Sinusoidal stimulation of carotid sinus baroreceptors and peripheral blood pressure in dogs

The role of pressure pulsatility in the carotid baroreflex control: a computer simulation study

The role of pressure pulsatility in the arterial pressure control by the carotid baroreflex was investigated by means of a mathematical model. The model describes the main hemodynamic properties of the cardiovascular system in pulsating conditions, the static and dynamic components of the carotid baroreflex, and their effect on systemic arterial resistance, heart frequency and systemic venous capacity. Experimental findings on the role of pressure pulsatility in the carotid baroreflex were reproduced in terms of a non-linear interaction between the static and dynamic components. Simulations of physiological experiments (mild haemorrhage, carotid occlusion manoeuvres, genesis of self-sustained arterial pressure waves) reveal that the role of pressure pulsatility is meaningful in response to those perturbations (such as carotid occlusion manoeuvres) characterized by different alterations in the mean and pulsating components.

Determinants of sensitization of carotid baroreceptors by pulsatile pressure in dogs

The threshold pressure of single baroreceptor units is decreased after compared with before exposure to pulsatile pressure according to previous studies in our laboratory. The purpose of the present study is to characterize the determinants of sensitization of arterial baroreceptors by pulsatile pressure. Carotid sinus nerve activity was recorded in dogs anesthetized with chloralose. Two indexes of baroreceptor "sensitivity" were obtained by comparing nerve activity before and immediately after exposure of the isolated carotid sinus to pulsatile pressure for periods up to 10 minutes. Sensitization occurred 1) when the threshold pressure of single baroreceptor units determined with a slow nonpulsatile ramp decreased after as compared with before pulsing and 2) when multiple unit activity increased after as compared with before pulsing at various mean levels of static pressure. Sensitization was evident after pulsing at mean pressure of 50 and 100 mm Hg, but not at 150 and 200 mm Hg, and was caused by the pulsatile change in diameter or deformation and not by the pulsatile change in wall tension. The magnitude of the effect was directly related to the duration of the pulsing period and to the frequency and amplitude of the pressure pulses. The sensitization could not be explained by increased diameter (sonomicrometers) or strain of the carotid sinus at the same pressure after pulsing; thus, there was an increase in "strain sensitivity" that outlasted the period of pulsing by up to several minutes. In most experiments the shift from static to pulsatile pressure at 50 and 100 mm Hg caused an increase in nerve activity, yet sensitization occurred after pulsing when one would have expected postexcitatory hyperpolarization or depression of activity upon return to static pressure. The sensitization was not caused by the release of prostacyclin from the endothelium since it was not reduced after endothelial denudation or inhibition of cyclooxygenase with indomethacin (30-80 microM) or ibuprofen (250 microM). We speculate that sensitization of baroreceptors by pulsatile pressure may contribute to the decreased sympathetic activity after periods of elevated pulse pressure (e.g., after exercise). We also propose that the decreased sensitivity of baroreceptors after acute elevation of arterial pressure (acute resetting) may be offset in part by the sensitizing effect of increased pulsatile stretch.

Effects of pulse frequency on single-unit baroreceptor activity during sine-wave and natural pulses in dogs

1. Previous studies using sine-wave pulses have shown that changes in pulse frequency do not alter single-unit activity per unit time. Since baroreceptor activity is phasic with most of the activity occurring in systole, we reasoned that the lack of effect of pulse frequency is due to the associated reciprocal change in duration of systole with sine-wave pulses. Therefore, we tested the hypothesis that change in frequency using natural arterial pulses alters baroreceptor unit activity per unit time. 2. In chloralose-anaesthetized dogs the isolated carotid sinus was connected to an electromagnetic pressure converter driven by a voltage generator that controlled pulse pressure and sine-wave pulse rate. To generate a natural pulse the converter was driven by the output from the pressure channel used to record arterial pressure. Systolic and diastolic pressures were maintained constant by a pressurized air source and an adjustable control on the voltage generator. The rate of natural pulses was decreased by electrical stimulation of the peripheral right vagus nerve which slowed the heart rate of the dog. 3. Decreases in sine-wave rate by approximately 73 pulses/min (from 156 +/- 2 to 83 +/- 1 pulses/min) at a mean pressure of 114 +/- 8 mmHg (n = 13) did not alter activity per unit time (28 +/- 3 vs. 29 +/- 3 spikes/s). 4. In contrast, decreases in natural pulse rate by an average of 78 pulses/min (from 182 +/- 7 to 104 +/- 15 pulses/min) at a comparable mean pressure (104 +/- 7 mmHg) (n = 9) decreased activity per unit time by 28% from 32 +/- 4 to 23 +/- 4 spikes/s (P less than 0.05). Similar findings were observed in five of six single units using an intact (normally perfused) carotid sinus preparation in dogs with heart block and cardiac pacing. 5. The results indicate that a change in natural pulse rate is an important determinant of single-unit baroreceptor activity.

New insights into the influence of pulsatile pressure on the arterial baroreceptor reflex

Pulsatile arterial pressure as compared to static pressure has important influences on baroreceptor activation and on the central mediation of the baroreflex. First, the static pressure-baroreceptor activity curve is sigmoid and the slope or gain peaks abruptly at pressures near 100 mmHg. In contrast, the pulsatile pressure-activity curve is linear over a wide range of pressure and the maximum gain is reduced. Second, exposure of baroreceptors to elevated levels of static pressure causes a rapid resetting or desensitization of the receptors within minutes. Baroreceptor resetting is absent or blunted when the elevated pressure is pulsatile. Third, inhibition of sympathetic activity is greater during pulsatile vs. static pressure even when baroreceptor activity per unit time is less. This central facilitation of the baroreflex is related to the lack of adaptation of central neurons to the pulse phasic afferent activity. The results indicate: 1) the extended linear range of the pressure-activity relation during pulsatile pressure may enable the baroreceptors to accurately detect changes in arterial pressure over a wider pressure range than that predicted from experiments using static pressure; and 2) pulsatile pressure can reduce resetting and promote reflex inhibition of sympathetic activity both by sensitizing the baroreceptors and by centrally facilitating the reflex.

Contrasting effects of static and pulsatile pressure on carotid baroreceptor activity in dogs

The purpose of this study was to contrast the effects of static and pulsatile pressure on carotid baroreceptor activity over a wide range of mean arterial pressure. Static and pulsatile pressure were applied to the isolated carotid sinus of dogs anesthetized with chloralose. Recordings were obtained from single baroreceptor units as well as from the whole sinus nerve or a large strand of the nerve. Three observations are reported. First, in single units the pulsatile pressure threshold, which averaged 48 +/- 8 (SEM) mm Hg, was far below the static pressure threshold, which averaged 79 +/- 8 mm Hg (p less than 0.05, n = 15). Thus, pulsatility decreased the threshold by an average of 31 mm Hg in contrast to the minimal or lack of decrease in threshold reported by others in aortic baroreceptors. Second, at moderate arterial pressures a shift from static to pulsatile pressure caused a decrease in single and multiple unit activities. In single units, the decrease approximated 15% (from 42.0 +/- 2.1 to 35.5 +/- 1.9 spikes/sec, p less than 0.05, n = 25). In all units, there was no diastolic nerve activity ("silence") when diastolic pressure was 1 to 10 mm Hg above static pressure threshold; 80% of the units exhibited "diastolic silence" when diastolic pressure was 20-30 mm Hg above threshold and 40% of the units showed silence at diastolic pressures 40-50 mm Hg above threshold. In whole nerve recordings, pulsatility increased activity from 57 +/- 15 to 142 +/- 29 spikes/sec (p less than 0.05) at low mean arterial pressures (50 and 75 mm Hg), as expected from the reduction in pressure threshold noted in single units, and decreased activity by approximately 15% (from 373 +/- 69 to 320 +/- 55 spikes/sec, p less than 0.05, n = 9) at mean arterial pressures of 125 and 150 mm Hg. This decrease in activity with a shift from static to pulsatile pressure at moderate arterial pressures has not been reported previously. Third, the static pressure-activity curve was sigmoid, and its gain peaked sharply at 75-100 mm Hg; in contrast, the pulsatile pressure-activity curve was linear between 25 and 150 mm Hg, and its maximum gain was half the maximum gain during static pressure. These differences between the static pressure-activity curve and the pulsatile pressure-activity curve were noted during both increases and decreases in carotid sinus pressure; both curves exhibited some hysteresis during the decreases in pressure.

Short-term regulation of arterial pressure and the calculation of open-loop gain in the intact anesthetized dog

Open-loop gain of the short-term systemic pressure regulation was determined under closed-loop conditions in the closed chest anesthetized dog (n = 5). For this purpose, cardiac output and mean systemic pressure were varied by ventricular pacing after the production of complete heart block. From the pressure-flow data resistance gain (the ratio of peripheral resistance change to pressure change in the steady state) was obtained by means of a simple model. The value of this gain was automatically estimated by fitting the pressure-flow relation described by the model to the experimental data. The model allows the pressure-flow relation to be straight or curved with or without a zero-flow pressure intercept. The best fit was obtained when the pressure-flow curve was convex to the pressure axis and had no intercept. When the model was linearized about the control values of pressure and flow (operating point), open-loop gain could be calculated from resistance gain. Its averaged value in the control condition, 1.63 +/- 0.45, is in agreement with values found by other investigators in open-loop conditions. During vasoconstriction open-loop gain, at the (new) operating point, increased to 2.51 +/- 0.51; during vasodilation it decreased to 1.17 +/- 0.27. Open-loop gain about an operating point thus can be determined in the intact animal from measurements of mean pressure and mean flow in the steady state.

The physiologically relevant information regarding systemic blood pressure encoded in the carotid sinus baroreceptor discharge pattern

1. The objective was to find out what kind of informatioon regarding systemic blood pressure is transduced by baroreceptors in vivo and how this information is coded in the receptor discharge. 2. Carotid sinus pressure, e.c.g., and receptor action potentials were recorded for fifty-two single fibre carotid sinus receptors found in twenty decerebrated unanaesthetized cats. 3. The inflation and gradual deflation of an intraaortic catheter tip balloon manipulated the blood pressure in the carotid sinus in a way as to define the full in vivo stimulus-response curve for each receptor. 4. Correlation coefficients were computed between stimulus and response variables for several points on the response curve of each receptor and for every possible combination of stimulus and response variables defined. 5. Stimulus variables were (a) systolic, (b) diastolic,, (c) mean, (d) pulse pressures and (e) peak positive dP/dt. Response variables were (a) average discharge rat, (b) peak instantaneous frequency, and (c) average burst frequency. 6. For every fibre in the sample only the correlations between systolic, diastolic and mean pressures vs. average discharge rate were consistently high and positive. All other correlations were numerically low and/or negative. 7. It was concluded that in vivo baroreceptors signal mainly pressure level (systolic, diastolic or mean) as opposed to pulse pressure or dP/dt, and that the average discharge rate is their best index of information content.