Baroreflex Control of Systemic Arterial Pressure and Vascular Bed

Non-additive effects of electrical stimulation of the dorsolateral prefrontal cortex and the vestibular system on muscle sympathetic nerve activity in humans

Sinusoidal galvanic vestibular stimulation (sGVS) induces robust modulation of muscle sympathetic nerve activity (MSNA) alongside perceptions of side-to-side movement, sometimes with an accompanying feeling of nausea. We recently showed that transcranial alternating current stimulation (tACS) of the dorsolateral prefrontal cortex (dlPFC) also modulates MSNA, but does not generate any perceptions. Here, we tested the hypothesis that when the two stimuli are given concurrently, the modulation of MSNA would be additive. MSNA was recorded from 11 awake participants via a tungsten microelectrode inserted percutaneously into the right common peroneal nerve at the fibular head. Sinusoidal stimuli (± 2 mA, 0.08 Hz, 100 cycles) were applied in randomised order as follows: (i) tACS of the dlPFC at electroencephalogram (EEG) site F4 and referenced to the nasion; (ii) bilateral sGVS applied to the vestibular apparatuses via the mastoid processes; and (iii) tACS and sGVS together. Previously obtained data from 12 participants supplemented the data for stimulation protocols (i) and (ii). Cross-correlation analysis revealed that each stimulation protocol caused significant modulation of MSNA (modulation index (paired data): 35.2 ± 19.4% for sGVS; 27.8 ± 15.2% for tACS), but there were no additive effects when tACS and sGVS were delivered concurrently (32.1 ± 18.5%). This implies that the vestibulosympathetic reflexes are attenuated with concurrent dlPFC stimulation. These results suggest that the dlPFC is capable of blocking the processing of vestibular inputs through the brainstem and, hence, the generation of vestibulosympathetic reflexes.

Differential relationship between decreased muscle oxygenation and blood pressure recovery during supraventricular and ventricular tachycardia

Vasoconstriction during tachyarrhythmia contributes to maintenance of arterial pressure (AP) by decreasing peripheral blood flow. This cross-sectional observational study aimed to ascertain whether the relationship between peripheral blood flow and AP recovery occurs during both paroxysmal supraventricular (PSVT, n = 19) and ventricular tachycardias (VT, n = 17). Peripheral blood flow was evaluated using forearm tissue oxygen index (TOI), and mean AP (MAP) was measured using a catheter inserted in the brachial or femoral artery during an electrophysiological study. PSVT and VT rapidly decreased MAP with a comparable heart rate (P = 0.194). MAP recovered to the baseline level at 40 s from PSVT onset, but not VT. The forearm TOI decreased during both tachyarrhythmias (P ≤ 0.029). The TOI response was correlated with MAPrecovery (i.e., MAP recovery from the initial rapid decrease) at 20-60 s from PSVT onset (r = -- 0.652 to - 0.814, P ≤ 0.0298); however, this association was not observed during VT. These findings persisted even after excluding patients who had taken vasoactive drugs. Thus, restricting peripheral blood flow was associated with MAP recovery during PSVT, but not VT. This indicates that AP recovery depends on the type of tachyarrhythmia: different cardiac output and/or vasoconstriction ability during tachyarrhythmia.

Anticipatory central command on standing decreases cerebral blood velocity causing hypocapnia in hyperpneic postural tachycardia syndrome

Fifty percent of patients with postural tachycardia syndrome (POTS) are hypocapnic during orthostasis related to initial orthostatic hypotension (iOH). We determined whether iOH drives hypocapnia in POTS by low BP or decreased cerebral blood velocity (CBv). We studied three groups; healthy volunteers (n = 32, 18 ± 3 yr) were compared with POTS, grouped by presence [POTS-low end-tidal CO2 (↓ETCO2), n = 26, 19 ± 2 yr] or absence [POTS-normal upright end-tidal carbon dioxide (nlCO2), n = 28, 19 ± 3 yr] of standing hypocapnia defined by end-tidal CO2 (ETCO2) ≤ 30 mmHg at steady-state, measuring middle cerebral artery CBv, heart rate (HR), and beat-to-beat blood pressure (BP). After 30 min supine, subjects stood for 5 min. Quantities were measured prestanding, at minimum CBv, minimum BP, peak HR, CBv recovery, BP recovery, minimum HR, steady-state, and 5 min. Baroreflex gain was estimated by α index. iOH occurred with similar frequency and minimum BP in POTS-↓ETCO2 and POTS-nlCO2. Minimum CBv was reduced significantly (P < 0.05) in POTS-↓ETCO2 (48 ± 3 cm/s) preceding hypocapnia compared with POTS-nlCO2 (61 ± 3 cm/s) or Control (60 ± 2 cm/s). The anticipatory increased BP was significantly larger (P < 0.05) in POTS (8 ± 1 mmHg vs. 2 ± 1) and began ∼8 s prestanding. HR increased in all subjects, CBv increased significantly (P < 0.05) in both POTS-nlCO2 (76 ± 2 to 85 ± 2 cm/s) and Control (75 ± 2 to 80 ± 2 cm/s) consistent with central command. CBv decreased in POTS-↓ETCO2 (76 ± 3 to 64 ± 3 cm/s) correlating with decreased baroreflex gain. Cerebral conductance [meanCBv/mean arterial blood pressure (MAP)] was reduced in POTS-↓ETCO2 throughout. Data support the hypothesis that excessively reduced CBv during iOH may intermittently reduce carotid body blood flow, sensitizing that organ and producing postural hyperventilation in POTS-↓ETCO2. Excessive fall in CBv occurs in part during prestanding central command and is a facet of defective parasympathetic regulation in POTS.NEW & NOTEWORTHY Dyspnea is frequent in postural tachycardia syndrome (POTS) and is associated with upright hyperpnea and hypocapnia that drives sinus tachycardia. It is initiated by an exaggerated reduction in cerebral conductance and decreased cerebral blood flow (CBF) that precedes the act of standing. This is a form of autonomically mediated "central command." Cerebral blood flow is further reduced by initial orthostatic hypotension common in POTS. Hypocapnia is maintained during the standing response and might account for persistent postural tachycardia.

Ageing and the Autonomic Nervous System

The vertebrate nervous system is divided into central (CNS) and peripheral (PNS) components. In turn, the PNS is divided into the autonomic (ANS) and enteric (ENS) nervous systems. Ageing implicates time-related changes to anatomy and physiology in reducing organismal fitness. In the case of the CNS, there exists substantial experimental evidence of the effects of age on individual neuronal and glial function. Although many such changes have yet to be experimentally observed in the PNS, there is considerable evidence of the role of ageing in the decline of ANS function over time. As such, this chapter will argue that the ANS constitutes a paradigm for the physiological consequences of ageing, as well as for their clinical implications.

Differential contributions of cardiac, coronary and pulmonary artery vagal mechanoreceptors to reflex control of the circulation

Distinct populations of stretch-sensitive mechanoreceptors attached to myelinated vagal afferents are found in the heart and adjoining coronary and pulmonary circulations. Receptors at atrio-venous junctions appear to be involved in control of intravascular volume. These atrial receptors influence sympathetic control of the heart and kidney, but contribute little to reflex control of systemic vascular resistance. Baroreceptors at the origins of the coronary circulation elicit reflex vasodilatation, like feedback control from systemic arterial baroreceptors, as well as having characteristics that could contribute to regulation of mean pressure. In contrast, feedback from baroreceptors in the pulmonary artery and bifurcation is excitatory and elicits a pressor response. Elevation of pulmonary arterial pressure resets the vasomotor limb of the systemic arterial baroreflex, which could be relevant for control of sympathetic vasoconstrictor outflow during exercise and other states associated with elevated pulmonary arterial pressure. Ventricular receptors, situated mainly in the inferior posterior wall of the left ventricle, and attached to unmyelinated vagal afferents, are relatively inactive under basal conditions. However, a change to the biochemical environment of cardiac tissue surrounding these receptors elicits a depressor response. Some ventricular receptors respond, modestly, to mechanical distortion. Probably, ventricular receptors contribute little to tonic feedback control; however, reflex bradycardia and hypotension in response to chemical activation may decrease the work of the heart during myocardial ischaemia. Overall, greater awareness of heterogeneous reflex effects originating from cardiac, coronary and pulmonary artery mechanoreceptors is required for a better understanding of integrated neural control of circulatory function and arterial blood pressure.

Central modulation of cardiac baroreflex moment-to-moment sensitivity during treadmill exercise in conscious cats

It remains undetermined whether the cardiac component of the entire arterial baroreflex is blunted even at the onset of low-intensity exercise. We sought to examine the moment-to-moment sensitivity of the cardiac baroreflex during walking at different speeds and the presumed mechanisms responsible for baroreflex modulation in conscious cats. Arterial baroreflex sensitivity for heart rate was estimated from the baroreflex ratio between changes in systolic arterial blood pressure and heart rate and from the slope of the baroreflex curve between the cardiovascular responses to brief occlusion of the abdominal aorta. Treadmill walking was performed for 1 min at three levels of speed (low: 20-30 m/min, moderate: 40 m/min, and high: 50-60 m/min) or for 3 min at the stepwise change of speed (low to high to low transition). Cardiac baroreflex sensitivity was blunted at the onset of walking, irrespective of speed. Thereafter, the blunted cardiac baroreflex sensitivity was restored around 15 s of walking at any speed, while the blunting occurred again at 45 s of high-speed walking. The inhibition of cardiac baroreflex sensitivity also occurred (1) during the speed transition from low to high and (2) at 45 s of high-speed exercise of the stepwise exercise. The blunted cardiac baroreflex sensitivity was restored immediately to the resting level during the speed transition from high to low, despite sustained pressor and tachycardiac responses. Therefore, moment-to-moment modulation of the cardiac baroreflex during exercise would occur in association with motor intention (i.e., exercise onset) and effort (i.e., treadmill speed).

Effects of the number of sit-stand maneuver repetitions on baroreflex sensitivity and cardiovascular risk assessments

Sit-stand maneuvers (SSM) have increasingly been used for baroreflex sensitivity (BRS) measurement in physiological research, but it remains unknown as to how many SSM need to be performed to measure BRS and assess its relation with cardiovascular disease (CVD) risk. Therefore, this study aimed to determine 1) the effect of the number of SSM repetitions on BRS and 2) the association between BRS and CVD risk factors. Data were collected from 174 individuals during 5 minutes of spontaneous rest and 5 minutes of repeated SSM at 0.05 Hz (i.e., 15 cycles of 10-second sit and 10-second stand). During SSM, BRS was calculated from the incremental cycles of 3, 6, 9, 12, and 15 SSM using transfer function analysis of heart rate (HR) and systolic blood pressure (SBP). General CVD risk factors, carotid arterial stiffness, and cardiorespiratory fitness were measured. In result, HR and SBP increased during SSM (p<0.05). The BRS remained at a similar level during the resting and SSM conditions, while the coherence function reached its peak after 3 cycles of SSM. BRS with ≥6 cycles of SSM was strongly correlated with age (r=-0.721 to -0.740), carotid distensibility (r=0.625 to 0.629), and cardiorespiratory fitness (r=0.333 to 0.351) (all p<0.001). Multiple regression analysis demonstrated that BRS with ≥6 cycles of SSM explained >60% of the variance in CVD risk factors. Therefore, our findings suggest that repeated SSM significantly strengthens the association between BRS and CVD risk factors. Particularly, BRS with ≥6 cycles of SSM is strongly associated with CVD risk.

A closed-loop approach to the study of the baroreflex dynamics during posture changes at rest and at exercise in humans

We hypothesized that during rapid uptilting at rest, due to vagal withdrawal, arterial baroreflex sensitivity (BRS) may decrease promptly and precede the operating point (OP) resetting, whereas different kinetics are expected during exercise steady state, due to lower vagal activity than at rest. To test this, eleven subjects were rapidly (<2 s) tilted from supine (S) to upright (U) and vice versa every 3 min, at rest and during steady-state 50 W pedaling. Mean arterial pressure (MAP) was measured by finger cuff (Portapres) and R-to-R interval (RRi) by electrocardiography. BRS was computed with the sequence method both during steady and unsteady states. At rest, BRS was 35.1 ms·mmHg^-1 (SD = 17.1) in S and 16.7 ms·mmHg^-1 (SD = 6.4) in U (P < 0.01), RRi was 901 ms (SD = 118) in S and 749 ms (SD = 98) in U (P < 0.01), and MAP was 76 mmHg (SD = 11) in S and 83 mmHg (SD = 8) in U (P < 0.01). During uptilt, BRS decreased promptly [first BRS sequence was 19.7 ms·mmHg^-1 (SD = 5.0)] and was followed by an OP resetting (MAP increase without changes in RRi). At exercise, BRS and OP did not differ between supine and upright positions [BRS was 7.7 ms·mmHg^-1 (SD = 3.0) and 7.7 ms·mmHg^-1 (SD = 3.5), MAP was 85 mmHg (SD = 13) and 88 mmHg (SD = 10), and RRi was 622 ms (SD = 61) and 600 ms (SD = 70), respectively]. The results support the tested hypothesis. The prompt BRS decrease during uptilt at rest may be ascribed to a vagal withdrawal, similarly to what occurs at exercise onset. The OP resetting may be due to a slower control mechanism, possibly an increase in sympathetic activity.

Exercise-Induced Modulation of Baroreflex Control of Sympathetic Nerve Activity

Exercise modulates arterial pressure (AP) regulation over various time spans. AP increases at the onset of exercise and this increase is then sustained during exercise. Once exercise is stopped, AP is suppressed for up to an hour afterwards. Prolonged endurance training is associated with dysfunction of the sympathetic regulation of AP in response to posture changes (orthostatic intolerance). Baroreflex control of sympathetic nerve activity (SNA) has been extensively studied to understand the mechanisms underlying exercise-induced changes in AP. We have previously presented entire baroreflex AP-SNA curves during and after exercise, and during central volume expansion, obtained using direct measurements of renal sympathetic nerve activity (RSNA) in conscious animals. In this review, we describe the modulatory effects of exercise on baroreflex control of AP based on these entire AP-RSNA baroreflex curves. We suggest that both acute and chronic exercise can have modulatory effects on the entire baroreflex curve for SNA, and that these effects differ among time periods.

Blood pressure regulation II: what happens when one system must serve two masters--oxygen delivery and pressure regulation?

During high-intensity dynamic exercise, O2 delivery to active skeletal muscles is enhanced through marked increases in both cardiac output and skeletal muscle blood flow. When the musculature is vigorously engaged in exercise, the human heart lacks the pumping capacity to meet the blood flow demands of both the skeletal muscles and other organs such as the brain. Vasoconstriction must therefore be induced through activation of sympathetic nervous activity to maintain blood flow to the brain and to produce the added driving pressure needed to increase flow to the skeletal muscles. In this review, we first briefly summarize the local vascular and neural control mechanisms operating during high-intensity exercise. This is followed by a review of the major neural mechanisms regulating blood pressure during high-intensity exercise, focusing mainly on the integrated activities of the arterial baroreflex and muscle metaboreflex. In high cardiac output situations, such as during high-intensity dynamic exercise, small changes in total peripheral resistance can induce large changes in blood pressure, which means that rapid and fine regulation is necessary to avoid unacceptable drops in blood pressure. To accomplish this rapid regulation, arterial baroreflex function may be modulated in various ways through activation of the muscle metaboreflex and/or other neural mechanisms. Moreover, this modulation of the arterial baroreflex may change over the time course of an exercise bout, or to accommodate changes in exercise intensity. Within this model, integration of arterial baroreflex modulation with other neural mechanisms plays an important role in cardiovascular control during high-intensity exercise.

Spontaneous baroreflex control of cardiac output during dynamic exercise, muscle metaboreflex activation, and heart failure

We have previously shown that spontaneous baroreflex-induced changes in heart rate (HR) do not always translate into changes in cardiac output (CO) at rest. We have also shown that heart failure (HF) decreases this linkage between changes in HR and CO. Whether dynamic exercise and muscle metaboreflex activation (via imposed reductions in hindlimb blood flow) further alter this translation in normal and HF conditions is unknown. We examined these questions using conscious, chronically instrumented dogs before and after pacing-induced HF during mild and moderate dynamic exercise with and without muscle metaboreflex activation. We measured left ventricular systolic pressure (LVSP), CO, and HR and analyzed the spontaneous HR-LVSP and CO-LVSP relationships. In normal animals, mild exercise significantly decreased HR-LVSP (-3.08 +/- 0.5 vs. -5.14 +/- 0.6 beats.min(-1).mmHg(-1); P < 0.05) and CO-LVSP (-134.74 +/- 24.5 vs. -208.6 +/- 22.2 ml.min(-1).mmHg(-1); P < 0.05). Moderate exercise further decreased both and, in addition, significantly reduced HR-CO translation (25.9 +/- 2.8% vs. 52.3 +/- 4.2%; P < 0.05). Muscle metaboreflex activation at both workloads decreased HR-LVSP, whereas it had no significant effect on CO-LVSP and the HR-CO translation. HF significantly decreased HR-LVSP, CO-LVSP, and the HR-CO translation in all situations. We conclude that spontaneous baroreflex HR responses do not always cause changes in CO during exercise. Moreover, muscle metaboreflex activation during mild and moderate dynamic exercise reduces this coupling. In addition, in HF the HR-CO translation also significantly decreases during both workloads and decreases even further with muscle metaboreflex activation.

Spontaneous baroreflex control of heart rate versus cardiac output: altered coupling in heart failure

Dynamic cardiac baroreflex responses are frequently investigated by analyzing the spontaneous reciprocal changes in arterial pressure and heart rate (HR). However, whether the spontaneous baroreflex-induced changes in HR translate into changes in cardiac output (CO) is unknown. In addition, this linkage between changes in HR and changes in CO may be different in subjects with heart failure (HF). We examined these questions using conscious dogs before and after pacing-induced HF. Spontaneous baroreflex sensitivity in the control of HR and CO was evaluated as the slopes of the linear relationships between HR or CO and left ventricular systolic pressure (LVSP) during spontaneous sequences of greater or equal to three consecutive beats when HR or CO changed inversely versus pressure. Furthermore, the translation of baroreflex HR responses into CO responses (HR-CO translation) was examined by computing the overlap between HR and CO sequences. In normal resting conditions, 44.0 +/- 4.4% of HR sequences overlapped with CO sequences, suggesting that only around half of the baroreflex HR responses cause CO responses. In HF, HR-LVSP, CO-LVSP, and the HR-CO translation significantly decreased compared with the normal condition (-2.29 +/- 0.5 vs. -5.78 +/- 0.7 beats.min(-1).mmHg(-1); -70.95 +/- 11.8 vs. -229.89 +/- 29.6 ml.min(-1).mmHg(-1); and 19.66 +/- 4.9 vs. 44.0 +/- 4.4%, respectively). We conclude that spontaneous baroreflex HR responses do not always cause changes in CO. In addition, HF significantly decreases HR-LVSP, CO-LVSP, and HR-CO translation.

Short-term autonomic control of the cardio-respiratory system: a summary with the help of a comprehensive mathematical model

The system which provides short-term cardiovascular regulation has a very complex structure, resulting from the non-linear interaction among several different mechanisms: they include baroreceptors, peripheral chemoreceptors, lung-stretch receptors, a direct CNS response to hypoxia and hypercapnia, local vessel response to changes in blood gas content. Furthermore, during dynamic exercise a feedforward mechanism anticipates cardiovascular requirements, and interacts with the respiratory and muscle pumps. Aim of this work is to summarize the complexity of this system, and to point out the role of individual mechanisms and their mutual relations, with the help of a comprehensive mathematical model developed by the authors in previous years. Examples of system response are discussed during various acute cardiovascular perturbations (pressure changes, changes in blood gas content, dynamic exercise) and model results compared with existing data in the literature. These examples emphasize the great complexity, richness and variability of the autonomic cardiovascular control system. Simulations suggest that mathematical models and computer simulation techniques may represent essential tools to comprehend and deepen our understanding of complex, multifactorial systems, the behaviour of which cannot be fully revealed by simple qualitative analysis. Models play an important role in modern physiology as a repository of knowledge and to integrate disparate data inside an integrative coherent structure.

Muscle metaboreflex attenuates spontaneous heart rate baroreflex sensitivity during dynamic exercise

Hypoperfusion of active skeletal muscle elicits a reflex pressor response termed the muscle metaboreflex. Dynamic exercise attenuates spontaneous baroreflex sensitivity (SBRS) in the control of heart rate (HR) during rapid, spontaneous changes in blood pressure (BP). Our objective was to determine whether muscle metaboreflex activation (MRA) further diminishes SBRS. Conscious dogs were chronically instrumented for measurement of HR, cardiac output, mean arterial pressure, and left ventricular systolic pressure (LVSP) at rest and during mild (3.2 km/h) or moderate (6.4 km/h at 10% grade) dynamic exercise before and after MRA (via partial reduction of hindlimb blood flow). SBRS was evaluated as the slopes of the linear relations (LRs) between HR and LVSP during spontaneous sequences of at least three consecutive beats when HR changed inversely vs. pressure (expressed as beats x min(-1) x mmHg(-1)). During mild exercise, these LRs shifted upward, with a significant decrease in SBRS (-3.0 +/- 0.4 vs. -5.2 +/- 0.4, P<0.05 vs. rest). MRA shifted LRs upward and rightward and decreased SBRS (-2.1 +/- 0.1, P<0.05 vs. mild exercise). Moderate exercise shifted LRs upward and rightward and significantly decreased SBRS (-1.2 +/- 0.1, P<0.05 vs. rest). MRA elicited further upward and rightward shifts of the LRs and reductions in SBRS (-0.9 +/- 0.1, P<0.05 vs. moderate exercise). We conclude that dynamic exercise resets the arterial baroreflex to higher BP and HR as exercise intensity increases. In addition, increases in exercise intensity, as well as MRA, attenuate SBRS.

Time-dependent modulation of arterial baroreflex control of muscle sympathetic nerve activity during isometric exercise in humans

We investigated the time-dependent modulation of arterial baroreflex (ABR) control of muscle sympathetic nerve activity (MSNA) that occurs during isometric handgrip exercise (IHG). Thirteen healthy subjects performed a 3-min IHG at 30% maximal voluntary contraction, which was followed by a period of imposed postexercise muscle ischemia (PEMI). The ABR control of MSNA (burst incidence and strength and total activity) was evaluated by analyzing the relationship between spontaneous variations in diastolic arterial pressure (DAP) and MSNA during supine rest, at each minute of IHG, and during PEMI. We found that 1) the linear relations between DAP and MSNA variables were shifted progressively rightward until the third minute of IHG (IHG3); 2) 2 min into IHG (IHG2), the DAP-MSNA relations were shifted upward and were shifted further upward at IHG3; 3) the sensitivity of the ABR control of total MSNA was increased at IHG2 and increased further at IHG3; and 4) during PEMI, the ABR operating pressure was slightly higher than at IHG2, and the sensitivity of the control of total MSNA was the same as at IHG2. During PEMI, the DAP-burst strength and DAP-total MSNA relations were shifted downward from the IHG3 level to the IHG2 level, whereas the DAP-burst incidence relation remained at the IHG3 level. These results indicate that during IHG, ABR control of MSNA is modulated in a time-dependent manner. We suggest that this modulation of ABR function is one of the mechanisms underlying the progressive increase in blood pressure and MSNA during the course of isometric exercise.

Cyclooxygenase inhibition and baroreflex sensitivity in humans

Animal studies suggest that prostanoids (i.e., such as prostacyclin) may sensitize or impair baroreceptor and/or baroreflex responsiveness depending on the site of administration and/or inhibition. We tested the hypothesis that acute inhibition of cyclooxygenase (COX), the rate-limiting enzyme in prostanoid synthesis, impairs baroreflex regulation of cardiac period (R-R interval) and muscle sympathetic nerve activity (MSNA) in humans and augments pressor reactivity. Baroreflex sensitivity (BRS) was determined at baseline (preinfusion) and 60 min after (postinfusion) intravenous infusion of a COX antagonist (ketorolac; 45 mg) (24 ± 1 yr; n = 12) or saline (25 ± 1 yr; n = 12). BRS was assessed by using the modified Oxford technique (bolus intravenous infusion of nitroprusside followed by phenylephrine). BRS was quantified as the slope of the linear portion of the 1) R-R interval-systolic blood pressure relation (cardiovagal BRS) and 2) MSNA-diastolic blood pressure relation (sympathetic BRS) during pharmacological changes in arterial blood pressure. Ketorolac did not alter cardiovagal (19.4 ± 2.1 vs. 18.4 ± 2.4 ms/mmHg preinfusion and postinfusion, respectively) or sympathetic BRS (−2.9 ± 0.7 vs. −2.6 ± 0.4 arbitrary units·beat−1·mmHg−1) but significantly decreased a plasma biomarker of prostanoid generation (plasma thromboxane B2) by 53 ± 11%. Cardiovagal BRS (21.3 ± 3.8 vs. 21.2 ± 3.0 ms/mmHg), sympathetic BRS (−3.4 ± 0.3 vs. −3.2 ± 0.2 arbitrary units·beat−1·mmHg−1), and thromboxane B2(change in −1 ± 12%) were unchanged in the control (saline infusion) group. Pressor responses to steady-state incremental (0.5, 1.0, and 1.5 μg·kg−1·min−1) infusion (5 min/dose) of phenylephrine were not altered by ketorolac ( n = 8). Collectively, these data indicate that acute pharmacological antagonism of the COX enzyme does not impair BRS (cardiovagal or sympathetic) or augment pressor reactivity in healthy young adults.

Muscle metaboreflex modulates the arterial baroreflex dynamic effects on peripheral vascular conductance in humans

We aimed to investigate the interaction between the arterial baroreflex and muscle metaboreflex [as reflected by alterations in the dynamic responses shown by leg blood flow (LBF: by the ultrasound Doppler method), leg vascular conductance (LVC), mean arterial blood pressure (MAP), and heart rate (HR)] in humans. In 12 healthy subjects (10 men and 2 women), who performed sustained 1-min handgrip exercise at 50% maximal voluntary contraction followed immediately by an imposed postexercise muscle ischemia (PEMI), 5-s periods of neck pressure (NP; 50 mmHg) or neck suction (NS; -60 mmHg) were used to evaluate carotid baroreflex function both at rest (Con) and during PEMI. First, the decreases in LVC and LBF and the augmentation of MAP elicited by NP were all greater during PEMI than in Con (DeltaLVC, -1.2 +/- 0.2 vs. -1.9 +/- 0.2 ml.min(-1).mmHg(-1); DeltaLBF, -97.3 +/- 11.2 vs. -177.0 +/- 21.8 ml/min; DeltaMAP, 6.7 +/- 1.2 vs. 11.5 +/- 1.4 mmHg, Con vs. PEMI; each P < 0.05). Second, in Con, NS significantly increased both LVC and LBF (DeltaLVC, 0.9 +/- 0.2 ml.min(-1).mmHg(-1); DeltaLBF, 46.6 +/- 9.8 ml/min; significant change from baseline: each P < 0.05), and, whereas during PEMI no significant increases in LVC and LBF occurred during NS itself (DeltaLVC, 0.2 +/- 0.1 ml.min(-1).mmHg(-1); DeltaLBF, 10.8 +/- 9.6 ml/min; each P > 0.05), a decrease was evident in each parameters at 5 s after the cessation of NS. Third, during PEMI, the decrease in MAP elicited by NS was smaller (DeltaMAP, -8.4 +/- 1.0 vs. -5.8 +/- 0.4 mmHg, Con vs. PEMI; P < 0.05), and it recovered to its initial level more quickly after NS (vs. Con). Finally, however, the HR responses to NS and NP were not different between PEMI and Con. These results suggest that during muscle metaboreflex activation in humans, the arterial baroreflex dynamic effect on peripheral vascular conductance is modulated, as exemplified by 1) an augmentation of the NP-induced LVC decrease, and 2) a loss of the NS-induced LVC increase.

Theoretical considerations in the dynamic closed-loop baroreflex and autoregulatory control of total peripheral resistance

The most important goal of this study is to enhance our understanding of the crucial functional relationships that determine the behavior of the systemic circulation and its underlying physiological regulatory mechanisms with minimal modeling. To the present day, much has been said about the indirect hydraulic effects of right atrial pressure (PRA) via cardiac output (CO) on arterial pressure (Pa) through the heart and pulmonary circulation or the direct regulatory effects of PRAon Pathrough the cardiopulmonary baroreflex; however, very little attention has been given to the hydraulic influence that PRAexerts directly through the systemic circulation. The experimental data reported by Guyton et al. in 1957 demonstrated that steady-state PRAand the rate at which blood passes through the systemic circulation are locked in a functional relationship independent of any consequence of altered PRAon cardiac function. With this in mind, we emphasize the analytic algebraic analysis of the systemic circulation composed of arteries, veins, and its underlying physiological regulatory mechanisms of baroreflex and autoregulatory modulation of total peripheral resistance (TPR), where the behavior of the system can be analytically synthesized from an understanding of its minimal elements. As a result of this analysis, we present a novel mathematical method to determine short-term TPR fluctuations, which accounts for the entirety of observed Pafluctuations, and propose a new cardiovascular system identification method to delineate the actual actions of the physiological mechanisms responsible for the dynamic couplings between CO, Pa, PRA, and TPR in an individual subject.

Test of dynamic closed-loop baroreflex and autoregulatory control of total peripheral resistance in intact and conscious sheep

This is the first study able to examine and delineate the actual actions of the physiological mechanisms responsible for the dynamic couplings between cardiac output (CO), arterial pressure (Pa), right atrial pressure (PRA), and total peripheral resistance (TPR) in an individual subject without altering the underlying regulatory mechanisms. Eight conscious male sheep were used, where both types of baroreceptors were independently exposed to simultaneous beat-to-beat pressure perturbations under intact closed-loop conditions while CO, Pa, PRA, and TPR were measured. We applied the cardiovascular system identification method proposed in a companion paper ( 4 ) to quantitatively characterize the dynamic closed-loop transfer relations CO→Pa, PRA→Pa, Pa→TPR, and PRA→TPR from the measured signals. To validate the dynamic properties of the estimated transfer relations, the essential parts of the linear dynamics of the model were independently and comprehensively evaluated via error model cross-validation, and the overall model's steady-state behavior was compared with a separate random effects regression approach. In addition to numerous physiological findings, we found that the cardiovascular system identification results were exceptionally consistent with the analytically derived solutions previously discussed in Ref. 4 . In conclusion, this study presents the first time validation of a cardiovascular system identification method by means of experimentally acquired animal data in the intact and conscious animal and offers a set of powerful quantitative tools essential to advancing our knowledge of cardiovascular regulatory physiology.

Heart failure alters the strength and mechanisms of arterial baroreflex pressor responses during dynamic exercise

Arterial baroreflex function is well preserved during dynamic exercise in normal subjects. In subjects with heart failure (HF), arterial baroreflex ability to regulate blood pressure is impaired at rest. However, whether exercise modifies the strength and mechanisms of baroreflex responses in HF is unknown. Therefore, we investigated the relative roles of cardiac output and peripheral vasoconstriction in eliciting the pressor response to bilateral carotid occlusion (BCO) in conscious, chronically instrumented dogs at rest and during treadmill exercise ranging from mild to heavy workloads. Experiments were performed in the same animals before and after rapid ventricular pacing-induced HF. At rest, the pressor response to BCO was significantly attenuated in HF (33.3 ± 1.2 vs. 18.7 ± 2.7 mmHg), and this difference persisted during exercise in part due to lower cardiac output responses in HF. However, both before and after the induction of HF, the contribution of vasoconstriction in active skeletal muscle toward the pressor response became progressively greater as workload increased. We conclude that, although there is an impaired ability of the baroreflex to regulate arterial pressure at rest and during exercise in HF, vasoconstriction in active skeletal muscle becomes progressively more important in mediating the baroreflex pressor response as workload increases with a pattern similar to that observed in normal subjects.

Modulation of arterial baroreflex control of muscle sympathetic nerve activity by muscle metaboreflex in humans

We aimed to investigate the interaction [with respect to the regulation of muscle sympathetic nerve activity (MSNA) and blood pressure] between the arterial baroreflex and muscle metaboreflex in humans. In 10 healthy subjects who performed a 1-min sustained handgrip exercise at 50% maximal voluntary contraction followed by forearm occlusion, arterial baroreflex control of MSNA (burst incidence and strength and total activity) was evaluated by analyzing the relationship between beat-by-beat spontaneous variations in diastolic arterial blood pressure (DAP) and MSNA both during supine rest (control) and during postexercise muscle ischemia (PEMI). During PEMI (vs. control), 1) the linear relationship between burst incidence and DAP was shifted rightward with no alteration in sensitivity, 2) the linear relationship between burst strength and DAP was shifted rightward and upward with no change in sensitivity, and 3) the linear relationship between total activity and DAP was shifted to a higher blood pressure and its sensitivity was increased. The modification of the control of total activity that occurs in PEMI could be a consequence of alterations in the baroreflex control of both MSNA burst incidence and burst strength. These results suggest that the arterial baroreflex and muscle metaboreflex interact to control both the occurrence and strength of MSNA bursts.

Augmentation of the push-pull effect by terminal aortic occlusion during head-down tilt

Tolerance to positive vertical acceleration (Gz) gravitational stress is reduced when positive Gz stress is preceded by exposure to hypogravity, which is called the "push-pull effect." The purpose of this study was to test the hypothesis that baroreceptor reflexes contribute to the push-pull effect by augmenting the magnitude of simulated hypogravity and thereby augmenting the stimulus to the baroreceptors. We used eye-level blood pressure as a measure of the effectiveness of the blood pressure regulatory systems. The approach was to augment the magnitude of the carotid hypertension (and the hindbody hypotension) when hypogravity was simulated by head-down tilt by mechanically occluding the terminal aorta and the inferior vena cava. Sixteen anesthetized Sprague-Dawley rats were instrumented with a carotid artery catheter and a pneumatic vascular occluder cuff surrounding the terminal aorta and inferior vena cava. Animals were restrained and subjected to a control gravitational (G) profile that consisted of rotation from 0 Gz to 90 degrees head-up tilt (+1 Gz) for 10 s and a push-pull G profile consisting of rotation from 0 Gz to 90 degrees head-down tilt (-1 Gz) for 2 s immediately preceding 10 s of +1 Gz stress. An augmented push-pull G profile consisted of terminal aortic vascular occlusion during 2 s of head-down tilt followed by 10 s of +1 Gz stress. After the onset of head-up tilt, the magnitude of the fall in eye-level blood pressure from baseline was -20 +/- 1.3, -23 +/- 0.7, and -28 +/- 1.6 mmHg for the control, push-pull, and augmented push-pull conditions, respectively, with all three pairwise comparisons achieving statistically significant differences (P < 0.01). Thus augmentation of negative Gz stress with vascular occlusion increased the magnitude of the push-pull effect in anesthetized rats subjected to tilting.

Ablation of NK1 receptors in rat nucleus tractus solitarii blocks baroreflexes

The neuropeptide substance P (SP) is found in vagal afferent nerves within the nucleus tractus solitarii, where it is released on stimulation of arterial baroreflexes. The neurokinin-1 receptors at which SP may act have been identified in the nucleus tractus solitarii, but there remains uncertainty if the neurons at which SP acts are critical to baroreflex transmission. By using SP conjugated with the toxin saporin, which kills the neurons at which SP may act, we sought to test the hypothesis that neurons expressing the neurokinin-1 receptor are critical to baroreflex transmission in the nucleus tractus solitarii. One and 2 weeks after injection of the toxin into the rat nucleus tractus solitarii, immunoreactivity for the neurokinin-1 receptor was lost. When the toxin had been injected bilaterally, the baroreflex gain was significantly reduced. Therefore, neurons that express SP receptors play a critical role in mediating baroreflexes through the nucleus tractus solitarii of rat.

Estimation of baroreflex gain using a baroreflex equilibrium diagram

Two types of closed-loop perturbations can be applied to the arterial baroreflex system. The first (P(D1)) is introduced into the baroreceptors without a direct effect on arterial pressure (AP), whereas the second (P(D2)) initially affects AP. Neck suction and hemorrhage are examples of P(D1) and P(D2), respectively. To estimate the baroreflex open-loop gain (G(Baro)) without knowing the absolute magnitudes of P(D1) and P(D2), we explored a new strategy to estimate G(Baro) by combining P(D1) and P(D2) in a baroreflex equilibrium diagram. In this diagram, the neural arc presents the input-output relationship between baroreceptor pressure input and sympathetic nerve activity (SNA). The peripheral arc presents the input-output relationship between SNA and AP. In 8 anesthetized rabbits, we estimated G(Baro) by multiplying the slopes of the peripheral arc determined from P(D1) and the neural arc determined from P(D2). We also estimated G(Baro) by a conventional open-loop analysis. The G(Baro) values estimated by the equilibrium diagram and the open-loop analysis showed a positive correlation (y = 0.80x + 0.22, r(2) = 0.95) and a standard error of estimate of 0.21 across the animals. We conclude that G(Baro) was estimated well by combining P(D1) and P(D2) in the equilibrium diagram.

A mathematical model of the carotid baroregulation in pulsating conditions

A mathematical model of short-term arterial pressure control by the carotid baroreceptors in vagotomized subjects is presented. It includes an elastance variable description of the left and right heart, the systemic and pulmonary circulations, the afferent carotid baroreceptor pathway, a central elaboration unit, and the action of five effector mechanisms. Simulation results suggest that the carotid baroreflex is able to significantly modulate the cardiac function curve, but this effect is masked in vivo by changes in arterial pressure and atrial pressure. During heart pacing, cardiac output increases with frequency at moderate levels of heart rate, then fails to increase further due to a reduction in stroke volume. Shifting from nonpulsatile to pulsatile perfusion of the carotid sinuses decreases the overall baroreflex gain. Finally, a sensitivity analysis suggests that venous unstressed volume control plays the major role in the early hemodynamic response to acute hemorrhage, whereas systemic resistance control is less important. In all cases, there has been satisfactory agreement between model and experimental results.

The gain of the hepatorenal reflex in anesthetized dogs

To determine the overall gain of the hepatorenal reflex, hypertonic NaCl solutions were infused into the portal vein or inferior vena cava of anesthetized dogs at a rate of 0.01 ml/kg/min for 30 min and the urinary Na excretion measured. Infusion of 2.25% NaCl into either the portal vein or inferior vena cava had no effect. Portal infusion of 4.5% or 9% NaCl produced an increase of 36 +/- 16 or 40 +/- 17 microeq/kg/30 min, respectively, while inferior vena caval infusion had no effect. With 13.5% NaCl infusion, a significant increase in the natriuretic response was seen with portal venous infusion, with no significant difference between portal venous and inferior vena caval infusion. Thus, the difference in the natriuretic response between portal venous and inferior vena caval infusion was dependent on the degree of input; when an appropriate input was given, natriuresis was induced only by portal venous infusion, probably mediated by the hepatorenal reflex. The gain of the hepatorenal reflex was calculated from the output divided by the input. When 4.5% or 9% NaCl was infused into the portal vein, the 2-h gain of the hepatorenal reflex was 0.38 +/- 0.15 or 0.34 +/- 0.14, respectively.

Accelerometer systolic time intervals as fast-response sensors of upright posture in the young

BACKGROUND

Sensors of posture may improve rate-adaptive pacing in syndromes where syncope occurs in the upright posture, particularly in the young. No sensor of posture has been described to date. Previous studies suggest that two sensors currently under investigation (preejection period [PEP] and left ventricular ejection time [LVET] systolic time intervals [STIs] and accelerometers) may be affected by posture. A PEP-sensing pacemaker is available commercially in which heart rate (HR) decreases with an increase in PEP (delta(HR)/delta[PEP]< 0). In patients with upright syncope, it is not known how such algorithms respond to posture. Also, it is not known whether STIs correlate with posture independent of autonomic tone.

METHODS AND RESULTS

We studied accelerometer-derived STIs in head-upright tilt-testing with beta-blockade and catecholamine stimulation in patients with syncope or presyncope using an ultra-low-frequency accelerometer placed on the chest. Thirty-two patients age 6 to 22 years with unexplained recurrent syncope or presyncope underwent tilt-testing involving two to four tilts (60 degrees) at baseline, during esmolol infusion (500 micrograms/kg load, 50 to 140 micrograms/kg per minute), after esmolol withdrawal, and during isoproterenol infusion if not contraindicated. PEP, LVET, and other indexes were quantified, and their relations to posture and to autonomic state were determined. With tilt, PEP increased from 98.9 +/- 2.2 to 109.1 +/- 2.8 msec (P < .0001), and LVET decreased (supine-to-upright) from 295.5 +/- 4.5 to 247.2 +/- 4.7 msec (P < .0001). PEP/LVET changed from 0.337 +/- 0.01 to 0.45 +/- 0.02 (P < .0001). Similar postural changes were observed during tilt with beta-blockade and esmolol withdrawal, and during isoproterenol infusion. STI changes occurred immediately on postural change and were stable. Postural change of PEP was greater than the beta-adrenergic effect by 6:1. Postural change of STIs was independent of vagal tone.

CONCLUSIONS

First, accelerometer-derived STIs detect postural changes. Because these changes are independent of autonomic tone and are rapid and stable, they may be useful as fast-response sensors of upright posture in rate-adaptive pacemakers. Second, with postural change, HR increases when PEP increases. However, PEP-sensing pacemakers presently under investigation assume the opposite (inverse) mathematical relationship. Therefore, current PEP-sensing pacemakers use an incorrect algorithm for physiological postural responses in syncope patients. These data predict a paradoxical tachycardic response to the supine posture in patients implanted with these devices.

High calcium diet prevents baroreflex impairment in salt-loaded spontaneously hypertensive rats

To investigate the role of the sympathetic control mechanism in the antihypertensive effect of dietary calcium supplementation, we examined whether a high calcium diet affected mean arterial pressure, renal sympathetic nerve activity, heart rate, and overall and central properties of the arterial baroreceptor reflex in salt-loaded young spontaneously hypertensive rats (SHR). Six-week-old SHR were fed either a normal (0.66%) or high (8.00%) salt diet with either a normal (1.17%) or high (4.07%) calcium content for 4 weeks. The arterial baroreceptor reflex was elicited with rats under halothane anesthesia by altering mean arterial pressure with nitroprusside or phenylephrine. The overall property of the arterial baroreceptor reflex was assessed by the median mean arterial pressure (MAP50) and maximal gain (Gmax) of the relation between mean arterial pressure and renal sympathetic nerve activity and between mean arterial pressure and heart rate. The central property of the arterial baroreceptor reflex was assessed by reflex inhibition of renal sympathetic nerve activity and heart rate elicited by electrical stimulation of the aortic depressor nerve. Compared with the control group fed a normal salt/normal calcium diet, the high salt/normal calcium group had significantly higher mean arterial pressure and renal sympathetic nerve activity but not heart rate. Moreover, the arterial baroreceptor reflex was impaired in the latter group, as evidenced by an increase in MAP50 and decrease in Gmax of the two relations and an attenuation of reflex inhibition of renal sympathetic nerve activity by aortic depressor nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic effects of carotid sinus baroreflex on ventriculoarterial coupling studied in anesthetized dogs

We evaluated dynamic effects of the carotid sinus baroreflex on ventriculoarterial coupling. In seven anesthetized, vagotomized dogs, we bilaterally isolated carotid sinuses and randomly changed carotid sinus pressure while measuring aortic pressure, aortic flow, and left ventricular pressure. Estimating left ventricular end-systolic elastance (Ees) and effective arterial elastance (Ea) on a beat-to-beat basis, we determined transfer functions from the carotid sinus pressure to Ees (HEes) and from the carotid sinus pressure to Ea (HEa) over the frequency range spanning 0.002-0.25 Hz. Both HEes and HEa exhibited characteristics of a second-order low-pass filter. The gains of HEes and HEa were 0.085 +/- 0.065 (mean +/- SD) and 0.081 +/- 0.049 mm Hg/ml/mm Hg, respectively. There were no significant differences in natural frequencies (0.039 +/- 0.013 versus 0.039 +/- 0.007 Hz) or damping ratios (0.65 +/- 0.11 versus 0.64 +/- 0.24). The results indicated that the carotid sinus baroreflex dynamically altered Ees and Ea to the same extent in the process of stabilizing arterial pressure. Because the arterial system extracts maximal external work from a given heart when Ea equals Ees, the carotid sinus baroreflex appeared to be designed to regulate the ventricular and arterial properties to optimize the energy transmission from the left ventricle to the arterial system in anesthetized, vagotomized dogs.

Acute resetting in two functionally different types of carotid baroreceptors

The presence of two types of carotid sinus baroreceptors, as characterized by two different stimulus-response curves in an earlier study, suggests that each type may play a different role in the regulation of blood pressure. The discontinuous hyperbolic curve of the type I baroreceptors, marked by higher firing rates and greater sensitivity than the sigmoidal curve of type II baroreceptors, suggests that these baroreceptors would contribute more to the buffering of arterial pressure changes than the "tonically" active type II baroreceptors, which fired over greater pressure ranges and generally had spontaneous subthreshold discharge. The firing characteristics of type II baroreceptors suggest that these receptors would contribute more to regulation of tonic, baseline levels of arterial pressure. If this functional differentiation exists, the acute resetting characteristics of the two types of baroreceptors could be different. Resetting is defined as a shift in the response curve of a baroreceptor, marked by shifts in pressure threshold, in the same direction as the change in pressure to which it is exposed. Type I baroreceptors would be more likely to reset in response to a sustained acute change in pressure, since their primary role would be to prevent the initial change in pressure. However, type II baroreceptors would not reset to the acute change in pressure, since their primary role would be to maintain consistent information on the level of existing pressure. Therefore, this study was performed to examine the acute resetting ability of both types of baroreceptors by using a vascularly isolated carotid sinus preparation in the dog.(ABSTRACT TRUNCATED AT 250 WORDS)

Experimental and modeling study of the excitability of carotid sinus baroreceptors

In this study we examined the effects of blockade of a transient K+ current with 4-aminopyridine (4-AP) on the static stimulus-response relation of myelinated carotid sinus baroreceptors (n = 8), using a vascularly isolated sinus preparation in sodium thiopental-anesthetized dogs. In one class of baroreceptors (type I), which did not fire spontaneously below the pressure threshold (Pth), 4-AP (10(-5) to 10(-4) M) decreased Pth in a dose-dependent manner and transformed the stimulus-response relation from a discontinuous, hyperbolic shape to a sigmoidal, continuous curve. After exposure to 10(-4) M of 4-AP, baroreceptors were spontaneously active below Pth. These effects of 4-AP were more pronounced in baroreceptors with a high control Pth and were independent of enhanced neurotransmitter release or changes in carotid sinus distensibility. In contrast, 4-AP had relatively little effect on type II baroreceptors, which under control conditions are characterized by a continuous, sigmoidal stimulus-response curve. We believe that these effects of 4-AP on baroreceptor discharge were mediated by blockade of a transient K+ conductance that was present at the receptor spike-initiation zone. This hypothesis was examined using a mathematical model based on the Hodgkin-Huxley axon, but modified to include the transient K+ conductance. The modeling results showed that the minimum current necessary to elicit action potential firing is an extremely sensitive function of the magnitude of this K+ conductance, supporting our experimental results obtained with 4-AP. Our findings suggest that a transient K+ conductance might play a role in the determination of Pth and that differences between type I and II receptors could be the result of differences in the effectiveness of this conductance in controlling spike-initiation zone excitability.

Firing characteristics of single-fiber carotid sinus baroreceptors

This study examined firing patterns of single-fiber carotid baroreceptors in response to slow ramp increases in carotid sinus pressure (1-2 mm Hg/sec) in vascularly isolated carotid sinus preparations in thiopental-anesthetized dogs (25 mg/kg, plus 10 mg/kg/hr). Two general types of baroreceptor discharge patterns were obtained: 1) type I, a discontinuous, hyperbolic pattern characterized by a sudden onset of discharge at threshold pressure with a relatively high threshold frequency, which gradually increased to a higher saturation firing rate and 2) type II, a continuous, sigmoidal pattern characterized by a gradual increase in discharge above threshold pressure and a relatively low threshold frequency and saturation firing rate. Type II baroreceptor curves typically showed spontaneous discharge below threshold pressure and significantly lower sensitivities, threshold frequency, saturation firing rate, and threshold pressure than those of type I receptors. However, the saturation pressures and operating ranges of the type II receptors were greater than those of type I receptors, and the pressure at which type II receptors had their greatest sensitivity was greater than that for type I receptors. Type I baroreceptors generally had large myelinated afferent A fibers; type II baroreceptors generally had smaller A and unmyelinated C fibers, based on conduction velocities. The presence of spontaneous activity with type II baroreceptors, combined with significantly lower sensitivities and wider pressure operating ranges seen relative to type I baroreceptors, suggests that these receptors may primarily serve to provide information on tonic, or baseline, levels of arterial blood pressure to the central cardiovascular centers. The sudden onset of discharge, higher sensitivities, and narrower operating ranges of type I baroreceptors suggest that their primary role may be to provide information on dynamic, sudden changes in arterial pressure.

Arterial baroreceptor reflex modulation of sympathetic-cardiovascular adjustments to heat stress

The purpose of this study was to determine if the arterial baroreceptor reflexes modulate the sympathocirculatory responses to acute heat stress. To address this, arterial pressure, heart rate, mesenteric and renal blood flow velocity (Doppler flow probes), arterial plasma norepinephrine, and colonic temperature were measured before and during whole body heating (42 degrees C ambient temperature) in groups of conscious, unrestrained rats with (sham) or without (sinoaortic deafferentation) intact arterial baroreceptor reflexes. Heating was stopped when a colonic temperature of 41 degrees C was attained. Baseline levels of arterial pressure were similar in the two groups, whereas heart rate was elevated in deafferented versus sham-operated rats (p less than 0.01). The increases above baseline for both arterial pressure (73 +/- 4 vs. 27 +/- 2 mm Hg) and heart rate (127 +/- 10 vs. 33 +/- 5 beats/min) were threefold to fourfold greater at the end of heating in the deafferented versus the sham group (p less than 0.01). Declines in mesenteric and renal blood flow were similar in the two groups during heating; however, deafferented rats had greater increases in both mesenteric and renal vascular resistance (p less than 0.05). Plasma norepinephrine was elevated at baseline in deafferented versus sham rats and increased in both groups during heating (p less than 0.01). The magnitude of the increase in plasma norepinephrine from baseline to 41 degrees C was fivefold greater in the deafferented versus the sham rats (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium channel antagonists on carotid sinus baroreflex control of arterial pressure and heart rate in anesthetized dogs

Our study was designed to determine whether the calcium channel antagonists verapamil, diltiazem, and nifedipine and the nitrate vasodilator sodium nitroprusside modulate carotid sinus (CS) baroreflex control of mean arterial pressure (MAP) and heart rate (HR). Pentobarbital-anesthetized, vagotomized dogs were surgically prepared for reversible vascular isolation of the CS regions. Open-loop performance of the CS baroreflex was determined under control conditions and after intravenous infusion of each agent for 20 minutes at four rates (nitroprusside: 0.3-10 micrograms/kg/min; verapamil and diltiazem: 1-30 micrograms/kg/min; nifedipine: 0.1-3 micrograms/kg/min). With the CS baroreflex loop closed, each vasodilator decreased MAP from control (nitroprusside: 127 +/- 3 to 69 +/- 5 mm Hg; verapamil: 137 +/- 7 to 86 +/- 5 mm Hg; diltiazem: 137 +/- 9 to 100 +/- 5 mm Hg; nifedipine: 140 +/- 6 to 109 +/- 7 mm Hg). Each compound also caused a dose-dependent downward shift in the open-loop CSP-MAP relations. The higher doses of each vasodilator also depressed the total range of control of MAP (i.e., maximum MAP minus minimum MAP) by the baroreflex and significantly attenuated the peak open-loop MAP/CSP gains (nitroprusside: 1.21 +/- 0.19 to 0.56 +/- 0.12; verapamil: 1.36 +/- 0.16 to 0.64 +/- 0.10; diltiazem: 1.52 +/- 0.34 to 0.89 +/- 0.11; nifedipine: 1.35 +/- 0.20 to 0.83 +/- 0.14) but did not alter the CSP at which the peak gain was manifest. Only verapamil and diltiazem significantly shifted downward the CSP-HR relations, whereas none of the drugs affected the total range of baroreflex control of HR (i.e., maximum HR minus minimum HR) or the peak open-loop HR/CSP gains. Our results suggest that 1) it is unlikely that calcium channel antagonists act directly on the baroreceptors or the neural components of the baroreflex loop (i.e., afferent, central and efferent nerves) because they impair CS baroreflex control of MAP but not HR and 2) the impairment of MAP control is predominantly due to a nonspecific blunting of adrenergic vasoconstriction.

Inconsistency of the slope and the volume intercept of the end-systolic pressure-volume relationship as individual indexes of inotropic state in conscious dogs: presentation of an index combining both variables

We tested the ability of the slope (Emax) and the volume intercept (Vo) of the end-systolic pressure-volume relationship (ESPVR) to indicate contractility changes in conscious dogs instrumented with sonomicrometers measuring left ventricular diameter in three orthogonal axes and a left ventricular pressure microtransducer. ESPVRs were generated by inferior vena caval occlusion under control conditions (C1 and C2) and during enhanced (I+) and depressed (I-) inotropic states achieved by infusion of dobutamine and injection of propranolol, respectively. No significant difference between the first control (C1) and I+ or between the second control (C2) and I- were found for either Emax (C1, 5.31 +/- 1.68 mm Hg/ml, mean +/- SD; I+, 5.37 +/- 1.44; C2, 5.20 +/- 1.62; I-, 4.18 +/- 1.32) or Vo (C1, 10.3 +/- 9.6 ml; I+, 7.3 +/- 9.1; C2, 9.9 +/- 9.0; I-, 12.7 +/- 12.5), despite significant changes in other indexes of contractility. Comparison of changes in Emax in individual animals in response to I+ and I- revealed that 63% were nonsignificant, 28% were significant and expected, and 9% were significant and paradoxical. Within defined volume limits and irrespective of individual changes in Emax and Vo, in all animals I+ shifted the ESPVR above and to the left of C1 and I- shifted the ESPVR below and to the right of C2. We thus integrated the changes in Emax and Vo by measuring the area beneath each ESPVR between defined limits of end-systolic volume. The values for area were: C1, 612 +/- 150 mm Hg.ml; I+, 745 +/- 191 (p less than .001); C2, 520 +/- 198; I-, 420 +/- 139 (p less than .001). We conclude that (1) neither Emax nor Vo are individually reliable indexes of changed contractility, and (2) the area beneath the ESPVR between defined end-systolic volume limits is a consistent indicator of variations in inotropic state.

Interaction of canine carotid sinus and aortic arch baroreflexes in the control of total peripheral resistance

Interaction of carotid sinus and aortic arch reflex control of total peripheral resistance was studied in eight dogs anesthetized with sodium pentobarbital and placed on constant flow cardiac bypass. Carotid sinus and aortic arch baroreceptor areas were isolated and separately perfused at controlled pressures. Combinations of carotid sinus and aortic arch pressures were delivered at random in steps of 25 mm Hg over the 50-225 mm Hg pressure range, and systemic arterial pressure was measured. Changes in arterial pressure reflected changes in total peripheral resistance. A multiple linear regression showed that both carotid sinus and aortic arch pressures exhibited a sigmoidal relationship with arterial pressure. Independent of carotid and aortic baroreceptor pressures, arterial pressure was found to be a periodic function of time (period = 2 hours) in all dogs. The average carotid sinus reflex open loop gain was found to be 0.231 +/- 0.092, while average aortic arch open loop gain was 0.141 +/- 0.088. The gain of either the carotid sinus or aortic arch reflex was not influenced by the absolute pressure level of the other receptor area. In a separate series of experiments performed in the same dogs, we tested the hypothesis that a nonlinear temporal summation of the reflex control of total peripheral resistance might exist when the inputs to carotid and aortic baroreceptors are changed simultaneously. With both inputs held at the region of maximum gain, 25 mm Hg step changes were imposed first on carotid sinus pressure, then on aortic arch pressure, and then on both simultaneously. A temporal inhibition of the two reflexes showed that simultaneous excitation of both receptors resulted in a smaller reflex response than the sum of individual responses.