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

Rasgrf2 controls noradrenergic involvement in the acute and subchronic effects of alcohol in the brain

RATIONALE

Alcohol addiction is a major psychiatric disease, and yet, the underlying molecular adaptations in the brain remain unclear. Recent evidence suggests a functional role for the ras-specific guanine-nucleotide releasing factor 2 (Rasgrf2) in alcoholism. Rasgrf2(-/-) mice consume less alcohol and show entirely absent dopamine responses to an alcohol challenge compared to wild types (WT).

OBJECTIVE

In order to further investigate how Rasgrf2 modifies the acute and subchronic effects of alcohol in the brain, we investigated its effects on the noradrenergic and serotonergic systems.

METHODS

We measured noradrenaline and serotonin activity in the brain by in vivo microdialysis and RNA expression by chip analysis and RT-PCR after acute and sub-chronic alcohol exposure in Rasgrf2(-/-) and WT mice.

RESULTS

In vivo microdialysis showed a significantly reduced noradrenergic response and an absent serotonergic response in the nucleus accumbens (NAcc) and caudate putamen (CPu) after an alcohol challenge in Rasgrf2(-/-) mice. A co-expression analysis showed that there is a high correlation between Rasgrf2 and α2 adrenoceptor RNA expression in the ventral striatum in naïve animals. Accordingly, we further assessed the role of Rasgrf2 in the response of the noradrenergic system to subchronic alcohol exposure. A decrease in β1 adrenoceptor gene expression was seen in Rasgrf2(+/+), but not Rasgrf2(-/-) mice following alcohol exposure. Conversely, alcohol resulted in a decrease in both β2 and α2 adrenoceptor gene expression in knockout but not WT Rasgrf2 mice.

CONCLUSIONS

These findings suggest that adaptations in the noradrenergic system contribute to the Rasgrf2 enhanced risk of alcoholism.

Role of the autonomic nervous system in atrial fibrillation: pathophysiology and therapy

Autonomic nervous system activation can induce significant and heterogeneous changes of atrial electrophysiology and induce atrial tachyarrhythmias, including atrial tachycardia and atrial fibrillation (AF). The importance of the autonomic nervous system in atrial arrhythmogenesis is also supported by circadian variation in the incidence of symptomatic AF in humans. Methods that reduce autonomic innervation or outflow have been shown to reduce the incidence of spontaneous or induced atrial arrhythmias, suggesting that neuromodulation may be helpful in controlling AF. In this review, we focus on the relationship between the autonomic nervous system and the pathophysiology of AF and the potential benefit and limitations of neuromodulation in the management of this arrhythmia. We conclude that autonomic nerve activity plays an important role in the initiation and maintenance of AF, and modulating autonomic nerve function may contribute to AF control. Potential therapeutic applications include ganglionated plexus ablation, renal sympathetic denervation, cervical vagal nerve stimulation, baroreflex stimulation, cutaneous stimulation, novel drug approaches, and biological therapies. Although the role of the autonomic nervous system has long been recognized, new science and new technologies promise exciting prospects for the future.

Altered expression of norepinephrine transporter and norepinephrine in human placenta cause pre-eclampsia through regulated trophoblast invasion

Objective

We investigated the norepinephrine transporter (NET) expression in normal and pre-eclamptic placentas and analyzed the invasion activity of trophoblastic cells based on norepinephrine (NE)-NET regulation.

Methods

NET and NE expression levels were examined by western blot and enzyme-linked immunosorbent assay, respectively. Trophoblast invasion activity, depending on NE-NET regulation, was determined by NET-small interfering RNA (siRNA) and NET transfection into the human extravillous trophoblast cells with or without NE treatment and invasion rates were analyzed by zymography and an invasion assay.

Results

NET mRNA was expressed at a low level in pre-eclamptic placentas compared with normal placentas and NE concentration in maternal plasma increased significantly in pre-eclamptic women compared to normal pregnant women (p<0.05). NET gene upregulation and NE treatment stimulated trophoblast cell invasion up to 2.5-fold (p<0.05) by stimulating matrix metalloproteinase-9 activity via the phosphoinositol-3-kinase/AKT signaling pathway, whereas NET-siRNA with NE treatment reduced invasion rates.

Conclusion

NET expression is reduced by inadequate regulation of NE levels during placental development. This suggests that a complementary balance between NET and NE regulates trophoblast cell invasion activities during placental development.

A urinary marker of oxidative stress covaries positively with hostility among midlife community volunteers

OBJECTIVE

Although not all findings are consistent, growing evidence suggests that individuals high in dispositional hostility are at elevated risk for cardiovascular disease and all-cause mortality; however, the mechanisms of these associations remain unclear. One possibility is that hostility is associated with oxidative stress. Here, we explore relationships between hostility and a measure of systemic oxidative stress among a mid-life sample.

METHODS

In a community sample of 223 adults aged 30 to 54 years (86% white, 50% female), oxidative stress was measured as the 24-hour urinary excretion of 8-hydroxy-2'-deoxyguanosine (8-OHdG). An abbreviated Cook Medley Hostility Scale was used to measure dimensions of hostility.

RESULTS

Regression analyses controlling for demographic characteristics and cardiovascular risk factors showed a positive relationship of 8-OHdG with total hostility (beta = 0.003, p = .03) and hostile affect (beta = 0.018, p < .001).

CONCLUSIONS

These findings provide evidence that dispositional hostility, and in particular, hostile affect, covary positively with systemic oxidative stress, raising the possibility that oxidative stress contributes to the pathogenicity of hostile attributes.

Cardiac sympathetic nerve terminal function in congestive heart failure

Increased cardiac release of norepinephrine (NE) and depleted cardiac stores of NE are two salient features of the human failing heart. Researches from my laboratory have shown that these changes are accompanied by a functional defect of NE uptake in the cardiac sympathetic nerve terminals. Our studies have shown that the decrease of NE uptake is caused by reduction of NE transporter density in the sympathetic nerve endings, and this change is responsible, at least in part, for the increased myocardial interstitial NE, decreased myocardial adrenoceptor density, and increased myocyte apoptosis in experimental cardiomyopathies. We have also provided evidence in both intact animals and cultured PC12 cells that the decrease of NE transporter is induced by the actions of oxidative metabolites of exogenous NE, involving endoplasmic reticulum stress and impaired N-glycosylation of the NE transporter. This change in the cardiac sympathetic NE uptake function, as demonstrated by [123I] metaiodobenzylguanidine in human studies, may not only serve as an important prognostic variable in patients with congestive heart failure, but also be used as a surrogate for the efficacies of various therapeutic interventions for heart failure. Finally, increasing evidence suggests and further studies are needed to show that the cardiac sympathetic nerve terminal function may be a direct target for pharmacologic treatment of congestive heart failure.

Temporal changes in myocardial adrenergic regulation with the progression to pump dysfunction after chronic β-adrenoreceptor activation in rats

We evaluated the relationship between myocardial norepinephrine release or inotropic responsiveness to adrenergic stimulation and intrinsic myocardial function after the progression to pump dysfunction induced by chronic beta-adrenoreceptor activation (isoproterenol [ISO], 0.1 mg/kg/day for 1 month or 6 months) in rats. Left ventricular (LV) systolic chamber dysfunction occurred after 6 months, but not after 1 month of beta-adrenoreceptor activation, as evidenced by reduced LV endocardial fractional shortening determined by echocardiography and a decrease in the slope of the LV systolic pressure-volume relations assessed in isolated, perfused heart preparations. A reduced pump function at 6 months of ISO administration was associated with chamber dilatation, while LV midwall fractional shortening (echocardiography) and the slope of the LV systolic stress-strain relations (isolated heart preparations), indices of intrinsic myocardial function, were unchanged. After 1 month of ISO administration, reduced beta1- and beta2-adrenoreceptor-mediated and sustained alpha-adrenoreceptor-mediated inotropic responses were noted. Nevertheless, increased inotropic potency of beta-adrenoreceptor agonists and upregulation of alpha-adrenoreceptor-mediated contractile responses were noted after 6 months of ISO administration. Increased adrenergic-inotropic responsiveness after 6 months of ISO administration was associated with depleted LV norepinephrine stores, as evidenced by reduced desipramine-stimulated norepinephrine concentrations in the coronary effluent. In conclusion, in the progression from compensated cardiac hypertrophy to pump dysfunction after chronic sympathetic activation, a preserved intrinsic myocardial contractility is accounted for by paradoxical upregulation of adrenergic-mediated contractile responses.

Correlation between cardiac oxidative stress and myocardial pathology due to acute and chronic norepinephrine administration in rats

Background: To investigate the cardiotoxic role of reactive oxygen species (ROS) and of products derived from catecholamines auto-oxidation, we studied: (1) the response of antioxidant cardiac cellular defence systems to oxidative stress induced by norepinephrine (NE) administration, (2) the effect of NE administration on cardiac β₁-adrenergic receptors by means of receptor binding assay, (3) the cellular morphological alterations related to the biologically cross-talk between the NE administration and cytokines [tumor necrosis factor-alpha (TNF-α), monocyte chemotactic protein-1 (MCP-1), interleukins IL6, IL8, IL10]Methods and Results: A total of 195 male rats was used in the experiment. All animals underwent electrocardiogram (EKG) before being sacrificed. The results obtained show that NE administration influences the antioxidant cellular defence system significantly increasing glutathione peroxidase (GPx) activity, glutathione reductase (GR) and superoxide dismutase (SOD). The oxidized glutathione (GSH/GSSG) ratio significantly decreases and malondialdehyde (MDA) levels increase showing a state of lipoperoxidation of cardiac tissue. We describe a significant apoptotic process randomly sparse in the damaged myocardium and the effect of ROS on the NE-mediated TNF-α, MCP-1, and IL6, IL8, IL10 production. Conclusions: Our results support the hypothesis that catecholamines may induce oxidative damage through reactive intermediates resulting from their auto-oxidation, irrespective of their interaction with adrenergic receptors, thus representing an important factor in the pathogenesis of catecholamines-induced cardiotoxicity. The rise of the cardioinhibitory cytokines may be interpreted as the adaptive response of jeopardized myocardium with respect to the cardiac dysfunction resulting from NE injection.

Cardiac dilatation and pump dysfunction without intrinsic myocardial systolic failure following chronic β-adrenoreceptor activation

There is no direct evidence to indicate that pump dysfunction in a dilated chamber reflects the impact of chamber dilatation rather than the degree of intrinsic systolic failure resulting from myocardial damage. In the present study, we explored the relative roles of intrinsic myocardial systolic dysfunction and chamber dilatation as mediators of left ventricular (LV) pump dysfunction. Administration of isoproterenol, a β-adrenoreceptor agonist, for 3 mo to rats (0.1 mg·kg−1·day−1) resulted in LV pump dysfunction as evidenced by a reduced LV endocardial fractional shortening (echocardiography) and a decrease in the slope of the LV systolic pressure-volume relation (isolated heart preparations). Although chronic β-adrenoreceptor activation induced cardiomyocyte damage (deoxynucleotidyl transferase-mediated dUTP nick-end labeling) as well as β1- and β2-adrenoreceptor inotropic downregulation (attenuated contractile responses to dobutamine and salbutamol), these changes failed to translate into alterations in intrinsic myocardial contractility. Indeed, LV midwall fractional shortening (echocardiography) and the slope of the LV systolic stress-strain relation (isolated heart preparations) were unchanged. A normal intrinsic myocardial systolic function, despite the presence of cardiomyocyte damage and β-adrenoreceptor inotropic downregulation, was ascribed to marked increases in myocardial norepinephrine release, to upregulation of α-adrenoreceptor-mediated contractile effects as determined by phenylephrine responsiveness, and to compensatory LV hypertrophy. LV pump failure was attributed to LV dilatation, as evidenced by increased LV internal dimensions (echocardiography), and a right shift and increased volume intercept of the LV diastolic pressure-volume relation. In conclusion, chronic sympathetic stimulation, despite reducing β-adrenoreceptor-mediated inotropic responses and promoting myocyte apoptosis, may nevertheless induce pump dysfunction primarily through LV dilatation, rather than intrinsic myocardial systolic failure.

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.

Beta1-adrenoceptor blockade mitigates excessive norepinephrine release into cardiac interstitium in mitral regurgitation in dog

Mitral regurgitation (MR) is associated with increased neuronal release of norepinephrine (NE) and epinephrine (EP) into myocardial interstitial fluid (ISF) that may be necessary in sustaining left ventricular (LV) function via activation of cardiomyocyte beta-adrenergic receptors (ARs). However, activation of neuronal beta-ARs on cardiac neurons may lead to further catecholamine release, with an attendant risk of functional deterioration. We hypothesize that a beneficial effect of beta-AR blockade may therefore mitigate excessive catecholamine release from cardiac adrenergic neurons in dogs with MR. We measured the effects of chronic beta-receptor blockade (beta-RB) on ISF NE and EP release using in vivo microdialysis in open-chest anesthetized dogs after 4 wk of MR with or without extended release of metoprolol succinate (100 mg/day) as well as in control dogs. Fractional shortening increased by 30% in both MR and MR + beta-RB dogs after 4 wk of MR. In MR + beta-RB dogs, stellate-stimulated heart rate change was attenuated compared with control and MR dogs, whereas peak change of LV pressure over time (+dP/dt) increased equally in all groups. Stellate-stimulated ISF NE increased fivefold over baseline in MR versus twofold in control dogs (< 0.05), but the NE release was significantly attenuated in MR + beta-RB dogs. In contrast, stellate-stimulated increases in ISF EP did not differ in control, MR, and MR + beta-RB dogs. This study demonstrates that beta-RB attenuates ISF NE release from cardiac neurons and that the LV functional response to MR is not dependent on an excess increase in ISF NE. Thus beta1-RB may exert a beneficial effect by attenuating untoward effects of excessive sympathetic efferent neural NE release while sustaining early LV functional adaptation to MR.

Mechanisms of preserved baseline cardiac systolic function in rats with adrenergic inotropic downregulation

Despite reductions in beta-adrenoreceptor (beta-AR)-mediated inotropic effects induced by sustained sympathetic activation in cardiac disease, whether these changes necessarily result in reductions in systolic function under resting conditions (baseline function) is not clear. Moreover, possible compensatory mechanisms which might contribute to maintaining the baseline systolic function despite reductions in beta-AR-mediated inotropic effects have not been systematically sought. In the present study, 1 month of daily administration of the beta-AR agonist, isoproterenol (0.05 mg/kg/day, i.p.), to rats resulted in an attenuation of left ventricular inotropic responses to isoproterenol over a wide range of concentrations (10(-8)-10(-4) M), whereas a decline of inotropic responses to norepinephrine, an endogenous inotrope, occurred only at high concentrations (10(-5)-10(-4) M). However, chronic isoproterenol administration failed to modify baseline systolic chamber and myocardial function, as determined in vivo using echocardiography (endocardial and midwall fractional shortening), and in isolated, perfused heart preparations (end-systolic chamber and myocardial elastance) Sustained baseline chamber function despite profound beta-AR-mediated inotropic downregulation was not attributed to alterations in cardiac loading conditions, resting heart rate, chamber remodeling, increased myocardial norepinephrine release, or enhanced contractile responses to alternative receptor/signal transduction pathways mediating positive inotropy (as assessed from histamine, serotonin, forskolin, angiotensin II or phenylephrine responsiveness). These findings indicate that baseline cardiac contractile function might be unaltered despite a profound impairment of beta-AR-induced responsiveness, an effect related to a preserved stimulatory influence of low physiological concentrations of endogenous norepinephrine constituting adrenergic tone at rest.

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.

Effects of early and late intravenous norepinephrine infusion on cerebral perfusion, microcirculation, brain-tissue oxygenation, and edema formation in brain-injured rats

OBJECTIVES

Reduction of cerebral perfusion during the early phase after traumatic brain injury is followed by a later phase of normal to increased perfusion. Thus, pharmacologically elevating mean arterial blood pressure with the aim of improving cerebral perfusion may exert different time-dependent effects on cortical perfusion, microcirculation, tissue oxygenation and brain edema formation after traumatic brain injury.

DESIGN

Randomized, placebo-controlled trial.

SETTING

Experimental laboratory at a university hospital.

SUBJECTS

A total of 37 male Sprague-Dawley rats subjected to a focal cortical contusion.

INTERVENTIONS

At 4 or 24 hrs after focal traumatic brain injury, mean arterial blood pressure was increased to 120 mm Hg for 90 mins by infusing norepinephrine. In rats receiving physiologic saline, mean arterial blood pressure remained unchanged. In the first series, pericontusional cortical perfusion was measured using the laser Doppler flowmetry scanning technique before injury and before, during, and after the infusion period. In a second series, intracranial and cerebral perfusion pressure and intraparenchymal perfusion and tissue oxygen measured within the contused and pericontusional cortex were recorded continuously before, during, and after norepinephrine infusion. Changes in cortical microcirculation were investigated by orthogonal polarization spectral imaging. At the end of each experiment, hemispheric swelling and water content were determined gravimetrically.

MEASUREMENTS AND MAIN RESULTS

At 4 and 24 hrs after traumatic brain injury, intravenous norepinephrine significantly increased pericontusional cortical perfusion, which was also reflected by an increase in diameters and flow velocities of pericontusional arterioles and venules. Cerebral perfusion pressure and intraparenchymal perfusion and tissue oxygen were significantly increased during norepinephrine infusion at 4 and 24 hrs. Hemispheric swelling and water content showed no difference between the groups.

CONCLUSIONS

After cortical impact injury, early and late intravenous norepinephrine infusion pressure-dependently increased cerebral perfusion and tissue oxygenation without aggravating or reducing brain edema formation. Future studies are warranted to determine long-term changes of short and prolonged norepinephrine-induced increases in mean arterial blood pressure and cerebral perfusion pressure.

Risk of mortality and heart failure exacerbations associated with inhaled beta-adrenoceptor agonists among patients with known left ventricular systolic dysfunction

OBJECTIVES

Recent studies suggest that myocardial beta(2)-adrenoceptors may be important in chronic heart failure. We sought to determine if use of selective beta(2)-agonists was associated with hospitalization for heart failure and all-cause mortality.

METHODS

We studied a cohort of patients with left ventricular systolic dysfunction (LVSD). The outcome was the first hospitalization with a primary diagnosis of chronic heart failure or death from any cause. The exposure was the average number of beta-agonist canisters filled per month in the 90 days prior to and 15 days after enrollment.

RESULTS

Among 1,529 subjects, the relative risk (RR) of chronic heart failure hospital admission associated with inhaled beta-agonists followed a dose-response relationship: RR for one canister per month, 1.4 (95% confidence interval [CI], 0.9 to 2.0), RR for two canisters per month, 1.7 (95% CI, 1.2 to 2.5), and RR for three canisters per month, 2.1 (95% CI, 1.4 to 3.1). The RR of death demonstrated a similar finding: RR for one canister per month, 0.9 (95% CI, 0.5 to 1.5), RR for two canisters per month, 1.3 (95% CI, 0.9 to 2.1), and RR for three canisters per month, 2.0 (95% CI, 1.3 to 3.1). Adjusting for potential confounding factors did not affect the estimates.

CONCLUSION

Among subjects with LVSD, inhaled beta-agonists were associated with an increased risk of heart failure hospitalization, and all-cause mortality. Clinicians should carefully consider the etiology of dyspnea when prescribing beta-agonists to patients with LVSD.

Ventricular hypertrophy plus neurohumoral activation is necessary to alter the cardiac beta-adrenoceptor system in experimental heart failure

Treatment of rats with monocrotaline (MCT) leads to pulmonary hypertension, right ventricular (RV) hypertrophy, and finally to RV heart failure. This is associated with characteristic changes in right ventricular beta-adrenoceptors (beta-AR), neuronal noradrenaline transporter (NAT) density and activity (uptake1), and G protein-coupled receptor kinase (GRK) activity. This study aimed to find out factors that determine beta-AR, uptake1, and GRK changes. Thus, 6-week-old rats were treated with 50 mg/kg MCT subcutaneous or 0.9% saline. Within 13 to 19 days after MCT application (group A), RV weight (222+/-6 versus 147+/-5 mg) and RV/left ventricular (LV) weight ratio (0.42+/-0.01 versus 0.29+/-0.01) were significantly increased, whereas plasma noradrenaline, RV beta-AR density, RV NAT density and activity, and RV GRK activity were not significantly altered. Twenty-one to twenty-eight days after MCT (group B), however, not only RV weight (316+/-4 versus 148+/-2 mg) and RV/LV weight ratio (0.61+/-0.01 versus 0.3+/-0.01) were markedly increased but also plasma noradrenaline (645+/-63 versus 278+/-18 pg/mL); now, RV beta-AR density (13.4+/-1.3 versus 26.5+/-1.1 fmol/mg protein), RV NAT density (50.9+/-11.3 versus 79.6+/-2.9 fmol/mg protein), and RV NAT activity (65.4+/-7.4 versus 111.8+/-15.9 pmol [3H]-NA/mg tissue slices/15 min) were significantly decreased and RV-membrane GRK activity (100+/-15 versus 67+/-6 [32P]-rhodopsin in cpm) significantly increased. LV parameters of MCT-treated rats were only marginally different from control LV. We conclude that in MCT-treated rats ventricular hypertrophy per se is not sufficient to cause characteristic alterations in the myocardial beta-AR system often seen in heart failure; only if ventricular hypertrophy is associated with neurohumoral activation beta-ARs are downregulated and GRK activity is increased.

Differential pre- and postsynaptic effects of desipramine on cardiac sympathetic nerve terminals in RHF

Right heart failure (RHF) is characterized by chamber-specific reductions of myocardial norepinephrine (NE) reuptake, beta-receptor density, and profiles of cardiac sympathetic nerve ending neurotransmitters. To study the functional linkage between NE uptake and the pre- and postsynaptic changes, we administered desipramine (225 mg/day), a NE uptake inhibitor, to dogs with RHF produced by tricuspid avulsion and progressive pulmonary constriction or sham-operated dogs for 6 wk. Animals receiving no desipramine were studied as controls. We measured myocardial NE uptake activity using [(3)H]NE, beta-receptor density by [(125)I]iodocyanopindolol, inotropic responses to dobutamine, and noradrenergic terminal neurotransmitter profiles by glyoxylic acid-induced histofluorescence for catecholamines, and immunocytochemical staining for tyrosine hydroxylase and neuropeptide Y. Desipramine decreased myocardial NE uptake activity and had no effect on the resting hemodynamics in both RHF and sham animals but decreased myocardial beta-adrenoceptor density and beta-adrenergic inotropic responses in both ventricles of the RHF animals. However, desipramine treatment prevented the reduction of sympathetic neurotransmitter profiles in the failing heart. Our results indicate that NE uptake inhibition facilitates the reduction of myocardial beta-adrenoceptor density and beta-adrenergic subsensitivity in RHF, probably by increasing interstitial NE concentrations, but protects the cardiac noradrenergic nerve endings from damage, probably via blockade of NE-derived neurotoxic metabolites into the nerve endings.

Differential alterations in cardiac adrenergic signaling in chronic hypoxia or norepinephrine infusion

Norepinephrine (NE)-induced desensitization of the adrenergic receptor pathway may mimic the effects of hypoxia on cardiac adrenoceptors. The mechanisms involved in this desensitization were evaluated in male Wistar rats kept in a hypobaric chamber (380 Torr) and in rats infused with NE (0.3 mg. kg(-1). h(-1)) for 21 days. Because NE treatment resulted in left ventricular (LV) hypertrophy, whereas hypoxia resulted in right (RV) hypertrophy, the selective hypertrophic response of hypoxia and NE was also evaluated. In hypoxia, alpha(1)-adrenergic receptors (AR) density increased by 35%, only in the LV. In NE, alpha(1)-AR density decreased by 43% in the RV. Both hypoxia and NE decreased beta-AR density. No difference was found in receptor apparent affinity. Stimulated maximal activity of adenylate cyclase decreased in both ventricles with hypoxia (LV, 41%; RV, 36%) but only in LV with NE infusion (42%). The functional activities of G(i) and G(s) proteins in cardiac membranes were assessed by incubation with pertussis toxin (PT) and cholera toxin (CT). PT had an important effect in abolishing the decrease in isoproterenol-induced stimulation of adenylate cyclase in hypoxia; however, pretreatment of the NE ventricle cells with PT failed to restore this stimulation. Although CT attenuates the basal activity of adenylate cyclase in the RV and the isoproterenol-stimulated activity in the LV, pretreatment of NE or hypoxic cardiac membranes with CT has a less clear effect on the adenylate cyclase pathway. The present study has demonstrated that 1) NE does not mimic the effects of hypoxia at the cellular level, i.e., hypoxia has specific effects on cardiac adrenergic signaling, and 2) changes in alpha- and beta-adrenergic pathways are chamber specific and may depend on the type of stimulation (hypoxia or adrenergic).

Alterations by norepinephrine of cardiac sympathetic nerve terminal function and myocardial beta-adrenergic receptor sensitivity in the ferret: normalization by antioxidant vitamins

BACKGROUND

Chronic excessive norepinephrine (NE) causes cardiac sympathetic nerve terminal abnormalities, myocardial beta-adrenergic receptor downregulation, and beta-adrenergic subsensitivity. The present study was carried out to determine whether these changes could be prevented by antioxidants.

METHODS AND RESULTS

Ferrets were administered either NE (1.33 mg/d) or vehicle by use of subcutaneous pellets for 4 weeks. Animals were simultaneously assigned to receive either antioxidant vitamins (beta-carotene, ascorbic acid, and alpha-tocopherol) or placebo pellets. NE increased plasma NE 4- to 5-fold but had no effect on heart rate, heart weight, arterial pressure, or left ventricular systolic function. However, myocardial NE uptake activity and NE uptake-1 site density were reduced, as well as cardiac neuronal NE, tyrosine hydroxylase, and neuropeptide Y. In addition, there was a decrease in myocardial beta-adrenergic receptor density with a selective decrease of the beta(1)-receptor subtype, reduction of the high-affinity site for isoproterenol, decreased basal adenylyl cyclase activity, and the adenylyl cyclase responses to isoproterenol, Gpp(NH)p, and forskolin. All of these changes were prevented by antioxidant vitamins. The effects of NE on myocardial beta-adrenergic receptor density, NE uptake-1 carrier site density, and neuronal NE were also prevented by superoxide dismutase or Trolox C.

CONCLUSIONS

The toxic effects of NE on the sympathetic nerve terminals are mediated via the formation of NE-derived oxygen free radicals. Preservation of the neuronal NE reuptake mechanism is functionally important, because the antioxidants also prevented myocardial beta-adrenergic receptor downregulation and postreceptor abnormalities. Thus, antioxidant therapy may be beneficial in heart failure, in which cardiac NE release is increased.