Comparison of the reflex vasomotor responses to separate and combined stimulation of the carotid sinus and aortic arch baroreceptors by pulsatile and non-pulsatile pressures in the dog

1. In the anaesthetized dog the carotid sinuses and aortic arch were isolated from the circulation and separately perfused with blood by a method which enabled the mean pressure, pulse pressure and pulse frequency to be varied independently in each vasosensory area. The systemic circulation was perfused at constant blood flow by means of a pump and the systemic venous blood was oxygenated by an extracorporeal isolated pump-perfused donor lung preparation.2. When the vasosensory areas were perfused at non-pulsatile pressures within the normal physiological range of mean pressures, the reflex reduction in systemic vascular resistance produced by a given rise in mean carotid sinus pressure was significantly greater than that resulting from the same rise of aortic arch pressure.3. On the other hand, when the vasosensory areas were perfused at normal pulsatile pressures and within the normal physiological range of mean pressures, there was no difference in the size of the reflex vascular responses elicited by the same rise in mean pressure in the carotid sinuses and in the aortic arch.4. Whereas the vasomotor responses elicited reflexly by changes in mean carotid sinus pressure are modified by alterations in pulse pressure, those evoked by the aortic arch baroreceptors through changes of mean pressure are only weakly affected by modifications in pulse pressure. Evidence for this was obtained from single stepwise changes of mean pressure in each vasosensory area during pulsatile and non-pulsatile perfusion, and from curves relating the mean pressure in the carotid sinuses or aortic arch and systemic arterial perfusion pressure.5. The vasomotor response elicited by combined stimulation of the carotid sinus and aortic arch baroreceptors was greater than either response resulting from their separate stimulation.6. When the mean perfusion pressures in the two vasosensory areas are changed together, the curve relating mean pressure to systemic arterial pressure during pulsatile perfusion of the areas is considerably flatter than that for non-pulsatile perfusion.7. Increasing the pulse pressure in the carotid sinuses or aortic arch caused a decrease in systemic vascular resistance, the response elicited from the carotid sinuses being the larger.8. Altering the phase angle between the pulse pressure waves in the carotid sinuses and aortic arch had no effect on systemic vascular resistance.9. In both vasosensory areas, increasing the pulse frequency caused a reduction in systemic vascular resistance.     

Interaction of aortic and carotid sinus baroreceptors: effect of activation times.

Changes in pulse-wave velocity were simulated by changing the relative timing between aortic and carotid sinus barorecptor activity in anesthetized rabbits and dogs. In the rabbit, electrical stimulation was used to vary the timing; in the dog, it was also varied by perfusing the carotid sinuses with externally generated pressure pulses that could be triggered in any portion of the cardiac cycle. Changing the relative delay between aortic and carotid sinsus nerve stimulation did not result in variations of blood pressure or heart rate in the rabbit. Varing the time of electrical stimulation of the carotid sinus nerve caused at most 5 mmHg change of blood pressure in the dog. Delay-related heart-rate changes could be usually observed only when the stimulus consisted of short, high-intensity bursts. When the carotid sinus was externally perfused with pulses of pressure, only one out of five dogs showed delay-related variations in blood pressure (3mmHg) and heart rate (6 beats/min). It is concluded that variations in pulse-wave velocity are unlikely to play a significant role in acute cardiovascular control.     

Circulatory responses to stimulation of the carotid body chemoreceptors in the cat.

Cardiovascular responses to carotid body chemoreceptor stimulation were followed in the 'curairzed', vagotomized, artificially ventilated cat. Stimulation of the chemoreceptors by perfusion of the carotid sinus regions with venous blood induced a reflex vasconstriction in skeletal muscle, kidney, intestine and skin, and, in most cases, an increased heart rate. A comparison of the chemoreceptor reflex responses with those obtained by direct electrical stimulation of the regional vasomotor fibres indicated that in chemorecptor reflexes the vasomotor fibre activity increased to the same extent in skeletal muscle and intestinal resistance vessels and, probably, in the nutritional skin vessels but to a smaller extent in the renal vessels and the skeletal muscle capacitance vessels. The renal vessels and the muscle capacitance vessels seemed, however, to respond more during chemoreceptor stimulation than when the barorecptors were unloaded, indicating that anexcitation of the bulbar vasomotor centre is more effective than the withdroawal of an inhibitory restraint in activating the vasomotor fibres to these vascular sections.     

Influence of carotid baroreceptors on different components of the vascular system

1. Reflex changes in wall tension of the lateral saphenous vein of one hind limb, the splenic veins and capsule, and the resistance vessels of the other hind limb caused by changes in baroreceptor activity were measured in vagotomized dogs under thiopentone-chloralose anaesthesia.2. Three different methods were used to alter pressure in one or both carotid sinuses. (1) Both carotid sinuses were vascularly isolated and filled with fully oxygenated Krebs-Ringer bicarbonate solution (pH 7.4) from a reservoir in which the pressure could be altered at will. (2) One sinus was denervated, and the contralateral sinus was perfused with arterial blood at different flow rates. (3) One sinus was denervated, and the innervated sinus was perfused with arterial blood at constant flow, the pressure being altered by changing the outflow resistance.3. The left saphenous vein was perfused at constant flow with autologous blood; changes in perfusion pressure were used as a measure of changes in veno-motor activity. The right common iliac artery was perfused at constant flow to measure changes in resistance vessel activity. Blood flow through the spleen was temporarily arrested, trapping a fixed volume of blood in the organ. Under these conditions, changes in splenic vein pressure were a measure of changes in smooth-muscle tension in the splenic capsule and veins.4. In order to assess the responses to baroreceptor stimulation in terms of alterations in sympathetic nerve traffic to different components of the peripheral vascular system, ;frequency-response curves' were constructed for spleen, saphenous vein, and limb resistance vessels by electrical stimulation of the splenic nerves and lumbar sympathetic chains.5. The saphenous vein showed no consistent response to changes in baroreceptor activity. Reduction in carotid sinus pressure from 180 to 100 mm Hg caused an increase in venous pressure in the isovolumetric spleen and in the iliac artery perfusion pressure. These results were confirmed by electrical stimulation of the carotid sinus nerve. Whereas the peak responses of the limb resistance vessels corresponded to an increase in lumbar sympathetic nerve traffic of 6-10 c/s, the maximal splenic responses were equivalent to an increase in splenic nerve traffic of 1-4 c/s. These results are consistent with selective autonomic nervous control of different components of the peripheral vascular system.     

Effect of Carotid Sinus Stimulation on Cardiac Output and Peripheral Vascular Resistance during Changes in Blood Volume Distribution in Man

In ten healthy subjects (mean age 29.6 years) the hemodynamic response to carotid sinus stimulation (neck suction - 40 mmHg) was studied under control conditions and during peripheral pooling of blood (lower body negative pressure). Heart rate, arterial and central venous pressure, cardiac output and forearm blood flow were measured. The time sequence of the heart rate response was studied separately in six healthy subjects. During control conditions, carotid sinus stimulation induced a significant decrease in arterial pressure and heart rate. The blood pressure decrease mainly reflected a reduction in cardiac output, total peripheral vascular resistance being essentially unchanged. However, in the skeletal muscle, represented by a forearm segment, vascular resistance decreased significantly. During lower body negative pressure (LBNP) the same stimulation of the carotid sinus induced a significantly greater fall in mean arterial pressure even though the reduction in cardiac output was slightly smaller on the average than in the control condition. The heart rate increased, probably secondary to a time dependent increase in heart rate elicited by the continuous LBNP stimulus. Total peripheral vascular resistance decreased significantly during LBNP, the reaction likewise differing significantly from that in the control condition. Thus the augmented blood pressure response was due to a more pronounced vasodilatation when the carotid sinus was stimulated during lower body negative pressure. The results indicate that the hemodynamic changes elicited by carotid sinus stimulation are modified by changes in the distribution of blood volume and in the tone of resistance vessels.     

Interaction of somatic and cardiopulmonary receptors in control of renal circulation.

The purpose of this study was to determine if there is an important interaction between somatic and cardipulmonary receptors in the control of vasomotor outflow to the kidney. This interaction was examined by determining the renal vasoconstrictor responses to afferent electrical stimulation (30 V, 1 ms, 20--40 Hz) of the sectioned sciatic nerve in 8 chloralose-anesthetized dogs with sinoaortic deafferentation. During isovolemia, sciatic stimulation resulted in significant increases in arterial pressure and heart rate, and in renal vasoconstriction. Volume expansion significantly attenuated and vagotomy significantly augmented the renal vasoconstrictor responses to sciatic stimulation. These interventions did not significantly influence the arterial pressure or heart rate responses to sciatic stimulation. In 4 dogs with aortic nerves sectioned and carotid sinuses isolated and perfused at constant pressure (135 mmHg), the renal vasoconstrictor responses to stimulation were attenuated by volume expansion and augmented by vagotomy. These data show that in the absence of the arterial baroreceptors or with intermediate levels of carotid baroreceptor activation, volume expansion and, thus, augementation of discharge of cardiopulmonary receptors (vagal afferents) markedly attenuated the renal vasoconstrictor responses to somatic afferent stimulation.     

Left ventricular inotropic and peripheral vasomotor responses from independent changes in pressure in the carotid sinuses and cerebral arteries in anaesthetized dogs

1. The pressure perfusing the isolated carotid sinuses and the pressure perfusing the cerebral circulation were changed independently, and the resulting inotropic responses in the left ventricle and peripheral vasomotor responses were determined.2. Inotropic responses were assessed by measuring changes in the maximum rate of change of left ventricular pressure (dP/dt max) with heart rate and mean aortic pressure held constant. Vascular resistance changes were usually assessed by perfusing the descending thoracic aorta at constant flow and measuring changes in perfusion pressure.3. Decreases in carotid sinus pressure over the baroreceptor sensitivity range resulted in a 45% increase in dP/dt max and a 59% increase in vascular resistance.4. Unless arterial oxygen tension was abnormally low, lowering cerebral perfusion pressure to 50 mm Hg resulted in little or no inotropic and vasomotor responses. In the presence of hypoxaemia (P(a,O2) < 60 mm Hg), lowering cerebral perfusion pressure to below about 80 mm Hg resulted in marked responses.5. These experiments suggest that, unless arterial oxygen tension is abnormally low, the carotid sinus reflex and not cerebral hypotension is important in the control of the inotropic state of the heart and of vasomotor activity. With hypoxaemia, responses from cerebral hypotension may also be important.     

Effects of graded pulsatile pressure on the reflex vasomotor responses elicited by changes of mean pressure in the perfused carotid sinus-aortic arch regions of the dog

1. In the anaesthetized dog, the carotid sinuses and aortic arch were isolated from the circulation and separately perfused with blood by a method which enabled the mean pressure, pulse pressure and pulse frequency to be varied independently in each vasosensory area. The systemic circulation was perfused at constant blood flow by means of a pump and the systemic venous blood was oxygenated by an extracorporeal isolated pump-perfused donor lung preparation.2. We have confirmed our previous observations that under steadystate conditions the vasomotor responses elicited reflexly by changes in mean carotid sinus pressure are modified by alterations in carotid sinus pulse pressure, whereas those evoked by changes of mean aortic arch pressure are only weakly affected by modifications of aortic pulse pressure.3. When the carotid sinus and aortic arch regions are perfused in combination at constant pulse frequency (110 c/min), the relationship between mean carotid sinus-aortic arch pressure and systemic arterial perfusion pressure is dependent on the size of the pulse pressure.4. Increasing the pulse pressure alters the curve relating the mean carotid sinus-aortic arch pressure to systemic arterial perfusion pressure in such a way that the perfusion pressure is lower at a given carotid sinus-aortic arch pressure within the range 80-150 mm Hg. The larger the pulse pressure, up to about 60 mm Hg, the greater the fall in systemic arterial perfusion pressure. Above a mean carotid sinus-aortic arch pressure of about 150 mm Hg, alterations of pulse pressure have little effect.5. There is a family of curves representing the relation between mean carotid sinus-aortic arch pressure and systemic vascular resistance, depending on the pulse pressure.     

Vasomotor inhibition in rabbits by vagal nonmedullated fibers from cardiopulmonary area.

In anesthetized rabbits with sinus and aortic nerve cut, when the cervical vagal nerves were cooled to 12, 8, 6, and 0degreeC, there were progressive increases in aortic blood pressure of 7 +/- 1, 15 +/- 2, 25 +/- 1, and 41 +/- 2 mmHg (SE), respectively. For comparison, during cooling of the aortic and vagal nerves, at 12degreesC there was a decrease in firing in the afferent fibers from aortic baroreceptors (48 +/- 4%) and pulmonary stretch receptors (57 +/- 5%), and at 6degreesC all activity was abolished. Thus, at 6degreesC the activity in medullated fibers from the aortic baroreceptors and pulmonary stretch receptors is blocked, but the increase in aortic blood pressure with vagal cooling is only 60% of that with cooling to 0degreeC. This demonstrates that cardiopulmonary receptors with nonmedullated vagal afferents (C fibers) contribute to the tonic inhibition of the vasomotor center. Because of overlap in sensitivity of different vagal fibers to cooling, the total contribution of these C fibers cannot be evaluated.     

Responses of abdominal vascular capacitance in the anaesthetized dog to changes in carotid sinus pressure.

1. The abdominal circulation of anaesthetized dogs was vascularly isolated without opening the abdomen, by cutting or tying all structures immediately above the diaphragm and tying the proximal ends of the hind limbs. The region was perfused at constant flow through the aorta and drained at constant pressure from the inferior vena cava. 2. Vascular resistance responses were expressed as the changes in perfusion pressure and capacitance responses were determined by integrating changes in vena caval outflow. 3. Decreasing the pressure in the isolated carotid sinuses over the whole baroreceptor sensitivity range increased mean perfusion pressure from 91 to 149 mmHg (a 67% increase in resistance) and decreased mean capacitance by 111 ml. (5 ml. kg-1). 4. The range of carotid sinus pressures over which capacitance responses occurred was at a significantly higher level than the corresponding range for resistance responses. 5. Comparison of the reflex responses with the responses to direct stimulation of efferent sympathetic nerves shows that quantitatively similar responses of resistance and capacitance to those induced by a large step decrease in carotid pressure could be produced by stimulating maximally the efferent sympathetic nerves at 5 Hz. These results also suggest that at all levels of carotid sinus pressure there is no difference in the impulse traffic to resistance and capacitance vessels.     

Studies on the central integration of excitatory chemoreceptor influences and inhibitory baroreceptor and cardiac receptor influences.

Experiments were performed on cats to explore the integrated cardiovascular responses when excitatory (chemoreceptor) and inhibitory (baroreceptor or cardiac receptor) influences are simultaneously presented to the medullary cardiovascular areas. At a given sinus pressure in the low or medium pressure range, the systemic blood pressure and the vascular resistance were higher when the chemoreceptors were stimulated, while a high, pulsating sinus pressure, i.e.a strong baroreceptor stimulation, could suppress completely even an intense chemoreceptor activation. Thus, the set point and the gain of the baroreflex were increased by a concomitant chemoreceptor activation. These effects are compatible with a simple, mutual 'summation' of excitatory and inhibitory influences on a common population of central vasomotor neurons. The reflex vasodilator effects elicited via vagal cardiac afferents were found to be more effectively suppressed by a concomitant chemoreceptor stimulation than were the baroreceptor effects, provided a primary chemoreceptor response (bradycardia) was at hand, while the heart rate responses were essentially uninfluenced by the prevailing chemoreceptor activity. This chemoreceptor suppression of the reflex vasodilatation from cardiac receptors, which may be of great importance in hypoxic situations, e.g. during a dive, suggests a more complex, neuronal interaction between the two reflex mechanisms in the CNS.     

The effects of changes in the carotid sinus baroreceptor activity on splanchnic blood flow in anesthetized man

In 9 patients being subjected to abdominal surgery, electromagnetic blood flow measurements were obtained from the hepatic, mesenteric and iliac beds while the carotid sinus baroreceptors were stimulated by carotid sinus massage. Carotid sinus stimulation produced an average maximum decrease in mean arterial pressure of 21%. Hepatic and mesenteric blood flows decreased by 15% and calculated vascular resistances were not significantly changed in these vascular beds. Iliac blood flow, on the other hand, showed a slight increase and iliac vascular resistance was decreased by 29%. It is concluded that the splanchnic vascular bed is of less importance in the carotid sinus baroreflex control of systemic arterial pressure in anesthetized man.     

Circulatory Responses to Stimulation of the Carotid Body Chemoreceptors in the Cat

Cardiovascular responses to carotid body chemoreceptor stimulation were followed in the ‘curarized’, vagotomized, artificially ventilated cat. Stimulation of the chemoreceptors by perfusion of the carotid sinus regions with venous blood induced a reflex vasoconstriction in skeletal muscle, kidney, intestine and skin, and, in most cases, an increased heart rate. A comparison of the chemoreceptor reflex responses with those obtained by direct electrical stimulation of the regional vasomotor fibres indicated that in chemoreceptor reflexes the vasomotor fibre activity increased to the same extent in skeletal muscle and intestinal resistance vessels and, probably, in the ‘nutritional’ skin vessels but to a smaller extent in the renal vessels and the skeletal muscle capacitancc vessels. The renal vessels and the muscle capacitance vessels seemed, however, to respond more during chemoreceptor stimulation than when the baroreceptors were unloaded, indicating that an excitation of the bul bar vasomotor centre is more effective than the withdrawal of an inhibitory restraint in activating the vasomotor fibres to these vascular sections.     

Lack of interaction between adenosine-induced vasodilatation and carotid baroreflex-induced changes in sympathetic activity in dog hindlimb artery

In anaesthetized dogs, a hindlimb was vascularly isolated and perfused at a constant flow rate of 7.7 +/- 1.9 ml min-1 100 g-1 (mean +/- S.E.M.; n = 5) through the femoral artery. The carotid sinuses were isolated and perfused at high (greater than 145 mmHg) or low (less than 75 mmHg) pressure to enable reflex sympathetic tone on the hindlimb vessels to be controlled. Both vagi were sectioned in the neck and mean aortic blood pressure was held constant by connection of the aorta to a reservoir. The responses to infusion of three doses of adenosine at high and low carotid sinus pressures were not significantly different: infusion of 0.60 +/- 0.16 microM-adenosine reduced femoral arterial perfusion pressure (FAPP) by 11.6 +/- 3.2% (n = 6) at high carotid sinus pressure and by 12.6 +/- 5.1% (n = 4) at low carotid sinus pressure, while 4.71 +/- 0.49 microM-adenosine reduced FAPP by 20.8 +/- 4.8% (n = 6) at high carotid sinus pressure and by 20.7 +/- 4.8% (n = 6) at low carotid sinus pressure; 50.1 +/- 7.3 microM-adenosine reduced FAPP by 36.7 +/- 5.5% (n = 6) at high carotid sinus pressure and by 27.7 +/- 7.8% (n = 5) at low carotid sinus pressure.(ABSTRACT TRUNCATED AT 250 WORDS)     

The vasomotor component of the carotid sinus baroreceptor reflex in the cat during stimulation of the hypothalamic defence area

1. The effect of moderate intensities of stimulation of the hypothalamic defence area on the baroreceptor reflex has been investigated in the cat by comparing the responses of arterial blood pressure and perfusion pressure of the isolated hind-limb muscle bed perfused at constant volume inflow, when the isolated carotid sinus was subjected to a series of non-pulsatile pressures with and without simultaneous hypothalamic stimulation.2. In the absence of hypothalamic stimulation the characteristic sigmoid curves relating sinus pressure to blood pressure or muscle perfusion pressure were obtained.3. With simultaneous stimulation of the hypothalamus a similar sigmoid relationship was found. There was no evidence of any reduction in the over-all power or maximum sensitivity of the baroreceptor reflex.4. However, in those cats which had been atropinized to abolish the cholinergically mediated muscle vasodilatation, the curves obtained during hypothalamic stimulation were displaced in such a manner as to suggest that, while baroreceptor modulation of vasoconstrictor tone continued during defence area stimulation, the blood pressure regulating mechanism had been ;reset' so that, within the physiological range of sinus pressures, any given level of sinus pressure was associated with a greater vasoconstrictor tone.5. In non-atropinized cats there was little displacement of the curves relating sinus pressure to blood pressure, while the curves relating sinus pressure to muscle perfusion pressure were displaced in the opposite direction so that over-all muscle vascular resistance was less than normal at each level of sinus pressure.     

Comparison of aortic and carotid baroreflexes in the dog

1. Experiments with vascularly isolated, blood-perfused aortic arch and carotid sinus preparations in sixteen dogs have provided evidence which suggests that, in the reflex regulation of normal arterial blood pressure, the aortic and carotid baroreflexes are not equivalent.2. Two different techniques were used. In one, a steel cannula was inserted into the ascending aorta and arch and fixed in position by ligatures about the aorta. The blood-filled space (aortic jacket) thus created could be distended with known pressures; the cardiac output passed through the cannula into the descending aorta. In the other, an extracorporeal circulation utilizing an isolated heart-lung preparation was used to separately perfuse the carotid sinuses, aortic arch, and systemic circulation of a test dog.3. Independent open-loop analysis of the aortic and carotid baroreflexes in each dog indicated that they were essentially similar in their over-all modus operandi but that there were quantitative differences between them which would suggest a predominant role for the carotid sinus reflex in the control of normal blood pressure.4. The carotid sinus Blutdruck-charakteristik curve was symmetrical about the range of normal blood pressure for the dog while the aortic arch curve was displaced to the right.5. The carotid sinus system had the greater gain (with reference to limb vascular resistance) and exhibited a greater maximal capacity to alter vascular resistance reflexly.6. When the carotid and aortic systems were activated simultaneously by distension, the reflex depressor responses were summed, essentially by a process of simple addition. A carotid-induced pressor response obscured a simultaneous aortic-induced depressor response of equal magnitude.7. In five dogs studied, the functional reflexogenic area of the aortic arch did not extend distally beyond the origin of the left subclavian artery.     

Baroreceptor and somatic sensory regulation of kidney function in two-kidney, one-clip Goldblatt hypertensive rats.

The aim of this investigation was to characterize the renal haemodynamic and tubular responses to somatic afferent nerve stimulation following the removal of afferent nerve input from the atria or the carotid sinuses in chloralose-urethane-anaesthetized Sprague-Dawley rats and two-kidney, one-clip Goldblatt hypertensive rats. Bilateral stimulation of the brachial nerve plexi at 15 V, 1.3 Hz for 0.2 ms resulted in an increase in systemic blood pressure in each group of 10-40%, while renal perfusion pressure was maintained at a constant level. There were significantly larger falls in left renal blood flow and combined left and right glomerular filtration rate in all groups following selective denervation of either the cardiopulmonary or the carotid sinus baroreceptors, respectively. Brachial nerve stimulation decreased urine flow rate and absolute and fractional sodium excretion from both kidneys in Sprague-Dawley intact animals by 53, 65 and 59%; in vagotomized animals by 68, 77 and 63%; and in carotid sinus denervated animals by 86, 90 and 48%, respectively. The renal response in the Goldblatt group were similar to the normotensive group, but the main contribution of the total response was from the untouched left kidney. The inhibitory influence of the vagus and carotid sinuses on the renal sympathetic nerve-mediated sodium and water resorption appeared to be enhanced in the Goldblatt hypertensive rats when compared with the normotensive rats. The renal functional responses to somatic afferent nerve stimulation appeared to be well preserved in the renovascular hypertensive rats, although there were important differences in the contributions to the responses from the left and right kidneys. Furthermore, the baroreceptors exerted a greater influence on basal renal function in the hypertensive rats.     

Coronary vascular and myocardial responses to carotid body stimulation in the dog.

Coronary vascular and myocardial responses to selective hypoxic and/or hypercapnic carotid chemoreceptor stimulation were investigated in constantly ventilated, pentobarbital or urethan-chloralose anesthetized dogs. Bilaterally isolated carotid chemoreceptors were perfused with autologous blood of varying O2 and CO2 tensions via an extracorporeal lung circuit. Systemic gas tensions were unchanged. Effects of carotid chemoreceptor stimulation on coronary vascular resistance, left ventricular dP/dt, and strain-gauge arch output were studied at natural coronary blood flow with the chest closed and during constant-flow perfusion of the left common coronary artery with the chest open. Carotid chemoreceptor stimulation slightly increased left ventricular dP/dt and slightly decreased the strain-gauge arch output, while markedly increasing systemic pressure. Coronary blood flow increased; however, coronary vascular resistance wa.as not affected. These studies show that local carotid body stimulation increases coronary blood flow but has little effect on the myocardium. The increase in coronary blood flow results mainly from an increase in systemic arterial pressure. Thus these data provide little evidence for increased sympathetic activity of the heart during local stimulation of the carotid chemoreceptors with hypoxic and hypercapnic blood.     

The carotid sinus baroreceptor reflex in the pregnant rabbit

1. The reflex responses to baroreceptor stimulation have been compared in eight pregnant and eight non-pregnant anaesthetized female rabbits.2. The vascularly isolated, innervated carotid sinus was exposed for 30 sec to a series of non-pulsatile pressures ranging from 30 to 230 mmHg. The contralateral sinus nerve and both aortic nerves were cut. Systemic arterial pressure and heart rate were measured at each sinus pressure.3. The range of arterial pressure change which could be evoked from the isolated innervated sinus was less in the pregnant than in the non-pregnant rabbits. Mean changes were 98 and 61 mmHg respectively. On the other hand changes in heart rate were similar in the two groups (45 and 43 beats/min respectively).4. The smaller blood pressure response in the pregnant animals resulted from a lesser rise in systemic arterial pressure at low levels of sinus pressure. At high sinus pressures the blood pressure fell to a similar level in both groups of animals.5. Pressure on the great vessels by the gravid uterus was not a factor since there was no consistent difference between the responses obtained with the rabbit lying on its back or on its side.6. Mechanisms which might be responsible for the difference found are discussed.     

Noise-enhanced heart rate and sympathetic nerve responses to oscillatory lower body negative pressure in humans

By injecting noise into the carotid sinus baroreceptors, we previously showed that heart rate (HR) responses to weak oscillatory tilt were enhanced via a mechanism known as "stochastic resonance." It remains unclear, however, whether the same responses would be observed when using oscillatory lower body negative pressure (LBNP), which would unload the cardiopulmonary baroreceptors with physically negligible effects on the arterial system. Also, the vasomotor sympathetic activity directly controlling peripheral resistance against hypotensive stimuli was not observed. We therefore investigated the effects of weak (0 to approximately -10 mmHg) oscillatory (0.03 Hz) LBNP on HR and muscle sympathetic nerve activity (MSNA) while adding incremental noise to the carotid sinus baroreceptors via a pneumatic neck chamber. The signal-to-noise ratio of HR, cardiac interbeat interval, and total MSNA were all significantly improved by increasing noise intensity, while there was no significant change in the arterial blood pressure in synchronized with the oscillatory LBNP. We conclude that the stochastic resonance, affecting both HR and MSNA, results from the interaction of noise with the signal in the brain stem, where the neuronal inputs from the arterial and cardiopulmonary baroreceptors first come together in the nucleus tractus solitarius. Also, these results indicate that the noise could induce functional improvement in human blood pressure regulatory system in overcoming given hypotensive stimuli.