Carotid Sinus Baroreceptor Reflex Research Articles

Arterial baroreceptor reflexes in young and old racing greyhounds.

The characteristics of the carotid sinus baroreceptor (CSB) reflex control of arterial pressure-flow relations were studied in young (18 mo) and old (10.5 yr) racing greyhounds anesthetized with chloralose. Pressure and flow were simultaneously measured in the ascending aorta, the celiac, superior mesenteric, renal, and iliac arteries under steady-state conditions of pulsatile perfusion of the isolated carotid sinuses before and after bilateral cervical vagotomy. Operating-point (OP) values of mean pressure were not significantly different between the young or old greyhounds, but OP values of cardiac output were lower and of peripheral resistance were higher in the older animals. OP values of regional resistances were higher in the older animals, but only those of the celiac and mesenteric beds were significantly different. Values of OP sensitivity of central hemodynamic variables were not decreased in the older animals. OP values of renal and iliac resistance sensitivity tended to be lower in the older animals. The overall range of control of the various hemodynamic variables by the CSB was lower in the older group only in the case of iliac resistance. The characteristics of the CSB were not uniformly depressed in the older animals but were relatively well maintained compared with those of the younger group.

The effect of the carotid sinus baroreceptor reflex on blood flow and volume redistribution in the total systemic vascular bed of the dog.

To quantify the relative contribution of blood flow redistribution and active changes in vascular capacity in the regulation of cardiac output, blood flow and volumes in two parallel vascular beds were measured in response to varying carotid sinus pressures. In nine dogs, carotid sinuses were isolated and intrasinus pressure was controlled. Two external reservoirs were placed between the caval veins and the right heart to measure changes in vascular capacity in splanchnic and extrasplanchnic vascular beds. At intrasinus pressures of 50 and 200 mm Hg, we have simultaneously measured arterial resistances, compliances, changes in flows, and "unstressed vascular volume," and time constants of venous drainage in the splanchnic and extrasplanchnic vascular beds. Compliances and time constants of venous drainage were found to be nearly equal in the two beds. A decrease in intrasinus pressure from 200 to 50 mm Hg resulted in a small redistribution of blood flow (about 5% of cardiac output) from the extrasplanchnic compartment to the splanchnic vascular bed. Changes in reservoir volumes were found to be around 7.0 ml/kg. The splanchnic vascular bed was responsible for a greater change in reservoir volume for a given change in intrasinus pressure. With any change in intrasinus pressure, the change in arterial resistance in the extrasplanchnic vascular bed was greater than that of the splanchnic vascular bed. Blood flow redistribution was not found to be a significant factor contributing to changes in reservoir volume. The changes in reservoir volume seen, must have been due to active changes in vascular capacity in the two channels chosen.

Effect of carotid sinus baroreceptor reflex on responses to phenylephrine and nitroprusside in anesthetized cats.

This study attempted to measure the extent to which the carotid sinus baroreceptor reflex attenuates the acute changes in arterial pressure during infusions of phenylephrine and nitroprusside, and to express these effects as equations which could be used in a simple computer model of the circulation. Arterial pressure and cardiac output were measured in vagotomized cats anesthetized with chloralose or pentobarbital. The carotid arteries were perfused to allow sinus pressure either to follow changes in systemic arterial pressure or to be held at the preinfusion control level, thereby eliminating the effects of the reflex. There was a linear relationship between arterial pressure in the presence and absence of the reflex, and the changes in arterial pressure were attenuated to one-third by the reflex while cardiac output was not significantly changed. These effects are described by simple equations involving peripheral resistance, heart rate, and venous compliance. The activity of the reflex was not significantly different in cats anesthetized with pentobarbital or chloralose, and our data do not support the view that these anesthetic agents depress the vasomotor component of the carotid sinus reflex.

Carotid sinus baroreceptor reflex control of the circulation in medial sclerotic and renal hypertensive rabbits and its modification by the aortic baroreceptors.

We studied the reflex control of blood pressure, heart rate, and hindlimb vascular resistance by the carotid sinus baroreceptors in normal (N), experimental renal hypertensive (RH, one kidney renal wrap model), and medial sclerotic (MS) rabbits under urethane anaesthesia using an isolated perfused carotid sinus preparation and auto-perfused hindlimb. The contralateral carotid sinus was denervated. Compared to N rabbits, the blood pressure and hindlimb vascular resistance of RH and MS rabbits were significantly elevated at all carotid sinus pressures 15 weeks after inducing the disease process. The maximum gains of the curves relating carotid sinus pressure to vascular resistance were significantly elevated in the MS and RH rabbits, but those relating carotid sinus pressure to heart rate were significantly reduced. The changes were greatest in the RH group in which the responses also were set to a higher carotid sinus pressure. In the three groups, division of the aortic nerves produced different changes in the sigmoid curves relating carotid sinus pressure to heart rate, blood pressure, and vascular resistance. There was a linear relationship between blood pressure and basal vascular resistance (correlation coefficient 0.88).

Epinephrine and the carotid sinus baroreceptor reflex. Influence on capacitive and resistive properties of the total systemic vascular bed of the dog.

Epinephrine and the carotid sinus baroreceptor reflex. Influence on capacitive and resistive properties of the total systemic vascular bed of the dog.

Carotid sinus control of pulsatile hemodynamics in halothane-anesthetized dogs.

The open-loop characteristics of the carotid sinus baroreceptor reflex control of pulsatile arterial pressure-flow relations were studied in halothane-anesthetized dogs. Pressures and flows were measured in the ascending aorta, the celiac, mesenteric, renal, and iliac arteries and were used to compute values of regional vascular impedance and hydraulic power. The carotid sinuses were bilaterally isolated and perfused under conditions of controlled mean pressure with a constant sinusoidal component. Measurements were made with the vagi intact and after bilateral vagotomy. Maximum values of open-loop gain averaged -0.78 +/- 0.08 before and -1.42 +/- 0.20 after vagotomy. Vagotomy produced significant increases in the variation of all hemodynamic variables with carotid sinus pressure that were nonuniformly affected in the various regional vascular beds. Aortic and regional vascular impedance showed significant variations with carotid sinus pressure that were augmented by vagotomy. Aortic impedance exhibited a minimum at the normal set point. These results indicate that a) carotid sinus baroreflexes are well preserved with halothane anesthesia, b) thoracic baroreceptor-mediated reflexes exert significant hemodynamic effects on systemic hemodynamics around normal set point values of arterial pressure, c) systemic baroreceptors exert control over large as well as small vessel properties, and d) the baroreceptor-mediated reflexes produce significant influences on hydraulic power and its components.

A study of the role of renal nerves in the renal responses to 60° head‐up tilt in the anaesthetized dog

1. Renal responses to 10 min of 60 degrees head-up tilt were measured in anaesthetized dogs in which renal perfusion pressure was maintained at a relatively constant value.2. Tilting was associated with a fall in systemic blood pressure and an increase in heart rate. Renal blood flow and glomerular filtration rate remained constant while there was a significant decrease in both absolute and fractional excretion of sodium.3. Animals which had undergone acute renal denervation were tilted. The cardiovascular responses were similar to intact animals. A fall in renal blood flow was observed but the glomerular filtration rate was maintained at a steady value during tilting. The decreased renal tubular excretion of sodium measured in intact animals was abolished.4. Alpha-adrenergic blockade of the kidney was achieved by infusion of phentolamine into the renal artery. Tilting of these animals caused cardiovascular changes similar to those observed in control animals but renal blood flow, glomerular filtration rate and sodium handling remained unchanged.5. Animals in which both carotid sinuses had been acutely denervated were tilted. Systemic blood pressure fell as in intact animals, but the rise in heart rate was significantly less. Renal blood flow, glomerular filtration rate and the rate of sodium excretion were unchanged.6. A 10 min period of 60 degrees head-up tilt in anaesthetized dogs resulted in an unchanged renal blood flow and glomerular filtration rate which was associated with a decrease in both fractional excretion of sodium and sodium excretion. The renal sympathetic nerves were shown to be responsible for these changes in tubular sodium handling which appeared to exert their action via renal tubular alpha-adrenergic receptors. This activation of the renal nerves appeared to be mediated by the carotid sinus baroreceptor reflex.

Changes in autonomic nerve activity during acute pericardial tamponade.

Acute experimental pericardial tamponade in anesthetized mongrel dogs produces marked alterations of autonomic reflex mechanisms. Afferent nerve activity from left ventricular receptors is inhibited during tamponade, whereas pericardial receptors are excited. Sympathetic efferent nerve activity traversing the ansae subclaviae is markedly increased during tamponade by the carotid sinus baroreceptor reflex and also by other undefined mechanisms. The increase in sympathetic activity produces a transient tachycardia and inotropic effect. A paradoxical increase vagal efferent nerve activity occurs during the period of maximal hypotension and appears to override the normal baroreceptor reflexes. This increase in vagal efferent activity corresponds to the development of a marked bradycardia during cardiac tamponade.

The carotid sinus baroreceptor reflex in conscious rabbits.

1. A method is described for altering the pressure across the wall of the carotid sinus in conscious rabbits by enclosing the carotid bifurcation in a rigid, fluid-filled capsule. The extracapsular arterial baroreceptors were denervated. 2. The baroreceptor--heart rate reflex, elicited by injecting vasoactive drugs or inflating aortic and vena caval cuffs, was used to test the new method. The function of the carotid sinus was shown to be unaffected by enclosure in the capsule. Denervation of the extracapsular baroreceptors reduced the gain of the baroreceptor--heart rate reflex two- to threefold. 3. The characteristics of the carotid baroreceptor reflex were studied in sixteen animals by the capsule method. Median estimates of maximum gain, and the range over which blood pressure changed, were 1.1 mmHg/mmHg and 57 mmHg respectively. There was good agreement between duplicate estimates made 1--20 days apart. 4. There was only a weak association between the effects on blood pressure and heart rate of altering carotid sinus transmural pressure. Autonomic blockade of the heart, so that its rate was fixed, did not reduce the gain or range of blood pressure change.

Interaction between carotid sinus baroreceptor and aortic nerve pressor reflexes in the dog

In dogs anesthetized with morphine-chloralose, strong, low-frequency (2 Hz) electrical stimulation of the aortic nerve (AN) causes pressure in the perfused hindlimbs to rise. This pressor response is followed by a large depressor phase upon cessation of stimulation. Simultaneous stimulation of the ipsilateral carotid sinus nerve (CSN) with intermittent trains of stimuli enhanced the AN pressor response by 113%. Similar stimulation of the contralateral CSN had little effect on the AN pressor response. Constant-frequency stimulation of the ipsilateral CSN also failed to enhance this response. Possible mechanisms of the interaction between these antagonistic reflexes were investigated. The results suggest that the augmentation of the AN pressor response results from a central neuronal interaction between these antagonistic reflexes.

Sinovagal interaction in arterial pressure restoration after 10% hemorrhage

The summation between the carotid sinus baroreceptor reflex system (CS system) and the vagally mediated reflex system (V system) was studied as they restore mean arterial pressure (MAP) after 10% quick hemorrhage in splenectomized conscious dogs chronically instrumented with catheters for pressure measurement and hemorrhage. The experiment was repeated under nerve-intact condition (intact), with cold block of the vagi ([V]), after carotid sinus denervation (CS), and CS plus [V] situations. MAP falls at 1.5 min after the hemorrhage were 7.2 in intact, 24.7 in [V], 36.0 in CS, and 67.6 in CS + [V] mmHg. When we calculated the open-loop gains of CS and V systems assuming a simply additive summation between them a self-contradiction occurred. To avoid this contradiction, it was necessary to assume that CS and V systems interact in a facilitatory manner. Mean open-loop gains calculated under this assumption were 1.64 for the CS system alone, 0.89 for the V system alone, and 6.59 for the interacting component between them. These intriguing results warrant further analysis of the summation between the two reflex systems.

Mechanism of the increased vascular capacity produced by mild perfusion hypothermia in the dog.

Mechanism of the increased vascular capacity produced by mild perfusion hypothermia in the dog.

A method for the analysis of sigmoid stimulus-response curves.

This paper describes a method, using an interactive computer programme, for fitting a curve to data which is expected to assume a sigmoid form. The programme is specifically designed to generate best estimates of the limits and maximum slope of stimulus-response curves for the carotid sinus baroreceptor reflex, but is applicable to curve-fitting tasks in other fields such as immunology, pharmacology, biochemistry and neurophysiology.

Lack of effect of isometric handgrip exercise on the responses of the carotid sinus baroreceptor reflex in man.

1. The change in arterial pressure and heart rate resulting from alteration of carotid sinus transmural pressure by a median--34 mmHg and +33 mmHg by means of a variable-pressure neck chamber was tested in seven male volunteer subjects, at rest and during exertion of 35, 45 and 65% of maximum voluntary handgrip. 2. During 60 s of 35 and 45%, and during 30 s of 65%, of maximal voluntary handgrip there was virtually no alteration of the response of blood pressure to alteration carotid sinus transmural pressure. 3. The bradycardic response to increase in carotid sinus transmural pressure was reduced at various times after the commencement of handgrip at 45 and 65% of maximum voluntary contraction. 4. It is concluded that a reduction in arterial baroreceptor reflex sensitivy does not play an important role in the initiation of the increase in arterial blood pressure and heart rate caused by isometric exercise. 5. The hypothesis is advanced that some of the cardiovascular changes in exercise may result from elevation of the central 'set point' for blood pressure.

Effect of isometric hand-grip exercise on the carotid sinus baroreceptor reflex in man.

1. The changes in R--R heart interval that result from step-increase and step-decrease in carotid sinus transmural pressure induced by a variable pressure neck chamber were measured in seven normal men. Observations were made at rest, and during isometric hand-grip exercise at 24%, 44% and 64% of maximal voluntary contraction. 2. The response of heart interval to increase in carotid sinus transmural pressure was progressively and markedly diminished according to the strength of hand-grip. This effect was fully developed from the moment of onset of the exertion. 3. The response of heart interval to decrease in carotid sinus transmural pressure was much less consistently affected by hand-grip exercise.

Actions of sodium and potassium ions on baroreceptors of normotensive and spontaneously hypertensive rats.

Actions of sodium and potassium ions on baroreceptors of normotensive and spontaneously hypertensive rats.

Changes in cardiac output and total peripheral resistance during the carotid sinus baroreceptor reflex in the pregnant rabbit.

1. The reflex cardiovascular effects of changes in pressure within the isolated carotid sinus have been examined in sixteen anaesthetized pregnant rabbits. 2. Comparison of the mean results at sinus pressures of 40 and 200 mmHg showed that heart rate fell by 32.1 beats min-1 and arterial pressure was reduced by 48.2 mmHg. Cardiac output, measured by thermal dilution, fell by 25.7 ml. min-1.kg-1 and total peripheral resistance was reduced by 0-292 mmHg. ml.-1 min. kg. 3. The corresponding changes previously reported in a group of seventeen non-pregnant female rabbits were a similar fall in heart rate of 34.5 beats min-1 but significantly greater reductions in arterial pressure of 69-8 mmHg and in total peripheral resistance of 0-432 mmHg. ml.-1 min.kg. However, the fall in cardiac output of 12.6 ml.min-1.kg-1 was significantly smaller. 4. These observations show that the smaller blood pressure response of the pregnant rabbit to alterations in sinus pressure is due to a reduced change in total peripheral resistance.

Circulatory reflexes from carotid and extracarotid baroreceptor areas in man.

The carotid sinus baroreceptor reflex was studied in 11 normotensive subjects, using a variable pressure neck chamber and correcting for imperfect pressure transmission to the carotid sinus. Decreased carotid baroreceptor stimulation caused a sustaineded rise in arterial pressure, and increased carotid baroreceptor stimulation caused a sustain fall. The responses were in linear relation to the stimulus, and, after reaching the steady state, greater for the reduced than for the increased baroreceptor stimulation. Thus the carotid sinus baroreceptor reflex of the normotensive man is an effective antihypotensive and antihypertensive feedback system, though the former function may have more sensitivity. The increased and decreased baroreceptor stimulation by the neck chamber also caused bradycardia and tachycardia which were modest in magnitude and often transient. In eight subjects the reflex changes in heart rate induced by the neck chamber were compared with those induced by altering transmural pressure not merely at the carotid sinus but throughout the arterial tree (injection of phenylephrine and trinitroglycerin). The slopes of these relations were 3 times as great in the latter circumstance. Thus the carotid baroreceptors play a lesser role in heart rate control than do extracarotid baroreceptors.

Modification of the flow-generating capability of the canine heart-lung compartment by the carotid sinus baroreceptor reflex.

To quantitatively understand how the carotid sinus baroreceptor reflex modifies the Starling curve (i.e., the aortic flow (AF)-mean right atrial pressure (MRAP) relationship), experiments were performed in closed-chest, naturally breathing, and anesthetized dogs before and after vagotomy. Mean aortic pressure (MAP) was fixed at approximately 100 mm Hg and the pressure in the isolated carotid sinus (ISP) was varied from 75 to 150 mm Hg in steps of 25 mm Hg. At each ISP, MRAP was slowly increased and decreased while measuring AF by a previously implanted electromagnetic flow probe. A family of AF-MRAP relation curves specified at the different ISP's were thus obtained. Third-order polynomials in MRAP and ISP adequately fit these curves. Whether the vagi were intact or cut, there was no significant difference between the AF-MRAP relation curves obtained at an ISP of 75 or 100 mm Hg. However, decreasing ISP from 125 to 100 mm Hg caused a 24% increase in AF, and increasing ISP to 150 mm Hg caused a 15% decrease in AF in the dogs with intact vagus nerves. For the vagotomized dogs, the same decrease or increase in ISP caused a 17% increase or a 21% decrease in AF, respectively. When MAP was allowed to change by the reflex, only insignificant changes in AF occurred. We conclude that the carotid sinus reflex significantly alters the flow-generating ability of the heart-lung compartment by as much as 40% but this becomes clearly observable only if the reflex change in aortic pressure is prevented.

The sinus nerve and baroreceptor input to the medulla of the cat.

Electrical stimulation within the medulla of cats revealed that myelinated primary afferent fibres of the sinus nerve terminated within the immediate vicinity of the tractus solitarius and its nucleus. 2. The activity of neurones within this area was also evoked on sinus nerve stimulation, although few (17%) were activated within a latency compatible with monosynaptic excitation. Additional projections over polysynaptic pathways have been shown to the parahypoglossal area and to the area of the nucleus ambiguus. 3. These three areas were shown to contain neurones whose activity was enhanced by stimulation of the baroreceptor endings of the ipsilateral carotid sinus. 4. No evidence for a projection of sinus nerve afferents to the medial reticular formation (an area extending medially from the hypoglossal nucleus and nerve tract and including the paramedian reticular nucleus) was obtained in either antidromic or orthodromic studies. 5. The organization of the central pathway of the carotid sinus baroreceptor reflex is discussed in the light of these results.