Cardiac Overexpression of the Norepinephrine Transporter Uptake-1 Results in Marked Improvement of Heart Failure

Dynamic monitoring of single-terminal norepinephrine transporter rate in the rodent cardiovascular system: A novel fluorescence imaging method

Here, we validate the use of a novel fluorescent norepinephrine transporter (NET) substrate for dynamic measurements of transporter function in rodent cardiovascular tissue; this technique avoids the use of radiotracers and provides single-terminal resolution. Rodent (Wistar rats and C57BL/6 mice) hearts and mesenteric arteries (MA) were isolated, loaded with NET substrate Neurotransmitter Transporter Uptake Assay (NTUA) ex vivo and imaged with confocal microscopy. NTUA labelled noradrenergic nerve terminals in all four chambers of the heart and on the surface of MA. In all tissues, a temperature-dependent, stable linear increase in intra-terminal fluorescence upon NTUA exposure was observed; this was abolished by NET inhibitor desipramine (1 μM) and reversed by indirectly-acting sympathomimetic amine tyramine (10 μM). NET reuptake rates were similar across the mouse cardiac chambers. In both species, cardiac NET activity was significantly greater than in MA (by 62 ± 29% (mouse) and 21 ± 16% (rat)). We also show that mouse NET reuptake rate was twice as fast as that in the rat (for example, in the heart, by 94 ± 30%). Finally, NET reuptake rate in the mouse heart was attenuated with muscarinic agonist carbachol (10 μM) thus demonstrating the potential for parasympathetic regulation of norepinephrine clearance. Our data provide the first demonstration of monitoring intra-terminal NET function in rodent cardiovascular tissue. This straightforward method allows dynamic measurements of transporter rate in response to varying physiological conditions and drug treatments; this offers the potential to study new mechanisms of sympathetic dysfunction associated with cardiovascular disease.

Advancements in the diagnostic workup, prognostic evaluation, and treatment of takotsubo syndrome

Takotsubo syndrome (TTS) is an acute and mostly reversible cardiomyopathy that mimics an acute coronary syndrome with left ventricular (LV) systolic dysfunction without relevant obstructive coronary artery disease. Its prevalence is probably underestimated and reaches 1.2-2% in patients with acute coronary syndrome undergoing coronary catheterization. Although supraphysiological epinephrine levels have been associated with TTS, the detailed pathophysiology is incompletely understood. Chest pain is the most common clinical presentation; however, cardiac decompensation, cardiogenic shock, and sudden cardiac death due to ventricular fibrillation may also be the first clinical manifestations. Patients are mostly postmenopausal women, in whom the condition is commonly associated with emotional triggers; however, men have a higher prevalence of TTS being associated with physical triggers, which has a worse prognosis compared with TTS associated with emotional triggers. As a diagnosis of exclusion, TTS has no single definitive diagnostic test. According to the distribution of LV wall motion abnormalities, various morphological subtypes have been identified. The final diagnosis depends on cardiac imaging with left ventricular angiography during acute heart catheterization, as well as on echocardiography and cardiac magnetic resonance. Most patients recover completely, albeit several factors have been associated with worse prognosis. Management is based on observational data, while randomized multicenter studies are still lacking. This review provides a general overview of TTS and focuses on the hypothesized pathophysiology, and especially on current practices in diagnosis, prognosis, and treatment.

Mechanisms underlying the increased cardiac norepinephrine spillover in heart failure

Patients with heart failure (HF) have increased levels of cardiac norepinephrine (NE) spillover, which is an independent predictor of mortality. We hypothesized that this increase in NE spillover in HF depends not only on increases in sympathetic nerve activity (SNA) but also on changes in the mechanisms controlling NE release and reuptake. Such changes would lead to differences between the increases in directly recorded SNA and NE spillover to the heart in HF. Experiments were conducted in conscious sheep implanted with electrodes to record cardiac SNA (CSNA). In addition, arterial pressure and cardiac NE spillover were determined. In HF, the levels of both CSNA (102 ± 8 vs. 45 ± 8 bursts/min, P < 0.05) and cardiac NE spillover (21.6 ± 3.8 vs. 3.9 ± 0.8 pmol/min, P < 0.05) were significantly higher than in normal control animals. In HF, baroreflex control of cardiac NE spillover was impaired, and when CSNA was abolished by increasing arterial pressure, there was no reduction in cardiac NE spillover. A decrease in cardiac filling pressures in the HF group led to a significant increase in CSNA, but it significantly decreased cardiac NE spillover. In HF, the levels of cardiac NE spillover were enhanced above those expected from the high level of SNA, suggesting that changes in mechanisms controlling NE release and reuptake further increase the high level of NE at the heart, which will act to enhance the deleterious effects of increased CSNA in HF. NEW & NOTEWORTHY This is the first study, to our knowledge, to compare direct recordings of cardiac sympathetic nerve activity with simultaneously measured cardiac norepinephrine (NE) spillover. Our results indicate that in heart failure, increased cardiac sympathetic nerve activity is a major contributor to the increased NE spillover. In addition, there is enhanced NE spillover for the levels of synaptic nerve activity.

Acute effects of arterial baroreflex on sympathetic nerve activity and plasma norepinephrine concentration

Arterial pressure (AP) elevates as a logarithmic function of exogenously administered dose of norepinephrine (NE). In contrast, AP is nearly linearly correlated with efferent sympathetic nerve activity (SNA) during acute baroreflex intervention. The present study aimed at quantifying the relationship between SNA and plasma NE concentration during acute baroreflex intervention. Carotid sinus regions were isolated from systemic circulation in five Wistar Kyoto rats, and carotid sinus pressure was changed among 60, 100, 120, 140, and 180 mm Hg every 2 min. Arterial blood (0.2 ml) was obtained at each pressure level for plasma NE measurement. Maximum AP and minimum AP were 153.34 ± 6.28 and 67.31 ± 4.92 mm Hg, respectively, in response to pressure perturbation. Plasma NE correlated linearly with SNA for individual animal data (slope: 0.957 ± 0.090 pg · ml(-1) · %(-1), intercept: 46.57 ± 7.22 pg/ml, r(2): ranged from 0.923 to 0.992) and also for group averaged data (NE = 0.956 × SNA + 47.97, r(2 )= 0.982). Blockade of neuronal NE uptake by intravenous desipramine (1 mg/kg) administration increased the slope (2.966 ± 0.686 pg · ml(-1) · %(-1), P < 0.05) and the intercept (168.73 ± 28.53 pg/ml, P < 0.01) of the plasma NE-SNA relationship. These results indicate that the relationship between SNA and plasma NE concentration was nearly linear within the normal physiological range of acute baroreflex control of AP. While plasma NE concentration can reflect changes in SNA, it may also overestimate the sympathetic outflow from the central nervous system when neuronal NE uptake is impaired systemically.

The effect of long-term left ventricular assist device support on myocardial sympathetic activity in patients with non-ischaemic dilated cardiomyopathy

AIMS

Dilated cardiomyopathy (DCM) patients have abundant levels of norepinephrine secondary to failure of the norepinephrine transporter uptake mechanism. Little is known about the effects of an LV assist device (LVAD) on cardiac sympathetic innervations and norepinephrine transporter dysfunction. This study examines the effects of continuous-flow HeartMate II LVAD on cardiac sympathetic innervations using [(123)I]metaiodobenzylguanidine ([(123)I]MIBG) nuclear imaging.

METHODS AND RESULTS

After injecting 431 ± 21 MBq of [(123)I]MIBG, planar scintigraphy was performed at 15 min and 4 h in 14 consecutive non-diabetic non-ischaemic DCM patients. Scans were executed early post-LVAD implantation (T1) and prior to either device explantation for myocardial recovery or transplant listing (T2). [(123)I]MIBG measured parameters included early and delayed heart-mediastinum (H/M) ratios and washout rate (W/O). Catecholamine levels were measured using liquid chromatography-mass spectrometry. Following 208.4 ± 85.5 days of LVAD support, both early and delayed H/M ratios increased by 42.1% (P < 0.001) and 54.7% (P < 0.001), respectively. The W/O rate decreased by 46% (P = 0.003). Plasma norepinephrine, epinephrine, and dopamine decreased significantly in correlation with [(123)I]MIBG parameters. Ten patients had recovered and had their device explanted as they had demonstrated a higher percentage change in delayed H/M ratio, W/O rate, and norepinephrine levels. Linear regression analysis revealed a strong correlation between percentage changes in both norepinephrine and epinephrine and myocardial recovery.

CONCLUSION

Combination therapy with LVAD and drug resulted in enhancement of [(123)I]MIBG uptake in DCM patients.

Exogenous nerve growth factor supplementation elevates myocardial immunoreactivity and attenuates cardiac remodeling in pressure-overload rats

It is postulated that supplementation of exogenous nerve growth factor (NGF) might mediate improvement of the cardiac sympathetic nerve function in heart failure (HF). Local intramuscular injection of NGF near the cardiac sympathetic ganglia could influence the innervation pattern, norepinephrine transporter (NET) gene expression, and improve the cardiac remodeling in experimental HF animals. In this study, we injected NGF into the scalenus medius muscles of Sprague-Dawley rats with abdominal aortic constriction (AC). The nerve innervated pattern, left ventricular morphology, and function following injection in rats with AC were investigated respectively by immunohistochemistry and echocardiography. Levels of mRNA expression of NET, growth associated protein 43 (GAP 43), NGF and its receptors TrkA and p75(NTR), and brain natriuretic peptide (BNP) were measured by real-time polymerase chain reaction. The results showed that myocardial NGF mRNA levels were comparable in rats with AC. Short-term supplementation of exogenous NGF raised the myocardial NGF immunoreactivity, but did not cause hyperinnervation and NET mRNA upregulation in the AC rats. Furthermore, myocardial TrkA mRNA was found to be remarkably decreased and p75(NTR) mRNA was increased. Myocardial TrkA downregulation may play a beneficial effect for avoiding the hyperinnervation, and it is reasonable to postulate that p75(NTR) can function as an NGF receptor in the absence of TrkA. Interestingly, local NGF administration into the neck muscles near the ganglia could attenuate cardiac remodeling and downregulate BNP mRNA. These results suggest that exogenous NGF can reach the target tissue along the axons anterogradely, and improve the cardiac remodeling.

Norepinephrine transporter function and human cardiovascular disease

Approximately 80-90% of the norepinephrine released in the brain or in peripheral tissues is taken up again through the neuronal norepinephrine transporter (NET). Pharmacological studies with NET inhibitors showed that NET has opposing effects on cardiovascular sympathetic regulation in the brain and in the periphery. Furthermore, NET is involved in the distribution of sympathetic activity between vasculature, heart, and kidney. Genetic NET dysfunction is a rare cause of the postural tachycardia syndrome. The condition is characterized by excessive adrenergic stimulation of the heart, particularly with standing. Conversely, NET inhibition may be beneficial in hypoadrenergic states, such as central autonomic failure or neurally mediated syncope, which results from acute sympathetic withdrawal. Biochemical studies suggested reduced NET function in some patients with essential hypertension. Furthermore, cardiac NET function appears to be reduced in common heart diseases, such as congestive heart failure, ischemic heart disease, and stress-induced cardiomyopathy. Whether NET dysfunction is a consequence or cause of progressive heart disease in human subjects requires further study. However, studies with the nonselective NET inhibitor sibutramine suggest that reduced NET function could have an adverse effect on the cardiovascular system. Given the widespread use of medications inhibiting NET, the issue deserves more attention.

Assessing left ventricular systolic dysfunction after myocardial infarction: are ejection fraction and dP/dt(max) complementary or redundant?

Among the various cardiac contractility parameters, left ventricular (LV) ejection fraction (EF) and maximum dP/dt (dP/dt(max)) are the simplest and most used. However, these parameters are often reported together, and it is not clear if they are complementary or redundant. We sought to compare the discriminative value of EF and dP/dt(max) in assessing systolic dysfunction after myocardial infarction (MI) in swine. A total of 220 measurements were obtained. All measurements included LV volumes and EF analysis by left ventriculography, invasive ventricular pressure tracings, and echocardiography. Baseline measurements were performed in 132 pigs, and 88 measurements were obtained at different time points after MI creation. Receiver operator characteristic (ROC) curves to distinguish the presence or absence of an MI revealed a good predictive value for EF [area under the curve (AUC): 0.998] but not by dP/dt(max) (AUC: 0.69, P < 0.001 vs. EF). Dividing dP/dt(max) by LV end-diastolic pressure and heart rate (HR) significantly increased the AUC to 0.87 (P < 0.001 vs. dP/dt(max) and P < 0.001 vs. EF). In naïve pigs, the coefficient of variation of dP/dt(max) was twice than that of EF (22.5% vs. 9.5%, respectively). Furthermore, in n = 19 pigs, dP/dt(max) increased after MI. However, echocardiographic strain analysis of 23 pigs with EF ranging only from 36% to 40% after MI revealed significant correlations between dP/dt(max) and strain parameters in the noninfarcted area (circumferential strain: r = 0.42, P = 0.05; radial strain: r = 0.71, P < 0.001). In conclusion, EF is a more accurate measure of systolic dysfunction than dP/dt(max) in a swine model of MI. Despite the variability of dP/dt(max) both in naïve pigs and after MI, it may sensitively reflect the small changes of myocardial contractility.

Sympathetic nervous dysregulation in the absence of systolic left ventricular dysfunction in a rat model of insulin resistance with hyperglycemia

Background

Diabetes mellitus is strongly associated with cardiovascular dysfunction, derived in part from impairment of sympathetic nervous system signaling. Glucose, insulin, and non-esterified fatty acids are potent stimulants of sympathetic activity and norepinephrine (NE) release. We hypothesized that sustained hyperglycemia in the high fat diet-fed streptozotocin (STZ) rat model of sustained hyperglycemia with insulin resistance would exhibit progressive sympathetic nervous dysfunction in parallel with deteriorating myocardial systolic and/or diastolic function.

Methods

Cardiac sympathetic nervous integrity was investigated in vivo via biodistribution of the positron emission tomography radiotracer and NE analogue [¹¹C]meta-hydroxyephedrine ([¹¹C]HED). Cardiac systolic and diastolic function was evaluated by echocardiography. Plasma and cardiac NE levels and NE reuptake transporter (NET) expression were evaluated as correlative measurements.

Results

The animal model displays insulin resistance, sustained hyperglycemia, and progressive hypoinsulinemia. After 8 weeks of persistent hyperglycemia, there was a significant 13-25% reduction in [¹¹C]HED retention in myocardium of STZ-treated hyperglycemic but not euglycemic rats as compared to controls. There was a parallel 17% reduction in immunoblot density for NE reuptake transporter, a 1.2 fold and 2.5 fold elevation of cardiac and plasma NE respectively, and no change in sympathetic nerve density. No change in ejection fraction or fractional area change was detected by echocardiography. Reduced heart rate, prolonged mitral valve deceleration time, and elevated transmitral early to atrial flow velocity ratio measured by pulse-wave Doppler in hyperglycemic rats suggest diastolic impairment of the left ventricle.

Conclusions

Taken together, these data suggest that sustained hyperglycemia is associated with elevated myocardial NE content and dysregulation of sympathetic nervous system signaling in the absence of systolic impairment.

Evaluation of LMI1195, a novel 18F-labeled cardiac neuronal PET imaging agent, in cells and animal models

BACKGROUND

Heart failure has been associated with impaired cardiac sympathetic neuronal function. Cardiac imaging with radiolabeled agents that are substrates for the neuronal norepinephrine transporter (NET) has demonstrated the potential to identify individuals at risk of cardiac events. N-[3-Bromo-4-(3-[18F]fluoro-propoxy)-benzyl]-guanidine (LMI1195) is a newly developed 18F-labeled NET substrate designed to allow cardiac neuronal imaging with the high sensitivity, resolution, and quantification afforded by positron emission tomography (PET).

METHODS AND RESULTS

LMI1195 was evaluated in comparison with norepinephrine (NE) in vitro and 123I-meta-iodobenzylguanidine (MIBG) in vivo. The affinity (Ki) of LMI1195 for NET was 5.16 ± 2.83 μmol/L, similar to that of NE (3.36 ± 2.77 μmol/L) in a cell membrane-binding assay. Similarly, LMI1195 uptake kinetics examined in a human neuroblastoma cell line had Km and Vmax values of 1.44 ± 0.76 μmol/L and 6.05 ± 3.09 pmol/million cells per minute, comparable to NE (2.01 ± 0.85 μmol/L and 6.23 ± 1.52 pmol/million cells per minute). In rats, LMI1195 heart uptake at 15 and 60 minutes after intravenous administration was 2.36 ± 0.38% and 2.16 ± 0.38% injected dose per gram of tissue (%ID/g), similar to 123I-MIBG (2.14 ± 0.30 and 2.19 ± 0.27%ID/g). However, the heart to liver and lung uptake ratios were significantly higher for LMI1195 than for 123I-MIBG. In rabbits, desipramine (1 mg/kg), a selective NET inhibitor, blocked LMI1195 heart uptake by 82%, which was more effective than 123I-MIBG (53%), at 1 hour after dosing. Sympathetic denervation with 6-hydroxydopamine, a neurotoxin, resulted in a marked (79%) decrease in LMI1195 heart uptake. Cardiac PET imaging with LMI1195 in rats, rabbits, and nonhuman primates revealed clear myocardium with low radioactivity levels in the blood, lung, and liver. Imaging in rabbits pretreated with desipramine showed reduced heart radioactivity levels in a dose-dependent manner. Additionally, imaging in sympathetically denervated rabbits resulted in low cardiac image intensity with LMI1195 but normal perfusion images with flurpiridaz F 18, a PET myocardial perfusion imaging agent. In nonhuman primates pretreated with desipramine (0.5 mg/kg), imaging with LMI1195 showed a 66% decrease in myocardial uptake. In a rat model of heart failure, the LMI1195 cardiac uptake decreased as heart failure progressed.

CONCLUSIONS

LMI1195 is a novel (18)F imaging agent retained in the heart through the NET and allowing evaluation of the cardiac sympathetic neuronal function by PET imaging.

Rabbit models of heart disease

Human heart disease is a major cause of death and disability. A variety of animal models of cardiac disease have been developed to better understand the etiology, cellular and molecular mechanisms of cardiac dysfunction and novel therapeutic strategies. The animal models have included large animals (e.g. pig and dog) and small rodents (e.g. mouse and rat) and the advantages of genetic manipulation in mice have appropriately encouraged the development of novel mouse models of cardiac disease. However, there are major differences between rodent and human hearts that raise cautions about the extrapolation of results from mouse to human. The rabbit is a medium-sized animal that has many cellular and molecular characteristics very much like human, and is a practical alternative to larger mammals. Numerous rabbit models of cardiac disease are discussed, including pressure or volume overload, ischemia, rapid-pacing, doxorubicin, drug-induced arrhythmias, transgenesis and infection. These models also lead to the assessment of therapeutic strategies which may become beneficial in human cardiac disease.

Ju Chen – University of California, San Diego, Department of Medicine, La Jolla, CA, USA

Robert Ross – University of California, San Diego, Cardiology Section, San Diego, CA, USA

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Evaluation of radioiodinated (2S,alphaS)-2-(alpha-(2-iodophenoxy)benzyl)morpholine as a radioligand for imaging of norepinephrine transporter in the heart

INTRODUCTION

The norepinephrine transporter (NET) is located presynaptically on noradrenergic nerve terminals and plays a critical role in the regulation of the synaptic norepinephrine (NE) concentration via the reuptake of NE. Changes in NET have been recently reported in several cardiac failures. Therefore, a NET-specific radioligand is useful for in vivo assessment of changes in NET density in various cardiac disorders. Recently, we developed a radioiodinated reboxetine analogue, (2S,alphaS)-2-(alpha-(2-iodophenoxy)benzyl)morpholine ((S,S)-IPBM), for NET imaging. In the current study, we assessed the applicability of radioiodinated (S,S)-IPBM to NET imaging in the heart.

METHODS

The NET affinity and selectivity were measured from the ability to displace specific [3H]nisoxetine and (S,S)-[125 I]IPBM binding to rat heart membrane, respectively. To evaluate the distribution of (S,S)-[125 I]IPBM in vivo, biodistribution experiment was performed in rats. With the use of several monoamine transporter binding agents, pharmacological blocking experiments were performed in rats.

RESULTS

In vitro binding assays showed that the affinity of (S,S)-IPBM to NET was similar to those of the well-known NET-specific binding agents, nisoxetine and desipramine. Furthermore, (S,S)-[125 I]IPBM binding was inhibited by nisoxetine and desipramine, but not by dopamine or serotonin transporter binding agents. These data indicated that (S,S)-IPBM had high affinity and selectivity for NET in vitro. Biodistribution studies in rats showed rapid and high uptake of (S,S)-[125 I]IPBM by the heart and rapid clearance from the blood. The heart-to-blood ratio was 31.9 at 180 min after the injection. The administration of nisoxetine and desipramine decreased (S,S)-[125 I]IPBM accumulation in the heart, but injection of fluoxetine and GBR12909 had little influence.

CONCLUSIONS

Radioiodinated (S,S)-IPBM is a potential radioligand for NET imaging in the heart.

Adenovirus-mediated delivery of relaxin reverses cardiac fibrosis

We have evaluated the effectiveness of systemic adenovirally delivered mouse relaxin on reversing fibrosis in a transgenic murine model of fibrotic cardiomyopathy due to beta(2)-adrenergic receptor (beta(2)AR) overexpression. Recombinant adenoviruses expressing green fluorescent protein (Ad-GFP), rat relaxin (Ad-rRLN) and mouse relaxin (Ad-mRLN) were generated and Ad-rRLN and Ad-mRLN were demonstrated to direct the expression of bioactive relaxin peptides in vitro. A single systemic injection of Ad-mRLN resulted in transgene expression in the liver and bioactive relaxin peptide in the plasma. Ad-mRLN, but not Ad-GFP, treatment reversed the increased left ventricular collagen content in beta(2)AR mice to control levels without affecting collagen levels in other heart chambers or in the lung and kidney. Hence a single systemic injection of adenovirus producing mouse relaxin reverses cardiac fibrosis without adversely affecting normal collagen levels in other organs and establishes the potential for the use of relaxin gene therapy for the treatment of cardiac fibrosis.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

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.

Determination of endogenous norepinephrine levels in different chambers of the rat heart by capillary electrophoresis coupled with amperometric detection

Capillary electrophoresis with end-column amperometric detection (CE-EC) was used to determine the regional distribution of norepinephrine (NE) in the hearts of sympathetically innervated (control) and chemically sympathectomized rats. Key features of the method are (i) the sample preparation and clean-up step that involved the application of off-line solid phase extraction (SPE) with a 95% NE recovery and (ii) the use of a diamond microelectrode for detection. NE was quantified in the left and right ventricle, the ventricular septum, and the left and right atrium. The NE concentration in the atria was three to five times higher than in the ventricles and ventricular septum of control rats. Basal NE levels in the left and right ventricle and the ventricular septum were reduced to below the detection limit (0.034 microg/g tissue) in tissues treated with the neurotoxin, 6-hydroxydopamine (6-OHDA), while only a moderate reduction was observed in the left and right atrium. Importantly, the diamond microelectrode provided low and stable background current and low peak-to-peak noise <or=0.65 pA at a detection potential of +0.86 V versus Ag/AgCl. A reproducible electrode response was observed for multiple injections of tissue homogenates with minimal response attenuation due to electrode fouling.

Functional alterations after cardiac sodium-calcium exchanger overexpression in heart failure

The sodium-calcium exchanger (NCX) is discussed as one of the key proteins involved in heart failure. However, the causal role and the extent to which NCX contributes to contractile dysfunction during heart failure are poorly understood. NCX overexpression was induced by infection with an adenovirus coding for NCX, which coexpressed green fluorescence protein (GFP) (AdNCX) by ex vivo gene transfer to nonfailing and failing rabbit cardiomyocytes. Myocardial gene transfer in rabbits in vivo was achieved by adenoviral delivery via aortic cross-clamping. Peak cell shortening of cardiomyocytes was determined photo-optically. Hemodynamic parameters in vivo were determined by echocardiography (fractional shortening) and tip catheter [maximal first derivative of left ventricular (LV) pressure (dP/d tmax); maximal negative derivative of LV pressure (−dP/d tmax)]. Peak cell shortening was depressed after NCX gene delivery in isolated nonfailing and in failing cardiomyocytes. In nonfailing rabbits in vivo, basal systolic contractility (fractional shortening and dP/d tmax) and maximum rate of LV relaxation (−dP/d tmax) in vivo were largely unaffected after NCX overexpression. However, during heart failure, long-term NCX overexpression over 2 wk significantly improved fractional shortening and dP/d tmax compared with AdGFP-infected rabbits, both without inotropic stimulation and after β-adrenergic stimulation with isoproterenol. −dP/d tmax was also improved after NCX overexpression in the failing rabbits group. These results indicate that short-term effects of NCX overexpression impair contractility of isolated failing and nonfailing rabbit cardiomyocytes. NCX overexpression over 2 wk in vivo does not seem to affect myocardial contractility in nonfailing rabbits. Interestingly, in vivo overexpression of NCX decreased the progression of systolic and diastolic contractile dysfunction and improved β-adrenoceptor-mediated contractile reserve in heart failure in rabbits in vivo.

Spironolactone preserves cardiac norepinephrine reuptake in salt-sensitive Dahl rats

An impairment of cardiac norepinephrine (NE) reuptake via the neuronal NE transporter (NET) enhances the effects of increased cardiac NE release in heart failure patients. Increasing evidence suggests that aldosterone and endothelins promote sympathetic overstimulation of failing hearts. Salt-sensitive Dahl rats (DS) fed a high-salt diet developed arterial hypertension and diastolic heart failure as well as elevated plasma levels of endothelin-1 and NE. Cardiac NE reuptake and NET-binding sites, as assessed by clearance of bolus-injected [(3)H]NE in isolated perfused rat hearts and [(3)H]mazindol binding, were reduced. Treatment of DS with the mineralocorticoid receptor antagonist spironolactone preserved the plasma levels of endothelin-1 and NE, cardiac NE reuptake, and myocardial NET density. Moreover, the ventricular function and survival of spironolactone-treated DS were significantly improved compared with untreated DS. The alpha(1)-inhibitor prazosin decreased blood pressure in DS similar to spironolactone treatment, but did not normalize the plasma levels of endothelin-1 and NE, NE reuptake, or ventricular function. In a heart failure-independent model, Wistar rats that were infused with aldosterone and fed a high-salt diet developed impaired cardiac NE reuptake. Treatment of these rats with the endothelin A receptor antagonist darusentan attenuated the impairment of NE reuptake. In conclusion, spironolactone preserves NET-dependent cardiac NE reuptake in salt-dependent heart failure. Evidence is provided that aldosterone inhibits NET function through an interaction with the endothelin system. Selective antagonism of the mineralocorticoid and/or the endothelin A receptor might represent therapeutic principles to prevent cardiac sympathetic overactivity in salt-dependent heart failure.