Deleterious effects of digitalis on reperfusion-induced arrhythmias and myocardial injury in ischemic rat hearts: possible involvements of myocardial Na+ and Ca2+ imbalance

Ca2+ influx mechanisms in caveolae vesicles of pulmonary smooth muscle plasma membrane under inhibition of alpha2beta1 isozyme of Na+/K+-ATPase by ouabain

AIMS

We sought to determine the mechanisms of an increase in Ca(2+) level in caveolae vesicles in pulmonary smooth muscle plasma membrane during Na(+)/K(+)-ATPase inhibition by ouabain.

MAIN METHODS

The caveolae vesicles isolated by density gradient centrifugation were characterized by electron microscopic and immunologic studies and determined ouabain induced increase in Na(+) and Ca(2+) levels in the vesicles with fluorescent probes, SBFI-AM and Fura2-AM, respectively.

KEY FINDINGS

We identified the alpha(2)beta(1) and alpha(1)beta(1) isozymes of Na(+)/K(+)-ATPase in caveolae vesicles, and only the alpha(1)beta(1) isozyme in noncaveolae fraction of the plasma membrane. The alpha(2)-isoform contributes solely to the enzyme inhibition in the caveolae vesicles at 40 nM ouabain. Methylisobutylamiloride (Na(+)/H(+)-exchange inhibitor) and tetrodotoxin (voltage-gated Na(+)-channel inhibitor) pretreatment prevented ouabain induced increase in Na(+) and Ca(2+) levels. Ouabain induced increase in Ca(2+) level was markedly, but not completely, inhibited by KB-R7943 (reverse-mode Na(+)/Ca(2+)-exchange inhibitor) and verapamil (L-type Ca(2+)-channel inhibitor). However, pretreatment with tetrodotoxin in conjunction with KB-R7943 and verapamil blunted ouabain induced increase in Ca(2+) level in the caveolae vesicles, indicating that apart from Na(+)/Ca(+)-exchanger and L-type Ca(2+)-channels, "slip-mode conductance" of Na(+) channels could also be involved in this scenario.

SIGNIFICANCE

Inhibition of alpha(2) isoform of Na(+)/K(+)-ATPase by ouabain plays a crucial role in modulating the Ca(2+) influx regulatory components in the caveolae microdomain for marked increase in (Ca(2+))(i) in the smooth muscle, which could be important for the manifestation of pulmonary hypertension.

Late sodium current inhibition as a new cardioprotective approach

There is increasing evidence that the late sodium current of the sodium channel in myocytes plays a critical role in the pathophysiology of myocardial ischemia and thus is a potential therapeutic target in patients with ischemic heart disease. Ranolazine, an inhibitor of the late sodium current, reduces the frequency and severity of anginal attacks and ST-segment depression in humans, and unlike other antianginal drugs, ranolazine does not alter heart rate or blood pressure. In experimental animal models, ranolazine has been shown to reduce myocardial infarct size and to improve left ventricular function after acute ischemia and chronic heart failure. This article reviews published data describing the role of late sodium current and its inhibition by ranolazine in clinical and experimental studies of myocardial ischemia.

Endothelin receptors, localized in sympathetic nerve terminals of the heart, modulate norepinephrine release and reperfusion arrhythmias

Endothelin (ET)-1 is an endogenous vasoconstrictor which modulates norepinephrine (NE) release in myocardial ischemia reperfusion. Recent studies have demonstrated the pro- or anti-arrhythmic effects in reperfusion. The present studies were undertaken to test the hypothesis that ET receptors located in sympathetic nerve terminals modulate NE release associated with reperfusion arrhythmias (ventricular fibrillation; VF). Immunohistochemical studies showed that both ETA and ETB receptors exist in the sympathetic nerve varicosities, which were stained positive for tyrosine hydroxylase (TH) in the left ventricular wall in guinea pigs. Isolated guinea pig hearts were subjected to 20 min of normothermic global ischemia followed by 30 min reperfusion. Exogenously applied ET-1 (0.1 and 1 nM) dose-dependently increased NE release and the duration of VF, but these responses were significantly suppressed with the Na(+)/H(+) exchanger inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride (10 microM). The ETA receptor antagonist (BQ123, 1 microM) and nonselective ET receptor antagonist (PD142893, 1 microM) significantly attenuated NE release and VF, whereas the ETB receptor antagonist (BQ788,300 nM) markedly elevated NE release but did not affect VF. These studies provide the first evidence that both ETA and ETB receptors, located in the sympathetic nerve varicosities, modulate NE release, at least in part, in association with reperfusion arrhythmias.

Positive inotropic effect of ouabain on isolated heart is accompanied by activation of signal pathways that link Na+/K+-ATPase to ERK1/2

Exposure of cultured rat cardiac myocytes to ouabain is known to cause the interaction of Na+/K+-ATPase with adjacent proteins, leading to activation of multiple signal transduction pathways, regulation of growth-related genes, and hypertrophy. The aim of this work was to determine if the proximal signaling events identified in cultured myocytes also occur in isolated intact hearts of rat and guinea pig in response to positive inotropic doses of ouabain. Langendorff rat heart preparations were exposed to 50 microM ouabain to produce positive inotropy without toxicity, and assayed for Src kinase, protein kinase C, and extracellular signal-regulated kinases 1 and 2 (ERK(1/2)). These kinases were rapidly activated by ouabain as in cultured cells. In isolated guinea pig hearts, 1 microM ouabain caused ERK(1/2) activation comparable to the effect of 50 microM ouabain in rat heart and consistent with the higher ouabain sensitivity of the contractility of guinea pig heart. These data show that the proximal ouabain-induced signal pathways previously noted in cultured cells are not artifacts of dispersion/culturing of myocytes, and are not the peculiar properties of the rat heart with its relatively low ouabain sensitivity. They also suggest that treatment with positive inotropic doses of cardiac glycosides is likely to be associated with changes in the cardiac phenotype.

A K(ATP) channel opener inhibited myocardial reperfusion action potential shortening and arrhythmias

Low concentrations of certain K(ATP) channel openers have been reported to exert a moderate inhibitory effect on arrhythmias during post-ischaemic early myocardial reperfusion, but the accompanying effects on the time course of changes in action potentials in intact hearts have not yet been studied. We report that in rat isolated hearts, reperfusion following 10 min of regional no-flow ischaemia was associated with both an acute, marked, but transient, shortening of ventricular repolarisation (by 63%) during reperfusion, and a high incidence (90%) of ventricular tachyarrhythmias. The K(ATP) channel opener Ro 31-6930 [2-(6-cyano-2,2-dimethyl-2H-1-benzopyran-4-yl)-pyridine 1-oxide], delivered prior to ischaemia at a relatively low concentration (0.5 microM), significantly reduced the incidence and duration of reperfusion arrhythmias, and prevented the associated acute action potential shortening during reperfusion, each in a glibenclamide (1 microM)-sensitive manner (P<0.05, n=10-15 hearts). This was associated with a moderate and non-arrhythmogenic action potential shortening during ischaemia (a potentially "cardioprotective" effect). However, these data highlight the potential harm these drugs may cause, since a higher concentration of Ro 31-6930 caused marked shortening of action potentials and significant pro-arrhythmia during ischaemia.

Relationship of elevated 23Na magnetic resonance image intensity to infarct size after acute reperfused myocardial infarction

BACKGROUND

Elevated 23Na MR image intensity after acute myocardial infarction has previously been shown to correspond to high tissue [Na+] and loss of myocardial viability. In this study, we explored the potential of in vivo 23Na MRI to assess infarct size and investigated possible mechanisms for elevated 23Na image intensity.

METHODS AND RESULTS

Thirteen dogs and 8 rabbits underwent in situ coronary artery occlusion and reperfusion and were imaged by 23Na MRI. For anatomically matched left ventricular short-axis cross sections (n=46), infarct size measured by in vivo 23Na MRI correlated well with triphenyltetrazolium chloride staining (r=0.87, y=0.92x+3.37, P<0.001). Elevated 23Na image intensity was observed in infarcted myocardium (206+/-37% of remote in dogs, P<0.001; 215+/-58% in rabbits, P<0.002) but was not observed after severe but reversible ischemic injury (101+/-11% of baseline, P=NS). High-resolution ex vivo imaging revealed that regions of elevated 23Na image intensity appeared to be identical to those of infarcted regions (r=0.97, y=0.92x+1.52, P<0.001). In infarcted regions, total tissue [Na+] was elevated (89+/-12 versus 37+/-9 mmol/L in control tissue, 156+/-60% increase, P<0.001) and was associated with increased intracellular sodium (254+/-68% of control, P<0.005) and an increased intracellular sodium/potassium ratio (868+/-512% of control, P<0.002). Morphometric analysis demonstrated only a minor increase in extracellular volume (17+/-8% versus 14+/-5%, P<0.05) in the infarcted territory.

CONCLUSIONS

Elevated 23Na MR image intensity in vivo measures infarct size after reperfused infarction in both a large and a small animal model. The mechanism of elevated 23Na image intensity is probably intracellular sodium accumulation secondary to loss of myocyte ionic homeostasis.

Na+/H+ exchangers. Molecular diversity and relevance to heart

During the last several years, significant advances have been made in our understanding of the molecular, cellular, and physiological diversity of mammalian Na+/H+ exchangers. This transporter forms a multigene family of at least six members (NHE1-NHE6) that share approximately 20-60% amino acid identity. NHE1 is the most predominant isoform expressed in heart and it contributes significantly to myocardial pHi homeostasis, which is important for maintaining contractility. However, hyperactivation of NHE1 during episodes of cardiac ischemia and reperfusion disrupts the intracellular ion balance, leading to cardiac dysfunction and damage in several animal models, but which can be prevented by pharmacological antagonists of NHE1. Molecular studies have indicated that the predicted transmembrane segments M4 and M9 contain several residues involved in drug sensitivity. Molecular dissection of the drug binding region should facilitate the rational design of more potent and isoform-specific drugs that may provide therapeutic benefit in the prevention of cardiac ischemia and reperfusion injuries.

Fast 23Na magnetic resonance imaging of acute reperfused myocardial infarction. Potential to assess myocardial viability

BACKGROUND

The ability of the myocyte to maintain an ionic concentration gradient is perhaps the best indication of myocardial viability. We studied the relationship of 23Na MRI intensity to viability and explored the potential of fast-imaging techniques to reduce 23Na imaging times in rabbits and dogs.

METHODS AND RESULTS

Eighteen rabbits underwent in situ coronary artery occlusion and reperfusion. The hearts were then either imaged following isolation and perfusion with cardioplegic solution (n = 6), imaged in vivo (n = 6), or analyzed for 23Na content and relaxation times (n = 12). Normal rabbits (n = 6) and dogs (n = 4) were imaged to examine the effect of animal size on 23Na image quality. 23Na imaging times were 7, 11, and 4 minutes for isolated rabbits, in vivo rabbits, and in vivo dogs, respectively. Infarcted, reperfused regions, identified by triphenyltetrazolium chloride staining, showed a significant elevation in 23Na image intensity compared with viable regions (isolated, 42 +/- 5%, P < .02; in vivo, 95 +/- 6%, P < .001), consistent with increased tissue sodium content. Similarly, 23Na MR spectroscopy showed that [Na+] was higher in nonviable than viable myocardium (isolated, 99 +/- 4 versus 61 +/- 2 mmol/L; in vivo, 91 +/- 2 versus 38 +/- 1 mmol/L; P < .001 for both). Image signal-to-noise ratios were higher in dogs than rabbits despite shorter imaging times, primarily due to larger voxels.

CONCLUSIONS

Following acute infarction with reperfusion, a regional increase in 23Na MR image intensity is associated with nonviable myocardium. Fast gradient-echo imaging techniques reduce 23Na imaging times to a few minutes, suggesting that 23Na MR imaging has the potential to become a useful experimental and clinical tool.

Veratridine-induced intoxication: an in vitro model for the characterization of anti-ischemic compounds?

Due to the complexity of ischemia-induced cellular dysfunction many different pharmacological approaches have been tested to improve cellular ischemia resistance. However, despite the importance of [Na+]i for ischemia-induced dysfunction, only very few studies have investigated the contribution of the Na+ channel to ischemia-induced failure of intracellular ion homeostasis. Since an activation of Na+ channels by veratridine also results in a failure of intracellular ion homeostasis, the veratridine- and ischemia-induced alterations of cellular function were compared. Moreover, despite the difference in the electrophysiological changes induced by veratridine and ischemia, the possible involvement of a slowly inactivating, less selective Na+ channel in both veratridine- and ischemia-induced cellular dysfunction is discussed. As a conclusion it is suggested that veratridine intoxication could be a helpful in vitro method for the characterization of putative anti-ischemic compounds.