The responses of aortic arch and right subclavian baroreceptors to changes of non-pulsatile pressure and their modification by hypothermia

1. A method is described for isolation of the aortic arch and right subclavian-carotid angle in situ in the rabbit and perfusion with Krebs-Henseleit solution or blood under controlled conditions of pressure and temperature.2. The characteristics of the baroreceptors of the aortic arch and right subclavian-carotid angle were studied by recording from single or few-fibre preparations of the left and right aortic nerves respectively. Curbes were plotted to show the relationship between the frequency of baroreceptor impulse activity and intra-aortic pressure during non-pulsatile perfusion under steady-state conditions.3. The aortic arch and right subclavian-carotid angle baroreceptors were found to have similar characteristics. Three types of response of the baroreceptors at the threshold pressure to a steady intra-aortic pressure are described.4. Increasing the intra-aortic pressure increased the frequency of impulses in fibres previously active and caused recruitment of other fibres in multi-fibre preparations. The relationship was linear at low pressures and a point of inflexion occurred at higher pressures in the majority of fibres.5. Lowering the temperature of the perfusate reduced the impulse frequency at any given pressure.6. The curves obtained during stepwise increases and decreases in intra-aortic arch pressure were dissimilar, particularly at the lower end of the pressure range. This phenomenon is probably due to properties of the arterial wall.7. When the aortic arch preparation was excised, changes occurred in the shape of the impulse frequency-pressure curves from baroreceptors in both areas. The point of inflexion was elevated and a higher percentage of fibres failed to reach a point of inflexion in the pressure range studied.     

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.     

The effects of altering mean pressure, pulse pressure and pulse frequency on the impulse activity in baroreceptor fibres from the aortic arch and right subclavian artery in the rabbit

1. A method is described for perfusing an isolated preparation of the rabbit aortic arch with independently controlled mean pressure, pulse pressure and pulse frequency.

2. Recordings made from single or few-fibre preparations from the aortic arch and right subclavian baroreceptor regions show that the number of impulses per second or per cycle in a single fibre is the same during pulsatile perfusion as during non-pulsatile perfusion if the pressure is above the threshold pressure to non-pulsatile perfusion during all phases of the pressure cycle.

3. In multi-fibre recordings the total number of impulses is greater during pulsatile perfusion than during non-pulsatile perfusion due largely to recruitment of fibres during systole.

4. The relationship between instantaneous impulse frequency and aortic arch pressure during one pressure cycle forms an elliptiform curve.

5. Increasing the pulse pressure increases the ellipse and causes additional recruitment of other fibres during systole, thus augmenting the total impulse activity.

6. Increasing the pulse frequency reduces the number of impulses per cycle for single fibres but produces a small increase in the total impulse frequency in one second period due to recruitment.

7. Increasing the rate of change of pressure by increasing the pulse pressure or pulse frequency produced a small reduction or no change of the threshold pressure. Similarly the `cut off' pressure was elevated in some fibres.

8. At low initial mean pressures, an increment of pressure, at constant pulse pressure and pulse frequency, increases the total impulse activity by increasing the frequency of impulses in single fibres already active during systole and diastole and by additional recruitment of other fibres. At higher mean pressures there is little increase in impulse activity as the maximum frequency of fibres is attained or superseded and there is little recruitment.

     

Influence of intravenous saline infusion on the aortic baroreceptor and left atrial B-type receptor activity in dogs

Influence of i.v. saline infusion on aortic arch pressure, left atrial mean pressure, heart rate, and the time-related characteristics of aortic arch baroreceptor and left atrial B-type receptor activities were studied in 20 beagle dogs. Saline infusion induced tachycardia in 10 dogs and bradycardia in 10. Aortic arch and left atrial pressures increased in both HR response groups. The average discharge rate of baroreceptors increased in both HR response groups, while the duration of baroreceptor burst, the number of baroreceptor action potentials/heart cycle and the average burst frequency did not increase significantly in the group that responded with tachycardia but increased significantly in the group that responded with bradycardia. The number of left atrial B-type action potentials/heart cycle, the average discharge rate and average burst frequency increased in both HR response groups. We conclude that the duration of baroreceptor burst, baroreceptor average burst frequency and the number of baroreceptor impulses/cardiac cycle are the important parameters eliciting baroreceptor reflex to i.v. saline infusion.     

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.     

PRES: The Controlled Noninvasiv Stimulation of the Carotid Baroreceptors in Humans

To study physiological and psychological effects of baroreceptor activity, the cervical neck cuff technique has been frequently used to stimulate the carotid baroreceptors mechanically. Using this technique, no satisfying control conditions to date have been available. Because the carotid stretch receptors are sensitive not only to the pressure level, but also to the rate of change, it is possible to manipulate the receptor firing through changes in carotid pulse amplitude. The device described here relies on the application of short changes in cuff pressure tied to different phases within the cardiac cycle (phase related external suction (PRES)). A brief external suction during systole has potent stimulatory effects on baroreceptors whereas the application of the very same pressure pulse during diastole inhibits the firing burst associated with the pulse wave. To allow an ongoing period of stimulation, a sequence of alternating negative/positive pressure pulses is applied. In the stimulation condition, the R-wave of the electrocardiogram triggers a negative pulse which is followed by a positive one during diastole. In the control condition this relationship is reversed. Two experiments are reported confirming different baroreceptor effects of the two conditions. PRES allows for blind or double-blind experiments to investigate effects of baroreceptor activity on physiology and behavior.     

Inotropic responses of the left ventricle to changes in aortic arch pressure in anaesthetized dogs

1. Inotropic changes in the left ventricle in chloralose anaesthetized dogs were determined in response to changes in non-pulsatile pressure perfusing the vascularly isolated aortic arch.2. Inotropic responses were assessed by measuring the maximum rate of change of left ventricular pressure (dP/dt max) in preparations in which heart rate, mean ascending aortic pressure and brachiocephalic (i.e. carotid sinus and cerebral) perfusion pressure were held constant.3. dP/dt max increased (average +43%) when aortic pressure was reduced from a level above that which produced maximum depression of the myocardium to a level below which no further responses could be obtained; responses occurred as aortic arch pressure was changed between 250 and 90 mm Hg.5. In the same preparations changes in the brachiocephalic artery perfusion pressure with aortic arch pressure held constant resulted in similar inotropic responses.6. It is suggested that aortic arch baroreceptors may be of importance in the control of the inotropic state of the heart.     

The KCNQ/M-current modulates arterial baroreceptor function at the sensory terminal in rats

The ion channels responsible for the pattern and frequency of discharge in arterial baroreceptor terminals are, with few exceptions, unknown. In this study we examined the contribution of KCNQ potassium channels that underlie the M-current to the function of the arterial baroreceptors. Labelled aortic baroreceptor neurons, immunohistochemistry and an isolated aortic arch preparation were used to demonstrate the presence and function of KCNQ2, KCNQ3 and KCNQ5 channels in aortic baroreceptors. An activator (retigabine) and an inhibitor (XE991) of the M-current were used to establish a role for these channels in setting the resting membrane potential and in regulating the response to ramp increases in arterial pressure. Retigabine raised the threshold for activation of arterial baroreceptors and shifted the pressure–response curve to higher aortic pressures. XE991, on the other hand, produced an increase in excitability as shown by an increase in discharge at elevated pressures as compared to control. We propose that KCNQ2, KCNQ3 and KCNQ5 channels provide a hyperpolarizing influence to offset the previously described depolarizing influence of the HCN channels in baroreceptor neurons and their terminals.     

Effect of acute intravenous volume loading on haemodynamics and aortic baroreceptor activity in dogs

This study evaluates the effect of acute intravenous volume loading on haemodynamics and aortic baroreceptor activity in order to determine the mechanistic factors responsible for divergent heart rate responses to volume loading. Eleven beagles were anaesthetized and instrumented for the recording of aortic pressure, diameter, flow and aortic baroreceptor activity. Isotonic saline, equal to 20% of the estimated blood volume, was infused intravenously within 60-90 s. The animals were divided into tachycardiac and bradycardic groups according to their heart rate response to volume loading. In the group developing the tachycardic response, aortic baroreceptor activity increased. Total peripheral resistance was reduced by 31% and variables representing aortic stretch, mean aortic pressure, diameter and tension remained unchanged. In contrast, in the group developing the bradycardiac response, aortic baroreceptor activity increased markedly. Also in this group peripheral resistance decreased, but only by 15%, which was significantly less than in the tachycardiac group. Factors determining aortic stretch, mean aortic pressure, diameter and tension also increased significantly. Earlier studies show that regardless of the resulting heart rate response, atrial receptor activity increases during volume loading. Thus, we conclude that during cardiac acceleration, marked peripheral vasodilation eliminates the stimulation of baroreceptors and therefore the tachycardic response caused by atrial receptors develops. In the case of cardiac slowing, vasodilation also takes place, but is not sufficient to prevent activation of baroreceptors. In consequence, increased baroreceptor activity overrides the tachycardic drive coming from the atrial receptors and a typical baroreceptor reflex becomes manifested.     

Haemodynamic information encoded in the aortic baroreceptor discharge during haemorrhage

This study concerns the function of aortic baroreceptors during slow haemorrhage, in particular the mechanisms by which baroreceptors respond to hypovolaemia in the absence of hypotension and the manner in which haemodynamic information is encoded in the aortic nerve discharge. Beagle dogs, anaesthetized with morphine and chloralose, were instrumented for the recording of aortic pressure, aortic diameter, aortic flow and aortic nerve activity. The animals were subjected to slow haemorrhage (0.4 ml kg-1 min-1) of 20% of the estimated blood volume. Mean aortic pressure remained unchanged, whereas mean aortic diameter, mean aortic tension and aortic baroreceptor activity decreased during the haemorrhage. Baroreceptor sensitivity to mean aortic pressure, mean aortic diameter and mean aortic tension decreased as the haemorrhage proceeded. Also, the sensitivity of baroreceptors to mean aortic pressure and mean aortic diameter during acute pressure stimulation was attenuated. The results indicate that during non-hypotensive haemorrhage aortic baroreceptor discharge is reduced by two mechanisms: firstly, via constriction and stiffening of aortic smooth muscle and, secondly, via direct effects of the compensatory mechanisms on the baroreceptors. The reduced sensitivity renders the baroreceptors capable of responding to hypovolaemia prior to the onset of hypotension. In the regression analysis, systolic aortic diameter proved to be the best determinant of baroreceptor activity (r = 0.877). Of the neural variables, impulses per cardiac cycle and average discharge rate were the most reliable, but no difference in the relative importance of these two parameters could be observed.     

The influence of aortic baroreceptors on venous tone in the perfused hind limb of the dog.

1. The aortic arch and both carotid sinuses were vascularly isolated and perfused. A hind limb was vascularly isolated and blood was pumped at constant flows into the femoral artery and the central end of a superficial metatarsal vein. 2. Large increases in aortic arch pressure resulted in decreases in arterial blood pressure, heart rate and femoral arterial perfusion pressure. The average response of the vein was a decrease of 11% in the pressure gradient between the perfused vein and the femoral vein. Similar responses were obtained when carotid sinus pressure was increased. 3. Crushing or cooling the lumbar sympathetic trunk caused responsed similar to those induced by increasing baroreceptor perfusion pressure. Stimulation at 1 HZ resulted in venous responses four times as great as the average reflex response, whereas frequencies of 2-5 Hz were required to produce changes in arterial resistance as great as those induced reflexly. 4. These experiments indicate, that although the large superficial veins of the dog's hind limb participate in the baroreceptor reflexes, the activities in the nerves supplying arterioles and veins must have been different.     

Adrenergic and pressure-induced modulation of carotid sinus baroreceptors in rabbit

The sympathetic control of the carotid sinus baroreceptor activity was studied in rabbits. Stimulation of the cervical sympathetic trunk (10 Hz, 1 ms, 4–12 V) elicited an increase in discharge frequency of the non-medullated baroreceptor afferents but not of the medullated fibers. An isolated carotid sinus area preparation was used to examine the influence of noradrenaline perfusion on baroreceptor activity. Non-medullated baroreceptor afferents, but not the medullated afferents, responded to noradrenaline perfusion (10^-6 g/ml) with a significant increase in firing rate. Short-term resetting of the baroreceptors was also studied with the same preparation. After perfusing the sinus at a hypertensive level for 30 min the pressure-response curves of both the non-medullated and the medullated baroreceptor afferents shifted to the right with increased thresholds and decreased maximal discharge frequencies. The extent of the short-term resetting was greater in non-medullated afferents than in medullated ones. It is concluded that the carotid sinus baroreceptors with non-medullated afferents are under the sympathetic control. The physiological significance of this is discussed.     

The effect of baroreceptors on the latency of evoked responses in sympathetic nerves during the cardiac cycle

1. A rapid increase in pressure in a vascularly isolated perfused carotid sinus has been shown to inhibit a reflex response in efferent sympathetic nerves of the dog evoked by electrical stimulation of the radial nerve.2. In intact preparations with the carotid baroreceptors innervated, the mean latency and the variance of the latency of reflex sympathetic nerve responses was reduced when the stimuli evoking the responses were applied at one point of both cardiac and respiratory cycles. When the baroreceptors were denervated there were no significant differences in the responses to random and synchronized stimuli.3. In intact preparations the latency of evoked responses in sympathetic nerves was found to vary progressively during a cardiac cycle; the maximum increase in latency was observed with the responses that occurred at that phase of the cardiac cycle when the baroreceptors exert maximal inhibition on spontaneous sympathetic activity. After denervation of the carotid sinuses a much smaller change during the cardiac cycle was still present, possibly due to effects produced by baroreceptors of the aortic arch and elsewhere.4. It was concluded that changes in baroreceptor activity, due to beat to beat fluctuations of the systemic arterial pressure are a major factor causing variations in the latency of responses in sympathetic nerves evoked by stimuli applied to a cutaneous nerve.     

The influence of heart rate on baroreceptor fibre activity in the carotid sinus and aortic depressor nerves of the rabbit

The arterial baroreceptors and their afferent fibres provide the sensory arm of the reflex that regulates systemic arterial pressure. We have examined whether the relationship between mean baroreceptor discharge and mean arterial pressure is altered when heart rate changes. Experiments were performed on pentobarbitone-anaesthetized rabbits. We recorded the activity of single and multifibre preparations of the carotid sinus (CSN) and aortic depressor nerves (ADN). Data were collected under control conditions and while heart rate was increased by approximately 30-35% by right atrial pacing. Baroreceptor regions were exposed to ramps of pressure (from approximately 25 to 140 mmHg, at approximately 0.5-1 mmHg s(-1)), generated by inflation and deflation of cuffs placed around the inferior vena cava and descending thoracic aorta. Response curves relating baroreceptor discharge to mean pressure were constructed and fitted with third-order polynomial expressions. To provide a measure of an effect of an increase in heart rate on the response curve in the region of the normal operating pressure, we calculated the position of the test response curve relative to the position of the control curve at 90 mmHg (deltaBP(90)). For the ADN, the activity of single fibres (presumptive myelinated fibres) was unaffected by increasing heart rate (deltaBP(90) = +0.1 +/- 1.0 mmHg), while single fibres in the CSN showed a small increase in activity (deltaBP(90) = -1.5 +/- 0.3 mmHg). In multifibre preparations there was a small increase in activity that may be attributable to additional activity in unmyelinated fibres (ADN, deltaBP(90) = -3.4 +/- 1.2 mmHg; CSN, deltaBP(90) = -5.2 +/- 0.9 mmHg). We conclude that the mean discharge of arterial baroreceptors remains a reliable index of mean arterial pressure in the presence of substantial changes in heart rate.     

Studies on blood-pressure regulation by the use of a baroreceptor analog

Summary In rabbits anesthetized with urethane the aortic and carotid sinus nerves were stimulated by an electric baroreceptor analog. In closed loop experiments the analog buffered blood pressure alterations and alterations of heart rate following the blockade of aortic nerves and clamping the common carotid arteries. The carotid sinus nerves appeared to be not more sensitive to baropacing by the analog than the aortic nerves. Blood pressure and heart rate oscillations induced by vagal stimulation with the analog were damped and finally disappeared when the electric receptor analog was used to stimulate simultaneously the depressor nerves. Analysis of the baroreceptor analog in open loop experiments gave the same results as obtained on natural baroreceptors: pulse pressure sensitivity changed with mean pressure, amplitude, and pulse rate. The transfer function of the analog is the same as transfer functions of carotid sinus nerves are. It is concluded, that the baroreceptor analog may be useful for closed loop baropacing treatment of hypertension.This work was supported by the Deutsche Forschungsgemeinschaft (Ze 67/1).     

Characteristics of brachiocephalic and carotid sinus baroreceptors with non-medullated afferents in rabbit

The characteristics of brachiocephalic and carotid sinus baroreceptors with non-medullated afferents have been studied in rabbits. The conduction velocities of 18 non-medullated fibers arising from the brachiocephalic baroreceptors were 0.4-0.8 m/s (0.59 +/- 0.03 m/s, mean +/- SE) and their thresholds were 74-104 mmHg (84 +/- 2 mmHg). 29 non-medullated carotid sinus baroreceptor afferents had thresholds ranging from 60 to 125 mmHg (92 +/- 3 mmHg). Their conduction velocities were 0.5-1.1 m/s (0.65 +/- 0.05 m/s). Blood pressure-baroreceptor response curves of both the brachiocephalic and the carotid sinus baroreceptors were constructed. As the aortic arch non-medullated baroreceptor afferents, the brachiocephalic and the carotid sinus baroreceptors with non-medullated afferents exhibited thresholds about 30 mmHg higher than the medullated ones, considerably lower firing rates, comparatively rapid adaptation and remarkable irregular discharge pattern. It is suggested that the non-medullated baroreceptor afferents are of little importance for the cardiovascular regulation under normal conditions but are likely to play an important role during a sudden rise in arterial pressure.     

Changes in sensitivity of aortic baroreceptors under the influence of the sympathetic nervous system

The dependence of activity of the aortic baroreceptors on the pressure in the aorta during stimulation of the branch from the stellate ganglion innervating the aortic baroreceptor zone was studied in experiments on cats. Stimulation of the sympathetic branch was shown not to change the threshold of activity of the baroreceptors (80 mm Hg) or their activity within the range from 80 to 140 mm Hg. With higher pressures in the aorta, stimulation of the sympathetic branch reduced baroreceptor activity, as a result of which the curve of baroreceptor activity as a function of pressure became flatter and reached maximal activity at a higher intraortic pressure (on the average 32 mm Hg higher). The mechanisms of the action of the sympathetic nerves on baroreceptor sensitivity and the physiological significance of this phenomenon are discussed.Laboratory of Physiology and Pathophysiology of the Circulation, All-Union Cardiological Scientific Center, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR E. I. Chazov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 83, No. 5, pp. 519–522, May, 1977.     

Influence of low- and high-pressure baroreceptors on plasma renin activity in humans

We evaluated effects of the low- and high-pressure baroreceptors on plasma renin activity (immunoassay) using graded lower body suction (LBS) in six healthy men. LBS at -10 and -20 mmHg for 10 min decreased central venous pressure without changing arterial pressure and thereby presumably reduced low- but not high-pressure baroreceptor inhibition of renin release. LBS at these levels produced forearm vasoconstriction, but did not increase renin. LBS at -40 mmHg decreased central venous and arterial pulse pressure and thus reduced both low- and high-pressure baroreceptor inhibition. LBS at this level produced forearm vasoconstriction and tachycardia and increased renin from 2.1 +/- 0.4 (mean +/- SE to 7.4 +/- 1.4 ng.ml-1.h-1 (P less than 0.05). In summary, reduction in low-pressure baroreceptor inhibition in humans did not increase renin in the presence of physiological tonic inhibition from high-pressure baroreceptors. Increases in renin did not occur until there was combined reduction of high- and low-pressure baroreceptor inhibition on plasma renin activity.     

Resetting of aortic baroreceptors with non-myelinated afferent fibers in spontaneously hypertensive rats

The phenomenon of resetting and the possible mechanisms responsible for it were investigated in aortic baroreceptors having unmyelinated axons. Recordings were obtained from 31 aortic baroreceptor C-fibers in 20 normotensive Wistar Kyoto rats (WKY) and 37 baroreceptor C-fibers in 11 spontaneously hypertensive rats (SHR). The recordings were obtained from the left aortic nerve of an in vitro aortic nerve-aortic arch preparation. The thresholds for activation of baroreceptor C-fibers were not different between SHR's and WKY's at 16 weeks of age (139±7 and 133±5 mmHg, respectively). However, the thresholds for activation in SHR at 36 weeks of age were considerably higher (163±4 mmHg) indicating resetting of the receptors in the older rats. The pressure-discharge curves were not different between SHR's and WKY's at 16 weeks of age and WKY's at 35 weeks of age, but were shifted to the right for SHR's at 36 weeks of age. The receptors showed greater responses to pulsatile pressures than to steady state pressures at the same mean level of pressure. We found that aortic C-fibers were not reset during the early established phase of hypertension in SHR's because of enhanced receptor strain sensitivity. This is likely to be of importance for tonic vasomotor control at this time. In later stages of hypertension and despite enhanced strain sensitivity the endings were reset to higher pressures probably due to an increased collagen content in the aortic wall.     

Mechanisms for activation of aortic baroreceptor C-fibres in rabbits and rats.

In an earlier study, we examined the pressure-response characteristics of rat aortic baroreceptors with C-fibre (non-medullated) afferents. Compared with aortic baroreceptor fibres with A-fibre (medullated) afferents, the C-fibres were activated at higher pressures and discharged more irregularly when stimulated with a steady level of pressure. Here we examine the relationship between discharge and the aortic diameter in these two types of afferents in rats and rabbits. An in vitro aortic arch/aortic nerve preparation was used to record single-fibre activity simultaneously with aortic arch pressure and diameter. Diameter was measured using a highly sensitive non-contact photoelectric device. Baroreceptor discharge was characterized by stimulating the nerve endings with either slow pressure ramps from subthreshold to 200-250 mmHg, at a rate of rise of 2 mmHg s-1, or pressure steps from subthreshold to suprathreshold levels, at amplitudes of 110-180 mmHg. In response to these inputs, C-fibres in rabbits (conduction velocities= 0.8-2.2 m s-1) behaved much like those in rats. The C-fibres had significantly higher pressure thresholds (95 +/- 3 mmHg vs. 53 +/- 2 mmHg; mean +/- SEM), lower threshold frequencies (2.4 +/- 0.5 vs. 27.7 +/- 1.8 spikes s-1), lower maximum discharge frequencies (22.7 +/- 2.3 vs. 65 +/- 5.8 spikes s-1) and more irregular discharge in response to a pressure step when compared with A-fibres (conduction velocities of 8-16 m s-1). When plotted against diameter, C-fibre ramp-evoked discharge increased gradually at first, and then rose steeply at increasingly higher ramp pressures where aortic diameter became relatively constant. In contrast, A-fibre discharge was linearly related to diameter over a wide range of pressure. These results suggest two interpretations: (1) The relation between stretch and C-fibre discharge is highly non-linear, with a marked increase in sensitivity at large diameters. (2) C-fibres are stimulated by changes in intramural stress rather than stretch.