Prevention of ischemic rigor contracture during coronary occlusion by inhibition of Na(+)-H+ exchange

Prevention of Ischemic Myocardial Contracture Through Hemodynamically Controlled DCD

Purpose

Ischemic myocardial contracture (IMC) or “stone heart” is a condition with rapid onset following circulatory death. It inhibits transplantability of hearts donated upon circulatory death (DCD). We investigate the effectiveness of hemodynamic normalization upon withdrawal of life-sustaining therapy (WLST) in a large-animal controlled DCD model, with the hypothesis that reduction in cardiac work delays the onset of IMC.

Methods

A large-animal study was conducted comprising of a control group (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$n=6$$\end{document}n=6) receiving no therapy upon WLST, and a test group (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$n=6$$\end{document}n=6) subjected to a protocol for fully automated computer-controlled hemodynamic drug administration. Onset of IMC within 1 h following circulatory death defined the primary end-point. Cardiac work estimates based on pressure-volume loop concepts were developed and used to provide insight into the effectiveness of the proposed computer-controlled therapy.

Results

No test group individual developed IMC within \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${1} \text { h}$$\end{document}1h, whereas all control group individuals did (4/6 within \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${30}{\text { min}}$$\end{document}30min).

Conclusion

Automatic dosing of hemodynamic drugs in the controlled DCD context has the potential to prevent onset of IMC up to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${1}{\text { h}}$$\end{document}1h, enabling ethical and medically safe organ procurement. This has the potential to increase the use of DCD heart transplantation, which has been widely recognized as a means of meeting the growing demand for donor hearts.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13239-021-00537-8.

Delayed ischaemic contracture onset by empagliflozin associates with NHE1 inhibition and is dependent on insulin in isolated mouse hearts

AIMS

Sodium glucose cotransporter 2 (SGLT2) inhibitors have sodium-hydrogen exchanger (NHE) inhibition properties in isolated cardiomyocytes, but it is unknown whether these properties extend to the intact heart during ischaemia-reperfusion (IR) conditions. NHE inhibitors as Cariporide delay time to onset of contracture (TOC) during ischaemia and reduce IR injury. We hypothesized that, in the ex vivo heart, Empagliflozin (Empa) mimics Cariporide during IR by delaying TOC and reducing IR injury. To facilitate translation to in vivo conditions with insulin present, effects were examined in the absence and presence of insulin.

METHODS AND RESULTS

Isolated C57Bl/6NCrl mouse hearts were subjected to 25 min I and 120 min R without and with 50 mU/L insulin. Without insulin, Empa and Cari delayed TOC by 100 and 129 s, respectively, yet only Cariporide reduced IR injury [infarct size (mean ± SEM in %) from 51 ± 6 to 34 ± 5]. Empa did not delay TOC in the presence of the NHE1 inhibitor Eniporide. Insulin perfusion increased tissue glycogen content at baseline (from 2 ± 2 µmol to 42 ± 1 µmol glycosyl units/g heart dry weight), amplified G6P and lactate accumulation at end-ischaemia, thereby decreased mtHKII and exacerbated IR injury. Under these conditions, Empa (1 µM) and Cariporide (10 µM) were without effect on TOC and IR injury. Empa and Cariporide both inhibited NHE activity, in isolated cardiomyocytes, independent of insulin.

CONCLUSIONS

In the absence of insulin, Empa and Cariporide strongly delayed the time to onset of contracture during ischaemia. In the presence of insulin, both Empa and Cari were without effect on IR, possibly because of severe ischaemic acidification. Insulin exacerbates IR injury through increased glycogen depletion during ischaemia and consequently mtHKII dissociation. The data suggest that also in the ex vivo intact heart Empa exerts direct cardiac effects by inhibiting NHE during ischaemia, but not during reperfusion.

SGLT2 inhibitors reduce infarct size in reperfused ischemic heart and improve cardiac function during ischemic episodes in preclinical models

The sodium-glucose cotransporter 2 (SGLT2) inhibitors are a new class of effective drugs managing patients, who suffer from type 2 diabetes (T2D): Landmark clinical trials including EMPA-REG, CANVAS and Declare-TIMI have demonstrated that SGLT2 inhibitors reduce cardiovascular mortality and re-hospitalization for heart failure (HF) in patients with T2D. It is well established that there is a strong independent relationship among infarct size measured within 1 month after reperfusion and all-cause death and hospitalization for HF: The fact that cardiovascular mortality was significantly reduced with the SGLT2 inhibitors, fuels the assumption that this class of therapies may attenuate myocardial infarct size. Experimental evidence demonstrates that SGLT2 inhibitors exert cardioprotective effects in animal models of acute myocardial infarction through improved function during the ischemic episode, reduction of infarct size and a subsequent attenuation of heart failure development. The aim of the present review is to outline the current state of preclinical research in terms of myocardial ischemia/reperfusion injury (I/R) and infarct size for clinically available SGLT2 inhibitors and summarize some of the proposed mechanisms of action (lowering intracellular Na⁺ and Ca²⁺, NHE inhibition, STAT3 and AMPK activation, CamKII inhibition, reduced inflammation and oxidative stress) that may contribute to the unexpected beneficial cardiovascular effects of this class of compounds.

Plasma Cytochrome c Detection Using a Highly Sensitive Electrochemiluminescence Enzyme-Linked Immunosorbent Assay

Background

Cytochrome c is an intermembrane mitochondrial protein that is released to the bloodstream following mitochondrial injury.

Methods and results

We developed an electrochemiluminescence immunoassay to measure cytochrome c in human and rat plasma, which showed high sensitivity with broad dynamic range (2-1200 ng/mL in humans and 5-500 ng/mL in rat) and high assay reproducibility (inter-assay coefficient <6% in humans and <10% in rat). In patients after blunt trauma, plasma cytochrome c directly correlated with injury severity. In rats after cardiac resuscitation, plasma cytochrome c inversely correlated with survival and responsiveness to mitochondrial protective interventions.

Conclusions

The cytochrome c assays herein presented have high sensitivity, wide dynamic range, and high reproducibility well suited for biomarker of mitochondrial injury.

Electrical plasticity and cardioprotection in myocardial ischemia--role of selective sodium channel blockers

The concept of electrical protection of the ischemic myocardium is in constant evolution and has recently been supported by experimental and clinical studies. Historically, antiplatelet agents, angiotensin-converting enzyme inhibitors, β-blockers, and statins have been all proposed as drugs conferring anti-ischemic cardioprotection. This was supported by the evidence consistently indicating that all these drugs were capable of reducing mortality and the risk of repeat myocardial infarction. The electrical plasticity paradigm is, however, a novel concept that depicts the benefits of improved sodium channel blockade with drugs such as ranolazine and cariporide. Although it has been hypothesized that the protective role of ranolazine depends on decreased fatty acid β-oxidation affecting preconditioning, we speculate against such a hypothesis, because inhibition of β-oxidation requires higher concentrations of the drug, above the therapeutic range. Rather, we discuss the key role of calcium overload reduction through inhibition of the late sodium current (I(Na)). Mechanisms driving cardioprotection involve the block of a cascade of complex ionic exchanges that can result in intracellular acidosis, excess cytosolic calcium, myocardial cellular dysfunction, and eventually cell injury and death. In this review we discuss the studies that demonstrate how electrical plasticity through sodium channel blockers can promote cardioprotection against ischemia in coronary heart disease.

Taurine detected using high-resolution magic angle spinning (1)H nuclear magnetic resonance: A potential indicator of early myocardial infarction

Magnetic resonance spectroscopy (MRS) is a unique non-invasive method for detecting cardiac metabolic changes. However, MRS in cardiac diagnosis is limited due to insensitivity and low efficiency. Taurine (Tau) is the most abundant free amino acid in the myocardium. We hypothesized that Tau levels may indicate myocardial ischemia and early infarction. Sprague-Dawley rats were divided into seven groups according to different time points during the course of myocardial ischemia, which was induced by left anterior descending coronary artery ligation. Infarcted myocardial tissue was obtained for high-resolution magic angle spinning (1)H nuclear magnetic resonance (NMR) analysis. Results were validated via high-performance liquid chromatography. The Tau levels in the ischemic myocardial tissue were reduced significantly within 5 min compared with those in the control group (relative ratio from 20.27±6.48 to 8.81±0.04, P<0.05) and were maintained for 6 h post-ischemia. Tau levels declined more markedly (56.5%) than creatine levels (48.5%) at 5 min after ligation. This suggests that Tau may have potential as an indicator in the early detection of myocardial ischemia by (1)H MRS.

Contribution of delayed intracellular pH recovery to ischemic postconditioning protection

Ischemic postconditioning (PoCo) has been proven to be a feasible approach to attenuate reperfusion injury and enhance myocardial salvage in patients with acute myocardial infarction, but its mechanisms have not been completely elucidated yet. Recent studies demonstrate that PoCo may delay the recovery of intracellular pH during initial reperfusion, and that its ability to limit infarct size critically depends on this effect. Prolongation of postischemic intracellular acidosis inhibits hypercontracture, mitochondrial permeability transition, calpain-mediated proteolysis, and gap junction-mediated spread of injury during the first minutes of reflow. This role of prolonged acidosis does not exclude the participation of other pathways in PoCo-induced cardioprotection. On the contrary, it may allow these pathways to act by preventing immediate reperfusion-induced cell death. Moreover, the existence of interactions between intracellular acidosis and endogenous protection signaling cannot be excluded and needs to be investigated. The role of prolonged acidosis in PoCo cardioprotection has important implications in the design of optimal PoCo protocols and in the translation of cardioprotective strategies to patients with on-going myocardial infarction receiving coronary reperfusion.

cGMP/PKG pathway mediates myocardial postconditioning protection in rat hearts by delaying normalization of intracellular acidosis during reperfusion

Ischemic postconditioning has been demonstrated to limit infarct size in patients, but its molecular mechanisms remain incompletely understood. Low intracellular pH (pHi) inhibits mitochondrial permeability transition, calpain activation and hypercontracture. Recently, delayed normalization of pHi during reperfusion has been shown to play an important role in postconditioning protection, but its relation with intracellular protective signaling cascades is unknown. The present study investigates the relation between the rate of pHi normalization and the cGMP/PKG pathway in postconditioned myocardium. In isolated Sprague-Dawley rat hearts submitted to transient ischemia both, postconditioning and acidic reperfusion protocols resulted in a similar delay in pHi recovery measured by (31)P-NMR spectroscopy (3.6±0.2min and 3.5±0.2min respectively vs. 1.4±0.2min in control group, P<0.01) and caused equivalent cardioprotection (48% and 41% of infarct reduction respectively, P<0.01), but only postconditioning increased myocardial cGMP levels (P=0.02) and activated PKG. Blockade of cGMP/PKG pathway by the addition of the guanylyl cyclase inhibitor ODQ or the PKG inhibitor KT5823 during reperfusion accelerated pHi recovery and abolished cardioprotection in postconditioned hearts, but had no effect in hearts subjected to acidic reperfusion suggesting that PKG signaling was upstream of delayed pHi normalization in postconditioned hearts. In isolated cardiomyocytes the cGMP analog 8-pCPT-cGMP delayed Na(+)/H(+)-exchange mediated pHi normalization after acidification induced by a NH(4)Cl pulse. These results demonstrate that the cGMP/PKG pathway contributes to postconditioning protection at least in part by delaying normalization of pHi during reperfusion, probably via PKG-dependent inhibition of Na(+)/H(+)-exchanger.

Na+/H+ exchange inhibitor cariporide attenuates skeletal muscle infarction when administered before ischemia or reperfusion

Administration of Na(+)/H(+) exchange isoform-1 (NHE-1) inhibitors before ischemia has been shown to attenuate myocardial infarction in several animal models of ischemia-reperfusion injury. However, controversy still exists as to the efficacy of NHE-1 inhibitors in protection of myocardial infarction when administered at the onset of reperfusion. Furthermore, the efficacy of NHE-1 inhibition in protection of skeletal muscle from infarction (necrosis) has not been studied. This information has potential clinical applications in prevention or salvage of skeletal muscle from ischemia-reperfusion injury in elective and trauma reconstructive surgery. The objective of this research project is to test our hypothesis that the NHE-1 inhibitor cariporide is effective in protection of skeletal muscle from infarction when administered at the onset of sustained ischemia or reperfusion and to study the mechanism of action of cariporide. In our studies, we observed that intravenous administration of cariporide 10 min before ischemia (1 or 3 mg/kg) or reperfusion (3 mg/kg) significantly reduced infarction in pig latissimus dorsi muscle flaps compared with the control, when these muscle flaps were subjected to 4 h of ischemia and 48 h of reperfusion (P < 0.05; n = 5 pigs/group). Both preischemic and postischemic cariporide treatment (3 mg/kg) induced a significant decrease in muscle myeloperoxidase activity and mitochondrial-free Ca(2+) content and a significant increase in muscle ATP content within 2 h of reperfusion (P < 0.05; n = 4 pigs/group). Preischemic and postischemic cariporide treatment (3 mg/kg) also significantly inhibited muscle NHE-1 protein expression within 2 h of reperfusion after 4 h of ischemia, compared with the control (P < 0.05; n = 3 pigs/group). These observations support our hypothesis that cariporide attenuates skeletal muscle infarction when administered at the onset of ischemia or reperfusion, and the mechanism involves attenuation of neutrophil accumulation and mitochondrial-free Ca(2+) overload and preservation of ATP synthesis in the early stage of reperfusion.

Effects of SM-20550 against myocardial infarction-induced arrhythmias, late infarct expansion, and left ventricular dysfunction

The aim of the study was to evaluate the cardioprotective and antiarrhythmic effects of intravenous Na+/H+ blockers (cariporide and SM-20550) in a rat model of ischemia and a long period reperfusion (14 days). This model allowed study of the role of Na+/H+ exchanger against late myocardial infarct expansion and left ventricular dysfunction. Each compound was administered 5 min before ischemia. Cariporide (from 0.16 mg/kg) and SM-20550 (from 0.04 mg/kg) significantly and dose-dependently reduced the number of ventricular premature beats during ischemia. The duration of ventricular tachycardia was importantly shortened in the presence of cariporide (0.63 mg/kg) and SM-20550 (0.16 mg/kg). Furthermore, cariporide (0.63 mg/kg) and SM-20550 (from 0.04 mg/kg) significantly reduced the infarct expansion: 43 +/- 2% in the cariporide group and 42 +/- 2% at 0.16 mg/kg SM-20550 versus 48 +/- 1% in the vehicle group. Cariporide and SM-20550 significantly prevented the left ventricular free wall thinning associated with the thickness ratio, suggesting a significant reduction of the ventricular dilation. Cariporide and SM-20550 significantly improved the negative dP/dtmax, suggesting a partial restoration of the cardiac relaxation. Collectively, Na+/H+ blockers administered before ischemia reduced arrhythmias and also prevented the remodeling process of the heart during postinfarction.

A halocin acting on Na+/H+ exchanger of haloarchaea as a new type of inhibitor in NHE of mammals

The capability of halocin H6 (a bacteriocin-like protein produced by haloarchaea Haloferax gibbonsii) to inhibit Na+/H+ exchanger (NHE) in mammalian cells and its cardio-protective efficacy on the ischemic and reperfused myocardium were evaluated in the present study. H6 inhibits NHE activity (measured by a flow cytometry method) in a dose-dependent form of cell lines of mammalian origin (HEK293, NIH3T3, Jurkat and HL-1) as well as in primary cell culture from human skeletal muscle (myocytes and fibroblasts). In vivo, an ischemia-reperfusion model in dogs by coronary arterial occlusion was used (two hours of regional ischemia and three hours of reperfusion). In animals treated with halocin H6 there was a significant reduction of premature ventricular ectopic beats and infarct size, whereas blood pressure and heart rate remained unchanged. Up to date, halocin H6 is the only described biological molecule that exerts a specific inhibitory activity in NHE of eukaryotic cells.

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.

Inflammation, proinflammatory mediators and myocardial ischemia-reperfusion Injury

Ischemic myocardium must be reperfused to terminate the ischemic event; otherwise the entire myocardium involved in the area at risk will not survive. However, there is a cost to reperfusion that may offset the intended clinical benefits of minimizing infarct size, postischemic endothelial and microvascular damage, blood flow defects, and contractile dysfunction. There are many contributors to this reperfusion injury. Targeting only one factor in the complex web of reperfusion injury is not effective because the untargeted mechanisms induce injury. An integrated strategy of reducing reperfusion injury in the catheterization laboratory involves controlling both the conditions and the composition of the reperfusate. Mechanical interventions such as gradually restoring blood flow or applying postconditioning may be used independently in or conjunction with various cardioprotective pharmaceuticals in an integrated strategy of reperfusion therapeutics to reduce postischemic injury.

Myocardial protection in reperfusion with postconditioning

Reperfusion is the definitive treatment for coronary occlusive disease. However, reperfusion carries the potential to exacerbate lethal injury, termed 'reperfusion injury'. Studies have suggested that reperfusion injury events are triggered during the early moments of reflow, and determine, in part, the severity of downstream manifestations of postischemic injury, including endothelial dysfunction, infarction and apoptosis. The application of brief iterative episodes of reflow (reoxygenation) and reocclusion (ischemia, hypoxia) at the immediate onset of reperfusion, which has been termed 'postconditioning' by the authors, reduces many manifestations of postischemic injury, notably infarct size, apoptosis, coronary vascular endothelial injury and reperfusion arrhythmias. Cardioprotection with postconditioning has been reported to be comparable with that observed using the gold standard maneuver ischemic preconditioning. In contrast to preconditioning, which exerts its effects primarily during the index ischemia, postconditioning appears to exert its effects during reperfusion alone. Postconditioning modifies the early phase of reperfusion in ways that are just beginning to be understood. It appears to first: reduce the oxidant burden and consequent oxidant-induced injury; secondly, attenuate the local inflammatory response to reperfusion; and thirdly, engage end effectors and signaling pathways implicated in other cardioprotective maneuvers, such as ischemic and pharmacologic preconditioning. Postconditioning seems to trigger the upregulation of survival kinases principally known to attenuate the pathogenesis of apoptosis and possibly necrosis. The postconditioning phenomenon has been reproduced by a number of independent laboratories and has been observed in both large and small animal in vivo models, as well as in ex vivo and cell culture models. In contrast to preconditioning, postconditioning may have widespread clinical application because it can be applied during reperfusion at the point of service for angioplasty, stenting, cardiac surgery and organ transplantation.

Cardioprotection of cariporide evaluated by plasma myoglobin and troponin I in myocardial infarction in pigs

The cardioprotective effects of cariporide were investigated against myoglobin and troponin I elevation in a model of myocardial infarction in pig, and the possible relationship between these markers and myocardial infarct size. The left circumflex coronary artery was ligated for 60-min and then reperfused for 48-h. Plasma levels of myoglobin and troponin I were quantified during reperfusion. Vehicle or cariporide (2.5 mg/kg) were administered i.v. before ischaemia and infused throughout ischaemia and for the beginning of reperfusion. In vehicle-treated pigs, the infarct size represented 26% +/- 3% of the area at risk. Cariporide significantly decreased the infarct size by 66% +/- 9%, and significantly reduced plasma levels of myoglobin and troponin I. A strongly correlated linear relationship between myocardial necrosis and plasma levels of myoglobin (R = 0.966, P < 0.0001) or troponin I (R = 0.855, P < 0.0001) was clearly identified. In conclusion, in our porcine model of myocardial infarction, even with small infarcts (in the presence of cariporide), plasma levels of myoglobin and troponin I are predictive of the presence of necrosis and its extent.

Biochemical dysfunction in heart mitochondria exposed to ischaemia and reperfusion

Heart tissue is remarkably sensitive to oxygen deprivation. Although heart cells, like those of most tissues, rapidly adapt to anoxic conditions, relatively short periods of ischaemia and subsequent reperfusion lead to extensive tissue death during cardiac infarction. Heart tissue is not readily regenerated, and permanent heart damage is the result. Although mitochondria maintain normal heart function by providing virtually all of the heart's ATP, they are also implicated in the development of ischaemic damage. While mitochondria do provide some mechanisms that protect against ischaemic damage (such as an endogenous inhibitor of the F1Fo-ATPase and antioxidant enzymes), they also possess a range of elements that exacerbate it, including ROS (reactive oxygen species) generators, the mitochondrial permeability transition pore, and their ability to release apoptotic factors. This review considers the process of ischaemic damage from a mitochondrial viewpoint. It considers ischaemic changes in the inner membrane complexes I-V, and how this might affect formation of ROS and high-energy phosphate production/degradation. We discuss the contribution of various mitochondrial cation channels to ionic imbalances which seem to be a major cause of reperfusion injury. The different roles of the H+, Ca2+ and the various K+ channel transporters are considered, particularly the K+(ATP) (ATP-dependent K+) channels. A possible role for the mitochondrial permeability transition pore in ischaemic damage is assessed. Finally, we summarize the metabolic and pharmacological interventions that have been used to alleviate the effects of ischaemic injury, highlighting the value of these or related interventions in possible therapeutics.

Reduction of myocardial infarct size by SM-197378, a novel Na+/H+ exchange inhibitor, in rabbits

The effects of SM-197378, 2-[[[amino(imino)methyl]amino]carbonyl]-1-methyl-4-trifluoromethyl-1H-indol-7-yl=hydrogen=sulfate monohydrate, a novel potent Na+/H+exchange inhibitor, on heart injury were studied using a rabbit model involving 30 min of myocardial ischemia and 5 h of reperfusion. Intravenous administration of SM-197378 before ischemia reduced the infarct size by approximately 30-50% in a dose-dependent manner. This anti-necrotic effect was achieved without significant hemodynamic changes. Moreover, administration of SM-197378 before reperfusion also resulted in a significant, approximately 30-40%, reduction in the infarct size. The anti-necrotic effect of pre-ischemic bolus treatment with SM-197378 was compared with that of nicorandil, a K+channel opener with nitrate-like activity, and ischemic preconditioning. With 30 and 60 min of ischemia, the anti-necrotic effects of SM-197378 and ischemic preconditioning were similar and superior to that of nicorandil. With 90 min of ischemia, the anti-necrotic effect of SM-197378 was superior to that of ischemic preconditioning.

Reduction of myocardial infarct size by SM-198110, a novel Na+/H+ exchange inhibitor, in rabbits

The effects of 3-[2-({[amino(imino)methyl]amino}carbonyl)-4-chloro-1H-indol-1-yl]-1-propanesulphonic acid monohydrate (SM-198110), a novel potent Na+/H+ exchange inhibitor, and cariporide (Hoe642), another Na+/H+ exchange inhibitor, were studied in a myocardial ischaemia and reperfusion injury model. Anaesthetized rabbits were subjected to occlusion of the coronary artery for 30 min followed by reperfusion for 5 h. SM-198110 or cariporide was administered before ischaemia and before reperfusion. We also assessed the anti-necrotic effect of SM-198110 when given before reperfusion, both alone and together with glibenclamide, a K(ATP) channel blocker, 5-hydroxydecanoate (5-HD), a mitochondrial K(ATP) channel-selective blocker and 8-(p-sulphophenyl)-theophylline (8-SPT), an adenosine receptor blocker. The infarct size was reduced dose-dependently by i.v. administration of SM-198110 before ischaemia, with a significant reduction in serum creatine phosphokinase activity. Infarct sizes, normalized to the size of the area-at-risk (means+/-SE) were: vehicle 56.6+/-3.7%; low-dose SM-198110 39.2+/-6.3%; mid-dose 32.8+/-7.4% (P < 0.05); high-dose 22.1+/-6.7% (P < 0.01). This anti-necrotic effect of SM-198110 was achieved without significant haemodynamic changes. Cariporide given before ischaemia also reduced infarct size significantly and dose-dependently. SM-198110 administered before reperfusion also resulted in a dose-dependent reduction in the infarct size. Infarct sizes were: vehicle 56.6+/-3.7%; low-dose SM-198110 44.5+/-5.7%; mid-dose 36.3+/-6.6% (P < 0.01); high-dose 34.7+/-3.8% (P < 0.01). In contrast, cariporide given before reperfusion did not reduce infarct sizes significantly. The anti-necrotic effect of SM-198110 was observed even when given 10 min after the beginning of reperfusion. Glibenclamide and 5-HD abolished the anti-necrotic effect of treatment before reperfusion with SM-198110. However, the co-administration of 8-SPT with SM-198110 did not affect infarct size. These results suggest that, in addition to Na+/H+ exchange inhibition, mitochondrial and/or sarcolemmal K(ATP) channels contribute to the anti-necrotic effect of SM-198110 when the latter is given before reperfusion.

Postconditioning A new link in nature’s armor against myocardial ischemia–reperfusion injury

Reperfusion injury is a complex process involving several cell types (endothelial cells, neutrophils, and cardiomyocytes), soluble proinflammatory mediators, oxidants, ionic and metabolic dyshomeostasis, and cellular and molecular signals. These participants in the pathobiology of reperfusion injury are not mutually exclusive. Some of these events take place during the very early moments of reperfusion, while others, seemingly triggered in part by the early events, are activated within a later timeframe. Postconditioning is a series of brief mechanical interruptions of reperfusion following a specific prescribed algorithm applied at the very onset of reperfusion. This algorithm lasts only from 1 to 3 minutes depending on species. Although associated with re-occlusion of the coronary artery or re-imposition of hypoxia in cell culture, the reference to ischemia has been dropped. Postconditioning has been observed to reduce infarct size and apoptosis as the "end games" in myocardial therapeutics; salvage of infarct size was similar to that achieved by the gold standard of protection, ischemic preconditioning. The cardioprotection was also associated with a reduction in: endothelial cell activation and dysfunction, tissue superoxide anion generation, neutrophil activation and accumulation in reperfused myocardium, microvascular injury, tissue edema, intracellular and mitochondrial calcium accumulation. Postconditioning sets in motion triggers and signals that are functionally related to reduced cell death. Adenosine has been implicated in the cardioprotection of postconditioning, as has e-NOS, nitric oxide and guanylyl cyclase, opening of K(ATP) channels and closing of the mitochondrial permeability transition pore. Cardioprotection by postconditioning has also been associated with the activation of intracellular survival pathways such as ERK1/2 and PI3 kinase - Akt pathways. Other pathways have yet to be identified. Although many of the pathways involved in postconditioning have also been identified in ischemic preconditioning, some may not be involved in preconditioning (ERK1/2). The timing of action of these pathways and other mediators of protection in postconditioning differs from that of preconditioning. In contrast to preconditioning, which requires a foreknowledge of the ischemic event, postconditioning can be applied at the onset of reperfusion at the point of clinical service, i.e. angioplasty, cardiac surgery, transplantation.

Cariporide enables hemodynamically more effective chest compression by leftward shift of its flow-depth relationship

When given during closed-chest resuscitation, cariporide (4-isopropyl-methylsulfonylbenzoyl-guanidine methanesulfonate; a selective inhibitor of the Na(+)/H(+) exchanger isoform-1) enables generation of viable perfusion pressures with less depth of compression. We hypothesized that this effect results from greater blood flows generated for a given depth of compression. Two series of 14 rats each underwent 10 min of untreated ventricular fibrillation followed by 8 min of chest compression before defibrillation was attempted. Compression depth was adjusted to maintain an aortic diastolic pressure (ADP) between 26 and 28 mmHg in the first series and between 36 and 38 mmHg in the second series. Within each series, rats were randomized to receive cariporide (3 mg/kg) or NaCl (0.9%; control) before chest compression was started. Blood flow was measured using 15-mum fluorescent microspheres. Less depth of compression was required to maintain the target ADP when cariporide was present in both series 1 (13.6 +/- 1.2 vs. 16.6 +/- 1.2 mm; P < 0.001) and series 2 (15.3 +/- 1.0 vs. 18.9 +/- 1.5 mm; P < 0.001). Despite less compression depth, the cardiac index in cariporide-treated rats was comparable to control rats in series 1 (11.1 +/- 0.7 vs. 11.3 +/- 1.4 ml.min(-1).kg(-1); P = not significant) but higher in series 2 (15.5 +/- 2.3 vs. 9.9 +/- 1.4 ml.min(-1).kg(-1); P < 0.05). Increases in compression depth (from series 1 to series 2) increased myocardial, cerebral, and adrenal blood flow in cariporide-treated rats. We conclude that cariporide enhances the efficacy of closed-chest resuscitation by leftward shift of the flow-depth relationship.

Neurologic Preservation by Na+-H+ Exchange Inhibition Prior to 90 Minutes of Hypothermic Circulatory Arrest

BACKGROUND

The effects of pretreatment with cariporide (HOE 642 Aventis Pharma, Strasbourg-Cedex, France), a Na+-H+ exchanger (NHE) blocker, were studied in a cerebral ischemia-reperfusion model of hypothermic arrest.

METHODS

Fifteen Yorkshire-Duroc pigs (37.1 +/- 4.2 kg) underwent femoral-jugular bypass and 90 minutes of deep hypothermic circulatory arrest at 19 degrees C. Ten animals were untreated, whereas 5 received 5 mg/kg of intravenous cariporide before cooling. After rewarming and off cardiopulmonary bypass, the pigs were weaned from anesthesia and followed for 24 hours. A standardized neurologic scoring system assessed brain functional recovery. Biochemical markers were used to analyze cellular injury. Control studies without circulatory arrest were done in 2 animals that underwent similar cooling and rewarming.

RESULTS

Neurologic recovery was rapid and complete in the nonischemic controls and in all pretreated animals. Conversely, at 24 hours, all untreated pigs exhibited a cloudy or stuporous level of consciousness, abnormal positioning, and with only one exception, could not sit or stand. The gradation of neurologic score (evaluating central nervous system, motor and sensory functions, respiration condition, level of consciousness, and behavior) was 0 +/- 0 (0 = normal, 500 = brain death) in the treated group, compared with 124 +/- 59 in the untreated animals. Biochemical analysis showed every variable of whole-body injury (including conjugated dienes (p < 0.05), serum aspartate amino transferase (p < 0.01), creatine kinase p < 0.001) and endothelin-1 (p < 0.001) to be higher in the untreated group.

CONCLUSIONS

NHE function alters experimental brain ischemia-reperfusion damage. These observations imply that NHE inhibition therapy before ischemia may improve neurologic protection in adult and infant patients undergoing cerebral ischemia during procedures that use hypothermic circulatory arrest.

Direct inhibition of the sodium/hydrogen exchanger after prolonged regional ischemia improves contractility on reperfusion independent of myocardial viability

BACKGROUND

A mechanism for myocardial dysfunction after ischemia and reperfusion is Na(+)/H(+) exchanger activation. Although past in vivo models of limited ischemia and reperfusion intervals demonstrate that Na(+)/H(+) exchanger inhibition confers myocardial protection when administered at the onset of ischemia, the effect of Na(+)/H(+) exchanger inhibition on myocardial function after prolonged ischemia and reperfusion remains unknown. This investigation tested the hypothesis that Na(+)/H(+) exchanger inhibition instituted at reperfusion and after prolonged coronary occlusion in pigs would influence myocardial contractility independent of myocardial viability.

METHODS

A coronary snare and sonomicrometry crystals were placed in pigs (n = 21, 32 kg). Coronary occlusion was instituted for 120 minutes followed by reperfusion for 180 minutes. At 105 minutes of ischemia, pigs were randomized to ischemia and reperfusion only (saline solution, n = 11) or Na(+)/H(+) exchanger inhibition (HOE-642, 3 mg/kg intravenously, n = 10). Myocardial injury was determined by tissue staining and measurement of plasma myocyte-specific enzymes. Myocardial contractility was determined by calculation of the regional end-systolic pressure-dimension relation (millimeters of mercury per centimeter) and by assessment of interregional shortening.

RESULTS

Infarct size was not different between groups (39% +/- 6%, P =.26). Moreover, at 180 minutes of reperfusion, plasma troponin-I and creatine kinase MB values had increased to identical levels in the ischemia and reperfusion-only and Na(+)/H(+) exchanger inhibition groups (300 +/- 35 and 50 +/- 6 ng/mL, respectively). At 90 minutes of ischemia, regional end-systolic pressure-dimension relation decreased from baseline (5.7 +/- 0.5 versus 2.7 +/- 0.3, P <.05) in the area at risk. By 30 minutes of reperfusion, regional end-systolic pressure-dimension relation decreased further in the ischemia and reperfusion-only group (1.6 +/- 0.2, P <.05), but improved with Na(+)/H(+) exchanger inhibition (4.4 +/- 0.7, P <.05).

CONCLUSIONS

Na(+)/H(+) exchanger inhibition instituted at reperfusion improved contractility independent of myocardial viability as assessed by absolute infarct size and myocyte-specific enzyme release. Thus, modulation of Na(+)/H(+) exchanger activity in the setting of prolonged ischemia and reperfusion may hold therapeutic potential.

Optimal dose and mode of delivery of Na+/H+ exchange-1 inhibitor are critical for reducing postsurgical ischemia-reperfusion injury

BACKGROUND

In clinical trials, perioperative intravenous Na(+)/H(+) exchange isoform-1 (NHE1) inhibitors were only moderately effective in high-risk patients undergoing surgical reperfusion (GUARDIAN trial). However, effective myocardial concentrations of NHE1 inhibitor may not have been achieved by parenteral administration alone. We tested the hypothesis that increasing doses of NHE1 inhibitor EMD 87580 ((2-methyl-4,5-di-(methylsulfonyl)-benzoyl)-guanidine) delivered in blood cardioplegia (BCP) and by parenteral route at reperfusion reduce myocardial injury after surgical reperfusion of evolving infarction.

METHODS

Twenty-six anesthetized dogs underwent 75 minutes of left anterior descending coronary artery occlusion, followed by cardiopulmonary bypass and 60 minutes of arrest with multidose 10 degrees C BCP. In the control group (n = 8), BCP was not supplemented. In the three EMD-BCP groups, BCP was supplemented with 10 micromol/L EMD 87580 (EMD-10, n = 5), 20 micromol/L EMD 87580 (EMD-20, n = 5), or 20 micromol/L EMD 87580 combined with an immediate reperfusion bolus (5 mg/kg intravenously) (EMD-20R, n = 8). The left anterior descending coronary artery occlusion was released just before the second infusion of BCP. Reperfusion continued for 120 minutes after discontinuation of cardiopulmonary bypass.

RESULTS

Postischemic systolic and diastolic function in the area at risk was dyskinetic in all groups. Infarct size (percentage of area at risk) was not significantly reduced in the EMD-10 (26.2% +/- 3.6%) and EMD-20 (22.5% +/- 2.4%) groups versus control (30.7% +/- 2.4%); however, infarct size was significantly reduced in the EMD-20R group (16.1% +/- 2.8%, p = 0.003). Edema in the area at risk in the EMD-10 (81.1% +/- 0.5% water content), EMD-20 (81.7% +/- 0.3%), and EMD-20R (81.9% +/- 0.3%) groups was less than in controls (83.2% +/- 0.2%), (p < 0.056). Neutrophil accumulation (myeloperoxidase activity) in postischemic area-at-risk myocardium was less in the EMD-20R group versus the control group (5.3 +/- 0.7 versus 8.7 +/- 1.4 absorbance units x min(-1) x g(-1); p = 0.05), which suggests an attenuated postischemic inflammatory response.

CONCLUSIONS

Optimal delivery of NHE1 inhibitor to the heart through combined cardioplegia and parenteral routes significantly attenuates myocardial injury after surgical reperfusion of regional ischemia. Timing, dose, and mode of delivery of NHE1 inhibitors are important to their efficacy.

A new role for cardioplegic buffering: should acidosis or calcium accumulation be counteracted to salvage jeopardized hearts?

OBJECTIVES

Thirty minutes of unprotected ischemia produced a jeopardized heart that was treated with a blood cardioplegic solution containing the natural erythrocyte and protein buffers. Cardioplegic pH was changed to 7.7 (buffered) or 7.2 (nonbuffered), and this was tested alone and after pretreatment with Na(+)-H(+) exchange blockade (cariporide) to define their protective effects.

METHODS

Twenty-four Yorkshire-Duroc pigs (27-34.5 kg) underwent 30 minutes of normothermic global ischemia, followed by 30 minutes of aortic clamping during protection with buffered (n = 12) or nonbuffered (n = 12) glutamate-aspartate-enriched blood cardioplegic solution. Twelve hearts (6 buffered and 6 nonbuffered) were pretreated with intravenous cariporide (5 mg/kg) 15 minutes before ischemia.

RESULTS

Severe and comparable left ventricle dysfunction followed buffered or nonbuffered cardioplegia: Preload recruitable stroke work recovered to 56% +/- 21% and 45% +/- 20% of baseline levels; creatine kinase MB, conjugated dienes, and myeloperoxidase activity markedly increased; moderate myocardial edema occurred; and endothelin-1 increased 2-fold more than baseline values. Cariporide pretreatment caused a similar return of preload recruitable stroke work to 86% +/- 9% and 90% +/- 6% after buffered or nonbuffered cardioplegia (P <.05 vs nonpretreated groups), allowed only minor creatine kinase MB and conjugated diene changes, and reduced endothelin-1 release 3-fold compared with hearts without sodium-hydrogen exchange blockage.

CONCLUSIONS

The severe ischemia-reperfusion injury of 30 minutes of normothermic ischemia is not altered by an acidic or alkalotic pH cardioplegic solution. Correction of damage is achieved by adding Na(+)-H(+) exchange blocker therapy before treatment with buffered and nonbuffered solutions; thus, sodium-hydrogen exchange inhibition plays a more vital role in recovery than pH management.

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.

The cardioprotective effects of Na+/H+ exchange inhibition and mitochondrial KATP channel activation are additive in the isolated rat heart

The mechanisms of recovery of the isolated rat heart were studied after 30 min of global ischemia. Functional recovery was assessed by the percentage recovery of developed pressure after 30 min reperfusion and by the magnitude of the contracture on reperfusion. After a control ischemia, developed pressure recovered to only 12+/-2% of pre-ischemic control and the reperfusion contracture was very large (81+/-6 mmHg). Activation of the mitochondrial KATP channel with 100 microM diazoxide present throughout ischemia and reperfusion improved recovery of developed pressure to 36+/-3% and reduced the reperfusion contracture (53+/-4 mmHg). Inhibition of the sodium/hydrogen exchanger with 10 microM cariporide caused a larger recovery of developed pressure to 72+/-4% and further reduced the reperfusion contracture (11+/-3 mmHg). The combination of both drugs increased recovery of developed pressure to 96+/-4% and the reperfusion contracture remained small (11+/-5 mmHg). The effectiveness of the timing of exposure to these drugs was explored. When both diazoxide and cariporide were applied 2 min before the end of ischaemia and remained present during reperfusion the recovery of developed pressure was 81+/-4% and the reperfusion contracture was small (12+/-3 mmHg); neither was significantly different to the recovery when both drugs were present throughout ischemia and reperfusion. We conclude that mitochondrial damage, blocked by diazoxide, and the coupled exchanger pathway, blocked by cariporide, are two of the principal damage pathways and functional recovery appears to be complete when both are blocked. The combination of these drugs is also highly effective when given 2 min before the end of ischemia.

Cariporide preserves mitochondrial proton gradient and delays ATP depletion in cardiomyocytes during ischemic conditions

The mechanism by which inhibition of Na+/H+ exchanger (NHE) reduces cell death in ischemic-reperfused myocardium remains controversial. This study investigated whether cariporide could inhibit mitochondrial NHE during ischemia, delaying H+ gradient dissipation and ATP exhaustion. Mouse cardiac myocytes (HL-1) were submitted to 1 h of simulated ischemia (SI) with NaCN/deoxyglucose (pH 6.4), with or without 7 microM cariporide, and mitochondrial concentration of Ca2+ (Rhod-2), 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and the charge difference across the mitochondrial membrane potential (Deltapsim, JC-1) were assessed. ATP content was measured by bioluminescence and mitochondrial swelling by spectrophotometry in isolated mitochondria. Cariporide significantly attenuated the acidification of the mitochondrial matrix induced by SI without modifying Deltapsim decay, and this effect was associated to a delayed ATP exhaustion and increased mitochondrial Ca2+ load. These effects were reproduced in sarcolemma-permeabilized cells exposed to SI. In these cells, cariporide markedly attenuated the fall in mitochondrial pH induced by removal of Na+ from the medium. In isolated mitochondria, cariporide significantly reduced the rate and magnitude of passive matrix swelling induced by Na+ acetate. In isolated rat hearts submitted to 40-min ischemia at different temperatures (35.5 degrees, 37 degrees, or 38.5 degrees C) pretreatment with cariporide limited ATP depletion during the first 10 min of ischemia and cell death (lactate dehydrogenase release) during reperfusion. These effects were mimicked when a similar ATP preservation was achieved by hypothermia and were abolished when the sparing effect of cariporide on ATP was suppressed by hyperthermia. We conclude that cariporide acts at the mitochondrial level, delaying mitochondrial matrix acidification and delaying ATP exhaustion during ischemia. These effects can contribute to reduce cell death secondary to ischemia-reperfusion.

Long-term magnetic resonance imaging/spectroscopy study of cariporide in a canine cardiac ischemia/reperfusion model

Using both 31P and 1H cardiac magnetic resonance techniques, it is possible to monitor the functional (ejection fraction [EF]) and biochemical (pH) status of the heart following a reperfused ischemic insult. This study assessed the effects of Na+/H+ exchange inhibition with cariporide in a closed-chest canine ischemia/reperfusion model. Dogs received 1-mg/kg cariporide treatments for 3 days after occlusion, but were monitored for 10 days. Baseline intracellular pH (+/-SEM) for the control and treated groups were 7.10 +/- 0.03 and 7.14 +/- 0.04, respectively, and dropped to 6.25 +/- 0.08 and 6.38 +/- 0.08 during occlusion. There was a significant increase in pH from occlusion to early reperfusion in the control group (P = 0.03) but, during the same time period, this increase was not seen in the cariporide group. There was a significant (P = 0.01) drug interaction in recovery of EF over the 10-day protocol. Individual time-point analysis revealed significant differences at immediate reperfusion through day 3 (73.9% +/- 2.5%, 84.5% +/- 3.1%; baseline normalized EF controls and cariporide, respectively). Neither pH nor EF measurements were significantly different between the groups at day 10. Despite early functional and metabolic benefits, infarct size, as measured at day 10, was 13.2% +/- 2.2% for the controls and 11.8% +/- 2.3% for the cariporide group (NS). Thus there were no long-term cariporide functional or biochemical benefits.

Basic biology and pharmacology of the cardiac sarcolemmal sodium/hydrogen exchanger

The Na+/H+ exchangers are a family of membrane proteins that transport sodium and hydrogen ions in opposite directions on a one-to-one basis, and play important roles in regulating cytoplasmic pH and cell volume and mediating sodium reabsorption in various tissues. In the myocardium, the physiological role of the exchanger is pH regulation. However, ischemic activation of the Na+/H+ exchanger in myocardium ultimately leads to intracellular calcium overload, a key mediator of ischemia and reperfusion injury. Studies in a wide variety of animal models have clearly shown that selective inhibition of the sarcolemmal Na+/H+ exchanger can delay progression of injury during ischemia, thereby reducing myocardial necrosis and improving recovery of ventricular function upon reperfusion. Furthermore, this inhibition does not adversely affect either the rate or degree of acidosis during ischemia. To be efficacious, Na+/H+ inhibition must be initiated before or during early ischemia; inhibition only during late ischemia and reperfusion has minimal to no beneficial effects. These preclinical data suggest that selective sodium hydrogen exchanger (NHE) inhibition may provide a new, efficacious treatment for acute myocardial ischemia in appropriate settings in humans.

Blood cardioplegia supplementation with the sodium-hydrogen ion exchange inhibitor cariporide to attenuate infarct size and coronary artery endothelial dysfunction after severe regional ischemia in a canine model

BACKGROUND

Activation of the sodium-hydrogen ion exchange mechanism results in accumulation of intracellular calcium through the sodium-calcium ion antiport mechanism. Administration of a sodium-hydrogen ion exchange inhibitor before or during ischemia attenuates myocardial ischemia and reperfusion injury. However, the cardioprotection exerted by sodium-hydrogen ion exchange inhibitors as adjuncts to cardioplegia without perioperative administration has not been tested in a model of surgical reperfusion of acute coronary occlusion with cardiopulmonary bypass. This study tested the hypothesis that sodium-hydrogen ion exchange inhibitor-supplemented blood cardioplegia would reduce postcardioplegia injury after severe regional ischemia.

METHODS

In anesthetized open-chest dogs, the left anterior descending coronary artery was occluded for 75 minutes, after which total cardiopulmonary bypass was initiated. After crossclamping, cold (4 degrees C) antegrade blood cardioplegia was delivered every 20 minutes for a total of 60 minutes of cardioplegic arrest. In 8 dogs, the blood cardioplegic solution was unsupplemented (vehicle group), whereas in 8 others the solution was supplemented with the sodium-hydrogen ion exchange inhibitor cariporide (10 micro mol/L, cariporide group).

RESULTS

In the in vitro studies, the direct effects of cariporide on neutrophil function were determined. Isolated canine neutrophils were stimulated by platelet activating factor. Cariporide attenuated superoxide anion production in a concentration-dependent manner, with no appreciable effect at 10 micro mol/L (the concentration used in blood cardioplegia) and a peak effect at 100 micro mol/L. In the in vivo cardiopulmonary bypass model, infarct size was significantly (P <.05) smaller in the cariporide group than in the vehicle group (22.4% +/- 3.5% vs 40.1% +/- 5.1% of area at risk), although there were no group differences in postischemic regional wall motion after 2 hours of reperfusion (0.1% +/- 0.9% vs -0.2% +/- 0.3% systolic shortening). Transmural myocardial edema in the area at risk was significantly decreased in the cariporide group (80.6% +/- 0.5%) relative to the vehicle group (83.1% +/- 0.6%). Myeloperoxidase activity in the area at risk, an index of neutrophil accumulation, was significantly lower in the cariporide group than in the vehicle group (4.7 +/- 0.9 absorbence units/[min. g tissue] vs 10.3 +/- 2.3 absorbence units/[min. g tissue]). In isolated postischemic left anterior descending coronary artery rings, maximum relaxation in response to the endothelium-dependent vasodilator acetylcholine was significantly greater in the cariporide group than in the vehicle group (77.5% +/- 7.4% vs 51.4% +/- 8.0%), whereas smooth muscle relaxation in response to nitroprusside was comparable between groups.

CONCLUSION

In this canine model, supplementation of blood cardioplegia with cariporide, a sodium-hydrogen ion exchange inhibitor, reduced infarct size, attenuated neutrophil accumulation in the area at risk, and reduced postischemic coronary artery endothelial dysfunction without directly inhibiting neutrophil activity. Cariporide as an adjunct to blood cardioplegia without perioperative administration attenuated surgical ischemia-reperfusion injury in jeopardized myocardium.

Myocyte and endothelial effects of preconditioning the jeopardized heart by inhibiting Na/H exchange

OBJECTIVES

The preconditioning effects of the adjunctive, cardiac-specific sodium-hydrogen ion exchange inhibitor cariporide (cariporide mesilate, HOE 642) were studied in hearts subjected to 30 minutes of normothermic ischemia and reperfusion to assess myocardial and endothelial changes.

METHODS

Sixteen Yorkshire-Duroc pigs (27-34 kg) receiving cardiopulmonary bypass underwent either cardiopulmonary bypass alone (control, n = 4) or 30 minutes of normothermic ischemia, followed by 30 minutes of blood reperfusion (n = 12). Six hearts were treated with 5 mg/kg cariporide administered intravenously 15 minutes before ischemia.

RESULTS

Cardiopulmonary bypass alone caused no changes. Conversely, 30 minutes of global normothermic ischemia caused 33% mortality and, in survivors, depression of left ventricular function to 22% +/- 6% of baseline preload recruitable stroke work and increased creatine kinase MB by 406% (88 +/- 13 U/L), conjugated dienes by 17% (161 +/- 0.2 AU/mL), and myeloperoxidase activity by 297% (0.036 +/- 0.005 U/g). Myocardial edema developed (3.5% water gain). Coronary sinus endothelin 1 increased by 111% (2.05 +/- 0.38 pg/mL), and nitric oxide production decreased by 10%. These adverse effects were limited by pretreatment with cariporide, which allowed complete survival and restored preload recruitable stroke work to 78% +/- 11%. Measurements of creatine kinase MB, conjugated dienes, myeloperoxidase, water, and endothelin 1 returned to baseline values, and nitric oxide production was accentuated 3-fold.

CONCLUSIONS

These observations show that adjunctive pretreatment with cariporide delays myocardial and endothelial injury during ischemia and reperfusion, limits oxygen-derived radical injury, restores function, reduces edema, and preserves endothelin and nitric oxide balance at normal values. The myeloperoxidase changes show that less white blood cell adherence supports reduced reperfusion endothelial damage.

Zoniporide: a potent and highly selective inhibitor of human Na(+)/H(+) exchanger-1

We evaluated the in vitro pharmacological profile of a novel, potent and highly selective Na(+)/H(+) exchanger-1 (NHE-1) inhibitor, [1-(Quinolin-5-yl)-5-cyclopropyl-1H-pyrazole-4-carbonyl]guanidine hydrochloride monohydrate (zoniporide or CP-597,396). The potency and selectivity of zoniporide were determined via inhibition of 22Na(+) uptake by PS-120 fibroblast cell lines overexpressing human NHE-1, -2 or rat NHE-3. Additionally, potency for endogenous NHE-1 was confirmed via ex vivo human platelet swelling assay (PSA), in which platelet swelling was induced by exposure to sodium propionate. The pharmacological profile of zoniporide was compared with that of eniporide and cariporide. Zoniporide inhibited 22Na(+) uptake in fibroblasts expressing human NHE-1 in a concentration-dependent manner (IC(50) = 14 nM) and was highly selective (157-fold and 15,700-fold vs. human NHE-2 and rat NHE-3, respectively). Zoniporide was 1.64- to 2.6-fold more potent at human NHE-1 than either eniporide or cariporide (IC(50) = 23 and 36 nM, respectively). Zoniporide was also more selective at inhibiting human NHE-1 vs. human NHE-2 than either eniporide or cariporide (157-fold selective compared with 27- and 49-fold, respectively). All three compounds inhibited human platelet swelling with IC(50) values in low nanomolar range. From these results, we conclude that zoniporide represents a novel, potent and highly selective NHE-1 inhibitor.

Myocardial protection during ventricular fibrillation by inhibition of the sodium-hydrogen exchanger isoform-1

Activation of the sarcolemmal sodium-hydrogen exchanger isoform-1 (NHE-1) in response to the intense intracellular acidosis that develops during ischemia has been identified as an important mechanism of myocardial cell injury. NHE-1 inhibition in the quiescent (nonfibrillating) heart ameliorates functional manifestation of ischemia and reperfusion injury. We investigated in isolated heart and intact rat models of ventricular fibrillation whether NHE-1 inhibition, by using the selective inhibitor cariporide, could ameliorate myocardial abnormalities that develop during ventricular fibrillation and limit resuscitability and survival. In the isolated rat heart, cariporide significantly reduced the magnitude of ischemic contracture during ventricular fibrillation and the accompanying increases in coronary vascular resistance. Hearts that had received cariporide during ventricular fibrillation had no diastolic dysfunction after resuscitation and recovered their systolic function earlier. In intact rats, cariporide given immediately before starting chest compression allowed generation of a coronary perfusion pressure and end-tidal Pco2 comparable with control rats but with significantly less depth of compression. Cariporide had an unprecedented effect in this rat model, prompting spontaneous defibrillation after approximately 8 mins of chest compression. After resuscitation, rats treated with cariporide had significantly less ventricular ectopic activity, better hemodynamic function, and higher survival rates (22 of 24 [94%] vs. 15 of 24 [63%] in control rats, p <.05). We conclude that NHE-1 inhibition may represent a novel and highly effective form of treatment for resuscitation from ventricular fibrillation.

Inhibition of Na+/H+ exchanger reduces infarct volume of focal cerebral ischemia in rats

Activation of Na+/H+ exchanger (NHE) may have an important role in ischemic cell death by means of intracellular overload of Na(+) and Ca(2+). Recent evidence has suggested that inhibitors of NHE have protective effects on myocardial ischemia both in vivo and in vitro. In this study, we tested the hypothesis that FR183998, an inhibitor of NHE, reduces infarct volume produced by focal cerebral ischemia in rats. We used 20 male spontaneously hypertensive rats. Either FR183998 (1 mg/kg; n=10), or vehicle (n=10) was given intravenously to the rats and the distal middle cerebral artery of each animal was occluded using a photothrombotic technique. We measured regional cerebral blood flow using laser-Doppler flowmetry throughout the experiments. After 3 days, infarct volume was measured in each animal group. To estimate the brain edema, we also calculated the cortical volume in both hemispheres. The infarct volume in the FR183998-treated group (82+/-8 mm(3), mean+/-S.E.M.) was significantly smaller than that in the control group (115+/-12 mm(3)) (P=0.034). The cortical volume of the occluded side in the FR183998-treated group (359+/-7 mm(3)) tended to be smaller than that in the control group (378+/-9 mm(3)) (P=0.116). The regional cerebral blood flow and physiological variables during ischemia were not significantly different between the two groups throughout the experiments. These results suggest that inhibition of NHE by FR183998 may have beneficial effects in reducing infarct volume and brain edema during cerebral ischemia. Thus, NHE may play an important role in the development of neuronal damage during acute cerebral ischemia.

The myocardial Na+/H+ exchanger: a potential therapeutic target for the prevention of myocardial ischaemic and reperfusion injury and attenuation of postinfarction heart failure

The myocardial Na+/H+ exchange (NHE) represents a major mechanism for pH regulation during normal physiological processes but especially during ischaemia and early reperfusion. However, there is now very compelling evidence that its activation contributes to paradoxical induction of cell injury. The mechanism for this most probably reflects the fact that activation of the exchanger is closely coupled to Na+ influx and therefore to elevation in intracellular Ca2+ concentrations through the Na+/Ca2+ exchange. The NHE is exquisitely sensitive to intracellular acidosis; however, other factors can also exhibit stimulatory effects via phosphorylation-dependent processes. These generally represent various autocrine and paracrine as well as hormonal factors such as endothelin-1, angiotensin II and alpha1-adrenoceptor agonists, which probably act through receptor-signal transduction processes. Thus far, 6 NHE isoforms have been identified and designated as NHE1 through NHE6. All except NHE6, which is located intracellularly, are restricted to the sarcolemmal membrane. In the mammalian myocardium the NHE1 subtype is the predominant isoform, although NHE6 has also been identified in the heart. The predominance of NHE1 in the myocardium is of some importance since, as discussed in this review, pharmacological development of NHE inhibitors for cardiac therapeutics has concentrated specifically on those agents which are selective for NHE1. These agents, as well as the earlier nonspecific amiloride derivatives have now been extensively demonstrated to possess excellent cardioprotective properties, which appear to be superior to other strategies, including the extensively studied phenomenon of ischaemic preconditioning. Moreover, the salutary effects of NHE inhibitors have been demonstrated using a variety of experimental models as well as animal species suggesting that the role of the NHE in mediating injury is not species specific. The success of NHE inhibitors in experimental studies has led to clinical trials for the evaluation of these agents in high risk patients with coronary artery disease as well as in patients with acute myocardial infarction (MI). Recent evidence also suggests that NHE inhibition may be conducive to attenuating the remodelling process after MI, independently of infarct size reduction, and attenuation of subsequent postinfarction heart failure. As such, inhibitors of NHE offer substantial promise for clinical development for attenuation of both acute responses to myocardial as well as chronic postinfarction responses resulting in the evolution to heart failure.

Myocardial protection during ventricular fibrillation by reduction of proton-driven sarcolemmal sodium influx

Although the inhibition of proton-driven sarcolemmal sodium influx ameliorates ischemic injury in the quiescent myocardium, the effects when ventricular fibrillation is present are largely unknown. We used an isolated rat heart model to investigate whether inhibition of the sodium-hydrogen exchanger isoform-1 (with the benzoylguanidine derivatives HOE-694 and cariporide) with or without concomitant inhibition of the sodium-bicarbonate co-transporter (with perfusate buffered with N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid [HEPES]) during ischemia and ventricular fibrillation could ameliorate functional myocardial abnormalities presumed to limit cardiac resuscitability. Ischemic contracture, which typically develops during ventricular fibrillation, was ameliorated by HOE-694 when either a bicarbonate-buffered (20 +/- 7 mm Hg vs 15 +/- 5 mm Hg, P <.05) or a HEPES-buffered (14 +/- 5 mm Hg vs 10 +/- 3 mm Hg, P <.04) perfusate was used. Maximal amelioration occurred when cariporide and HEPES-buffered perfusate were used simultaneously (25 +/- 14 mm Hg vs 11 +/- 3 mm Hg, P <.01), and this was accompanied by lesser leftward shifts of the end-diastolic pressure-volume curves after defibrillation. Intramyocardial sodium increases of 76% during ischemia and ventricular fibrillation (P <.05) were ameliorated by the sodium-influx-limiting interventions. Thus interventions limiting sarcolemmal sodium influx during ischemia and ventricular fibrillation may facilitate successful resuscitation from ventricular fibrillation.

Na(+)/H(+) exchange inhibitor cariporide attenuates cell injury predominantly during ischemia and not at onset of reperfusion in porcine hearts with low residual blood flow

BACKGROUND

This study investigated whether myocardial protection by inhibition of Na(+)/H(+) exchange (NHE) occurs during ischemia and/or during reperfusion.

METHODS AND RESULTS

The left anterior descending coronary artery was occluded in 32 pigs for 60 minutes and then reperfused for 24 hours. Infarct sizes (nitroblue tetrazolium [NBT] stain, histology) were determined at the end of the experiments. An extracorporeal bypass was used to achieve a constant residual blood flow of 3 mL/min in the myocardium at risk during ischemia. The NHE-1 inhibitor cariporide or distilled water was infused into the extracorporeal bypass system. In group 1, active treatment was administered from the onset of ischemia until 10 minutes of reperfusion (n=8). In group 2, active treatment was infused during the first 30 minutes of ischemia only (n=8). The group 3 animals (n=8) received intracoronary cariporide after 45 minutes of ischemia until 10 minutes of reperfusion. The control animals (group 4, n=7) were treated similarly to group 1 animals, with the cariporide solution being replaced by distilled water. Infarct sizes of group 1 (NBT stain, 41.5+/-20%; histology, 44. 6+/-12%) and group 2 (NBT stain, 33.5+/-14%; histology 34.9+/-15%) differed significantly (at least P:=0.012) from infarct sizes of group 3 (NBT stain, 71.6+/-15%; histology, 69.2+/-12%) and the control group (NBT stain, 76+/-9%; histology 72.4+/-12%). Cariporide treatment in group 1 and group 2 significantly improved functional recovery after 24 hours of reperfusion.

CONCLUSIONS

Myocardial protection by cariporide is predominantly achieved by NHE inhibition during ischemia and not during early reperfusion.

Na(+)/H(+) exchange inhibition-induced cardioprotection in dogs: effects on neutrophils versus cardiomyocytes

Numerous studies have examined the effect of Na(+)/H(+) exchanger (NHE) inhibition on the myocardium; however, the effect of NHE-1 inhibition on neutrophil function has not been adequately examined. An in vivo canine model of myocardial ischemia-reperfusion injury in which 60 min of left anterior descending coronary artery occlusion followed by 3 h of reperfusion was used to examine the effect of NHE-1 inhibition on infarct size (IS) and neutrophil function. BIIB-513, a selective inhibitor of NHE-1, was infused before ischemia. IS was expressed as a percentage of area at risk (IS/AAR). NHE-1 inhibition significantly reduced IS/AAR and reduced neutrophil accumulation in the ischemic myocardium. NHE-1 inhibition attenuated both phorbol 12-myristate 13-acetate- and platelet-activating factor-induced neutrophil respiratory burst but not CD18 upregulation. Furthermore, NHE-1 inhibition directly protected cardiomyocytes against metabolic inhibition-induced lactate dehydrogenase release and hypercontracture. This study provides evidence that the cardioprotection induced by NHE-1 inhibition is likely due to specific protection of cardiomyocytes and attenuation of neutrophil activity.

Reduction of myocardial infarct size by SM-20550, a novel Na(+)/H(+) exchange inhibitor, in rabbits

The effects of N-(aminoiminomethyl)-1, 4-dimethyl-1H-indole-2-carboxamide methanesulfonic acid (SM-20550), a novel potent Na(+)/H(+) exchanger, and nicorandil, a K(+) channel opener with nitrate-like activity, were studied in a myocardial ischemia and reperfusion injury model. Anesthetized rabbits underwent occlusion of the coronary artery (30 min) followed by reperfusion (5 h). Intravenous administration of SM-20550 before ischemia reduced the infarct size by approximately 30-70% in a dose-dependent manner, with a significant reduction in serum creatine phosphokinase activity. Similarly, intravenous administration of nicorandil before ischemia reduced the infarct size by 33% with a significant reduction in serum creatine phosphokinase activity. Moreover, intravenous administration of SM-20550 after ischemia resulted in a significant, approximately 20-40% reduction in the infarct size, but the administration of nicorandil after ischemia did not reduce the infarct size. These results indicate that SM-20550 reduced myocardial necrosis when administered either before or after ischemia.

Pharmacology and clinical assessment of cariporide for the treatment coronary artery diseases

Myocardial protection through pharmacological approaches represents a large therapeutic challenge and is an important therapeutic strategy in patients with coronary artery disease, particularly after myocardial infarction. Extensive animal experiments have repeatedly demonstrated the efficacy of sodium-hydrogen exchange (NHE) inhibition as a potent cardioprotective approach. The heart possesses primarily the NHE1 isoform which has led to the development of NHE1 specific inhibitors for cardiovascular therapeutics. Cariporide (HOE 642) is the first of such agents to have been developed and subjected to clinical trial. Preclinical studies with cariporide revealed excellent protection against necrosis, apoptosis, arrhythmias and mechanical dysfunction in hearts subjected to ischaemia and reperfusion. Cariporide has recently been evaluated in a large dose-finding Phase II/Phase III clinical trial (GUARDIAN) to assess its efficacy in patients with acute coronary syndromes. Overall results failed to demonstrate protection but sub-group analysis revealed significant risk reductions with the highest cariporide dose (120 mg t.i.d.) especially in high risk patients undergoing coronary artery bypass surgery. This suggests that insufficient dosage may have accounted, at least in part, for the less than optimum results. Another NHE1 inhibitor, eniporide, is currently in Phase II clinical trial (ESCAMI) in patients with acute myocardial infarction (MI) who are given angioplasty or thrombolysis. Although the study has not been completed interim findings appear positive. Both drugs were well-tolerated and produced no excess side effects compared with placebo. Further studies are needed to confirm the efficacy of NHE1 inhibitors for the treatment of coronary heart disease, even so initial results are encouraging.

Importance of bicarbonate transport for protection of cardiomyocytes against reoxygenation injury

Isolated cardiomyocytes from adult rats were incubated in anoxic bicarbonate-buffered media at extracellular pH (pH(o)) 6.4 until a cytosolic Ca(2+) overload and intracellular pH (pH(i)) of 6.4 were reached. On reoxygenation, the pH of the medium was changed to 7.4 to activate the Na(+)/H(+)exchanger (NHE) and the Na(+)-HCO(-)(3) symporter (NBS). The reoxygenation was performed in the absence or presence of the NHE inhibitor HOE-642 (3 micromol/l) and/or the NBS inhibitor DIDS (0.5 mmol/l), as in bicarbonate-free media. In reoxygenated control cells pH(i) rapidly recovered to the preanoxic level, and a burst of spontaneous oscillations of cytosolic Ca(2+) occurred, accompanied by the development of hypercontracture. When NBS and NHE were simultaneously inhibited during reoxygenation, pH(i) recovery was prevented, Ca(2+) oscillations were attenuated, and hypercontracture was abolished. Sole inhibition of NBS or NHE showed no protection against hypercontracture. In the absence of cytosolic acidosis, HOE-642 or DIDS did not prevent hypercontracture induced by Ca(2+) overload. The results demonstrate that simultaneous inhibition of NHE and NBS is needed to protect myocardial cells against reoxygenation-induced hypercontracture.