-binding protein C to the same extent. Neither the SH oxidation nor the carbonylation of the giant sarcomeric protein titin was affected by these treatments. MPO activation induces a cardiomyocyte dysfunction by affecting Ca(2+)-regulated active and Ca(2+)-independent passive force production and myofilament Ca(2+) sensitivity, independent of protein SH oxidation and carbonylation. The MPO-induced deleterious functional alterations can be prevented by the MPO-I and Met. Inhibition of MPO may be a promising therapeutic target to limit myocardial contractile dysfunction during inflammation.

Stimulus-dependent regulation of nuclear Ca2+ signaling in cardiomyocytes: a role of neuronal calcium sensor-1

In cardiomyocytes, intracellular calcium (Ca²⁺) transients are elicited by electrical and receptor stimulations, leading to muscle contraction and gene expression, respectively. Although such elevations of Ca²⁺levels ([Ca²⁺]) also occur in the nucleus, the precise mechanism of nuclear [Ca²⁺] regulation during different kinds of stimuli, and its relationship with cytoplasmic [Ca²⁺] regulation are not fully understood. To address these issues, we used a new region-specific fluorescent protein-based Ca²⁺ indicator, GECO, together with the conventional probe Fluo-4 AM. We confirmed that nuclear Ca²⁺ transients were elicited by both electrical and receptor stimulations in neonatal mouse ventricular myocytes. Kinetic analysis revealed that electrical stimulation-elicited nuclear Ca²⁺ transients are slower than cytoplasmic Ca²⁺ transients, and chelating cytoplasmic Ca²⁺ abolished nuclear Ca²⁺ transients, suggesting that nuclear Ca²⁺ are mainly derived from the cytoplasm during electrical stimulation. On the other hand, receptor stimulation such as with insulin-like growth factor-1 (IGF-1) preferentially increased nuclear [Ca²⁺] compared to cytoplasmic [Ca²⁺]. Experiments using inhibitors revealed that electrical and receptor stimulation-elicited Ca²⁺ transients were mainly mediated by ryanodine receptors and inositol 1,4,5-trisphosphate receptors (IP3Rs), respectively, suggesting different mechanisms for the two signals. Furthermore, IGF-1-elicited nuclear Ca²⁺ transient amplitude was significantly lower in myocytes lacking neuronal Ca²⁺ sensor-1 (NCS-1), a Ca²⁺ binding protein implicated in IP₃R-mediated pathway in the heart. Moreover, IGF-1 strengthened the interaction between NCS-1 and IP₃R. These results suggest a novel mechanism for receptor stimulation-induced nuclear [Ca²⁺] regulation mediated by IP3R and NCS-1 that may further fine-tune cardiac Ca²⁺ signal regulation.

Cardiomyopathy in offspring of pregestational diabetic mouse pregnancy

PURPOSE

To investigate cardiomyopathy in offspring in a mouse model of pregestational type 1 diabetic pregnancy.

METHODS

Pregestational diabetes was induced with STZ administration in female C57BL6/J mice that were subsequently mated with healthy C57BL6/J males. Offspring were sacrificed at embryonic day 18.5 and 6-week adolescent and 12-week adult stages. The size and number of cardiomyocyte nuclei and also the extent of collagen deposition within the hearts of diabetic and control offspring were assessed following cardiac tissue staining with either haematoxylin and eosin or Picrosirius red and subsequently quantified using automated digital image analysis.

RESULTS

Offspring from diabetic mice at embryonic day 18.5 had a significantly higher number of cardiomyocyte nuclei present compared to controls. These nuclei were also significantly smaller than controls. Collagen deposition was shown to be significantly increased in the hearts of diabetic offspring at the same age. No significant differences were found between the groups at 6 and 12 weeks.

CONCLUSIONS

Our results from offspring of type 1 diabetic mice show increased myocardial collagen deposition in late gestation and have increased myocardial nuclear counts (hyperplasia) as opposed to increased myocardial nuclear size (hypertrophy) in late gestation. These changes normalize postpartum after removal from the maternal intrauterine environment.

Cardiomyocyte culture - an update on the in vitro cardiovascular model and future challenges

The success of any work with isolated cardiomyocytes depends on the reproducibility of cell isolation, because the cells do not divide. To date, there is no suitable in vitro model to study human adult cardiac cell biology. Although embryonic stem cells and induced pluripotent stem cells are able to differentiate into cardiomyocytes in vitro, the efficiency of this process is low. Isolation and expansion of human cardiomyocyte progenitor cells from cardiac surgical waste or, alternatively, from fetal heart tissue is another option. However, to overcome various issues related to human tissue usage, especially ethical concerns, researchers use large- and small-animal models to study cardiac pathophysiology. A simple model to study the changes at the cellular level is cultures of cardiomyocytes. Although primary murine cardiomyocyte cultures have their own advantages and drawbacks, alternative strategies have been developed in the last two decades to minimise animal usage and interspecies differences. This review discusses the use of freshly isolated murine cardiomyocytes and cardiomyocyte alternatives for use in cardiac disease models and other related studies.

The molecular mechanisms of offspring effects from obese pregnancy

The incidence of obesity, increased weight gain and the popularity of high-fat / high-sugar diets are seriously impacting upon the global population. Billions of individuals are affected, and although diet and lifestyle are of paramount importance to the development of adult obesity, compelling evidence is emerging which suggests that maternal obesity and related disorders may be passed on to the next generation by non-genetic means. The processes acting within the uteri of obese mothers may permanently predispose offspring to a diverse plethora of diseases ranging from obesity and diabetes to psychiatric disorders. This review aims to summarise some of the molecular mechanisms and active processes currently known about maternal obesity and its effect on foetal and neonatal physiology and metabolism. Complex and multifactorial networks of molecules are intertwined and culminate in a pathologically synergistic manner to cause disruption and disorganisation of foetal physiology. This altered phenotype may potentiate the cycle of intergenerational transmission of obesity and related disorders.

Perilipin 5, a lipid droplet-binding protein, protects heart from oxidative burden by sequestering fatty acid from excessive oxidation

Lipid droplets (LDs) are ubiquitous organelles storing neutral lipids, including triacylglycerol (TAG) and cholesterol ester. The properties of LDs vary greatly among tissues, and LD-binding proteins, the perilipin family in particular, play critical roles in determining such diversity. Overaccumulation of TAG in LDs of non-adipose tissues may cause lipotoxicity, leading to diseases such as diabetes and cardiomyopathy. However, the physiological significance of non-adipose LDs in a normal state is poorly understood. To address this issue, we generated and characterized mice deficient in perilipin 5 (Plin5), a member of the perilipin family particularly abundant in the heart. The mutant mice lacked detectable LDs, containing significantly less TAG in the heart. Particulate structures containing another LD-binding protein, Plin2, but negative for lipid staining, remained in mutant mice hearts. LDs were recovered by perfusing the heart with an inhibitor of lipase. Cultured cardiomyocytes from Plin5-null mice more actively oxidized fatty acid than those of wild-type mice. Production of reactive oxygen species was increased in the mutant mice hearts, leading to a greater decline in heart function with age. This was, however, reduced by the administration of N-acetylcysteine, a precursor of an antioxidant, glutathione. Thus, we conclude that Plin5 is essential for maintaining LDs at detectable sizes in the heart, by antagonizing lipase(s). LDs in turn prevent excess reactive oxygen species production by sequestering fatty acid from oxidation and hence suppress oxidative burden to the heart.

Protective effects of coenzyme q(10) on decreased oxidative stress resistance induced by simvastatin

The effects of simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase), on oxidative stress resistance and the protective effects of coenzyme Q (CoQ) were investigated. When simvastatin was administered orally to mice, the levels of oxidized and reduced CoQ₉ and CoQ₁₀ in serum, liver, and heart, decreased significantly when compared to those of control. The levels of thiobarbituric acid reactive substances induced by Fe²⁺-ascorbate in liver and heart mitochondria also increased significantly with simvastatin. Furthermore, cultured cardiac myocytes treated with simvastatin exhibited less resistance to oxidative stress, decreased time to the cessation of spontaneous beating in response to H₂O₂ addition, and decreased responsiveness to electrical field stimulation. These results suggested that oral administration of simvastatin suppresses the biosynthesis of CoQ, which shares the same biosynthesis pathway as cholesterol up to farnesyl pyrophosphate, thus compromising the physiological function of reduced CoQ, which possesses antioxidant activity. However, these undesirable effects induced by simvastatin were alleviated by coadministering CoQ₁₀ with simvastatin to mice. Simvastatin also reduced the activity of NADPH-CoQ reductase, a biological enzyme that converts oxidized CoQ to the corresponding reduced CoQ, while CoQ₁₀ administration improved it. These findings may also support the efficacy of coadministering CoQ₁₀ with statins.

Differential contribution of mitochondria, NADPH oxidases, and glycolysis to region-specific oxidant stress in the anoxic-reoxygenated embryonic heart

The ability of the developing myocardium to tolerate oxidative stress during early gestation is an important issue with regard to possible detrimental consequences for the fetus. In the embryonic heart, antioxidant defences are low, whereas glycolytic flux is high. The pro- and antioxidant mechanisms and their dependency on glucose metabolism remain to be explored. Isolated hearts of 4-day-old chick embryos were exposed to normoxia (30 min), anoxia (30 min), and hyperoxic reoxygenation (60 min). The time course of ROS production in the whole heart and in the atria, ventricle, and outflow tract was established using lucigenin-enhanced chemiluminescence. Cardiac rhythm, conduction, and arrhythmias were determined. The activity of superoxide dismutase, catalase, gutathione reductase, and glutathione peroxidase as well as the content of reduced and oxidized glutathione were measured. The relative contribution of the ROS-generating systems was assessed by inhibition of mitochondrial complexes I and III (rotenone and myxothiazol), NADPH oxidases (diphenylene iodonium and apocynine), and nitric oxide synthases ( N-monomethyl-l-arginine and N-iminoethyl-l-ornithine). The effects of glycolysis inhibition (iodoacetate), glucose deprivation, glycogen depletion, and lactate accumulation were also investigated. In untreated hearts, ROS production peaked at 10.8 ± 3.3, 9 ± 0.8, and 4.8 ± 0.4 min (means ± SD; n = 4) of reoxygenation in the atria, ventricle, and outflow tract, respectively, and was associated with arrhythmias. Functional recovery was complete after 30–40 min. At reoxygenation, 1) the respiratory chain and NADPH oxidases were the main sources of ROS in the atria and outflow tract, respectively; 2) glucose deprivation decreased, whereas glycogen depletion increased, oxidative stress; 3) lactate worsened oxidant stress via NADPH oxidase activation; 4) glycolysis blockade enhanced ROS production; 5) no nitrosative stress was detectable; and 6) the glutathione redox cycle appeared to be a major antioxidant system. Thus, the glycolytic pathway plays a predominant role in reoxygenation-induced oxidative stress during early cardiogenesis. The relative contribution of mitochondria and extramitochondrial systems to ROS generation varies from one region to another and throughout reoxygenation.

Predicting the no-reflow phenomenon following successful percutaneous coronary intervention

In the setting of acute myocardial infarction, early and adequate reopening of an infarct-related artery is not necessarily followed by a complete restoration of myocardial perfusion. This condition is usually defined as ‘no-reflow’. The pathophysiology of no-reflow is multifactorial since extravascular compression, microvascular vasoconstriction, embolization during percutaneous coronary intervention, and platelet and neutrophil aggregates are involved. In the clinical arena, angiographic findings and easily available clinical parameters can predict the risk of no-reflow. More recently, several studies have demonstrated that biomarkers, especially those related to the pathogenetic components of no-reflow, could also have a prognostic role in the prediction and in the full understanding of the multiple mechanisms of this phenomenon. Thus, in this article, we investigate the role of several biomarkers on admission in predicting the occurrence of no-reflow following successful percutaneous coronary intervention.

The dynamin-related GTPase Drp1 is required for embryonic and brain development in mice

Brain-specific Drp1 knockout mice demonstrate that Drp1-mediated organelle division is important for development, mitochondrial morphogenesis, and apoptosis.

The dynamin-related guanosine triphosphatase Drp1 mediates the division of mitochondria and peroxisomes. To understand the in vivo function of Drp1, complete and tissue-specific mouse knockouts of Drp1 were generated. Drp1-null mice die by embryonic day 11.5. This embryonic lethality is not likely caused by gross energy deprivation, as Drp1-null cells showed normal intracellular adenosine triphosphate levels. In support of the role of Drp1 in organelle division, mitochondria formed extensive networks, and peroxisomes were elongated in Drp1-null embryonic fibroblasts. Brain-specific Drp1 ablation caused developmental defects of the cerebellum in which Purkinje cells contained few giant mitochondria instead of the many short tubular mitochondria observed in control cells. In addition, Drp1-null embryos failed to undergo developmentally regulated apoptosis during neural tube formation in vivo. However, Drp1-null embryonic fibroblasts have normal responses to apoptotic stimuli in vitro, suggesting that the apoptotic function of Drp1 depends on physiological cues. These findings clearly demonstrate the physiological importance of Drp1-mediated organelle division in mice.

Protection of cardiomyocytes from ischemic/hypoxic cell death via Drbp1 and pMe2GlyDH in cardio-specific ARC transgenic mice

The ischemic death of cardiomyocytes is associated in heart disease and heart failure. However, the molecular mechanism underlying ischemic cell death is not well defined. To examine the function of apoptosis repressor with a caspase recruitment domain (ARC) in the ischemic/hypoxic damage of cardiomyocytes, we generated cardio-specific ARC transgenic mice using a mouse alpha-myosin heavy chain promoter. Compared with the control, the hearts of ARC transgenic mice showed a 3-fold overexpression of ARC. Langendoff preparation showed that the hearts isolated from ARC transgenic mice exhibited improved recovery of contractile performance during reperfusion. The cardiomyocytes cultured from neonatal ARC transgenic mice were significantly resistant to hypoxic cell death. Furthermore, the ARC C-terminal calcium-binding domain was as potent to protect cardiomyocytes from hypoxic cell death as ARC. Genome-wide RNA expression profiling uncovered a list of genes whose expression was changed (>2-fold) in ARC transgenic mice. Among them, expressional regulation of developmentally regulated RNA-binding protein 1 (Drbp1) or the dimethylglycine dehydrogenase precursor (pMe(2)GlyDH) affected hypoxic death of cardiomyocytes. These results suggest that ARC may protect cardiomyocytes from hypoxic cell death by regulating its downstream, Drbp1 and pMe(2)GlyDH, shedding new insights into the protection of heart from hypoxic damages.

H(2)O(2)-induced left ventricular dysfunction in isolated working rat hearts is independent of calcium accumulation

Reactive oxygen species (ROS) and intracellular Ca(2+) overload play key roles in myocardial ischemia-reperfusion (IR) injury but the relationships among ROS, Ca(2+) overload and LV mechanical dysfunction remain unclear. We tested the hypothesis that H(2)O(2) impairs LV function by causing Ca(2+) overload by increasing late sodium current (I(Na)), similar to Sea Anemone Toxin II (ATX-II). Diastolic and systolic Ca(2+) concentrations (d[Ca(2+)](i) and s[Ca(2+)](i)) were measured by indo-1 fluorescence simultaneously with LV work in isolated working rat hearts. H(2)O(2) (100 microM, 30 min) increased d[Ca(2+)](i) and s[Ca(2+)](i). LV work increased transiently then declined to 32% of baseline before recovering to 70%. ATX-II (12 nM, 30 min) caused greater increases in d[Ca(2+)](i) and s[Ca(2+)](i). LV work increased transiently before declining gradually to 17%. Ouabain (80 microM) exerted similar effects to ATX-II. Late I(Na) inhibitors, lidocaine (10 microM) or R56865 (2 microM), reduced effects of ATX-II on [Ca(2+)](i) and LV function, but did not alter effects of H(2)O(2). The antioxidant, N-(2-mercaptopropionyl)glycine (MPG, 1 mM) prevented H(2)O(2)-induced LV dysfunction, but did not alter [Ca(2+)](i). Paradoxically, further increases in [Ca(2+)](i) by ATX-II or ouabain, given 10 min after H(2)O(2), improved function. The failure of late I(Na) inhibitors to prevent H(2)O(2)-induced LV dysfunction, and the ability of MPG to prevent H(2)O(2)-induced LV dysfunction independent of changes in [Ca(2+)](i) indicate that impaired contractility is not due to Ca(2+) overload. The ability of further increases in [Ca(2+)](i) to reverse H(2)O(2)-induced LV dysfunction suggests that Ca(2+) desensitization is the predominant mechanism of ROS-induced contractile dysfunction.

Magnesium deficiency augments myocardial response to reactive oxygen species

Magnesium (Mg) deficiency and oxidative stress are independently implicated in the etiopathogenesis of various cardiovascular disorders. This study was undertaken to examine the hypothesis that Mg deficiency augments the myocardial response to oxidative stress. Electrically stimulated rat papillary muscle was used for recording the contractile variation. Biochemical variables of energy metabolism (adenosine triphosphate (ATP) and creatine phosphate) and markers of tissue injury (lactate dehydrogenase (LDH) release and lipidperoxidation), which can affect myocardial contractility, were assayed in Langendorff-perfused rat hearts. Hydrogen peroxide (100 micromol/L) was used as the source of reactive oxygen species. The negative inotropic response to H2O2 was significantly higher in Mg deficiency (0.48 mmol Mg/L) than in Mg sufficiency (1.2 mmol Mg/L). Low Mg levels did not affect ATP levels or tissue lipid peroxidation. However, H2O2 induced a decrease in ATP; enhanced lipid peroxidation and the release of LDH were augmented by Mg deficiency. Increased lipid peroxidation associated with a decrease in available energy might be responsible for the augmentation of the negative inotropic response to H2O2 in Mg deficiency. The observations from this study validate the hypothesis that myocardial response to oxidative stress is augmented by Mg deficiency. This observation has significance in ischemia-reperfusion injury, where Mg deficiency can have an additive effect on the debilitating consequences.

Protective effects of non-mitogenic human acidic fibroblast growth factor on hydrogen peroxide-induced damage to cardiomyocytes in vitro

AIM: To study the protective effect of non-mitogenic human acidic fibroblast growth factor (FGF) on cardiac oxidative injury in vivo.

METHODS: Ventricular cardiomyocytes were isolated from 1- to 3-d-old neonatal SD mice and cultured in Dulbecco’s minimum essential medium supplemented with 15% fetal bovine serum under an atmosphere of 50 mL/L CO₂-95% air at 37 °C, as well as assessed by immunocyto-chemical assay. We constructed the cardiomyocyte injury model by exposure to a certain concentration of H₂O₂. Cellular viability, superoxide dismutase (SOD) activity, leakage of maleic dialdehyde and anti-apoptosis effect were included to evaluate the cardiac protective effect of non-mitogenic human acidic FGF.

RESULTS: Over 50% of the cardiomyocytes beat spontaneously on the 2^nd d of culture and synchronously beat after being cultured for 3 d. Forty-eight hours after plating was completed, the purity of such cultures was 95% myocytes, assessed by an immunocytochemical assay. Cellular viability dramatically decreased with the increasing of the concentration of H₂O₂. Non-mitogenic human acidic FGF showed significant resistance to the toxic effect of H₂O₂, significantly increased the cellular viability as well as the activity of SOD, and dramatically decreased the leakage of maleic dialdehyde as well as the cellular apoptosis rate.

CONCLUSION: Hydrogen peroxide shows strong cytotoxicity to the cultured cardiac myocytes, and non-mitogenic human acidic FGF shows strong cardio-protective effect when exposed to a certain concentration of H₂O₂.

Endothelium-derived reactive oxygen species: their relationship to endothelium-dependent hyperpolarization and vascular tone

Opinions on the role of reactive oxygen species (ROS) in the vasculature have shifted in recent years, such that they are no longer merely regarded as indicators of cellular damage or byproducts of metabolism--they may also be putative mediators of physiological functions. Hydrogen peroxide (H2O2), in particular, can initiate vascular myocyte proliferation (and, incongruously, apoptosis), hyperplasia, cell adhesion, migration, and the regulation of smooth muscle tone. Endothelial cells express enzymes that produce ROS in response to various stimuli, and H2O2 is a potent relaxant of vascular smooth muscle. H2O2 itself can mediate endothelium-dependent relaxations in some vascular beds. Although nitric oxide (NO) is well recognized as an endothelium-derived dilator, it is also well established, particularly in the microvasculature, that another factor, endothelium-derived hyperpolarizing factor (EDHF), is a significant determinant of vasodilatory tone. This review primarily focuses on the hypothesis that H2O2 is an EDHF in resistance arteries. Putative endothelial sources of H2O2 and the effects of H2O2 on potassium channels, calcium homeostasis, and vascular smooth muscle tone are discussed. Furthermore, given the perception that ROS can more likely elicit cytotoxic effects than perform signalling functions, the arguments for and against H2O2 being an endogenous vasodilator are assessed.

Prevention of renal damage by alpha tocopherol in ischemia and reperfusion models of rats

Ischemia-reperfusion injury in rat kidneys most probably comes from oxidative stress, but the possible preventive effect of alpha-tocopherol (AT) treatment on this injury has not yet been established. Forty male Wistar rats were randomly divided into four groups. The left renal arteries of all rats except the controls were clamped to induce renal ischemia. The left kidneys of the rats in the ischemia group were removed following 40 min ischemia. The rats in the ischemia-reperfusion and ischemia-reperfusion-AT groups were treated similarly, but in these groups the renal arteries were re-perfused for 1 h following ischemia. The rats in the ischemia-reperfusion-AT group also received 10 mg/kg AT 3 h prior to ischemia. The specimens were examined histopathologically and ultrastructurally, and the tissue calcium levels were measured. Light microscope and ultrastructural examination showed that the greatest damage occurred in the ischemia-reperfusion group. The highest level of tissue calcium was also found in this group. In the ischemia-reperfusion-AT-treated group, less tissue damage and a lower tissue calcium concentration was found compared to both the ischemia and ischemia-reperfusion groups. Our results indicate that AT can reduce tissue damage after ischemia-reperfusion injury.

Oxidative injury of isolated cardiomyocytes: dependence on free radical species

The contribution of lipid peroxidation to myocardial injury by free radicals (FR) is still unclear. Consequently, we examined the functional damages inflicted on cultured rat cardiomyocytes (CM) during FR stress provoked by the xanthine/xanthine oxidase system (X/XO) or by a hydroperoxidized fatty acid ((9 Z, 11 E, 13 (S), 15 Z)-13-hydroperoxyocta-decatrienoic acid; 13-HpOTrE), in order to simulate in vitro the initial phase and the propagation phase of the FR attack, respectively. Transmembrane potentials were recorded with glass microelectrodes and contractions were monitored photometrically. The EPR spectroscopy showed that X/XO produced superoxide and hydroxyl radicals during 10 min. The X/XO system altered sharply and irreversibly the spontaneous electrical and mechanical activities of the CM. However, the gas chromatographic analysis showed that these drastic functional damages were associated with comparatively moderate membrane PUFA degradation. Moreover, the EPR analysis did not reveal the production of lipid-derived FR. 13-HpOTrE induced a moderate and reversible decrease in electrical parameters, with no change in CM contractions. These results indicate that the functional consequences of FR attack are dependent on the radical species present and do not support the idea that the membrane lipid breakdown is a major factor of myocardial oxidant dysfunction.

Vinculin, Talin, Integrin alpha6beta1 and laminin can serve as components of attachment complex mediating contraction force transmission from cardiomyocytes to extracellular matrix

Recently, we reported that cardiomyocytes adhere to extracellular matrix at costameres, the striated distribution of vinculin between Z-lines and the sarcolemma, where transmission of contraction forces from myofibrils to the extracellular matrix occurs. To identify other molecules involved in force transmission at costameres, we examined adult rat and embryonic chick cardiomyocytes cultured on coverslips or flexible thin silicone rubber substrata. Immunolocalization of talin showed a costameric, striated distribution, which corresponded to dark contacts with interference reflection microscopy. The molecules involved in substrate adhesion were cross-linked with the non-penetrating cross-linking agent Bis(sulfosuccinimidyl)-suberate and detected by immunohistochemical staining with anti-alpha6, alpha3, alphav, or beta1 integrin antibodies. Both alpha6 and beta1 showed costameric distributions, but alpha3 and alpha(v) did not. The distribution of laminin after cross-linking and extraction also showed a costameric distribution. When anti-integrin beta1 antibody was added to live cardiomyocytes grown on the silicone rubber substratum, the transmission of contraction forces was inhibited. These findings suggest that vinculin, talin, integrin alpha6beta1 and laminin system can be involved in transmission of contraction force to the extracellular matrix.

Peroxynitrite is a positive inotropic agent in atrial and ventricular fibres of the frog heart

1. We report opposite inotropic effects of NO donors in frog cardiac fibres. The negative effect, elicited by either 3-morpholino-sydnonimine (SIN-1) or S-nitroso-N-acetyl-penicillamine (SNAP), involved cyclic GMP (cGMP) production. However, SIN-1, unlike SNAP, could elicit a positive effect, in a superoxide dismutase (SOD)-sensitive manner. SIN-1, unlike SNAP, can release both NO and superoxide anion, the precursors of peroxynitrite (OONO-). The role of these messengers was examined. 2. Catalase did not reduce the positive inotropic effect of SIN-1. Thus, a conversion of superoxide anion into hydrogen peroxide was not involved in this effect. In addition, catalase did not modify the negative effects of SIN-1 plus SOD, or SNAP plus SOD. 3. LY 83583, a superoxide anion generator, elicited a positive inotropic effect, like SIN-1. The effect of LY 83583 was additive to the negative effects of SIN-1 or SNAP, and to the positive effect of SIN-1. Thus, superoxide anion generation, per se, did not account for the positive effect of SIN-1. 4. Authentic peroxynitrite (OONO-), but not mock-OONO- (negative control plus decomposed OONO-), exerted a dramatic positive inotropic effect in cardiac fibres. The effect of OONO- was larger in atrial fibres, as compared with ventricular fibres. 5. The positive effect of OONO- was not additive with that of SIN-1, suggesting a common mechanism of action. In contrast, the effects of either OONO- or SIN-1 were additive with the negative inotropic effect of SNAP. Furthermore, the effect of OONO-, like that of SIN-1, was not antagonized by 1H-[1,2,4]xidiazolo[4, 3-a]quinoxaline-1-one (ODQ; 10 microM), the guanylyl cyclase inhibitor. 6. The positive inotropic effects of SIN-1 and OONO- were not modified by hydroxyl radical scavengers, such as dimethyl-thio-urea (DMTU; 10 mM). 7. The positive inotropic effect of SIN-1 (100 microM) was abolished in sodium-free solutions, a treatment that eliminates the activity of the sodium-calcium exchanger. In contrast, the effect of SIN-1 was unchanged by a potassium channel inhibitor (tetraethyl-ammonium, 20 mM), or a sodium-potassium pump inhibitor (ouabain 10 microM). 8. We conclude that OONO- is a positive inotropic agent in frog cardiac fibres. The generation of OONO- accounts for the positive inotropic effect of SIN-1. OONO- itself was responsible for the positive inotropic effect, and appeared to modulate the activity of the sodium-calcium exchanger.

Protective effect of gamma-glutamylcysteinylethyl ester on dysfunction of the selenium-deficient rat heart

We investigated the protective effect of intracellular GSH against cardiac dysfunction in selenium (Se)-deficient neonatal rats and cultured fetal rat myocytes. A Se-deficient diet with or without daily subcutaneous injections of gamma-glutamylcysteinylethyl ester (gammay-GCE) (a membrane-permeating GSH precursor) was given to rats from gestation day 4 via the dam to postnatal day 14. Se deficiency induced a 62% incidence of electrocardiographic abnormalities such as sinus arrhythmias or extrasystole, a 63% reduction in dP/dt in the left ventricle, and an increase in thiobarbituric acid reacting substances (TBARS), but no ultrastructural cardiac lesions were observed. Administration of gamma-GCE increased the intracellular GSH concentration ([GSH]i) of both neonatal rat hearts and cultured fetal rat cardiac myocytes. gamma-GCE-like sodium selenite prevented the cardiac dysfunction and the TBARS increment. gamma-GCE also prevented H2O2 toxicity in the cultured myocytes. The Vmax, but not the Km, for GSH of Se-dependent GSH peroxidase (Se-Gpx) activity in Se-deficient rat heart homogenates was one-third that of normal rat heart homogenates. Although gamma-GCE did not affect the Se-Gpx Vmax and Km for GSH, it did induce a substantial and significant increase in [GSH]i, which was postulated to increase the velocity of H2O2 decomposition by Se-Gpx activity 1.6-fold. These data suggest that the increase in [GSH]i may have played a role in preventing the TBARS increase and cardiac dysfunction in Se-deficient rats.

Interference of H2O2 with stimulus-secretion coupling in mouse pancreatic beta-cells

1. We have reported previously that in mouse pancreatic beta-cells H2O2 hyperpolarizes the membrane and increases the ATP-sensitive K+ current recorded in the perforated patch configuration of the patch-clamp technique. The present study was undertaken to elucidate the underlying mechanisms. 2. The intracellular ATP concentration measured by chemoluminescence was reduced by H2O2. The ADP concentration increased in parallel during the first 10 min, resulting in a pronounced decrease in the ATP/ADP ratio. 3. Consistent with these results, glucose-stimulated insulin secretion from isolated islets was inhibited by H2O2. 4. Membrane hyperpolarization measured with intracellular microelectrodes in intact islets and inhibition of insulin secretion were counteracted by tolbutamide, indicating that the channels are still responsive to inhibitors and that the ATP concentration is not too low to trigger exocytosis. However, the sensitivity of the beta-cells to tolbutamide was reduced after treatment with H2O2. 5. H2O2 increased the intracellular Ca2+ activity ([Ca2+]i) in a biphasic manner. A first transient rise in [Ca2+]i due to mobilization of Ca2+ from intracellular stores was followed by a sustained increase, which was at least partly dependent on Ca2+ influx. The first phase seems to reflect Ca2+ mobilization from mitochondria. 6. Our results demonstrate that H2O2 interferes with glucose metabolism, which influences the membrane potential and ATP-sensitive K+ current via the intracellular concentration of ATP. These events finally lead to an inhibition of insulin secretion despite an increase in [Ca2+]i.

Metallothionein-overexpressing neonatal mouse cardiomyocytes are resistant to H2O2 toxicity

To study cellular and molecular events of cardiac protection by metallothionein (MT) from oxidative injury, a primary neonatal cardiomyocyte culture was established from a specific cardiac MT-overexpressing transgenic mouse model. Ventricular cardiomyocytes were isolated from 1- to 3-day-old neonatal mice and cultured in an Eagle's minimum essential medium supplemented with 20% fetal bovine serum under an atmosphere of 5% CO2-95% air at 37 degreesC. Forty-eight hours after plating was completed, the purity of such cultures was 95% myocytes, assessed by an immunocytochemical assay. Over 80% of the cardiomyocytes beat spontaneously on the first day of culture and synchronously in a confluent monolayer after the sixth day of culture. Cellular MT concentrations in the transgenic cardiomyocytes before culturing and on the sixth day postculturing were about seven- and twofold higher than nontransgenic controls, respectively. However, there were no significant differences in cell morphology, glutathione content, and antioxidant enzymatic activities between these two types of cardiomyocytes. When these cells were challenged by H2O2, the transgenic cardiomyocytes displayed a significant resistance to the toxic effect of this oxidant, as measured by cell viability, lactate dehydrogenase leakage, and morphological alterations. In addition, the transgenic cells were highly protected from H2O2-induced lipid peroxidation. These observations demonstrate that MT protects the cultured cardiomyocytes from H2O2 toxicity by preventing its interaction with macromolecules such as lipids, and this cultured primary neonatal mouse cardiomyocyte system provides a valuable tool to directly study cellular and molecular events of MT in cardiac protection against oxidative injury.

Activation of angiotensinogen gene in cardiac myocytes by angiotensin II and mechanical stretch

Circulating and cardiac renin-angiotensin systems (RAS) play important roles in the development of cardiac hypertrophy. Mechanical stretch of cardiac myocytes induces secretion of ANG II and evokes hypertrophic responses. Angiotensinogen is a unique substrate of the RAS. This study was performed to examine the regulation of the angiotensinogen gene in cardiac myocytes in response to ANG II and stretch. ANG II and stretch significantly increased the levels of angiotensinogen mRNA in cardiac myocytes. Actinomycin D completely inhibited ANG II- and stretch-mediated increases in angiotensinogen mRNA. Although CV-11974 abolished ANG II-mediated increases in mRNA level and promoter activity of the angiotensinogen gene, the inhibition of stretch-mediated activation by CV-11974 was significant but not complete. These results indicate that ANG II activates transcription of the angiotensinogen gene exclusively via ANG II type 1-receptor pathway and that stretch activates such transcription mainly via the same pathway in cardiac myocytes. Furthermore, factors other than ANG II may also be involved in stretch-mediated activation of the angiotensinogen gene in cardiac myocytes.

gp130 is involved in stretch-induced MAP kinase activation in cardiac myocytes

We have recently reported that mitogen activated protein kinase (MAP kinase) is activated by the stretch of the cultured cardiac myocytes in the angiotensin II deficient state in the angiotensinogen-deficient mice (Atg-/-), suggesting that factors other than the cardiac renin-angiotensin system are involved in the stretch-induced MAP kinase activation. We examined the contribution of cytokines using RX435, an anti-gp130 antibody. Leukemia inhibitory factor, which is one of the cytokines and has the common receptor subunit gp130, activated MAP kinase and the response was completely blocked by pretreatment of the Atg-/- cardiac myocytes with RX435. RX435 pretreatment greatly reduced stretch-induced activation of MAP kinase in Atg-/- cardiac myocytes. Interestingly, the same results were obtained in the cardiac myocytes of control mice. These results suggest that cytokine-gp130 may play a role in the stretch-induced MAP kinase activation independently of Ang II in cardiac myocytes.

Correlation between direct ESR spectroscopic measurements and electromechanical and biochemical assessments of exogenous free radical injury in isolated rat cardiac myocytes

Reactive free radical species appear to be involved in the ischemic injury of cardiac muscle, although the mechanisms by which oxygen-derived free radicals affect the heart cell function are not known. In the present study, cultured ventricular myocytes were exposed to an exogenous oxygen radical generating system. The myocyte-enriched, primary cultures were prepared from ventricles of new-born rat heart and exposed to a xanthine/xanthine oxidase (X+XO) system. The transmembrane potentials were recorded with glass microelectrodes. Cell contractions were monitored photometrically. The release of lactate dehydrogenase (LDH) in the medium was analysed. Quantitative measurement and the time course of the radical generation were performed by the electron paramagnetic resonance (EPR) spin trapping technique with the spin trap 5,5-dimethyl-1-pyroline-N-oxide (DMPO). We verified that X and XO alone had no significant functional and biochemical effects. The X+XO system produced a rapid decrease in the action potential amplitude. This effect was accompanied by a strong decrease in contractility and spontaneous rate. The time course of these functional defects were correlated with a progressive efflux of LDH from the cardiomyocytes. Prolonging the exposure to the X+XO system provoked the cessation of the spontaneous beatings and the progressive loss of the resting diastolic potential, together with a near total release of the cellular LDH. The LDH release and the functional depression were both efficiently prevented by catalase. On the contrary, superoxide dismutase (SOD) slowed down but did not protect against the functional and biochemical effects of the free radicals. In comparison, the EPR spectra obtained indicated that the X+XO system was associated with an important generation of superoxide anions but also with a small hydroxyl production. SOD scavenged the superoxide but a small .OH production persisted. Catalase (CAT) did not modify the superoxide generation but decreased the hydroxyl adduct formation. These results suggest that, although the generation of superoxide anions by the X+XO system was higher than the hydroxyl production, the functional injury and enzyme leakage seemed mainly mediated through a hydrogen peroxide-hydroxyl radical pathway. Cultured ventricular myocytes can be thus used as a valuable model to investigate the cellular mechanism of oxidant-induced damage in the heart.

Stretch-induced MAP kinase activation in cardiomyocytes of angiotensinogen-deficient mice

The renin-angiotensin system plays an important role in the hypertrophic responses in cardiac myocytes through the activation of signal transduction pathways and expression of oncogenes. In the present study, we examined mechanical stretch-induced activation of mitogen-activated protein kinases (MAP kinases) using cultured cardiac myocytes derived from neonatal angiotensinogen gene deficient mice (Agt-/-) and neonatal wild type mice (Agt+/+). Within 2 minutes of being added to cardiac myocytes, angiotensin II activated MAP kinases and the response was completely blocked by pretreatment of the cardiac myocytes with CV-11974, a selective antagonist of angiotensin II type 1 receptors. Interestingly, mechanical stretch resulted in significantly greater activation of MAP kinases in Agt-/- cardiac myocytes than in Agt+/+ cardiac myocytes. CV-11974 failed to suppress the stretch-induced activation of MAP kinases in Agt-/- cardiac myocytes while it inhibited the activation in Agt+/+ cardiac myocytes. BQ123, an endothelin type A receptor antagonist, had no effect on stretch-induced activation of MAP kinases in cardiac myocytes from either mouse strain. These results suggest that cardiac RAS is important for stretch-induced MAP kinase activation in Agt+/+ cardiac myocytes; however, angiotensin II is not indispensable for mechanical stretch-induced activation of MAP kinases in Agt-/- cardiac myocytes.

Detachment of cultured cells from the substratum induced by the neutrophil-derived oxidant NH2Cl: synergistic role of phosphotyrosine and intracellular Ca2+ concentration

The neutrophil-derived, membrane-permeating oxidant, NH2Cl, (but not the non-membrane-permeating chloramine, taurine-NHCl) induced detachment of fetal mouse cardiac myocytes and other cell types (fibroblasts, epithelial cells, and endothelial cells) from the culture dish, concomitant with cell shrinkage ("peeling off"). Stimulated human neutrophils also induced peeling off of cultured mouse cardiac myocytes when the latter were pretreated with inhibitors of .OH and elastase. Immunofluorescence microscopy revealed that the NH2Cl-induced peeling off of WI-38 fibroblasts is accompanied by disorganization of integrin alpha 5 beta 1, vinculin, stress fibers, and phosphotyrosine (p-Tyr)-containing proteins. Decrease in the content of the p-Tyr-containing proteins of the NH2Cl-treated cells was analyzed by immunoblotting techniques. Coating of fibronectin on the culture dish prevented both NH2Cl-induced peeling off and a decrease in p-Tyr content. Preincubation with a protein-tyrosine phosphatase inhibitor, sodium orthovanadate (Na3VO4), also prevented NH2Cl-induced peeling off, suggesting that dephosphorylation of p-Tyr is necessary for peeling off. NH2Cl-induced peeling off was accompanied by an increase in intracellular Ca2+ concentration ([Ca2+]i) in mouse cardiac myocytes and WI-38 fibroblasts. The absence of extracellular Ca2+ prevented both NH2Cl-induced peeling off and increased [Ca2+]i, both of which did occur on subsequent incubation of the cells in Ca2+-containing medium. These observations suggest that an increase in [Ca2+]i is also necessary for peeling off. Depletion of microsomal and cytosolic Ca2+ by incubation with the microsomal Ca2+-ATPase inhibitor 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ) plus EGTA prevented both NH2Cl-induced increases in [Ca2+]i and peeling off. Direct inhibition of microsomal Ca2+ pump activity by NH2Cl may participate in the NH2Cl-induced [Ca2+]i increment. A combination of p-Tyr dephosphorylation by genistein (an inhibitor of tyrosine kinase) and an increase in [Ca2+]i by BHQ could also induce peeling off. All these observations suggest a synergism between p-Tyr dephosphorylation and increased [Ca2+]i in NH2Cl-induced peeling off.