Age-related changes in muscle sympathetic nerve activity in essential hypertension

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.

How to assess sympathetic activity in humans

Sympathetic factors play a central role not only in cardiovascular homeostatic control but also in the pathogenesis and/or in the progression of several cardiovascular diseases, such as essential hypertension, myocardial infarction, cardiac arrhythmias and congestive heart failure. This explains why assessment of adrenergic neural function in humans has been, and certainly still remains, one of the major fields in cardiovascular research. The present paper will review in detail the haemodynamic, pharmacological, biochemical, neurophysiological, neurochemical and neural imaging techniques by which sympathetic activity is assessed in humans, highlighting the main advantages and limitations of each of them. Although plasma noradrenaline measurement represents a useful guide to assess sympathetic neural function, direct recording of sympathetic nerve traffic via microneurography and noradrenaline radiotracer methods have in recent years largely supplanted the plasma noradrenaline approach. This is because they allow (1) discrimination between the central or peripheral nature of increased plasma noradrenaline levels, and (2) precise estimation of the behaviour of regional sympathetic neural function both under physiological and pathological conditions. In contrast, the approach based on spectral analysis of heart rate and blood pressure signals has been shown to have important limitations which prevent the method from faithfully reflecting sympathetic cardiovascular drive. Neural imaging techniques, which require expensive technical support, allow direct visualization of sympathetic enervation of human organs, thus providing information on the 'in vivo' metabolism of noradrenaline in different cardiovascular districts. Although technical improvements have allowed a more precise assessment of human adrenergic function, no technique so far available can be viewed as a 'gold standard' with which the others might be compared. Limitations and disadvantages of the various techniques may be reduced if these methods are seen as being complementary and employed in combination, allowing more reliable information to be achieved on the sympathetic abnormalities characterizing cardiovascular diseases, and thus hopefully providing a stronger rationale for newer therapeutic approaches involving pharmacological modification of the sympathetic nervous system and adrenoreceptors.

Human sympathetic and vagal baroreflex responses to sequential nitroprusside and phenylephrine

We evaluated a method of baroreflex testing involving sequential intravenous bolus injections of nitroprusside followed by phenylephrine and phenylephrine followed by nitroprusside in 18 healthy men and women, and we drew inferences regarding human sympathetic and vagal baroreflex mechanisms. We recorded the electrocardiogram, photoplethysmographic finger arterial pressure, and peroneal nerve muscle sympathetic activity. We then contrasted least squares linear regression slopes derived from the depressor (nitroprusside) and pressor (phenylephrine) phases with 1) slopes derived from spontaneous fluctuations of systolic arterial pressures and R-R intervals, and 2) baroreflex gain derived from cross-spectral analyses of systolic pressures and R-R intervals. We calculated sympathetic baroreflex gain from integrated muscle sympathetic nerve activity and diastolic pressures. We found that vagal baroreflex slopes are less when arterial pressures are falling than when they are rising and that this hysteresis exists over pressure ranges both below and above baseline levels. Although pharmacological and spontaneous vagal baroreflex responses correlate closely, pharmacological baroreflex slopes tend to be lower than those derived from spontaneous fluctuations. Sympathetic baroreflex slopes are similar when arterial pressure is falling and rising; however, small pressure elevations above baseline silence sympathetic motoneurons. Vagal, but not sympathetic baroreflex gains vary inversely with subjects' ages and their baseline arterial pressures. There is no correlation between sympathetic and vagal baroreflex gains. We recommend repeated sequential nitroprusside followed by phenylephrine doses as a simple, efficientmeans to provoke and characterize human vagal and sympathetic baroreflex responses.

Sympathetic nervous system activity and alpha-adrenergic responsiveness in older hypertensive humans

We have previously demonstrated in normotensive humans an age-associated increase in sympathetic nervous system (SNS) activity combined with appropriate downregulation of alpha-adrenergic responsiveness. Impaired downregulation of alpha-adrenergic responsiveness, despite a comparable level of SNS activity, could contribute to higher blood pressure in older hypertensive humans. We measured arterial plasma norepinephrine (NE) levels and the extravascular NE release rate (NE2) derived from [3H]NE kinetics (to assess systemic SNS activity), and platelet and forearm arterial adrenergic responsiveness in 20 normotensive (N) and in 24 hypertensive (H), otherwise healthy, older subjects (60-75 yr). Although plasma NE levels were similar (N 357 +/- 27 vs. H 322 +/- 22 pg/ml; P = 0.37), NE2 tended to be greater in the hypertensive group (H 2.23 +/- 0.21 vs. N 1.64 +/- 0.20 microgram. min-1. m-2; P = 0. 11), and the NE metabolic clearance rate was greater (H 1,100 +/- 30 vs. N 900 +/- 50 ml/m2; P = 0.004). In the hypertensive group, there was a greater alpha-agonist-mediated inhibition of platelet membrane adenylyl cyclase activity and a NE- but not ANG II-mediated decrease in forearm blood flow. Compared with normotensive subjects, in older hypertensive subjects 1) NE metabolic clearance rate is increased, 2) systemic SNS activity tends to be increased, and 3) arterial and platelet alpha-adrenergic responsiveness is enhanced. These results suggest that heightened SNS activity coupled with enhanced alpha-adrenergic responsiveness may contribute to elevated blood pressure in older hypertensive humans.

Platelet alpha2-adrenoceptor alterations in patients with essential hypertension

AIMS

The purpose of this study was to determine whether human platelet alpha2-adrenoceptors were altered in essential hypertension. A systematic analysis was carried out on 165 normotensives and 124 untreated primary hypertensives.

METHODS

The study was performed at different levels: i) density and affinity of platelet alpha2-adrenoceptors were determined by receptor binding assays using the full alpha2-adrenoceptor agonist [3H]-UK 14304 and a thermodynamic analysis of data was carried out to evaluate if binding mechanisms at the molecular level were altered during hypertension; ii) the functionality of Gi proteins coupled to alpha2-adrenoceptors and iii) forskolin-stimulated cAMP levels were measured.

RESULTS

Platelet alpha2-adrenoceptors mean density (Bmax) and affinity (Kd) (+/-s.e.mean) were significantly lower and higher, respectively, in normotensive than in hypertensive subjects [Bmax=327+/-4 vs 435+/-5 fmol mg(-1) of protein (P<0.01) and Kd=3.76+/-10.05 vs 6.50+/-0.15 nM (P<0.01), respectively]. The 50% stimulating concentration of adrenaline on [35S]-GTPgammaS binding to Gi proteins was significantly (P<0.01) lower in normotensives (12+/-2 nM) than in hypertensives (110+/-10 nM). The 50% inhibiting concentration of adrenaline on forskolin-stimulated cAMP levels was significantly (P<0.01) lower in normotensive (22+/-2 nM) than in hypertensive subjects (200+/-25 nM).

CONCLUSIONS

Present analysis, including receptorial and functional data, provides evidence that marked alterations occur in platelet alpha2-adrenoceptors of hypertensive subjects.

I1-imidazoline agonist moxonidine decreases sympathetic nerve activity and blood pressure in hypertensives

Moxonidine is an I1-imidazoline receptor agonist that reduces blood pressure in hypertensives. Experimental data suggest that moxonidine inhibits central sympathetic activity. However, whether such a mechanism is involved in vivo in humans is still unclear. We investigated the effects of 0.4 mg moxonidine orally on muscle sympathetic nerve activity and heart rate in an open study in 8 healthy volunteers. Furthermore, we studied the effects of 0.4 mg moxonidine on muscle sympathetic nerve activity, heart rate, blood pressure, 24-hour blood pressure profile, and hormone plasma levels in 25 untreated hypertensives in a double-blind, placebo-controlled study. Moxonidine decreased muscle sympathetic nerve activity in both healthy volunteers (P<0.05 versus baseline) and hypertensives (P<0.02 versus placebo). Plasma norepinephrine also decreased (P<0. 01), whereas plasma epinephrine and renin levels did not change (P=NS). Furthermore, moxonidine decreased systolic (P<0.0001) and diastolic (P<0.001) blood pressure. Heart rate decreased after moxonidine in healthy subjects (P<0.05); in hypertensives, heart rate decreased during the night hours (P<0.05) but not during daytime (P=NS). Plasma levels of LDL, HDL, and total cholesterol were not influenced by the drug (P=NS). Moxonidine decreases systolic and diastolic blood pressure by inhibiting central nervous sympathetic activity. This makes this new drug suitable for the treatment of human hypertension and possibly for other cardiovascular diseases with increased sympathetic nerve activity, ie, ischemic heart disease and heart failure.

Gender difference in age-related changes in muscle sympathetic nerve activity in healthy subjects

Muscle sympathetic nerve activity (MSNA) was measured directly along with blood pressure at rest in 69 healthy women (20-79 yr old) and 76 age-matched healthy men (16-80 yr old). All were nonobese and normotensive. In the women and men the MSNA was positively correlated with age (women: y = 0.788x - 5.418, r = 0.846, P < 0. 0001; men: y = 0.452x + 12.565, r = 0.751, P < 0.0001). The regression intercept of y was significantly lower (P < 0.0001) in the women than in the men, and the regression slope was significantly steeper (P < 0.0001) in the women. The MSNA was lower in women than in men among those <30 (P = 0.0012), 30-39 (P = 0. 0126), and 40-49 yr old (P = 0.0462) but was similar in women and men among those 50-59 (P = 0.1911, NS) and >/=60 yr old (P = 0.1739, NS). The results suggest that MSNA increases with age in women and men and that the activity is markedly lower in young women than in men but is markedly accelerated with age.

Increased sympathetic nervous system activity and its therapeutic reduction in arterial hypertension, portal hypertension and heart failure

Although the underlying mechanisms no doubt differ, activation of the sympathetic nervous system is an important pathophysiological feature in primary arterial hypertension, in portal hypertension accompanying hepatic cirrhosis, and in heart failure, and is a logical therapeutic target for centrally acting sympathetic nervous system suppressant drugs. Portal hypertension: The sympathetic outflows to skeletal muscle vasculature, the heart, the kidneys and to the hepatomesenteric circulation are stimulated in patients with alcoholic cirrhosis of the liver, perhaps as a reflex response to the vasodilatation and vascular shunting present. Acute dosing with clonidine produces dose dependent reduction in noradrenaline spillover from visceral organs and reduction in hepatic vein wedge pressure, with preservation of hepatic blood flow and negligible fall in arterial pressure. These findings indicate the clinical potential of drugs such as clonidine, moxonidine and rilmenidine for chronically lowering portal venous pressure in cirrhosis. Arterial hypertension: Activation of the sympathetic outflow to the heart, kidneys and skeletal muscle vasculature is commonly present in younger (< 45 years) patients with essential hypertension. The sympathetic stimulation appears to have adverse consequences in hypertensive patients beyond blood pressure elevation. Neural vasoconstriction in skeletal muscle has metabolic effects by impairing glucose delivery, which is a basis for insulin resistance and hyperinsulinemia. Within the heart a trophic effect of sympathetic activation on cardiac growth, contributing to the development of left ventricular hypertrophy, and an arrhythmogenic effect are also likely. Cardiac failure: The cardiac sympathetic nerves are preferentially stimulated in severe heart failure, with norepinephrine release from the failing heart at rest being increased as much as 50-fold, similar to the level seen in healthy people during near maximum exercise. This preferential activation of the cardiac sympathetic outflow contributes to arrhythmogenesis and possibly to progression of the heart failure, and has been directly linked to mortality; a high rate of spillover of noradrenaline from the heart is a strong, independent predictor of poor prognosis in severe cardiac failure. The mechanisms underlying sympathetic nervous stimulation are not entirely clear. Increased intracardiac diastolic pressure seems to be one peripheral signal, and increased forebrain norepinephrine turnover an important central mechanism. Following the demonstration of the beneficial effect of the beta-adrenergic blocker, carvedilol, and with second generation centrally acting sympathetic suppressants now under clinical investigation, elucidation of the abnormalities in central nervous control of sympathetic outflow in heart failure has become clinically relevant.

Comparison of effects of enalapril and nitrendipine on cardiac sympathetic nervous system in essential hypertension

OBJECTIVES

The purpose of this study was to assess the effects of enalapril and nitrendipine on the cardiac sympathetic nervous system.

BACKGROUND

Angiotensin-converting enzyme inhibitors and long-acting calcium channel blockers have been widely used in the treatment of cardiovascular diseases, in some of which sympathetic overactivity plays a major role in the pathophysiology and prognosis. However, little information is available on the effects of these drugs on the cardiac sympathetic nervous system.

METHODS

123I-metaiodobenzylguanidine (MIBG) cardiac imaging was performed before and 3 months after drug administration in 46 patients with mild essential hypertension. Twenty-two patients were treated with 5 to 10 mg of enalapril once a day, and the other 24 with 5 to 10 mg of nitrendipine once a day. For comparison, 20 normotensive subjects were also studied.

RESULTS

There were no significant differences between the basal characteristics in the 2 hypertensive groups. In both hypertensive groups, both systolic and diastolic blood pressures were significantly reduced to similar levels after the 3-month drug treatment. Before the drug treatment, the 2 hypertensive groups had a significantly higher washout rate and lower MIBG uptake than the normotensive subjects. The heart-to-mediastinum ratio significantly increased (p < 0.0001), with decreased (p < 0.002) washout rate after drug treatment in the enalapril group, but with no significant changes in the nitrendipine group.

CONCLUSION

Enalapril could suppress cardiac sympathetic activity and nitrendipine had no effect on it. The knowledge of antihypertensive drugs on the cardiac sympathetic nervous system appears to be helpful in selecting appropriate treatment in cardiovascular diseases.

Intense sympathetic nerve activity in adults with hypopituitarism and untreated growth hormone deficiency

Perturbations in the sympathetic nervous system may be anticipated in adults with hypopituitarism and untreated GH deficiency, because the syndrome is associated with both peripheral and central factors known to modulate sympathetic traffic. The higher prevalence of hypertension and increased cardiovascular morbidity/mortality reported in GH-deficient patients may suggest increased activity of the sympathetic nervous system. We recorded muscle sympathetic nerve activity (MSNA) in 10 hypopituitary adults with adequate hormonal replacement therapy except GH and in 10 healthy controls matched for age, gender, and body mass index to test whether hormonal aberrations in hypopituitarism and untreated GH deficiency are associated with an increase in sympathetic nerve traffic. Blood samples for insulin-like growth factor I, free T4, and TSH were taken after an overnight fast, followed by an oral glucose tolerance test. Direct intraneural recordings of MSNA were performed with a tungsten microelectrode from the peroneal nerve. The hypopituitary subjects had markedly increased MSNA (54 +/- 4 bursts/min vs. 34 +/- 4 in controls; P < 0.002), which was not related to abdominal obesity or altered glucose metabolism. When assessed for the whole study group, MSNA was inversely correlated to serum insulin-like growth factor I (r = -0.59; P < 0.006) and TSH (r = -0.46; P < 0.04). MSNA was positively correlated to diastolic blood pressure (r = 0.80; P < 0.0005) in patients, but not in controls. The intense sympathetic discharge is suggested to be of central origin and may be an important underlying mechanism for the secondary hypertension and increased cardiovascular morbidity/mortality in this patient group.

Microneurographic research on sympathetic nerve responses to environmental stimuli in humans

The sympathetic nervous system plays an important role to maintain the homeostasis of vital functions in humans against environmental stimuli. Sympathetic nerve responses to environmental stimuli in humans have been assessed conventionally using rather indirect methods by analyzing the responses of effector organs or by measuring the changes in plasma norepinephrine level. Meanwhile, the microneurography technique has enabled us to approach the sympathetic nervous system in humans more directly. By applying this technique, it has become possible to investigate how the human sympathetic nervous system responds to different kinds of environmental stimuli. In this paper, the usefulness of microneurography as a research tool in environmental physiology is shown together with a review of microneurographic findings on sympathetic nerve responses to environmental stimuli in humans.

Muscle sympathetic nerve activity during cold pressor test in patients with cerebrovascular accidents

BACKGROUND AND PURPOSE

Autonomic dysfunction is frequently present in patients with cerebrovascular accidents (CVA). However, the pathophysiological mechanisms of these disorders are not clear. The purpose of the study was to assess the effects of CVA on the autonomic nervous system.

METHODS

In eight male patients with a history of CVA with damage of the cortical or subcortical structures, we measured the cold pressor response during recording of muscle sympathetic nerve activity (MSNA) from the peroneal nerve on the hemiplegic side. We also studied 10 age-matched male control subjects. Tests were performed before, during, and after immersion of the nonhemiplegic hand in ice water for a period of 3 minutes in each phase. We also recorded changes in heart rate (HR), arterial blood pressure, skin temperature of the middle finger, and perception of pain using the Borg's score.

RESULTS

During the control period, the mean burst count of MSNA in CVA (57.2 +/- 3.9 beats/100 HR) was higher than in control subjects (36.3 +/- 3.2 beats/100 HR) (P<.05). Total MSNA (the mean burst amplitude per minute times burst rate) increased significantly in CVA and control during the immersion period by 79.9 +/- 18.4% and 133.1 +/- 25.6%, respectively. The percent change in total MSNA in CVA was attenuated during immersion compared with control subjects. The HR and skin temperature responses as well as the Borg's score were similar in both groups during control, hand immersion, and recovery periods.

CONCLUSIONS

The present results suggest that increased MSNA in CVA may be due to damage of cortical or subcortical structures or stroke-related changes in other areas or nonspecific changes that cause continuous increase in basal MSNA.

The stress hormone adrenocorticotropin enhances sympathetic outflow to the muscle vascular bed in humans

OBJECTIVE

To examine the role of adrenocorticotropin in the regulation of the sympathetic outflow to the muscle vascular bed in healthy female humans.

DESIGN

Eight healthy, nonsmoking female subjects (aged 18-33 years) were examined before and after injection of 0.25 mg adrenocorticotropin 1-24 or placebo according to a balanced, double-blind cross-over protocol.

METHODS

Muscle sympathetic nerve activity, arterial pressure, and heart rate were continuously recorded both under basal conditions and during a 50 min period after injection of each substance. Furthermore, sympathoexcitatory capacities of inspiratory apneas and cold pressure tests performed before and after injection of adrenocorticotropin were determined.

RESULTS

The injection of adrenocorticotropin rapidly increased burst frequency of muscle sympathetic nerve activity (P < 0.01). The maximal effect of adrenocorticotropin, with an increase in burst frequency of 63%, occurred during the third minute after injection and waned subsequently, but muscle sympathetic nerve activity remained significantly increased during the first 10 min after injection. The stimulatory effect of adrenocorticotropin had disappeared 40 min after injection. The sympathoexcitatory capacity of a maximal inspiratory apnea and a cold pressure test, respectively, remained unchanged 10 and 45 min after the administration of adrenocorticotropin compared with control. Neither blood pressure nor heart rate was significantly affected by administration of the peptide.

CONCLUSIONS

The data establish that the stress hormone adrenocorticotropin acutely increases sympathetic outflow to the muscle vascular bed in female humans. This effect is most likely mediated via central nervous system autonomic centers. The influence of adrenocorticotropin on the sympathetic nervous system might contribute to the alteration of response to stress in the course of the development of hypertension and could also add to the hypertensiogenic effects of corticosteroids and mineralocorticoids in states with excess adrenocorticotropin.

Renal nerves and D1-dopamine receptor-mediated natriuresis

The resistance of the spontaneously hypertensive rat (SHR) kidney to the natriuretic effect of dopamine and D1 agonists may be due to increased renal nerve activity. Therefore, we compared the effects of the intrarenal arterial infusion of the D1 agonist, SKF 38383, into the denervated (DNX) kidney of saline-loaded-anesthetized SHR and its control, the Wistar-Kyoto (WKY) rat. In both WKY and SHR, DNX of the left kidney slightly decreased urine flow (UV) and absolute (UNaV) and fractional sodium excretion (FENa) in the innervated right kidney; neither vehicle nor D1 agonist infusion exerted any effect. In the left kidney, denervation increased UV, UNaV, and FENa to a similar degree in WKY and SHR (2-fold), without affecting renal blood flow, glomerular filtration rate, or blood pressure. In WKY but not in SHR, after DNX, the D1 agonist dose-dependently increased UV, UNaV, and FENa in the denervated kidney. We conclude that the decreased natriuretic effect of D1 agonists in the SHR is not due to increased renal nerve activity. These data support our previous studies implicating a defect of the D1 receptor or its regulation in the kidney in genetic hypertension.

Baroreflex control of sympathetic nerve activity in essential and secondary hypertension

Studies performed in experimental animals and in humans have documented that high blood pressure markedly impairs baroreceptor control of heart rate. Whether a similar impairment also characterizes baroreceptor control of sympathetic activity modulating peripheral vasomotor tone is still unknown. In 28 untreated essential hypertensive subjects [14 of moderate and 14 of more severe degree, age 51.6+/-2.4 and 52.6+/-2.1 years (mean+/-SEM)] and in 13 untreated secondary hypertensives (renovascular or pheochromocytoma, age 50.1+/-4.6 years), we measured beat-to-beat arterial blood pressure (finger photoplethysmographic device), heart rate (electrocardiogram), and efferent postganglionic muscle sympathetic nerve activity (microneurography) at rest and during baroreceptor stimulation and deactivation induced by stepwise intravenous infusions of phenylephrine and nitroprusside, respectively. Data were compared with those obtained in 15 age-matched normotensive control subjects. Muscle sympathetic nerve activity (bursts per 100 heart beats) showed a progressive and significant (P<.01) increase from normotension (40.3+/-3.3) to moderate (55.6+/-4.1) and more severe essential hypertension (68.2+/-4.1), paralleling the progressive increase in blood pressure values. In contrast, muscle sympathetic nerve activity was not increased in secondary hypertensives (40.5+/-6.7) despite blood pressure values similar to or even greater than those of severe essential hypertensives. In both essential and secondary hypertensives, baroreceptor-heart rate control was displaced toward elevated blood pressure values and markedly impaired compared with normotensive subjects (average reduction, 38.5%). In contrast, the sympathoinhibitory and sympathoexcitatory responses to baroreceptor stimulation and deactivation were displaced toward elevated blood pressure values but similar in all groups. Thus, sympathetic activation characterizes essential but not secondary hypertension. Regardless of its nature, however, hypertension is not accompanied by an impairment of baroreceptor modulation of sympathetic activity.

Dissociation between muscle and skin sympathetic nerve activity in essential hypertension, obesity, and congestive heart failure

Essential hypertension, obesity, and congestive heart failure are characterized by an increase in muscle sympathetic nerve activity. Whether in these conditions skin sympathetic nerve activity is also increased has never been systematically examined, however. In 10 untreated mild essential hypertensive, 12 untreated normotensive obese, 10 mild (New York Heart Association class II) heart failure, and 10 normotensive lean healthy control subjects, we measured beat-to-beat arterial blood pressure (Finapres technique), body mass index, and postganglionic sympathetic nerve activity in skeletal muscle and skin areas (microneurographic technique, peroneal nerve). The muscle and skin nerve measurements were made in a randomized sequence. All data were obtained with the subject supine in a quiet, semidark environment at constant temperature over two periods of 30 minutes each, separated by a 20- to 30-minute interval. Blood pressure was increased only in hypertensive and body mass index only in obese subjects. Muscle sympathetic nerve activity quantified as bursts/min was markedly and significantly (P<.01) greater in essential hypertensive (33.3+/-1.7), obese (42.2+/-2.8), and congestive heart failure subjects (55.8+/-4.3) in comparison with control subjects (23.9+/-1.6). This was the case also for muscle sympathetic nerve activity, quantified as bursts per 100 heart beats. In contrast, skin sympathetic nerve activity (bursts per minute) was superimposable in hypertensive, obese, heart failure, and control subjects, its ability to increase being documented in all four groups by the marked response to an acoustic stimulus. Thus, in various diseases, muscle but not skin sympathetic activity is increased, with the sympathetic activation not being uniformly distributed over the whole cardiovascular system.

Alpha-2 adrenergic activity in perimenopausal women

Lipid alterations and increased blood pressure may occur during perimenopause. No data are available in perimenopausal women on the alpha-2 adrenergic activity which affects norepinephrine secretion. We studied cardiovascular and catecholamine responses to clonidine (300 mg per os) in a group of 15 perimenopausal women (PeriMW) and in a control group of 13 premenopausal women (PreMW). Nine of the perimenopausal women were also studied after 4-month percutaneous estrogen replacement therapy (PeriMWE). Systolic and diastolic blood pressure (SBP, DBP), heart rate (HR), plasma norepinephrine (NE) and epinephrine (E) were evaluated before and at 120 min, 130 min, 140 min after clonidine administration. Basal values of SBP, DBP and HR were not different (F = 0.7, p = NS; F = 0.2, p = NS and F = 0.1, p = NS respectively) between PeriMW both before and after therapy and PreMW. Resting levels of E were similar in PreMW and in PeriMW before and during estrogen therapy (F = 0.8, p = NS); PeriMW showed higher basal NE levels both before and during estrogen therapy than PreMW (F = 12; p < 0.001). Clonidine administration decreased SBP, DBP and NE levels in PreMW, in PeriMW and in PeriMWE without any difference between the groups (F = 1.2, p = NS; F = 0.5, p = NS and F = 1.3, P = NS respectively). HR decreased significantly after clonidine in PreMW (F = 5.4, p < 0.03) but not in PeriMW before (F = 1.0, p = NS) and during estrogen therapy (F = 0.5, p = NS). Clonidine did not affect plasma E in the three groups studied (F = 2.8, p = NS; F = 2.2, P = NS and F = 0.1, p = NS). The present study demonstrates that increased basal plasma NE levels are present in PeriMW. The cardiovascular and catecholamine response to clonidine in PeriMW both before and during estrogen therapy are similar to those observed in PreMW, suggesting a normal inhibitory alpha-2 receptor pathway.

The sympathetic nervous system is involved in the maintenance but not initiation of the hypertension induced by N(omega)-nitro-L-arginine methyl ester

Studies in anesthetized animals have advanced the theory that there is an important neurogenic component to the hypertension caused by pharmacological inhibition of nitric oxide, but studies in conscious animals have produced conflicting evidence for and against this theory. To try to reconcile the seemingly contradictory data, we hypothesized that the neurogenic component of this hypertension is time dependent such that the sympathetic nervous system is involved primarily in the maintenance, rather than the initiation, of the hypertension. We measured intra-arterial pressure in conscious, unrestrained rats with and without guanethidine-induced sympathectomy during varying durations of intravenous N(omega)-nitro-L-arginine methyl ester (L-NAME). The major new finding is that sympathectomy had no effect on the hypertensive response to bolus injections of L-NAME but in the same rats it produced a greater than 50% attenuation in the hypertension seen after 6 days of continuous L-NAME (change in mean arterial pressure, 23+/-4 versus 55+/-4 mm Hg, P<.01, sympathectomy versus control). Using 8-hour infusions of L-NAME, we found that 60 minutes was the minimum time required for detecting a sympathectomy-sensitive component of L-NAME-induced hypertension. Furthermore, we demonstrate that the magnitude of this component increases further between 8 hours to 6 days of continuous L-NAME: it accounted for only 18% of the total hypertensive response at 8 hours but 61% after 6 days. From these experiments, we conclude that the importance of the sympathetic system in the pathogenesis of L-NAME-induced hypertension accrues slowly over hours and days, and thus its importance can be overlooked by focusing on the initial phase of the hypertension.

Sympathetic nerve traffic correlates with the release of nitric oxide in humans: implications for blood pressure control

1. Resting human sympathetic vasoconstrictor traffic displays large reproducible inter-individual differences which are similar in nerves to muscle, heart and kidney. In spite of this there is no correlation between levels of blood pressure and sympathetic traffic. To test the hypothesis that the pressor effect of the vasoconstrictor activity is counteracted by a circulating dilating factor we measured muscle nerve sympathetic activity (MSA) and an indicator of nitric oxide release (plasma nitrate) in healthy young males. 2. Sympathetic activity was recorded with the microneurographic technique in the peroneal nerve and a forearm venous plasma sample was obtained in twenty-one normotensive males aged 21-28 years. Plasma nitrate was analysed by gas chromatography and mass spectrometry. 3. There was a positive linear correlation between the plasma nitrate concentration and the strength of MSA both when the nerve activity was expressed as bursts per minute and bursts per 100 heart beats (r = 0.51, P = 0.02 and r = 0.46, P = 0.04, respectively). 4. The data suggest that the stronger the sympathetic activity the higher the release of the dilating substance, nitric oxide. This would be expected to counteract vasoconstrictor effects of the nerve traffic and thereby contribute to the lack of relationship between resting levels of MSA and blood pressure. We speculate that altered coupling between sympathetic traffic and nitric oxide release may cause abnormal peripheral resistance, e.g. in hypertension.

Norepinephrine spillover at rest and during submaximal exercise in young and old subjects

Aging is associated with elevations in plasma norepinephrine concentrations. The purpose of this investigation was to examine total body and regional norepinephrine spillover as an indicator of sympathetic nerve activity. Eight young (26 +/- 3 yr) and seven old (69 +/- 5 yr) male subjects were studied at rest and during 20 min of submaximal cycling exercise at 50% of peak work capacity. Norepinephrine spillover was determined by continuous intravenous infusion of [3H]norepinephrine. Arterial norepinephrine concentrations were significantly greater at rest for old vs. young subjects (280 +/- 36 vs. 196 +/- 27 ng/ml, respectively). Whereas total norepinephrine spillover did not differ between groups at rest, hepatomesenteric norepinephrine spillover was 50% greater in old subjects compared with their young counterparts (51 +/- 7 vs. 34 +/- 5 ng/min, respectively). Additionally, norepinephrine clearance rates at rest were significantly lower for the old subjects (-23%). During exercise, plasma norepinephrine concentrations increased compared with rest, with old subjects again demonstrating greater values than the young group. Hepatomesenteric norepinephrine spillover was significantly greater (+36%) during exercise for old subjects compared with young; however, no difference was found for whole body spillover rates between age groups. Norepinephrine clearance rates remained depressed (-80%) in the old subjects during exercise. Clearance of epinephrine mirrored that for norepinephrine both at rest and during exercise across age groups. It was concluded that in old subjects, a reduction in norepinephrine clearance and an increase in regional norepinephrine spillover can account for the higher plasma norepinephrine concentrations observed at rest. This relationship is not exacerbated by the stress imposed during an acute bout of exercise.

Cerebral noradrenaline spillover and its relation to muscle sympathetic nervous activity in healthy human subjects

Studies using internal jugular vein blood sampling in human subjects have demonstrated the release of noradrenaline from the brain and have provided a link between central nervous system noradrenergic neuronal activity and renal, cardiac and total body sympathetic activity. The aim of this study was to further categorise the dependence of regional sympathetic nervous function on central nervous system noradrenergic neuronal processes by combining measures of internal jugular venous noradrenaline spillover, as an indicator of brain noradrenaline release, and cerebral blood flow scans with measures of the overall integrated neuronal firing rate for the body as a whole, the spillover of noradrenaline into the coronary sinus and with measurements of resting muscle sympathetic nerve activity. Positive veno-arterial plasma noradrenaline gradients were found across the brain, with the plasma concentration being 17 +/- 3% (p < 0.01) greater in the internal jugular vein. Linear regression analysis revealed a significant relationship between the degree of muscle sympathetic nerve activity and the spillover of noradrenaline from subcortical brain regions (y = 0.1 x + 16.0; r = 0.81, p < 0.02). The rate of spillover of noradrenaline for the body as a whole also bore a significant association with the rate of subcortical noradrenaline spillover (y = 0.01x + 2.33; r = 0.71, p < 0.05). Cortical noradrenaline spillover was not related to any of the sympathetic nervous system parameters measured in this study. The demonstration of a direct relationship between the rate of peroneal nerve firing and the spillover of noradrenaline from subcortical brain regions provides further support for the concept of central nervous system noradrenergic cell groups behaving in a sympathoexcitatory role.

Attenuation of contractile responses to sympathetic co-transmitters in veins from subjects with essential hypertension

Neuropeptide Y (NPY), noradrenaline (NA) and ATP are cotransmitters of the sympathetic nervous system and exert vasocontractile effects. The aim of this study was to determine the role of these sympathetic co-transmitters in human hypertension. Subcutaneous vessels from 12 patients with essential hypertension and 12 matched controls were studied in vitro. Vascular contractile responses to NPY, NA, alpha,beta-methylene ATP (alpha,beta-mATP) and potassium were studied in isolated arteries and veins (diameter 0.1-1.1 mm) with intact endothelium. The dilatory effect of acetylcholine was used to test the endothelial function. There was no difference in potency (pD2) or contractile response to NPY, NA or alpha,beta-mATP between hypertensive and control arteries. In veins, however, the contractile response to NPY was significantly reduced in hypertensives and the responses to NA were unchanged. Furthermore, the sensitivity (pD2) to alpha,beta-mATP was significantly reduced in veins from hypertensives. There was no difference in the dilatory response to acetylcholine between the hypertensives and the controls, neither in the arteries nor in the veins, indicating that the observed changes in vascular reactivity to NPY, NA and alpha,beta-mATP were not endothelium-dependent. In conclusion, the postjunctional contractile effect of NPY and sensitivity (pD2) to alpha,beta-mATP, co-transmitters of the peripheral sympathetic nervous system, are attenuated in veins in essential hypertension.

Age-dependent changes in cardiac muscarinic receptor function in healthy volunteers

OBJECTIVES

This study was conducted to determine possible age-dependent changes in the responsiveness of human cardiac muscarinic receptors.

BACKGROUND

It is well known that the baroreflex activity decreases with aging. However, the mechanisms underlying this phenomenon are not completely understood at present.

METHODS

In six healthy young (mean [+/-SEM] age 26 +/- 2 years) and six healthy older volunteers (mean age 60 +/- 2 years), we determined 1) the effects of graded doses of atropine (bolus application, six doses, each for 20 min, range 0.03 to 0.96 mg) and the M1-cholinoceptor selective antagonist pirenzepine (bolus application, eight doses, each for 20 min, range 0.04 to 10 mg) on heart rate, blood pressure and systolic time intervals (as measure of inotropism); and 2) the baroreflex activity by assessing the bradycardic response to phenylephrine.

RESULTS

Atropine and pirenzepine caused biphasic effects on heart rate: At lower doses (< 0.12 mg for atropine, < 5 mg for pirenzepine) they decreased heart rate, whereas at higher doses they increased heart rate. Heart rate decreases induced by both antimuscarinic drugs were significantly larger in the young volunteers than in the older volunteers, whereas heart rate increases were not significantly different for both drugs. Atropine and pirenzepine did not significantly affect blood pressure and systolic time intervals. Infusion of graded doses of phenylephrine (four doses ranging from 0.1 to 1.0 microgram/kg body weight per min for 15 min each) caused a higher increase in systolic blood pressure and a smaller decrease in heart rate at each dose in the older volunteers than in the young volunteers. The slopes of the regression lines were 16 +/- 2.3 ms/mm Hg for the young and 6 +/- 0.5 ms/mm Hg for the older volunteers (p < 0.01).

CONCLUSIONS

Human cardiac muscarinic receptor activity is diminished with increasing age; such decreased cardiac muscarinic receptor activity could contribute to the decrease in baroreflex activity with aging. In contrast, antimuscarinic drugs seem to have no effect on human cardiac contractility.

Increase in muscle nerve sympathetic activity after glucose intake is blunted in the elderly

Muscle nerve sympathetic activity (MSA; involved in blood pressure regulation) was recorded by microneurography in the peroneal nerve for 90 min after ingestion of 100 g D-glucose in three groups of healthy subjects: young subjects (mean age 26 years) and 70-year-old men with normal and reduced insulin sensitivity as assessed by euglycaemic insulin clamp. Muscle nerve sympathetic activity at rest was lowest in the young and highest in the insulin-resistant subjects (burst frequencies 19.8 +/- 6.0, 47.7 +/- 7.0 and 55.1 +/- 11.5 bursts/min for the three groups, respectively). The young subjects responded to glucose intake with a pronounced increase in MSA, a response that was blunted in the elderly and weakest in the insulin-resistant subjects. A similar relationship was observed during a Valsalva manoeuvre, indicating that the blunted response in the elderly is a generalized phenomenon. Blood pressure remained stable in the young subjects but fell slightly and significantly in the elderly subjects. It is concluded that old subjects utilize their total capacity for MSA close to maximum at rest. Thus, the reserve for response to stimuli normally evoking a strong increase in MSA is restricted. This restriction may contribute to postprandial hypotension in the healthy elderly.

Renal noradrenaline spillover correlates with muscle sympathetic activity in humans

1. To study the relationship between indices of resting sympathetic traffic in nerves to skeletal muscles and the kidneys, simultaneous measurements were made of muscle sympathetic activity in the peroneal nerve and renal noradrenaline spillover in ten healthy normotensive males aged 18-69 years (mean 42 years). 2. Group mean levels (+/-S.D.) of muscle sympathetic activity and renal spillover were 22 +/- 17 bursts min-1 and 105 +/- 49 ng min-1, respectively. There were significant positive correlations between individual values of muscle sympathetic activity and renal noradrenaline spillover (r = 0.76, P < 0.01) and similarly between muscle sympathetic activity and renal venous plasma concentration of noradrenaline(r = 0.79, P < 0.007). 3. The results indicate that, although the sympathetic system has the capacity for selective activation of different subdivisions, in healthy human subjects resting traffic is similar or proportional in sympathetic nerves to skeletal muscles and the kidney.

Gender differences in muscle sympathetic nerve activity: effect of body fat distribution

Muscle sympathetic nerve activity (MSNA) has been correlated with percent body fat (%BF) in males. Because MSNA is typically lower and %BF higher in females, we tested whether this relationship could be generalized to females. Because abdominal-visceral body fat in men may be responsible for elevated sympathetic activity, we hypothesized that an estimate [waist-to-thigh ratio (W/T)] would correlate positively with MSNA in both genders and account for higher MSNA in males. Microneurography, hydrodensitometry, and W/T measures were obtained in 14 males and 14 females with a large range of %BF and W/T. Regression analyses revealed positive correlations between MSNA and %BF in males (r = 0.55, P = 0.04) and in females (r = 0.63, P = 0.02), with no difference in the slopes of the regression lines but a higher intercept in males (P < 0.01). When genders were pooled, MSNA and W/T were correlated (r = 0.68, P < 0.0001); this positive correlation was also found in males (r = 0.57, P = 0.04) but not as strongly in females (r = 0.49, P = 0.07). Forward stepwise multiple-regression analysis using %BF, W/T, gender, and age indicated that W/T was the primary factor related to MSNA (R2 = 0.46); the other factors were not independent predictors. It is concluded that %BF is related to MSNA in both males and females but that the regression line is shifted downward in females because of lower levels of MSNA. W/T is a better correlate of MSNA than %BF and partially explains the higher MSNA in males. These findings may be relevant to the cardiovascular and metabolic disease risk associated with abdominal obesity.

Sympathetic nerve activity and insulin in obese normotensive and hypertensive men

The relationship between resting levels of muscle sympathetic nerve activity (MSA) and blood pressure is a matter of controversy. Body weight has recently been identified as an independent determinant of muscle sympathetic discharge, which may have influenced previous studies focused on MSA and mechanisms of hypertension. In the present study, we measured resting MSA and plasma insulin levels in 18 obese (body mass index, 32 +/- 4 kg/m2) (mean +/- SD), middle-aged (52 +/- 6 years), hypertensive (155 +/- 11/97 +/- 8 mm Hg) subjects and 16 age- and body mass index-matched normotensive control subjects. In the postabsorptive state, resting MSA was similar in the hypertensive and normotensive groups (43 +/- 4 versus 39 +/- 3 bursts per minute, 69 +/- 5 versus 64 +/- 5 bursts per 100 heart beats, P = NS) (mean +/- SEM) and did not correlate with either systolic or diastolic blood pressure. Weak but significant positive correlations were found between resting MSA and both fasting insulin levels (P < .05) and body mass index (P = .05) in hypertensive but not normotensive subjects. There was a strong positive correlation between fasting insulin and body mass index in both normotensive subjects and the entire study group (P < .005). Fasting insulin and body mass index correlated with diastolic blood pressure (P < .05) in the entire study group. In conclusion, a relationship between fasting insulin, body mass index, and blood pressure was confirmed, whereas only a weak correlation was found between MSA and fasting insulin in hypertensive but not normotensive subjects. The fact that MSA was similar in the two groups argues strongly against augmented MSA being important for the maintenance of hypertension, at least in middle-aged, obese men.

Central blood volume: a determinant of early cardiac adaptation in arterial hypertension?

OBJECTIVES

This study was undertaken to assess the influence of the fluid volume state on cardiac adaptation to hypertension.

BACKGROUND

Left ventricular hypertrophy is an important predictor of hypertensive complications. We analyzed volume status and its impact on cardiac structural changes in early hypertension.

METHODS

In 33 normotensive subjects, 40 patients with borderline hypertension and 63 patients with established essential hypertension, mean arterial pressure was measured invasively; total blood volume was measured by iodine-125-labeled plasma albumin and hematocrit; central blood volume by indocyanine green dye dilution curve; and diastolic diameter and left ventricular mass by two-dimensional-guided M-mode echocardiography.

RESULTS

Central blood volume was approximately 20% higher in patients with stage I borderline hypertension than in normotensive subjects ([mean +/- SD] 3,001 +/- 663 vs. 2,493 +/- 542 ml, p < 0.05), whereas total blood volume was similar in all three groups. This shift in intravascular volume toward the cardiopulmonary circulation was accompanied by a significant increase in diastolic diameter (5.29 +/- 0.80 vs. 4.86 +/- 0.77 cm, p < 0.05) and in left ventricular mass (239.4 +/- 90.6 vs. 183.5 +/- 68.8 g, p < 0.05) in patients with borderline hypertension compared with subjects with normotension. In patients with established essential hypertension, volume status of stroke volume and diastolic dimension returned to normal values, whereas left ventricular mass increased further.

CONCLUSIONS

We conclude that the early phase of hypertension is characterized by centripetal distribution of intravascular volume, leading to an increased preload to the left ventricle. This change in volume status appears to be related to cardiac structural adaptation to an increase in arterial pressure.

Sympathetic neural mechanisms in obstructive sleep apnea

Blood pressure, heart rate, sympathetic nerve activity, and polysomnography were recorded during wakefulness and sleep in 10 patients with obstructive sleep apnea. Measurements were also obtained after treatment with continuous positive airway pressure (CPAP) in four patients. Awake sympathetic activity was also measured in 10 age- and sex-matched control subjects and in 5 obese subjects without a history of sleep apnea. Patients with sleep apnea had high levels of nerve activity even when awake (P < 0.001). Blood pressure and sympathetic nerve activity did not fall during any stage of sleep. Mean blood pressure was 92 +/- 4.5 mmHg when awake and reached peak levels of 116 +/- 5 and 127 +/- 7 mmHg during stage II sleep (n = 10) and rapid eye movement (REM) sleep (n = 5), respectively (P < 0.001). Sympathetic activity increased during sleep (P = 0.01) especially during stage II (133 +/- 9% above wakefulness; P = 0.006) and REM (141 +/- 13%; P = 0.007). Peak sympathetic activity (measured over the last 10 s of each apneic event) increased to 299 +/- 96% during stage II sleep and to 246 +/- 36% during REM sleep (both P < 0.001). CPAP decreased sympathetic activity and blood pressure during sleep (P < 0.03). We conclude that patients with obstructive sleep apnea have high sympathetic activity when awake, with further increases in blood pressure and sympathetic activity during sleep. These increases are attenuated by treatment with CPAP.

Increased muscle sympathetic nerve activity after glucagon administration in man

Muscle sympathetic nerve activity (MSNA) after glucagon administration was quantitatively measured in five healthy male volunteers (age: 20-28 years old). After an overnight fasting, 1 mg glucagon dissolved in a 1-ml vehicle was injected i.v. as a bolus. MSNA, ECG and blood pressure were simultaneously recorded 15 min before and after the injection. MSNA (bursts/min), heart rate (HR), and systolic and diastolic blood pressures (dBP and sBP) were significantly increased at 2-3 min after glucagon injection. Their means and standard deviations of control for 15 min vs. 3 min after the injection were in the five subjects; MSNA, 10 +/- 6 vs. 30 +/- 10 bursts/min, HR, 61 +/- 10 vs. 76 +/- 8 beats/min, dBP, 66 +/- 9 vs. 91 +/- 9 mmHg and sBP, 122 +/- 12 vs. 152 +/- 11 mmHg. These results suggest that glucagon has a direct facilitatory effect on the central sympathetic nervous system.

Effects of aging on the responsiveness of the human cardiac sympathetic nerves to stressors

BACKGROUND

Aging increases human sympathetic nervous activity at rest. Beause of the probable importance of neural stress responses in the heart as triggers for clinical end points of coronary artery disease, it is pertinent to investigate whether sympathetic nervous responses to stresses are increased by aging.

METHODS AND RESULTS

We applied kinetic methods for measuring the fluxes to plasma of neurochemicals relevant to sympathetic neurotransmission in younger (aged 20 to 30 years) and older (aged 60 to 75 years) healthy men during mental stress (difficult mental arithmetic), isometric exercise (sustained handgrip), and dynamic exercise (supine cycling). The increase in total norepinephrine spillover to plasma with mental stress was unaffected by age. In contrast, the increase in cardiac norepinephrine spillover was two to three times higher in the older subjects (P < .05). The probable mechanism of this higher cardiac norepinephrine spillover was reduced neuronal reuptake of the transmitter, because age had no influence on the overflow of the norepinephrine precursor, dihydroxyphenylalanine, or intraneuronal metabolite, dihydroxyphenylglycol (levels of these two substances reflect rates of cardiac norepinephrine synthesis and intraneuronal metabolism), and the transcardiac extraction of plasma radiolabeled norepinephrine was lower in the older subjects (P < .05). An almost identical pattern of neurochemical response was seen with isometric exercise. During cycling, total norepinephrine spillover was 16% lower in the older men, but cardiac norepinephrine spillover was 53% higher.

CONCLUSIONS

Reduced norepinephrine reuptake increases the overflow of the neurotransmitter to plasma from the aging heart during stimulation of the cardiac sympathetic outflow. Failure of transmitter inactivation at postjunctional receptors with aging would amplify the neural signal, and in the presence of myocardial disease could trigger adverse stress-induced cardiovascular events, particularly when accompanied by an age-dependent reduction in vagal tone. Reduction of postsynaptic adrenergic responsiveness with aging, however, might protect against this, as indicated by our finding that in no case was the heart rate increase during stress greater in older men, despite their having larger increases in cardiac norepinephrine spillover.

Central nervous system noradrenergic control of sympathetic outflow in normotensive and hypertensive humans

We applied transmitter washout methodology, sampling internal jugular venous plasma via a percutaneously placed catheter, to study CNS norepinephrine release in humans and its relation to peripheral sympathetic activity. Norepinephrine overflows into the venous drainage of the brain, as do its precursor, DOPA, and metabolites DHPG and MHPG, indicating that the blood-brain barrier provides an incomplete impediment to their outward flux from the brain. Pharmacological testing with two drugs which altered CNS norepinephrine turnover, the tricyclic antidepressant desipramine and the ganglionic blocker, trimethaphan, demonstrated a direct relation existed between CNS norepinephrine release and sympathetic nerve firing rates. In essential hypertension, the sympathetic activation commonly present was associated with, and possibly caused by increased CNS release of norepinephrine, manifested in elevated overflow of norepinephrine, MHPG and DHPG from the brain. Bilateral jugular sampling, coupled with a cerebral venous sinus scan to delineate the drainage pattern, demonstrated that this increased norepinephrine release was confined to subcortical forebrain regions.

Efficacy and safety of rilmenidine in elderly patients--comparison with hydrochlorothiazide. The Belgian Multicentre Study Group

The aim of this trial was to study the treatment of hypertension in the elderly, comparing a new oxazoline antihypertensive agent, rilmenidine, with the diuretic hydrochlorothiazide (HCZ). After 2 weeks on placebo, 88 elderly patients (mean age 75 years; 65 women), corresponding to strict inclusion criteria, were randomized to 8 weeks double-blind monotherapy with rilmenidine 1-2 mg/day (n = 46) or HCZ 25-50 mg/day (n = 42), with administration of potassium supplements as required. Particular emphasis was placed on the evaluation of safety: blood screens were repeated after 2, 4, and 8 weeks of treatment and symptoms were systematically evaluated every 2 weeks. The rilmenidine and HCZ groups were comparable at randomization, with baseline supine systolic/diastolic blood pressures of 167/101 mm Hg and 172/101 mm Hg, respectively. Both drugs induced a significant decrease in blood pressure: at 8 weeks, supine blood pressure had decreased to 154/89 mm Hg and to 155/87 mm Hg in the rilmenidine and HCZ groups, respectively (difference not significant between groups). Changes in heart rate did not differ significantly between groups (-3 bpm at 8 weeks). Drug-related symptoms were rare and the incidence was similar in both groups. Weight decreased significantly in the HCZ group by 1 kg (p < 0.001) and did not change in the rilmenidine group. After 8 weeks of monotherapy, expected variations in serum biochemistry were detected in the HCZ group, resulting in a significant difference in comparison with the rilmenidine group: serum potassium and chloride decreased significantly and uric acid levels increased significantly in the HCZ group.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of metformin and insulin on sympathetic nerve activity, norepinephrine spillover and blood pressure in obese, insulin resistant, normoglycemic, hypertensive men

To evaluate the effect of metformin on insulin sensitivity and to further examine the relationship between insulin resistance, sympathetic nerve activity and blood pressure, 6 obese insulin resistant, normoglycemic hypertensive men were investigated (age 49 +/- 2 years, BMI 27.6 +/- 1.2, mean +/- SEM). The study had a placebo controlled, double blind, cross over design with 6 weeks' metformin treatment (850 mg b.i.d) vs placebo. Blood pressure was measured weekly. At the end of each treatment period, glucose infusion rate (GIR), muscle sympathetic nerve activity (MSA) and renal and total body norepinephrine (NE) kinetics (radioisotope dilution) were examined during euglycemic hyperinsulinemic clamp. Fasting insulin was 13 +/- 3 and 10 +/- 2 mU/l and fasting glucose 5.3 +/- 0.2 and 5.1 +/- 0.1 mmol/l after placebo and metformin treatment, respectively (ns). GIR during the last hour of the insulin clamp was 3.7 +/- 0.6 vs 3.6 +/- 0.6 mg/kg x min (ns). Resting MSA, total body and right renal NE spillover did not differ significantly after placebo and metformin treatment. Systolic and diastolic blood pressures were 151 +/- 10/95 +/- 5 mmHg after placebo and 146 +/- 5/94 +/- 5 mmHg after metformin treatment (ns). Thus metformin treatment did not have any significant effect on insulin sensitivity, blood pressure or sympathetic activity in this small group of patients. Renal plasma flow and MSA increased significantly during the insulin clamp, whereas renal NE and total body NE spillover remained unchanged, suggesting nonuniform regional sympathetic nerve responses to acute hyperinsulinemia.

Neuropeptide Y stimulates proliferation of human vascular smooth muscle cells: cooperation with noradrenaline and ATP

Since the sympathetic nervous system has been shown to exert a trophic influence on vascular smooth muscle cells (SMC), we studied the growth regulating effects of neuropeptide Y (NPY) in cooperation with the sympathetic co-transmitters noradrenaline and adenosine triphosphate (ATP) in human vascular SMC. NPY stimulated DNA synthesis in human SMC grown from subcutaneous arteries and veins (diameter: 0.4 mm) measured by [3H]thymidine incorporation. Also cell number and protein synthesis were stimulated. The effect was mediated through the Y1-receptor and not Y2 or Y3 since the Y1-selective NPY analogue Pro34-NPY and peptide YY stimulated mitogenesis in the same magnitude as NPY while the NPY-fragment NPY13-36 only had minor effects. The effect was blocked by pretreating the cells with pertussis toxin indicating a Gi/o-coupled effect. The other sympathetic co-transmitters, noradrenaline and ATP, also stimulated mitogenesis in the human SMC in a similar magnitude as NPY. When added together NPY and noradrenaline potentiated each other in the mitogenic response. ATP had mainly additive effects. This is the first demonstration that NPY, noradrenaline and ATP stimulates growth in human vascular SMC. This suggests a role of the sympathetic cotransmitters in modulating vascular tone, but also by inducing hypertrophy/hyperplasia with possible clinical consequences.

Monoaminergic neuronal activity in subcortical brain regions in essential hypertension

In this study we aimed to elucidate the role of central noradrenergic, dopaminergic, adrenergic and serotonergic neuronal systems in the development of essential hypertension. Fifteen untreated essential hypertensive subjects (aged 44 +/- 3 years) and 32 healthy volunteers (aged 38 +/- 3 years) participated in this study. By combining direct blood sampling techniques with cerebral blood flow scans we were able to differentiate between cortical and subcortical venous drainage of the brain. Veno-arterial MHPG, HVA and 5-HIAA plasma concentration gradients combined with internal jugular vein plasma flows were used, according to the Fick Principle, to derive metabolite spillovers which in turn were used as indicators of central noradrenergic, dopaminergic and serotonergic neuronal activity, respectively. These amine systems, in both the brainstem and forebrain, have been implicated in the regulation of sympathetic outflow and blood pressure. Total body noradrenaline spillover to plasma was concurrently measured to assess the relationship between central monoamine turnover and sympathetic activity. Compared to their healthy counterparts the hypertensive subjects had an elevated release of MHPG from subcortical brain regions (1.4 +/- 0.3 v 0.5 +/- 0.2 nmol/min, p < 0.05). An inverse relationship between blood pressure and subcortical HVA overflow existed, with the HVA overflow being significantly lower in the hypertensives (0.5 +/- 0.2 v 2.1 +/- 0.5 nmol/min, p < 0.05). Subcortical 5-HUAA overflow did not differ between the two groups, and adrenaline spillover from the brain was not detected in either group. Subcortical MHPG overflow was significantly correlated with total body NA spillover to plasma (p < 0.05). These results indicate that reciprocal aberrations in subcortical noradrenaline and dopamine turnover exist in essential hypertension. Although the physiological significance of this remains to be unequivocally elucidated we postulate that elevated subcortical noradrenergic activity, presumably in the forebrain where noradrenergic neurons are pressor, may cause sympathoexcitation and play a role in the development of essential hypertension.

Systemic hemodynamics during sleep in young or middle-aged and elderly patients with essential hypertension

Age-related changes in cardiovascular regulatory mechanisms may affect blood pressure homeostasis during sleep and in the daytime. This study compared systemic hemodynamics during the daytime and sleep between 12 young or middle-aged patients (young, 42.1 +/- 13.9 years old, mean +/- SD, less than 56 years old) and 12 elderly patients with essential hypertension (old, 65.3 +/- 2.8, 60 to 70 years old). They were all hospitalized and placed on a diet containing approximately 7 g sodium chloride per day. Intra-arterial blood pressure and electrocardiogram were recorded for 24 hours, and electroencephalogram and electroophthalmogram were recorded during the night with a telemetric method. Cardiac output was measured with patients in the supine position by the cuvette method during the daytime and stage 3 or 4 sleep at night. The averaged 24-hour blood pressure was similar in the two groups (140 +/- 2 [SEM]/85 +/- 3 mm Hg in the young group and 144 +/- 4/81 +/- 2 mm Hg in the old group). The reduction in mean blood pressure during sleep was also comparable in both groups (-18 +/- 2 in the young group and -20 +/- 2 mm Hg in the old group). Cardiac index was smaller in the old group than the young group during both the daytime and sleep (daytime, 2.3 +/- 0.1 versus 3.2 +/- 0.2 [L/min]/m2, P < .01; sleep, 2.1 +/- 0.1 versus 2.6 +/- 0.2 [L/min]/m2, P < .01). The reduction in cardiac index during sleep was greater in the young than the old group (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical aspects of sympathetic activation and parasympathetic withdrawal in heart failure

Proposed reflex mechanisms for generalized neurohumoral activation in heart failure include decreased input from inhibitory baroreceptor afferent vessels and increased input from excitatory afferent vessels arising from arterial chemoreceptors, skeletal muscle metaboreceptors or the lungs. Not all subjects with left ventricular dysfunction have increased sympathetic nerve activity, but the magnitude of sympathoneural activation appears to independently predict survival. This association suggests both a causative mechanism linking sympathetic activation with adverse outcome and a therapeutic opportunity to improve the prognosis of such patients by inhibiting central sympathetic outflow. Generalized sympathetic activation is not unique to heart failure, and its functional consequences appear to be both organ- and condition-specific. Sympathetic activation is present in other disorders such as mild hypertension, cirrhosis and aging that do not share the dim prognosis of congestive heart failure. The adverse effects of adrenergic activation on the diseased myocardium may be a function of the magnitude and time course of increases in cardiac sympathetic nerve activity, the mechanical and electrophysiologic consequences of nonuniform abnormalities of sympathetic innervation in the failing heart and the absence of specific countervailing mechanisms present in other conditions also characterized by increased sympathetic traffic. The hypotheses that activation of adrenergic drive to the diseased myocardium is the causative mechanism linking sympathoexcitation to adverse outcome and that interventions that inhibit sympathetic outflow to the heart will improve the prognosis of patients with congestive heart failure have not been specifically tested. Greater understanding of the mechanisms responsible for the heterogeneity of sympathetic activation in response to ventricular dysfunction, for cardiac-specific and generalized activation of the sympathetic nervous system and for the stimulation or suppression of countervailing mechanisms capable of resisting its adverse effects is fundamental to the development of better therapies for congestive heart failure.

Reduced sympathetic nervous activity. A potential mechanism predisposing to body weight gain

The sympathetic nervous system is recognized to play a role in the etiology of animal and possibly human obesity through its impact on energy expenditure and/or food intake. We, therefore, measured fasting muscle sympathetic nerve activity (MSNA) in the peroneal nerve and its relationship with energy expenditure and body composition in 25 relatively lean Pima Indian males (means +/- SD; 26 +/- 6 yr, 82 +/- 19 kg, 28 +/- 10% body fat) and 19 Caucasian males (29 +/- 5 yr, 81 +/- 13 kg, 24 +/- 9% body fat). 24-h energy expenditure, sleeping metabolic rate, and resting metabolic rate were measured in a respiratory chamber, whereas body composition was estimated by hydrodensitometry. Pima Indians had lower MSNA than Caucasians (23 +/- 6 vs 33 +/- 10 bursts/min, P = 0.0007). MSNA was significantly related to percent body fat in Caucasians (r = 0.55, P = 0.01) but not in Pimas. MSNA also correlated with energy expenditure adjusted for fat-free mass, fat mass, and age in Caucasians (r = 0.51, P = 0.03; r = 0.54, P = 0.02; and r = 0.53, P = 0.02 for adjusted 24-h energy expenditure, sleeping metabolic rate, and resting metabolic rate, respectively) but not in Pima Indians. In conclusion, the activity of the sympathetic nervous system is a determinant of energy expenditure in Caucasians. Individuals with low resting MSNA may be at risk for body weight gain resulting from a lower metabolic rate. A low resting MSNA and the lack of impact of MSNA on metabolic rate might play a role in the etiology of obesity in Pima Indians.

Possible genetic influence on the strength of human muscle nerve sympathetic activity at rest

Large reproducible interindividual differences in the strength of human muscle nerve sympathetic activity have been demonstrated previously without satisfactory explanation. We undertook the present study to investigate whether a genetic influence may be a factor of importance. Microneurographic recordings of sympathetic impulse traffic were made in the peroneal nerve in nine pairs of monozygotic male twins and eight pairs of age-matched male subjects without family relationship. The strength of the sympathetic activity was quantitated as number of sympathetic bursts per 100 heart beats and bursts per minute. Group mean values of muscle sympathetic activity, heart rate, and blood pressure were similar in the two groups. Intrapair differences (mean +/- SEM) of sympathetic activity were 5.4 +/- 1.7 bursts per 100 heart beats (1.7 +/- 0.5 bursts per minute) for the twins and 19.4 +/- 3.2 bursts per 100 heart beats (11.8 +/- 2.5 bursts per minute) for the control subjects (P < .01 for both). The degree of reproducibility between twins is similar to that reported previously between repeated recordings in the same subject. The finding may indicate that the strength of sympathetic outflow to muscle is controlled genetically. If so, we speculate that this may contribute to the heritability of blood pressure in both normotensive and hypertensive subjects.

Cyclosporine therapy after cardiac transplantation causes hypertension and renal vasoconstriction without sympathetic activation

BACKGROUND

Hypertension frequently complicates the use of cyclosporine A (CyA) therapy, and it has been suggested that sympathoexcitation may be the underlying mechanism in this form of hypertension.

METHODS AND RESULTS

To further investigate the possibility of a neurogenic mechanism for this hypertensive effect, we studied the effects of CyA on renal blood flow (n = 11), forearm blood flow (n = 8), and sympathetic nervous system activity, assessed by renal and whole-body radiolabeled norepinephrine plasma kinetics and muscle sympathetic nerve firing (using microneurography) in cardiac transplant recipients receiving CyA and a reference group of healthy age-matched control subjects (n = 17). In 11 cardiac transplant patients (2 hours after cyclosporine dose), renal blood flow was significantly lower than that in 8 control subjects (680 +/- 88 vs 1285 +/- 58 mL/min, P < .001). In 5 of these transplant patients, renal blood flow was measured before and for 2 hours after oral cyclosporine and fell progressively over this period, by 37% (P < .01). Total body and renal norepinephrine spillover rates in transplant patients were similar to those in control subjects (3070 +/- 538 vs 2618 +/- 313 pmol/min and 579 +/- 124 vs 573 +/- 95 pmol/min, respectively), and there was no progressive effect in the 2 hours after cyclosporine dosing. Forearm blood flow was increased 2 hours after CyA administration (1.74 +/- 0.31 to 3.12 +/- 0.50 mL x 100 mL-1 x min-1, P < .001), whereas mean arterial blood pressure and noninvasively determined cardiac output (indirect Fick method) were unchanged. Muscle sympathetic nerve discharge rates recorded in 6 of these transplant patients were not different from those in 9 healthy control subjects (37.9 +/- 10.1 vs 41.3 +/- 2.3 bursts per 100 beats per minute). During 90 to 120 minutes of recording after cyclosporine dosing, nerve firing rates remained unchanged.

CONCLUSIONS

CyA therapy causes acute renal vasoconstriction without accompanying systemic hemodynamic effects. These renal effects are nonneural, not being attributable to sympathoexcitation.

Elevated sympathetic nerve activity in patients with accelerated essential hypertension

To determine if an abnormality exists in the sympathetic nervous system of patients with accelerated hypertension, we recorded muscle sympathetic nerve activity (MSNA) from the tibial nerve by microneurography in eight benign essential hypertensives and seven accelerated essential hypertensives. Basal MSNA, plasma renin activity, and plasma angiotensin II levels were significantly higher in accelerated hypertensives than in benign hypertensives (P < 0.05). To clarify the relationship between the renin-angiotensin axis and sympathetic nervous system in the accelerated hypertensives, we measured the MSNA after 7 d of oral administration of captopril (75 mg/d) for antihypertensive treatment in the benign hypertensives and accelerated hypertensives. After administering captopril, the arterial pressure decreased significantly in the benign hypertensives and accelerated hypertensives with decreases in plasma angiotensin II levels, and the decreases in arterial pressure were greater in the accelerated hypertensive than in the benign hypertensives. After captopril administration, the MSNA decreased significantly in the accelerated hypertensives but did not change in the benign hypertensives. Thus, in accelerated hypertensives, sympathetic tone is elevated, and the elevated sympathetic tone is closely related to the activated renin-angiotensin axis tone.

Augmented resting sympathetic activity in awake patients with obstructive sleep apnea

Muscle nerve sympathetic activity (MSA) was recorded during wakefulness in 11 patients with obstructive sleep apnea (OSA) and in 9 sex- and age-matched healthy control subjects. Plasma levels of norepinephrine (NE) and neuropeptide Y were analyzed. Five patients had established hypertension (resting supine systolic BP/diastolic BP > or = 160/95 mm Hg). The investigation was performed after a minimum of 3 weeks' washout period of antihypertensive medication. Muscle sympathetic activity during supine rest was higher in patients compared with controls (p < 0.01) with no difference between normotensive and hypertensive patients. However, systolic, but not diastolic, BP was positively related to resting MSA (n = 20, p < 0.01). There was no significant correlation between body mass index and MSA. Resting MSA was unrelated to disease severity expressed as apnea frequency or minimum SaO2 during the overnight recording. Both the arterial and venous plasma norepinephrine was higher in patients compared with controls (p < 0.05). Plasma levels of NE correlated to resting MSA (p < 0.01) in the whole study group (patients and controls) but not within the respective subgroups. No significant correlation, however, was found between plasma NE (arterial and venous) and BP. Plasma neuropeptide Y-like immunoreactivity was similar in patients and controls. However, one patient with hypertension had approximately twice this level in repeated samples. It is concluded that neurogenic sympathetic activity as well as circulating plasma NE is increased in patients with OSA. This increased sympathetic activity during awake supine rest may reflect a pathophysiologic adaptation to hypoxia and hemodynamic changes occurring at repetitive apneas during sleep. The correlation between MSA and systolic BP implies that this mechanism may be directly or indirectly involved in the development of cardiovascular complications in OSA.

Long-term variability and reproducibility of resting human muscle nerve sympathetic activity at rest, as reassessed after a decade

Human muscle nerve sympathetic activity measured by microneurography during supine rest is known to vary considerably between healthy subjects, whereas in a given individual the level of muscle nerve sympathetic activity is stable over weeks and months. To further characterize long-term variability or reproducibility microneurographic recordings of muscle nerve sympathetic activity were performed in 15 healthy, normotensive subjects (mean age 51 years) who had undergone the same procedure between 10 and 14 years earlier (mean 12 years). The range of muscle nerve sympathetic activity was 9-59 in the first and 13-61 bursts/min in the second recording. Subjects maintained the level of muscle nerve sympathetic activity displayed previously, although with a slight but significant tendency to a higher outflow with increasing age. It is concluded that muscle nerve sympathetic activity is characterized by large inter-individual differences and strong intra-individual reproducibility over many years, with a tendency to increase with age. The age relationship is only in a minor part responsible for the variability, the cause of which remains unexplained. Because of the marked difference between individuals, strict normality criteria are difficult to define when comparing groups of subjects. There remains the risk of either obtaining spurious differences or obscuring a true abnormality. This is unlikely to apply when results in individual subjects are compared.

Age and gender influence muscle sympathetic nerve activity at rest in healthy humans

Muscle sympathetic nerve activity at rest increases with age in humans. The respective influences of the aging process per se and gender on this increase and whether age and gender effects on muscle sympathetic nerve activity can be identified with plasma norepinephrine concentrations, however, have not been established. To examine these issues, nine young women (aged 24 +/- 1 years; mean +/- SEM), eight young men (aged 26 +/- 1 years), seven older women (aged 63 +/- 1 years), and eight older men (aged 66 +/- 1 years) were studied. All were healthy, normotensive (blood pressure < 140/90 mm Hg), nonobese (< 20% above ideal weight), unmedicated, nonsmokers engaged in minimal to recreational levels of chronic physical activity. Arterial blood pressure (manual sphygmomanometry, brachial artery), heart rate, muscle sympathetic nerve activity (peroneal microneurography), and antecubital venous plasma norepinephrine concentrations (radioenzymatic assay) were determined during quiet supine resting conditions. Body weight was higher in men, but there were no age-related differences, whereas estimated body fat (sum of skinfolds) was higher in women and in the older groups (p < 0.05). Estimated daily energy expenditure, arterial blood pressure, and heart rate were not different among the groups. Both muscle sympathetic nerve activity burst frequency and burst incidence at rest were progressively higher in the young women, young men, older women, and older men (10 +/- 1 versus 18 +/- 2 versus 25 +/- 3 versus 39 +/- 5 bursts/min and 16 +/- 1 versus 30 +/- 4 versus 40 +/- 3 versus 61 +/- 6 bursts/100 heartbeats, respectively; all p < 0.05 versus each other).(ABSTRACT TRUNCATED AT 250 WORDS)

Catecholamines and essential hypertension

Given the ubiquitous distribution of catecholamines in mammals, and their importance in a range of physiological processes pivotal to blood pressure regulation, the subject of catecholamines and essential hypertension has a broader context than simply consideration of sympathetic nervous system and adrenal medullary dysfunction. These further matters are the likely involvement in hypertension pathogenesis of the CNS catecholaminergic neurones influencing peripheral sympathetic outflow, the possible pathogenetic significance of adrenaline released as a cotransmitter in sympathetic nerves, and the natriuretic renal tubular dopamine mechanisms for regulating body sodium balance which appear to be impaired in patients with essential hypertension. The central consideration, however, remains the important issue of the causes and consequences of the now well-documented sympathetic nervous overactivity which characterizes the early developmental phases of essential hypertension.

Hypertension in blacks: socioeconomic stress and sympathetic nervous system activity

Primary hypertension is almost twice as prevalent among American blacks as among whites. Causes of this increased prevalence of hypertension remain elusive. Elevation of sympathetic nervous system activity, in part secondary to increased levels of socioeconomic stress, is hypothesized as playing a role. Increased sympathetic nervous system activity may increase peripheral vascular resistance directly or through increased vascular reactivity. Microneurography allows direct measurement of peripheral sympathetic nervous system activity. Application of this technique will allow comparison of sympathetic nervous system activity in black and white subjects and provide additional insight into the role of the sympathetic nervous system in the development of hypertension in blacks.