Reflex Control Of Circulation Research Articles

Altered cardiovascular reflexes responses in conscious Angiotensin-(1-7) receptor Mas-knockout mice

This study evaluated the physiological importance of Angiotensin-(1-7) receptor Mas on reflex control of circulation. Experiments were performed in male Mas-knockout (Mas-KO) and Wild Type (WT) conscious mice (12–20 wk of age). Baroreceptor reflex was evaluated by the bradycardic response induced by phenylephrine (0.25 μg/5 μl, i.v.). Bezold–Jarisch reflex was evaluated by phenylbiguanide (0.5 μg/5 μl, i.v.) and chemoreflex by potassium cyanide (2.5 μg/5 μl, i.v.). Baseline mean arterial pressure was higher in Mas-KO ( n = 14) as compared with WT mice ( n = 18) (118 ± 1 mm Hg vs. 109 ± 2 mm Hg); however, heart rate was similar in both strains (615 ± 30 bpm vs. 648 ± 13 bpm). Baroreflex bradycardia was lower (0.78 ± 0.44 ms/mm Hg vs. 1.30 ± 0.14 ms/mm Hg) in Mas-KO compared with WT mice. The depressor (−17 ± 5 mm Hg vs. −45 ± 6 mm Hg) and bradycardic (−212 ± 36 bpm vs. −391 ± 29 bpm) components of the Bezold–Jarisch reflex were also lower in Mas-KO mice. In addition, chemoreflex pressor response (+20 ± 3 mm Hg vs. +12 ± 0.8 mm Hg) and bradycardic response (−250 ± 74 bpm vs. −52 ± 26 bpm) were significantly higher in Mas-KO. These results further advances previous studies by showing that the lack of Mas receptor induced important imbalance in the neural control of blood pressure, altering not only the baroreflex but also the chemo- and Bezold–Jarisch reflexes.

Reflex control of circulation in the elderly

Baroreceptor control of heart rate is markedly reduced in elderly subjects. However, the effects of aging on baroreceptor control of blood pressure and on the vascular and neurohumoral influences of volume cardiopulmonary receptors are unknown. In this paper we report evidence that in both conscious rats and humans aging is associated with a fall in the baroreceptor ability to rapidly cause blood pressure changes, but that the more long-term carotid baroreceptor control of blood pressure remains similar to that observed in younger individuals. Early and late cardiopulmonary receptor modulation of vascular resistance is impaired by aging, which also reduces the influence of this reflex on renin secretion. These dynamic and steady-state alterations in reflex cardiovascular control account for several hemodynamic abnormalities of the advanced age.

Effects of diazepam on the carotid sinus baroreflex control of circulation in rabbits

To examine the effects of diazepam on the carotid sinus baroreflex control of circulation, bilateral carotid occlusion was performed on 14 conscious rabbits with aortic denervation. The responses of mean arterial pressure, heart rate, cardiac output and total peripheral resistance were obtained. The haemodynamic responses to carotid occlusion were evaluated at cumulative doses of 0.5 and 1.0 mg kg-1 of diazepam. The administration of diazepam decreased cardiac output and increased total peripheral resistance significantly, but did not affect the arterial pressure and heart rate. The response of total peripheral resistance to carotid occlusion was significantly increased from 0.118 +/- 0.018 (mean +/- SE) to 0.154 +/- 0.026 mmHg min ml-1 at 1.0 mg kg-1 of diazepam. The heart rate response was attenuated significantly from 41 +/- 5 to 24 +/- 4 beats min-1 at 1.0 mg kg-1 of diazepam. Diazepam did not alter the response of arterial pressure to carotid occlusion. We suggest that the dissociated effects of diazepam on the reflex control of circulation reflect the dissociated influences of diazepam on the central sympathetic and vagal-mediated pathways.

Cardiogenic reflexes and left ventricular hypertrophy

Studies performed in both animal and human species have shown that receptors anatomically located in the cardiac walls and in the pulmonary vascular bed exert a powerful influence on sympathetic vasoconstrictor tone and renin release from the kidney. This paper will review recent data collected by our group on the effects on this homeostatic reflex function of two different models of cardiac hypertrophy, i.e. that associated with arterial hypertension and that induced by prolonged physical training. It will also examine whether and to what extent cardiopulmonary reflex control of circulation can be restored after an effective antihypertensive pharmacological treatment has induced a regression of the structural alterations of the heart. The results of these studies suggest that cardiopulmonary reflex control of circulation is markedly impaired in the presence of cardiac hypertrophy, with (severe hypertensive subjects) or without (athletes) high blood pressure values. In hypertensives, furthermore, therapeutic regression of cardiac hypertrophy seems to restore, although not fully normalize, this reflex function. Taken together these findings suggest that cardiac hypertrophy, per se, is a condition characterized by the loss of the reflex functions exerted by cardiac receptors, with adverse consequences on cardiovascular homeostasis.

Effects of Celiprolol on Reflex Control of the Cardiovascular System in Essential Hypertension

We have previously shown that baroreceptor control of the cardiovascular system and the cardiopulmonary receptor control of peripheral circulation are preserved or only moderately reduced during antihypertensive treatment with acebutolol or nadolol, which indicates that treatment with beta blockers with or without intrinsic sympathomimetic activity does not impair fundamental neural mechanisms involved in circulatory homeostasis. In the present study we have investigated the reflex control of circulation before and during antihypertensive treatment with celiprolol, which, in addition to its beta-blocking action, has vasodilator properties that may stem from interference with neural cardiovascular control. In six essential hypertensive subjects we measured blood pressure (intraarterial catheter), heart rate (ECG recording), central venous pressure (right atrial catheter), and forearm blood flow and resistance (plethysmography) before and during alterations in the activity of the arterial baroreceptors obtained by means of lower body suction and passive leg raising. The study was performed before and after 5-7 days of oral administration of celiprolol at 200-400 mg once a day. Compared to the values obtained in the control, condition celiprolol caused a reduction in blood pressure, a slight change in heart rate, and an increase in forearm blood flow, which indicated the occurrence of a clear-cut forearm vasodilatation. The heart rate responses to arterial baroreceptor manipulation were unchanged by celiprolol which reset the carotid baroreflex so that its tonic restraint on blood pressure increased despite the hypotension induced by the drug. The inhibitory restraint tonically exerted by the cardiopulmonary receptors on peripheral circulation was also increased by celiprolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Splenic and renal sympathetic responses to stimulation of splenic receptors in cats.

The effect of selective stimulation of splenic receptors on reflex responses of splenic and renal efferent nerves was studied in anesthetized, vagotomized, sinoaortic denervated cats. The following substances were injected into the artery or vein of vascularly isolated spleens: warm physiological saline (congestion), capsaicin (CAPS), bradykinin (BK), and norepinephrine (NE). Splenic congestion increased efferent activity of splenic and renal nerves, splenic venous pressure, systemic arterial pressure, and heart rate. CAPS and BK elicited responses similar to those produced by congestion but caused greater excitation of splenic than renal nerves. Reflex responses were eliminated by section of splenic nerves. Injection of NE increased splenic venous pressure but did not elicit reflex responses. Finally, in contrast to a previous report light- and electron-microscopic examination of splenic nerves revealed myelinated, as well as unmyelinated, fibers. These results have demonstrated that activation of splenic receptors elicits reflex cardiovascular responses and excitation of splenic and renal efferent nerves, contraction of the spleen does not produce reflex responses, the adequate stimulus for reflex responses is stretch of vessels and possibly capsule, and splenic receptors play a role in the reflex control of circulation.

Preface

Although William Harvey in 1628 used a truly comparative approach in his study on ‘The Movement of the Heart and Blood’, mentioning crustaceans, fishes, amphibians, reptiles and birds as well as mammals, physiologists now tend to be classified as either mammalian or comparative. One objective of the discussion meeting and this associated review volume was to bring together experienced scientists from either side of this arbitrary division who between them covered a large proportion of the animal kingdom. The major objective, however, was to view the separate central and peripheral mechanisms controlling the respiratory and circulatory systems, and then to see how these systems are controlled and co-ordinated during various types of behaviour and activities in order that an adequate supply of oxygen to the metabolizing tissues may be maintained, presumably with the least expenditure of energy.Such ‘goal-directed’ control and co-ordination may seem an obvious necessity during exercise, particularly for those air-breathing animals that spend much of their time underwater or for those that live at high altitude. However, a similar objective may be achieved during that most energy-conserving of all activities, sleep. The rhythmic ventilation of most mammals, which is generated by medullary neurones and which modulates output to the cardiovascular system, becomes irregular during desynchronized sleep. This irregular pattern may be controlled by a different central generator from that functioning in the awake animal, but its irregular nature is similar to that seen in a number of resting poikilothermic animals and in the marine mammals. In these animals wide fluctuations in blood gases are experienced, so the concept of internal respiratory homeostasis is not appropriate. Interestingly, the reflex control of circulation during sleep is mainly by way of the peripheral chemoreceptors, which is similar to the condition in the foetus where the baroreceptors do not function. However, the presence of a baroreflex in aquatic vertebrates indicates that it has not evolved solely in response to the influence of gravity on the circulatory system. The general level of arterial blood pressure may, in fact, be set centrally, with the peripheral receptors merely ‘fine-tuning’ the system.Respiratory and cardiovascular adjustments associated with some patterns of behaviour can be studied in traditional ways with anaesthetized or restrained animals. The defence-arousal response and a pattern of sympathetic activity similar to that seen during desynchronized sleep can be obtained by stimulating specific regions of the brain. Other types of activity cannot be so usefully studied under such conditions. It is now clear that the physiological responses to submersion (diving) are different when the animal dives spontaneously compared with forced submersion.At this meeting we heard from scientists studying topics as widely different as the connexions and interactions of central neurones, peripheral sense organs and whole animal activities in crustaceans, fishes, reptiles, birds and mammals (including man). At first sight this may appear an impossibly broad area to cover at a three-day meeting and in a relatively slim volume. In the event it was clear that important mechanisms for compensation are widespread throughout the animal kingdom (e.g. both the cray-fish, Austropotamobius, in hypoxic water and man at high altitude benefit from the leftward shift of the blood oxygen equilibrium curve that results from a respiratory alkalosis). It is hoped that the truly comparative approach to physiology will be fostered by this volume. As Harvey found, by studying a wide range of animals we are better able to understand the mechanisms which determine our own responses to the environment and, maybe, our aspirations within it.

Calcium-entry blockade, beta-adrenergic blockade and the reflex control of circulation.

Calcium-entry blockade, beta-adrenergic blockade and the reflex control of circulation.