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

Evening but not morning aerobic training improves sympathetic activity and baroreflex sensitivity in elderly patients with treated hypertension

The blood pressure-lowering effect of aerobic training is preceded by improving cardiovascular autonomic control. We previously demonstrated that aerobic training conducted in the evening (ET) induces a greater decrease in blood pressure than morning training (MT). To study whether the greater blood pressure decrease after ET occurs through better cardiovascular autonomic regulation, this study aimed to compare MT versus ET on muscle sympathetic nerve activity (MSNA) and baroreflex sensitivity (BRS) in treated patients with hypertension. Elderly patients treated for hypertension were randomly allocated into MT (n = 12, 07.00-10.00 h) or ET (n = 11, 17.00-20.00 h) groups. Both groups trained for 10 weeks, 3 times/week, cycling for 45 min at moderate intensity. Beat-to-beat blood pressure (finger photoplethysmography), heart rate (electrocardiography) and MSNA (microneurography) were assessed at the initial and final phases of the study at baseline and during sequential bolus infusions of sodium nitroprusside and phenylephrine (modified-Oxford technique) to evaluate cardiac and sympathetic BRS. Mean blood pressure decreased significantly after ET but not after MT (-9 ± 11 vs. -1 ± 8 mmHg, P = 0.042). MSNA decreased significantly only after ET with no change after MT (-12 ± 5 vs. -3 ± 7 bursts/100 heart beats, P = 0.013). Sympathetic BRS improved after ET but not after MT (-0.8 ± 0.7 vs. 0.0 ± 0.8 bursts/100 heart beats/mmHg, P = 0.052). Cardiac BRS improved similarly in both groups (ET: +1.7 ± 1.8 vs. MT: +1.4 ± 1.9 ms/mmHg, Pphase  ≤ 0.001). In elderly patients treated for hypertension, only ET decreased mean blood pressure and MSNA and improved sympathetic BRS. These findings revealed that the sympathetic nervous system has a key role in ET's superiority to MT in blood pressure-lowering effect. KEY POINTS: Reducing muscle nerve sympathetic activity and increasing sympathetic baroreflex sensitivity plays a key role in promoting the greater blood pressure reduction observed with evening training. These findings indicated that simply changing the timing of exercise training may offer additional benefits beyond antihypertensive medications, such as protection against sympathetic overdrive and loss of baroreflex sensitivity, independent markers of mortality. Our new findings also suggest new avenues of investigation, such as the possibility that evening aerobic training may be beneficial in other clinical conditions with sympathetic overdrive, such as congestive heart failure and hypertrophic cardiomyopathy.

Autonomic nervous system activity changes in patients with hypertension and overweight: role and therapeutic implications

The incidence and prevalence of hypertension is increasing worldwide, with approximately 1.13 billion of people currently affected by the disease, often in association with other diseases such as diabetes mellitus, chronic kidney disease, dyslipidemia/hypercholesterolemia, and obesity. The autonomic nervous system has been implicated in the pathophysiology of hypertension, and treatments targeting the sympathetic nervous system (SNS), a key component of the autonomic nervous system, have been developed; however, current recommendations provide little guidance on their use. This review discusses the etiology of hypertension, and more specifically the role of the SNS in the pathophysiology of hypertension and its associated disorders. In addition, the effects of current antihypertensive management strategies, including pharmacotherapies, on the SNS are examined, with a focus on imidazoline receptor agonists.

Normotension, hypertension and body fluid regulation: brain and kidney

The fraction of hypertensive patients with essential hypertension (EH) is decreasing as the knowledge of mechanisms of secondary hypertension increases, but in most new cases of hypertension the pathophysiology remains unknown. Separate neurocentric and renocentric concepts of aetiology have prevailed without much interaction. In this regard, several questions regarding the relationships between body fluid and blood pressure regulation are pertinent. Are all forms of EH associated with sympathetic overdrive or a shift in the pressure-natriuresis curve? Is body fluid homoeostasis normally driven by the influence of arterial blood pressure directly on the kidney? Does plasma renin activity, driven by renal nerve activity and renal arterial pressure, provide a key to stratification of EH? Our review indicates that (i) a narrow definition of EH is useful; (ii) in EH, indices of cardiovascular sympathetic activity are elevated in about 50% of cases; (iii) in EH as in normal conditions, mediators other than arterial blood pressure are the major determinants of renal sodium excretion; (iv) chronic hypertension is always associated with a shift in the pressure-natriuresis curve, but this may be an epiphenomenon; (v) plasma renin levels are useful in the analysis of EH only after metabolic standardization and then determination of the renin function line (plasma renin as a function of sodium intake); and (vi) angiotensin II-mediated hypertension is not a model of EH. Recent studies of baroreceptors and renal nerves as well as sodium intake and renin secretion help bridge the gap between the neurocentric and renocentric concepts.

Sympathomodulatory Effects of Antihypertensive Drug Treatment

BACKGROUND

An activation of sympathetic neural influences to the heart and peripheral circulation has been shown to represent a hallmark of the essential hypertensive state, adrenergic neural factors participating together with other variables at the development and progression of the high blood pressure state as well as of the hypertension-related target organ damage. This represents the rationale for employing in hypertension treatment drugs which combine the blood pressure-lowering properties with the modulatory effects on the sympathetic neural function.

METHODS AND RESULTS

Several studies published during the past 40 years have investigated the impact of antihypertensive drugs on the sympathetic target as assessed by indirect and direct approaches. In the present paper, the effects of different monotherapies or combination drug treatment used in hypertension to lower elevated blood pressure values on various adrenergic markers will be examined. This will be followed by a discussion of the (i) hemodynamic and nonhemodynamic consequences of employing antihypertensive drugs with sympathomodulatory or sympathoexcitatory properties and (ii) mechanisms potentially responsible for the adrenergic responses to a given antihypertensive drug. The final part of this review will address the questions still open related to the impact of antihypertensive drug treatment on sympathetic function. Two questions in particular will be examined, i.e., whether antihypertensive drugs with sympathomodulatory properties may be capable to fully restore a "normal" adrenergic drive and how far sympathetic activity should be reduced in hypertensive patients.

CONCLUSION

Future investigations aimed at answering these questions will be needed in order to improve cardiovascular protection in treated hypertensive patients.

The sympathetic nervous system as a target for the treatment of hypertension and cardiometabolic diseases

The regulation of blood pressure by the sympathetic nervous system is reviewed with an emphasis on the role of the sympathetic nervous system in the development and maintenance of hypertension. Evidence from patients and animal models is summarized. Because it is clear that there is a neural contribution to many types of human hypertension and other cardiometabolic diseases, the case is presented for a renewed emphasis on the development of sympatholytic approaches for the treatment of hypertension and other conditions associated with hyperactivity of the sympathetic nervous system.

Quantification of cardiac autonomic nervous activities in ambulatory dogs by eliminating cardiac electric activities using cubic smoothing spline

With the development of an implantable radio transmitter system, direct measurement of cardiac autonomic nervous activities (CANAs) became possible for ambulatory animals for a couple of months. However, measured CANAs include not only CANA but also cardiac electric activity (CEA) that can affect the quantification of CANAs. In this study, we propose a novel CEA removal method using moving standard deviation and cubic smoothing spline. This method consisted of two steps of detecting CEA segments and eliminating CEAs in detected segments. Using implanted devices, we recorded stellate ganglion nerve activity (SGNA), vagal nerve activity (VNA) and superior left ganglionated plexi nerve activity (SLGPNA) directly from four ambulatory dogs. The CEA-removal performance of the proposed method was evaluated and compared with commonly used high-pass filtration (HPF) for various heart rates and CANA amplitudes. Results tested with simulated CEA and simulated true CANA revealed stable and excellent performance of the suggested method compared to the HPF method. The averaged relative error percentages of the proposed method were less than 0.67%, 0.65% and 1.76% for SGNA, VNA and SLGPNA, respectively.

Brain angiotensin peptides regulate sympathetic tone and blood pressure

Brain angiotensin II (Ang II) induces tonic sympathoexcitatory effects through AT1 receptor stimulation of glutamatergic neurons and sympathoinhibitory effects via GABAergic neurons in the rostral ventrolateral medulla, the brainstem 'pressor area'. NADPH-derived superoxide production and reactive oxygen species signalling is critical in these actions, and AT2 receptors in the rostral ventrolateral medulla appear to mediate opposing effects on sympathetic outflow. In the hypothalamic paraventricular nucleus, Ang II has AT1 receptor-mediated sympathoexcitatory effects and enhances nitric oxide formation, which in turn inhibits the Ang II effects through a GABAergic mechanism. Ang II also decreases the tonic sympathoinhibitory effect of gamma amino butyric acid within the paraventricular nucleus. Angiotensin III and Angiotensin IV increase blood pressure via brain AT1 receptor stimulation. Angiotensin (1-7) influences cardiovascular function through a specific Mas-receptor. This review examines the evidence that brain angiotensin peptides, glutamate, gamma amino butyric acid and nitric oxide interact within the rostral ventrolateral medulla and paraventricular nucleus to control sympathetic tone and blood pressure.

Is kidney ischemia the central mechanism in parallel activation of the renin and sympathetic system?

In chronic kidney disease simultaneous activation of the renin-angiotensin and sympathetic systems occurs. Kidney ischemia seems to play a key role in the pathogenesis. This review firstly summarizes experimental and clinical evidence in chronic kidney disease supporting this idea and addresses the possibility that this mechanism is also relevant in some other disease conditions.

Circulating angiotensin II and dietary salt: converging signals for neurogenic hypertension

Circulating angiotensin II (Ang II) combined with high salt intake increases sympathetic nerve activity (SNA) in some forms of hypertension. Ang II-induced increases in SNA are modest, delayed, and specific to certain vascular beds. The brain targets for circulating Ang II are neurons in the area postrema (AP), subfornical organ (SFO), and possibly other circumventricular organs. Ang II signaling is integrated with sodium-sensitive neurons in the SFO and/or organum vasculosum of the lamina terminalis (OVLT) and drives sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM) via the paraventricular nucleus (PVN). It is likely that, over time, new patterns of gene expression emerge within neurons of the SFO-PVN-RVLM pathway that transform their signaling properties. This transformation is critical in maintaining increased SNA. Identification of a novel gene supporting this process may provide new targets for treatment of neurogenic hypertension.

Decreased renal sympathetic activity in response to cardiac unloading with nitroglycerin in patients with heart failure*

AIMS

To examine changes in renal sympathetic outflow in response to cardiac unloading with nitroglycerin (GTN) in patients with chronic heart failure (CHF) and healthy subjects (HS).

METHODS AND RESULTS

Renal (RNAsp) and total body (TBNAsp) noradrenaline (NA) spillover were measured with radiotracer methods in 16 patients with CHF (50+/-3 years, LVEF 20+/-1%) and nine HS (57+/-2 years) during right heart and renal vein catheterisation. Low dose GTN decreased mean pulmonary artery pressure (PAm: CHF -7+/-2 mm Hg, HS -4+/-1 mm Hg, p<0.05 vs. baseline) but not mean arterial pressure (MAP: CHF -2+/-1 mm Hg, HS -2+/-1 mm Hg) and did not affect RNAsp in any of the study groups. High dose GTN lowered MAP (CHF -12+/-1 mm Hg, HS -12+/-2 mm Hg, p<0.05 vs. baseline) and PAm (CHF -13+/-2 mm Hg, HS -5+/-1 mm Hg, p<0.05 vs. baseline) and was accompanied by a significant reduction in RNAsp only in CHF (1.3+/-0.1 nmol/min baseline to 0.9+/-0.2 nmol/min, p<0.05), whereas RNAsp in HS remained unchanged.

CONCLUSIONS

In spite of a reduction in both arterial pressure and cardiac filling pressures, renal sympathetic activity decreased in CHF and did not increase in HS. These findings suggest that the altered loading conditions resulting from high-dose GTN infusion have renal sympathoinhibitory effects.

Nitric oxide in hypertension

Hypertension is a major risk factor for cardiovascular disease, and reduction of elevated blood pressure significantly reduces the risk of cardiovascular events. Endothelial dysfunction, which is characterized by impairment of nitric oxide (NO) bioavailability, is an important risk factor for both hypertension and cardiovascular disease and may represent a major link between the conditions. Evidence suggests that NO plays a major role in regulating blood pressure and that impaired NO bioactivity is an important component of hypertension. Mice with disruption of the gene for endothelial NO synthase have elevated blood pressure levels compared with control animals, suggesting a genetic component to the link between impaired NO bioactivity and hypertension. Clinical studies have shown that patients with hypertension have a blunted arterial vasodilatory response to infusion of endothelium-dependent vasodilators and that inhibition of NO raises blood pressure. Impaired NO bioactivity is also implicated in arterial stiffness, a major mechanism of systolic hypertension. Clarification of the mechanisms of impaired NO bioactivity in hypertension could have important implications for the treatment of hypertension.

Microneurography as a tool in clinical neurophysiology to investigate peripheral neural traffic in humans

Microneurography is a method using metal microelectrodes to investigate directly identified neural traffic in myelinated as well as unmyelinated efferent and afferent nerves leading to and coming from muscle and skin in human peripheral nerves in situ. The present paper reviews how this technique has been used in clinical neurophysiology to elucidate the neural mechanisms of autonomic regulation, motor control and sensory functions in humans under physiological and pathological conditions. Microneurography is particularly important to investigate efferent and afferent neural traffic in unmyelinated C fibers. The recording of efferent discharges in postganglionic sympathetic C efferent fibers innervating muscle and skin (muscle sympathetic nerve activity; MSNA and skin sympathetic nerve activity; SSNA) provides direct information about neural control of autonomic effector organs including blood vessels and sweat glands. Sympathetic microneurography has become a potent tool to reveal neural functions and dysfunctions concerning blood pressure control and thermoregulation. This recording has been used not only in wake conditions but also in sleep to investigate changes in sympathetic neural traffic during sleep and sleep-related events such as sleep apnea. The same recording was also successfully carried out by astronauts during spaceflight. Recordings of afferent discharges from muscle mechanoreceptors have been used to understand the mechanisms of motor control. Muscle spindle afferent information is particularly important for the control of fine precise movements. It may also play important roles to predict behavior outcomes during learning of a motor task. Recordings of discharges in myelinated afferent fibers from skin mechanoreceptors have provided not only objective information about mechanoreceptive cutaneous sensation but also the roles of these signals in fine motor control. Unmyelinated mechanoreceptive afferent discharges from hairy skin seem to be important to convey cutaneous sensation to the central structures related to emotion. Recordings of afferent discharges in thin myelinated and unmyelinated fibers from nociceptors in muscle and skin have been used to provide information concerning pain. Recordings of afferent discharges of different types of cutaneous C-nociceptors identified by marking method have become an important tool to reveal the neural mechanisms of cutaneous sensations such as an itch. No direct microneurographic evidence has been so far proved regarding the effects of sympathoexcitation on sensitization of muscle and skin sensory receptors at least in healthy humans.

Influence of central inhibition of sympathetic nervous activity on myocardial metabolism in chronic heart failure: acute effects of the imidazoline I1-receptor agonist moxonidine

Although β-adrenergic blockade is beneficial in heart failure, inhibition of central sympathetic outflow using moxonidine has been associated with increased mortality. In the present study, we studied the acute effects of the imidazoline-receptor agonist moxonidine on haemodynamics, NA (noradrenaline) kinetics and myocardial metabolism. Fifteen patients with CHF (chronic heart failure) were randomized to a single dose of 0.6 mg of sustained-release moxonidine or matching placebo. Haemodynamics, NA kinetics and myocardial metabolism were studied over a 2.5 h time period. There was a significant reduction in pulmonary and systemic arterial pressures, together with a decrease in cardiac index in the moxonidine group. Furthermore, there was a simultaneous reduction in systemic and cardiac net spillover of NA in the moxonidine group. Analysis of myocardial consumption of substrates in the moxonidine group showed a significant increase in non-esterified fatty acid consumption and a possible trend towards an increase in myocardial oxygen consumption compared with the placebo group (P=0.16). We conclude that a single dose of moxonidine (0.6 mg) in patients already treated with a β-blocker reduced cardiac and overall sympathetic activity. The finding of increased lipid consumption without decreased myocardial oxygen consumption indicates a lack of positive effects on myocardial metabolism under these conditions. We suggest this might be a reason for the failure of moxonidine to prevent deaths in long-term studies in CHF.

Peripheral sympatholytic actions of four AT1 antagonists: are they relevant for long-term antihypertensive efficacy?

BACKGROUND

Angiotensin II causes hypertension not only by direct constriction of vascular smooth muscle, but also by facilitating the release of noradrenaline from sympathetic terminals and by enhancing vascular noradrenaline sensitivity. AT1 receptor antagonists attenuate all these actions, but display some evidence of substance-related selectivities.

OBJECTIVE

The contribution of pre- or postsynaptic impairment of sympathetic transmission to long-term antihypertensive efficacy should be determined for four structurally different, clinically approved AT1 antagonists.

DESIGN

Spontaneously hypertensive rats were treated with candesartan, eprosartan, irbesartan, or losartan via osmotic minipumps for 4 weeks at doses yielding identical reductions of blood pressure. Maximum efficacy was obtained with a tripled dose of candesartan.

METHODS

In the pithed rat model, stimulus/response dependencies were determined for vasopressor effectivity of preganglionic electrical stimulation, and of intravenous bolus applications of noradrenaline and angiotensin II.

RESULTS

Losartan, irbesartan, eprosartan, and candesartan at doses of 5, 40, 20, and 0.05 mg/kg per day, were equally effective in reducing basal systolic blood pressure (-42 mmHg), and the vasopressor potency of angiotensin II (approximately 10-fold). The efficacies of preganglionic stimulation and exogenous noradrenaline were unaltered, with the exception of irbesartan, which reduced vascular noradrenaline sensitivity. The tripled dose of candesartan further reduced basal and angiotensin II-stimulated blood pressures, and significantly attenuated vascular noradrenaline sensitivity.

CONCLUSION

AT1 antagonists at doses that effectively reduce blood pressure in chronic therapy do not generally suppress peripheral sympathetic function. A potential interaction consists in a reduction of vascular noradrenaline sensitivity, which can be considered as a class effect of AT1 antagonists at high dosage.

Role of a Responsive Sympathetic Nervous System in the Chronic Hypotensive Effects of Losartan in Normal Rats

We have previously demonstrated the chronic hypotensive effects of the AT1 antagonist losartan in normotensive, salt-replete rats. We hypothesized that the chronic effects of losartan are mediated in part by blockade of the central sympathoexcitatory actions of angiotensin II. To test this hypothesis, we have used a novel approach to effectively "clamp" the sympathetic nervous system at a fixed level through chronic administration of the ganglionic blocking agent hexamethonium (15 mg/kg/h) and the alpha agonist phenylephrine (2.26 mg/kg/d). Two of 3 groups of rats [CON and CLAMP(NNa)] were placed on (0.1%) NaCl diets, whereas the third [CLAMP(LNa)] was placed on a low (0.002%)-sodium diet. Continuous measurements of mean arterial pressure (MAP) were made via radiotelemetry. After 9 days of hexamethonium plus phenylephrine treatment in CLAMP(NNa) and CLAMP(LNa) rats, baseline MAP was not different in all 3 groups of rats: CON (104+/-4 mm Hg), CLAMP(NNa) (104+/-4 mm Hg), and CLAMP(LNa) (106+/-2 mm Hg). After 5 days of subsequent losartan treatment, a change in MAP of only -7+/-2 mm Hg was observed in CLAMP(NNa) rats compared with -22+/-2 mm Hg in CON and CLAMP(LNa) rats. These results do not support the hypothesis that the hypotensive actions of losartan are entirely dependent on a responsive sympathetic nervous system rats.

HYPOTHESIS: SET-POINTS and LONG-TERM CONTROL OF ARTERIAL PRESSURE. A THEORETICAL ARGUMENT FOR A LONG-TERM ARTERIAL PRESSURE CONTROL SYSTEM IN THE BRAIN RATHER THAN THE KIDNEY

1. It has been hypothesised that the 'set-point' for the long-term control of mean arterial (MAP) resides within the kidney. In this model, the set-point of the 'chronic renal function curve' establishes the steady state relationship between renal perfusion pressure and urinary excretion of sodium and water, which, in turn, affects blood volume and cardiac output. The 'renal-MAP set-point' theory predicts that the kidney controls MAP to maintain its own excretory function and that long-term regulation of blood volume and cardiac output are paramount to the regulation of arterial pressure. 2. An alternative hypothesis is proposed in which the 'set-point' for the long-term control of MAP resides within the central nervous system (CNS) rather than the kidney. In contrast with the 'renal-MAP set-point' model, the 'CNS-MAP set-point' model dictates that the brain controls MAP to maintain cerebral blood flow and CNS function. 3. The 'CNS-MAP set-point hypothesis' predicts that long-term regulation of MAP is paramount to the regulation of blood volume and cardiac output. It is proposed that the 'CNS-MAP set-point' system operates independently of the arterial baroreceptor reflex, which is a short-term controller of MAP. 4. The precise mechanisms by which the CNS 'senses' MAP are complex and remain to be discovered. The MAP 'sensor' likely involves integration of hormone levels linked to body fluid homeostasis and osmoreceptor and baroreceptor inputs. It is also proposed that an as yet undiscovered 'central baroreceptor' exists within the brain itself. 5. The 'CNS-MAP set-point hypothesis' predicts that many forms of experimental and essential hypertension are due to a primary shift in the CNS-MAP set-point.

Regional norepinephrine spillover in response to angiotensin-converting enzyme inhibition in healthy subjects

OBJECTIVES

Even though most previous studies have shown that central nervous angiotensin II causes sympatho-excitation, there are data suggesting that blockade of the renin-angiotensin system (RAS) could activate the renal sympathetic nerves. The aim of the present study was to investigate overall, cardiac and renal sympathetic nerve activity, before and after intravenous enalaprilat, in healthy normotensive subjects without an activated RAS.

METHODS

Thirty healthy subjects underwent catheterization of the radial artery, right renal and coronary sinus veins with blood sampling at baseline and 30-40 min after 1.25 or 2.5 mg of intravenous enalaprilat, respectively. Regional and overall sympathetic nervous activity was estimated using isotope dilution, calculating spillovers of norepinephrine.

RESULTS

Mean arterial blood pressure decreased by 5% (P < 0.05) after the higher dose and remained unchanged after the lower dose of enalaprilat, whereas renal norepinephrine spillover increased after both doses by 49 and 26% respectively (P < 0.05 for both). Cardiac and total body norepinephrine spillover remained unchanged after both doses of enalaprilat. Pulmonary capillary wedge pressure, which was measured in eight subjects after 2.5 mg enalaprilat, fell by 43% (P < 0.05).

CONCLUSIONS

In the absence of, or after a minor, blood pressure fall, intravenous enalaprilat selectively activates the renal nerves in healthy subjects without an activated RAS. Unloading of the low-pressure baroreceptor system and/or a central nervous effect of enalaprilat may be responsible for this differentiated sympathetic nervous response.

Persistent lowering of arterial pressure after continuous and intermittent therapy

OBJECTIVE

The present study investigates the impact of antihypertensive treatment on persistent reduction of arterial pressure after cessation of drug treatment.

DESIGN AND METHODS

Specifically, adult spontaneously hypertensive rats (SHR) were treated for 6 weeks with inhibitors of the renin-angiotensin system (RAS), or combination therapy (hydralazine, nifedipine, hydrochlorothiazide) and following a 14-week 'drug holiday', were re-treated for 4 weeks. Mean arterial pressure (MAP) was continuously monitored via radiotelemetry.

RESULTS

Comparison in the first off-treatment period revealed that RAS inhibitor drugs produced a 16-18% persistent lowering of arterial pressure, whereas the triple therapy induced a 10% lowering of MAP relative to untreated SHR. The drug re-challenge induced a further 9% reduction in the 'off'-treatment level of MAP such that in all treatment groups MAP was reduced by more than 30 mmHg compared with controls.

CONCLUSIONS

This study provides new evidence that combination therapy, not directly targeting the RAS, can be efficacious in persistently reducing MAP off-treatment. Furthermore, we demonstrated that the 6-week treatment with RAS inhibitors induced equivalent persistent changes as a 10-week treatment. That is, the additional 4 weeks of continuous therapy was ineffective in further altering the off-treatment MAP. In contrast, with the intermittent treatment protocol (the 14-week 'drug holiday') a further effect on persistent lowering of MAP was regained. These findings suggest continuous long-term treatment with antihypertensive drugs may not be the most effective means of reversing underlying circulatory abnormalities and that the introduction of a drug holiday may be beneficial.

Comparison of the vascular and antiadrenergic activities of four angiotensin II type 1 antagonists in the pithed rat

BACKGROUND

Angiotensin II is known to facilitate the release of catecholamines from peripheral sympathetic neurons by stimulating presynaptically located receptors. Although inhibitor studies have revealed these to be angiotensin II type 1 (AT1) receptors, they do in fact appear to display peculiar susceptibilities to various AT1 receptor antagonists, which might correspond to different neuronal and vascular receptor subtypes.

OBJECTIVE

A direct comparison of the pre- and postsynaptic potencies of four AT1 antagonists was performed to characterize these receptors further.

DESIGN

We studied angiotensin II-induced catecholamine release and vasoconstriction in pithed, spontaneously hypertensive rats under the influence of candesartan, eprosartan, EXP 3174, and irbesartan. The effect of AT1 blockade on postsynaptic vascular sensitivity to noradrenaline (NA) was also determined.

METHODS

Pithed rats received repeated intravenous applications of either angiotensin II or NA, preceded by cumulatively increasing doses of the AT1 antagonists. Vasoconstriction and catecholamine release were quantified by the measurement of acute increases in blood pressure and plasma NA, respectively.

RESULTS

All AT1 antagonists dose-dependently suppressed angiotensin II-induced vasoconstriction and release of NA. Although the antagonists differed greatly in their inhibitory potencies (ID50 range 7-445 microg/kg), each displayed a similar potency at both neuronal and vascular angiotensin receptors. In a higher dose range, all AT1 antagonists attenuated the blood pressure increase in response to NA by up to 70%. The order of potencies for all inhibitory effects was: candesartan > eprosartan > EXP 3174 > irbesartan.

CONCLUSION

The AT1 antagonists tested do not discriminate between presynaptic neuronal and postsynaptic vascular angiotensin II receptors - a fact that refutes the existence of tissue-specific AT1 receptor subtypes. A marked reduction in vascular sensitivity to NA may contribute to the antihypertensive and cardioprotective mechanisms of AT1 antagonists.

Sympathetic nervous system and the kidney in hypertension

Long-term control of arterial pressure has been attributed to the kidney by virtue of its ability to couple the regulation of blood volume to the maintenance of sodium and water balance by the mechanisms of pressure natriuresis and diuresis. In the presence of a defect in renal excretory function, hypertension arises as the consequence of the need for an increase in arterial pressure to offset the abnormal pressure natriuresis and diuresis mechanisms, and to maintain sodium and water balance. There is growing evidence that an important cause of the defect in renal excretory function in hypertension is an increase in renal sympathetic nerve activity (RSNA). First, increased RSNA is found in animal models of hypertension and hypertensive humans. Second, renal denervation prevents or alleviates hypertension in virtually all animal models of hypertension. Finally, increased RSNA results in reduced renal excretory function by virtue of effects on the renal vasculature, the tubules, and the juxtaglomerular granular cells. The increase in RSNA is of central nervous system origin, with one of the stimuli being the action of angiotensin II, probably of central origin. By acting on brain stem nuclei that are important in the control of peripheral sympathetic vasomotor tone (e.g. rostral ventrolateral medulla), angiotensin II increases the basal level of RSNA and impairs its arterial baroreflex regulation. Therefore, the renal sympathetic nerves may serve as the link between central sympathetic nervous system regulatory sites and the kidney in contributing to the renal excretory defect in the development of hypertension.