Short-term hibernation in adult cardiomyocytes is PO(2) dependent and Ca(2+) mediated

Molecular basis and clinical implications of HIFs in cardiovascular diseases

Oxygen maintains the homeostasis of an organism in a delicate balance in different tissues and organs. Under hypoxic conditions, hypoxia-inducible factors (HIFs) are specific and dominant factors in the spatiotemporal regulation of oxygen homeostasis. As the most basic functional unit of the heart at the cellular level, the cardiomyocyte relies on oxygen and nutrients delivered by the microvasculature to keep the heart functioning properly. Under hypoxic stress, HIFs are involved in acute and chronic myocardial pathology because of their spatiotemporal specificity, thus granting them therapeutic potential. Most adult animals lack the ability to regenerate their myocardium entirely following injury, and complete regeneration has long been a goal of clinical treatment for heart failure. The precise manipulation of HIFs (considering their dynamic balance and transformation) and the development of HIF-targeted drugs is therefore an extremely attractive cardioprotective therapy for protecting against myocardial ischemic and hypoxic injury, avoiding myocardial remodeling and heart failure, and promoting recovery of cardiac function.

Myocardial stunning and hibernation revisited

Unlike acute myocardial infarction with reperfusion, in which infarct size is the end point reflecting irreversible injury, myocardial stunning and hibernation result from reversible myocardial ischaemia-reperfusion injury, and contractile dysfunction is the obvious end point. Stunned myocardium is characterized by a disproportionately long-lasting, yet fully reversible, contractile dysfunction that follows brief bouts of myocardial ischaemia. Reperfusion precipitates a burst of reactive oxygen species formation and alterations in excitation-contraction coupling, which interact and cause the contractile dysfunction. Hibernating myocardium is characterized by reduced regional contractile function and blood flow, which both recover after reperfusion or revascularization. Short-term myocardial hibernation is an adaptation of contractile function to the reduced blood flow such that energy and substrate metabolism recover during the ongoing ischaemia. Chronic myocardial hibernation is characterized by severe morphological alterations and altered expression of metabolic and pro-survival proteins. Myocardial stunning is observed clinically and must be recognized but is rarely haemodynamically compromising and does not require treatment. Myocardial hibernation is clinically identified with the use of imaging techniques, and the myocardium recovers after revascularization. Several trials in the past two decades have challenged the superiority of revascularization over medical therapy for symptomatic relief and prognosis in patients with chronic coronary syndromes. A better understanding of the pathophysiology of myocardial stunning and hibernation is important for a more precise indication of revascularization and its consequences. Therefore, this Review summarizes the current knowledge of the pathophysiology of these characteristic reperfusion phenomena and highlights their clinical implications.

beta2-Adrenoceptor confers cardioprotection against hypoxia in isolated ventricular myocytes and the effects depend on estrogenic environment

Estrogen plays a cardioprotective role in female rat hearts subjected to ischemia/reperfusion injury. The its effects are, at least partially, associated with decreased cardiomyocyte contraction and increased expression of beta(2)-adrenoceptor (beta(2)-AR). We tested whether beta(2)-AR could be involved in cardioprotection against ischemic damage and whether the roles of beta(2)-AR were dependent on estrogenic environment. We first determined the effects of hypoxia/reoxygenation (H/R) on cardiomyocyte shortening in female rats. We then determined the roles of beta(2)-AR in cardiomyocyte shortening, lactate dehydrogenase (LDH) release in culture medium, and cell death during hypoxia in isolated myocytes from female rats. We further determined the effects of estrogen on the roles of beta(2)-AR during hypoxia. H/R induced short-term hibernation and stunning at the level of ventricular myocytes from normal female rats. Inhibition of beta(2)-AR with ICI118,551 significantly elevated adrenergic contractile reserve, myocardial injury, and cell death in normal female rats during hypoxia, whereas ovariectomy (OVX) prominently enhanced myocyte contraction, myocardial injury, and cell death, and deprived the alternations in normal female rats. These changes were restored to normal by estrogen replacement (OVX+E(2)). These data suggest that beta(2)-AR may be involved in the cardioprotection against ischemic damage, and the cardioprotection may depend on estrogenic environment.

Microvascular transport model predicts oxygenation changes in the infarcted heart after treatment

Chronic heart failure is most commonly due to ischemic cardiomyopathy after a previous myocardial infarction (MI). Rebuilding lost myocardium to prevent heart failure mandates a neovasculature able to nourish new cardiomyocytes. Previously we have used a series of novel techniques to directly measure the ability of the scar neovasculature to deliver and exchange oxygen at 1–4 wk after MI in rats following left coronary artery ligation. In this study, we have developed a morphologically realistic mathematical model of oxygen transport in cardiac tissue to help in deciding what angiogenic strategies should be used to rebuild the vasculature. The model utilizes microvascular morphology of cardiac tissue based on available morphometric images and is used to simulate experimentally measured oxygen levels after MI. Model simulations of relative oxygenation match experimental measurements closely and can be used to simulate distributions of oxygen concentration in normal and infarcted rat hearts. Our findings indicate that both vascular density and vascular spatial distribution play important roles in cardiac tissue oxygenation after MI. Furthermore, the model can simulate relative changes in tissue oxygen levels in infarcted tissue treated with proangiogenic compounds such as losartan. From the minimum oxygen concentration myocytes need to maintain their normal function, we estimate that 2 wk after MI 29% of the myocardium is severely hypoxic and that the vascular density of the infarcted tissue should reach 75% of normal tissue to ensure that no areas of the myocardium are critically hypoxic.

Do rat cardiac myocytes release ATP on contraction?

ATP is released by numerous cell types in response to mechanical strain. It then acts as a paracrine or autocrine signaling molecule, inducing a variety of biological responses. In this work, we addressed the question whether mechanical force acting on the membranes of contracting cardiomyocytes during periodic longitudinal shortening can stimulate the release of ATP. Electrically stimulated isolated adult rat cardiomyocytes as well as spontaneously contracting mouse cardiomyocytes derived from embryonic stem (ES) cells were assayed for ATP release with the use of luciferase and a sensitive charge-coupled device camera. Sensitivity of soluble luciferase in the supernatant of cardiomyocytes was 100 nM ATP, which is approximately 10-fold below the EC(50) values for most purinergic receptors expressed in the heart (1.5-20 microM). Light intensities were not different between resting or contracting adult rat cardiomyocytes. Similar results were obtained with ES-cell-derived contracting mouse cardiomyocytes. ATP release was measurable only from obviously damaged or permeabilized cells. To increase selectivity and sensitivity of ATP detection we have targeted a recombinant luciferase to the sarcolemmal membrane using a wheat germ agglutinin-IgG linker. Contraction of labeled adult rat cardiomyocytes was not associated with measurable bioluminescence. However, when human umbilical vein endothelial cells were targeted with membrane-bound luciferase, shear stress-induced ATP release could be clearly detected, demonstrating the sensitivity of the detection method. In the present study, we did not detect ATP release from contracting cardiomyocytes on the single cell level, despite adequate sensitivity of the detection system. Thus deformation of the contracting cardiomyocyte is not a key stimulus for the release of cellular ATP.

Mitochondrial respiratory control can compensate for intracellular O(2) gradients in cardiomyocytes at low PO(2)

In isolated single cardiomyocytes with moderately elevated mitochondrial respiration, direct evidence for intracellular radial gradients of oxygen concentration was obtained by subcellular spectrophotometry of myoglobin (Mb). When oxygen consumption was increased by carbonyl cyanide m-chlorophenylhydrazone (CCCP) during superfusion of cells with 4% oxygen, PO(2) at the cell core dropped to 2.3 mmHg, whereas Mb near the plasma membrane was almost fully saturated with oxygen. Subcellular NADH fluorometry demonstrated corresponding intracellular heterogeneities of NADH, indicating suppression of mitochondrial oxidative metabolism due to relatively slow intracellular oxygen diffusion. When oxygen consumption was increased by electrical pacing in 2% oxygen, radial oxygen gradients of similar magnitude were demonstrated (cell core PO(2) = 2.6 mmHg). However, an increase in NADH fluorescence at the cell core was not detected. Because CCCP abolished mitochondrial respiratory control while it was intact in electrically paced cardiomyocytes, we conclude that mitochondria with intact respiratory control can sustain electron transfer with reduced oxygen supply. Thus mitochondrial intrinsic regulation can compensate for relatively slow oxygen diffusion within cardiomyocytes.

Nitric oxide reduces energy supply by direct action on the respiratory chain in isolated cardiomyocytes

To investigate the effect of nitric oxide (NO) on cardiac energy metabolism, isolated cardiomyocytes of Wistar rats were incubated in an Oxystat system at a constant ambient PO2 (25 mmHg) and oxygen consumption (VO2); free intracellular Ca(2+) (fura 2), free cytosolic adenosine [S-adenosylhomocysteine (SAH) method], and mitochondrial NADH (autofluorescence) were measured after application of the NO donor morpholinosydnonimine (SIN-1). In Na(+)-free medium (contracting cardiomyocytes), VO2 increased from 7.9 +/- 1.2 to 26.4 +/- 3.1 nmol x min(-1) x mg protein(-1). SIN-1 (100 micromol/l) decreased VO2 in contracting (-21 +/- 3%) and in quiescent cells (-24 +/- 7%) by the same extent. Inhibition of VO2 was dose dependent (EC(50): 10(-7) mol/l). S-nitroso-N-acetyl-penicillamine, another NO donor, also inhibited VO2, whereas SIN-1C (100 micromol/l), the degradation product of SIN-1, displayed no inhibitory effect. Intracellular Ca(2+) remained unchanged, and inhibition of protein kinases G, A, or C did not antagonize the effect of NO. Mitochondrial NADH increased with NO, indicating a reduced flux through the respiratory chain. In quiescent but not in contracting cardiomyocytes, NO significantly increased adenosine, indicating a reduced energy status. These data suggest the following. 1) NO decreases cardiac respiration, most likely via direct inhibition of the respiratory chain. 2) Whereas in quiescent cardiomyocytes the inhibition of aerobic ATP formation by NO causes reduction in energy status, contracting cells are able to compensate for the NO-induced inhibition of oxidative phosphorylation, maintaining energy status constant.