Total, and organ-specific, noradrenaline plasma kinetics in essential hypertension

EXPRESS: Cardiac Sympathetic Dysfunction in Pulmonary Arterial Hypertension: Lesson from Left-sided Heart Failure

Sympathetic nervous system hyperactivity has a well-recognized role in the pathophysiology of heart failure with reduced left ventricular ejection fraction. Alterations in sympathetic nervous system have been related to the pathophysiology of pulmonary arterial hypertension, but it is unclear whether cardiac sympathetic nervous system is impaired and how sympathetic dysfunction correlates with hemodynamics and clinical status in pulmonary arterial hypertension patients. The aim of this study was to evaluate the cardiac sympathetic nervous system activity by means of 123Iodine-metaiodobenzylguanidine nuclear imaging in pulmonary arterial hypertension patients and to explore its possible correlation with markers of disease severity. Twelve consecutive pulmonary arterial hypertension patients (nine women, median age 56.5 (17.8), eight idiopathic and four connective tissue-associated pulmonary arterial hypertension) underwent cardiac 123Iodine-metaiodobenzylguanidine scintigraphy. The results were compared with those of 12 subjects with a negative history of cardiovascular or pulmonary disease who underwent the same nuclear imaging test because of a suspected paraganglioma or pheochromocytoma, with a negative result (controls), and 12 patients with heart failure with reduced left ventricular ejection fraction. Hemodynamics, echocardiography, six-minute walking distance, cardiopulmonary exercise testing, and N-terminal pro brain natriuretic peptide were collected in pulmonary arterial hypertension patients within one week from 123Iodine-metaiodobenzylguanidine scintigraphy. Cardiac 123Iodine-metaiodobenzylguanidine uptake, assessed as early and late heart-to-mediastinum ratio, was significantly lower in pulmonary arterial hypertension compared to controls (p = 0.001), but similar to heart failure with reduced left ventricular ejection fraction. Myocardial 123Iodine-metaiodobenzylguanidine turnover, expressed as washout rate, was similar in pulmonary arterial hypertension and heart failure with reduced left ventricular ejection fraction and significantly higher compared to controls (p = 0.016). In the pulmonary arterial hypertension group, both early and late heart-to-mediastinum ratios and washout rate correlated with parameters of pulmonary arterial hypertension severity including pulmonary vascular resistance, right atrial pressure, tricuspid annular plane systolic excursion, N-terminal pro brain natriuretic peptide, and peak VO2. Although we evaluated a small number of subjects, our study showed a significant impairment in cardiac sympathetic nervous system in pulmonary arterial hypertension, similarly to that observed in heart failure with reduced left ventricular ejection fraction. This impairment correlated with indices of pulmonary arterial hypertension severity. Cardiac sympathetic dysfunction may be a contributing factor to the development of right-sided heart failure in pulmonary arterial hypertension.

Impacts of Surgically Performed Renal Denervation on the Cardiovascular and Electrophysiological Variables in the Chronic Atrioventricular Block Dogs - Comparison With Those of Amiodarone Treatment

BACKGROUND

In order to begin to precisely clarify the impact of renal denervation on the blood pressure, atrial fibrillation and ventricular tachyarrhythmias, in addition to proarrhythmic potential, its cardiovascular effects were assessed by using the chronic complete atrioventricular block dogs.

METHODS AND RESULTS

Cardiohemodynamic and electrophysiological effects, together with neurohumoral factors and/or electrolytes, were assessed before and 4 weeks after either renal denervation (n=5) or amiodarone treatment (n=6). Amiodarone hydrochloride was given orally to the animals every day in a dose of 200 mg/day for the first 7 days followed by 100 mg/day for the following 21 days. The renal denervation decreased the systolic pressure, idioventricular rate, prolonged ventricular effective refractory period, and slightly suppressed the adrenergic tone and the renin-angiotensin-aldosterone system, but hardly affected the atrial effective refractory period and terminal repolarization period. Amiodarone prolonged the atrial effective refractory period, whereas no significant change was detected in the other variables.

CONCLUSIONS

Surgically performed renal denervation may possess the anti-ventricular tachyarrhythmic rather than anti-atrial fibrillatory potentials, and it also modestly decreased the blood pressure. Thus, currently obtained information may be used as guidance for better understanding the utility and limitation of renal denervation against various types of cardiovascular diseases. (Circ J 2016; 80: 1556-1563).

Sympathoneural and adrenomedullary responses to mental stress

This concept-based review provides historical perspectives and updates about sympathetic noradrenergic and sympathetic adrenergic responses to mental stress. The topic of this review has incited perennial debate, because of disagreements over definitions, controversial inferences, and limited availability of relevant measurement tools. The discussion begins appropriately with Cannon's "homeostasis" and his pioneering work in the area. This is followed by mental stress as a scientific idea and the relatively new notions of allostasis and allostatic load. Experimental models of mental stress in rodents and humans are discussed, with particular attention to ethical constraints in humans. Sections follow on sympathoneural responses to mental stress, reactivity of catecholamine systems, clinical pathophysiologic states, and the cardiovascular reactivity hypothesis. Future advancement of the field will require integrative approaches and coordinated efforts between physiologists and psychologists on this interdisciplinary topic.

Renal sympathetic nerve ablation for treatment-resistant hypertension

Hypertension is a major risk factor for increased cardiovascular events with accelerated sympathetic nerve activity implicated in the pathogenesis and progression of disease. Blood pressure is not adequately controlled in many patients, despite the availability of effective pharmacotherapy. Novel procedure- as well as device-based strategies, such as percutaneous renal sympathetic nerve denervation, have been developed to improve blood pressure in these refractory patients. Renal sympathetic denervation not only reduces blood pressure but also renal as well as systemic sympathetic nerve activity in such patients. The reduction in blood pressure appears to be sustained over 3 years after the procedure, which suggests absence of re-innervation of renal sympathetic nerves. Safety appears to be adequate. This approach may also have potential in other disorders associated with enhanced sympathetic nerve activity such as congestive heart failure, chronic kidney disease and metabolic syndrome. This review will focus on the current status of percutaneous renal sympathetic nerve denervation, clinical efficacy and safety outcomes and prospects beyond refractory hypertension.

The role of renal denervation for the treatment of resistant hypertension

Hypertension affects more than one-fourth of the adult population worldwide and is a major risk factor for cardiovascular and kidney disease. Currently, the majority of patients with hypertension do not reach goal blood pressure (BP) targets, and cardiovascular risk is increased further for patients with treatment-resistant hypertension, defined as office BP above goal despite pharmacological treatment with three or more antihypertensive medications at optimal doses including a diuretic. Although missed diagnosis of secondary forms of hypertension, physician inertia and non-adherence with prescribed medication are important contributors to the phenomenon of resistant hypertension that need to be addressed, there is a need for alternative therapeutic approaches. Renal sympathetic denervation is a minimally invasive endovascular procedure that disrupts renal efferent and afferent neural connections, both of which are important regulators of BP control. Limited data from recent clinical trials indicate that this approach is safe and effectively lowers BP in patients with treatment-resistant hypertension. Accumulating data is emerging to suggest that renal sympathetic denervation may also have utility beyond treatment-resistant hypertension. This review aims to briefly summarize the existing evidence for the use of renal denervation (RDN) in patients with treatment-resistant hypertension and to explore the potential utility of RDN in other pathological states associated with sympathetic dysfunction.

Catheter-based renal denervation for resistant hypertension: rationale and design of the SYMPLICITY HTN-3 Trial

Hypertension represents a significant global public health concern, contributing to vascular and renal morbidity, cardiovascular mortality, and economic burden. The opportunity to influence clinical outcomes through hypertension management is therefore paramount. Despite adherence to multiple available medical therapies, a significant proportion of patients have persistent blood pressure elevation, a condition termed resistant hypertension. Recent recognition of the importance of the renal sympathetic and somatic nerves in modulating blood pressure and the development of a novel procedure that selectively removes these contributors to resistant hypertension represents an opportunity to provide clinically meaningful benefit across wide and varied patient populations. Early clinical evaluation with catheter-based, selective renal sympathetic denervation in patients with resistant hypertension has mechanistically correlated sympathetic efferent denervation with decreased renal norepinephrine spillover and renin activity, increased renal plasma flow, and has demonstrated clinically significant, sustained reductions in blood pressure. The SYMPLICITY HTN-3 Trial is a pivotal study designed as a prospective, randomized, masked procedure, single-blind trial evaluating the safety and effectiveness of catheter-based bilateral renal denervation for the treatment of uncontrolled hypertension despite compliance with at least 3 antihypertensive medications of different classes (at least one of which is a diuretic) at maximal tolerable doses. The primary effectiveness endpoint is measured as the change in office-based systolic blood pressure from baseline to 6 months. This manuscript describes the design and methodology of a regulatory trial of selective renal denervation for the treatment of hypertension among patients who have failed pharmacologic therapy.

Cardiac sympathetic activation in patients with pulmonary arterial hypertension

Patients with congestive heart failure (CHF) due to left ventricular (LV) dysfunction have sympathetic activation specifically directed to the myocardium. Although pulmonary arterial hypertension (PAH) is associated with increased systemic sympathetic activity, its impact on sympathetic drive to ventricular myocardium is unknown. Fifteen patients with PAH (9 women; 54 ± 12 years) were studied: 10 with idiopathic PAH and 5 with a connective tissue disorder. We measured hemodynamics, as well as radiolabeled and endogenous concentrations of arterial and coronary sinus norepinephrine (NE). These measures were repeated after inhaled nitric oxide (NO). Measurement of transcardiac NE concentrations and the cardiac extraction of radiolabeled NE allowed calculation of the corrected transcardiac gradient of NE (CTCG of NE). Comparative data were collected from 15 patients (9 women: 55 ± 12 yr) with normal LV function and 15 patients with CHF (10 women; 53 ± 12 yr). PAH patients had elevated arterial NE concentrations compared with those with normal LV function but were similar to those with CHF. The CTCG of NE was higher in those with PAH than in the normal LV group (3.6 ± 2.2 vs. 1.5 ± 0.9 pmol/ml; P < 0.01) but similar to that seen in those with CHF (3.3 ± 1.4; P = NS). Inhaled NO, which reduced pulmonary artery pressure and increased cardiac output, had no effect on cardiac sympathetic activity. Therefore, cardiac sympathetic activation occurs in PAH. The mechanism of this activation remains uncertain but does not involve elevations in left heart filling pressure.

Adrenergic mechanisms do not contribute to age-related decreases in calf venous compliance

Limb venous compliance decreases with advancing age, even in healthy humans. To test the hypothesis that adrenergic mechanisms contribute to age-associated reductions in limb venous compliance, we measured calf venous compliance before and during acute systemic α- and β-adrenergic blockade in eight young (27 ± 1 yr old, mean ± SE) and eight older healthy men (67 ± 2 yr old). Calf venous compliance was determined in supine subjects by inflating a thigh-collecting cuff to 60 mmHg for 8 min and then decreasing it (1 mmHg/s) to 0 mmHg while calf volume was indexed with a strain gauge. The slope (·10⁻³) of the pressure-compliance relation (compliance= β₁ + 2·β₂·cuff pressure), which is the first derivative of the quadratic pressure-volume relation [(Δlimb volume) = β₀+ β₁·(cuff pressure) + β₂·(cuff pressure)²] during reductions in cuff pressure, was used to quantify calf venous compliance. Calf venous compliance was ∼30% lower (P < 0.01) in older compared with young men before adrenergic blockade. In response to adrenergic blockade calf venous compliance did not increase in young (-2.62 ± 0.14 and -2.29 ± 0.18 ml·dl⁻¹·mmHg⁻¹, before and during blockade, respectively) or older men (-1.78 ± 0.27 and -1.68 ± 0.21 ml·dl⁻¹ ·mmHg⁻¹). Moreover, during adrenergic blockade differences in calf venous compliance between young and older men observed before adrenergic blockade persisted. Collectively, these data strongly suggest that adrenergic mechanisms neither directly restrain calf venous compliance in young or older men nor do they contribute to age-associated reductions in calf venous compliance in healthy men.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Whole body norepinephrine kinetics in ANG II-salt hypertension in the rat

The purpose of this study was to investigate total body norepinephrine (NE) kinetics as an index of global sympathetic nervous system (SNS) outflow in a rat model of chronic ANG II-salt hypertension. Male Sprague-Dawley rats fed a 0.4% (normal salt, NS) or 2% (HS) NaCl diet were instrumented with arterial and venous catheters. After 5 days of recovery and a 3-day control period, ANG II (150 ng.kg(-1).min(-1)) was given subcutaneously by minipump for 14 days. Plasma NE levels and total body NE spillover and clearance were determined on control day 3 and ANG II infusion days 7 and 14 using radioisotope dilution principles. To perform this analysis, 3H-NE and NE were measured in arterial plasma after a 90-min infusion of tracer amounts of 3H-NE. Mean arterial pressure (MAP) was similar during the control period in NS and HS rats; however, MAP increased to a higher level in HS rats. During the control period, plasma NE tended to be lower in rats on HS, whereas NE clearance tended to be higher in HS rats. As a result NE spillover was similar in NS and HS rats during the control period. In NS rats, plasma NE, NE spillover, and NE clearance were unchanged by ANG II. In contrast, in rats on the HS diet, plasma NE and NE spillover increased during ANG II infusion, whereas NE clearance was unchanged. In conclusion, a HS diet alone or ANG II infusion in animals fed NS do not affect global sympathetic outflow. However, the additional hypertensive response to ANG II in animals fed HS is accompanied by SNS activation.

Hypertension caused by prenatal testosterone excess in female sheep

Polycystic ovary syndrome (PCOS), a leading cause of infertility, affects approximately 10% of women of reproductive age. The etiology and pathophysiology of PCOS are poorly understood. PCOS is multifaceted and includes reproductive abnormalities and components of the metabolic syndrome such as insulin resistance, obesity, dyslipidemia, and hypertension. Exposure to excess testosterone (T) during the prenatal period may predispose individuals to PCOS phenotype. The goal of this study was to determine whether hypertension and dyslipidemia occur in a well-characterized model of PCOS produced by prenatal treatment of sheep with T. Radiotelemetry was used to measure blood pressure over a 24-h period in conscious, undisturbed animals. To normalize circulating estradiol levels across treatment, control (n = 4) and prenatal T-treated (100 mg T propionate im twice weekly from days 30 to 90 of fetal life, n = 4) 2-yr-old females were ovariectomized, instrumented with a radiotelemetry transmitter, and clamped with early follicular phase levels of estrogen using an implant. Six days later, a 24-h recording period commenced. Prenatal T-treated sheep were hypertensive compared with control sheep, and heart rate tended to be higher. T-treated sheep had hyperglycemia, insulin resistance, hypernatremia, and hyperchloremia, and both total and LDL cholesterol tended to be higher. Plasma aldosterone and epinephrine were significantly lower in T-treated sheep, whereas norepinephrine was unchanged. This first-ever use of radiotelemetric blood pressure recordings in sheep demonstrates that mild hypertension, a risk factor reported in some women with PCOS, is also a feature of the sheep model of PCOS produced by prenatal T treatment.

Regional sympathetic activity in pre-hypertensive phase of spontaneously hypertensive rats

Imbalances in central and peripheral sympathetic nervous system (SNS) activity have been observed in essential and experimental hypertension. This study was carried out in order to evaluate SNS activity in two distinct tissue types of spontaneously hypertensive rats (SHR), compared to Wistar-Kyoto normotensive (WKY) rats, in the pre-hypertensive phase (4-5 weeks of age). Interstitial concentrations of norepinephrine (NE) and other catecholamines were measured by microdialysis in striated muscle, whose tone is controlled by baroreflexes, and in the subcutaneous adipose tissue where sympathetic output mainly controls metabolism. Two groups of SHR and WKY male rats were studied, aged 4-5 weeks, with a mean body weight of 92 and 86 g respectively. Systolic blood pressure (SBP, tail-cuff) values were 113 mm Hg (SD +/- 6.2) in SHR and 108 mm Hg (SD +/- 7.3) in WKY rats (p=0.28,t test). Two microdialysis probes were positioned in the subcutaneous fatty tissue and in the striated muscle of the parascapular region and perfused with Ringers' solution. The dialysate was collected every thirty minutes for 3 hours and analyzed in HPLC-ED to determine the content of NE and other catecholamines. Interstitial levels of NE were higher in SH than in WKY rats in both tissues. Mean NE values from subcutaneous adipose tissue in 4-5 week-old SHR were 1162 +/- 193 pg/ml compared to 496 + 188 pg/ml in WKY rats (p<0.001, t test). Muscle tissue NE levels in SHR were 1241 +/- 337 pg/ml vs. 521 +/- 138 pg/ml in WKY rats (p<0.001, t test). Plasma NE concentrations (279 +/- 61 pg/ml in SHR vs 246 + 69 pg/ml in WKY P = 0.65, t test) were not significantly different between the two strains at this young age. These findings suggest SNS hyperactivity in young SHR, though still normotensive, possibly dissociated from regional components of regulation (baroreceptor control in striated muscle and metabolic control in subcutaneous adipose tissue).

Responses of serum insulin and blood pressure to cold and handgrip in obese patients

A close correlation between body weight and blood pressure has been frequently observed in both clinical and epidemiological studies. The aim of this clinical trial was to evaluate whether, in obese patients, there is any relationship between blood pressure, at rest or during sympathetic stimulation, and blood glucose and serum insulin, both while fasting and during an oral glucose challenge. Twenty obese patients (age 26-65 years, body weight 97 +/- 16 kg, 11 normotensive and 9 hypertensive) entered the study. After a 4-week run-in period on an isocaloric diet with normal intake of sodium, blood pressure and heart rate were measured at rest and during sympathetic stimulation induced by cold and isometric testing. Responses of glucose and insulin to a standardized 75 g oral glucose tolerance test were also evaluated. The responses of glucose and insulin to glucose challenge were not statistically different in normotensive and hypertensive obese patients. Levels of insulin in the serum in the serum in the fasting state and during glucose load were significantly correlated with the response of blood pressure to cold and isometric exercise, but not to blood pressure at rest. The response of heart rate to cold was closely related to insulin only in the subgroup of normotensives. The present findings support the hypothesis that the sympathetic nervous system, which influences secretion of insulin and regulation of blood pressure, is involved in the pathophysiology of the association of obesity and hypertension.

Neuroendocrine and beta-adrenoceptor response to chronic ethanol and aggression in rats

Male rats were administered either ethanol (6-8 g/kg/day) or dextrin-maltose, an isocaloric equivalent, for two weeks prior to a 24-hour resident-intruder test. After the first 20 minutes of the aggression test residents showed a greater increase in norepinephrine than intruders (216% vs. 97%), while intruders showed a greater increase in epinephrine (394% vs. 51%) and corticosterone (338% vs. 129%) than residents. Ethanol administration increased the initial epinephrine response of intruders almost two-fold compared to dextrin-maltose intruders. After 24 hours of aggression testing plasma norepinephrine was still elevated in residents (92%) and intruders (71%), however, only intruders continued to show an elevation in plasma corticosterone (98%) and epinephrine (107%). Using a cumulative dose-response technique, the dose of isoproterenol required to produce 50% of the maximal heart rate response (ED50) increased in intruders by 108% following aggression testing with ethanol intruders showing significantly smaller mean change. The increase in ED50 was related to drug type, behavior, and plasma corticosterone and epinephrine levels. Rats treated with ethanol had a greater beta-adrenoceptor density than control rats. However, no relationship was found between receptor density and the other measures in this study.

Renal sympathetic responses to changes in arterial pressure in conscious rabbits

Renal sympathetic nerve activity (RSNA) at rest and in response to changes in mean arterial pressure (MAP) were examined by measuring norepinephrine (NE) spillover rate across the kidney in 14 conscious rabbits. NE spillover rate was calculated from veno-arterial difference in plasma NE concentrations across the kidney corrected by the fractional extraction of infused tritiated NE. Renal NE spillover rate (mean +/- S.E.M.) at rest in 14 conscious rabbits was 14.7 +/- 0.9 ng/min. During sodium nitroprusside infusions at 10 and 20 micrograms/kg/min, renal NE spillover rate significantly increased by 55 and 101% in response to falls in MAP of 15 and 19% respectively. During phenylephrine infusion at 2 micrograms/kg/min, renal NE spillover rate significantly decreased by 40% in response to a 14% rise in MAP. The relative contribution of renal to total NE spillover rate decreased during sympathetic stimulation, while this ratio was unchanged during sympathetic inhibition. This study demonstrates that the renal NE spillover method is sufficiently sensitive and reliable to detect the responses in RSNA to physiological stimuli in conscious rabbits. The difference observed in the degree of stimulation of renal versus total NE spillover rate during hypotension produces confirmation that sympathetic responses are not uniformly distributed in the body.

Direct evidence from intraneural recordings for increased central sympathetic outflow in patients with heart failure

Patients with heart failure have increased vascular resistance and evidence for increased neurohumoral drive. High levels of circulating norepinephrine are found in patients with heart failure, but it is not known whether they reflect increased sympathetic neural activity or result from altered synthesis, release, or metabolism of norepinephrine. We used microneurography (peroneal nerve) to directly record sympathetic nerve activity to muscle (mSNA) and also measured plasma norepinephrine levels in patients with heart failure and in normal control subjects. Our goal was to determine whether sympathetic nerve activity is increased in patients with heart failure and whether plasma norepinephrine levels correlate with levels of mSNA in heart failure. Resting muscle sympathetic nerve activity in 16 patients with moderate to severe heart failure (54 +/- 5 bursts/min, mean +/- SE) was significantly higher (p less than .01) than the levels of activity in either nine age-matched normal control subjects (25 +/- 4 bursts/min) or 19 "young" normal control subjects (24 +/- 2 bursts/min). We found a significant correlation between plasma norepinephrine levels and mSNA (r = .73, p less than .05). Neither mSNA nor plasma norepinephrine levels correlated with total systemic vascular resistance, cardiac index, left ventricular ejection fraction, or heart rate. However, both mSNA and plasma norepinephrine levels showed significant positive correlations (p less than .05) with left ventricular filling pressures (r = .80, mSNA vs filling pressures; r = .82, norepinephrine levels vs filling pressures) and mean right atrial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Indices of sympathetic vascular innervation in sympathectomized patients

We measured arterial and venous plasma catecholamines and used laser-Doppler flowmetry to measure cutaneous microcirculatory flow in the sympathectomized and in the intact limbs of 3 patients who had undergone regional sympathectomies. Venous concentrations of norepinephrine, the sympathetic neurotransmitter, exceeded arterial concentrations in the intact limbs--a normal finding--but invariably were less than arterial in the sympathectomized limbs of the same patients, both during baseline conditions and during sympathetic stimulation using tilt, standing and the cold pressor test (mean arteriovenous decrement about 40%). Arterial epinephrine levels exceeded venous levels with or without sympathectomy. Skin microvascular flow rapidly decreased during the cold pressor test and the Valsalva maneuver in the intact but not in the sympathectomized limbs, and spontaneous flow oscillations occurred in the sympathectomized limbs. The results suggest that an arteriovenous increment in plasma norepinephrine reflects local release of norepinephrine from sympathetic nerve endings, whereas removal of circulating catecholamines can occur with or without sympathetic neural impulses. Laser-Doppler flowmetry can measure reflexive sympathetically mediated responses of skin microvascular flow and so can detect sympathetic denervation. Spontaneous oscillations in this flow may not depend exclusively on oscillations in the activity of the sympathetic microvascular innervation.

Measurement of regional neuronal removal of norepinephrine in man

We describe here and validate an in vivo technique to measure the regional proportionate removal of norepinephrine (NE) by neuronal uptake (Uptake1) in man. The measurement is based on the steady-state arterial and venous concentrations of tritiated NE and tritiated isoproterenol (ISO) during simultaneous infusion of both. The validity of this technique depends on the removal of circulating NE, but not of ISO, by sympathetic nerve endings and on there being no other factor contributing to the net difference in the plasma disappearance of these catecholamines. To test these hypotheses, we compared the removal of NE in the arm with that of ISO in 14 people and the effects of pretreatment with the specific inhibitor of Uptake1, desipramine, in 8 people. In all the subjects a greater percent of NE than of ISO was removed during passage of blood through the forearm (54 vs. 46%, P less than 0.0001). Pretreatment with desipramine decreased significantly the removal of NE to virtually exactly that of ISO. The difference in NE and ISO clearances by arm tissues was therefore completely accounted for by Uptake1. About 15% of infused NE which is removed in the arm is removed by Uptake1. The ability to measure regional Uptake1 should contribute to better understanding of the relationship between circulating levels of plasma NE and sympathetic neural activity and may allow detection of abnormalities of neuronal norepinephrine removal in clinical disease states.

Plasma noradrenaline kinetics in humans

Recently developed radiotracer methods for measuring the overall rate of release of noradrenaline to plasma, for the body as a whole, can be used to estimate 'total sympathetic nervous system activity' in humans. These techniques find application in clinical studies of sympathetic nervous physiology and pharmacology. The inherent weakness of any biochemical test of global sympathetic tone such as this lies in the fact that sympathetic nervous system responses typically show regional differentiation. Biochemical indices of overall sympathetic activity are insufficiently discriminating to delineate patterns of sympathetic nervous response, representing instead an algebraic sum of all regional increases or decreases in sympathetic tone. Modification of the whole-body radiotracer methodology enables organ-specific sympathetic nervous system activity to be estimated, from measurements of regional release of noradrenaline to plasma. This should facilitate investigation of possible sympathetic pathophysiology in disease states. Illustrative of potential application of the method are preliminary findings of increased renal sympathetic nervous tone in young patients with essential hypertension, and of selective activation of sympathetic nerves to the kidney by diuretics.