Simultaneous determination of plasma noradrenaline and adrenaline kinetics. Responses to nitroprusside-induced hypotension and 2-deoxyglucose-induced glucopenia in the rabbit

Evaluation of elevated heart rate as a sympathetic nervous system biomarker in essential hypertension

BACKGROUND

There is a need for an easily accessible biomarker of sympathetic nervous activation in essential hypertension, but none exists. Heart rate (HR) has been suggested, but requires validation, now doubly important as an elevated HR in hypertension has emerged as an independent cardiovascular risk factor.

METHODS

Isotope dilution methodology was used to measure total and regional noradrenaline spillover and adrenaline secretion rates in 30 patients with unmedicated essential hypertension and in a comparator group of 48 healthy participants with normal blood pressure. The particular interest was in the relationship of measured HR to cardiac noradrenaline spillover, the measure of cardiac sympathetic activity.

RESULTS

Sympathetic activation was present in the patients with essential hypertension, evident in significantly increased mean cardiac, renal and total noradrenaline spillover rates. Adrenaline secretion was normal. HR in hypertension correlated directly with cardiac noradrenaline spillover (r = 0.82, P = 9.3 × 10), but not with renal noradrenaline spillover or adrenaline secretion. 67% of the variance in HR was attributable to differences in cardiac sympathetic activity. Among hypertensive patients there was no internal correlation between cardiac noradrenaline spillover, renal noradrenaline spillover and adrenaline secretion; the sympathetic activation commonly was not 'global'. In healthy participants HR did not correlate with measures of sympathetic activity or adrenaline secretion.

CONCLUSION

When sympathetic activation exists in essential hypertension it is differentiated, not necessarily involving all sympathetic outflows. An elevated HR proved to be a biomarker of cardiac sympathetic activation but not activation of the renal sympathetic outflow. Identifying activation of the cardiac sympathetic outflow as the prime mechanism of hypertension tachycardia is relevant to therapies which should now be considered to minimize cardiovascular risk in this clinical setting. Is an elevated HR a valid biomarker of sympathetic activation in essential hypertension? Yes, but only for the cardiac sympathetic outflow. The unavoidable principle is that regional differentiation of sympathetic responses in essential hypertension means that no simple test can ever represent each and every sympathetic outflow.

The adrenal medulla in cardiovascular medicine: an untold story

Unlike noradrenaline, the sympathetic neurotransmitter which overflows to the circulation, adrenaline (ADR) is a secreted hormone, with a low plasma concentration, and plasma concentration for biological action a log order lower than that of noradrenaline. The venous drainage of the left adrenal medulla into the left renal vein does expose this vein to uniquely high plasma ADR concentrations and possible risk of thrombosis at high rates of ADR secretion. There is typically a different timeframe for adrenal medullary and sympathetic nervous system responses: ADR release is short term in contrast with sympathetic activation persisting for years in heart failure and hypertension. The historic view of Walter Cannon, subject to recent review, that the sympathoadrenal system is a unified biological system, was deconstructed further with demonstration of frequent mismatching of adrenal medullary and sympathetic nervous responses. Under gravity stimulation with standing, there is prompt sympathetic activation without ADR release. In many diseases, notably obesity, hypertension, heart failure and depressive illness, an activated sympathetic nervous system and silent adrenal medulla coexist. The therapeutic corollary of this is that ADR blockade is much less commonly needed clinically than pharmacological antagonism of the sympathetic nervous system.

Does sympathetic dysfunction occur before denervation in pure autonomic failure?

Pure autonomic failure (PAF) is a rare sporadic disorder characterized by autonomic failure in the absence of a movement disorder or dementia and is associated with very low plasma norepinephrine (NE) levels-suggesting widespread sympathetic denervation, however due to its rarity the pathology remains poorly elucidated. We sought to correlate clinical and neurochemical findings with sympathetic nerve protein abundances, accessed by way of a forearm vein biopsy, in patients with PAF and in healthy controls and patients with multiple systems atrophy (MSA) in whom sympathetic nerves are considered intact. The abundance of sympathetic nerve proteins, extracted from forearm vein biopsy specimens, in 11 patients with PAF, 8 patients with MSA and 9 age-matched healthy control participants was performed following a clinical evaluation and detailed evaluation of sympathetic nervous system function, which included head-up tilt (HUT) testing with measurement of plasma catecholamines and muscle sympathetic nerve activity (MSNA) in addition to haemodynamic assessment to confirm the clinical phenotype. PAF participants were found to have normal abundance of the NE transporter (NET) protein, together with very low levels of tyrosine hydroxylase (TH) (P<0.0001) and reduced vesicular monoamine transporter 2 (VMAT2) (P<0.05) protein expression compared with control and MSA participants. These findings were associated with a significantly higher ratio of plasma 3,4-dihydroxyphenylglycol (DHPG):NE in PAF participants when compared with controls (P<0.05). The finding of normal NET abundance in PAF suggests intact sympathetic nerves but with reduced NE synthesis. The finding of elevated plasma ratio of DHPG:NE and reduced VMAT2 in PAF indicates a shift towards intraneuronal NE metabolism over sequestration in sympathetic nerves and suggests that sympathetic dysfunction may occur ahead of denervation.

Cardiac autonomic function in adolescents operated by arterial switch surgery

BACKGROUND

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.

METHODS AND RESULTS

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).

CONCLUSIONS

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.

Enhanced responses to ganglion blockade do not reflect sympathetic nervous system contribution to angiotensin II-induced hypertension

OBJECTIVE

We examined whether a specific increase in sympathetic nervous system (SNS) activity accounts for the enhanced depressor response to ganglion blockade in angiotensin II (AngII)-induced hypertension in rabbits or whether it reflects a general increased sensitivity of arterial pressure to vasodilatation.

METHODS

Rabbits were renal denervated or sham-operated and 2 weeks later AngII (50 ng/kg per min) infusion commenced. Mean arterial pressure (MAP) responses to ganglion blockade (pentolinium) and vasodilators nitroprusside and adenosine were measured 2-4 weeks later.

RESULTS

Basal MAP was 74 +/- 2 mmHg and maximum hypotensive responses to pentolinium, nitroprusside and adenosine were -17 +/- 2, -17 +/- 1 and -21 +/- 2 mmHg. AngII increased MAP similarly in intact and renal denervated rabbits (+25 +/- 4 mmHg and +31 +/- 4 mmHg, respectively). In intact rabbits, depressor responses to pentolinium were augmented by 75% during AngII infusion but responses to vasodilators also increased by 73-106% suggesting general augmentation of vascular reactivity rather than a specific increase in SNS neural activity. Consistent with this notion, total noradrenaline spillover was similar in normal and AngII-treated rabbits. In renal denervated rabbits, AngII enhanced depressor responses to vasodilators but not pentolinium, suggesting that sympathetic activity may be reduced by AngII hypertension when renal nerves are absent. In anaesthetized rabbits, methoxamine-induced decreases in hindlimb vascular conductance were greater in hypertensive than normotensive rabbits suggesting the presence of vascular hypertrophy of sufficient magnitude to explain increased responses to ganglion blockade and vasodilators.

CONCLUSION

Enhanced depressor responses to ganglion blockade in AngII hypertension do not reflect augmented SNS activity, but rather, augmented sympathetic vasoconstriction mediated by a vascular amplifier effect.

Sympathetic activity in major depressive disorder: identifying those at increased cardiac risk?

BACKGROUND

Evidence exists linking major depressive disorder (MDD) with clinical cardiovascular events. The importance of the sympathetic nervous system in the generation of cardiac risk in other contexts is established.

OBJECTIVE

To examine the importance of the sympathetic nervous system in the generation of cardiac risk in patients with major depressive disorder (MDD).

METHODS

Studies were performed in 39 patients meeting the Diagnostic and Statistical Manual of Mental Disorders IV (DSM-IV) criteria for MDD and in 76 healthy subjects. Treatment for patients consisted of selective serotonin reuptake inhibition (SSRI) for 12 weeks. Whole body and cardiac sympathetic activity were examined using noradrenaline isotope dilution methodology and sympathetic nerve recording techniques. Measurement of the extraction of infused tritiated noradrenaline by the heart, and estimation of cardiac dihydroxyphenylglycol production provided direct quantification of neuronal noradrenaline reuptake.

RESULTS

Sympathetic activity, particularly in the heart and for the whole body, in patients with MDD followed a bimodal distribution. Elevated values were observed in patients with co-morbid panic disorder (P = 0.006). Consistent with a defect in noradrenaline reuptake, the cardiac extraction of tritiated noradrenaline (0.80 +/- 0.01 versus 0.56 +/- 0.04%, P < 0.001) and cardiac dihydroxyphenylglycol overflow (109 +/- 8 versus 73 +/- 11, P = 0.01) were reduced in patients with MDD. SSRI therapy abolished the excessive sympathetic activation, with whole body noradrenaline spillover falling from 518 +/- 83 to 290 +/- 41 ng/min (P = 0.008).

CONCLUSIONS

We have identified a subset of patients with MDD in whom sympathetic nervous activity is extraordinarily high, including in the sympathetic outflow to the heart. Treatment with an SSRI may reduce sympathetic activity in a manner likely to reduce cardiac risk.

Responses of the hypothalamopituitary adrenal axis and the sympathoadrenal system to isolation/restraint stress in sheep of different adiposity

There is evidence that levels of adipose tissue can influence responses of the hypothalamopituitary-adrenal (HPA) axis to stress in humans and rats but this has not been explored in sheep. Also, little is known about the sympathoadrenal responses to stress in individuals with relatively different levels of adipose tissue. We tested the hypothesis that the stress-induced activation of the HPA axis and sympathoadrenal system is lower in ovariectomized ewes with low levels of body fat (lean) than ovariectomized ewes with high levels of body fat (fat). Ewes underwent dietary manipulation for 3 months to yield a group of lean ewes (n = 7) with a mean (+/-SEM) live weight of 39.1 +/- 0.9 kg and body fat of 8.9 +/- 0.6% and fat ewes (n = 7) with a mean (+/-SEM) live weight of 69.0 +/- 1.8 kg and body fat of 31.7 +/- 3.4%. Fat ewes also had higher circulating concentrations of leptin than lean ewes. Blood samples were collected every 15 min over 8 h when no stress was imposed (control day) and on a separate day when 4 h of isolation/restraint was imposed after 4 h of pretreatment sampling (stress day). Plasma concentrations of adrenocorticotropic hormone (ACTH), cortisol, epinephrine and norepinephrine did not change significantly over the control day and did not differ between lean and fat ewes. Stress did not affect plasma leptin levels. All stress hormones increased significantly during isolation/restraint stress. The ACTH, cortisol and epinephrine responses were greater in fat ewes than lean ewes but norepinephrine responses were similar. Our results suggest that relative levels of adipose tissue influence the stress-induced activity of the hypothalamopituitary-adrenal axis and some aspects of the sympathoadrenal system with fat animals having higher responses than lean animals.

Renal responses to acute reflex activation of renal sympathetic nerve activity and renal denervation in secondary hypertension

We tested whether the responsiveness of the kidney to basal renal sympathetic nerve activity (RSNA) or hypoxia-induced reflex increases in RSNA, is enhanced in angiotensin-dependent hypertension in rabbits. Mean arterial pressure, measured in conscious rabbits, was similarly increased (+16 ± 3 mmHg) 4 wk after clipping the left ( n = 6) or right ( n = 5) renal artery or commencing a subcutaneous ANG II infusion ( n = 9) but was not increased after sham surgery ( n = 10). Under pentobarbital sodium anesthesia, reflex increases in RSNA (51 ± 7%) and whole body norepinephrine spillover (90 ± 17%), and the reductions in glomerular filtration rate (−27 ± 5%), urine flow (−43 ± 7%), sodium excretion (−40 ± 7%), and renal cortical perfusion (−7 ± 3%) produced by hypoxia were similar in normotensive and hypertensive groups. Hypoxia-induced increases in renal norepinephrine spillover tended to be less in hypertensive (1.1 ± 0.5 ng/min) than normotensive (3.7 ± 1.2 ng/min) rabbits, but basal overflow of endogenous and exogenous dihydroxyphenolglycol was greater. Renal plasma renin activity (PRA) overflow increased less in hypertensive (22 ± 29 ng/min) than normotensive rabbits (253 ± 88 ng/min) during hypoxia. Acute renal denervation did not alter renal hemodynamics or excretory function but reduced renal PRA overflow. Renal vascular and excretory responses to reflex increases in RSNA induced by hypoxia are relatively normal in angiotensin-dependent hypertension, possibly due to the combined effects of reduced neural norepinephrine release and increased postjunctional reactivity. In contrast, neurally mediated renin release is attenuated. These findings do not support the hypothesis that enhanced neural control of renal function contributes to maintenance of hypertension associated with activation of the renin-angiotensin system.

Renal Sympathetic Neuroeffector Function in Renovascular and Angiotensin II–Dependent Hypertension in Rabbits

We tested the hypotheses that the gains of specific renal sympathetic neuroeffector mechanisms are altered in secondary hypertension and that the nature of these alterations depends on the precise experimental setting of the kidney. Rabbits were sham operated, or made comparably hypertensive (mean arterial pressure increased 17% to 24%) by clipping the left or right renal artery or by chronic infusion of angiotensin II (20 to 50 ng kg −1 min −1 SC). Four to 6 weeks later, under pentobarbital anesthesia, the left renal nerves were sectioned and electrically stimulated at low (0 to 2 Hz) and high (4 to 8 Hz) frequencies. Neurally evoked reductions in total renal blood flow, cortical perfusion, urine flow, and sodium excretion and increases in renal norepinephrine spillover were not significantly greater in kidneys of hypertensive rabbits than normotensive controls. Neurally evoked increases in renal renin release and the slope of the relationship between renin release and norepinephrine spillover were less in kidneys of hypertensive rabbits than normotensive controls. Low-frequency renal nerve stimulation reduced medullary perfusion, which was negatively correlated with renal norepinephrine spillover in kidneys from all 3 groups of hypertensive rabbits but not normotensive controls. Two-hertz stimulation reduced medullary perfusion by 19% in hypertensive rabbits but not in normotensive rabbits. Thus, of all of the renal sympathetic neuroeffector mechanisms studied, only neural control of medullary perfusion was enhanced in these models of secondary hypertension. This effect appears to be mediated postjunctionally, not through enhanced neural norepinephrine release, and may contribute to the development and/or maintenance of hypertension in these models.

Sex differences in postnatal growth and renal development in offspring of rabbit mothers with chronic secondary hypertension

Previously, we demonstrated that adult blood pressure was increased in offspring of rabbit mothers with chronic secondary renal hypertension. Our study identified sex-specific differences in the programming of hypertension, with female, not male, offspring, having increased blood pressure at 30 wk of age. The aim of this study was to characterize the maternal hypertension during pregnancy to determine potential programming stimuli. Further, we examined the impact of chronic maternal hypertension on offspring birth weight, nephron number, and renal noradrenaline content (as an index of renal innervation density). Three groups of mothers and their offspring were studied: two-kidney, one-wrap (2K-1W, n = 9 mothers) hypertensive, two-kidney, two-wrap (2K-2W, n = 8) hypertensive, and a sham-operated group (n = 9). Mean arterial blood pressure was increased by approximately 20 mmHg throughout pregnancy in both hypertensive groups compared with sham mothers (P(G) < 0.001). Plasma renin activity (PRA; P(G) < 0.05) and aldosterone (P(G) < 0.05) levels were increased during gestation in the 2K-1W, but not the 2K-2W mothers. Birth weight was increased by approximately 20% in offspring of both groups of hypertensive mothers (P(T) < 0.001), though this was associated with a reduction in litter size. Renal noradrenaline content was increased ( approximately 40%, P < 0.05) at 5 wk of age in female 2K-1W offspring compared with sham offspring. Glomerular number was not reduced in female offspring of either group of hypertensive mothers; however, glomerular tuft volume was reduced in female 2K-2W offspring (P < 0.05), indicative of a reduction in glomerular filtration surface area. In conclusion, the two models of renal hypertension produced differential effects on the offspring. The impact of a stimulated maternal renin-angiotensin system in the 2K-1W model of hypertension may influence development of the renal sympathetic nerves and contribute to programming of adult hypertension.

Cardiovascular responses to combined treatment with selective monoamine oxidase type B inhibitors and L-DOPA in the rat

BACKGROUND AND PURPOSE

Postural hypotension is a common side-effect of L-DOPA treatment of Parkinson's disease, and may be potentiated when L-DOPA is combined with selegiline, a selective inhibitor of monoamine oxidase B (MAO-B). Rasagiline is a new, potent and selective MAO-B inhibitor, which does not possess the sympathomimetic effects of selegiline. We have studied the effects of these selective MAO inhibitors, L-DOPA and dopamine on the cardiovascular system of the rat.

EXPERIMENTAL APPROACH

Blood pressure and heart rate was measured in conscious rats following acute or chronic administration of rasagiline, selegiline and L-DOPA, by comparison with the selective MAO-A inhibitor clorgyline, or the MAO-A/B inhibitor tranylcypromine. Cardiovascular responses, catecholamine release, and modification of pressor response to dopamine were studied in pithed rats.

KEY RESULTS

In conscious rats neither rasagiline nor selegiline caused significant potentiation of the effects of L-DOPA (50, 100, 150 mg.kg(-1)) on blood pressure or heart rate at doses which selectively inhibited MAO-B, but L-DOPA responses were potentiated by clorgyline and tranylcypromine. In rats treated twice daily for 8 days with L-DOPA and carbidopa, selegiline (5 mg.kg(-1)) but not rasagiline (0.2 mg.kg(-1)) caused a significant hypotensive response to L-DOPA and carbidopa, although both drugs caused similar inhibition of MAO-A and MAO-B. In pithed rats, selegiline but not rasagiline increased catecholamine release and heart rate, and potentiated dopamine pressor response at MAO-B selective dose.

CONCLUSIONS AND IMPLICATIONS

The different responses to the two MAO-B inhibitors may be explained by the amine releasing effect of amphetamine metabolites formed from selegiline.

Psychophysiological mechanisms in panic disorder: a correlative analysis of noradrenaline spillover, neuronal noradrenaline reuptake, power spectral analysis of heart rate variability, and psychological variables

BACKGROUND

The risk of adverse clinical cardiac events is increased in patients with panic disorder (PD). We evaluated possible mechanistic links between PD and heart disease. We estimated cardiac vagal activity from heart rate variability (HRV) measurements and quantified sympathetic nervous system (SNS) activity using plasma noradrenaline tracer kinetics methodology.

METHODS

Thirty-nine people with PD and 39 age- and gender-matched healthy volunteers were studied. In 19 participants with PD, both HRV and plasma noradrenaline kinetics were tested; in 20 with PD and 20 healthy volunteers, HRV measurements only were made, whereas in 19 healthy volunteers, noradrenaline kinetics only was tested. All panic disorder participants completed psychological measures of anxiety sensitivity and state and trait anxiety; healthy volunteers in whom HRV was measured also provided psychological measures.

RESULTS

Sympathetic nervous tone in the heart, based on rates of cardiac noradrenaline spillover, was normal in PD. Noradrenaline and adrenaline plasma clearance and plasma tritiated noradrenaline and adrenaline extraction in transit through the heart, all dependent on the noradrenaline transporter (NET), were reduced in PD. Psychometric testing linked inhibition of anger to this deficit in NET functioning. Anxiety sensitivity was specifically associated with impaired cardiac NET. High- and low-frequency heart rate spectral power was unrelated to all plasma noradrenaline kinetics measurements.

CONCLUSION

Defective neuronal reuptake of noradrenaline, by augmenting the sympathetic neural signal in the heart, might have a dual effect, sensitizing the heart such as to lead to symptom development (and thus perhaps causing panic disorder) and, second, potentially contributing to adverse cardiac events in established PD.

Effect of angiotensin II receptor blockade on autonomic nervous system function in patients with essential hypertension

It has long been proposed that the renin-angiotensin system exerts a stimulatory influence on the sympathetic nervous system, including augmentation of central sympathetic outflow and presynaptic facilitation of norepinephrine release from sympathetic nerves. We tested this proposition in 19 patients with essential hypertension, evaluating whether the angiotensin receptor blockers (ARBs) eprosartan and losartan had identifiable antiadrenergic properties. This was done in a prospective, randomized, three-way placebo-controlled study of crossover design. Patients were randomized to 600 mg of eprosartan daily, 50 mg of losartan daily, or placebo. The treatment period was 4 wk, with 2-wk washout periods. Multiunit firing rates in efferent sympathetic nerves distributed to skeletal muscle vasculature (muscle sympathetic nerve activity, MSNA) were measured with microneurography, testing whether ARBs inhibit central sympathetic outflow. In parallel, isotope dilution methodology was used to measure whole body norepinephrine spillover to plasma. Mean blood pressure on placebo was 151/98 mmHg, with both ARBs causing reductions of approximately 11 mmHg systolic and 6 mmHg diastolic pressure, placebo corrected. Both MSNA [35 +/- 12 bursts/min (mean +/- SD) on placebo] and whole body norepinephrine spillover [366 +/- 247 ng/min] were unchanged by ARB administration, indicating that the ARBs did not materially inhibit central sympathetic outflow or act presynaptically to reduce norepinephrine release at existing rates of nerve firing. These findings contrast with the easily demonstrable reduction in sympathetic nervous activity produced by antihypertensive drugs of the imidazoline-binding class, which are known to act within the brain to inhibit sympathetic nervous outflow. We conclude that sympathetic nervous inhibition is not a major component of the blood pressure-lowering action of ARBs in essential hypertension.

Cardiovascular actions of the venom from the Irukandji (Carukia barnesi) jellyfish: effects in human, rat and guinea-pig tissues in vitro and in pigs in vitro

1. We have investigated the cardiovascular pharmacology of the crude venom extract (CVE) from the potentially lethal, very small carybdeid jellyfish Carukia barnesi, in rat, guinea-pig and human isolated tissues and anaesthetized piglets. 2. In rat and guinea-pig isolated right atria, CVE (0.1-10 microg/mL) caused tachycardia in the presence of atropine (1 micromol/L), a response almost completely abolished by pretreatment with tetrodotoxin (TTX; 0.1 micromol/L). In paced left atria from guinea-pig or rat, CVE (0.1-3 microg/mL) caused a positive inotropic response in the presence of atropine (1 micromol/L). 3. In rat mesenteric small arteries, CVE (0.1-30 microg/mL) caused concentration-dependent contractions that were unaffected by 0.1 micromol/L TTX, 0.3 micromol/L prazosin or 0.1 micromol/L omega-conotoxin GVIA. 4. Neither the rat right atria tachycardic response nor the contraction of rat mesenteric arteries to CVE were affected by the presence of box jellyfish (Chironex fleckeri) antivenom (92.6 units/mL). 5. In human isolated driven right atrial trabeculae muscle strips, CVE (10 microg/mL) tended to cause an initial fall, followed by a more sustained increase, in contractile force. In the presence of atropine (1 micromol/L), CVE only caused a positive inotropic response. In separate experiments in the presence of propranolol (0.2 micromol/L), the negative inotropic effect of CVE was enhanced, whereas the positive inotropic response was markedly decreased. 6. In anaesthetized piglets, CVE (67 microg/kg, i.v.) caused sustained tachycardia and systemic and pulmonary hypertension. Venous blood samples demonstrated a marked elevation in circulating levels of noradrenaline and adrenaline. 7. We conclude that C. barnesi venom may contain a neural sodium channel activator (blocked by TTX) that, in isolated atrial tissue (and in vivo), causes the release of transmitter (and circulating) catecholamines. The venom may also contain a 'direct' vasoconstrictor component. These observations explain, at least in part, the clinical features of the potentially deadly Irukandji syndrome.

Sympathetic nerve function--assessment by radioisotope dilution analysis

Radioisotope dilution measurements of norepinephrine spillover (rate of entry of the transmitter into plasma) provide more accurate assessments of sympathoneural transmitter release than allowed by measurements of plasma catecholamine concentrations alone. Measurements of total body norepinephrine spillover, as an index of global sympathetic outflow, allow effects on plasma clearance to be distinguished from effects on release of catecholamines into plasma, while spillovers from specific tissues enable examination of regionalized sympathetic responses. However, spillovers of norepinephrine represent only a fraction of the transmitter that escapes neuronal and extraneuronal uptake after release by nerves. Numerous factors may influence this fraction and measures spillovers independently of transmitter release by nerves. Modified radioisotope dilution methods for assessment of rate processes operating within and between intracellular and extracellular compartments have further improved our understanding of the relationships of norepinephrine release, uptake, spillover, turnover, and metabolism. This article reviews the breadth of information about sympathetic nerve function attainable using catecholamine radioisotope dilution analyses against a backdrop of the relative advantages and methodological limitations associated with the methodology.

Atrial fibrillation impairs cardiac sympathetic response to baroreceptor unloading in congestive heart failure

AIMS

In this study, we investigated for a potential mechanism by which atrial fibrillation (AF) might convey a worse prognosis in congestive heart failure (CHF). Specifically, we aimed to determine whether AF impaired cardiac sympathetic response to baroreceptor unloading in comparison to sinus rhythm (SR) in CHF.

METHODS AND RESULTS

Eighteen CHF patients (ejection fraction 30+/-2%, age 59+/- 2 years), nine in SR and nine in AF, were enrolled. A right heart study and cardiac sympathetic tone assessment by coronary sinus catheter were performed at baseline and after 10 min of 20 degrees and 30 degrees of passive head up tilt (HUT). Filling pressures fell significantly during HUT in both SR and AF groups (AF, P=0.002; SR, P<0.001). The cardiac sympathetic response to HUT was significantly attenuated by AF compared with SR (P=0.014). In conjunction, right atrial appendages were collected from 23 cardiac surgery patients, 12 in SR and 11 in AF to investigate the presence of fibrosis. AF was associated with a significant increase in the collagen density (P=0.025).

CONCLUSION

AF is associated with impaired cardiac sympathetic response to baroreceptor unloading compared with SR in CHF, possibly secondary to atrial fibrosis.

Preserved left ventricular structure and function in mice with cardiac sympathetic hyperinnervation

Cardiac-specific overexpression of nerve growth factor (NGF), a neurotrophin, leads to sympathetic hyperinnervation of heart. As a consequence, adverse functional changes that occur after chronically enhanced sympathoadrenergic stimulation of heart might develop in this model. However, NGF also facilitates synaptic transmission and norepinephrine uptake, effects that would be expected to restrain such deleterious outcomes. To test this, we examined 5- to 6-mo-old transgenic (TG) mice that overexpress NGF in heart and their wild-type (WT) littermates using echocardiography, invasive catheterization, histology, and catecholamine assays. In TG mice, hypertrophy of the right ventricle was evident (+67%), but the left ventricle was only mildly affected (+17%). Left ventricular (LV) fractional shortening and fractional area change values as indicated by echocardiography were similar between the two groups. Catheterization experiments revealed that LV +/-dP/dt values were comparable between TG and WT mice and responded similarly upon isoproterenol stimulation, which indicates lack of beta-adrenergic receptor dysfunction. Although norepinephrine levels in TG LV tissue were approximately twofold those of WT tissue, TG plasma levels of the neuronal norepinephrine metabolite dihydroxyphenylglycol were fivefold those of WT plasma. A greater neuronal uptake activity was also observed in TG LV tissue. In conclusion, overexpression of NGF in heart leads to sympathetic hyperinnervation that is not associated with detrimental effects on LV performance and is likely due to concomitantly enhanced norepinephrine neuronal uptake.

Long-term outcome in relation to renal sympathetic activity in patients with chronic heart failure

AIMS

Although cardiac sympathetic activation is associated with adverse outcome in patients with chronic heart failure (CHF), the influence of renal sympathetic activity on outcome is unknown. We assessed the hypothesis that renal noradrenaline (NA) spillover is a predictor of the combined endpoint of all-cause mortality and heart transplantation in CHF.

METHODS AND RESULTS

Sixty-one patients with CHF, New York Heart Association (NYHA) I-IV (66% NYHA III-IV), and left ventricular ejection fraction (LVEF) 26+/-9% (mean+/-SD) were studied with cardiac and renal catheterizations at baseline and followed for 5.5+/-3.7 years (median 5.5 years, range 12 days to 11.6 years). Nineteen deaths and 13 cases of heart transplantation were registered. Only renal NA spillover above median, 1.19 (interquartile range 0.77-1.43) nmol/min, was independently associated with an increased relative risk (RR) of the combined endpoint (RR 3.1, 95% CI 1.2-7.6, P=0.01) in a model also including total body NA spillover, LVEF, glomerular filtration rate (GFR), renal blood flow, cardiac index, aetiology, and age.

CONCLUSION

Renal noradrenergic activation has a strong negative predictive value on outcome independent of overall sympathetic activity, GFR, and LVEF. These findings suggest that treatment regimens that further reduce renal noradrenergic stimulation could be advantageous by improving survival in patients with CHF.

Rilmenidine sympatholytic activity preserves mental stress, orthostatic sympathetic responses and adrenaline secretion

BACKGROUND

Heightened central sympathetic nervous outflow is common in essential hypertension, contributing to hypertension development and possibly also to complications. Acute sympathetic nervous activation is a proven trigger for adverse cardiovascular events. Accordingly, antihypertensive drugs inhibiting sympathetic outflow represent a theoretically attractive therapeutic option.

OBJECTIVES

To study the sympatholytic and blood pressure-lowering activity of the imidazoline binding agent rilmenidine at rest and during reflex sympathetic activation.

DESIGN AND METHODS

We used a randomized, double-blind, 6-week cross-over study, with a 1-week placebo run-in period, two 2-week active treatment intervals (rilmenidine 1 mg twice daily or placebo) and intervening 1-week placebo washout. In 15 hypertensive patients, noradrenaline and adrenaline plasma kinetics and intra-arterial blood pressure measurements were performed at rest, after mental stress (difficult mental arithmetic) and during head-up tilting, at the end of the 2-week dosing periods.

RESULTS

The noradrenaline spillover rate, indicative of whole body sympathetic activity, was reduced 35% by rilmenidine at rest (P < 0.01) and remained significantly lower during mental stress and tilting, although the increases in noradrenaline spillover with both stimuli were preserved. The effects on intra-arterial blood pressure ran in parallel, a fall in supine resting pressure, but no reduction in blood pressure rise during mental stress and a lack of fall in blood pressure with tilting. On placebo, adrenaline secretion was 0.88 +/- 0.15 nmol/min (mean +/- SE) at rest, increased by 0.42 +/- 0.23 nmol/min with mental stress (P = 0.019) and was unchanged with tilting. Rilmenidine left adrenaline secretion untouched under all conditions.

CONCLUSIONS

The present study confirms a sympatholytic effect of rilmenidine during supine rest but preservation of sympathetic responses during mental stress and tilting, with the latter underlying a freedom from postural hypotension on the drug. The absence of suppression of reflexive sympathetic responses contrasts with the described effects of rilmenidine in experimental animals, and emphasizes the previously demonstrated unique importance in humans of suprabulbar noradrenergic neuronal projections from the brainstem in regulating tonic sympathetic activity, with these being inhibited by imidazoline binding agents. Sympathetic nervous inhibition with rilmenidine contrasted with an absence of suppression of adrenaline secretion, affirming that sympathetic nervous and adrenal medullary function can be disconnected.

Percutaneous mitral annular reduction provides continued benefit in an ovine model of dilated cardiomyopathy

BACKGROUND

Functional mitral valve regurgitation plays a key role in the symptomatic severity and progression of heart failure. In an ovine model of dilated cardiomyopathy, we examined the chronic functional consequences of mitral regurgitation reduction using a recently developed novel percutaneous mitral annular reduction (PMAR) device.

METHODS AND RESULTS

Fourteen adult sheep were paced right ventricularly at 180 to 190 bpm for 5 weeks, leading to the development of moderate mitral valve regurgitation. After echocardiographic, hemodynamic, and neurohormonal analysis, 9 animals underwent PMAR. All animals were subsequently paced for another 28 days, and a final echocardiographic and hemodynamic study was conducted. Animals that had undergone PMAR showed significantly increased negative and positive dP/dt, whereas pulmonary capillary wedge pressure and mitral valve regurgitation were significantly reduced compared with those at device implant despite continued pacing. In conjunction, significant improvements in plasma norepinephrine and brain natriuretic peptide were apparent.

CONCLUSIONS

The application of PMAR in animals with pacing-induced dilated cardiomyopathy and functional mitral valve regurgitation resulted in continued improvements in hemodynamic and neurohormonal parameters.

Extra-adipocyte leptin release in human obesity and its relation to sympathoadrenal function

The link between the human sympathoadrenalmedullary system and the adipocyte hormone leptin is controversial. We measured total and regional norepinephrine spillover, epinephrine secretion rate, and extra-adipocyte leptin release in 22 lean [body mass index (BMI) < 26] and 20 obese (BMI > 28) normotensive men who underwent arterial and central venous catheterization. Because plasma clearance of leptin is primarily by renal removal, for men at steady state we could estimate whole body leptin release to plasma from renal plasma leptin extraction. Whole body leptin release was 1,950 +/- 643 (means +/- SE) ng/min in obese men and 382 +/- 124 ng/min in lean men (P < 0.05). Total and renal norepinephrine spillover rates correlated directly with whole body leptin secretion rate. Leptin is released from multiple nonadipocyte sites, which we tested by use of simultaneous arteriovenous blood sampling. We found a surprisingly large contribution of brain leptin release to the plasma leptin pool, 529 +/- 175 ng/min (> 40% whole body leptin release), with greater leptin release in obese than in lean men, 935 +/- 321 vs. 160 +/- 59 ng/min (P = 0.045). In parallel with leptin measurements, we also quantified brain serotonin turnover and jugular overflow of neuropeptide Y (NPY). Brain serotonin turnover was higher in obese than in lean men, 227 +/- 112 vs. 21 +/- 14 ng/min (P = 0.019), as was overflow of NPY from the brain, 12.9 +/- 1.4 vs. 5.3 +/- 2.2 ng/min (P = 0.042). These results suggest that leptin is released within the brain and at an increased rate in obese humans, in whom activation of brain serotonergic and NPY mechanisms also exists.

Method for simultaneous measurement of norepinephrine, 3-methoxy-4-hydroxyphenylglycol and 3,4-dihydroxyphenylglycol by liquid chromatography with electrochemical detection: application in rat cerebral cortex and plasma after lithium chloride treatment

An assay was developed to quantify norepinephrine (NE) and its metabolites (MHPG and DHPG) by high-performance liquid chromatography with electrochemical detection method (HPLC-ECD) in brain tissue and plasma of rats treated by LiCl. Separation on C(18) column was obtained by a mobile phase consisting of 4.5% methanol in buffer (0.1 M sodium acetate, 0.2 M citric acid) containing 0.2 mM ethylenediaminetetraacetic acid disodium salt (EDTA Na(2)) and 0.4 mM sodium octylsulfate, operated at a flow rate of 0.8 ml/min. A potential of +0.78 V was applied across the working and reference electrodes of the detector. The precision was in the range 2.88-4.35% for NE, 5.94-11.0% for MHPG and 1.97-4.40% for DHPG. Accuracy was 98.8-99.3% for NE, 97.4-100% for MHPG and 96.1-101% for DHPG. The limit of detection was 0.6 ng/ml for NE, 0.5 ng/ml for MHPG and 0.2 ng/ml for DHPG. The linearity is over the range 20-60 ng/ml for NE, 7-23 ng/ml for MHPG and 6-20 ng/ml for DHPG. The assay has been applied successfully to measure simultaneously cortex and plasmas concentrations of these three catecholamines in rats.

Pre-weaning carvedilol treatment in spontaneously hypertensive rats

Hypertension in spontaneously hypertensive rats (SHR) has been permanently abolished by aggressive treatment regimens targeted against the sympathetic nervous system and adrenal medulla, initiated during the pre-weaning period (guanethidine and nerve growth factor antiserum combined with either adrenal demedullation or prazosin treatment). To investigate the components of the sympatho-adrenal system involved, we treated pre-weaning SHR with the combined alpha(1)- and beta-adrenoceptor antagonist carvedilol (60 mg/kg/day s.c.; postnatal days 1-21). Carvedilol treatment significantly blocked adrenoceptors during the treatment period, delayed development (eye opening), reduced growth, and reduced arterial pressure and heart rate. However, there was only modest attenuation of the subsequent development of hypertension at 10 weeks of age (mean arterial pressure 129.5+/-1.8 versus 136.1+/-1.6 mm Hg in vehicle-treated littermates; P<0.05). Thus pre-weaning carvedilol treatment slightly but significantly attenuated the development of SHR hypertension at 10 weeks, suggesting that the profound antihypertensive effects of pre-weaning sympatho-adrenal ablation are attributable to factors other than alpha(1)- and beta-adrenoceptor-mediated effects of catecholamines during this period.

Seasonal differences in the effect of isolation and restraint stress on the luteinizing hormone response to gonadotropin-releasing hormone in hypothalamopituitary disconnected, gonadectomized rams and ewes

Stress responses are thought to act within the hypothalamopituitary unit to impair the reproductive system, and the sites of action may differ between sexes. The effect of isolation and restraint stress on pituitary responsiveness to GnRH in sheep was investigated, with emphasis on possible sex differences. Experiments were conducted during the breeding season and the nonbreeding season. In both experiments, 125 ng of GnRH was injected i.v. every 2 h into hypothalamopituitary disconnected, gonadectomized rams and ewes on 3 experimental days, with each day divided into two periods. During the second period on Day 2, isolation and restraint stress was imposed for 5.5 h. Plasma concentrations of LH and cortisol were measured in samples of blood collected from the jugular vein. In the second experiment (nonbreeding season), plasma concentrations of epinephrine, norepinephrine, 3,4-dihydroxyphenylalanine, and 3,4-dihydroxyphenylglycol were also measured. In both experiments, there was no effect of isolation and restraint stress on plasma concentrations of cortisol in either sex. During the breeding season, there was no effect of isolation and restraint stress on plasma concentrations of LH in either sex. During the nonbreeding season, the amplitude of the first LH pulse after the commencement of stress was significantly reduced (P < 0.05) in rams and ewes. In the second experiment, during stress there was a significant increase (P < 0.05) in plasma concentrations of epinephrine in rams and ewes and significantly higher (P < 0.05) basal concentrations of norepinephrine in ewes than in rams. These results suggest that in sheep stress reduces responsiveness of the pituitary gland to exogenous GnRH during the nonbreeding season but not during the breeding season, possibly because of mediators of the stress response other than those of the hypothalamus-pituitary-adrenal gland axis.

Interactions between leptin and the human sympathetic nervous system

Results from animal experimentation suggest a 2-way interaction between leptin and the sympathetic nervous system, with leptin causing sympathetic activation and conversely, with the sympathetic system exercising regulatory feedback inhibition over leptin release. We have now tested this hypothesis in humans. In the absence of results from leptin infusions, to test for sympathetic stimulation of leptin release, we sought a quantitative naturalistic linkage of sympathetic activity with leptin plasma concentration across a broad range of leptin values in men of widely differing adiposity. Renal norepinephrine spillover was correlated with plasma leptin (r=0.628, P<0.01), but other measures of sympathoadrenal function did not. To test for sympathetic and adrenomedullary inhibition of leptin release, we studied clinical models of high sympathetic tone, heart failure, and essential hypertension, in which lowered plasma leptin levels might have been expected but were not found; a model of low sympathetic activity, pure autonomic failure, in which plasma leptin level was normal (6.1+/-1.2 vs 12.8+/-3.1 ng/mL in healthy subjects); and a clinical model of reduced epinephrine secretion, healthy aging, in which plasma leptin level again was normal (5.7+/-1.1 ng/mL vs 4.0+/-0.9 ng/mL in men >60 years and <35 years, respectively). Paradoxically, leptin concentration was elevated in heart failure, caused entirely by reduced renal clearance of leptin release, 142.0+/-30.5 mL/min, compared with 56.9+/-18.9 mL/min (P<0.05). These results provide some support for the view that leptin stimulates the sympathetic nervous system, at least for renal sympathetic outflow, but do not confirm the concept of regulatory feedback inhibition of leptin release by the sympathetic nervous system.

Acute electrophysiologic effects of intravenous amiodarone are independent of a sympatholytic action in humans

Previous experiments in animals demonstrated a novel sympatholytic action of acute intravascular amiodarone (AM). It is not known if this action also occurs in humans. Twelve male volunteers performed handgrip for 10 min before and after 300 mg intravenous (IV) AM over 60 min. The effect of handgrip was determined from changes in blood pressure (BP), heart rate (HR), and cardiac noradrenaline (NA) spillover. Changes in cardiac spillover of dihydroxyphenylglycol (DHPG), the metabolite of NA, were measured during AM infusion. The electrophysiological effects of AM were determined from changes to the A-H intervals during right atrial stimulation (100 beats/min). Handgrip increased HR (63 +/- 2 to 84 +/- 5 beats/min and 65 +/- 3 to 84 +/- 4 beats/min), systolic BP (141 +/- 4 to 179 +/- 6 mm Hg and 140 +/- 4 to 179 +/- 7 mm Hg), and cardiac NA spillover (11.9 +/- 4 to 44.3 +/- 13 ng/min and 17.3 +/- 4 to 55.5 +/- 11 ng/min) before and after AM, respectively (P < 0.02 in all groups). There was good correlation between increases in cardiac NA spillover and HR (r2 = 0.86) and systolic BP (r2 = 0.87). AM increased the A-H interval (95.5 +/- 18 to 107.8 +/- 20 ms, P < 0.02). There was no difference in hemodynamic or NA response to handgrip before or after the AM infusion. There was also no change in DHPG cardiac spillover during AM infusion. Acute IV AM did not exert a sympatholytic action in humans, with no attenuation in hemodynamic or NA response to handgrip or increase in DHPG production, despite producing an electrophysiologic response.

An ovine model of tachycardia-induced degenerative dilated cardiomyopathy and heart failure with prolonged onset

BACKGROUND

The aim of this study was to develop a model of long-term progressive heart failure (HF).

METHODS AND RESULTS

A cardiac output flowprobe was implanted on the pulmonary artery of 9 adult sheep weighing 40 to 50 kg. Rapid ventricular pacing for 21 days at 160 to 190 bpm (rate A) resulted in moderate HF. Animals were then paced at 205 to 215 bpm (rate B) for 42 days (severe HF) and for 28 days at rate B (advanced HF). Data were collected at baseline and moderate, severe, and advanced HF during submaximal exercise testing and by transthoracic echocardiography in sinus rhythm. There were marked increases in left ventricular (LV) area, mitral valve regurgitation, and LV end-diastolic pressure and decreases in LV wall thickness, LV ejection fraction, positive and negative dP/dt(max), and positive (dP/dt(max))/P throughout the pacing protocol.

CONCLUSIONS

This ovine HF model incorporates the progressive nature of human HF and allows examination of both structural changes and hemodynamic parameters of HF during and after exercise challenge.

The influence of aging on the human sympathetic nervous system and brain norepinephrine turnover

Investigating aging effects on the sympathetic nervous system and ascertaining underlying central nervous system (CNS) mechanisms mediating sympathetic stimulation is clinically pertinent because of the possible interconnection of cardiovascular disease development with age-dependent sympathetic nervous changes. Because of previous evidence linking human CNS neuronal noradrenergic function and sympathetic activity, we investigated the influence of aging on brain norepinephrine turnover in 22 healthy men aged 20-30 yr and 16 healthy men aged 60-75 yr by measuring the internal jugular venous overflow of norepinephrine and its lipophilic metabolites. Sympathoneural and adrenal medullary function was also studied, using plasma catecholamine isotope dilution methodology and regional central venous sampling. In the older men there was increased norepinephrine turnover in suprabulbar subcortical brain regions, 317 +/- 50 ng/min compared with 107 +/- 18 ng/min in younger men. A differentiated sympathetic nervous activation was also present in older men. Overall, levels of both cardiac and hepatomesenteric norepinephrine spillover were directly correlated with subcortical norepinephrine turnover. These findings suggest that in sympathetic nervous activation accompanying aging, as has previously been demonstrated with the sympathetic nervous stimulation in human hypertension and heart failure, there is an underlying sympathoexcitatory influence of noradrenergic projections to suprabulbar subcortical regions.

Sympathetic nervous activation following subarachnoid hemorrhage: Influence of intravenous clonidine

BACKGROUND

Subarachnoid hemorrhage is often accompanied by systemic complications and cerebral vasospasm. Elevated levels of circulating catecholamines may be involved in the pathophysiology behind these events. The alpha-2-agonist clonidine inhibits sympathetic outflow by a central mechanism. Unrestricted sympathoexcitation may be detrimental and administration of clonidine may be beneficial in these patients.

METHODS

Using isotope dilution methodology, norepinephrine kinetic determinations, comprising determination of arterial norepinephrine concentration and rates of norepinephrine spillover to and removal, or clearance, from plasma, were performed on three occasions during the first week after subarachnoid hemorrhage in 25 patients. Eleven of these patients received clonidine (continuous i.v. infusion 5.8 +/- 0.7 microg x kg(-1) x 24 h(-1)) and the remainder, standard therapy. Initial results were compared with 17 healthy age-matched subjects and eight patients suffering from severe traumatic brain injury without traumatic subarachnoid hemorrhage.

RESULTS

Subarachnoid hemorrhage patients exhibited markedly elevated arterial plasma norepinephrine concentrations [3.74 +/- 0.48, P < 0.001 vs. healthy subjects (1.59 +/- 0.11 nmol/L) and P < 0.05 vs. head trauma patients (1.94 +/- 0.29 nmol/L)]. The rate of clearance of norepinephrine from plasma in the subarachnoid patients was also significantly greater than that observed in the healthy subjects (2.66 +/- 0.15 vs. 2.14 +/- 0.15 L/min, P < 0.05) and the head trauma patients (2.00 +/- 0.12 L/min, P < 0.05). Compared with both control groups, on admission the rate of spillover of norepinephrine to plasma following subarachnoid hemorrhage was markedly elevated (9.11 +/- 1.12, P < 0.001). Clonidine treatment (continuous i.v. infusion 5.8 +/- 0.7 microg x kg(-1) x 24 h(-1)) did not reduce the increased rate of spillover of norepinephrine to plasma following subarachnoid hemorrhage.

CONCLUSION

Sympathetic nervous activity is markedly elevated following subarachnoid bleeding. Clonidine had no effect on the rate of norepinephrine spillover to, or clearance from, plasma in these patients. Clearly, further studies are required to elucidate the mechanisms responsible for generating sympathetic nervous activation following subarachnoid hemorrhage.

Reduced spontaneous baroreceptor sensitivity in patients with renovascular hypertension

OBJECTIVE

Sympathetic nerve activity is increased in hypertensive patients with renal artery stenosis. Less is known about cardiac vagal function in these patients before and after renal angioplasty. The aim of the present study was to investigate cardiac baroreceptor reflex sensitivity together with total body noradrenaline (NA) spillover in hypertensive patients with renal artery stenosis before, and in some patients, 1 year after renal angioplasty.

MATERIAL AND METHODS

Spontaneous baroreceptor reflex sensitivity and total body noradrenaline (NA) spillover were measured in patients with renovascular hypertension before intervention (n = 18), patients being cured/improved 1 year after renal angioplasty (n = 5) and age-matched healthy subjects (n = 25).

RESULTS

Hypertensive patients with renal artery stenosis had higher total body NA spillover (4630 +/- 619 versus 3132 +/- 210 pmol/min, P < 0.05) and reduced cardiac baroreceptor reflex sensitivity (6.1 +/- 1.0 versus 10.7 +/- 1.0 ms/mmHg, P < 0.01) compared with healthy subjects. Similar results were obtained (before intervention) in a subgroup of patients (n = 9) with renovascular hypertension defined as cured/improved 1 year following renal angioplasty. Baroreceptor reflex sensitivity improved after renal angioplasty in a subset of patients showing good blood pressure control 1 year after intervention (6.4 +/- 0.7 to 9.4 +/- 1.7 ms/mmHg, P < 0.05).

CONCLUSIONS

Patients with renovascular hypertension showed reduced cardiac baroreceptor reflex sensitivity and increased noradrenergic activity, which to some extent was reversed 1 year following successful renal angioplasty.

Bradykinin facilitates noradrenaline spillover during contraction in the canine gracilis muscle

Noradrenaline spillover from skeletal muscle vascular areas increases during exercise but the underlying mechanisms are not well understood. Muscle contraction itself causes changes in many factors that could affect noradrenaline spillover. For instance, it has been reported that bradykinin is synthesized in skeletal muscle areas during contraction. Because the B2 bradykinin receptor facilitates noradrenaline spillover, it may be involved in the increase associated with contraction. In this experiment, we studied the effect of bradykinin on noradrenaline spillover in the in situ canine gracilis muscle, using the specific B2 antagonist HOE 140. The drug did not modify noradrenaline spillover at rest, but did cause a significant decrease during muscle contraction, from 558 to 181 pg·min–1. As reported previously in the literature, fractional extraction of noradrenaline decreased during muscle contraction. This effect was independent of HOE 140 treatment. In light of our results, it seems that bradykinin formation during muscle contraction may play an important part in the observed increase in noradrenaline spillover but does not affect fractional extraction.Key words: skeletal muscle, fractional extraction, stimulation, HOE 140, B2 receptors.

Norepinephrine kinetics in freely moving rats

Norepinephrine (NE) kinetics were investigated in freely moving (FM) and minimally stressed (MS) rats with the isotope dilution technique. 1) The mean NE spillover rate (NE-SOR) was 79 +/- 6 ng. kg(-1). min(-1), and the mean NE metabolic clearance rate (NE-MCR) 179 +/- 9 ml. kg(-1). min(-1) (n = 31). Thus the NE kinetics in FM and MS rats are much faster than in human beings, probably related to a higher sympathetic drive. 2) Whether the magnitude of NE-MCR is related to the level of plasma NE concentration was investigated. No significant correlation was calculated between plasma NE concentration and NE-MCR in 31 control rats. When plasma NE concentration was varied during either acute or chronic infusion of exogenous NE, NE-MCR remained unchanged as long as animal hemodynamics were not altered. When plasma NE concentration was high enough to increase mean arterial pressure (MAP), NE-MCR was decreased. However, when MAP was increased within comparable magnitude, NE-MCR was decreased during NE and increased during epinephrine (Epi) infusion. Thus the existence of an alpha-/beta-adrenergic mechanism involved in the regulation of NE-MCR independent of known hemodynamic mechanisms is suggested. 3) The "epinephrine hypothesis" was revisited in FM and MS rats. At variance with humans, very high plasma Epi concentrations have to be induced to increase NE-SOR in resting rats. Furthermore, NE-MCR was also increased, accounting for the nonsignificant increase of plasma NE concentration. Within the range of Epi concentrations with no effect on NE-SOR, an increase of NE release was revealed when the presynaptic alpha(2)-adrenoreceptors were partially inhibited by yohimbine. This suggests the existence of a second epinephrine hypothesis.

Mechanisms of Carvedilol Action in Human Congestive Heart Failure

Abstract —The precise mechanism by which β-adrenoceptor blockers exert their beneficial actions in patients with heart failure remains unclear. Several possibilities have been proposed, including heart rate reduction, β2-adrenoceptor–mediated modulation of catecholamine release, antagonism of the receptor-mediated toxic actions of norepinephrine on the myocardium, and favorable effects on myocardial energetics. In the present study we evaluated the effect of 3 months of carvedilol therapy on hemodynamics, total systemic and cardiac norepinephrine spillover (isotope dilution method), and myocardial metabolism (myocardial oxygen consumption and carbon dioxide release) in 10 patients with severe congestive heart failure. Although carvedilol treatment was associated with a significant improvement in left ventricular ejection fraction (17±1% to 28±3%; P <0.01) and left ventricular stroke work (87±13 to 119±21 g · m per beat; P <0.05), this effect was unrelated to changes in total systemic or cardiac norepinephrine spillover. The rise in left ventricular stroke work was accompanied by a modest rise in myocardial oxygen consumption per beat (0.33±0.04 to 0.42±0.04; P =0.05), although contractile efficiency was unchanged. The favorable effects of carvedilol on ventricular function in the failing heart are not explained by alterations in norepinephrine release or by changes in myocardial contractile efficiency.

Cardiovascular and behavioural responses to psychological stress in spontaneously hypertensive rats: effect of treatment with DSP-4

We used a model of psychological stress combining exposure to an open-field novel environment, radio-telemetric measurement of blood pressure and heart rate, and behavioural tracking analysis of behavioural parameters. All rats showed significant increases in blood pressure and heart rate for the duration of open-field exposure, with spontaneously hypertensive rats (SHR) showing markedly greater pressor responses and tachycardia when compared to either Wistar-Kyoto (WKY) or Sprague-Dawley rats (SD rats). Behavioural responses in the open-field were unrelated to the magnitude of cardiovascular responses. Open-field exposure on 4 consecutive days induced similar pressor responses and tachycardia on each day. By contrast, behavioural responses were reduced from the second day of open-field exposure. Treatment of SHR and WKY rats with DSP-4, to deplete central noradrenaline levels, did not affect cardiovascular responses in SHR, whereas WKY rats showed a trend towards inhibition. However, in WKY rats, but not SHR, DSP-4 treatment caused a marked reduction in behavioural activity in the open-field. In conclusion, these data show that: (1) SHR display marked cardiovascular hyperreactivity to psychological open-field stress when compared to two normotensive rat strains; (2) unlike behavioural responses, cardiovascular stress responses do not habituate upon repeated stress exposure; and (3) noradrenergic projections from the locus coeruleus do not appear to play a major role in cardiovascular stress responses in SHR or WKY rats, although they may be involved in behavioural responses in WKY rats.

Differentiated response of the sympathetic nervous system to angiotensin-converting enzyme inhibition in hypertension

Hypertension with renal artery stenosis is associated with both an activated renin-angiotensin system and elevated sympathetic activity. Therefore, in this condition it may be favorable to use a therapeutic modality that does not reflexly increase heart rate, renin secretion, and sympathetic nervous activity. The purpose of the present study was to assess overall, renal, and muscle sympathetic activity after short-term administration of an angiotensin-converting enzyme inhibitor (enalaprilat) and a nonspecific vasodilator (dihydralazine) to hypertensive patients with renal artery stenosis. Forty-eight patients undergoing a clinical investigation for renovascular hypertension were included in the study. An isotope dilution technique for assessing norepinephrine spillover was used to estimate overall and bilateral renal sympathetic nerve activity. In 11 patients simultaneous intraneural recordings of efferent muscle sympathetic nerve activity were performed. Thirty minutes after dihydralazine administration, mean arterial pressure fell by 15%, whereas plasma angiotensin II, muscle sympathetic nerve activity, heart rate, and total body norepinephrine spillover increased (P<0.05 for all). In contrast, after enalaprilat administration a fall in arterial pressure similar to that for dihydralazine was followed by decreased angiotensin II levels and unchanged muscle sympathetic nerve activity, heart rate, and total body norepinephrine spillover, whereas renal norepinephrine spillover increased by 44% (P<0.05). Acute blood pressure reduction by an angiotensin-converting enzyme inhibitor provokes a differentiated sympathetic response in patients with hypertension and renal artery stenosis, inasmuch that overall and muscle sympathetic reflex activation are blunted, whereas the reflex renal sympathetic response to blood pressure reduction is preserved.

The 'adrenaline hypothesis' of hypertension revisited: evidence for adrenaline release from the heart of patients with essential hypertension

OBJECTIVE

Whether adrenaline acts as a sympathetic nervous cotransmitter in humans and stimulates beta2-adrenoceptors to augment neuronal noradrenaline release remains a subject of considerable dispute. The aim of this study was to test if adrenaline is released from regional sympathetic nerves (in the heart) in patients with essential hypertension, and to investigate whether locally released adrenaline might enhance cardiac noradrenaline release.

METHODS

Using dual isotope dilution methodology, adrenaline and noradrenaline plasma kinetics was measured for the whole body and in the heart in 13 untreated patients with essential hypertension and 27 healthy volunteers. All research participants underwent cardiac catheterization under resting conditions.

RESULTS

At rest, there was negligible adrenaline release from the sympathetic nerves of the heart in healthy subjects, 0.27 +/- 1.62 ng/min. In contrast, in patients with essential hypertension, adrenaline was released from the heart at a rate of 1.46 +/- 1.73 ng/min, equivalent on a molar basis to approximately 5% of the associated cardiac noradrenaline spillover value. Cardiac noradrenaline spillover was higher in hypertensive patients, 24.9 +/- 17.0 ng/min compared to 15.4 +/- 11.7 ng/min in healthy volunteers (P< 0.05). Among patients, rates of cardiac adrenaline and noradrenaline spillover correlated directly (r= 0.59, P< 0.05).

CONCLUSIONS

This study, in demonstrating release of adrenaline from the heart in patients with essential hypertension, and in disclosing a proportionality between rates of cardiac adrenaline and noradrenaline release, provides perhaps the most direct evidence to date in support of the 'adrenaline hypothesis' of essential hypertension.

Endogenous renin and related short-term blood pressure variability in the conscious rat

This study was designed to investigate, by use of spectral analysis, the blood pressure variability changes induced in the conscious rat by activation of plasmatic renin activity. Rats were surgically prepared with a supra-renal catheter inserted via the left carotid artery to perform the infusions, and with a femoral artery catheter to measure blood pressure and heart rate. Secretion of renin was induced using beta-adrenoceptor stimulation produced by isoprenaline. A first group (n=8) was infused with isoprenaline: 0.003, 10, 100 and 300 ng/kg/min, at a rate of 20 microl/min. A second group (n=8) was given a bolus injection of the angiotensin AT(1) receptor antagonist, valsartan (2 mg/kg, i.a.), prior to isoprenaline infusions. The lack of effect of infusion per se was checked in additional animals (n=8) infused with saline only (20 microl/min). Five other groups of animals were prepared with arterial catheters as mentioned previously. Each group received one concentration of infused isoprenaline and samples of blood were collected for further determinations of plasma renin activity and catecholamine concentrations. Blood pressure recordings were analysed using the fast Fourier transform on 2048 points time series (204.8 s). Isoprenaline increased plasma renin activity and did not modify plasma catecholamine concentrations. The low-frequency (0.02-0.2 Hz) component of the systolic blood pressure variability was amplified by isoprenaline (10 ng/kg/min isoprenaline: 4.16+/-0.62 mm Hg(2) vs. 2.90+/-0.44 mm Hg(2) for control value, P<0.05), a concentration that did not alter either blood pressure or heart rate levels. Isoprenaline lowered blood pressure and increased heart rate, starting at concentrations of 100 ng/kg/min. Valsartan, whose principal effect was generation of tachycardia (+25 bpm) modified neither blood pressure levels nor blood pressure variability. Valsartan prevented the amplification of the low-frequency oscillations of systolic blood pressure induced by isoprenaline (10 ng/kg/min isoprenaline: 2.53+/-0.38 mm Hg(2) vs. 2.20+/-0.25 mm Hg(2) for control value (valsartan, ns). We conclude that a moderate increase of plasma renin activity enhanced systolic blood pressure variability in the low-frequency range, without affecting blood pressure and heart rate levels.

Increased sympathetic nervous activity in patients with nontraumatic subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Activation of the sympathetic nervous system, which leads to elevation of circulating catecholamines, is implicated in the genesis of cerebral vasospasm and cardiac aberrations after subarachnoid hemorrhage. To this juncture, sympathetic nervous testing has relied on indirect methods only.

METHODS

We used an isotope dilution technique to estimate the magnitude and time course of sympathoadrenal activation in 18 subarachnoid patients.

RESULTS

Compared with 2 different control groups, the patients with subarachnoid hemorrhage exhibited an approximately 3-fold increase in total-body norepinephrine spillover into plasma within 48 hours after insult (3.2+/-0.3 and 4.2+/-0.7 versus 10.2+/-1.4 nmol/L; P<0.05 versus both). This sympathetic activation persisted throughout the 7- to 10-day examination period and was normalized at the 6-month follow-up visit.

CONCLUSIONS

The present study has established that massive sympathetic nervous activation occurs in patients after subarachnoid hemorrhage. This overactivation may relate to the well-known cardiac complications described in subarachnoid hemorrhage.

Effect of blood flow and muscle contraction on noradrenaline spillover in the canine gracilis muscle

Many authors have reported that, during exercise, noradrenaline spillover increases and fractional extraction decreases. It has been suggested that the increase in blood flow to active muscles may contribute to these effects. Muscle contraction also causes changes in many factors that may affect noradrenaline spillover and fractional extraction. In this experiment, we studied the effect of muscle contraction and blood flow on noradrenaline and adrenaline spillover and fractional extraction in the in situ canine gracilis muscle. The low intensity stimulation protocol enabled us to have muscle contractions without any effect on the local concentration of noradrenaline, as measured by microdialysis, and noradrenaline spillover. Fractional extraction of both noradrenaline and adrenaline was unaffected by increasing blood flow three and four times its resting value. In addition, noradrenaline spillover was increased by the higher blood flow, from 188 to 452 pg·min-1 at rest and from 246 to 880 pg·min-1 during stimulation. Stimulation of muscle contraction caused a significant increase in fractional extraction of noradrenaline and a nonsignificant increase in adrenaline extraction. In addition, an adrenaline spillover was observed in certain conditions. In light of our results, it seems that blood flow may not be the main factor decreasing fractional extraction of noradrenaline during exercise. However, blood flow could contribute to the increase in noradrenaline spillover observed in the active muscles during exercise.Key words: skeletal muscle, spillover, fractional extraction, stimulation, adrenaline.

Effect of intrauterine growth restriction on blood pressure, glucose tolerance and sympathetic nervous system activity in the rat at 3-4 months of age

OBJECTIVE

Epidemiological studies suggest that intrauterine growth restriction (IUGR) due to maternal undernutrition during pregnancy represents a major risk factor for hypertension and diabetes in adult age. However, placental insufficiency, rather than maternal malnutrition, is the main cause of IUGR in the Western world. We therefore studied the relationship between birth weight and adult blood pressure and glucose tolerance in an established animal model of placental insufficiency.

DESIGN

IUGR was induced by uterine artery ligation in pregnant rats and the offspring were studied at 3-4 months of age.

METHODS

In one subgroup of animals (n = 41, birth weight range 3.2-6.6 g) blood pressure was recorded over 72 h using telemetry and hypothalamic tissue levels of noradrenaline was measured. In another subgroup (n = 30, birth weight range 3.0-6.8 g) the activity of the sympathetic nervous system (SNS) was assessed by noradrenaline isotope dilution techniques and glucose tolerance determined by an intravenous glucose load.

RESULTS

Adult blood pressure was independent of birth weight Haemodynamic responses of IUGR rats to moderate sound stress was unaltered. In male rats neither SNS activity, hypothalamic noradrenaline concentrations nor glucose tolerance was associated with birth weight In contrast, IUGR in female rats was associated with increased SNS activity, elevated fasting blood glucose as well as lower insulin and higher glucose levels in response to a glucose load.

CONCLUSION

IUGR is not linked to an elevated blood pressure at 3-4 months of age in this model. However, in female rats, IUGR is associated with increased SNS activity and impaired glucose tolerance in adult life.

The effect of histamine receptor antagonists on stress-induced catecholamine secretion: an adrenomedullary microdialysis study in the rat

The effects of pretreatment with selective histamine receptor antagonists on changes in sympathoadrenal activity and haemodynamics, induced by 60-min immobilization stress, were studied in conscious rats. Using adrenomedullary microdialysis, it was shown that ranitidine (5 mg/kg, i.v.), a histamine H2 receptor antagonist, selectively suppressed stress-stimulated noradrenaline secretion without affecting adrenaline response, whereas triprolidine (10 mg/kg, i.v.), a histamine H1 receptor antagonist, had little effect on stress-induced secretion of both catecholamines. Neither triprolidine nor ranitidine changed the pressor response to 60-min stress. The stress-induced increase in heart rate was not altered by triprolidine, whereas ranitidine reduced it after 30 min of stress. To test whether the anti-secretory effect of ranitidine could be of peripheral origin, in a separate experimental series, a local catecholamine secretion was stimulated by histamine (0.5 mM) perfused through the adrenomedullary dialysis probe. It appeared that triprolidine, but not ranitidine, reduced this effect of histamine. Thus, the present results suggest that during stress, the activity of the central histaminergic system, via histamine H2-receptors, may selectively modulate noradrenaline secretion by the adrenal gland.

Neural mechanisms in human obesity-related hypertension

OBJECTIVE

Two hypotheses concerning mechanisms of weight gain and of blood pressure elevation in obesity were tested. The first hypothesis is that in human obesity sympathetic nervous system underactivity is present, as a metabolic basis for the obesity. The second hypothesis, attributable to Landsberg, is that sympathetic nervous activation occurs with chronic overeating, elevating blood pressure. These are not mutually exclusive hypotheses, since obesity is a heterogeneous disorder.

DESIGN AND METHODS

Whole body and regional sympathetic nervous system activity, in the kidneys and heart, was measured at rest using noradrenaline isotope dilution methodology in a total of 86 research voluteers in four different subject groups, in lean and in obese people who either did, or did not, have high blood pressure.

RESULTS

In the lean hypertensive patients, noradrenaline spillover for the whole body, and from the heart and kidneys was substantially higher than in the healthy lean volunteers. In normotensive obesity, the whole body noradrenaline spillover rate was normal, mean renal noradrenaline spillover was elevated (twice normal), and cardiac noradrenaline spillover reduced by approximately 50%. In obesity-related hypertension, there was elevation of renal noradrenaline spillover, comparable to that present in normotensive obese individuals but not accompanied by suppression of cardiac noradrenaline spillover, which was more than double that of normotensive obese individuals (P<0.05), and 25% higher than in healthy volunteers. There was a parallel elevation of heart rate in hypertensive obese individuals.

CONCLUSIONS

The sympathetic underactivity hypothesis of obesity causation now looks untenable, as based on measures of noradrenaline spillover, sympathetic nervous system activity was normal for the whole body and increased for the kidneys; the low sympathetic activity in the heart would have only a trifling impact on total energy balance. The increase in renal sympathetic activity in obesity may possibly be a necessary cause for the development of hypertension in obese individuals, although clearly not a sufficient cause, being present in both normotensive and hypertensive obese individuals. The discriminating feature of obesity-related hypertension was an absence of the suppression of the cardiac sympathetic outflow seen in normotensive obese individuals. Sympathetic nervous changes in obesity-related hypertension conformed rather closely to those expected from the Landsberg hypothesis.

Antiadrenergic effect of chronic amiodarone therapy in human heart failure

OBJECTIVES

The aim of the present study was to evaluate the influence of amiodarone on neurochemical parameters of sympathetic nervous activity in patients with congestive heart failure.

BACKGROUND

Unlike most antiarrhythmic agents, amiodarone has been shown to exert a beneficial effect on survival in some studies of patients with congestive heart failure. The pharmacology of this agent is complex, and as such, the mode of its action is unclear in humans. Some experimental studies suggest that amiodarone exerts a sympatholytic effect.

METHODS

To evaluate the effect of amiodarone on sympathetic nervous activity, we measured the total systemic and cardiac norepinephrine (NE) spillover rate by isotope dilution in 58 patients with severe heart failure (left ventricular ejection fraction 20 +/- 1%), 22 of whom were receiving chronic amiodarone treatment. Release rates for dihydroxyphenylalanine (DOPA, a precursor of NE), and endogenous and radiolabeled dihydroxyphenylglycol (DHPG and 3H-DHPG, intraneuronal metabolites of NE and 3H-NE, respectively) were also determined to assess sympathetic neuronal integrity.

RESULTS

Amiodarone-treated patients had significantly lower cardiac spillover rates for NE (42%, p = 0.001), DOPA (74%, p < 0.001), DHPG (44%, p < 0.01) and 3H-DHPG (51%, p < 0.01) than those patients not treated with amiodarone. Hemodynamic assessment of amiodarone-treated patients revealed higher cardiac output (4.4 +/- 0.2 vs. 3.7 +/- 0.2 liters/min, p < 0.01), and slightly lower pulmonary capillary wedge pressure (18 +/- 2 vs. 22 +/- 1, p = NS) than in untreated patients. After correction for the potential confounding effect of hemodynamic differences, amiodarone-treated patients continued to demonstrate significantly lower spillover rates of NE, DOPA and DHPG from the heart.

CONCLUSIONS

These data indicate that amiodarone may exert beneficial effects on the failing human heart through a sympatholytic process, and this action appears to be relatively cardioselective.

The effects of physical treatment on induced fever in humans

PURPOSE

Initial treatments for fever include the amelioration of underlying causes and administration of antipyretic medications. However, patients who fail these treatments are often actively cooled, which may be counterproductive because decreasing skin temperature increases the thermoregulatory core target temperature. Cooling may also provoke metabolic and autonomic stress and thermal discomfort.

SUBJECTS AND METHODS

We studied 9 subjects, each on 3 days. Fever was induced each day with 100,000 IU/kg of interleukin-2 administered intravenously (elapsed time zero). Randomly assigned treatments were 1) control (a cotton blanket), 2) cooling (forced air at 15 degrees C), or 3) self-adjust (forced-air warming adjusted to comfort). Treatments were maintained for 3 to 8 elapsed hours.

RESULTS

Peak core temperatures (mean +/- SD) were 38.4 +/- 0.5 degrees C on the control day, 38.1 +/- 0.5 degrees C on the cooling day, and 38.5 +/- 0.4 degrees C on the self-adjust day. Integrated core temperatures were 6.0 +/- 1.6 degrees C x h on the control day, 5.7 +/- 2.2 degrees C x h on the cooling day, and 6.4 +/- 1.2 degrees C x h on the self-adjust day. Neither peak nor integrated core temperatures differed significantly on the 3 days. Shivering was common on the cooling day but otherwise rare. Oxygen consumption was normal on the control and self-adjust days but increased 35% to 40% during cooling (P = 0.0001). Mean arterial pressure and plasma norepinephrine and epinephrine concentrations were significantly greater during cooling (P <0.05). On a self-reported thermal comfort scale, the subjects were miserable during cooling and significantly more comfortable on the self-adjust than control day (P <0.05).

CONCLUSION

We conclude that active cooling should be avoided in unsedated patients with moderate fever, because it does not reduce core temperature but does increase metabolic rate, activate the autonomic nervous system, and provoke thermal discomfort.