Pre-treatment with the Na+/H+ exchange inhibitor cariporide delays cell-to-cell electrical uncoupling during myocardial ischemia

Autosis: a new form of cell death in myocardial ischemia-reperfusion injury

Cardiomyocytes undergo a variety of cell death events during myocardial ischemia‒reperfusion injury (MIRI). Understanding the causes of cardiomyocyte mortality is critical for the prevention and treatment of MIRI. Among the various types of cell death, autosis is a recently identified type of autophagic cell death with distinct morphological and chemical characteristics. Autosis can be attenuated by autophagy inhibitors but not reversed by apoptosis or necrosis inhibitors. In recent years, it has been shown that during the late phase of reperfusion, autosis is activated, which exacerbates myocardial injury. This article describes the characteristics of autosis, autophagic cell death, and the relationship between autophagic cell death and autosis; reviews the mechanism of autosis in MIRI; and discusses its clinical significance.

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

Ischemic Postconditioning Reduces Reperfusion Arrhythmias by Adenosine Receptors and Protein Kinase C Activation but Is Independent of KATP Channels or Connexin 43

Ischemic postconditioning (IPoC) reduces reperfusion arrhythmias but the antiarrhythmic mechanisms remain unknown. The aim of this study was to analyze IPoC electrophysiological effects and the role played by adenosine A1, A2A and A3 receptors, protein kinase C, ATP-dependent potassium (KATP) channels, and connexin 43. IPoC reduced reperfusion arrhythmias (mainly sustained ventricular fibrillation) in isolated rat hearts, an effect associated with a transient delay in epicardial electrical activation, and with action potential shortening. Electrical impedance measurements and Lucifer-Yellow diffusion assays agreed with such activation delay. However, this delay persisted during IPoC in isolated mouse hearts in which connexin 43 was replaced by connexin 32 and in mice with conditional deletion of connexin 43. Adenosine A1, A2A and A3 receptor blockade antagonized the antiarrhythmic effect of IPoC and the associated action potential shortening, whereas exogenous adenosine reduced reperfusion arrhythmias and shortened action potential duration. Protein kinase C inhibition by chelerythrine abolished the protective effect of IPoC but did not modify the effects on action potential duration. On the other hand, glibenclamide, a KATP inhibitor, antagonized the action potential shortening but did not interfere with the antiarrhythmic effect. The antiarrhythmic mechanisms of IPoC involve adenosine receptor activation and are associated with action potential shortening. However, this action potential shortening is not essential for protection, as it persisted during protein kinase C inhibition, a maneuver that abolished IPoC protection. Furthermore, glibenclamide induced the opposite effects. In addition, IPoC delays electrical activation and electrical impedance recovery during reperfusion, but these effects are independent of connexin 43.

Evaluating Novel Targets of Ischemia Reperfusion Injury in Pig Models

Coronary heart diseases are of high relevance for health care systems in developed countries regarding patient numbers and costs. Disappointingly, the enormous effort put into the development of innovative therapies and the high numbers of clinical studies conducted are counteracted by the low numbers of therapies that become clinically effective. Evidently, pre-clinical research in its present form does not appear informative of the performance of treatments in the clinic and, even more relevant, it appears that there is hardly any consent about how to improve the predictive capacity of pre-clinical experiments. According to the steadily increasing relevance that pig models have gained in biomedical research in the recent past, we anticipate that research in pigs can be highly predictive for ischemia-reperfusion injury (IRI) therapies as well. Thus, we here describe the significance of pig models in IRI, give an overview about recent developments in evaluating such models by clinically relevant methods and present the latest insight into therapies applied to pigs under IRI.

Selective Inhibition of Succinate Dehydrogenase in Reperfused Myocardium with Intracoronary Malonate Reduces Infarct Size

Inhibition of succinate dehydrogenase (SDH) with malonate during reperfusion reduces infarct size in isolated mice hearts submitted to global ischemia. However, malonate has toxic effects that preclude its systemic administration in animals. Here we investigated the effect of intracoronary malonate on infarct size in pigs submitted to transient coronary occlusion. Under baseline conditions, 50 mmol/L of intracoronary disodium malonate, but not lower concentrations, transiently reduced systolic segment shortening in the region perfused by the left anterior descending coronary artery (LAD) in open-chest pigs. To assess the effects of SDH inhibition on reperfusion injury, saline or malonate 10 mmol/L were selectively infused into the area at risk in 38 animals submitted to ischemia-reperfusion. Malonate improved systolic shortening in the area at risk two hours after 15 min of ischemia (0.18 ± 0.07 vs 0.00 ± 0.01 a.u., p = 0.025, n = 3). In animals submitted to 40 min of ischemia, malonate reduced reactive oxygen species production (MitoSOX staining) during initial reperfusion and limited infarct size (36.46 ± 5.35 vs 59.62 ± 4.00%, p = 0.002, n = 11), without modifying reperfusion arrhythmias. In conclusion, inhibition of SDH with intracoronary malonate during early reperfusion limits reperfusion injury and infarct size in pigs submitted to transient coronary occlusion without modifying reperfusion arrhythmias or contractile function in distant myocardium.

Effect of global cardiac ischemia on human ventricular fibrillation: insights from a multi-scale mechanistic model of the human heart

Acute regional ischemia in the heart can lead to cardiac arrhythmias such as ventricular fibrillation (VF), which in turn compromise cardiac output and result in secondary global cardiac ischemia. The secondary ischemia may influence the underlying arrhythmia mechanism. A recent clinical study documents the effect of global cardiac ischaemia on the mechanisms of VF. During 150 seconds of global ischemia the dominant frequency of activation decreased, while after reperfusion it increased rapidly. At the same time the complexity of epicardial excitation, measured as the number of epicardical phase singularity points, remained approximately constant during ischemia. Here we perform numerical studies based on these clinical data and propose explanations for the observed dynamics of the period and complexity of activation patterns. In particular, we study the effects on ischemia in pseudo-1D and 2D cardiac tissue models as well as in an anatomically accurate model of human heart ventricles. We demonstrate that the fall of dominant frequency in VF during secondary ischemia can be explained by an increase in extracellular potassium, while the increase during reperfusion is consistent with washout of potassium and continued activation of the ATP-dependent potassium channels. We also suggest that memory effects are responsible for the observed complexity dynamics. In addition, we present unpublished clinical results of individual patient recordings and propose a way of estimating extracellular potassium and activation of ATP-dependent potassium channels from these measurements.

Protection against myocardial ischemia-reperfusion injury in clinical practice

Even when reperfusion therapy is applied as early as possible, survival and quality of life are compromised in a considerable number of patients with ST-segment elevation acute myocardial infarction. Some cell death following transient coronary occlusion occurs during reperfusion, due to poor handling of calcium in the sarcoplasmic reticulum-mitochondria system, calpain activation, oxidative stress, and mitochondrial failure, all promoted by rapid normalization of intracellular pH. Various clinical trials have shown that infarct size can be limited by nonpharmacological strategies--such as ischemic postconditioning and remote ischemic conditioning--or by drugs--such as cyclosporine, insulin, glucagon-like peptide-1 agonists, beta-blockers, or stimulation of cyclic guanosine monophosphate synthesis. However, some clinical studies have yielded negative results, largely due to a lack of consistent preclinical data or a poor design, especially delayed administration. Large-scale clinical trials are therefore necessary, particularly those with primary clinical variables and combined therapies that consider age, sex, and comorbidities, to convert protection against reperfusion injury into a standard treatment for patients with ST-segment elevation acute myocardial infarction.

Disruption of chronic cariporide treatment abrogates myocardial ion homeostasis during acute ischemia reperfusion

Cariporide, an Na/H exchanger inhibitor, is a drug with cardioprotective properties. However, chronic treatment with cariporide may modify the protein phenotype of the cardiomyocytes. Disruption of the equilibrium between a cariporide-modified phenotype and the supply of cariporide could be deleterious. The aim of this study was to test the effects of this equilibrium rupture (EqR) on cardiac function at baseline and acute ischemia reperfusion. Rats were chronically treated with cariporide (2.5 mg·kg·d) or with placebo for 21 days, after which isolated Langendorff-mode heart perfusion experiments utilized cariporide-free buffer. During this type of perfusion, the drug is rapidly cleared from the cellular environment. After 30 minutes of stabilization, the hearts were subjected to global zero-flow ischemia (25 minutes) followed by reperfusion (45 minutes). Measures of mechanical function, oxygen consumption, lactate plus pyruvate, CO2 and proton release into the coronary effluent were determined. The gene and protein expression of proton extruders was also evaluated. Chronic cariporide administration followed by EqR reduced the expression of the Na/H exchanger, increased the expression of the HCO3 or Na exchanger, decreased monocarboxylate/H carrier expression, reduced the lactate plus pyruvate release but did not change the glucose oxidation rate and mechanical function compared with baseline conditions. The resulting low glycolytic rate was associated with a stronger contracture during ischemia. During reperfusion, the early release of acidic forms was higher and redirected toward the use of the Na/H and HCO3 /Na exchangers to the detriment of the safe monocarboxylate/H carrier. Both phenomena were assumed to increase the Na uptake and activate the Na/Ca exchanger, resulting in Na and Ca overload and further cellular damage. This explains the impaired recovery of the contractile function observed in the EqR group during reperfusion. In conclusion, although cariporide is usually cardioprotective, a disruption of its chronic treatment followed by an ischemia/reperfusion event can become deleterious.

Inhibition of the Na+/H+ exchanger protects the immature rabbit myocardium from ischemia and reperfusion injury

BACKGROUND

This study investigated the cardioprotective effects of pharmacologic pretreatment with HOE642, a selective Na(+)/H(+ )exchanger (NHE) isoform-1 inhibitor, in immature rabbit hearts, as compared with ischemic preconditioning (IPC).

METHODS

For this study, 36 isolated immature New Zealand white rabbit hearts were equilibrated on the Langendorff apparatus. They were randomly divided into three groups: control group, IPC group, and HOE642 group. The hearts in each group were subjected to 60 min of ischemia plus 60 min of reperfusion (I/R). In the IPC group, the hearts were preconditioned by 5 min of ischemia followed by 10 min of reperfusion before I/R. In the HOE642 group, the hearts were pretreated with HOE642 (5 mumol/l) for 15 min before I/R. Left ventricular performance (LVDP, +dp/dt(max), -dp/dt(max)), coronory artery flow (CF), myocardial water content, adenosine triphosphate (ATP), cardiac-specific enzymes (creatine kinase [CK], CK fraction MB [CK-MB], and lacate dehydrogenase [LDH]), and intracellular calcium content were measured. Myocardial ultrastructure was observed under transmission electron microscopy.

RESULTS

The recovery rates for left ventricular performance and CF in both the HOE642 and the IPC groups increased compared with those for the control subjects (p < 0.05). Moreover, the recovery rates for LVDP, +dp/dt(max), -dp/dt(max), and CF in the HOE642 group were markedly higher than in the IPC group at most time points of reperfusion (p < 0.05). Compared with the control group, CK, CK-MB, and LDH in the HOE642 group were decreased significantly (p < 0.05), whereas only LDH was reduced in the IPC group (p < 0.05). Water content was significantly reduced and ATP reserve was significantly increased in both the IPC and HOE642 groups (p < 0.05). However, compared with the IPC group, water content in the HOE642 group was significantly lower (81.26% +/- 1.26% vs 83.58% +/- 1.27%; p < 0.05) and ATP was significantly higher (21.46 +/- 2.40 vs 17.66 +/- 1.50 mug/g; p < 0.05). The HOE642 pretreatment exerted a better effect of reducing calcium overload than IPC (265.8 +/- 41.1 vs 408.5 +/- 56.8 mg/kg dry weight; p < 0.05). The blinded ultrastructural assessment under transmission electron microscopy showed that HOE642 brought about more myocyte salvage than IPC.

CONCLUSION

This study demonstrated that HOE642 pretreatment is superior to IPC against ischemia and reperfusion injury in isolated immature rabbit myocardium.

Reperfusion injury as a therapeutic challenge in patients with acute myocardial infarction

Cardiomyocyte death secondary to transient ischemia occurs mainly during the first minutes of reperfusion, in the form of contraction band necrosis involving sarcolemmal rupture. Cardiomyocyte hypercontracture caused by re-energisation and pH recovery in the presence of impaired cytosolic Ca(2+) control as well as calpain-mediated cytoskeletal fragility play prominent roles in this type of cell death. Hypercontracture can propagate to adjacent cells through gap junctions. More recently, opening of the mitochondrial permeability transition pore has been shown to participate in reperfusion-induced necrosis, although its precise relation with hypercontracture has not been established. Experimental studies have convincingly demonstrated that infarct size can be markedly reduced by therapeutic interventions applied at the time of reperfusion, including contractile blockers, inhibitors of Na(+)/Ca(2+) exchange, gap junction blockers, or particulate guanylyl cyclase agonists. However, in most cases drugs for use in humans have not been developed and tested for these targets, while the effect of existing drugs with potential cardioprotective effect is not well established or understood. Research effort should be addressed to elucidate the unsolved issues of the molecular mechanisms of reperfusion-induced cell death, to identify and validate new targets and to develop appropriate drugs. The potential benefits of limiting infarct size in patients with acute myocardial infarction receiving reperfusion therapy are enormous.

The modulatory effects of connexin 43 on cell death/survival beyond cell coupling

Connexins form a diverse and ubiquitous family of integral membrane proteins. Characteristically, connexins are assembled into intercellular channels that aggregate into discrete cell-cell contact areas termed gap junctions (GJ), allowing intercellular chemical communication, and are essential for propagation of electrical impulses in excitable tissues, including, prominently, myocardium, where connexin 43 (Cx43) is the most important isoform. Previous studies have shown that GJ-mediated communication has an important role in the cellular response to stress or ischemia. However, recent evidence suggests that connexins, and in particular Cx43, may have additional effects that may be important in cell death and survival by mechanisms independent of cell to cell communication. Connexin hemichannels, located at the plasma membrane, may be important in paracrine signaling that could influence intracellular calcium and cell survival by releasing intracellular mediators as ATP, NAD(+), or glutamate. In addition, recent studies have shown the presence of connexins in cell structures other than the plasma membrane, including the cell nucleus, where it has been suggested that Cx43 influences cell growth and differentiation. In addition, translocation of Cx43 to mitochondria appears to be important for certain forms of cardioprotection. These findings open a new field of research of previously unsuspected roles of Cx43 intracellular signaling.

Prostaglandin-mediated inhibition of Na+/H+ exchanger isoform 2 stimulates recovery of barrier function in ischemia-injured intestine

Prostaglandins stimulate repair of the ischemia-injured intestinal barrier in the porcine ileum through a mechanism involving cAMP-dependent Cl- secretion and inhibition of electroneutral Na+/H+ exchanger (NHE) activity. In the present study, we focused on the role of individual NHE isoforms in the recovery of barrier function. Ischemia-injured porcine ileal mucosa was mounted on Ussing chambers. Short-circuit current (I(sc)), transepithelial electrical resistance (TER), and isotopic fluxes of 22Na were measured in response to PGE2 and selective inhibitors of epithelial NHE isoforms. Immunoassays were used to assess the expression of NHE isoforms. Forty-five minutes of intestinal ischemia resulted in a 45% reduction in TER (P < 0.01). Near-complete restitution occurred within 60 min. Inhibition of NHE2 with HOE-694 (25 microM) added to the mucosal surface of the injured ileum stimulated significant elevations in TER, independent of changes in I(sc) and histological evidence of restitution. Pharmacological inhibition of NHE3 or NHE1 with mucosal S-3226 (20 microM) or serosal cariporide (25 microM), respectively, had no effect. Ischemia-injured tissues treated with mucosal S-3226 or HOE-694 exhibited equivalent reductions in mucosal-to-serosal fluxes of 22Na+ (by approximately 35%) compared with nontreated ischemia-injured control tissues (P < 0.05). Intestinal ischemia resulted in increased expression of the cytoplasmic NHE regulatory factor EBP50 in NHE2 but not in NHE3 immunoprecipitates. Selective inhibition of NHE2, and not NHE3, induces recovery of barrier function in the ischemia-injured intestine.

Acute and Chronic Regulation of the Renal Na+/H+ Exchanger NHE3 in Rats with STZ-Induced Diabetes mellitus

BACKGROUND

Early stages of diabetic nephropathy are characterized by alterations of glomerular filtration, increased tubular sodium and water reabsorption, and systemic volume expansion, which may be a major cause for the development of hypertension. As a significant fraction of renal salt and water transport is mediated by the proximal tubular Na+/H+ exchanger NHE3, we investigated its regulation in rats with STZ-induced diabetes mellitus.

METHODS

Male Sprague-Dawley rats were injected +/- streptozotocin (STZ, 60 mg/kg), and sacrificed after 2, 7 or 14 days. Renal cortical BBM vesicles were prepared to measure Na+/H+ exchange (NHE) activity and NHE3 protein abundance. Cortical NHE3 mRNA was extracted to perform Northern blot analysis. Pharmacological inhibitors were used in vivo and in vitro in order to identify isoform specificity conferring changes in NHE activity mediated by the diabetic milieu.

RESULTS

Compared to control rats, STZ rats were clearly hyperglycemic at all time points studied. NHE activity was significantly increased by 40 and 37% in diabetic rats after 7 and 14 days, respectively, but not after 2 days. The increase in Na+/H+ exchange activity was not inhibited by HOE-642 (3 microM). Administration of exogenous insulin to diabetic rats resulted in lower blood sugars, but not NHE activity. Moreover, serum glucose concentration did not correlate with NHE activity in any subgroup nor in all animals analyzed together. However, in STZ rats supplemented with exogenous insulin NHE activity was positively correlated with serum insulin concentrations (r = 0.86, p < 0.01). In vivo, the increase in NHE activity induced by STZ could be completely inhibited when rats were fed 6 ppm of HOE-642 with the diet over 14 days. The changes in Na+/H+ exchange activity were not paralleled by changes in NHE3 protein or mRNA abundance in diabetic rats at any of the time points investigated.

CONCLUSIONS

These results suggest that proximal tubular Na/H exchange activity is modified in the early stage of diabetes mellitus.

Ischemia-induced arrhythmia: the role of connexins, gap junctions, and attendant changes in impulse propagation

Sudden cardiac death accounts for more than half of all cardiovascular deaths in the US, and a large proportion of these deaths are attributed to ischemia-induced ventricular fibrillation. As such, the mechanisms underlying the initiation and maintenance of these lethal rhythms are of significant clinical and scientific interest. In large animal hearts, regional ischemia induces two phases of ventricular arrhythmia. The first phase (1A) occurs between 5 and 7 min after arrest of perfusion. This phase is associated with membrane depolarization, a mild intracellular and extracellular acidification and a small membrane depolarization. A second phase (1B) of ventricular arrhythmia occurs between 20 and 30 minutes after arrest of perfusion. This phase occurs at a time when ischemia-induced K+ and pH changes are relatively stable. The arrhythmia is presumed to relate to the process of cell-to-cell electrical uncoupling because a rapid increase of tissue impedance precedes the onset of the arrhythmia. Of note is that tissue resistance is primarily determined by the conductance properties of the gap junctions accounting for cell-to-cell coupling. Impulse propagation in heart is determined by active and passive membrane properties. An important passive cable property that is modulated by ischemia is intercellular resistance and is determined primarily by gap junctional conductance. As such changes in Impulse propagation during myocardial ischemia are determined by contemporaneous changes in active and passive membrane properties. Cellular K loss, intracellular and extracellular acidosis and membrane depolarization are important factors decreasing excitatory currents, while the collapse of the extracellular compartment and cell-to-cell electrical uncoupling increase the resistance to current flow. The time-course of cellular coupling is closely linked to a number of physiological processes including depletion of ATP, and accumulation of intracellular Ca2+. Hence, interventions such as ischemic preconditioning attenuate the effect of subsequent ischemia, delay the onset of cell-to-cell electrical uncoupling and likewise delay the onset of ischemia-induced arrhythmia.

Myocardial ischemia/reperfusion-injury, a clinical view on a complex pathophysiological process

Myocardial infarction is the major cause of death in the world. Over the last two decades, coronary reperfusion therapy has become established for the management of acute myocardial infarction (AMI). However, restoration of blood flow to previously ischemic myocardium results in the so-called ischemia/reperfusion (IR)-injury. The different clinical manifestations of this injury include myocardial necrosis, arrhythmia, myocardial stunning and endothelial- and microvascular dysfunction including the no-reflow phenomenon. The pathogenesis of ischemia/reperfusion injury consists of many mechanisms. Recently, there's increasing evidence for an important role in IR-injury on hypercontracture induced by high levels of cytosolic calcium or by low concentrations of ATP. In the last years, many studies on experimental models were investigated, but the clinical trials confirming these effects remain spare. Recently, the beneficial effect of Na(+)/H(+)-exchange inhibitor cariporide and of the oxygen-derived free radical (ODFR) scavenger vitamin E on coronary bypass surgery-induced IR-injury were demonstrated. Also recently, the beneficial effect of allopurinol on the recovery of left ventricular function after rescue balloon-dilatation was demonstrated. The beneficial effect of magnesium and trimetazidine on IR-injury remains controversial. The beneficial effect of adenosine remains to be further confirmed. There's also increasing interest in agentia combining the property of upregulating NO-synthase (e.g. L-arginine) and restoring the balance between NO and free radicals (e.g. tetrahydrobiopterin). One of such agents could be folic acid. In this review article the authors give an overview of the recent insights concerning pathogenesis and therapeutic possibilities to prevent IR-induced injury.

Effect of sarcolemmal rupture on myocardial electrical impedance during oxygen deprivation

Plasma membrane disruption is a characteristic feature of cell death induced by hypoxia or ischemia. Here, we investigated whether analysis of tissue electrical impedance allows detection of ongoing cell membrane rupture and necrotic cell death in hypoxic or ischemic myocardium. Twenty-eight isolated rat hearts were submitted to 5 h of ischemia (n = 8) or hypoxia (n = 20). Myocardial electrical impedance and lactate dehydrogenase (LDH) release were monitored. The time course of hypoxia-induced cell death was modified by altering pH (pH 7.4 or 6.4, 5 h) or by adding 3 or 10 mM glycine. Ischemia and hypoxia induced an increase in electrical impedance, followed by a plateau, and later a reduction. During hypoxia, LDH release started after a prolonged lapse of time (80.00 +/- 8.37 min at pH 7.4 and 122.50 +/- 11.82 min at pH 6.4). The onset of LDH release was followed by the onset of the late reduction in electrical impedance, and both were delayed by acidic pH (P < 0.05) and by glycine (P < 0.05). The times of onset of LDH release and of late electrical changes were significantly correlated (r = 0.752, P < 0.001). In separate experiments, induction of sarcolemmal rupture with Triton X-100 (n = 6) mimicked the late effects of ischemia or hypoxia on tissue impedance. The protective effects of glycine and acidosis on membrane disruption were confirmed (propidium iodide) in energy-deprived HL-1 cardiomyocytes. These results describe for the first time a late fall in electrical impedance in myocardium submitted to prolonged oxygen deprivation and demonstrate that this fall allows detection of ongoing cell necrosis.

Glycine protects cardiomyocytes against lethal reoxygenation injury by inhibiting mitochondrial permeability transition

Post-ischaemic reperfusion may precipitate cardiomyocyte death upon correction of intracellular acidosis due in part to mitochondrial permeability transition. We investigated whether glycine, an amino acid with poorly understood cytoprotective properties, may interfere with this mechanism. In cardiomyocyte cultures, addition of glycine during re-energization following 1 h of simulated ischaemia (NaCN/2-deoxyglucose, pH 6.4) completely prevented necrotic cell death associated with pH normalization. Glycine also protected against cell death associated with pH normalization in reoxygenated rat hearts. Glycine prevented cyclosporin-sensitive swelling and calcein release associated with re-energization in rat heart mitochondria submitted to simulated ischaemia or to Ca(2+) stress under normoxia. NMR spectroscopy revealed a marked glycine depletion in re-energized cardiomyocytes that was reversed by exposure to 3 mm glycine. These results suggest that intracellular glycine exerts a previously unrecognized inhibition on mitochondrial permeability transition in cardiac myocytes, and that intracellular glycine depletion during myocardial hypoxia/reoxygenation makes the cell more vulnerable to necrotic death.

Enhanced effect of gap junction uncouplers on macroscopic electrical properties of reperfused myocardium

Transient inhibition of gap junction (GJ)-mediated communication with heptanol during myocardial reperfusion limits infarct size. However, inhibition of cell coupling in normal myocardium may be arrhythmogenic. The purpose of this study was to test the hypothesis that the consequences of GJ inhibition may be magnified in reperfused myocardium compared with normal tissue, thus allowing the inhibition of GJs in reperfused tissue while only minimally modifying overall macroscopic cell coupling in normal myocardium. Concentration-response curves were defined for the effects of heptanol, 18alpha-glycyrrhetinic acid, halothane, and palmitoleic acid on conduction velocity, tissue electrical impedance, developed tension and lactate dehydrogenase (LDH) release in normoxically perfused rat hearts (n= 17). Concentrations lacking significant effects on tissue impedance were added during the initial 15 min of reperfusion in hearts submitted to 60 min (n= 43) or 30 min (n= 35) of ischaemia. These concentrations markedly increased myocardial electrical impedance (resistivity and phase angle) in myocardium reperfused after either 30 or 60 min of ischaemia, and reduced reperfusion-induced LDH release after 1 h of ischaemia by 83.6, 57.9, 51.7 and 52.5% for heptanol, 18alpha-glycyrrhetinic acid, halothane and palmitoleic acid, respectively. LDH release was minimal in hearts submitted to 30 min of ischaemia, independently of group allocation. In conclusion, the present results strongly support the hypothesis that intercellular communication in postischaemic myocardium may be effectively reduced by concentrations of GJ inhibitors affecting only minimally overall electrical impedance in normal myocardium. Reduction of cell coupling during initial reperfusion was consistently associated with attenuated lethal reperfusion injury.

Protection afforded by ischemic preconditioning is not mediated by effects on cell-to-cell electrical coupling during myocardial ischemia-reperfusion

The end-effectors of ischemic preconditioning (IPC) are not well known. It has been recently shown that transgenic mice underexpressing the gap junction protein connexin43 (Cx43) cannot be preconditioned. Because gap junctions allow spreading of cell death during ischemia-reperfusion in different tissues, including myocardium, we hypothesized that the protection afforded by IPC is mediated by effects on gap junction-mediated intercellular communication. To test this hypothesis, we analyzed the effect of IPC (5 min ischemia-5 min reperfusion x 2) on the changes in electrical impedance (four electrode probe) and impulse propagation velocity (transmembrane action potential) induced by ischemia (60 min) and reperfusion (60 min) in isolated rat hearts. IPC (n = 8) reduced reperfusion-induced lactate dehydrogenase release by 65.8% with respect to control hearts (n = 9) (P = 0.04) but had no effect on the time of onset of rigor contracture (increase in diastolic tension), electrical uncoupling (sharp changes in tissue resistivity and phase angle in impedance recordings), or block of impulse propagation during ischemia. Normalization of electrical impedance during reperfusion was also unaffected by IPC. The lack of effect of IPC on ischemic rigor contracture and on changes in tissue impedance during ischemia-reperfusion were validated under in vivo conditions in pigs submitted to 48 min of coronary occlusion and 120 min of reperfusion. IPC (n = 12) reduced infarct size (triphenyltetrazolium) by 64.9% (P = 0.01) with respect to controls (n = 17). We conclude that the protection afforded by IPC is not mediated by effects on electrical coupling. This result is consistent with recent findings suggesting that Cx43 could have effects on cell survival independent on changes in cell-to-cell communication.