Cardioprotective-Mimetics Reduce Myocardial Infarct Size in Animals Resistant to Ischemic Preconditioning

Increased sarcolemmal Na(+)/H(+) exchange activity in hypertrophied myocytes from dogs with chronic atrioventricular block

Dogs with compensated biventricular hypertrophy due to chronic atrioventricular block (cAVB), are more susceptible to develop drug-induced Torsade-de-Pointes arrhythmias and sudden cardiac death. It has been suggested that the increased Na(+) influx in hypertrophied cAVB ventricular myocytes contribute to these lethal arrhythmias. The increased Na(+) influx was not mediated by Na(+) channels, in fact the Na(+) current proved reduced in cAVB myocytes. Here we tested the hypothesis that increased activity of the Na(+)/H(+) exchanger type 1 (NHE-1), commonly observed in hypertrophic hearts, causes the elevated Na(+) influx. Cardiac acid-base transport was studied with a pH-sensitive fluorescent dye in ventricular myocytes isolated from control and hypertrophied cAVB hearts; the H(+) equivalent flux through NHE-1, Na(+)-HCO(-) 3 cotransport (NBC), Cl(-)/OH(-) exchange (CHE), and Cl(-)/HCO(-) 3 exchange (AE) were determined and normalized per liter cell water and corrected for surface-to-volume ratio. In cAVB, sarcolemmal NHE-1 flux was increased by 65 ± 6.3% in the pH i interval 6.3-7.2 and NBC, AE, and CHE fluxes remained unchanged. Accordingly, at steady-state intracellular pH the total sarcolemmal Na(+) influx by NHE-1 + NBC increased from 8.5 ± 1.5 amol/μm(2)/min in normal myocytes to 15 ± 2.4 amol/μm(2)/min in hypertrophied cAVB myocytes. We conclude that compensated cardiac hypertrophy in cAVB dogs is accompanied with an increased sarcolemmal NHE-1 activity. This in conjunction with unchanged activity of the other acid-base transporters will raise the intracellular Na(+) in hypertrophied cAVB myocytes.

Pathophysiology of myocardial reperfusion injury: preconditioning, postconditioning, and translational aspects of protective measures

Heart diseases due to myocardial ischemia, such as myocardial infarction or ischemic heart failure, are major causes of death in developed countries, and their number is unfortunately still growing. Preliminary exploration into the pathophysiology of ischemia-reperfusion injury, together with the accumulation of clinical evidence, led to the discovery of ischemic preconditioning, which has been the main hypothesis for over three decades for how ischemia-reperfusion injury can be attenuated. The subcellular pathophysiological mechanism of ischemia-reperfusion injury and preconditioning-induced cardioprotection is not well understood, but extensive research into components, including autacoids, ion channels, receptors, subcellular signaling cascades, and mitochondrial modulators, as well as strategies for modulating these components, has made evolutional progress. Owing to the accumulation of both basic and clinical evidence, the idea of ischemic postconditioning with a cardioprotective potential has been discovered and established, making it possible to apply this knowledge in the clinical setting after ischemia-reperfusion insult. Another a great outcome has been the launch of translational studies that apply basic findings for manipulating ischemia-reperfusion injury into practical clinical treatments against ischemic heart diseases. In this review, we discuss the current findings regarding the fundamental pathophysiological mechanisms of ischemia-reperfusion injury, the associated protective mechanisms of ischemic pre- and postconditioning, and the potential seeds for molecular, pharmacological, or mechanical treatments against ischemia-reperfusion injury, as well as subsequent adverse outcomes by modulation of subcellular signaling mechanisms (especially mitochondrial function). We also review emerging translational clinical trials and the subsistent clinical comorbidities that need to be overcome to make these trials applicable in clinical medicine.

Effect on ex vivo platelet aggregation and in vivo cyclic flow with Na+/H+ exchange inhibition: Gumina, NHE-1 inhibition and platelet aggregation

BIIB 513 and EMD 85131 are selective inhibitors of the Na+/H+ exchanger-1 (NHE-1) that are benzoylguanidine derivatives of the clinically employed diuretic amiloride. Prior studies have suggested a role for NHE-1 activity in platelet activation and aggregation using amiloride or its non- benzoylguanidines derivatives. However, the concentrations employed in these prior studies were at levels known to exert effects on other ion transport systems besides the NHE-1. Therefore, the purpose of this study was to examine the effects of more selective NHE-1 inhibitors, BIIB 513 and EMD 85131, on platelet aggregation and in vivo cyclic flow following arterial injury. BIIB 513 and EMD 85131 effects on ex vivo canine and human platelet aggregation in response to various agents was monitored via platelet aggregation. For analysis of in vivo thrombus formation, a femoral artery crush injury model was employed and a flow meter was used to monitor the effect of BIIB 513 on cyclic blood flow. Treatment of either canine or human platelets with up to 1 mM of BIIB 513 had no effect on aggregation induced by platelet activating factor (PAF), thrombin receptor activator peptide (TRAP), or adenosine diphosphate (ADP). Additionally, the structurally related compound EMD 85131 at up to 1 mM failed to inhibit TRAP induced platelet aggregation. In vivo administration of up to 9 mg/kg of BIIB 513 intravenously failed to affect cyclic flow in a canine model of femoral artery injury. These data demonstrate that the specific and selective NHE-1 inhibitors BIIB 513 or EMD 85131 have no effect on ex vivo platelet aggregation or in vivo cyclic flow following arterial injury.

Interaction between pre- and postconditioning in the in vivo rat heart

Patients with an impending myocardial infarction may be preconditioned by pre-infarct angina. Hence, it is important to establish whether ischemic postconditioning is still effective in preconditioned hearts. We therefore studied in anesthetized rats the effect of postconditioning after coronary artery occlusions (CAO) of 60 min in control hearts, hearts preconditioned by a single 15-min CAO (1IPC15) or a triple 3-min CAO (3IPC3). Furthermore, we studied the effect of postconditioning in hearts that had been pharmacologically preconditioned with intravenous adenosine and in hearts that had become tolerant to 1IPC15. Postconditioning limited infarct size in control hearts, but did not afford additional protection in preconditioned hearts, irrespective of the IPC stimulus. NO synthase inhibition abolished the cardioprotection by postconditioning, both IPC stimuli, and the combination of postconditioning and either IPC stimulus. Postconditioning also failed to afford cardioprotection in hearts protected by adenosine, and in hearts that had become tolerant to cardioprotection by 1IPC15. In accordance with previous observations, postconditioning paradoxically increased infarct size following a 30-min CAO. This detrimental effect was prevented by either IPC stimulus, in a NO synthase-dependent manner. In conclusion, postconditioning does not afford additional protection in preconditioned hearts, irrespective of the preconditioning stimulus and the presence of tolerance to preconditioning. Lack of additional protection may be related to the observation that postconditioning and preconditioning are both mediated via NO synthase. In contrast, the increase in infarct size by postconditioning following a 30-min CAO is abolished by either IPC stimulus. These findings indicate that the interaction between preconditioning and postconditioning is highly dependent on the duration of index ischemia, but independent of the preconditioning stimulus.

Aging and cardioprotection

Advanced age is a strong independent predictor for death, disability, and morbidity in patients with structural heart disease. With the projected increase in the elderly population and the prevalence of age-related cardiovascular disabilities worldwide, the need to understand the biology of the aging heart, the mechanisms for age-mediated cardiac vulnerability, and the development of strategies to limit myocardial dysfunction in the elderly have never been more urgent. Experimental evidence in animal models indicate attenuation in cardioprotective pathways with aging, yet limited information is available regarding age-related changes in the human heart. Human cardiac aging generates a complex phenotype, only partially replicated in animal models. Here, we summarize current understanding of the aging heart stemming from clinical and experimental studies, and we highlight targets for protection of the vulnerable senescent myocardium. Further progress mandates assessment of human tissue to dissect specific aging-associated genomic and proteomic dynamics, and their functional consequences leading to increased susceptibility of the heart to injury, a critical step toward designing novel therapeutic interventions to limit age-related myocardial dysfunction and promote healthy aging.

Effect of a hypoglycemic agent on ischemic preconditioning in patients with type 2 diabetes and stable angina pectoris

OBJECTIVE

Ischemic preconditioning is an increased tolerance to myocardial ischemia during the second of two consecutive exercise tests. ATP-sensitive K(+) channel blockers, such as glinides and sulfonylurea drugs, can induce loss of ischemic preconditioning. This study aimed to investigate the effects of repaglinide, a hypoglycemic agent with an affinity for myocardial ATP-sensitive K (+)channels, on the results of consecutive exercise tests in patients with diabetes and multivessel coronary artery disease.

METHODS

Forty-two patients with type 2 diabetes and chronic stable angina pectoris, and two-vessel or three-vessel disease participated in this study. The patients underwent two consecutive treadmill exercise tests (phase 1). On the day after these exercise tests, 2 mg of oral repaglinide was given to the patients. One week later, two exercise tests were repeated consecutively (phase 2).

RESULTS

All patients achieved 1.0-mm ST-segment depression during the four exercise tests (T1, T2, T3, and T4). In phase 2, seven patients improved in time to onset of 1.0-mm ST-segment depression. The worsening of the time to onset of 1.0-mm ST-segment depression in phase 2 demonstrated ischemic preconditioning block in 83.3% of patients (P=0.0001). Even the postexercise electrocardiographic parameters (ST-segment depression morphology and magnitude and arrhythmias) were significantly different between the groups with and without pharmacologic ischemic preconditioning block (P=0.031).

CONCLUSIONS

Repaglinide, an oral hypoglycemic agent with ATP-sensitive K(+) channel-blocker activity, eliminated the myocardial ischemic preconditioning in patients with coronary disease and diabetes.

Chronic preconditioning: a novel approach for cardiac protection

Ischemic preconditioning is the most powerful protective mechanism known against lethal ischemia. Unfortunately, the protection lasts for only a few hours. Here we tested the hypothesis that the heart can be kept in a preconditioned state for constant protection against ischemia. In this study we chose BMS-191095 (BMS), a highly selective opener of mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channels. BMS (1 mg/kg ip) was administered to rats every 24 h until 96 h. In other groups, BMS plus wortmannin (WTN, 15 microg/kg ip), an inhibitor of the phosphatidylinositol 3-kinase (PI3-K), or BMS plus 5-hydroxydecanoic acid (5-HD, 5 mg/kg ip), an inhibitor of mitoK(ATP), or BMS plus N(omega)-nitro-L-arginine methyl ester (L-NAME) (30 microg/kg ip), an inhibitor of nitric oxide (NO) synthase, were administered to rats. Rats were then subjected to 30-min left anterior descending coronary artery occlusion and 120-min reperfusion. Cardiac function, infarct size, pathological changes, and apoptosis were assessed at the end of treatments. Saline-treated hearts displayed marked contractile dysfunction and underwent pathological changes. BMS-treated rats showed significant improvement in cardiac function, and infarct size was significantly reduced in BMS-treated hearts. However, protection by BMS was abolished by 5-HD, WTN, or L-NAME. These data demonstrate that hearts can be chronically preconditioned and retain their ability to remain resistant against lethal ischemia and that this protection is mediated by activation of mitoK(ATP) via NO and PI3-K/Akt signaling pathways.