Prevention of ischemia/reperfusion-induced cardiac apoptosis and injury by melatonin is independent of glutathione peroxdiase 1

Abstract: Free‐radical generation is one of the primary causes of myocardial ischemia/reperfusion (I/R) injury. Melatonin is an efficient free‐radical scavenger and induces the expression of antioxidant enzymes. We have previously shown that melatonin can prevent free‐radical‐induced myocardial injury. To date, the mechanism underlying melatonin’s cardioprotective effect is not clear. In this study, we assessed the ability of melatonin to protect against I/R injury in mice deficient in glutathione peroxidase 1 (Gpx1). Mice hearts were subjected to 40 min of global ischemia in vitro followed by 45 min of reperfusion. Myocardial I/R injury (expressed as % of recovery of left ventricular developed pressure × heart rate) was exacerbated in mice deficient in Gpx1 (51 ± 3% for Gpx1^+/+ mice versus 31 ± 6% for Gpx1^?/? mice, P < 0.05). Administration of melatonin for 30 min protected against I/R injury in both Gpx1^+/+ mice (72 ± 4.8%) and Gpx1^?/? mice (63 ± 4.7%). This protection was accompanied by a significant improvement in left ventricular end‐diastolic pressure and a twofold decrease in lactate dehydrogenase (LDH) level released from melatonin‐treated hearts. In another set of experiments, mice were subjected to 50 min of ligation of the left descending anterior coronary artery in vivo followed by 4 hr of reperfusion. The infarct sizes, expressed as the percentage of the area at risk, were significantly larger in Gpx1^?/? mice than in Gpx1^+/+ mice (75 ± 9% versus 54 ± 6%, P < 0.05) and were reduced significantly in melatonin‐treated mice (31 ± 3.7% Gpx1^?/? mice and 33 ± 6.0% Gpx1^+/+ mice). In hearts subjected to 30 min of coronary artery occlusion followed by 3 hr of reperfusion, melatonin‐treated hearts had significantly fewer in situ oligo ligation‐positive myocytes and less protein nitration. Our results demonstrate that the cardioprotective function of melatonin is independent of Gpx1.     

SR-targeted CaMKII inhibition improves SR Ca²+ handling, but accelerates cardiac remodeling in mice overexpressing CaMKIIδC

Cardiac myocyte overexpression of CaMKIIδ_C leads to cardiac hypertrophy and heart failure (HF) possibly caused by altered myocyte Ca²⁺ handling. A central defect might be the marked CaMKII-induced increase in diastolic sarcoplasmic reticulum (SR) Ca²⁺ leak which decreases SR Ca²⁺ load and Ca²⁺ transient amplitude. We hypothesized that inhibition of CaMKII near the SR membrane would decrease the leak, improve Ca²⁺ handling and prevent the development of contractile dysfunction and HF. To test this hypothesis we crossbred CaMKIIδ_C overexpressing mice (CaMK) with mice expressing the CaMKII-inhibitor AIP targeted to the SR via a modified phospholamban (PLB)-transmembrane-domain (SR-AIP). There was a selective decrease in the amount of activated CaMKII in the microsomal (SR/membrane) fraction prepared from these double-transgenic mice (CaMK/SR-AIP) mice. In ventricular cardiomyocytes from CaMK/SR-AIP mice, SR Ca²⁺ leak, assessed both as diastolic Ca²⁺ shift into SR upon tetracaine in intact myocytes or integrated Ca²⁺ spark release in permeabilized myocytes, was significantly reduced. The reduced leak was accompanied by enhanced SR Ca²⁺ load and twitch amplitude in double-transgenic mice (vs. CaMK), without changes in SERCA expression or NCX function. However, despite the improved myocyte Ca²⁺ handling, cardiac hypertrophy and remodeling was accelerated in CaMK/SR-AIP and cardiac function worsened. We conclude that while inhibition of SR localized CaMKII in CaMK mice improves Ca²⁺ handling, it does not necessarily rescue the HF phenotype. This implies that a non-SR CaMKIIδ_C exerts SR-independent effects that contribute to hypertrophy and HF, and this CaMKII pathway may be exacerbated by the global enhancement of Ca transients.     

Melatonin ameliorates calcium homeostasis in myocardial and ischemia-reperfusion injury in chronically hypoxic rats

Chronic hypoxia (CH) leads to the deterioration of myocardial functions with impaired calcium handling in the sarcoplasmic reticulum (SR), which may be mediated by oxidative stress. We hypothesized that administration of antioxidant melatonin would protect against cardiac and ischemia-reperfusion (I/R) injury by ameliorating SR calcium handling. Adult Sprague-Dawley rats that had received a daily injection of melatonin or vehicle were exposed to 10% oxygen for 4 wk. The heart of each rat was then dissected and perfused using a Langendorff apparatus. The ratio of heart-to-body weight, ventricular hypertrophy and hematocrit were increased in the hypoxic rats compared with the normoxic controls. Malondialdehyde levels were also increased in the heart of hypoxic rats and were lowered by the treatment of melatonin. The hearts were subjected to left coronary artery ischemia (30 min) followed by 120-min reperfusion. Lactate dehydrogenase leakage before ischemia, during I/R and infarct size of the isolated perfused hearts were significantly elevated in the vehicle-treated hypoxic rats but not in the melatonin-treated rats. Spectroflurometric studies showed that resting calcium levels and I/R-induced calcium overload in the cardiomyocytes were more significantly altered in the hypoxic rats than the normoxic controls. Also, the hypoxic group had decreased levels of the SR calcium content and reduced amplitude and decay time of electrically induced calcium transients, indicating impaired contractility and SR calcium re-uptake. Moreover, there were reductions in protein expression of calcium handling proteins, markedly shown at the level of SR-Ca(2+) ATPase (SERCA) in the heart of hypoxic rats. Melatonin treatment significantly mitigated the calcium handling in the hypoxic rats by preserving SERCA expression. The results suggest that melatonin is cardioprotective against CH-induced myocardial injury by improving calcium handling in the SR of cardiomyocytes via an antioxidant mechanism.     

Melatonin induces calcium release from CCK‐8‐ and thapsigargin‐sensitive cytosolic stores in pancreatic AR42J cells

Abstract: Melatonin is produced following circadian rhythm with high levels being released at night and has been implicated in the regulation of physiological processes in major tissues, including the pancreas. The aim of our study was to examine the effects of melatonin on intracellular free Ca²⁺ concentration ([Ca²⁺]_c) in AR42J pancreatic cells. Our results show that stimulation of cells with 1 nm cholecystokinin (CCK)‐8 led to a transient increase in [Ca²⁺]_c followed by a decrease towards a value close to the prestimulation level. Melatonin (at the concentrations 1, 10, 100 μm and 1 mm ) induced changes in [Ca²⁺]_c that consisted of single or short lasting spikes in the form of oscillations or slow transient increases followed by a slow reduction towards a value close to the resting level. Depletion of intracellular Ca²⁺ stores by stimulation of cells with 1 nm CCK‐8 or 1 μm thapsigargin (Tps) blocked Ca²⁺ responses evoked by melatonin in the majority of cells. Conversely, prior stimulation of cells with 1 mm melatonin in the absence of extracellular Ca²⁺ inhibited Ca²⁺ mobilization in response to a secondary application of CCK‐8 or Tps. In summary, our results show that melatonin releases Ca²⁺ from intracellular stores and can therefore modulate the responses of the pancreas to CCK‐8. The source for Ca²⁺ mobilization most probably is the endoplasmic reticulum. These data raise the possibility that melatonin also involves Ca²⁺ signalling, in addition to other intracellular messengers, to modulate cellular function.     

Phytomelatonin receptor PMTR1‐mediated signaling regulates stomatal closure in Arabidopsis thaliana

Melatonin has been detected in plants in 1995; however, the function and signaling pathway of this putative phytohormone are largely undetermined due to a lack of knowledge about its receptor. Here, we discovered the first phytomelatonin receptor (CAND2/PMTR1) in Arabidopsis thaliana and found that melatonin governs the receptor‐dependent stomatal closure. The application of melatonin induced stomatal closure through the heterotrimeric G protein α subunit‐regulated H₂O₂ and Ca²⁺ signals. The Arabidopsis mutant lines lacking AtCand2 that encodes a candidate G protein‐coupled receptor were insensitive to melatonin‐induced stomatal closure. Accordingly, the melatonin‐induced H₂O₂ production and Ca²⁺ influx were completely abolished in cand2. CAND2 is a membrane protein that interacts with GPA1 and the expression of AtCand2 was tightly regulated by melatonin in various organs and guard cells. CAND2 showed saturable and specific ¹²⁵I‐melatonin binding, with apparent K_d (dissociation constant) of 0.73 ± 0.10 nmol/L (r² = .99), demonstrating this protein is a phytomelatonin receptor (PMTR1). Our results suggest that the phytomelatonin regulation of stomatal closure is dependent on its receptor CAND2/PMTR1‐mediated H₂O₂ and Ca²⁺signaling transduction cascade.     

Histidine-rich calcium binding protein: The new regulator of sarcoplasmic reticulum calcium cycling

The histidine-rich calcium binding protein (HRC) is a novel regulator of sarcoplasmic reticulum (SR) Ca²⁺-uptake, storage and release. Residing in the SR lumen, HRC binds Ca²⁺ with high capacity but low affinity. In vitro phosphorylation of HRC affects ryanodine affinity of the ryanodine receptor (RyR), suggesting a functional role of HRC on SR Ca²⁺-release. Indeed, acute HRC overexpression in isolated rodent cardiomyocytes decreases Ca²⁺-induced Ca²⁺-release, increases SR Ca²⁺-load, and impairs contractility. The HRC effects on RyR may be regulated by the Ca²⁺-sensitivity of its interaction with triadin. However, HRC also affects the SR Ca²⁺-ATPase, as shown by HRC overexpression in transgenic mouse hearts, which resulted in reduced SR Ca²⁺-uptake rates, cardiac remodeling and hypertrophy. In fact, in vitro generated evidence suggests that HRC directly interacts with SR Ca²⁺-ATPase2, supporting a dual role of HRC in Ca²⁺-homeostasis: regulation of both SR Ca²⁺-uptake and Ca²⁺-release. Furthermore, HRC plays an important role in myocyte differentiation and in antiapoptotic cardioprotection against ischemia/reperfusion induced cardiac injury. Interestingly, HRC has been linked with familiar cardiac conduction disease and an HRC polymorphism was shown to associate with malignant ventricular arrhythmias in the background of idiopathic dilated cardiomyopathy. This review summarizes studies, which have established the critical role of HRC in Ca²⁺-homeostasis, suggesting its importance in cardiac physiology and pathophysiology.     

Melatonin triggers PKA activation in the rodent malaria parasite Plasmodium chabaudi

Abstract: Calcium (Ca²⁺) is a critical regulator of many aspects of the Plasmodium reproductive cycle. In particular, intra‐erythrocyte Plasmodium parasites respond to circulating levels of the melatonin in a process mediated partly by intracellular Ca²⁺. Melatonin promotes the development and synchronicity of parasites, thereby enhancing their spread and worsening the clinical implications. The signalling mechanisms underlying the effects of melatonin are not fully established, although both Ca²⁺ and cyclic AMP (cAMP) have been implicated. Furthermore, it is not clear whether different strains of Plasmodium use the same, or divergent, signals to control their development. The aim of this study was to explore the signalling mechanisms engaged by melatonin in P. chabaudi, a virulent rodent parasite. Using parasites at the throphozoite stage acutely isolated from mice erythrocytes, we demonstrate that melatonin triggers cAMP production and protein kinase A (PKA) activation. Interestingly, the stimulation of cAMP/PKA signalling by melatonin was dependent on elevation of Ca²⁺ within the parasite, because buffering Ca²⁺ changes using the chelator BAPTA prevented cAMP production in response to melatonin. Incubation with melatonin evoked robust Ca²⁺ signals within the parasite, as did the application of a membrane‐permeant analogue of cAMP. Our data suggest that P. chabaudi engages both Ca²⁺ and cAMP signalling systems when stimulated by melatonin. Furthermore, there is positive feedback between these messengers, because Ca²⁺ evokes cAMP elevation and vice versa. Melatonin more than doubled the observed extent of parasitemia, and the increase in cAMP concentration and PKA activation was essential for this effect. These data support the possibility to use melatonin antagonists or derivates in therapeutic approach.     

Mitochondrial free calcium regulation in hypoxia and reoxygenation: relation to cellular injury

Summary The role of mitochondria in myocardial ischemic and hypoxic injury is discussed. Increases in mitochondrial Ca content and ionized Ca²⁺ concentration are observed during and after ischemic and hypoxic exposure and have traditionally been considered to impair mitochondrial function. New data are discussed in which it is shown that increases in mitochondrial [Ca²⁺] do not necessarily reflect irreversible myocyte injury. Further, it is shown that irreversible cellular injury may occur in hypoxic myocytes in association with increases in mitochondrial [Ca²⁺] that would ordinarily be considered to fall within a physiologic range. The significance of these observations is considered in context with observations relating to the assessment of post-hypoxic mitochondrial function.Supported in part by National Heart, Lung and Blood Institute Grants R01 HL-42050 and K08 HL-02539     

Increased susceptibility to Ca2+‐induced permeability transition and to cytochrome c release in rat heart mitochondria with aging: effect of melatonin

Abstract: Aging is associated with a decline of cardiac function. The mitochondrial permeability transition (MPT) may be a factor in cardiac dysfunction associated with aging. We investigated the effect of aging and long‐term treatment with melatonin (approximately 10 mg/kg b.w./day for 2 months), a known natural antioxidant, on the susceptibility to Ca²⁺‐induced MPT opening and cytochrome c release in rat heart mitochondria. The mitochondrial content of normal and oxidized cardiolipin as a function of aging and melatonin treatment was also analyzed. Mitochondria from aged rats (24 month old) displayed an increased susceptibility to Ca²⁺‐induced MPT opening, associated with an elevated release of cytochrome c, when compared with young control animals (5 month old). Melatonin treatment counteracted both these processes. Aging was also associated with an oxidation/depletion of cardiolipin which could be counteracted as well by melatonin. It is proposed that the increased level of oxidized cardiolipin could be responsible, at least in part, for the increased susceptibility to Ca²⁺‐induced MPT opening and cytochrome c release in rat heart mitochondria with aging. Melatonin treatment counteracts both these processes, most likely, by preventing the oxidation/depletion of cardiolipin. Our results might have implications in the necrotic and apoptotic myocytes cell death in aged myocardium, particularly in ischemia/reperfusion injury.     

Visualization of melatonin's multiple mitochondrial levels of protection against mitochondrial Ca2+-mediated permeability transition and beyond in rat brain astrocytes

Abstract: Melatonin protects cells against various types of oxidative stress‐induced apoptosis due primarily to its ability to effectively scavenge pathological and disease condition‐augmented generation of mitochondrial reactive oxygen species (mROS). Once produced, mROS indiscriminately damage mitochondrial components and more importantly they crucially activate directly the mitochondrial permeability transition (MPT), one of the critical mechanisms for initiating post mitochondrial apoptotic signaling. Whether or not melatonin targets directly the MPT, however, remains inconclusive, particularly during oxidative stress. This study, thus, investigated this possibility of an ‘oxidation free Ca²⁺ stress’ in the presence of vitamin E after ionomycin exposure as a sole Ca²⁺‐mediated MPT in order to exclude melatonin’s primary antioxidative effects as well as Ca²⁺‐mediated oxidative stress. The studies were carried out using cultured rat brain astrocytes RBA‐1. With the application of laser scanning multiple fluorescence imaging microscopy, we visualized for the first time multiple mitochondrial protective effects provided by melatonin during Ca²⁺ stress. First, melatonin, due to its primary antioxidative actions, completely prevented mCa²⁺‐induced mROS formation during ionomycin exposure. Secondly, when melatonin^’s antioxidative effects were prevented due to the addition of vitamin E, melatonin significantly prevented mCa²⁺‐mediated MPT and apoptosis suggesting its direct targeting of the MPT. Surprisingly, in the presence of cyclosporin A, a MPT inhibitor, melatonin reduced further mCa²⁺‐mediated apoptosis during ionomycin exposure also suggesting its targeting beyond the MPT. As astrocytes are actively involve in regulating synaptic transmission and neurovascular coupling in the CNS, these multiple mitochondrial layers of protection provided by melatonin against mCa²⁺‐and/or mROS‐mediated apoptosis in astrocytes may be crucial for future therapeutic prevention and treatment of astrocyte‐mediated neurodegenerative diseases in the CNS.     

Sarcoplasmic Reticulum Function in Murine Ventricular Myocytes Overexpressing SR CaATPase

To examine the effects of the overexpression of sarcoplasmic reticulum (SR) CaATPase on function of the SR and Ca²⁺homeostasis, we measured [Ca²⁺]_itransients (fluo-3), and L-type Ca²⁺currents (I_Ca,L), Na/Ca exchanger currents (I_Na/Ca), and SR Ca²⁺content with voltage clamp in ventricular myocytes isolated from wild type (WT) mice and transgenic (SRTG) mice. The amplitude of [Ca²⁺]_itransients was insignificantly increased in SRTG myocytes, while the diastolic [Ca²⁺]_itended to be lower. The initial and terminal declines of [Ca²⁺]_itransients were significantly accelerated in SRTG myocytes, implying a functional upregulation of the SR CaATPase. We examined the functional contribution of only the SR CaATPase to the initial and the terminal phase of the decline of [Ca²⁺]_i, by abruptly inhibiting Na/Ca exchange with a rapid switcher device. The rate of [Ca²⁺] decline mediated by the SR CaATPase was increased by 40% in SRTG compared with WT myocytes. The function of the L-type Ca²⁺channel was unchanged in SRTG myocytes, while INa/Ca density was slightly (10%) decreased. Measured SR Ca²⁺content was significantly increased by 29% in SRTG myocytes. Thus, overexpression of SR CaATPase markedly accelerates the decline of [Ca²⁺]_itransients, and induces an increase in SR Ca²⁺content, with some downregulation of the Na/Ca exchanger.     

Prevention of Ventricular Arrhythmia and Calcium Dysregulation in a Catecholaminergic Polymorphic Ventricular Tachycardia Mouse Model Carrying Calsequestrin‐2 Mutation

Arrhythmia Prevention in CPVT . Background: Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a familial arrhythmic syndrome caused by mutations in genes encoding the calcium‐regulation proteins cardiac ryanodine receptor (RyR2) or calsequestrin‐2 (CASQ2). Mechanistic studies indicate that CPVT is mediated by diastolic Ca²⁺ overload and increased Ca²⁺ leak through the RyR2 channel, implying that treatment targeting these defects might be efficacious in CPVT. Method and results: CPVT mouse models that lack CASQ2 were treated with Ca²⁺‐channel inhibitors, β‐adrenergic inhibitors, or Mg²⁺. Treatment effects on ventricular arrhythmia, sarcoplasmic reticulum (SR) protein expression and Ca²⁺ transients of isolated myocytes were assessed. Each study agent reduced the frequency of stress‐induced ventricular arrhythmia in mutant mice. The Ca²⁺ channel blocker verapamil was most efficacious and completely prevented arrhythmia in 85% of mice. Verapamil significantly increased the SR Ca²⁺ content in mutant myocytes, diminished diastolic Ca²⁺ overload, increased systolic Ca²⁺ amplitude, and prevented Ca²⁺ oscillations in stressed mutant myocytes. Conclusions: Ca²⁺ channel inhibition by verapamil rectified abnormal calcium handling in CPVT myocytes and prevented ventricular arrhythmias. Verapamil‐induced partial normalization of SR Ca²⁺ content in mutant myocytes implicates CASQ2 as modulator of RyR2 activity, rather than or in addition to, Ca²⁺ buffer protein. Agents such as verapamil that attenuate cardiomyocyte calcium overload are appropriate for assessing clinical efficacy in human CPVT . (J Cardiovasc Electrophysiol, Vol. 22, pp. 316‐324, March 2011)     

Mediated protective effect of electroacupuncture pretreatment by miR-214 on myocardial ischemia/reperfusion injury

Background Electroacupuncture pretreatment plays a protective role in myocardial ischemia/reperfusion （I/R） injury and microRNAs （miRNAs） could act on various facets of cardiac function. However, the role of miRNAs in the cardioprotection by electroacupuncture pre-treatment on myocardial I/R injury remains unknown. The purpose of the study was to examine whether miR-214 was involved in cardio-protection by electroacupuncture. Methods Using rat myocardial I/R model, we examined the role of electroacupuncture pretreatment in myocardial I/R injury and analyzed the changes in the expression of miR-214. In addition, I/R was simulated in vitro by performing oxy-gen-glucose deprivation （OGD） on H9c2 cell cultures, and the effect of electroacupuncture pretreatment on I/R injury as well as expressional level of miR-214 were examined in vitro. Furthermore, the miR-214 mimic was transfected into OGD-treated H9c2 cells, we analyzed the cell apoptosis, lactate dehydrogenase （LDH） and creatine kinase （CK） activities, intracellular free Ca2＋concentration （[Ca2＋]i） as well as the relative protein levels of sodium/calcium exchanger 1（NCX1）, BCL2-like 11 （BIM）, calmodulin-dependent protein kinase IIδ（CaMKIIδ） and Cyclophilin D （CypD）. Results The in vivo results revealed that compared with the I/R group, the electroacupuncture pretreatment group showed significant decreased myocardial infarct size, as well as the increased indices of the cardiac function, including heart rate, mean arterial pressure, left ventricular systolic pressure and maximal rate for left ventricular pressure rising and declining （±dp/dt max）. In addition, electroacupuncture pretreatment could inhibit the elevation of LDH and CK activities induced by I/R injury. The quantitative PCR （qPCR） results demonstrated electroacupuncture pretreatment could provide cardioprotection against myocardial I/R injury in rats with miR-214 up-regulation. In the meanwhile, in vitro, electroacupuncture pretreatment protected     

Taxol,a Microtubule Stabilizer,Improves Cardiac Functional Recovery during Postischemic Reperfusion in Rat in Vitro

Aims: Microtubule disruption contributes to cellular and organic dysfunction, and is implicated in ischemia/reperfusion (I/R) injury. The purpose of this study was to explore the effects of taxol, a microtubule stabilizer, on cardiac functional recovery during reperfusion. Methods: Left ventricular developed pressure, left ventricular end‐diastolic pressure, maximal time derivatives of pressure and the severity of ventricular arrhythmias were analyzed in isolated rat heart. Microtubule structure was immunohistochemically measured. Apoptosis and necrosis was identified with TUNEL or TTC staining, respectively. Mitochondrial permeability transition pore (mPTP) mRNA expression was examined by real‐time polymerase chain reactions. mPTP opening, reactive oxygen species (ROS), and oxidative enzyme activities were measured with fluorometric or spectrophotometric techniques. Intracellular calcium concentration ([Ca²⁺]_i) and Ca²⁺ transients were examined by Fura‐2‐AM and Fluo‐3‐AM, respectively. Cytosolic cytochrome c, sarcoplasmic reticulum Ca²⁺‐ATPase (SERCA2), ryanodine receptors (RyR), phospholamban (PLB), and PLB phosphorylation were analyzed by Western blot. Effective refractory period (ERP) and afterpotential‐mediated activity were detected using microelectrode. Results: Taxol improved the functional recovery of post‐I/R. Taxol preserved the intact microtubule structure in reperfusion. mPTP mRNA expression was unchanged while the mPTP opening was reduced by taxol, and this effect was accompanied by the decreased ROS level caused by oxidative enzymes activities’ changes. Taxol reduced apoptosis and the level of cytosolic cytochrome c in reperfusion. Taxol also promoted rapid recovery of [Ca²⁺]_i, prevented reduction of the amplitude of Ca²⁺ transients and shortened the decay time of Ca²⁺ transients. The protein expression of SERCA2, RyR, and PLB remained unchanged in reperfusion. Taxol prevented the increase of Phospho‐Thr17‐PLB and Phospho‐Ser16‐PLB in reperfusion. In addition, taxol facilitated rapid recovery of ERP and counter‐acted afterpotential‐mediated activity. Conclusion: Taxol may effectively improve cardiac functional recovery during reperfusion via inhibiting mPTP opening, ameliorating abnormal calcium homeostasis, and reducing the substrates associated with arrhythmias.     

Ca2 + Sparks and Ca2 + waves are the subcellular events underlying Ca2 + overload during ischemia and reperfusion in perfused intact hearts

Abnormal intracellular Ca²⁺ cycling plays a key role in cardiac dysfunction, particularly during the setting of ischemia/reperfusion (I/R). During ischemia, there is an increase in cytosolic and sarcoplasmic reticulum (SR) Ca²⁺. At the onset of reperfusion, there is a transient and abrupt increase in cytosolic Ca^2+ +, which occurs timely associated with reperfusion arrhythmias. However, little is known about the subcellular dynamics of Ca²⁺ increase during I/R, and a possible role of the SR as a mechanism underlying this increase has been previously overlooked. The aim of the present work is to test two main hypotheses:  (1) An increase diastolic Ca²⁺ sparks frequency (cspf) constitutes a mayor substrate for the ischemia-induced diastolic Ca²⁺ increase; (2) an increase in cytosolic Ca²⁺ pro-arrhythmogenic events (Ca²⁺ waves), mediates the abrupt diastolic Ca²⁺ rise at the onset of reperfusion. We used confocal microscopy on mouse intact hearts loaded with Fluo-4. Hearts were submitted to global I/R (12/30 min) to assess epicardial Ca²⁺ sparks in the whole heart. Intact heart sparks were faster than in isolated myocytes whereas cspf was not different. During ischemia, cspf significantly increased relative to preischemia (2.07 ± 0.33 vs. 1.13 ± 0.20 sp/s/100 μm, n = 29/34, 7 hearts). Reperfusion significantly changed Ca²⁺ sparks kinetics, by prolonging Ca²⁺ sparks rise time and decreased cspf. However, it significantly increased Ca²⁺ wave frequency relative to ischemia (0.71 ± 0.14 vs. 0.38 ± 0.06 w/s/100 μm, n = 32/33, 7 hearts). The results show for the first time the assessment of intact perfused heart Ca^2 + sparks and provides direct evidence of increased Ca²⁺ sparks in ischemia that transform into Ca²⁺ waves during reperfusion. These waves may constitute a main trigger for reperfusion arrhythmias.     

Toll-like receptor-4 deficiency alleviates chronic intermittent hypoxia-induced renal injury,inflammation, and fibrosis

Background

Obstructive sleep apnea (OSA)-associated chronic kidney disease is mainly caused by chronic intermittent hypoxia (CIH) triggered renal damage. This study aims to investigate the role of toll-like receptor-4 (TLR4) in underlying mechanism involved chronic intermittent hypoxia (CIH)-induced renal damage.

Methods

C57BL/6J mice with normal TLR4 (TLR4 WT) or deficient TLR4 (TLR4 KO) were divided into four groups and exposed to normal air (NA) and CIH: TLR4 WT + NA, TLR4 KO + NA, TLR4 WT + CIH, and TLR4 KO + CIH. CIH lasted for 8 h/day and 7 days/week for 6 weeks. Renal injury and inflammation were evaluated by histology and ELISA. Renal tubular apoptosis, macrophages, and fibroblasts recruitment were determined by TUNEL assay, immunofluorescence, and western blot.

Results

In response to CIH, TLR4 deficiency alleviated renal histological injury, renal dysfunction, and fibrosis. TLR4 deficiency ameliorated renal dysfunction (serum BUN and creatinine) and tubular endothelial apoptosis determined by immunofluorescence staining of CD31 and TUNEL, and western blot of apoptotic protein (caspase-3, c-caspase-3, and Bax/Bcl-2 ratio). Furthermore, we also found TLR4 deficiency abrogated CIH-induced macrophages (CD68) and fibroblasts (α-SMA) recruitment, further reducing expression of extra-cellular matrix protein (collagen I and collagen IV) and inflammatory cytokines release (IL-6, TNF-α, and MCP-1). Finally, we used immunohistochemistry to demonstrate that TLR4 deficiency attenuated increased expression of MyD88 and NF-kB p65 after CIH treatment.

Conclusions

Our data suggest that TLR4 plays a vital role in CIH-induced renal injury, inflammation and fibrosis, and inhibition of TLR4 probably provides a therapeutic potential for CIH-induced kidney damage.

     

Local control of Ca-induced Ca release in mouse sinoatrial node cells

Emerging evidence from large animal models implicates Ca²⁺ regulation, particularly intracellular sarcoplasmic reticulum (SR) Ca²⁺ release, as essential for sinoatrial node (SAN) automaticity. However, despite the apparent importance of SR Ca²⁺ release to SAN cell function it is uncertain how SR Ca²⁺ release is controlled in SAN cells from mouse. Understanding mouse SAN SR Ca²⁺ release mechanism will allow improved understanding of results in studies on SAN from genetic mouse models of Ca²⁺ homeostatic proteins. Here we investigated the functional relationship between sarcolemmal Ca²⁺ influx and SR Ca²⁺ release at the level of single SAN cell, using simultaneous patch-clamp current recording and high resolution confocal Ca²⁺ imaging techniques. In mouse SAN cells, both Ca²⁺ channel currents and triggered SR Ca²⁺ transients displayed bell-shaped, graded function with the membrane potential. Moreover, the gain function for Ca²⁺-induced Ca²⁺ release (CICR) displayed a monotonically decreasing function with strong voltage dependence, consistent with a “local control” mechanism for CICR. In addition, we observed numerous discrete Ca²⁺ sparks at the voltage range of diastolic depolarization, in sharp contrast to the much lower frequency of sparks observed at resting potentials. We concluded that the “local control” mechanism of CICR is responsible for both local Ca²⁺ release during diastolic depolarization and the synchronized Ca²⁺ transients observed during action potential in SAN cells.     

The nuclear melatonin receptor RORα is a novel endogenous defender against myocardial ischemia/reperfusion injury

Circadian rhythm disruption or decrease in levels of circadian hormones such as melatonin increases ischemic heart disease risk. The nuclear melatonin receptors RORs are pivotally involved in circadian rhythm regulation and melatonin effects mediation. However, the functional roles of RORs in the heart have never been investigated and were therefore the subject of this study on myocardial ischemia/reperfusion (MI/R) injury pathogenesis. RORα and RORγ subtypes were detected in the adult mouse heart, and RORα but not RORγ was downregulated after MI/R. To determine the pathological consequence of MI/R‐induced reduction of RORα, we subjected RORα‐deficient staggerer mice and wild‐type (WT) littermates to MI/R injury, resulting in significantly increased myocardial infarct size, myocardial apoptosis and exacerbated contractile dysfunction in the former. Mechanistically, RORα deficiency promoted MI/R‐induced endoplasmic reticulum stress, mitochondrial impairments, and autophagy dysfunction. Moreover, RORα deficiency augmented MI/R‐induced oxidative/nitrative stress. Given the emerging evidence of RORα as an essential melatonin effects mediator, we further investigated the RORα roles in melatonin‐exerted cardioprotection, in particular against MI/R injury, which was significantly attenuated in RORα‐deficient mice, but negligibly affected by cardiac‐specific silencing of RORγ. Finally, to determine cell type‐specific effects of RORα, we generated mice with cardiomyocyte‐specific RORα overexpression and they were less vulnerable to MI/R injury. In summary, our study provides the first direct evidence that the nuclear melatonin receptor RORα is a novel endogenous protective receptor against MI/R injury and an important mediator of melatonin‐exerted cardioprotection; melatonin‐RORα axis signaling thus appears important in protection against ischemic heart injury.     

Effects of adrenalectomy and in vivo administration of dexamethasone on ATP-dependent calcium accumulation by sarcoplasmic reticulum from rat heart

ATP-dependent Ca²⁺ accumulation and (Mg²⁺ + Ca²⁺)ATPase activities were determined in cardiac sarcoplasmic reticulum (SR) from control, adrenalectomized and adrenalectomized-dexamethasone-treated rats. SR from adrenalectomized rats displayed significantly diminished rates (～40 to 50%, P < 0.001) of Ca²⁺ accumulation compared to SR from control rats. The decreased Ca²⁺ accumulating activity of the membranes from adrenalectomized compared to control rats could be observed at varying Ca²⁺ concentrations (0.5 to 11.9 μm); kinetic analysis showed that the velocity of Ca²⁺ transport but not the apparent affinity of the transport system for Ca²⁺ was altered by adrenalectomy. ATPase activities (with Mg²⁺ or with Mg²⁺ + Ca²⁺) of SR were not altered significantly following adrenalectomy. In vivo administration of dexamethasone to adrenalectomized rats led to a partial (75 to 80%) yet significant (P < 0.01 to 0.05) restoration of Ca²⁺ accumulating activity of SR towards normal; dexamethasone-treatment caused decrease in basal (Mg²⁺)ATPase activity and enhancement in (Mg²⁺ + Ca²⁺)ATPase activity. When added in vitro, dexamethasone had no effect on Ca²⁺ accumulating and ATPase activities of SR from control or adrenalectomized rats. These findings imply an important role for glucocorticoids in the maintenance of membrane Ca²⁺ transport function, and therefore, normal myocardial contractility.