Cardiac Norepinephrine Reuptake Research Articles

Depletion of cardiac catecholamine stores impairs cardiac norepinephrine re-uptake by downregulation of the norepinephrine transporter.

In heart failure (HF), a disturbed cardiac norepinephrine (NE) homeostasis is characterized by depleted cardiac NE stores, impairment of the cardiac NE re-uptake by the neuronal norepinephrine transporter (NET) and enhanced cardiac NE net release. Reduced cardiac NE content appears to be caused by enhanced cardiac NE net release from sympathetic neurons in HF, triggered by neurohumoral activation. However, it remains unclear whether reduced NE itself has an impact on cardiac NE re-uptake, independent of neurohumoral activation. Here, we evaluated whether depletion of cardiac NE stores alone can regulate cardiac NE re-uptake. Treatment of Wistar rats with reserpine (5 mg/kg/d) for one (1d) or five days (5d) resulted in markedly reduced cardiac NE content, comparable to NE stores in experimental HF due to pressure overload. In order to assess cardiac NE re-uptake, the specific cardiac [3H]-NE uptake via the NET in a Langendorff preparation was measured. Reserpine treatment led to decreased NE re-uptake at 1d and 5d compared to saline treatment. Expression of tyrosine hydroxylase (TH), the rate-limiting enzyme of the NE synthesis, was elevated in left stellate ganglia after reserpine. Mechanistically, measurement of NET mRNA expression in left stellate ganglia and myocardial NET density revealed a post-transcriptional downregulation of the NET by reserpine. In summary, present data demonstrate that depletion of cardiac NE stores alone is sufficient to impair cardiac NE re-uptake via downregulation of the NET, independent of systemic neurohumoral activation. Knowledge about the regulation of the cardiac NE homeostasis may offer novel therapeutic strategies in HF.

Aldosterone augments Na+-induced reduction of cardiac norepinephrine reuptake.

Impairment of the cardiac norepinephrine (NE) reuptake by the neuronal NE transporter contributes to enhanced cardiac NE net release in congestive heart failure. Elevated plasma levels of aldosterone (AL) promote sympathetic overstimulation in failing hearts by unclear mechanisms. Our aim was to evaluate if elevated AL and/or alterations in Na(+) intake regulate cardiac NE reuptake. To test the effects of AL and Na(+) on cardiac NE reuptake, Wistar rats were fed a normal-salt (NS) diet (0.2% NaCl), a low-salt (LS) diet (0.015% NaCl), or a high-salt (HS) diet (8% NaCl). Another group of animals received AL infusion alone (0.75 μg/h) or AL infusion plus HS diet. Specific cardiac [(3)H]NE uptake via the NE transporter in a Langendorff preparation and AL plasma levels were measured at different time points between 5 and 42 days of treatment. To compare these findings from healthy animals with a disease model, Dahl salt-sensitive rats were investigated as a model of congestive heart failure with endogenously elevated AL. In summary, neither exogenous nor endogenous elevations of AL alone were sufficient to reduce cardiac NE reuptake. Only the HS diet induced a reduction of NE reuptake by 26%; additional infusion of AL augmented this effect to a further reduction of NE reuptake by 36%. In concordance, Dahl salt-sensitive rats treated with a HS diet displayed elevated AL and a marked reduction of NE reuptake. We conclude that exogenous or endogenous AL elevations alone do not reduce cardiac NE reuptake, but AL serves as an additional factor that negatively regulates cardiac NE reuptake in concert with HS intake.

Cardiac autonomic function in adolescents operated by arterial switch surgery

Children with transposition of the great arteries, in whom an arterial switch operation (ASO) is performed, have been shown to have an increased incidence of sudden death, which may be due to cardiac autonomic imbalance and repolarisation instability. We hypothesised that i) cardiac norepinephrine (NE) kinetics and ii) arterial baroreflex sensitivity (BRS), reflecting sympathetic activity and vagal function respectively, are altered in this group. 17 children (15.8 ± 1.5 years of age) with ASO-surgery in the neonatal period were studied. 17 had cardiac BRS assessed by spontaneous fluctuations of systolic blood pressure and RR-interval, and repolarisation was measured as QT variability index. Matched healthy subjects were controls. Cardiac vagal function and repolarisation pattern were unchanged following ASO-surgery. At cardiac catheterisation, we infused tritiated NE in 8 of these children to examine total body and cardiac sympathetic function at baseline and following 5 min of adenosine infusion to induce reflex sympathetic activation. Blood was sampled simultaneously from the aorta and coronary sinus. Cardiac fractional extraction of ([3H])NE was substantially lower in operated children, being 56 ± 10 vs. 82 ± 9% (p=0.0001). Following i.v. adenosine in the operated group, NE total body spillover doubled vs. baseline (p<0.002) and the coronary venous-arterial concentration gradient of ([3H])dihydroxyphenylglycol increased 4-fold (p=0.04). Arterial switch operation performed neonatally appears to leave cardiac vagal function intact and, although cardiac sympathetic activation in response to adenosine occurs, cardiac neuronal NE reuptake is impaired. This may be pro-arrhythmic by reducing removal capacity of NE from the cardiac synaptic cleft.

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

BackgroundDiabetes 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.MethodsCardiac 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.ResultsThe 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.ConclusionsTaken 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.

Differential expression of cardiac neurotrophic factors and sympathetic nerve ending abnormalities within the failing heart

In congestive heart failure (CHF), cardiac sympathetic nerve endings transdifferentiate from a balanced norepinephrine (NE) storage/release/uptake apparatus to a nerve that predominantly releases NE. Little is known about the neurotrophic factors that may trigger this process. In the present study, we evaluated the cardiac expression pattern of nerve growth factor (NGF), neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF) in salt-sensitive Dahl rats (DS), which are characterized by profound alterations of the cardiac sympathetic nervous system. Experiments were performed in male DS and salt-resistant Dahl rats (DR) 30, 40 and 50 days after onset of high-salt intake. The sympathetic nerve density was measured by glyoxylic acid-induced histofluorescence. Cardiac NE re-uptake was assessed by isolated heart perfusion with [3H]-NE and norepinephrine transporter (NET) mRNA by real-time PCR. Cardiac expression of neurotrophic factors was determined by ribonuclease protection assay and Western blot analysis. DS rats displayed reduced left ventricular sympathetic nerve endings 40 days after onset of high-salt intake, which was preceded by an impaired cardiac [3H]-NE uptake. NGF, a positive regulator of NE re-uptake, and NT-3 were down-regulated already 30 days after onset of high-salt intake, whereas BDNF and CNTF protein expression were increased not before 40 days after onset of high-salt intake. In conclusion, during the development of CHF, a dysregulated NE storage/release/uptake apparatus within the sympathetic nerve endings might be triggered by differential expression of cardiac neurotrophic factors.

Preserved Norepinephrine Reuptake but Reduced Sympathetic Nerve Endings in Hypertrophic Volume-Overloaded Rat Hearts

Background In congestive heart failure (CHF), an activation of the cardiac sympathetic nervous system results in depleted cardiac norepinephrine (NE) stores. The underlying regulatory mechanisms are discussed controversially and were investigated in the present study in CHF resulting from volume overload. Methods and Results Aorto-caval shunt (AVS) was performed in rats. Plasma NE levels were determined by radioenzymatic assay, left ventricular NE by high-performance liquid chromatography, endothelin-1 by enzyme-linked immunosorbent assay. Tyrosine-hydroxylase (TH)– and nerve growth factor (NGF)–mRNA was determined by Northern blot analysis and ribonuclease-assay. Cardiac [ 3H]-NE uptake was measured in isolated perfused hearts. Glyoxylic acid–induced histofluorescence was used to quantify cardiac sympathetic nerves. Compared with sham-operated animals (SH), AVS rats were characterized by depleted cardiac NE stores and enhanced NE plasma levels. Neither TH-mRNA levels in stellate ganglia, nor cardiac [ 3H]-NE-uptake were reduced in AVS. The left ventricular density of sympathetic nerves was markedly decreased. Gene expression of myocardial NGF (a positive regulator of NE reuptake and cardiac sympathetic nerve density) and left ventricular endothelin-1 (a negative regulator of NE reuptake and positive regulator of cardiac NGF expression) were unchanged. Conclusion In volume-overloaded hypertrophic hearts, depletion of cardiac NE stores is caused by a reduction of the sympathetic nerve density, whereas cardiac NE reuptake is preserved.

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.

Endothelin-1 inhibits the neuronal norepinephrine transporter in hearts of male rats

Endothelin-1 (ET-1) potentiates norepinephrine (NE)-induced contractile responses. An impairment of cardiac NE re-uptake by the neuronal NE transporter (NET) contributes to an increased NE net release in failing hearts. We hypothesized that both phenomena are caused by ET-1-mediated inhibition of NET. [3H]-NE-uptake, electrical field stimulation-evoked NE overflow and left ventricular contractility (LV-dp/dt(max)) were measured in isolated perfused rat hearts. NET density on cardiac plasma membranes was determined by [3H]-mazindol binding. Experimental heart failure in rats was induced by transverse aortic constriction (TAC). ET-1 inhibited cardiac [3H]-NE-uptake in a concentration- and time-dependent manner. The endothelin A receptor (ET(A)) antagonist BQ123 but not the endothelin B receptor (ET(B)) antagonist BQ788 abolished ET-1-induced reduction of [3H]-NE-uptake. Likewise, ET-1, but not the ET(B) agonist sarafotoxin S6c, enhanced the stimulated overflow of endogenous NE. In contrast, ET-1 inhibited the stimulated NE overflow during NET blockade (exocytotic NE release) via activation of ET(B). In isovolumically contracting healthy hearts, ET-1 potentiated the NE- but not isoprenaline-induced increase in LV-dp/dt(max). Since isoprenaline is not a NET substrate, the enhanced LV-dp/dt(max) response to NE thus depends on NET. In TAC rats, ET(A) antagonism by darusentan improved both impairment of cardiac [3H]-NE-uptake and reduction of [3H]-mazindol binding sites. ET-1 inhibits cardiac NE re-uptake via ET(A) but attenuates exocytotic NE release via ET(B), resulting in opposite effects on cardiac NE net release. In healthy hearts, ET(A)-mediated inhibition of NE re-uptake exceeds ET(B)-mediated silencing of NE release and potentiates the NE-induced increase in left ventricular contractility. In TAC rats, endogenous ET-1 impairs NE re-uptake and promotes sympathetic overstimulation of failing hearts.

Sympathetic augmentation in hypertension: role of nerve firing, norepinephrine reuptake, and Angiotensin neuromodulation.

There is growing evidence that essential hypertension is commonly neurogenic and is initiated and sustained by sympathetic nervous system overactivity. Potential mechanisms include increased central sympathetic outflow, altered norepinephrine (NE) neuronal reuptake, diminished arterial baroreflex dampening of sympathetic nerve traffic, and sympathetic neuromodulation by angiotensin II. To address this issue, we used microneurography and radiotracer dilution methodology to measure regional sympathetic activity in 22 hypertensive patients and 11 normotensive control subjects. The NE transport inhibitor desipramine was infused to directly assess the potential role of impaired neuronal NE reuptake. To evaluate possible angiotensin sympathetic neuromodulation, the relation of arterial and coronary sinus plasma concentrations of angiotensin II to sympathetic activity was investigated. Hypertensive patients displayed increased muscle sympathetic nerve activity and elevated total systemic, cardiac, and renal NE spillover. Cardiac neuronal NE reuptake was decreased in hypertensive subjects. In response to desipramine, both the reduction of fractional transcardiac 3[H]NE extraction and the increase in cardiac NE spillover were less pronounced in hypertensive patients. DNA sequencing analysis of the NE transporter gene revealed no mutations that could account for reduced transporter activity. Arterial baroreflex control of sympathetic nerve traffic was not diminished in hypertensive subjects. Angiotensin II plasma concentrations were similar in both groups and were not related to indexes of sympathetic activation. Increased rates of sympathetic nerve firing and reduced neuronal NE reuptake both contribute to sympathetic activation in hypertension, whereas a role for dampened arterial baroreflex restraint on sympathetic nerve traffic and a peripheral neuromodulating influence of angiotensin II appear to be excluded.

Increased cardiac norepinephrine release in spontaneously hypertensive rats: role of presynaptic alpha-2A adrenoceptors.

An increased sympathoadrenergic activation is thought to contribute to the maintenance of elevated blood pressure levels in hypertension. Therefore, the regulation of cardiac presynaptic sympathetic neurotransmission was investigated in spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY). Electrical field stimulation (1 min, 4 Hz) evoked a higher norepinephrine (NE) overflow from isolated perfused SHR than from WKY hearts (171 +/- 78 versus 111 +/- 27 nmol/g; means +/- SD, n = 7, P < 0.05). The difference in stimulation-evoked NE overflow was neither due to increased NE stores nor to a higher density of sympathetic nerve endings in SHR hearts. Furthermore, impairment of cardiac NE re-uptake was ruled out, as pharmacological inhibition of NE re-uptake by desipramine (300 nmol/l) similarly increased NE overflow from SHR (+ 54 +/- 17%) and WKY hearts (+ 59 +/- 18%). However, inhibition of presynaptic alpha-2 adrenoceptors (alpha-2R) with yohimbine (1 micromol/l) resulted in a significantly larger increase in NE overflow from WKY (+ 244 +/- 42%) than from SHR hearts (+ 162 +/- 47%, P < 0.05 versus WKY), indicating impairment of presynaptic inhibitory effect of alpha-2R in SHR. Supporting this notion, mRNA concentrations of alpha-2(A), the predominant presynaptic alpha-2R subtype, were reduced in SHR compared with WKY (738 +/- 251 versus 1468 +/- 518 mRNA molecules/10 ng, n = 7, P < 0.01), as quantified by competitive reverse transcription-polymerase chain reaction derived from left stellate ganglia. The impairment of the alpha-2R mediated presynaptic negative feedback mechanism by a reduced expression of the alpha-2R subtype A may increase cardiac net secretion of NE in SHR and could therefore contribute to their hypertensive phenotype.

Evidence for increased renal norepinephrine overflow during sodium restriction in humans.

To investigate the differentiated pattern of efferent sympathetic nerve activity by means of analyzing norepinephrine kinetics in response to sodium restriction, cardiorenal sympathetic activity during rest and mental stress was studied in 12 subjects (33.3 +/- 2.6 years old, SEM) exposed to a low and a normal sodium diet; 5-40 mmol and 160-200 mmol/24 hours, respectively (crossover design). Organ norepinephrine release was calculated from organ plasma flow, arteriovenous plasma concentration gradient across the organ and the organ's fractional extraction of radiolabeled norepinephrine. Body weight and urinary sodium/24 hr fell significantly and urinary potassium/24 hr and both supine and standing blood pressure remained unchanged. Total norepinephrine release to plasma and norepinephrine plasma clearance were similar in both phases (approximately 460 ng/min and 1.90 l/min, respectively). A 138% increase in renal norepinephrine overflow was observed during sodium restriction (from 112 to 267 ng/min, p less than 0.025), which was due to elevated renal vein norepinephrine (434 versus 290 pg/ml, p less than 0.01) because renal plasma flow and renal norepinephrine extraction were unaltered. Similarly, sodium restriction caused a 168% elevation of renal renin secretion (p less than 0.05). Resting cardiac norepinephrine spillover and cardiac norepinephrine reuptake were unchanged between the two salt phases. Total and cardiac norepinephrine release, supine blood pressure, and heart rate increased to about the same extent in response to mental testing regardless of salt phase. In conclusion, sodium restriction induced a differential and physiological increase in resting renal sympathetic nervous activity, leaving cardiac norepinephrine overflow unchanged. Cardiac norepinephrine uptake was normal, which further supports the concept of a true increase of efferent renal nerve activity.