Modification of the reflex response to stimulation of carotid sinus baroreceptors during and following stimulation of the hypothalamic defence area in the cat

1. The effects of stimulation of the hypothalamic defence area on carotid sinus baroreceptor reflexes have been investigated by examining the cardiovascular responses to a 15 sec period of increased pressure within the vascularly isolated carotid sinus before, during, and immediately following a 25 sec period of hypothalamic stimulation.2. Identification of the hypothalamic defence area was based on the occurence of atropine-sensitive muscle vasodilatation. Electrode positions were confirmed by histological examination.3. During hypothalamic stimulation the reflex fall in blood pressure resulting from a rise in sinus pressure was found to be undiminished whether sinus pressure was raised at the onset or at the 10th sec of hypothalamic stimulation.4. By contrast, in at least half the cats in which a reflex bradycardia could be evoked from the sinus, this bradycardia was largely if not completely suppressed during hypothalamic stimulation. This suppression of reflex bradycardia occurred when sinus pressure was raised at the onset as well as at the 10th sec of stimulation.5. During the first 5 sec after hypothalamic stimulation the hypotensive response to an increase in carotid sinus pressure was much reduced; on the other hand the reduction in heart rate was exaggerated, sometimes to a very marked degree.6. The results suggest that stimulation of the hypothalamic defence area can modify baroreceptor reflexes and that this modification can include selective alterations in the various components of the reflex response.     

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.     

Cardiovascular responses to carotid sinus baroreceptor stimulation during moderate to severe exercise in man

Our objective was to assess the importance of arterial baroreflexes in maintaining vasoconstriction in active muscle during moderate to severe exercise. Eight subjects exercised for 8-15 min on a cycle ergometer at three levels (averages 94, 194, 261 W) requiring 40-88% of VO2 max. Four times during each exercise level pulsatile negative pressure (-50 mmHg) was applied over the carotid sinuses for 30 s; suction was applied at each ECG R-wave for 250-400 ms. Before and during each neck suction, femoral venous blood flow (FVBF) was measured by constant infusion thermal dilution. At 94 W neck suction significantly reduced blood pressure (BP) (15 mmHg) and heart rate (HR) (7 beats min-1), and raised leg vascular conductance (LVC) (11.4%) without changing FVBF. At 194 W, neck suction reduced BP (9 mmHg), HR (4 beats min-1) and FVBF (5.1%, 240 ml min-1), and raised LVC (5.2%). At 261 W, LVC was unchanged by neck suction, but BP and FVBF both fell (9 mmHg and 650 ml min-1 or 7.4%). We conclude that competing local vasodilation and sympathetic vasoconstriction control muscle blood flow during moderate exercise, and vasoconstrictor tone can be withdrawn by baroreceptor stimulation. High levels of vasoconstrictor outflow to muscle in severe exercise may not originate from baroreflexes.     

The localization of the first synapse in the carotid sinus baroreceptor reflex pathway and its alteration of the afferent input

Summary In 78 anaesthetized cats the lower brain stem was explored with microelectrodes for localization of the secondary neurones of the carotid sinus primary afferents. By means of recording single unit activities and evoked potentials (E.P.) by stimulation of the carotid sinus nerve (CSN) these neurones were located exclusively in the nucleus tractus solitarius (NTS) at the level of the obex.The E. P. was recorded with both ipsilateral and contralateral CSN stimulation.During repetitive stimulation of the CSN the frequency transmission across the synapse showed a sharp frequency limitation. With 10 Imp/sec the amplitude of the E.P. was only 50% of the maximal amplitude obtained with 1 Imp/sec. This frequency limitation is also manifested in the frequency effectiveness of the whole baroreceptor reflex measured as blood pressure reduction. It showed a maximum with frequencies of 20–30 Imp/sec, while stimulation of the secondary neurones in the NTS showed an increase in blood pressure reduction up to the tested frequencies of 120 Imp/sec.The E.P. was tested in the different phases of the respiratory cycle. No difference in amplitude or latency between the E.P. was noted.The relevance of these findings to the arrangement of the central neural substrates for the cardiovascular control is discussed.This work was supported by the Deutsche Forschungsgemeinschaft.A summary of these studies was presented at the 34. Tagung der Deutschen Physiologischen Gesellschaft [46].     

Effect of cervical sympathetic nerve stimulation on canine carotid sinus reflex.

In the chloralose-anesthetized dog the carotid sinus on one side of the neck was isolated vascularly. Pressure in the isolated sinus [carotid sinus pressure (CSP)], electrocardiogram, and systemic arterial pressure were recorded. Both vagosympathetic trunks were cut and the contralateral common carotid artery was occluded or the contralateral sinus nerve was cut to reduce reflex buffering of arterial pressure changes. By varying CSP from 50 to 250 mmHg the full range of the reflex response was examined. Electrical stimulation of the peripheral end of the cut ipsilateral cervical sympathetic nerve brought about a rapid decrease in mean arterial pressure (MAP) and heart rate (HR) at lower CSPs, no change in these variables at midrange CSPs, and a gradual increase at higher CSPs, such that the gain of the reflex was reduced (1.89 +/- 0.19 to 1.33 +/- 0.15 mmHg/mmHg). The decrease in MAP and HR at lower CSPs implies an increase in baroreceptor activity whereas the converse would appear to occur at higher CSPs. These responses attained a maximum value at low stimulus frequencies (less than 10 Hz).     

Amplitude dependence of the carotid sinus reflex

Summary Both carotid arteries of anesthetized mongrel dogs were perfused with arterial blood by a servo controlled peristaltic pump. This preparation allowed to examine the carotid sinus pressure reflex under open loop condition as well as under closed loop condition, without touching the receptor area. The experiments were performed before and after vagotomy. The response of the systemic arterial pressure to changes of the carotid sinus pressure was recorded and evaluated. In order to test the open loop characteristics of the system, the perfusion pressure in the carotid arteries was varied sinusoidally (0.01 to 0.2 Hz). It was found that the amplitude of the carotid sinus pressure had a marked influence on the gain of the reflex. The gain decreased with increasing amplitudes of the carotid sinus pressure at all frequencies tested. Vagotomy increased the gain especially at the low frequencies.By applying a hybrid computer system to control the perfusion pump, the same preparation could be examined under closed loop condition. Particularly, the effect of an artificial time delay (1–35 sec) between systemic and carotid sinus pressure was tested. We found that this procedure generates autooscillations of the arterial pressure. The period of the oscillations is proportional to the artificial time delay. From open loop results the closed loop behaviour of the system was calculated and compared with the actual experimental results. We could show that the amplitude dependence of the carotid sinus pressure reflex is important to explain the generation of oscillations in the closed loop system by an artificial time delay. However, whereas the predicted autooscillation frequencies agreed quite well with the experimental recordings, the prediction of the amplitudes of the oscillations showed remarkable errors, which probably are due to still unexplained nonlinearities.It can be concluded that 1) the amplitude dependence of the carotid sinus reflex is an important property of the pressure control system. 2. The application and comparison of open loop and closed loop methods in the same experiment allows to check results and predictions more thoroughly than either one of these methods alone.Supported by NIH grants HE 11747 and GM 1919 and Österreichischer Fonds zur Förderung der wissenschaftlichen Forschung.Supported by NIH research career development award HE 50288-01.     

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.