Hélder Mauad

Neural reflex regulation of arterial pressure in pathophysiological conditions: interplay among the baroreflex, the cardiopulmonary reflexes and the chemoreflex

The maintenance of arterial pressure at levels adequate to perfuse the tissues is a basic requirement for the constancy of the internal environment and survival. The objective of the present review was to provide information about the basic reflex mechanisms that are responsible for the moment-to-moment regulation of the cardiovascular system. We demonstrate that this control is largely provided by the action of arterial and non-arterial reflexes that detect and correct changes in arterial pressure (baroreflex), blood volume or chemical composition (mechano- and chemosensitive cardiopulmonary reflexes), and changes in blood-gas composition (chemoreceptor reflex). The importance of the integration of these cardiovascular reflexes is well understood and it is clear that processing mainly occurs in the nucleus tractus solitarii, although the mechanism is poorly understood. There are several indications that the interactions of baroreflex, chemoreflex and Bezold-Jarisch reflex inputs, and the central nervous system control the activity of autonomic preganglionic neurons through parallel afferent and efferent pathways to achieve cardiovascular homeostasis. It is surprising that so little appears in the literature about the integration of these neural reflexes in cardiovascular function. Thus, our purpose was to review the interplay between peripheral neural reflex mechanisms of arterial blood pressure and blood volume regulation in physiological and pathophysiological states. Special emphasis is placed on the experimental model of arterial hypertension induced by N-nitro-L-arginine methyl ester (L-NAME) in which the interplay of these three reflexes is demonstrable.

ORGANIZATION OF ELECTRICALLY AND CHEMICALLY EVOKED DEFENSIVE BEHAVIORS WITHIN THE DEEPER COLLICULAR LAYERS AS COMPARED TO THE PERIAQUEDUCTAL GRAY MATTER OF THE RAT

Stimulation of the periaqueductal gray matter (PAG) and the deeper layers of superior colliculus (SC) produces both freezing (tense immobility) and flight (trotting, galloping and jumping) behaviors along with exophthalmus (fully opened bulging eyes) and, less often, micturition and defecation. The topography of these behaviors within the distinct layers of SC remains unclear. Therefore, this study compared the defensive repertoire of intermediate (ILSC) and deep (DLSC) layers of SC to those of dorsolateral periaqueductal gray matter (DLPAG) and lateral periaqueductal gray matter (LPAG) [Neuroscience 125 (2004) 71]. Electrical stimulation was carried out through intensity- (0-70 microA) and frequency-varying (0-130 Hz) pulses. Chemical stimulation employed a slow microinfusion of N-methyl-d-aspartic acid (NMDA, 0-2.3 nmol, 0.5 nmol/min). Probability curves of intensity-, frequency- and NMDA-evoked behaviors, as well as the unbiased estimates of median stimuli, were obtained by threshold logistic analysis. Compared with the PAG, the most important differences were the lack of frequency-evoked jumping in both layers of SC and the lack of NMDA-evoked galloping in the ILSC. Moreover, although galloping and jumping were also elicited by NMDA stimulation of DLSC, effective doses were about three times higher than those of DLPAG, suggesting the spreading of the injectate to the latter structure. In contrast, exophthalmus, immobility and trotting were evoked throughout the tectum structures. However, whatever the response and kind of stimulus, the lowest thresholds were always found in the DLPAG and the highest ones in the ILSC. Besides, neither the appetitive, nor the offensive, muricide or male reproductive behaviors were produced by any kind of stimulus in the presence of appropriate targets. Accordingly, the present data suggest that the deeper layers of SC are most likely involved in the increased attentiveness (exophthalmus, immobility) or restlessness (trotting) behaviors that herald a full-blown flight reaction (galloping, jumping) mediated in the PAG.

Differential chronotropic and dromotropic responses to focal stimulation of cardiac vagal ganglia in the rat

Vagal cardioinhibition is exerted through a reduction not only in the heart rate but also in the rate of propagation of the cardiac action potential and in myocardial contractility. In several species, such effects can be produced independently by selective activation of ganglia in identified ‘fat pads’. In this study we investigate differential control of heart rate and atrioventricular conduction by two ganglionic clusters in the rat, a species increasingly important in studies of cardiovascular control. Epicardial sites producing low‐threshold changes in P‐P and P‐R interval of the ECG in an arterially perfused preparation were explored with concentric bipolar stimulating electrodes. Stimulation sites centred on two principal ganglia, the sinoatrial (SA) ganglion at the junction of the right superior vena cava and right atrium, and the atrioventricular (AV) ganglion at the junction of the inferior pulmonary veins and left atrium. Stimulation of the SA ganglion decreased heart rate in all preparations, with little or no effect on AV conduction in one‐third. Stimulation of the AV ganglion consistently slowed conduction without eliciting a comparable bradycardia. Responses survived blockade of ganglionic transmission by trimetaphan, with an enhanced chronotropic selectivity to SA ganglion stimulation, suggesting that co‐excitation of preganglionic elements en passant may have contributed to the earlier mixed responses. Effective stimulation sites were precisely circumscribed and corresponded to principal ganglionic clusters confirmed histologically. We conclude that cardiac vagal ganglia in the rat show a topographical functional organisation and are amenable to investigation using the arterially perfused preparation.

Cardiovascular effects of scorpionfish (Scorpaena plumieri) venom

The aim of the present study was to investigate the cardiovascular activity of Scorpaena plumieri venom in both in vivo and in vitro models. In anesthetized rats, doses of the venom (14-216 microg protein/kg) induced a transient increase in the mean arterial pressure. However at higher dose (338 microg protein/kg) this effect was followed by a sudden hypotension and the animal evolved to death. The heart rate was temporarily increased and followed by bradycardia using doses > or =108 microg/kg. In isolated rat hearts the crude venom (5-80 microg protein) produced dose-dependent positive ventricular chronotropic, inotropic, lusitropic and coronary vasoconstriction responses. Partial purification of an active fraction (CF, cardiovascular fraction) which reproduced the cardiovascular effects induced by crude venom on isolated hearts was achieved by conventional gel filtration chromatography. Adrenergic blockades, prazosin and propranolol, significantly attenuated these responses. The coronary vasoconstriction response to CF was also attenuated by chemical endothelium denudation. In conclusion, the data showed that S. plumieri fish venom induces disorders in the cardiovascular system. It also suggests that alpha(1) and beta-adrenergic receptors, and the vascular endothelium, are involved at least partially, in these cardiac effects.

Effects of myocardial hypertrophy on neural reflexes controlling cardiovascular function

There are clinical and experimental evidences that the cardiopulmonary reflex function is impaired in chronic hypertension, but it could be due to myocardial hypertrophy rather than to hypertension itself. To test this hypothesis we evaluated the Bezold-Jarisch reflex in experimental conditions of myocardial hypertrophy and arterial normotension. Adult male Wistar rats were subjected to myocardial hypertrophy (MHR) treating them with the beta-adrenoceptor agonist isoproterenol (0.3 mg/kg/day, s.c.) for 15 days and compared with vehicle injected control rats (CR). No significant changes in body weight (283+/-14 vs. 299+/-9 g), resting mean arterial pressure (104+/-4 vs. 110+3 mm Hg) or heart rate (330+/-11 vs. 358+/-18 bpm) were observed in MHR compared to CR. As expected, MHR showed left and right ventricular and left atrial hypertrophy when compared to CR. The bradycardia and hypotension that characterizes the Bezold-Jarisch reflex, induced by the 5-HT3, agonist phenyldiguanide (1.5-24.0 microg/kg, i.v.), were significantly decreased in MHR compared to CR. Cardiac muscarinic responsiveness, which was assessed by electrical stimulation of the efferent vagus in anesthetized animals or by stimulation of muscarinic receptors in isolated hearts, was unchanged or increased, respectively, in MHR compared to CR. Additional studies showed that the baroreflex and chemoreflex were also attenuated in MHR compared to CR. These data indicate that cardiac hypertrophy impairs the Bezold-Jarisch reflex probably due to changes at central integrative areas of the reflex.

Involvement of the ipsilateral rostral ventrolateral medulla in the pressor response to L-glutamate microinjection into the nucleus tractus solitarii of awake rats.

Microinjection of L-glutamate into the lateral commissural nucleus tractus solitarii (NTS) of unanesthetized rats evokes increases in mean arterial pressure (MAP) and a bradycardia. In a previous study we verified that this increase in MAP is mediated sympathetically because prazosin (i.v.) blocks this response. The aim of the present study was to evaluate the role of the rostral ventrolateral medulla (RVLM) in the pressor response produced by L-glutamate microinjected into the NTS of unanesthetized rats. L-Glutamate was microinjected into the NTS before and 15 and 90 min after microinjection of kynurenic acid into the ipsilateral RVLM. Pressor (+24+/-3 vs. +6+/-3 mm Hg) and bradycardic (-101+/-10 vs. -3+/-12 bpm) responses to L-glutamate microinjected into the NTS (n = 8) were almost abolished 15 min after microinjection of kynurenic acid into the RVLM when compared with control responses. Both pressor (+23+/-6 mm Hg) and bradycardic (-93+/-16 bpm) responses to L-glutamate into the NTS returned to control values 90 min after microinjection of kynurenic acid into the RVLM. These data indicate that the pressor response to L-glutamate into the NTS is essentially dependent on the ipsilateral RVLM and also that this sympatho-excitatory response is mediated by excitatory amino acid receptors in RVLM neurons.

Cardiovascular changes following acute and chronic chemical lesions of the dorsal periaqueductal gray in conscious rats.

This study was carried out to investigate the effects of chemical lesions of dorsal periaqueductal gray (DPAG) on resting arterial pressure (AP) and heart rate (HR) as well as on cardiac baroreflex of conscious normotensive rats. Lesions were performed by bilateral microinjections of 150 mM NMDA into the DPAG (DPAG-lesion group). Controls were similarly injected with 165 mM NaCl (DPAG-sham group). Animals with chronic lesions confined only to the superior colliculus (SC-lesion group) were also used as controls of DPAG-lesion. Cardiovascular parameters were recorded 1 or 7 days after the microinjections of NMDA in acute and chronic groups, respectively. Cardiac baroreflex was assessed by measuring the HR responses to the intravenous injection of phenylephrine or sodium nitroprusside. Baroreflex was estimated by sigmoidal curve fitting of HR responses. An increased baroreflex gain was observed in chronic DPAG-lesion rats compared to both DPAG-sham (p < 0.01) and SC-lesion (p < 0.05) chronic groups. The chronic DPAG-lesion group showed also an elevation of both the tachycardia (p < 0.05) and bradycardia (p < 0.01) plateaus compared to chronic DPAG-sham rats, while the SC-lesion group showed an elevation of the bradycardia plateau only (p < 0.01). Similar results on baroreflex function were observed following acute lesion of the DPAG, i.e. an increase in baroreflex gain (p < 0.01) and the elevation of both tachycardia (p < 0.05) and bradycardia plateaus (p < 0.01) compared to the acute DPAG-sham group. Resting AP and HR did not differ among the chronic groups. In contrast, the acute lesion of the DPAG produced a reduction in AP (p < 0.01) accompanied by an increase in HR (p < 0.01). The present data suggest that the DPAG is involved in the tonic and reflex control of AP and HR in conscious rats. In addition, the SC seems to contribute to the baroreflex cardioinhibition.

Combined aliskiren and L-arginine treatment has antihypertensive effects and prevents vascular endothelial dysfunction in a model of renovascular hypertension.

Angiotensin II is a key player in the pathogenesis of renovascular hypertension, a condition associated with endothelial dysfunction. We investigated aliskiren (ALSK) and L-arginine treatment both alone and in combination on blood pressure (BP), and vascular reactivity in aortic rings. Hypertension was induced in 40 male Wistar rats by clipping the left renal artery. Animals were divided into Sham, 2-kidney, 1-clip (2K1C) hypertension, 2K1C+ALSK (ALSK), 2K1C+L-arginine (L-arg), and 2K1C+ALSK+L-arginine (ALSK+L-arg) treatment groups. For 4 weeks, BP was monitored and endothelium-dependent and independent vasoconstriction and relaxation were assessed in aortic rings. ALSK+L-arg reduced BP and the contractile response to phenylephrine and improved acetylcholine relaxation. Endothelium removal and incubation with N-nitro-L-arginine methyl ester (L-NAME) increased the response to phenylephrine in all groups, but the effect was greater in the ALSK+L-arg group. Losartan reduced the contractile response in all groups, apocynin reduced the contractile response in the 2K1C, ALSK and ALSK+L-arg groups, and incubation with superoxide dismutase reduced the phenylephrine response in the 2K1C and ALSK groups. eNOS expression increased in the 2K1C and L-arg groups, and iNOS was increased significantly only in the 2K1C group compared with other groups. AT1 expression increased in the 2K1C compared with the Sham, ALSK and ALSK+L-arg groups, AT2 expression increased in the ALSK+L-arg group compared with the Sham and L-arg groups, and gp91phox decreased in the ALSK+L-arg group compared with the 2K1C and ALSK groups. In conclusion, combined ALSK+L-arg was effective in reducing BP and preventing endothelial dysfunction in aortic rings of 2K1C hypertensive rats. The responsible mechanisms appear to be related to the modulation of the local renin-angiotensin system, which is associated with a reduction in endothelial oxidative stress.

Pomegranate peel extract attenuates oxidative stress by decreasing coronary angiotensin-converting enzyme (ACE) activity in hypertensive female rats

ABSTRACT Based on the antioxidant properties of pomegranate, this study was designed to investigate the effects of pomegranate peel extract on damage associated with hypertension and aging in a spontaneously hypertensive rat (SHR) model. The influence of pomegranate consumption was examined on systolic blood pressure (SBP), angiotensin-converting enzyme (ACE) coronary activity, oxidative stress, and vascular morphology. Four- or 28-wk-old SHR model rats were treated for 30 d, with terminal experimental animal age being 8 and 32 wk, respectively, with either pomegranate extract (SHR-PG) or filtered water (SHR). Data showed significant reduction in SBP and coronary ACE activity in both age groups. The levels of superoxide anion, a measure of oxidative stress, were significantly lower in animals in the SHR-PG group compared to SHR alone. Coronary morphology demonstrated total increases in vascular wall areas were in the SHR group, and pomegranate peel extract diminished this effect. Pomegranate peel extract consumption conferred protection against hypertension in the SHR model. This finding was demonstrated by marked reduction in coronary ACE activity, oxidative stress, and vascular remodelling. In addition, treatment was able to reduce SBP in both groups. Evidence indicates that the use of pomegranate peel extract may prove beneficial in alleviating coronary heart disease.

Role of pulmonary stretch receptors and sympathetic system in the inhibition of reflex bradycardia produced by chemical stimulation of the periaqueductal gray matter of the rat.

The present study examined the role of the sympathetic system and pulmonary afferent feedback in the baroreflex inhibition by chemical stimulation of the dorsal periaqueductal gray matter (DPAG) of the anesthetized rat. The baroreflex bradycardia was induced by phenylephrine infusions (PHE, 50 μg/ml/min, i.v.) given either alone or combined with glutamate microinjections (GLU, 10 nmol/100 nl) into the DPAG. GLU microinjections alone produced marked increases in respiratory amplitude (67±19%), but barely changed the respiratory frequency (15±3 cpm) and blood pressure (14±2 mm Hg), and did not affect the heart rate. In contrast, the same injections produced a 92% inhibition of PHE-induced bradycardia (from -62 to -5 bpm). Because GLU microinjections per se had little effects on blood pressure, the baroreflex inhibition should be credited to the deactivation of both the vagal and sympathetic reflex pathways at the medulla. Indeed, the baroreflex was inhibited in only 47% following the DPAG stimulation of atenolol-treated rats. The GLU-evoked inhibition of baroreflex was also correlated with concomitant increases in respiratory amplitude. The role of pulmonary feedback in baroreflex inhibition was thus examined before and after the neuromuscular blockade of atenolol-treated rats. In spontaneously breathing rats, GLU microinjections reversed PHE-induced bradycardia to tachycardia, thereby producing a 153% inhibition of reflex bradycardia (from -38 bpm to +20 bpm). In contrast, the baroreflex inhibition was attenuated in only 53% after neuromuscular blockade (from -34 to -16 bpm). Data are the first evidence of the contribution of pulmonary stretch receptor feedback in DPAG-evoked inhibition of reflex bradycardia.