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(Circulation. 1996;93:1667-1676.)
© 1996 American Heart Association, Inc.

Articles

Cardiac Sympathetic Nerve Function in Congestive Heart Failure

Graeme Eisenhofer, PhD; Peter Friberg, MD, PhD; Bengt Rundqvist, MD; Arshed A. Quyyumi, MD; Gavin Lambert, PhD; David M. Kaye, MBBS, PhD; Irwin J. Kopin, MD; David S. Goldstein, MD, PhD; Murray D. Esler, MBBS, PhD

From the Clinical Neuroscience Branch, National Institute of Neurological Disorders and Stroke (G.E., I.J.K., D.S.G.), Bethesda, Md; the Cardiology Branch, National Heart, Lung, and Blood Institute (A.A.Q.), National Institutes of Health, Bethesda, Md; the Departments of Cardiology and Clinical Physiology, University of Göteborg, Göteborg, Sweden (P.F., B.R.); and the Baker Medical Research Institute, Prahran, Victoria, Australia (G.L., D.M.K., M.D.E.).

Correspondence to Graeme Eisenhofer, Bldg 10, Room 5N-214, 10 Center Dr MSC 1424, National Institutes of Health, Bethesda, MD 20892-1424. E-mail ge@box-g.nih.gov.

	Abstract

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Background Increased availability of norepinephrine (NE) for activation of cardiac adrenoceptors (increased cardiac adrenergic drive) and depletion of myocardial NE stores may contribute to the pathophysiology and progression of congestive heart failure. This study used a comprehensive neurochemical approach to examine the mechanisms responsible for these abnormalities.

Methods and Results Subjects with and without congestive heart failure received intravenous infusions of [³H]NE. Cardiac spillover, reuptake, vesicular-axoplasmic exchange, and tissue stores of NE were assessed from arterial and coronary venous plasma concentrations of endogenous and [³H]-labeled NE and dihydroxyphenylglycol. Tyrosine hydroxylase activity was assessed from plasma dopa, and NE turnover was assessed from measurements of NE metabolites. NE release and reuptake were both increased in the failing heart; however, the efficiency of NE reuptake was reduced such that cardiac spillover of NE was increased disproportionately more than neuronal release of NE. Cardiac NE stores were 47% lower and the rate of vesicular leakage of NE was 42% lower in the failing than in the normal heart. Cardiac spillover of dopa and NE turnover were increased similarly in congestive heart failure.

Conclusions Increased neuronal release of NE and decreased efficiency of NE reuptake both contribute to increased cardiac adrenergic drive in congestive heart failure. Decreased vesicular leakage of NE, secondary to decreased myocardial stores of NE, limits the increase in cardiac NE turnover in CHF. Decreased NE store size in the failing heart appears to result not from insufficient tyrosine hydroxylation but from chronically increased NE turnover and reduced efficiency of NE reuptake and storage.

Key Words: norepinephrine • heart failure • nervous system, autonomic • radioisotopes

	Introduction

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Elevated plasma NE,¹ increased spillover of NE into plasma,^{2 3} and increased directly recorded muscle sympathetic nerve firing⁴ all demonstrate sympathetic activation in CHF. Increased cardiac NE spillover^{5 6 7} and desensitization of cardiac ß-adrenoceptors^{8 9} are also consistent with increased availability of NE for activation of cardiac adrenoceptors (increased cardiac adrenergic drive). Another repeatedly confirmed observation in CHF is that cardiac stores of NE are depleted.^{10 11 12 13 14 15}

Although it is indisputable that adrenergic drive to the failing heart is increased and cardiac NE stores are depleted, the underlying mechanisms of these changes are unclear. Given the sympathetic activation that accompanies CHF, it has been assumed that increased adrenergic drive in the failing heart reflects increased neuronal release of NE. However, the amount of NE available after release also depends importantly on subsequent inactivation by neuronal uptake. Thus, Rose et al¹⁶ reported that neuronal release of NE in the failing heart is actually decreased and that this is obscured by a concomitant decrease in NE reuptake. Although this conclusion is disputed,⁶ reduced efficiency of cardiac NE uptake in CHF is now supported by numerous other studies^{17 18 19 20} and could explain many of the abnormalities of the failing heart, including increased adrenergic drive, desensitization of ß-adrenoceptors, and depletion of NE stores. In turn, depletion of NE stores has been proposed to contribute to decreased cardiac neuronal release of NE²¹ and insufficient inotropic support of the failing myocardium.²²

The present study applied a previously documented method^{23 24} to assess cardiac NE reuptake and release in CHF. The method relies on measurements of [³H]-labeled and endogenous DHPG, the intraneuronal NE metabolite, in arterial and coronary venous plasma during intravenous infusion of [³H]NE. This also enables examination of the exchange of NE between the storage vesicles and axoplasm. As the main determinant of NE turnover,²⁴ this exchange could play a crucial role in the depletion of cardiac NE stores in CHF. Measurements of plasma dopa, an index of tyrosine hydroxylase activity,²⁵ were compared with estimates of NE turnover derived from measurements of NE and its metabolites. Finally, measurements of time-dependent changes in the specific activity of [³H]DHPG were used to estimate the size of cardiac NE stores. The current study provides a comprehensive assessment of cardiac sympathetic nerve function to elucidate the mechanisms of cardiac adrenergic activation and depletion of myocardial NE stores in CHF.

	Methods

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Study Population
Subjects included 54 patients with CHF (42 men, 12 women) aged 29 to 75 years (mean age, 52 years) and 57 control subjects (53 men, 4 women) aged 18 to 74 years (mean age, 50 years). All patients with CHF had left ventricular ejection fractions <35% (mean±SD, 22±7%). Most CHF patients (n=45) were in New York Heart Association functional class III, 6 were in class II, and 3 in class IV. Ischemic cardiomyopathy was demonstrated in 25 patients by angiography, 1 patient had CHF secondary to valvular heart disease, and the rest had idiopathic dilated cardiomyopathy. At the time studied, most patients were receiving two or more of the following agents: diuretics, angiotensin-converting enzyme inhibitors, anticoagulants, ß-blockers, or digoxin. Control subjects included 32 normal volunteers and 25 patients with angina pectoris (15 patients with chest pain and coronary artery disease with >50% narrowing of the lumen of any major coronary artery, and 10 patients with chest pain but without coronary artery disease). Subjects were studied as part of ongoing protocols at three different institutions: the National Institutes of Health (Bethesda, Md); Sahlgrenska University Hospital (Göteborg, Sweden); and the Baker Medical Research Institute (Prahran, Victoria, Australia). All procedures were approved by the appropriate review committees, and subjects provided informed consent to participate in studies.

Catheterization
Subjects were studied in the morning in a cardiac catheterization laboratory after refraining from smoking and consumption of caffeinated beverages overnight. Medications were withheld for 12 hours before studies. A cannula inserted under local anesthesia in a radial, brachial, or femoral artery was used to monitor arterial pressure and obtain arterial blood samples. A thermodilution catheter advanced under fluoroscopic guidance into the coronary sinus was used to sample coronary venous blood and measure coronary sinus blood flow. A forearm venous cannula was used for infusion of [³H]NE and desipramine.

Radiotracer Infusion
All subjects received an intravenous infusion of [³H]NE (levo-[2,5,6-³H]NE or levo-[7-³H]NE; New England Nuclear) delivered at a rate of 0.5 to 1.5 µCi/min. In most studies, the [³H]NE was infused simultaneously with [³H]-labeled epinephrine (levo-N-methyl-[³H]epinephrine, also from New England Nuclear). Since deamination of [³H]epinephrine results in loss of³H, production of [³H]DHPG from [³H]NE is not obscured by simultaneous administration of [³H]epinephrine.

Blood Samples and Coronary Blood Flow
Arterial and coronary venous blood samples (10 to 20 mL) were drawn simultaneously starting 15 minutes after radiotracer infusions were begun. Samples were collected into ice-chilled tubes containing heparin or EDTA. Plasma was separated by centrifugation and stored at -80°C. Coronary sinus blood flow was measured by thermodilution before each blood collection.

Desipramine Administration
Desipramine hydrochloride (Ciba-Geigy) was administered by intravenous infusion to 16 CHF patients and 19 normal volunteers to inhibit neuronal uptake of NE. Infusions began immediately after withdrawal of baseline arterial and coronary sinus blood samples and lasted 15 to 30 minutes (cumulative dose of 0.25 to 0.5 mg/kg). Arterial and coronary venous blood samples were drawn within 30 minutes after the infusion.

Cycling Exercise
Twenty-four CHF patients and 31 normal control subjects performed supine cycling exercise during radiotracer infusions. Cycling was performed for 10 to 20 minutes at 50% of each subject's maximum work capacity. Arterial and coronary venous blood samples were drawn before exercise and during the last minute of exercise. Increases in cardiac spillovers of DHPG relative to those of NE provided indexes of NE reuptake.²⁴ Increases in cardiac dopa spillover in relation to increases in NE turnover served as indexes of NE biosynthesis.²⁵

Assays
Catechols in plasma (1 mL) and samples of the infusion preparation (10 µL) were adsorbed onto alumina and quantified by LCED.²⁶ Timed collections of the eluant as it left the electrochemical cell enabled separation of [³H]-labeled DHPG, NE, and epinephrine for assay by liquid scintillation spectrometry. Interassay CVs were 8.4% for DHPG, 6.5% for NE, 5.9% for dopa, 11.6% for DA, and 11.6% for DOPAC. Intra-assay CVs were 4.8% for DHPG, 1.9% for NE, 3.8% for dopa, 8.1% for DA, and 3.9% for DOPAC.

Plasma concentrations of NMN and MN were determined by LCED after extraction by cation-exchange chromatography.²⁷ Concentrations of [³H]-labeled NMN and MN were measured as described above. Interassay CVs were 12.2% for NMN and 11.2% for MN. Intra-assay CVs were 4.2% for NMN and 3.3% for MN.

LCED was also used to measure plasma MHPG after its extraction into ethyl acetate.²⁴ The interassay CV was 7.0% and the intra-assay CV was 4.8%.

Plasma concentrations of HVA and VMA were determined by gas chromatography/mass spectrometry after ethyl acetate extraction and derivatization with pentafluoropropionic anhydride.²⁸ Interassay CVs were 6.8% for HVA and 3.9% for VMA. Intra-assay CVs were 3.8% for HVA and 2.9% for VMA.

Concentrations of sulfate-conjugated catecholamines and metabolites were determined by each of the above procedures after enzymatic hydrolysis of the conjugates by incubation of plasma samples with saturating quantities of sulfatase (Sigma Chemical Co).

Cardiac Spillover of NE
Cardiac spillover of NE into plasma (SP_NE)—the rate of entry into the coronary venous drainage of the NE released by cardiac tissues (pmol/min)—was estimated by a rearrangement of the equation described by Esler et al.²⁹

(1)

The numerator—the rate of entry of [³H]NE into the venous drainage (dpm/min)—was estimated from the product of the coronary venous plasma concentration of [³H]NE ([³H]NE_V, dpm/mL) and the coronary sinus plasma flow (Q_p, mL/min). The denominator—the specific activity of [³H]NE (dpm/pmol) that resulted from dilution of the [³H]NE entering the venous drainage by the endogenous NE released by cardiac tissues—was estimated from the specific activities (dpm/pmol) of [³H]NE in arterial (SA_A) and coronary venous plasma (SA_V). These specific activities were calculated from the respective arterial or venous plasma concentrations of [³H]NE (dpm/mL) and endogenous NE (pmol/mL).

Cardiac Spillovers of NMN, DHPG, MHPG, Dopa, and DOPAC
Cardiac spillover of NMN was estimated after correction for the amount removed by use of the extraction of MN.³⁰ The NMN formed in the heart is derived mainly from metabolism of locally released NE. The small portion of NMN spillover derived from circulating NE was estimated as described elsewhere.³⁰ Cardiac spillovers of DHPG, MHPG, dopa, and DOPAC—none of which are extracted appreciably by the heart—were estimated from the product of the arterial-venous difference in plasma concentrations and coronary plasma or blood flow as appropriate.^{24 25}

Cardiac NE Turnover
Cardiac NE turnover was estimated from the sum of differences in rates at which NE and its metabolites entered and left the coronary circulation. This represents the rate of net loss of NE (pmol/min) by metabolism and loss into plasma, which at steady state is matched by an equal rate of NE synthesis.

Cardiac [³H]NE Extraction
The fractional cardiac extraction of [³H]NE (F)—the proportion of [³H]NE removed from plasma during passage through the coronary circulation—was calculated from

(2)

where [³H]NE_A and [³H]NE_V are the plasma concentrations of [³H]NE (dpm/mL) in arterial and coronary venous plasma, respectively.

Specific Activity of [³H]DHPG Produced in the Heart
The specific activity of [³H]DHPG produced in the heart (SA_DHPG)—which represents the specific activity of the [³H]NE precursor in the cardiac sympathetic axoplasm (dpm/pmol)—was estimated according to the equation

(3)

where [³H]DHPG_AV and DHPG_AV are the respective arterial-coronary venous increments in plasma concentrations of [³H]-labeled DHPG (dpm/mL) and endogenous DHPG (pmol/mL).

Cardiac Neuronal Removal of [³H]NE and Spillover of [³H]DHPG
The desipramine-sensitive removal of [³H]NE ([³H]NE_U)—the rate of entry of [³H]NE into cardiac sympathetic neurons (dpm/min)—was estimated by

(4)

where F and F_dmi are the fractional extractions of [³H]NE before and after desipramine, respectively (estimated by use of Equation 2), Q_p is the plasma flow (mL/min), and [³H]NE_A is the arterial plasma concentration of [³H]NE (dpm/mL).

The cardiac spillover of [³H]DHPG derived from [³H]NE removed by cardiac sympathetic neurons and metabolized before storage ([³H]DHPG_S), ie, not that derived from [³H]NE leaking from storage vesicles (dpm/min), was estimated from differences in cardiac spillovers of [³H]DHPG immediately before and after desipramine using the equation

(5)

where Q and Q_dmi are coronary blood flows (mL/min) before and after desipramine, respectively, and [³H]DHPG_AV and [³H]DHPG_AVdmi represent the arterial-coronary venous increases in plasma concentrations (dpm/mL) of [³H]DHPG immediately before and after desipramine, respectively.

Statistical Methods
Results are expressed as mean±SEM. Differences were assessed by ANOVA or by paired or unpaired Student's t test as appropriate. Linear regression analysis was by least squares. Statistical significance was defined as a value of P<.05.

	Results

Top Abstract Introduction Methods Results Discussion References

 
Baseline Plasma Catechols and Metabolites
Patients with CHF had elevated (P<.005) arterial plasma concentrations of NE and most of its metabolites, including DHPG, NMN, MHPG, VMA, and the sulfate conjugates of NE, NMN, and MHPG (Table 1). Arterial plasma concentrations of dopa, HVA, and DA sulfate were also higher (P<.02) in CHF patients than in control subjects. There were significant (P<.02) arterial-coronary venous increments in plasma concentrations of DHPG, NMN, MHPG, dopa, and DOPAC in both subject groups; however, only CHF patients had arterial-venous increments (P<.0001) in plasma NE, and only control subjects had increments (P<.0001) in DHPG sulfate. There were no arterial-coronary venous increments in plasma VMA, HVA, or the sulfate conjugates of NE, NMN, or DA in either group. Arterial-coronary venous increments in plasma NE and MHPG were larger (P<.005) in CHF patients than in control subjects, whereas control subjects had larger (P<.01) arterial-coronary venous increments in plasma DHPG, DHPG sulfate, and DOPAC.

View this table: [in this window] [in a new window]   Table 1. Arterial and Coronary Venous Plasma Concentrations of NE, DA, Their Metabolites, and Dopa

Baseline Cardiac Spillovers of NE, Metabolites, and Dopa
Cardiac NE spillover was fourfold higher (P<.0001) in CHF patients than in control subjects, whereas spillovers of other compounds showed variable differences between the two groups (Table 2). Cardiac spillover of NMN was 60% higher (P<.03) and MHPG 91% higher (P<.0001) in CHF patients. Cardiac spillovers of unconjugated DHPG did not differ, whereas those of DHPG sulfate were higher (P<.03) in control subjects. There was a nonsignificant trend (P=.13) for the cardiac spillover of DOPAC to be higher in control subjects. Cardiac spillover of dopa was 32% higher (P<.05) in CHF patients than in control subjects, a difference matched closely by the 31% higher (P<.006) rate of cardiac NE turnover. There were no significant differences in cardiac spillovers of NE or its metabolites or of dopa and DOPAC between CHF patients with ischemic cardiomyopathy and those with idiopathic dilated cardiomyopathy (data not shown).

View this table: [in this window] [in a new window]   Table 2. Baseline Coronary Blood and Plasma Flows, Cardiac Spillovers of NE and Other Compounds, and Cardiac Turnover of NE

Cardiac Removal of NE and Production of NMN
The cardiac removal of endogenous NE from the circulation was much larger (P<.0001) in CHF patients than in control subjects (Table 3), reflecting the higher rate of delivery (flowxconcentration) of NE to the coronary bed. The cardiac spillover of NMN that was derived from extracted NE was also considerably larger (P<.0001) in CHF patients than in control subjects.

View this table: [in this window] [in a new window]   Table 3. Cardiac Extraction of NE and Spillover of NMN Derived From Extracted NE

Cardiac Extraction of [³H]NE and Production of [³H]DHPG
In control subjects, 79.3±1.3% of the [³H]NE entering the coronary circulation was extracted by the heart. In patients with CHF, the fractional cardiac extraction, at 60.2±2.0%, was 24% lower (P<.0001) than in control subjects.

Arterial-coronary venous increments in plasma concentrations of [³H]DHPG increased with time of the [³H]NE infusion; this resulted in time-dependent increases in the specific activity of [³H]DHPG produced in the hearts of both control subjects and CHF patients (Fig 1). In CHF patients, the rate of increase in the specific activity of [³H]DHPG produced by the heart was twice that in control subjects (0.128 versus 0.066 dpm·pmol^-1·min^-1).

View larger version (27K): [in this window] [in a new window]   Figure 1. Specific activity of [3H]-labeled DHPG produced in the hearts of control subjects () and patients with cardiac failure () as a function of time after the start of infusion of [3H]-labeled NE. Data were derived from simultaneously obtained samples of arterial and coronary venous plasma (186 paired samples from CHF patients and 426 paired samples from control subjects) drawn at different times after the start of [3H]NE infusions and partitioned into 20-minute periods. Concentrations of [3H]DHPG were normalized to arterial plasma concentrations of [3H]NE of 1000 dpm/mL. The arterial-coronary venous increment in normalized plasma [3H]DHPG concentrations was divided by the arterial-coronary venous increment in plasma DHPG concentrations to estimate the specific activity of [3H]DHPG according to Equation 3 in "Methods." The regression lines describing the relations between time of infusion and specific activity of [3H]DHPG were estimated from the data collected starting 30 minutes after the beginning of [3H]NE infusions.

Cardiac Neuronal Removal of NE and Production of DHPG
Cardiac extractions of [³H]NE were lower in CHF patients before (P<.0001) and after (P<.03) administration of desipramine (Fig 2A). The cardiac extraction of [³H]NE was decreased (P<.0001) considerably by desipramine in both groups of subjects. The desipramine-induced decrease in cardiac extraction of [³H]NE was 22% smaller (P<.0001) in CHF patients than in control subjects (ratio of 0.457 versus 0.589).

View larger version (37K): [in this window] [in a new window]   Figure 2. Desipramine-induced decreases in cardiac extractions of [3H]-labeled NE (A) and arterial-coronary venous increments in plasma concentrations of [3H]-labeled DHPG (B) and endogenous DHPG (C) in control subjects (left) and patients with CHF (right). Results represent mean±SEM values for the 19 control subjects and 16 CHF patients who received desipramine. *P<.03 vs control subjects. P<.01 vs baseline.

Desipramine reduced (P<.01) the arterial-coronary venous increments in plasma concentrations of [³H]DHPG by 46% in control subjects and by 39% in CHF patients (Fig 2B). Arterial-coronary venous increments in plasma [³H]DHPG did not differ between control subjects and CHF patients, before or after desipramine.

Desipramine reduced (P<.0002) the arterial-coronary venous increments in plasma concentrations of endogenous DHPG by 20% in control subjects and by 31% in CHF patients (Fig 2C). Arterial-coronary venous increments in plasma DHPG were smaller (P<.001) in CHF patients, both before and after desipramine.

Comparison of the rates of removal of [³H]NE by cardiac sympathetic neurons with the much smaller cardiac spillovers of [³H]DHPG (Table 4) indicated that only a small portion of the [³H]NE removed by cardiac sympathetic nerves was deaminated immediately after uptake and appeared in coronary venous plasma as [³H]DHPG. In control subjects, 5.5±1.3% (2845 of 51 538 dpm/min) of the [³H]NE removed by cardiac sympathetic neurons appeared in coronary venous plasma as [³H]DHPG. In CHF patients, this value was 7.1±1.6% (3475 of 48 768 dpm/min).

View this table: [in this window] [in a new window]   Table 4. Cardiac Neuronal Removal of [3H]NE and Spillover of [3H]DHPG Derived From [3H]NE Immediately After Reuptake

Cardiac Production of DHPG From Recaptured NE During Exercise
Cycling exercise caused large increases (P<.0001) in cardiac spillovers of NE in both groups (Fig 3A). Spillovers of NE were higher (P<.0001) at baseline in CHF patients than in control subjects but did not differ during exercise. The absolute increments in cardiac spillovers of NE during exercise were similar in both groups (1330±276 versus 1363±151 pmol/min).

View larger version (31K): [in this window] [in a new window]   Figure 3. Exercise-induced increases in cardiac spillovers of NE (A) and DHPG (B) and ratios of increases in DHPG spillovers to those of NE spillovers (C) in control subjects (left) and patients with CHF (right). Results represent mean±SEM values for the 31 control subjects and 24 CHF patients who performed supine cycling exercise. *P<.01 vs control subjects. P<.005 vs rest.

Exercise also increased (P<.005) cardiac spillovers of DHPG in both groups (Fig 3B). Spillovers of DHPG were similar at baseline but were lower (P<.006) during cycling exercise in CHF patients. The absolute increment in cardiac spillover of DHPG during exercise was also smaller (P<.002) in CHF patients than in control subjects (571±174 versus 930±98 pmol/min).

The ratio of the exercise-induced increase in cardiac spillover of DHPG to that of NE (an index of NE reuptake efficiency) was 44% lower (P<.001) in CHF patients than in control subjects (0.413±0.057 versus 0.735±0.060) (Fig 3C).

NE Turnover and Tyrosine Hydroxylase Activity During Exercise
In contrast to the manyfold increases in cardiac spillover of NE during exercise (Fig 3A), cardiac turnover of NE increased (P<.0001) consistently but by only 2-fold in CHF patients and 2.8-fold in control subjects (Fig 4A). Cardiac spillover of dopa also increased (P<.003) during exercise in both groups (Fig 4B). The magnitude of increases in dopa spillover was similar to that in cardiac NE turnover; in control subjects, cardiac dopa spillover increased by 2.4-fold and in CHF patients by 2-fold. The ratio of the exercise-induced increase in cardiac spillover of dopa to the increase in NE turnover did not differ among the two groups.

View larger version (32K): [in this window] [in a new window]   Figure 4. Exercise-induced increases in cardiac turnovers of NE (A), increases in cardiac spillovers of dopa (B), and ratios of increases in dopa spillovers to those of turnovers of NE (C) in control subjects (left) and patients with CHF (right). Results represent mean±SEM values for the 31 control subjects and 24 CHF patients who performed supine cycling exercise. *P<.0001 vs control subjects. P<.005 vs rest.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
The differences in disposition and metabolism of NE in control subjects and CHF patients indicate that sympathetic nerves of the failing myocardium release more NE and remove it less efficiently by reuptake. Cardiac NE stores are depleted, and there is a corresponding decrease in leakage of NE from vesicular stores but an adequate capacity for tyrosine hydroxylation.

Neuronal Reuptake of NE
Use of radiotracer-dilution analysis to estimate the rate of entry of NE into a compartment depends on a general formula³¹ consisting of a numerator (the rate of entry of [³H]NE into the compartment) and a denominator (the specific activity of [³H]NE resulting from dilution of the [³H]-labeled NE with the endogenous NE entering the compartment) (eg, see Equation 1 in "Methods"). As described elsewhere,^{23 24} the specific activity of [³H]NE that enters cardiac sympathetic nerves by neuronal uptake equals that of the [³H]DHPG produced from the same source. DHPG, however, has two intraneuronal sources: NE recaptured after release and NE that is leaked from storage vesicles.²⁴ Blockade of neuronal uptake with desipramine enables separate estimation of these two components.

The ratios of desipramine-induced decrements in arterial-coronary venous gradients of plasma [³H]-labeled DHPG to endogenous DHPG (Fig 2) provide estimates of the specific activity of [³H]NE removed by neuronal uptake in control subjects (21.2 dpm/pmol) and CHF patients (14.65 dpm/pmol). Division of these specific activities into the rates of cardiac neuronal removal of [³H]NE (Table 4) yields rates of NE reuptake. Estimated NE reuptake was higher in CHF patients than in control subjects (3329 versus 2431 pmol/min) and, in both groups, manyfold higher than the corresponding rate of NE spillover (Fig 5).

View larger version (41K): [in this window] [in a new window]   Figure 5. The processes of synthesis, vesicular-axoplasmic exchange, metabolism, release, neuronal and extraneuronal uptakes, spillover, and turnover of NE for sympathetic nerves of the normal (left) and failing (right) human heart at rest. U1 indicates neuronal uptake; U2, extraneuronal uptake; TYR, tyrosine; DHPG-SO4, DHPG sulfate; MAO, monoamine oxidase; COMT, catechol-O-methyltransferase. Numbers with each arrow represent the rates of each process in picomoles per minute.

Extraneuronal Uptake of NE
NMN is produced in extraneuronal tissues from both circulating and locally released NE.³⁰ The data in Table 3 indicate that in control subjects, 0.29% (0.29 of 99 pmol/min) of the NE removed by the heart is O methylated and appears in outflowing coronary venous plasma as NMN. This compares with 0.67% (1.47 of 221 pmol/min) in CHF patients. These low conversion rates reflect the greater importance of neuronal than extraneuronal removal mechanisms in the heart.³² From these conversion rates, together with the total cardiac spillovers of NMN (Table 2), it can be estimated³⁰ that the rate of extraneuronal uptake of NE is 96 pmol/min in control subjects and 169 pmol/min in CHF patients. In keeping with other findings,³¹ these rates are much lower than rates of NE reuptake but similar to those of NE spillover (Fig 5). Thus, differences in extraneuronal uptake are too small to explain overall group differences in the disposition of NE.

NE Release and Efficiency of NE Reuptake
The higher rates of neuronal reuptake, extraneuronal uptake, and spillover of NE indicate increased cardiac neuronal release of NE in CHF (Fig 5). However, larger proportional increases in cardiac spillover than in reuptake of NE (270% versus 37% increase) indicate decreased efficiency of NE reuptake in CHF. Decreased cardiac extractions of [³H]NE in CHF are also consistent with decreased efficiency of cardiac NE reuptake. Thus, increased adrenergic drive in the failing heart results from both increased neuronal release of NE and decreased efficiency of NE reuptake.

Twofold larger exercise-induced increases in cardiac DHPG relative to NE spillover in control subjects than in CHF patients (Fig 3) provide further evidence for decreased efficiency of NE reuptake in the failing heart. At rest, most DHPG is derived from NE that is leaked from vesicles, whereas during sympathetic activation, increases in DHPG production depend entirely on NE reuptake.^{23 24} Ratios of increases in cardiac DHPG to NE spillover (Fig 3) and of cardiac [³H]DHPG spillover to [³H]NE removal (Table 4) indicate²⁴ that during exercise, rates of cardiac NE reuptake were 13.4-fold (0.735 divided by 0.055) higher than NE spillover rates in control subjects and 5.8-fold (0.413 divided by 0.071) higher in CHF patients. Thus, NE reuptake is less efficient in the failing than in the normal heart, both at rest and during sympathetic activation.

Decreased efficiency of NE reuptake during exercise, as reflected by decreased ratios of reuptake to spillover, could reflect a washout effect, in which increased blood flow increases the proportion of released NE that enters the venous drainage.^{33 34} Thus, decreased efficiency of NE reuptake in the failing heart could reflect changes of cardiac ultrastructure, microvascular hemodynamics, or density of sympathetic nerve endings and does not necessarily imply abnormal function of the membrane transporter.

Most CHF patients in the present study remained medicated during the investigation. Cardiac spillovers of NE are much higher in untreated^{5 6} than in treated CHF patients,⁷ probably reflecting more sympathetic recruitment in untreated patients. Thus, although decreased efficiency of NE reuptake contributed substantially to increased cardiac adrenergic drive in the treated CHF patients of the present study, increased NE release might contribute more to elevated cardiac NE spillovers in untreated patients.^{5 6}

Vesicular Leakage of NE
Comparison of the desipramine-sensitive cardiac neuronal removal of [³H]NE and spillover of [³H]DHPG (Table 4) indicates that in control subjects, 18.1-fold more NE enters the cardiac sympathetic axoplasm than is released into plasma as DHPG, compared with 14.0-fold more in patients with CHF. Thus, from the cardiac spillovers of endogenous DHPG (Table 2), rates of entry of NE into the cardiac sympathetic axoplasm can be estimated to be 12 761 pmol/min (18.1x705) in control subjects and 9296 pmol/min (14x664) in CHF patients. Subtraction of the corresponding neuronal reuptake rates shows that vesicular leakage of NE is higher in control subjects than in patients with CHF (10 330 versus 5967 pmol/min). Thus, although rates of neuronal release, reuptake, and spillover of NE are higher in the failing than in the normal heart, the rate of leakage of transmitter from cardiac storage vesicles is lower (Fig 5), presumably reflecting depletion of cardiac NE stores.^{10 11 12 13 14 15}

Cardiac NE Stores
Reduced tissue stores of NE in the failing heart are reflected by differences in the time-dependent increases in specific activity of [³H]DHPG produced in the hearts of control subjects and CHF patients (Fig 1). The linear time-dependent increase in [³H]DHPG specific activity reflects the increase in the specific activity of [³H]NE in vesicular stores secondary to accumulation of infused [³H]NE.²⁴ The rate of increase varies directly with the rate of entry of [³H]NE into vesicular stores and inversely with the amount of endogenous NE available to dilute the sequestered [³H]NE. The amount of NE in vesicular stores may therefore be estimated by dividing the rate of vesicular sequestration of [³H]NE by the rate of increase in specific activity of [³H]DHPG (Table 5). The NE content of the normal human myocardium is between 4 and 8 nmol/g,^{12 13} or 1 to 2 µmol in a normal 250-g left ventricle. The present estimates (Table 5) are close to this range; however, they reflect the NE content of only that region of the myocardium drained by the coronary sinus. The 50% lower cardiac NE content in the failing heart agrees with previous findings.^{10 11 12 13 14 15}

View this table: [in this window] [in a new window]   Table 5. Cardiac Stores and Kinetics of Turnover and Vesicular Leakage of NE

Cardiac NE Turnover
As illustrated in Fig 5, NE turnover depends much less on neuronal release of NE than on leakage of NE from storage vesicles. The present data show that the contributions of neuronal release and vesicular leakage to NE turnover are altered reciprocally in CHF; thus, the overall net increase in NE turnover is small. However, in keeping with other findings,³⁵ the rate constant for NE turnover is increased substantially in CHF, resulting in considerable reduction of the half-life of cardiac NE stores (Table 5).

The calculated rate constants for NE leakage (Table 5) are close to that of 0.018 min^-1 calculated by Ulli Trendelenburg, MD, PhD (unpublished data, 1993), from fractional rates of loss of NE in the isolated rat vas deferens.³⁶ Similar rate constants for vesicular leakage in control subjects and CHF patients indicate that the decreased rate of NE leakage in the failing heart reflects the depleted state of NE stores. Furthermore, the similar rate constants for vesicular leakage indicate that the greater rate constant for cardiac NE turnover in CHF must be due to greater neuronal release of NE in the failing heart. Less efficient NE reuptake (85% versus 92%) and vesicular sequestration (88% versus 92%) in the failing than in the normal heart could also contribute to the increased rate constant for cardiac NE turnover in CHF.

Cardiac NE Synthesis
Results of previous studies in animal models of CHF suggested that reduced tyrosine hydroxylase activity^{11 37} or impaired conversion of DA to NE¹⁴ could contribute to depletion of NE stores in the failing heart. In contrast, the present results suggest that depletion of NE stores may reflect attainment of a new steady state in response to chronic alterations in the determinants of NE turnover. Reduced cardiac tissue stores of NE lessen the contribution of vesicular leakage to the increase in NE turnover and thus minimize the increase in catecholamine biosynthesis required to maintain constant stores of transmitter. Thus, the small 31% increase in the overall rate of NE turnover in the failing heart was matched closely by the 32% increase in dopa spillover (Table 2), indicating that the activity of tyrosine hydroxylase was increased to a level adequate to maintain cardiac NE stores at their lowered level.

Lack of cardiac dopa production in patients with heart transplants³⁸ or pure autonomic failure,³⁹ and positive relationships between cardiac NE turnover and cardiac dopa spillover^{25 40} support the view that cardiac dopa spillover reliably reflects cardiac tyrosine hydroxylase activity. This is also supported in the present study by the magnitude of the increases in cardiac dopa spillover during exercise that matched closely those in NE turnover (Fig 4). Similar relative increases in cardiac dopa spillover to NE turnover in both groups indicate adequate capacity of tyrosine hydroxylase to respond to increased NE turnover in the failing heart.

Conclusions
Increased neuronal release of NE and decreased efficiency of NE reuptake both contribute to cardiac adrenergic activation in CHF. Decreased vesicular leakage of NE in the failing heart due to depleted cardiac NE stores limits the increase in cardiac NE turnover that results from increased NE release. Chronically increased NE release combined with reduced efficiency of NE reuptake and storage may contribute to depletion of NE stores in CHF.

	Selected Abbreviations and Acronyms

 

CHF = congestive heart failure CV = coefficient of variation DA = dopamine DHPG = dihydroxyphenylglycol DOPAC = dihydroxyphenylacetic acid dpm = disintegrations per minute HVA = homovanillic acid LCED = liquid chromatography with electrochemical detection MHPG = methoxyhydroxyphenylglycol MN = metanephrine NE = norepinephrine NMN = normetanephrine VMA = vanillylmandelic acid

	Acknowledgments

 
The authors gratefully acknowledge the expert technical assistance of Anneli Ambring, Helen Cox, Douglas Hooper, and Andrea Turner.

Received August 31, 1995; revision received October 25, 1995; accepted November 3, 1995.

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