Functional significance of presynaptic alpha-adrenergic receptors in failing and nonfailing human left ventricle

Circulating noradrenaline leads to release of neuropeptide Y from cardiac sympathetic nerve terminals via activation of β-adrenergic receptors

Cardiac disease is marked by sympathoexcitation and elevated levels of noradrenaline (NA) and cotransmitter neuropeptide Y (NPY). Increased NPY levels are associated with a greater risk of ventricular arrhythmias and mortality. Nonetheless, the factors that cause NPY release remain poorly understood. We hypothesized that circulating catecholamines might lead to NPY release from myocardial sympathetic nerve terminals via a β-receptor-mediated mechanism that enhances sympathoexcitation. Ventricular interstitial NA and NPY levels were measured in six Yorkshire pigs after i.v. administration of NA (1 mg) and before and after propranolol infusion (1 mg/kg). Real-time interstitial NPY levels were measured using ventricular capacitive immunoprobes (CIs) affixed with NPY antibodies and quantified as the change in CI input current (INPY ) upon binding of NPY. Interstitial NA was measured with adjacent fast-scan cyclic voltammetry probes (INA ). A left ventricular pressure catheter and continuous ECGs were used for haemodynamic recordings, and an epicardial 56-electrode sock was used for measurements of activation recovery interval, a surrogate of action potential duration. Upon administration of NA, heart rate and left ventricular pressure increased, and activation recovery interval shortened. Notably, NA significantly increased interstitial myocardial NPY levels. After propranolol, changes in heart rate and activation recovery interval were largely mitigated. The INA increased to a similar extent post-propranolol vs. pre-propranolol, but changes in INPY were significantly reduced post-propranolol. Coronary sinus plasma analyses confirmed fast-scan cyclic voltammetry and CI findings. Hence, this study demonstrates that circulating NA induces NPY release from ventricular sympathetic nerve terminals, the mechanism for which is mediated via β-adrenergic receptors and can be blocked by the non-selective β-blocker, propranolol. KEY POINTS: Cardiovascular disease is characterized by sympathovagal imbalance, with increased plasma noradrenaline (NA) and neuropeptide Y (NPY) concentrations. Increased NPY levels are associated with increased ventricular arrhythmias and mortality in heart failure. Limited data are available on the specific factors that cause NPY release. In this study, fast-scan cyclic voltammetry and capacitive immunoprobes were used to allow for real-time in vivo measurements of interstitial myocardial neurotransmitters and neuropeptides, respectively. Using an in vivo porcine model with cardiac fast-scan cyclic voltammetry and capacitive immunoprobes, it was shown that systemic NA can increase ventricular interstitial NPY levels, suggesting that NA induces NPY release from postganglionic sympathetic nerves. The release of NPY was blocked by administration of the non-selective β-blocker propranolol, suggesting that release of NPY is dependent on activation of β-adrenergic receptors by NA.

Autonomic nervous system and cardiac neuro-signaling pathway modulation in cardiovascular disorders and Alzheimer's disease

The heart is a functional syncytium controlled by a delicate and sophisticated balance ensured by the tight coordination of its several cell subpopulations. Accordingly, cardiomyocytes together with the surrounding microenvironment participate in the heart tissue homeostasis. In the right atrium, the sinoatrial nodal cells regulate the cardiac impulse propagation through cardiomyocytes, thus ensuring the maintenance of the electric network in the heart tissue. Notably, the central nervous system (CNS) modulates the cardiac rhythm through the two limbs of the autonomic nervous system (ANS): the parasympathetic and sympathetic compartments. The autonomic nervous system exerts non-voluntary effects on different peripheral organs. The main neuromodulator of the Sympathetic Nervous System (SNS) is norepinephrine, while the principal neurotransmitter of the Parasympathetic Nervous System (PNS) is acetylcholine. Through these two main neurohormones, the ANS can gradually regulate cardiac, vascular, visceral, and glandular functions by turning on one of its two branches (adrenergic and/or cholinergic), which exert opposite effects on targeted organs. Besides these neuromodulators, the cardiac nervous system is ruled by specific neuropeptides (neurotrophic factors) that help to preserve innervation homeostasis through the myocardial layers (from epicardium to endocardium). Interestingly, the dysregulation of this neuro-signaling pathway may expose the cardiac tissue to severe disorders of different etiology and nature. Specifically, a maladaptive remodeling of the cardiac nervous system may culminate in a progressive loss of neurotrophins, thus leading to severe myocardial denervation, as observed in different cardiometabolic and neurodegenerative diseases (myocardial infarction, heart failure, Alzheimer's disease). This review analyzes the current knowledge on the pathophysiological processes involved in cardiac nervous system impairment from the perspectives of both cardiac disorders and a widely diffused and devastating neurodegenerative disorder, Alzheimer's disease, proposing a relationship between neurodegeneration, loss of neurotrophic factors, and cardiac nervous system impairment. This overview is conducive to a more comprehensive understanding of the process of cardiac neuro-signaling dysfunction, while bringing to light potential therapeutic scenarios to correct or delay the adverse cardiovascular remodeling, thus improving the cardiac prognosis and quality of life in patients with heart or neurodegenerative disorders.

Effect of α2-Adrenoceptor Stimulation on Functional Parameters of Langendorff-Isolated Rat Heart

We studied the effect of α₂-adrenoreceptor agonist clonidine hydrochloride in concentrations of 10^-9-10^-6 M on inotropy, chronotropy, and coronary flow in Langendorff-isolated heart of adult rats. It was found that α₂-adrenoreceptor agonist changed all studied parameters. Left ventricular myocardium contraction force decreased after application of all tested concentrations, the maximum effect was observed at a concentration of 10^-6 M. Stimulation of α₂-adrenergic receptors in concentrations of 10^-8, 10^-7, and 10^-6 M produced a two-phase effect (initial increase and a subsequent decrease) on the coronary flow. Clonidine hydrochloride in the maximum concentration (10^-6 M) caused a decrease in HR in one group and an increase in the other.

Peripheral and central effects of circulating catecholamines

Physical challenges, emotional arousal, increased physical activity, or changes in the environment can evoke stress, requiring altered activity of visceral organs, glands, and smooth muscles. These alterations are necessary for the organism to function appropriately under these abnormal conditions and to restore homeostasis. These changes in activity comprise the "fight-or-flight" response and must occur rapidly or the organism may not survive. The rapid responses are mediated primarily via the catecholamines, epinephrine, and norepinephrine, secreted from the adrenal medulla. The catecholamine neurohormones interact with adrenergic receptors present on cell membranes of all visceral organs and smooth muscles, leading to activation of signaling pathways and consequent alterations in organ function and smooth muscle tone. During the "fight-or-flight response," the rise in circulating epinephrine and norepinephrine from the adrenal medulla and norepinephrine secreted from sympathetic nerve terminals cause increased blood pressure and cardiac output, relaxation of bronchial, intestinal and many other smooth muscles, mydriasis, and metabolic changes that increase levels of blood glucose and free fatty acids. Circulating catecholamines can also alter memory via effects on afferent sensory nerves impacting central nervous system function. While these rapid responses may be necessary for survival, sustained elevation of circulating catecholamines for prolonged periods of time can also produce pathological conditions, such as cardiac hypertrophy and heart failure, hypertension, and posttraumatic stress disorder. In this review, we discuss the present knowledge of the effects of circulating catecholamines on peripheral organs and tissues, as well as on memory in the brain.

Sympathetic nervous system activation in human heart failure: clinical implications of an updated model

Disturbances in cardiovascular neural regulation, influencing both disease course and survival, progress as heart failure worsens. Heart failure due to left ventricular systolic dysfunction has long been considered a state of generalized sympathetic activation, itself a reflex response to alterations in cardiac and peripheral hemodynamics that is initially appropriate, but ultimately pathological. Because arterial baroreceptor reflex vagal control of heart rate is impaired early in heart failure, a parallel reduction in its reflex buffering of sympathetic outflow has been assumed. However, it is now recognized that: 1) the time course and magnitude of sympathetic activation are target organ-specific, not generalized, and independent of ventricular systolic function; and 2) human heart failure is characterized by rapidly responsive arterial baroreflex regulation of muscle sympathetic nerve activity (MSNA), attenuated cardiopulmonary reflex modulation of MSNA, a cardiac sympathoexcitatory reflex related to increased cardiopulmonary filling pressure, and by individual variation in nonbaroreflex-mediated sympathoexcitatory mechanisms, including coexisting sleep apnea, myocardial ischemia, obesity, and reflexes from exercising muscle. Thus, sympathetic activation in the setting of impaired systolic function reflects the net balance and interaction between appropriate reflex compensatory responses to impaired systolic function and excitatory stimuli that elicit adrenergic responses in excess of homeostatic requirements. Recent observations have been incorporated into an updated model of cardiovascular neural regulation in chronic heart failure due to ventricular systolic dysfunction, with implications for the clinical evaluation of patients, application of current treatment, and development of new therapies.

The role of adrenergic receptor polymorphisms in heart failure

The main function of the cardiac adrenergic system is to regulate cardiac work both in physiologic and pathologic states. A better understanding of this system has permitted the elucidation of its role in the development and progression of heart failure. Regardless of the initial insult, depressed cardiac output results in sympathetic activation. Adrenergic receptors provide a limiting step to this activation and their sustained recruitment in chronic heart failure has proven to be deleterious to the failing heart. This concept has been confirmed by examining the effect of beta-blockers on the progression of heart failure. Studies of adrenergic receptor polymorphisms have recently focused on their impact on the adrenergic system regarding its adaptive mechanisms, susceptibilities and pharmacological responses. In this article, we review the function of the adrenergic system and its maladaptive responses in heart failure. Next, we discuss major adrenergic receptor polymorphisms and their consequences for heart failure risk, progression and prognosis. Finally, we discuss possible therapeutic implications resulting from the understanding of polymorphisms and the identification of individual genetic characteristics.

Cardiac adrenoceptors: physiological and pathophysiological relevance

At present, nine adrenoceptor (AR) subtypes have been identified: alpha(1A)-, alpha(1B)-, alpha(1D)-, alpha(2A)-, alpha(2B)-, alpha(2C)-, beta(1)-, beta(2)-, and beta(3)AR. In the human heart, beta(1)- and beta(2)AR are the most powerful physiologic mechanism to acutely increase cardiac performance. Changes in betaAR play an important role in chronic heart failure (CHF). Thus, due to increased sympathetic activity in CHF, betaAR are chronically (over)stimulated, and that results in beta(1)AR desensitization and alterations of down-stream mechanisms. However, several questions remain open: What is the role of beta(2)AR in CHF? What is the role of increases in cardiac G(i)-protein in CHF? Do increases in G-protein-coupled receptor kinase (GRK)s play a role in CHF? Does betaAR-blocker treatment cause its beneficial effects in CHF, at least partly, by reducing GRK-activity? In this review these aspects of cardiac AR pharmacology in CHF are discussed. In addition, new insights into the functional importance of beta(1)- and beta(2)AR gene polymorphisms are discussed. At present it seems that for cardiovascular diseases, betaAR polymorphisms do not play a role as disease-causing genes; however, they might be risk factors, might modify disease, and/or might influence progression of disease. Furthermore, betaAR polymorphisms might influence drug responses. Thus, evidence has accumulated that a beta(1)AR polymorphism (the Arg389Gly beta(1)AR) may affect the response to betaAR-blocker treatment.

A screen of candidate genes and influence of β2-adrenergic receptor genotypes in postural tachycardia syndrome

OBJECTIVE

To screen candidate genes, encoding beta2-adrenergic receptor (beta2AR), alpha2C-adrenergic receptor (alpha(2C)AR), norepinephrine transporter (NET), and mitochondrial complex I (COI), for common single nucleotide polymorphisms (SNPs) in patients with postural tachycardia syndrome (POTS); alterations could potentially cause or aggravate orthostatic tachycardia and to relate beta2AR SNPs, known to effect venomotor tone, to heart rate (HR) and blood pressure measurements during 10-min head-up tilt.

METHODS

(a) DNA extraction from leukocytes of 29 patients with POTS; (b) Denaturing high performance liquid chromatography analysis to screen for the 12-bp deletion (Del322-325) in alpha(2C)AR and for the alanine to proline mutation at amino acid 457 (Ala457Pro) in NET; (c) Systematic direct sequence analysis to screen for SNPs in beta2AR, NET, and COI.

RESULTS

Three common polymorphisms were abundant in at least one allele in beta2AR resulting in a cysteine to arginine in the 5' promoter region (72% of patients), an arginine to glycine at amino acid-16 (Gly16; 86%), and a glutamine to glutamic acid at amino acid-27 (Glu27; 66%), a frequency that was no different to the normal Caucasian population. Orthostatic HR was significantly greater in patients with Glu27. Diastolic blood pressure (DBP) was significantly lower in a subset of patients with Gly16 whose HR were > or =120 beats/min with head-up tilt. All patients did not show the Ala457Pro mutation of NET; all sequence variants detected in alpha(2C)AR, NET, and COI were not considered causally related to POTS.

CONCLUSIONS

Of the candidate genes screened, none harbored a SNP considered to be causally related to POTS. There was significant association of HR and DBP with SNPs of the gene encoding beta2AR; Gly16 or Glu27 could aggravate orthostatic tachycardia by excessive venous pooling.

β-adrenergic receptor blockade during exercise decreases intestinal lymphocyte apoptosis but not cell loss in mice

Catecholamines induce apoptosis in various lymphoid populations. This process can occur with both α- and β-adrenoreceptors. Heavy exercise increases plasma catecholamine concentrations, and is also a cause of lymphocyte apoptosis, a possible explanation for postexercise lymphocytopenia. The purpose of this study was to examine the effects of adrenoreceptor antagonism on exercise-induced decreases and apoptosis of intestinal lymphocytes. Mice received an intraperitoneal injection of phentolamine (a nonselective α-blocker), nadolol (a nonselective β-blocker), or saline (vehicle) prior to an exhaustive bout of exercise. Total intestinal lymphocyte numbers, percent and number of CD3+ lymphocytes, and cell viability were assessed. Neither α- nor β-antagonism prevented exercise-induced cell loss in the intestine; however, pretreatment with nadolol significantly reduced the number of apoptotic and necrotic cells. Phentolamine administration appeared to increase the incidence of cell death among intestinal lymphocytes. Both drugs decreased the percentage of CD3+ intestinal lymphocytes. Our study suggests that catecholamines are not responsible for postexercise lymphocytopenia, but β-adrenoceptor blockade may confer protection against exercise-induced apoptosis of intestinal lymphocytes.Key words: catecholamines, exhaustive exercise, apoptosis, intestinal lymphocytes, rodents.

Effect of Clonidine on Cardiac Norepinephrine Spillover in Isolated Rat Heart

The purpose of this study is to determine the effect of clonidine on cardiac norepinephrine spillover utilizing an isolated rat heart preparation with attached cardiac sympathetic nerves. Following a 20-minute stabilization period, the sympathetic ganglion for each heart preparation was electrically stimulated with 10V and 2 Hz for 30 seconds (S1: 60 pulses). Heart rate, left ventricular developed pressure, and coronary perfusion pressure was allowed to return to baseline and the perfusate was randomly switched to Krebs buffer containing one of two treatments: placebo or clonidine (1 microM). After 10 minutes of treatment, the sympathetic ganglion was again electrically stimulated with 10V and 2 Hz for 30 seconds (S2: 60 pulses). The perfusate exiting the heart before, during, and after each electrical stimulation was collected for the determination of cardiac norepinephrine spillover. Clonidine administration significantly reduced cardiac norepinephrine spillover by approximately 50% (P < 0.05) and was associated with a 36% reduction in heart rate (P < 0.05). These findings provide evidence that clonidine can directly suppress NE spillover from cardiac sympathetic nerve terminals. Thus, suppression of cardiac NE by clonidine may be due to stimulation of presynaptic alpha2-adrenergic receptors or imidazoline subtype I receptors located on cardiac sympathetic nerve terminals. Results from our study demonstrate a reduction in cardiac NE spillover by clonidine and provide additional evidence that it can directly suppress peripheral sympathetic activity in that our results were obtained utilizing an isolated perfused heart preparation with attached cardiac sympathetic nerves devoid of any CNS input.

Presynaptic modulation of evoked NE release contributes to sympathetic activation after pressure overload

Activation of the sympathetic nervous system is well documented in heart failure. Our previous studies demonstrated an increase in evoked norepinephrine (NE) release from left ventricle (LV) slices at 10 days of pressure overload. The purpose of this study was to test the hypothesis that presynaptic modulation of NE release contributes to sympathetic activation after pressure overload. We examined the functional status of the presynaptic α2- and β2-receptors and ANG II subtype 1 (AT1) receptors in LV slices from 10-day aortic constricted (AC) and sham-operated (SO) rats. Evoked 3H overflow from LV slices preloaded with [3H]NE was increased in AC rats. The α2-agonist UK-14,304 decreased evoked 3H overflow with no differences between groups. The β2-agonist salbutamol increased evoked 3H overflow with greater sensitivity in slices from AC rats. The β-antagonist propranolol decreased evoked 3H overflow from LV slices of AC rats but not controls. ANG II increased evoked 3H overflow with greater sensitivity in slices from AC rats. These data support the hypothesis that aberrant presynaptic modulation of catecholamine release contributes to sympathetic activation after pressure overload.

Activity of the uptake-1 norepinephrine transporter as measured by I-123 MIBG in heart failure patients with a loss-of-function polymorphism of the presynaptic alpha2C-adrenergic receptor

BACKGROUND

Patients with a deletion of 4 consecutive amino acids in the gene encoding for the alpha(2C)-adrenergic receptor (alpha(2C)Del322-325) have an increased prevalence of clinical heart failure, worse clinical status, and a lower left ventricular ejection fraction compared with patients without this deletion. We postulated that patients with the alpha(2C)Del322-325 polymorphism would have a compensatory increase in norepinephrine uptake-1 transporter activity as measured by iodine 123 metaiodobenzylguanidine (MIBG).

METHODS AND RESULTS

Thirty-nine patients with heart failure related to idiopathic dilated cardiomyopathy were studied. Demographic characteristics, left ventricular ejection fraction, maximum oxygen consumption, exercise duration, and plasma norepinephrine levels did not differ between patients with the alpha(2C) receptor polymorphism (n = 9) and those without it (n = 30). Patients with the alpha(2C)Del322-325 polymorphism had significantly greater heart-to-mediastinum ratios of I-123 MIBG at 4 hours after tracer injection (1.60 +/- 0.19 vs 1.41 +/- 0.19, P =.0117) and greater background-corrected heart counts per pixel at 4 hours compared with patients without the polymorphism.

CONCLUSIONS

Patients with genetic impairment of the alpha(2C)-adrenergic receptor have augmented activity of the norepinephrine uptake-1 transporter as measured by I-123 MIBG. Further studies are needed to clarify the mechanism by which uptake-1 transporter activity is increased in this setting.

Sympathetic nervous system activity and ventricular tachyarrhythmias: recent advances

Sympathetic nervous system activity (SNSA) is believed to participate in the genesis of ventricular tachyarrhythmias (VTA) but understanding has been impeded by the number and complexity of effects and the paucity of data from humans. New information from studies of genetic disorders, animal models, and spontaneous human arrhythmias indicates the importance of the temporal pattern of SNSA in arrhythmia development. The proarrhythmic effects of short-term elevations of SNSA are exemplified by genetic disorders and include enhancement of early and delayed afterdepolarizations and increased dispersion of repolarization. The role of long-term elevations of SNSA is suggested by animal models of enhanced SNSA signaling that results in apoptosis, hypertrophy, and fibrosis, and sympathetic nerve sprouting caused by infusion of nerve growth factor. Processes that overlap short- and long-term effects are suggested by changes in R-R interval variability (RRV) that precede VTA in patients by several hours. SNSA-mediated alterations in gene expression of ion channels may account for some intermediate-term effects. The propensity for VTA is highest when short-, intermediate, and long-term changes are superimposed. Because the proarrhythmic effects are related to the duration and intensity of SNSA, normal regulatory processes such as parasympathetic activity that inhibits SNSA, and oscillations that continuously vary the intensity of SNSA may provide vital antiarrhythmic protection that is lost in severe heart failure and other disorders. These observations may have therapeutic implications. The recommended use of beta-adrenergic receptor blockers to achieve a constant level of inhibition does not take into account the temporal patterns and regional heterogeneity of SNSA, the proarrhythmic effects of alpha-adrenergic receptor stimulation, or the potential proarrhythmic effects of beta-adrenergic receptor blockade. Further research is needed to determine if other approaches to SNSA modulation can enhance the antiarrhythmic effects.

Synergistic polymorphisms of beta1- and alpha2C-adrenergic receptors and the risk of congestive heart failure

BACKGROUND

Sustained cardiac adrenergic stimulation has been implicated in the development and progression of heart failure. Release of norepinephrine is controlled by negative feedback from presynaptic alpha2-adrenergic receptors, and the targets of the released norepinephrine on myocytes are beta1-adrenergic receptors. In transfected cells, a polymorphic alpha2C-adrenergic receptor (alpha2CDel322-325) has decreased function, and a variant of the beta1-adrenergic receptor (beta1Arg389) has increased function. We hypothesized that this combination of receptor variants, which results in increased synaptic norepinephrine release and enhanced receptor function at the myocyte, would predispose persons to heart failure.

METHODS

Genotyping at these loci was performed in 159 patients with heart failure and 189 controls. Logistic-regression methods were used to determine the potential effect of each genotype and the interaction between them on the risk of heart failure.

RESULTS

Among black subjects, the adjusted odds ratio for heart failure among persons who were homozygous for alpha2CDel322-325 as compared with those with the other alpha2C-adrenergic receptor genotypes was 5.65 (95 percent confidence interval, 2.67 to 11.95; P<0.001). There was no increase in risk with beta1Arg389 alone. However, there was a marked increase in the risk of heart failure among persons who were homozygous for both variants (adjusted odds ratio, 10.11; 95 percent confidence interval, 2.11 to 48.53; P=0.004). The patients with heart failure did not differ from the controls in the frequencies of nine short tandem-repeat alleles. Among white subjects, there were too few who were homozygous for both polymorphisms to allow an adequate assessment of risk.

CONCLUSIONS

The alpha2CDel322-325 and beta1Arg389 receptors act synergistically to increase the risk of heart failure in blacks. Genotyping at these two loci may be a useful approach for identification of persons at risk for heart failure or its progression, who may be candidates for early preventive measures.

Altered beta-adrenergic receptor gene regulation and signaling in chronic heart failure

J. D. Port and M. R. Bristow. Altered Beta-adrenergic Receptor Gene Regulation and Signaling in Chronic Heart Failure. Journal of Molecular and Cellular Cardiology (2001) 33, 887-905. Beta adrenergic receptors (beta -ARs) are critical regulators of cardiac function in both normal and pathophysiological states. Under normal conditions, beta -ARs and their signaling pathways modulate both the rate and force of myocardial contraction and relaxation, allowing individuals to respond appropriately to physiological stress or exercise. However, in chronic heart failure, sustained activation of the beta -AR signaling pathways can have overtly negative biological consequences. This notion is reinforced by the positive outcomes of a number of clinical trials demonstrating the usefulness of beta-blocker therapy in chronic congestive heart failure. During the last few years, significant progress has been made in understanding the molecular biological basis of beta -AR function, both at the biochemical and genetic levels. In this review, the biological basis of adrenergic signaling and how this changes in heart failure is discussed. Aspects of adrenergic receptor pharmacology relevant to heart failure are reviewed, including the recently emerging differences described for beta(1)- v beta(2)-AR signaling pathways. Highlighting these differences is recent evidence that over-stimulation of the beta(1)-AR pathway in cardiac myocytes appears to be pro-apoptotic, whereas stimulation of the beta(2)-AR pathway may be anti-apoptotic. Overview of beta -AR gene regulation, transgenic models of beta -AR overexpression, and beta -AR polymorphisms as they relate to heart failure progression are also discussed.

Evidence for functional presynaptic alpha-2 adrenoceptors and their down-regulation in human heart failure

OBJECTIVES

The aim of this study was to investigate the role of peripheral presynaptic alpha-2 adrenergic receptors in modulating norepinephrine (NE) release in congestive heart failure (CHF).

BACKGROUND

Activation of the sympathetic nervous system is a hallmark of CHF. Clonidine, an imidazoline and adrenergic agonist with high selectivity for the alpha-2 adrenoceptor, has been shown to reduce generalized sympathetic activity in heart failure after parenteral administration. If it could be shown that peripheral presynaptic alpha-2 adrenoceptors are inhibitory to NE release, then they could be targeted for future therapy, and as a corollary, potentially circumvent unwanted side effects arising from stimulation of alpha-2 adrenoceptors in the brain. Additionally, it could be concluded that these receptors form the basis for an auto-inhibitory feedback to further NE release.

METHODS

Fifteen healthy volunteers and 10 patients with heart failure received intra-arterial clonidine via the brachial artery (0.05 microg and 0.48 microg/100 ml forearm/min). Radio-tracer techniques were employed for studying NE kinetics.

RESULTS

Intra-arterial clonidine caused a dose-dependent decrease in forearm spillover of NE in healthy individuals (low dose, high dose: 26%, 49%: p < 0.05, p < 0.001, respectively). In the patient group, no decrease in forearm spillover was demonstrated after local administration. The difference in response between the two groups was statistically significant (p = 0.004).

CONCLUSIONS

Peripheral sympathoneural alpha-2 adrenoceptors are functionally important in inhibiting NE release in the healthy human. In heart failure, this function is lost. This finding offers further insights into the mechanisms responsible for high circulating levels of NE in patients with heart failure. In addition, it suggests that selective targeting of peripheral presynaptic alpha-2 adrenoceptors will not achieve sympathoinhibition in heart failure.

Norepinephrine concentrations in the epicardial transudate reflect early changes in adrenergic activity in the isolated perfused heart

The aim of this study was to establish whether epicardial transudates could be used to uncover small, but physiologically important changes in interstitial NE concentrations under normal and pathological conditions. Norepinephrine (NE) concentrations measured in epicardial transudate fluid were compared to NE levels in the coronary effluent in normal and pressure overload hypertrophied (POH) rat hearts. Hearts were isolated together with the stellate ganglion and perfused in the inverted position. Epicardial surface transudates, representative fluid of the interstitial myocardial compartment, and coronary effluents were collected for determination of NE levels in the presence and absence of stellate ganglion stimulation. The same protocol was repeated in the presence and absence of nisoxetine, a NE uptake blocker. NE concentrations in epicardial transudates were 16- and 19-fold higher than in the coronary effluent in both sham and POH groups, respectively. NE concentrations in the transudates but not in the coronary effluents were significantly higher (1.6-fold) in hearts with POH when compared to normal hearts. Likewise, nisoxetine (10(-5)m) increased (1.3-fold) NE concentrations in the transudates but not in the effluents of sham animals. As expected, stellate ganglion stimulation increased NE concentrations in both transudates and effluents in sham and POH hearts. In conclusion, determination of NE concentrations in epicardial transudates represents a simple, rapid and sensitive method to detect increases in adrenergic activity in normal and abnormal hearts.

Acute hemodynamic and neurohumoral effects of moxonidine in congestive heart failure secondary to ischemic or idiopathic dilated cardiomyopathy

Elevated plasma norepinephrine (PNE) has been shown to be an important predictor of morbidity and mortality in patients with congestive heart failure (CHF). Moxonidine selectively stimulates imidazoline receptors located in the medulla, which centrally inhibit sympathetic outflow. PNE is suppressed and peripheral vasodilation reduces systemic blood pressure. This study evaluated the acute neurohumoral and hemodynamic effects of a single dose of oral moxonidine in 32 patients (22 men, mean +/- SD age 66 +/- 10 years) with CHF. All patients were in New York Heart Association functional class III and stabilized on chronic therapy with diuretics, digitalis, and angiotensin-converting enzyme inhibitors. The mean PNE concentration was 509 +/- 304 pg/ml at baseline. Patients underwent invasive hemodynamic monitoring after double-blind randomization to either placebo (n = 12), moxonidine 0.4 mg (n = 9), or moxonidine 0.6 mg (n = 11). Moxonidine produced a dose-dependent, vasodilator response compared with placebo. Analysis of the time-averaged change from baseline over 6 hours demonstrated that moxonidine 0.6 mg caused significant reductions in mean systemic arterial pressure (p <0.0001), mean pulmonary arterial pressure (p <0.005), systemic vascular resistance (p <0.05), pulmonary vascular resistance (p <0.01), and heart rate (p <0.05). Stroke volume was unchanged. PNE was reduced substantially (-180 pg/ml at 4 hours, p <0.005) and the reduction was highly correlated with the baseline level (r = -0.968). Moxonidine was well tolerated in this single-dose study and resulted in a modest, dose-dependent, vasodilator response, with substantial reductions in systemic and pulmonary arterial blood pressure. Trials designed to evaluate the clinical efficacy of chronic moxonidine therapy in CHF added to conventional therapy would be appropriate.

Cardiac and systemic sympathetic activity in response to clonidine in human heart failure

OBJECTIVES

We studied the effects of clonidine on cardiac sympathetic activity and left ventricular function in patients with congestive heart failure (CHF).

BACKGROUND

Sympathetic activation has major prognostic implications in patients with heart failure. Clonidine, an imidazoline and alpha2-receptor agonist, has been shown to cause a reduction in generalized sympathetic activity.

METHODS

Nine patients with CHF (left ventricular ejection fraction 22+/-4% [mean+/-SEM]) received a 50 microg and 100 microg bolus of clonidine intravenously. Study measurements included right and left heart hemodynamics, cardiac output, rate of rise in left ventricular peak positive pressure (LV + dP/dt) and tau, along with cardiac and total body norepinephrine spillover. The radiotracer method was used for calculation of norepinephrine spillover.

RESULTS

Right and left heart filling pressures did not change in response to either dose of clonidine. Mean arterial pressure fell after the second dose of clonidine, from 94+/-8 to 82+/-6 mm Hg (p < 0.05). The LV + dP/dt was reduced from 737+/-53 to 629+/-43 mm Hg/s (p < 0.05). Clonidine also caused a significant increase in tau, as measured by the method of Weiss (65+/-3 vs. 74+/-4 ms, p < 0.01) and the direct pressure half-time technique (48+/-2 vs. 54+/-3 ms, p < 0.01). Cardiac norepinephrine spillover fell from 121+/-29 to 52+/-20 pmol/min in response to 100 microg of clonidine (p < 0.01 vs. control).

CONCLUSIONS

Despite a significant fall in arterial pressure, clonidine caused a marked reduction in sympathetic activity directed at the heart. The negative inotropic and lusitropic effects appear to be secondary to this reduction in sympathetic drive. Because increased cardiac and generalized sympathetic activity are strong predictors of an adverse outcome in patients with CHF, the role of centrally active sympathoinhibitory agents in the therapy of CHF deserves further exploration.

Medical therapy of chronic heart failure. Role of ACE inhibitors and beta-blockers

Heart failure has long been considered to have a progressive downhill course leading inexorably to an early demise. This course often occurs silently, in the absence of any obvious cardiac insults. The reason for this is a combination of cell loss, myocyte dysfunction, impaired energetics, and pathologic remodeling of the chamber. Improved clinical outcome should result from strategies that reduce the biologic signals responsible for myocyte growth, dysfunction, and loss and chamber remodeling. Clinicians should no longer attempt to treat chronic heart failure with pharmacologic growth and remodeling process. In time, it may be possible for the clinician to view the treatment of heart failure largely as a matter of improving the biologic function of the myocardium.

Adrenergic nervous system in heart failure

Recent demonstration that the level of sympathetic nervous drive to the failing heart in patients with severe heart failure is a major determinant of prognosis, and that mortality in heart failure is decreased by beta-adrenergic blockade with carvedilol, indicates the clinical relevance of cardiac neuroscience research. Important initial findings were observations that the plasma concentration of the sympathetic neurotransmitter, norepinephrine, is elevated in heart failure, and that overall clinical outcome is related to plasma norepinephrine concentration (although heart failure severity may be a confounder). Sympathetic nerve recording (clinical microneurography) and radiotracer methods measuring regional sympathetic activity in the heart (cardiac norepinephrine "spillover") have now largely supplanted antecubital venous norepinephrine measurements as research tools, with newer methods providing information on regional sympathetic function that was previously lacking. The cardiac sympathetic nerves are preferentially stimulated in severe heart failure, with norepinephrine release from the failing heart at rest in untreated patients increased up to 50-fold, which is similar to the level of release in healthy hearts during near maximal exercise. There is lesser stimulation of the sympathetic outflows to the kidneys and skeletal muscle. In early mild heart failure, it is only the cardiac sympathetic nerves that are activated. This preferential activation of cardiac sympathetic outflow contributes to arrhythmia development and probably to progression of heart failure and has been linked to mortality in mild and severe heart failure. The central nervous system mechanisms involved in the sympathetic nervous activation present in patients with heart failure remain uncertain. Increased intracardiac diastolic pressure seems to be one peripheral reflex stimulus with increased forebrain norepinephrine turnover being an important central mechanism.

Ischemic versus idiopathic cardiomyopathy: differing neurohumoral profiles despite comparable peak oxygen uptake

OBJECTIVE

We tested the hypothesis that neurohormonal and immunological activation differs in ischemic and idiopathic dilated cardiomyopathy since recent intervention trials indicate that ischemic cardiomyopathy seems to carry a worse prognosis than idiopathic cardiomyopathy of comparable clinical severity.

METHODS

In ten patients with ischemic cardiomyopathy undergoing spiroergometric evaluation venous levels of norepinephrine, epinephrine, renin, angiotensin, atrial natriuretic peptide as well as soluble interleukin-2-receptor were determined before, during and 10 min after exercise. Results were compared to sixteen patients with idiopathic cardiomyopathy with similar peak oxygen uptake (13.3+/-3 vs. 13.6+/-3 ml/kg/min; P=ns).

RESULTS

In ischemic patients, norepinephrine, angiotensin, and interleukin-2 receptor levels were significantly higher before, during and after exercise. Interleukin-2-receptor levels correlated with angiotensin.

CONCLUSIONS

We conclude that in ischemic as compared to idiopathic cardiomyopathy, a more pronounced activation of the sympathetic, renin-angiotensin and T-cell immune system is present at rest, during and after exercise. These data may contribute to explain differences in response to intervention and in prognosis. They warrant further investigation.