Myocardial cell protection : a challenging time for action and a challenging time for clinical research

Predictors of no- reflow during primary angioplasty for acute myocardial infarction, from Medical College Hospital, Trivandrum

Background

Primary angioplasty (PCI) for acute myocardial infarction is associated with no-reflow phenomenon, in about 5–25% of cases. Here we analysed the factors predicting no reflow .

Methods

This was a case control study of consecutive patients with acute myocardial infarction who underwent Primary PCI from August 2014 to February 2015.

Results

Of 181 patients who underwent primary PCI, 47 (25.9%) showed an angiographic no-reflow phenomenon. The mean age was 59.19 ± 10.25 years and females were 11%.

Univariate predictors of no reflow were age >60 years (OR = 6.146, 95%CI 2.937–12.86, P = 0<0.001), reperfusion time >6 h (OR = 21.94, 95%CI 9.402–51.2, P = < 0.001), low initial TIMI flow (≤1) (OR = 12.12, 95%CI 4.117–35.65, P < 0.001), low initial TMPG flow (≤1) (OR = 36.19, 95%CI 4.847–270.2, P < 0.001) a high thrombus burden (OR = 11.04,95%CI 5.124–23.8, P < 0.001), a long target lesion (OR = 8.54, 95%CI 3.794–19.23, P < 0.001), Killip Class III/IV(OR = 2.937,95%CI 1.112–7.756,P = 0.025) and overlap stenting(OR = 3.733,95%CI 1.186–11.75,P = 0.017).

Multiple stepwise logistic regression analysis predictors were: longer reperfusion time > 6 h (OR = 13.844, 95%CI 3.214–59.636, P = <0.001), age >60 years (OR = 8.886, 95%CI 2.145–36.80, P = 0.003), a long target lesion (OR = 8.637, 95%CI 1.975–37.768, P = 0.004), low initial TIMI flow (≤1) (OR = 20.861, 95%CI 1.739–250.290, P = 0.017).

Conclusions

It is important to minimize trauma to the vessel, avoid repetitive balloon dilatations use direct stenting and use the shortest stent if possible.

Effects and mechanism of Xin Mai Jia in a rabbit model of atherosclerosis

The aim of this study was to investigate the protective effects of Xin Mai Jia (XMJ) on atherosclerosis (AS) in rabbits and to explore the underlying mechanisms in order to provide experimental evidence for the clinical application of XMJ. An intraperitoneal injection of vitamin D3, combined with a high-fat diet and sacculus injury, was utilized to establish the AS rabbit model. Following the oral administration of lovastatin, Zhibituo and different dosages of XMJ, respectively, blood was drawn from each rabbit for the detection of blood rheological indicators, such as serum lipids. The pathological changes in the right common carotid artery were observed. Vascular function experiments and the expression detection of common carotid artery-related proteins by immunohistochemistry were conducted. XMJ was observed to decrease the blood lipid levels of the AS rabbits; increase the concentration of high-density lipoprotein and apolipoprotein A; decrease blood viscosity, erythrocyte sedimentation rate and hematocrit; elevate the levels of endothelial nitric oxide synthase (eNOS) and Na⁺/H⁺ exchanger 1 in vascular tissues and decrease the levels of angiotensin II receptor, type 1 (AT-1) and endothelin-1 (ET-1). In conclusion, XMJ was shown to lower the blood lipid levels of the experimental AS rabbits, improve the abnormal changes in hemorheology, increase the eNOS content in the vascular tissue, decrease the AT-1 and ET-1 levels and increase the endothelium-dependent vasodilation reaction. XMJ therefore has an anti-AS effect.

Cardiac diseases complicating community-acquired pneumonia

PURPOSE OF REVIEW

The aim is to evaluate basic mechanisms, prevalence, risk factors, outcomes, and potential treatments of cardiovascular events (CVEs) in patients with community-acquired pneumonia (CAP).

RECENT FINDINGS

In this review, we present a new model to evaluate the pathophysiology of cardiac disease in patients with pneumonia based on plaque-related events, such as acute myocardial infarction, versus plaque-unrelated events, such as arrhythmias and heart failure. CAP increases the risk for both plaque-related and plaque-unrelated events with an absolute rate of CVE across different cohorts that varies broadly from 10 to 30%. These complications may happen among both ambulatory patients and inpatients, either on admission or during hospitalization, and/or after discharge. CVEs represent a major cause for increased mortality in CAP patients, contributing to more than 30% of deaths at long-term follow-up.

SUMMARY

From a clinical perspective, especially during the first 24 h after hospitalization, CAP patients should be tested for the probability to have or develop during hospitalization a cardiac event. From a research point of view, there is an urgent need to prospectively evaluate cardioprotective interventions.

Translation of TRO40303 from myocardial infarction models to demonstration of safety and tolerance in a randomized Phase I trial

BACKGROUND

Although reperfusion injury has been shown to be responsible for cardiomyocytes death after an acute myocardial infarction, there is currently no drug on the market that reduces this type of injury. TRO40303 is a new cardioprotective compound that was shown to inhibit the opening of the mitochondrial permeability transition pore and reduce infarct size after ischemia-reperfusion in a rat model of cardiac ischemia-reperfusion injury.

METHODS

In the rat model, the therapeutic window and the dose effect relationship were investigated in order to select the proper dose and design for clinical investigations. To evaluate post-ischemic functional recovery, TRO40303 was tested in a model of isolated rat heart. Additionally, TRO40303 was investigated in a Phase I randomized, double-blind, placebo controlled study to assess the safety, tolerability and pharmacokinetics of single intravenous ascending doses of the compound (0.5 to 13 mg/kg) in 72 healthy male, post-menopausal and hysterectomized female subjects at flow rates from 0.04 to 35 mL/min (EudraCT number: 2010-021453-39). This work was supported in part by the French Agence Nationale de la Recherche.

RESULTS

In the vivo model, TRO40303 reduced infarct size by 40% at 1 mg/kg and by 50% at 3 and 10 mg/kg given by intravenous bolus and was only active when administered before reperfusion. Additionally, TRO40303 provided functional recovery and reduced oxidative stress in the isolated rat heart model.These results, together with pharmacokinetic based allometry to human and non-clinical toxicology data, were used to design the Phase I trial. All the tested doses and flow rates were well tolerated clinically. There were no serious adverse events reported. No relevant changes in vital signs, electrocardiogram parameters, laboratory tests or physical examinations were observed at any time in any dose group. Pharmacokinetics was linear up to 6 mg/kg and slightly ~1.5-fold, hyper-proportional from 6 to 13 mg/kg.

CONCLUSIONS

These data demonstrated that TRO40303 can be safely administered by the intravenous route in humans at doses expected to be pharmacologically active. These results allowed evaluating the expected active dose in human at 6 mg/kg, used in a Phase II proof-of-concept study currently ongoing.

Inhibition of O-GlcNAcase in perfused rat hearts by NAG-thiazolines at the time of reperfusion is cardioprotective in an O-GlcNAc-dependent manner

Acute increases in O-linked β-N-acetylglucosamine (O-GlcNAc) levels of cardiac proteins exert protective effects against ischemia-reperfusion (I/R) injury. One strategy to rapidly increase cellular O-GlcNAc levels is inhibition of O-GlcNAcase (OGA), which catalyzes O-GlcNAc removal. Here we tested the cardioprotective efficacy of two novel and highly selective OGA inhibitors, the NAG-thiazoline derivatives NAG-Bt and NAG-Ae. Isolated perfused rat hearts were subjected to 20 min global ischemia followed by 60 min reperfusion. At the time of reperfusion, hearts were assigned to the following four groups: 1) untreated control; 2) 50 μM NAG-Bt; 3) 100 μM NAG-Bt; or 4) 50 μM NAG-Ae. All treatment groups significantly increased total O-GlcNAc levels (P < 0.05 vs. control), and this was significantly correlated with improved contractile function and reduced cardiac troponin I release (P < 0.05). Immunohistochemistry of normoxic hearts showed intense nuclear O-GlcNAc staining and higher intensity at Z-lines with colocalization of O-GlcNAc and the Z-line proteins desmin and vinculin. After I/R, there was a marked loss of both cytosolic and nuclear O-GlcNAcylation and disruption of normal striated Z-line structures. OGA inhibition largely preserved structural integrity and attenuated the loss of O-GlcNAcylation; however, nuclear O-GlcNAc levels remained low. Immunoblot analysis confirmed ∼50% loss in both nuclear and cytosolic O-GlcNAcylation following I/R, which was significantly attenuated by OGA inhibition (P < 0.05). These data provide further support for the notion that increasing cardiac O-GlcNAc levels by inhibiting OGA may be a clinically relevant approach for ischemic cardioprotection, in part, by preserving the integrity of O-GlcNAc-associated Z-line protein structures.

Dimethyl amiloride, a Na+-H+ exchange inhibitor, and its cardioprotective effects in hemorrhagic shock in in vivo resuscitated rats

Stimulation of the Na(+)-H(+) exchanger plays an important role in the pathway of myocardial dysfunction and injury following hemorrhagic shock. Inhibition of the Na(+)-H(+) exchanger appears to be a new pharmacological tool for myocardial protection. Despite the extensive research that has been done on the role of the Na(+)-H(+) exchanger in ischemia reperfusion, little is known about the role of the exchanger following hemorrhagic shock. The purpose of this study was to examine the protective effects of blocking the cardiac Na(+)-H(+) exchanger, using 20 microM dimethyl amiloride (DMA), a specific Na(+)-H(+) exchanger blocker, on myocardial contractile function after ex vivo perfusion of isolated rat heart following 1 h of hemorrhagic shock. Sprague-Dawley rats were assigned to hemorrhage + DMA, hemorrhage, sham hemorrhage + DMA and sham hemorrhage groups (n = 6 per group). Hearts were perfused with a balanced salt solution for 60 min. In the DMA treated group, 20 microM DMA was added for the first 5 min of the 60-min ex vivo heart resuscitation. The results showed that inhibition of the Na(+)-H(+) exchanger for 5 min on ex vivo perfusion of the isolated hearts following hemorrhagic shock using 20 microM DMA improved myocardial contractile function. Blocking the Na(+)-H(+) exchanger on ex vivo perfusion of isolated hearts using 20 muM DMA has protective effects on myocardial contractile function.

No-reflow phenomenon and prognosis in patients with acute myocardial infarction

The rapid restoration of coronary flow to the jeopardized myocardium has become an essential part of therapy after acute myocardial infarction. Despite an open infarct-related artery, breakdown of or obstruction to coronary microvasculature can markedly reduce blood flow to the infarct zone. This effect is known as the no-reflow phenomenon. Advances in imaging modalities have improved visualization of no reflow, showing its frequency to be higher than was estimated by clinical judgment alone. This phenomenon is important because it correlates with infarct size and provides useful prognostic information. No reflow is associated with reduced left ventricular ejection fraction, left ventricular remodeling, and poor clinical outcomes, placing patients with this effect in a high-risk group among reperfused patients. The focus of reperfusion therapy is shifting towards improved myocardial perfusion, which could promote functional recovery of viable muscle, reduce infarct expansion, and increase the delivery of blood-borne components, thereby accelerating the healing process. Various pharmacologic interventions and catheter-based devices to retrieve embolic materials have been proposed. Further studies to improve understanding of the pathophysiology of microvascular dysfunction will, however, help in the further development of preventive and therapeutic strategies. In this article, I discuss in depth the data available on the no-reflow phenomenon.

Impact of low-flow ischemia on substrate oxidation and glycolysis in the isolated perfused rat heart

Interventions that stimulate carbohydrate oxidation appear to be beneficial in the setting of myocardial ischemia or infarction. However, the mechanisms underlying this protective effect have not been defined, in part because of our limited understanding of substrate utilization under ischemic conditions. Therefore, we used (1)H and (13)C NMR spectroscopy to investigate substrate oxidation and glycolytic rates in a global low-flow model of myocardial ischemia. Isolated male Sprague-Dawley rat hearts were perfused for 30 min under conditions of normal flow (control) and low-flow ischemia (LFI, 0.3 ml/min) with insulin and (13)C-labeled lactate, pyruvate, palmitate, and glucose at concentrations representative of the physiological fed state. Despite a approximately 50-fold reduction in substrate delivery and oxygen consumption, oxidation of all exogenous substrates plus glycogen occurred during LFI. Oxidative metabolism accounted for 97% of total calculated ATP production in the control group and approximately 30% in the LFI group. For controls, lactate oxidation was the major source of ATP; however, in LFI, this shifted to a combination of oxidative and nonoxidative glycogen metabolism. Interestingly, in the LFI group, anaplerosis relative to citrate synthase increased sevenfold compared with controls. These results demonstrate the importance of oxidative energy metabolism for ATP production, even during very-low-flow ischemia. We believe that the approach described here will be valuable for future investigations into the underlying mechanisms related to the protective effect of increasing cardiac carbohydrate utilization and may ultimately lead to identification of new therapeutic targets for treatment of myocardial ischemia.

Urocortin-II and urocortin-III are cardioprotective against ischemia reperfusion injury: an essential endogenous cardioprotective role for corticotropin releasing factor receptor type 2 in the murine heart

Corticotropin-releasing factor (CRF) receptor type 2beta (CRFR2beta) is expressed in the heart. Urocortin (Ucn)-I activation of CRFR2beta is cardioprotective against ischemic reperfusion (I/R) injury by stimulation of the ERKs1/2 p42, 44. However, by binding CRF receptor type 1, Ucn-I can also activate the hypothalamic stress axis. Ucn-II/stresscopin related peptide and Ucn-III/stresscopin are two new members of the CRF/Ucn-I gene family and are selective for CRFR2beta. We propose that CRFR2beta selective Ucn-II or Ucn-III will protect cardiomyocytes and the ex vivo Langendorff perfused rat heart from I/R injury by activation of ERK1/2-p42, 44. Ucn-II is expressed in mouse cardiomyocytes, and Ucn-II or Ucn-III can bind to CRFR2beta, resulting in ERK1/2-p42, p-44 phosphorylation and cAMP stimulation. Phosphorylation of ERK1/2-p42, p-44 is regulated by the Ras/Raf-1 kinase pathway, independent of adenylate cyclase and, therefore, cAMP activation. Ucn-II and Ucn-III protect cardiomyocytes from I/R injury and reduce the percentage of infarct size:risk ratio in Langendorff perfused rat hearts exposed to regional I/R (P<0.001). The CRFR2 selective antagonist astressin2-B and an ERK1/2-p42, 44 inhibitor abolish the cardioprotective actions of Ucn-II and Ucn-III in reperfusion. Cardiomyocytes isolated from CRFR2-null mice are less resistant to I/R injury, compared with wild-type cardiomyocytes. We propose the use of CRFR2 selective agonists, Ucn-II and Ucn-III, to treat ischemic heart disease because of their potent cardioprotective effects in the murine heart and their minimal impact on the hypothalamic stress axis. We emphasize an important endogenous cardioprotective role for CRFR2beta in the murine heart.

Cardioprotection against reperfusion injury is maximal with only two minutes of sevoflurane administration in rats

PURPOSE

Volatile anesthetics can protect the heart against reperfusion injury. When sevoflurane is given for the first 15 min of reperfusion, a concentration corresponding to one minimum alveolar concentration (MAC) provides a maximum protective effect. The present study addresses the question of how long sevoflurane has to be administered to achieve the best cardioprotection.

METHODS

Chloralose anesthetized rats were subjected to a 25-min occlusion of a major coronary artery, followed by 90 min of reperfusion. During the initial phase of reperfusion, an end-tidal concentration of 2.4 vol.% of sevoflurane (1 MAC) was given for two (n = 8), five (n = 8) or ten minutes (n = 7). Seven rats served as untreated controls. We measured left ventricular (LV) pressure, mean aortic pressure and infarct size (triphenyltetrazolium staining).

RESULTS

Administration of sevoflurane for two minutes resulted in the greatest reduction of infarct size to 15% (8-22 [mean (95% confidence interval)] of the area at risk compared with controls [51 (47-55) %, P < 0.001]. Five or ten minutes of sevoflurane administration reduced infarct size to 26 (18-34) and 26 (18-35) % [P < 0.05], respectively. The cardiodepressant effect of sevoflurane varied with the duration of its administration: LV dP/dt was reduced from 6332 mmHg x sec(-1) (5771-6894) during baseline to 4211 mmHg x sec(-1) (3031-5391), 3811 mmHg x sec(-1) (2081-5540) and 3612 mmHg x sec(-1) (2864-4359) after two, five and ten minutes of reperfusion, respectively.

CONCLUSION

Administration of 1 MAC sevoflurane for the first two minutes of reperfusion effectively protects the heart against reperfusion injury in rats in vivo. A longer administration time had lesser cardioprotective effects in this experimental model.

Characterization of sabiporide, a new specific NHE-1 inhibitor exhibiting slow dissociation kinetics and cardioprotective effects

Sabiporide, a new benzoguanidine, was characterized on fibroblasts stably expressing the Na(+)/H(+) exchanger isoforms NHE-1, NHE-2 and NHE-3. 22Na(+) uptake experiments show that this compound possesses a K(i) of 5+/-1.2 x 10(-8) M for NHE-1, and discriminates efficiently between the NHE-1, -2 and -3 isoforms (K(i) for NHE-2: 3+/-0.9 x 10(-6) M, and K(i)>1 mM for NHE-3). Similar K(i) values are obtained on rat cardiomyocytes and human platelets expressing NHE-1 (K(i)'s of 7+/-1 x 10(-9) and 2.7+/-0.4 x 10(-8) M respectively). Interestingly, when compared with amiloride and cariporide, sabiporide inhibition persists even after this molecule had been rinsed out (half time of 7 h for sabiporide, and of 1 and 2.5 min for amiloride and cariporide, respectively), the decay of all these molecules exhibiting a complex multiexponential behavior. Thus, sabiporide, which possesses remarkable cardioprotective properties, is a specific NHE-1 inhibitor possessing unique binding kinetics.

Effect of endovascular cooling on myocardial temperature, infarct size, and cardiac output in human-sized pigs

Mild hypothermia reduces myocardial infarct size in small animals; however, the extent of myocardial protection in large animals with greater thermal mass remains unknown. We evaluated the effects of mild endovascular cooling on myocardial temperature, infarct size, and cardiac output in 60- to 80-kg isoflurane-anesthetized pigs. We occluded the left anterior descending coronary artery for 60 min, followed by reperfusion for 3 h. An endovascular heat-exchange catheter was used to either lower core body temperature to 34 degrees C (n = 11) or maintain temperature at 38 degrees C (n = 11). Additional studies assessed myocardial viability and microvascular perfusion with (99m)Tc-sestamibi autoradiography. Endovascular cooling reduced infarct size compared with normothermia (9 +/- 6% vs. 45 +/- 8% of the area at risk; P < 0.001), whereas the area at risk was comparable (19 +/- 3% vs. 20 +/- 7%; P = 0.65). Salvaged myocardium showed normal sestamibi uptake, confirming intact microvascular flow and myocyte viability. Cardiac output was maintained in hypothermic hearts because of an increase in stroke volume, despite a decrease in heart rate. Mild endovascular cooling to 34 degrees C lowers myocardial temperature sufficiently in human-sized hearts to cause a substantial cardioprotective effect, preserve microvascular flow, and maintain cardiac output.

Cariporide (HOE 642) attenuates leukocyte activation in ischemia and reperfusion

Cariporide (HOE 642) ameliorates myocardial ischemia/reperfusion (I/R) injury, by the well established reduction of cytosolic [Ca(2+)] in cardiac myocytes through inhibition of Na(+)/H(+) exchange. However, postischemic inflammation also contributes to I/R injury. We tested the hypothesis that cariporide also modulates the inflammatory response. The effect of cariporide on L-selectin expression by human leukocytes in vitro and leukocyte adhesion and emigration in the reperfused rat cremaster muscle in vivo were studied. The rat cremaster muscle was exteriorized for intravital videomicroscopy, induction of ischemia (90 min), and reperfusion (90 min). Eleven rats were pretreated with cariporide (9 mg/kg body weight IV) whereas 11 rats received saline. Leukocyte adhesion was quantified offline. Human venous blood was incubated with cariporide (3 micromol/L) or saline, stimulated with formyl- methionine-leucine-phenylalanine (10(-10)-10(-6) mol/L), and granulocyte L-selectin expression was analyzed by flow cytometry. Cariporide reduced leukocyte rolling and adhesion by approximately 35% and 45%, respectively, after 30 min of reperfusion. Leukocyte extravasation was decreased by approximately 85% after 90 min. Cariporide increased L-selectin shedding at each formyl-methionine-leucine-phenylalanine concentration, reducing the 50% effective dose from 9.95 to 4.68 nmol/L. Thus, cariporide may ameliorate I/R injury not only by the known reduction of cytosolic [Ca(2+)] in cardiomyocytes, but also by attenuating leukocyte-dependent inflammatory responses. Promotion of L-selectin shedding from activated leukocytes may present a mechanism underlying this newly detected effect.

IMPLICATIONS

This study provides evidence that inhibition of Na(+)/H(+) exchange by cariporide (HOE 642) attenuates the postischemic inflammatory response. Leukocyte adhesion and emigration, assessed by in vivo microscopy, were markedly reduced in rat cremaster muscle, possibly because of increased L-selectin shedding of activated leukocytes as demonstrated by flow cytometry.

The Na(+)/H(+) exchange inhibitor eniporide as an adjunct to early reperfusion therapy for acute myocardial infarction. Results of the evaluation of the safety and cardioprotective effects of eniporide in acute myocardial infarction (ESCAMI) trial

OBJECTIVES

We conducted an international, prospective, randomized, double-blind, placebo-controlled phase 2 trial in patients undergoing thrombolytic therapy or primary angioplasty for acute ST-elevation myocardial infarction (MI) to investigate the effect of eniporide on infarct size and clinical outcome.

BACKGROUND

Experimental studies suggest that the activity of the Na(+)/H(+) exchange (NHE) plays an important role in the unfavorable sequels of myocardial ischemia and reperfusion. Eniporide specifically inhibits the NHE-1 isoform and has been shown to limit infarct size in experimental models.

METHODS

The primary efficacy end point was the infarct size measured by the cumulative release of alpha-hydroxybutyrate dehydrogenase (alpha-HDBH) (area under the curve [AUC] 0 to 72 h). In stage 1, 50, 100, 150 or 200 mg eniporide given as a 10-min infusion before start of reperfusion therapy were compared with placebo in 430 patients, and in stage 2, 100 and 150 mg eniporide were compared with placebo in 959 patients.

RESULTS

In stage 1, the administration of 100 mg and 150 mg eniporide resulted in smaller infarct sizes (mean alpha-HBDH AUC in U/ml x h, placebo: 44.2, 100 mg eniporide: 40.2, 150 mg eniporide: 33.9), especially in the angioplasty group. In contrast, in stage 2 there was no difference in the enzymatic infarct size between the three groups (placebo: 41.2, 100 mg eniporide: 43.0, 150 mg eniporide: 41.5). Overall there was no effect of eniporide on clinical outcome (death, cardiogenic shock, heart failure, life-threatening arrhythmias). However, there was a significant reduction of the incidence of heart failure in patients reperfused late (>4 h).

CONCLUSIONS

In this large study administration of the NHE-1 inhibitor eniporide, before reperfusion therapy in patients with acute ST elevation MI, did not limit infarct size or improve clinical outcome.

Coupling of histamine H3 receptors to neuronal Na+/H+ exchange: a novel protective mechanism in myocardial ischemia

In myocardial ischemia, adrenergic nerves release excessive amounts of norepinephrine (NE), causing dysfunction and arrhythmias. With anoxia and the concomitant ATP depletion, vesicular storage of NE is impaired, resulting in accumulation of free NE in the axoplasm of sympathetic nerves. Intraneuronal acidosis activates the Na(+)/H(+) exchanger (NHE), leading to increased Na(+) entry in the nerve terminals. These conditions favor availability of the NE transporter to the axoplasmic side of the membrane, causing massive carrier-mediated efflux of free NE. Neuronal NHE activation is pivotal in this process; NHE inhibitors attenuate carrier-mediated NE release. We previously reported that activation of histamine H(3) receptors (H(3)R) on cardiac sympathetic nerves also reduces carrier-mediated NE release and alleviates arrhythmias. Thus, H(3)R activation may be negatively coupled to NHE. We tested this hypothesis in individual human SKNMC neuroblastoma cells stably transfected with H(3)R cDNA, loaded with the intracellular pH (pH(i)) indicator BCECF. These cells possess amiloride-sensitive NHE. NHE activity was measured as the rate of Na(+)-dependent pH(i) recovery in response to an acute acid pulse (NH(4)Cl). We found that the selective H(3)R-agonist imetit markedly diminished NHE activity, and so did the amiloride derivative EIPA. The selective H(3)R antagonist thioperamide abolished the imetit-induced NHE attenuation. Thus, our results provide a link between H(3)R and NHE, which may limit the excessive release of NE during protracted myocardial ischemia. Our previous and present findings uncover a novel mechanism of cardioprotection: NHE inhibition in cardiac adrenergic neurons as a means to prevent ischemic arrhythmias associated with carrier-mediated NE release.

Therapeutic potential of H(3)-receptor agonists in myocardial infarction

Sympathetic over-activity accompanied by excessive noradrenaline (NA) release within the heart is a recognised cause of dysfunction in myocardial ischaemia. Myocardial infarction is often accompanied by arrhythmias with high morbidity and mortality. Indeed, NA enhances intracellular Ca(2+) by increasing its influx through voltage-dependent channels, mobilising it from intracellular stores and favouring its inward transport by Na(+)/Ca(2+) exchange. Ca(2+) overload eventually results in dysrhythmia and uncoordinated myocyte contraction. Moreover, NA increases metabolic demand. In concert with other contributing factors, this will aggravate the primary ischaemia and initiate a vicious cycle that can culminate in myocardial damage and heart failure. Therefore, reduction of NA release from cardiac sympathetic nerves is an important protective measure. Adrenergic nerves possess inhibitory receptors, such as alpha(2)-adrenoceptors, adenosine A(1)-receptors and histamine H(3)-receptors (H(3)R). In myocardial infarction, NA is released by both exocytotic (Ca(2+)-dependent) and carrier-mediated (Na(+)/H(+) exchange-dependent) mechanisms, associated with short-term and protracted ischaemia, respectively. Unlike alpha(2)-adrenoceptor agonists that reduce NA exocytosis, but enhance carrier-mediated NA release, H(3)R agonists inhibit both exocytotic and carrier-mediated NA release. Moreover, unlike adenosine A(1)-receptor agonists, H(3)R agonists do not depress sinoatrial and atrioventricular nodes, nor cause bronchoconstriction. Therefore, stimulation of H(3)R on cardiac sympathetic nerve endings is an important new way to protect the heart from the consequences of ischaemia and infarction. Although H(3)R agonists alleviate reperfusion arrhythmias in isolated hearts by reducing NA release, this protective action needs to be demonstrated in classical in vivo models of occlusion/reperfusion. Regardless, H(3)R agonists offer the promise of a novel strategy in the treatment of myocardial ischaemia and infarction.

In vivo models of myocardial ischemia and reperfusion injury: application to drug discovery and evaluation

This review discusses the pharmacology of regional myocardial ischemia and reperfusion (I/R) injury and the utilization of in vivo animal models in the preclinical development of novel therapeutic compounds. The manuscript aims to provide an overview of a number of different cardioprotective strategies that have been successful from a preclinical perspective and to also present where possible results of clinical trials of the respective compounds. Myocardial ischemia reperfusion injury may be manifested as myocardial stunning, ventricular arrhythmias, coronary vascular dysfunction, or the development of a myocardial infarct. This review is principally concerned with preclinical studies related to reduction of infarct size. The pathophysiology of the reperfusion injury process is complex, including primarily cellular and humoral components of inflammation, as well as myocellular ionic and metabolic disturbances. This review will discuss strategies directed at oxygen-derived free radicals, neutrophils, adenosine, and the sodium-hydrogen exchanger (NHE). The results of preclinical cardioprotective studies are influenced by the paradigm used therefore methodological considerations will also be presented where appropriate.