Elevated myocardial interstitial norepinephrine concentration contributes to the regulation of Na+,K(+)-ATPase in heart failure

The biology of neurotrophins: cardiovascular function

This chapter addresses the role of neurotrophins in the development of the heart, blood vessels, and neural circuits that control cardiovascular function, as well as the role of neurotrophins in the mature cardiovascular system. The cardiovascular system includes the heart and vasculature whose functions are tightly controlled by the nervous system. Neurons, cardiomyocytes, endothelial cells, vascular smooth muscle cells, and pericytes are all targets for neurotrophin action during development. Neurotrophin expression continues throughout life, and several common pathologies that impact cardiovascular function involve changes in the expression or activity of neurotrophins. These include atherosclerosis, hypertension, diabetes, acute myocardial infarction, and heart failure. In many of these conditions, altered expression of neurotrophins and/or neurotrophin receptors has direct effects on vascular endothelial and smooth muscle cells in addition to effects on nerves that modulate vascular resistance and cardiac function. This chapter summarizes the effects of neurotrophins in cardiovascular physiology and pathophysiology.

Cardiac hypertrophy induced by sustained β-adrenoreceptor activation: pathophysiological aspects

Cardiac hypertrophy is promoted by adrenergic over-activation and represents an independent risk factor for cardiovascular morbidity and mortality. The basic knowledge about mechanisms by which sustained adrenergic activation promotes myocardial growth, as well as understanding how structural changes in hypertrophied myocardium could affect myocardial function has been acquired from studies using an animal model of chronic systemic beta-adrenoreceptor agonist administration. Sustained beta-adrenoreceptor activation was shown to enhance the synthesis of myocardial proteins, an effect mediated via stimulation of myocardial growth factors, up-regulation of nuclear proto-oncogenes, induction of cardiac oxidative stress, as well as activation of mitogen-activated protein kinases and phosphatidylinositol 3-kinase. Sustained beta-adrenoreceptor activation contributes to impaired cardiac autonomic regulation as evidenced by blunted parasympathetically-mediated cardiovascular reflexes as well as abnormal storage of myocardial catecholamines. Catecholamine-induced cardiac hypertrophy is associated with reduced contractile responses to adrenergic agonists, an effect attributed to downregulation of myocardial beta-adrenoreceptors, uncoupling of beta-adrenoreceptors and adenylate cyclase, as well as modifications of downstream cAMP-mediated signaling. In compensated cardiac hypertrophy, these changes are associated with preserved or even enhanced basal ventricular systolic function due to increased sarcoplasmic reticulum Ca(2+) content and Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. The increased availability of Ca(2+) to maintain cardiomyocyte contraction is attributed to prolongation of the action potential due to inhibition of the transient outward potassium current as well as stimulation of the reverse mode of the Na(+)-Ca(2+) exchange. Further progression of cardiac hypertrophy towards heart failure is due to abnormalities in Ca(2+) handling, necrotic myocardial injury, and increased myocardial stiffness due to interstitial fibrosis.

Modification of sarcolemmal Na+-K+-ATPase and Na+/Ca2+exchanger expression in heart failure by blockade of renin-angiotensin system

The activities of both sarcolemmal (SL) Na+-K+-ATPase and Na+/Ca2+exchanger, which maintain the intracellular cation homeostasis, have been shown to be depressed in heart failure due to myocardial infarction (MI). Because the renin-angiotensin system (RAS) is activated in heart failure, this study tested the hypothesis that attenuation of cardiac SL changes in congestive heart failure (CHF) by angiotensin-converting enzyme (ACE) inhibitors is associated with prevention of alterations in gene expression for SL Na+-K+-ATPase and Na+/Ca2+exchanger. CHF in rats due to MI was induced by occluding the coronary artery, and 3 wk later the animals were treated with an ACE inhibitor, imidapril (1 mg·kg−1·day−1), for 4 wk. Heart dysfunction and cardiac hypertrophy in the infarcted animals were associated with depressed SL Na+-K+-ATPase and Na+/Ca2+exchange activities. Protein content and mRNA levels for Na+/Ca2+exchanger as well as Na+-K+-ATPase α1-, α2- and β1-isoforms were depressed, whereas those for α3-isoform were increased in the failing heart. These changes in SL activities, protein content, and gene expression were attenuated by treating the infarcted animals with imidapril. The beneficial effects of imidapril treatment on heart function and cardiac hypertrophy as well as SL Na+-K+-ATPase and Na+/Ca2+exchange activities in the infarcted animals were simulated by enalapril, an ACE inhibitor, and losartan, an angiotensin receptor antagonist. These results suggest that blockade of RAS in CHF improves SL Na+-K+-ATPase and Na+/Ca2+exchange activities in the failing heart by preventing changes in gene expression for SL proteins.

Extracellular norepinephrine reduces neuronal uptake of norepinephrine by oxidative stress in PC12 cells

Cardiac norepinephrine (NE) uptake activity is reduced in congestive heart failure. Our studies in intact animals suggest that this effect on the cardiac sympathetic nerve endings is caused by oxidative stress and/or NE toxic metabolites derived from NE. In this study, we investigated the direct effects of NE on neuronal NE uptake activity and NE transporter (NET), using undifferentiated PC12 cells. Cells were incubated with NE (1–500 μM) either alone or in combination of Cu2+ sulfate (1 μM), which promotes free radical formation by Fenton reaction for 24 h. NE uptake activity was measured using [3H]NE. Cell viability was determined with the use of Trypan blue exclusion and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay, and cellular oxidative stress by dichlorodihydrofluorescein fluorescence and the GSH/GSSG ratio. Cell viability was reduced by NE >100 μM. At lower doses, NE produced oxidative stress and a dose-dependent reduction of NE uptake activity without affecting cell viability significantly. Cu2+, which has no direct effect on NE uptake activity, potentiated oxidative stress and reduction of NE uptake activity produced by NE. This decrease of NE uptake activity was associated with reductions of NE uptake binding sites and NET protein expression by using the radioligand assay and Western blot analysis, but no changes in NET gene expression. In addition, the free-radical scavenger mannitol, and antioxidant enzymes superoxide dismutase and catalase, reduced oxidative stress and attenuated the reductions of NE uptake activity and NET protein produced by NE/Cu. Thus our results support a functional role of oxidative stress in mediating the neuronal NE uptake reducing effect of NE and that this effect of NE on NET is a posttranscriptional event.

Myocardial beta-adrenoceptor down-regulation by norepinephrine is linked to reduced norepinephrine uptake activity

Chronic administration of norepinephrine for 8 weeks has been shown to reduce neuronal norepinephrine uptake activity and increase interstitial norepinephrine concentration in the heart. To determine whether the changes could lead to myocardial beta-adrenoceptor down-regulation or beta-adrenergic subsensitivity, we measured left ventricular contractile responses to dobutamine, myocardial beta-adrenoceptor density, beta subtype distribution, competitive inhibition agonist binding, and adenylyl cyclase activity activation by isoproterenol, 5'-guanylylimidodiphosphate, and forskolin in dogs after a norepinephrine or saline infusion for 8 weeks. We found that norepinephrine infusion reduced myocardial beta-adrenoceptor density, beta(1)-adrenoceptor subtype density, and high-affinity site for isoproterenol. Left ventricular contractile responses to dobutamine were reduced in the norepinephrine-infused animals. In addition, norepinephrine infusion decreased the basal adenylyl cyclase activity and the adenylyl cyclase responses to isoproterenol, 5'-guanylylimidodiphosphate, and forskolin. The findings indicate that a decrease in cardiac norepinephrine uptake predisposes the heart to norepinephrine-induced myocardial beta-adrenoceptor down-regulation, and that norepinephrine, when present in a sufficient amount over a long period as it is in chronic heart failure, can reduce myocardial beta-adrenergic responsiveness by both homologous and heterologous desensitization.

Myocardial region (right or left ventricle) and aetiology of heart failure can influence the inotropic effect of ouabain in failing human myocardium

AIMS

To investigate whether the inotropic effect of ouabain in failing human myocardium varies according to the heart chamber tested (right or left ventricle) or the aetiology of the heart disease, i.e. ischaemic or idiopathic.

METHODS

The inotropic effect of ouabain was measured, as the percentage change in baseline tension, in myocardial strips isolated from right (RV; n=21) and left ventricles (LV; n=21) of hearts explanted from patients with idiopathic (IDC; n=11) and ischaemic cardiomyopathy (CAD; n=10). Concentration-effect curves obtained with ouabain (0.05-1.6 micromol l-1 ) were analysed using the Emax sigmoidal model, and the following parameters were calculated: Emax, EC50, n and EC10 (threshold concentration). The influence of ventricular chamber and heart failure aetiology on these parameters was evaluated by means of a two-way anova.

RESULTS

Age and baseline haemodynamic parameters did not differ between IDC and CAD patients. Baseline strip contractility was highly variable (range: 0.48-10.0 mN), but neither ventricular chamber nor aetiology could explain such variability. A two-way anova showed that EC10 was greater in CAD than in IDC preparations (0.097+/-0.013 micromol l-1 vs 0.059+/-0. 009 micromol l-1; 95% C.I. for difference 0.043, 0.071) and Emax was lower in RV than in LV (121+/-21% vs 250+/-38%; 95% C.I. -221, -36), while EC50 and n were not significantly different between groups.

CONCLUSIONS

The inotropic effect of ouabain in human myocardium may vary according to aetiology of heart failure and the ventricle being tested. Although our results do not support the hypothesis of increased sensitivity to cardiac glycosides in CAD patients, they may explain the diminished effect observed in patients with RV failure.

Isoform-specific alterations in cardiac and erythrocyte Na+,K+-ATPase activity induced by norepinephrine

BACKGROUND

Myocardial Na+,K+-ATPase activities are decreased in congestive heart failure because of an increase in plasma norepinephrine levels, but it is difficult to monitor the activities in the clinical setting.

METHODS AND RESULTS

This study investigated whether erythrocyte Na+,K+-ATPase activity can reflect myocardial enzyme activity and whether isoform-specific alterations occur in the presence of catecholamine. Na+,K+-ATPase activity was measured by the colorimetric method by using the left ventricular myocardium and erythrocytes prepared from eight rabbits given norepinephrine for 7 days and from eight control rabbits that received saline. The protein levels of total catalytic subunit and alpha1- or alpha3-isoform of Na+,K+-ATPase were determined by Western blot analysis. Na+,K+-ATPase activity was lower in both myocardium and erythrocytes from norepinephrine-treated rabbits than control rabbits (P < .01 and P < .01, respectively). There was a close correlation in Na+,K+-ATPase activity between myocardium and erythrocytes (r = .963). Total catalytic subunit protein level was lower in myocardium from norepinephrine-treated rabbits than control rabbits, but the alpha1-isoform level was similar between the two groups. The alpha3-isoform level was lower in norepinephrine-treated rabbits than control rabbits. In erythrocytes, alpha1-isoform was lower in norepinephrine-treated rabbits than control rabbits.

CONCLUSIONS

Na+,K+-ATPase activity in myocardium could be reflected in erythrocyte membrane, although there was a difference in isoform-specific regulation between the two.

Protein changes observed in pacing-induced heart failure using two-dimensional electrophoresis

Rapid ventricular pacing in dogs results in a low output cardiomyopathic state which is similar to idiopathic dilated cardiomyopathy in man. However, the pathophysiological mechanisms which cause this failure following pacing are unknown. Five dogs underwent rapid ventricular pacing. Hearts were stimulated at 245 beats per min (bpm) for four weeks and then reduced to 190 bpm to stabilize the failure. Six unoperated dogs were used as controls. This paper compares the two-dimensional gel electrophoresis (2-DE) protein patterns of left ventricular samples from the paced myocardium with the control dogs. Changes in protein expression were analyzed qualitatively and semi-quantitatively. In the paced dog samples 69 protein spots were significantly altered of which 42 were decreased and 27 were elevated. One qualitative change was observed: elongation factor Tu was present only the control hearts. Of these proteins, 20 have been identified by a combination of N-terminal protein microsequencing, peptide mass profiling by mass spectrometry, amino acid compositional analysis, and by comparison with databases of canine and human ventricular proteins. Ten of these are associated with mitochondria and energy production, including: pyruvate dehydrogenase E1 component, isocitrate dehydrogenase subunit alpha, HSP60 and HSP70, creatine kinase M and fatty acid binding protein. The cytoskeletal protein desmin was detected in reduced quantities and a spot corresponding to a fragment of desmin was increased. These results indicate that the development of heart failure in the paced dog involves alterations in mitochondrial energy production, the cytoskeleton and calcium activation.