Muscle mechanoreflex induces the pressor response by resetting the arterial baroreflex neural arc

The role of peripheral venous distension reflex in regulating hemodynamics: mini review

Significant volume is pooled in veins in humans and the amount is dramatically altered by various physiological stresses and diseases. Several animal and human studies demonstrated that limb venous distension evoked significant increases in blood pressure and sympathetic nerve activity (venous distension reflex, VDR). VDR has attracted much attention because of its potential to explain the still unknown mechanism of autonomic dysfunction in several diseases, which would lead to a new treatment approach. This mini review discusses accumulated evidence of VDR at this point and what should be investigated in the future to apply the current understanding of VDR in clinical practice.

Baroreflex responses to limb venous distension in humans

The venous distension reflex (VDR) is a pressor response evoked by peripheral venous distension and accompanied by increased muscle sympathetic nerve activity (MSNA). The effects of venous distension on the baroreflex, an important modulator of blood pressure (BP), has not been examined. The purpose of this study was to examine the effect of the VDR on baroreflex sensitivity (BRS). We hypothesized that the VDR will increase the sympathetic BRS (SBRS). Beat-by-beat heart rate (HR), BP and MSNA were recorded in 16 female and 19 male young healthy subjects. To induce venous distension, normal saline equivalent to 5% of the forearm volume was infused into the veins of the occluded forearm. SBRS was assessed from the relationship between diastolic BP and MSNA during spontaneous BP variations. Cardiovagal BRS (CBRS) was assessed with the sequence technique. Venous distension evoked significant increases in BP and MSNA. Compared to baseline, during the maximal VDR response period, SBRS was significantly increased (-3.1 ± 1.5 to -4.5 ± 1.6 bursts･100 heartbeat^-1･mmHg^-1, P < 0.01) and CBRS was significantly decreased (16.6 ± 5.4 to 13.8 ± 6.1 ms･mmHg-¹, P < 0.01). No sex differences were observed in the effect of the VDR on SBRS or CBRS. These results indicate that in addition to its pressor effect, the VDR altered both SBRS and CBRS. We speculate that these changes in baroreflex function contribute to the modulation of MSNA and BP during limb venous distension.

What type of physical exercise should be recommended for improving arterial stiffness on adult population? A network meta-analysis

AIMS

Physical exercise has been associated with a reduction in arterial stiffness, a subclinical process underlying cardiovascular disease. However, the effect of different types of exercise (aerobic, resistance, combined, interval training, stretching, or mind-body modalities) on arterial stiffness is unclear. This network meta-analysis aimed to examine the effectiveness of different types of exercise on arterial stiffness as measured by pulse wave velocity in adults.

METHODS AND RESULTS

We searched Cochrane Central Register of Controlled Trials, CINAHL, MEDLINE (via Pubmed), Embase, and Web of Science databases, for randomized clinical trials including at least a comparison group, from their inception to 30 June 2020. A frequentist network meta-analysis was performed to compare the effect of different types of physical exercise on arterial stiffness as measured by pulse wave velocity. Finally, 35 studies, with a total of 1125 participants for exercise intervention and 633 participants for the control group, were included. In the pairwise meta-analyses, the exercises that improved arterial stiffness were: interval training [effect size (ES) 0.37; 95% confidence interval (CI) 0.01-0.73], aerobic exercise (ES 0.30; 95% CI 0.13-0.48) and combined exercise (ES 0.22; 95% CI 0.04-0.40). Furthermore, the network meta-analysis showed that mind-body interventions were the most effective type of exercise to reduce the pulse wave velocity (ES 0.86; 95% CI 0.04-1.69). In addition, combined exercise (ES 0.35; 95% CI 0.08-0.62), aerobic exercise (ES 0.33; 95% CI 0.09-0.57), and interval training (ES 0.33; 95% CI 0.02-0.64) showed significant improvements.

CONCLUSION

Our findings showed that aerobic exercise, combined exercise, interval training, and mind-body exercises were the most effective exercise modalities for reducing arterial stiffness, assuming an important role in the prevention of cardiovascular diseases.

Reflex arc of the teeth clenching-induced pressor response in rats

Although "teeth clenching" induces pressor response, the reflex tracts of the response are unknown. In this study, dantrolene administration inhibited teeth clenching generated by electrical stimulation of the masseter muscles and completely abolished the pressor response. In addition, trigeminal ganglion block or hexamethonium administration completely abolished the pressor response. Local anesthesia of molar regions significantly reduced the pressor response to 27 ± 10%. Gadolinium (mechanoreceptor blocker of group III muscle afferents) entrapment in masticatory muscles also significantly reduced the pressor response to 62 ± 7%. Although atropine methyl nitrate administration did not change the pressor response, a significant dose-dependent augmentation of heart rate was observed. These results indicate that both periodontal membrane and mechanoreceptors in masticatory muscles are the receptors for the pressor response, and that the afferent and efferent pathways of the pressor response pass through the trigeminal afferent nerves and sympathetic nerves, respectively.

Central chemoreflex activation induces sympatho-excitation without altering static or dynamic baroreflex function in normal rats

Central chemoreflex activation induces sympatho-excitation. However, how central chemoreflex interacts with baroreflex function remains unknown. This study aimed to examine the impact of central chemoreflex on the dynamic as well as static baroreflex functions under open-loop conditions. In 15 anesthetized, vagotomized Sprague-Dawley rats, we isolated bilateral carotid sinuses and controlled intra-sinus pressure (CSP). We then recorded sympathetic nerve activity (SNA) at the celiac ganglia, and activated central chemoreflex by a gas mixture containing various concentrations of CO₂ Under the baroreflex open-loop condition (CSP = 100 mmHg), central chemoreflex activation linearly increased SNA and arterial pressure (AP). To examine the static baroreflex function, we increased CSP stepwise from 60 to 170 mmHg and measured steady-state SNA responses to CSP (mechanoneural arc), and AP responses to SNA (neuromechanical arc). Central chemoreflex activation by inhaling 3% CO₂ significantly increased SNA irrespective of CSP, indicating resetting of the mechanoneural arc, but did not change the neuromechanical arc. As a result, central chemoreflex activation did not change baroreflex maximum total loop gain significantly (-1.29 ± 0.27 vs. -1.68 ± 0.74, N.S.). To examine the dynamic baroreflex function, we randomly perturbed CSP and estimated transfer functions from 0.01 to 1.0 Hz. The transfer function of the mechanoneural arc approximated a high-pass filter, while those of the neuromechanical arc and total (CSP-AP relationship) arcs approximated a low-pass filter. In conclusion, central chemoreflex activation did not alter the transfer function of the mechanoneural, neuromechanical, or total arcs. Central chemoreflex modifies hemodynamics via sympatho-excitation without compromising dynamic or static baroreflex AP buffering function.

Greater Progression of Age-Related Aortic Stiffening in Adults with Poor Trunk Flexibility: A 5-Year Longitudinal Study

Purpose: Having a low level of physical fitness, especially cardiorespiratory fitness, appears to accelerate age-related aortic stiffening. Whereas, some studies have reported that trunk flexibility is a component of physical fitness, it is also negatively associated with arterial stiffening independent of cardiorespiratory fitness in cross-sectional studies. However, no long-term longitudinal study has determined whether poor trunk flexibility accelerates the progression of age-related aortic stiffening. We examined trunk flexibility and aortic stiffness progression in a 5-year longitudinal study.

Methods and Results: A total of 305 apparently healthy men and women participated in this study (49.6 ± 9.5 years of age). Trunk flexibility was measured using a sit-and-reach test. Aortic stiffness was assessed using carotid-femoral pulse wave velocity (cfPWV) at baseline and after 5 years. Analysis of covariance (ANCOVA) was used to assess the association of the annual rate of cfPWV across flexibility levels (low, middle, high). There were no significant differences in baseline cfPWV among the three groups (835 ± 164, 853 ± 140, 855 ± 2.68 cm/s; P = 0.577). Annual ΔcfPWV was significantly higher in the low-flexibility group than in the high-flexibility group (P = 0.009). ANCOVA revealed an inverse relationship between flexibility level and annual ΔcfPWV (14.41 ± 2.73, 9.79 ± 2.59, 2.62 ± 2.68 cm/s/year; P for trend = 0.011). Multiple regression analysis revealed that baseline sit and reach (β = −0.12, −0.70 to −0.01) was independently correlated with ΔcfPWV following adjustment for baseline peak oxygen uptake, age, sex, body fat, heart rate, and cfPWV. The 5-year change in cfPWV was not significantly correlated with 5-year change in sit-and-reach performance (P = 0.859).

Conclusion: Poor trunk flexibility is associated with greater progression of age-related aortic stiffening in healthy adults. However, we failed to confirm a significant association between 5-year change in aortic stiffness and 5-year change in trunk flexibility. The association between increased age-related increase in aortic stiffness and deterioration in flexibility due to age may require observation for more than 5 years.

Open-loop static and dynamic characteristics of the arterial baroreflex system in rabbits and rats

The arterial baroreflex system is the most important negative feedback system for stabilizing arterial pressure (AP). This system serves as a key link between the autonomic nervous system and the cardiovascular system, and is thus essential for understanding the pathophysiology of cardiovascular diseases and accompanying autonomic abnormalities. This article focuses on an open-loop systems analysis using a baroreceptor isolation preparation to identify the characteristics of two principal subsystems of the arterial baroreflex system, namely, the neural arc from pressure input to efferent sympathetic nerve activity (SNA) and the peripheral arc from SNA to AP. Studies on the static and dynamic characteristics of the two arcs under normal physiological conditions and also under various interventions including diseased conditions are to be reviewed. Quantitative understanding of the arterial baroreflex function under diseased conditions would help develop new treatment strategies such as electrical activation of the carotid sinus baroreflex for drug-resistant hypertension.

Supine effect of passive cycling movement induces vagal withdrawal

[Purpose] The purpose of this study was to examine changes in vagal tone during passive exercise while supine. [Subjects and Methods] Eleven healthy males lay supine for 5 min and then performed passive cycling for 10 min using a passive cycling machine. The lower legs moved through a range of motion defined by 90° and 180° knee joint angles at 60 rpm. Respiratory rates were maintained at 0.25 Hz to elicit respiratory sinus arrhythmia. Heart rate variability was analyzed using the time domain analysis, as the root mean squared standard differences between adjacent R-R intervals (rMSSD), and spectrum domain analysis of the high frequency (HF) component. [Results] Compared to rest, passive cycling decreased rMSSD (rest, 66.6 ± 92.6 ms; passive exercise, 53.5 ± 32.5 ms). However, no significant changes in HR or HF were observed (rest, 68.2 ± 6.9 bpm, 65.6 ± 12.0 n.u.; passive exercise, 70.2 ± 7.2 bpm, 67.9 ± 10.0 n.u.). [Conclusion] These results suggest that passive exercise decreases rMMSD through supine-stimulated mechanoreceptors with no effect on HR or HF. Therefore, rMSSD is not affected by hydrostatic pressure during passive cycling in the supine position.

Afferent vagal nerve stimulation resets baroreflex neural arc and inhibits sympathetic nerve activity

It has been established that vagal nerve stimulation (VNS) benefits patients and/or animals with heart failure. However, the impact of VNS on sympathetic nerve activity (SNA) remains unknown. In this study, we investigated how vagal afferent stimulation (AVNS) impacts baroreflex control of SNA. In 12 anesthetized Sprague–Dawley rats, we controlled the pressure in isolated bilateral carotid sinuses (CSP), and measured splanchnic SNA and arterial pressure (AP). Under a constant CSP, increasing the voltage of AVNS dose dependently decreased SNA and AP. The averaged maximal inhibition of SNA was ‐28.0 ± 10.3%. To evaluate the dynamic impacts of AVNS on SNA, we performed random AVNS using binary white noise sequences, and identified the transfer function from AVNS to SNA and that from SNA to AP. We also identified transfer functions of the native baroreflex from CSP to SNA (neural arc) and from SNA to AP (peripheral arc). The transfer function from AVNS to SNA strikingly resembled the baroreflex neural arc and the transfer functions of SNA to AP were indistinguishable whether we perturbed ANVS or CSP, indicating that they likely share common central and peripheral neural mechanisms. To examine the impact of AVNS on baroreflex, we changed CSP stepwise and measured SNA and AP responses with or without AVNS. AVNS resets the sigmoidal neural arc downward, but did not affect the linear peripheral arc. In conclusion, AVNS resets the baroreflex neural arc and induces sympathoinhibition in the same manner as the control of SNA and AP by the native baroreflex.

Afferent vagal nerve stimulation resets the baroreflex neural arc and inhibits sympathetic nerve activity.

Interaction between vestibulo-cardiovascular reflex and arterial baroreflex during postural change in rats

To examine a cooperative role for the baroreflex and the vestibular system in controlling arterial pressure (AP) during voluntary postural change, AP was measured in freely moving conscious rats, with or without sinoaortic baroreceptor denervation (SAD) and/or peripheral vestibular lesion (VL). Voluntary rear-up induced a slight decrease in AP (-5.6 ± 0.8 mmHg), which was significantly augmented by SAD (-14.7 ± 1.0 mmHg) and further augmented by a combination of VL and SAD (-21 ± 1.0 mmHg). Thus we hypothesized that the vestibular system sensitizes the baroreflex during postural change. To test this hypothesis, open-loop baroreflex analysis was conducted on anesthetized sham-treated and VL rats. The isolated carotid sinus pressure was increased stepwise from 60 to 180 mmHg while rats were placed horizontal prone or in a 60° head-up tilt (HUT) position. HUT shifted the carotid sinus pressure-sympathetic nerve activity (SNA) relationship (neural arc) to a higher SNA, shifted the SNA-AP relationship (peripheral arc) to a lower AP, and, consequently, moved the operating point to a higher SNA while maintaining AP (from 113 ± 5 to 114 ± 5 mmHg). The HUT-induced neural arc shift was completely abolished in VL rats, whereas the peripheral arc shifted to a lower AP and the operating point moved to a lower AP (from 116 ± 3 to 84 ± 5 mmHg). These results indicate that the vestibular system elicits sympathoexcitation, shifting the baroreflex neural arc to a higher SNA and maintaining AP during HUT.

Open-loop dynamic and static characteristics of the carotid sinus baroreflex in rats with chronic heart failure after myocardial infarction

We estimated open-loop dynamic characteristics of the carotid sinus baroreflex in normal control rats and chronic heart failure (CHF) rats after myocardial infarction. First, the neural arc transfer function from carotid sinus pressure to splanchnic sympathetic nerve activity (SNA) and its corresponding step response were examined. Although the steady-state response was attenuated in CHF, the negative peak response and the time to peak did not change significantly, suggesting preserved neural arc dynamic characteristics. Next, the peripheral arc transfer function from SNA to arterial pressure (AP) and its corresponding step response were examined. The steady-state response and the initial slope were reduced in CHF, suggesting impaired end-organ responses. In a simulation study based on the dynamic and static characteristics, the percent recovery of AP was reduced progressively as the size of disturbance increased in CHF, suggesting that a reserve for AP buffering is lost in CHF despite relatively maintained baseline AP.

High levels of circulating angiotensin II shift the open-loop baroreflex control of splanchnic sympathetic nerve activity, heart rate and arterial pressure in anesthetized rats

Although an acute arterial pressure (AP) elevation induced by intravenous angiotensin II (ANG II) does not inhibit sympathetic nerve activity (SNA) compared to an equivalent AP elevation induced by phenylephrine, there are conflicting reports as to how circulating ANG II affects the baroreflex control of SNA. Because most studies have estimated the baroreflex function under closed-loop conditions, differences in the rate of input pressure change and the magnitude of pulsatility may have biased the estimation results. We examined the effects of intravenous ANG II (10 microg kg(-1) h(-1)) on the open-loop system characteristics of the carotid sinus baroreflex in anesthetized and vagotomized rats. Carotid sinus pressure (CSP) was raised from 60 to 180 mmHg in increments of 20 mmHg every minute, and steady-state responses in systemic AP, splanchnic SNA and heart rate (HR) were analyzed using a four-parameter logistic function. ANG II significantly increased the minimum values of AP (67.6 +/- 4.6 vs. 101.4 +/- 10.9 mmHg, P < 0.01), SNA (33.3 +/- 5.4 vs. 56.5 +/- 11.5%, P < 0.05) and HR (391.1 +/- 13.7 vs. 417.4 +/- 11.5 beats/min, P < 0.01). ANG II, however, did not attenuate the response range for AP (56.2 +/- 7.2 vs. 49.7 +/- 6.2 mmHg), SNA (69.6 +/- 5.7 vs. 78.9 +/- 9.1%) or HR (41.7 +/- 5.1 vs. 51.2 +/- 3.8 beats/min). The maximum gain was not affected for AP (1.57 +/- 0.28 vs. 1.20 +/- 0.25), SNA (1.94 +/- 0.34 vs. 2.04 +/- 0.42%/mmHg) or HR (1.11 +/- 0.12 vs. 1.28 +/- 0.19 beats min(-1) mmHg(-1)). It is concluded that high levels of circulating ANG II did not attenuate the response range of open-loop carotid sinus baroreflex control for AP, SNA or HR in anesthetized and vagotomized rats.

Poor trunk flexibility is associated with arterial stiffening

Flexibility is one of the components of physical fitness as well as cardiorespiratory fitness and muscular strength and endurance. Flexibility has long been considered a major component in the preventive treatment of musculotendinous strains. The present study investigated a new aspect of flexibility. Using a cross-sectional study design, we tested the hypothesis that a less flexible body would have arterial stiffening. A total of 526 adults, 20 to 39 yr of age (young), 40 to 59 yr of age (middle-aged), and 60 to 83 yr of age (older), participated in this study. Subjects in each age category were divided into either poor- or high-flexibility groups on the basis of a sit-and-reach test. Arterial stiffness was assessed by brachial-ankle pulse wave velocity (baPWV). Two-way ANOVA indicated a significant interaction between age and flexibility in determining baPWV ( P < 0.01). In middle-aged and older subjects, baPWV was higher in poor-flexibility than in high-flexibility groups (middle-aged, 1,260 ± 141 vs. 1,200 ± 124 cm/s, P < 0.01; and older, 1,485 ± 224 vs. 1,384 ± 199 cm/s, P < 0.01). In young subjects, there was no significant difference between the two flexibility groups. A stepwise multiple-regression analysis ( n = 316) revealed that among the components of fitness (cardiorespiratory fitness, muscular strength, and flexibility) and age, all components and age were independent correlates of baPWV. These findings suggest that flexibility may be a predictor of arterial stiffening, independent of other components of fitness.

Bionic cardiology: exploration into a wealth of controllable body parts in the cardiovascular system

Bionic cardiology is the medical science of exploring electronic control of the body, usually via the neural system. Mimicking or modifying biological regulation is a strategy used to combat diseases. Control of ventricular rate during atrial fibrillation by selective vagal stimulation, suppression of ischemia-related ventricular fibrillation by vagal stimulation, and reproduction of neurally commanded heart rate are some examples of bionic treatment for arrhythmia. Implantable radio-frequency-coupled on-demand carotid sinus stimulators succeeded in interrupting or preventing anginal attacks but were replaced later by coronary revascularization. Similar but fixed-intensity carotid sinus stimulators were used for hypertension but were also replaced by drugs. Recently, however, a self-powered implantable device has been reappraised for the treatment of drug-resistant hypertension. Closed-loop spinal cord stimulation has successfully treated severe orthostatic hypotension in a limited number of patients. Vagal nerve stimulation is effective in treating heart failure in animals, and a small-size clinical trial has just started. Simultaneous corrections of multiple hemodynamic abnormalities in an acute decompensated state are accomplished simply by quantifying fundamental cardiovascular parameters and controlling these parameters. Bionic cardiology will continue to promote the development of more sophisticated device-based therapies for otherwise untreatable diseases and will inspire more intricate applications in the twenty-first century.

The vestibular system is integral in regulating plastic alterations in the pressor response to free drop mediated by the nonvestibular system

Microgravity resulting from free drop elicits a pressor response that involves both vestibular and nonvestibular pathways. In rats reared under a 3G environment for 2 weeks, plastic alterations in both vestibular- and nonvestibular-mediated responses are induced; specifically, the pressor responses involving both pathways are reduced [C. Abe, K. Tanaka, C. Awazu, H. Chen, H. Morita, Plastic alteration of vestibulo-cardiovascular reflex induced by 2 weeks of 3-G load in conscious rats, Exp. Brain Res. 181 (2007) 639-646]. It is currently unknown whether plastic alterations in the nonvestibular system depend on the vestibular system. To examine this topic, the pressor response to free drop was compared between rats with and without vestibular lesion (VL) reared under 1G or 3G environments. The pressor response to free drop was 34+/-3mmHg in vestibular intact rats reared under 1G, and was significantly attenuated in rats reared under a 3G environment for 2 weeks (13+/-3mmHg); however, the pressor response was similar between VL-1G (18+/-3mmHg) and VL-3G (19+/-3mmHg) rats. Therefore, the 3G environment induced plastic alterations in the pressor response to free drop mediated by both the vestibular and nonvestibular systems, and the vestibular system is indispensable for induction of the plastic alteration of the nonvestibular-meidated pressor response to free drop.

Muscle mechanoreflex augments arterial baroreflex-mediated dynamic sympathetic response to carotid sinus pressure

Although the muscle mechanoreflex is one of the pressor reflexes during exercise, its interaction with dynamic characteristics of the arterial baroreflex remains to be quantitatively analyzed. In anesthetized, vagotomized, and aortic-denervated rabbits ( n = 7), we randomly perturbed isolated carotid sinus pressure (CSP) using binary white noise while recording renal sympathetic nerve activity (SNA) and arterial pressure (AP). We estimated the transfer functions of the baroreflex neural arc (CSP to SNA) and peripheral arc (SNA to AP) under conditions of control and muscle stretch of the hindlimb (5 kg of tension). The muscle stretch increased the dynamic gain of the neural arc while maintaining the derivative characteristics [gain at 0.01 Hz: 1.0 ± 0.2 vs. 1.4 ± 0.6 arbitrary units (au)/mmHg, gain at 1 Hz: 1.7 ± 0.6 vs. 2.7 ± 1.4 au/mmHg; P < 0.05, control vs. stretch]. In contrast, muscle stretch did not affect the peripheral arc. In the time domain, muscle stretch augmented the steady-state response at 50 s (−1.1 ± 0.3 vs. −1.7 ± 0.7 au; P < 0.05, control vs. stretch) and negative peak response (−2.1 ± 0.5 vs. −3.1 ± 1.5 au; P < 0.05, control vs. stretch) in the SNA step response. A simulation experiment using the results indicated that the muscle mechanoreflex would accelerate the closed-loop AP regulation via the arterial baroreflex.

Impairment of vestibular-mediated cardiovascular response and motor coordination in rats born and reared under hypergravity

It is well known that environmental stimulation is important for the proper development of sensory function. The vestibular system senses gravitational acceleration and then alters cardiovascular and motor functions through reflex pathways. The development of vestibular-mediated cardiovascular and motor functions may depend on the gravitational environment present at birth and during subsequent growth. To examine this hypothesis, arterial pressure (AP) and renal sympathetic nerve activity (RSNA) were monitored during horizontal linear acceleration and performance in a motor coordination task in rats born and reared in 1-G or 2-G environments. Linear acceleration of +/-1 G increased AP and RSNA. These responses were attenuated in rats with a vestibular lesion, suggesting that the vestibular system mediated AP and RSNA responses. These responses were also attenuated in rats born in a 2-G environment. AP and RSNA responses were partially restored in these rats when the hypergravity load was removed, and the rats were maintained in a 1-G environment for 1 wk. The AP response to compressed air, which is mediated independently of the vestibular system, did not change in the 2-G environment. Motor coordination was also impaired in the 2-G environment and remained impaired even after 1 wk of unloading. These results indicate that hypergravity impaired both the vestibulo-cardiovascular reflex and motor coordination. The vestibulo-cardiovascular reflex was only impaired temporarily and partially recovered following 1 wk of unloading. In contrast, motor coordination did not return to normal in response to unloading.

Modulation of the control of muscle sympathetic nerve activity during incremental leg cycling

We tested the hypotheses that arterial baroreflex (ABR) control over muscle sympathetic nerve activity (MSNA) in humans does not remain constant throughout a bout of leg cycling ranging in intensity from very mild to exhausting. ABR control over MSNA (burst incidence, burst strength and total MSNA) was evaluated by analysing the relationship between beat-to-beat spontaneous variations in diastolic arterial pressure (DAP) and MSNA in 15 healthy subjects at rest and during leg cycling in a seated position at five workloads: very mild (10 W), mild (82 +/- 5.0 W), moderate (126 +/- 10.2 W), heavy (156 +/- 14.3 W), and exhausting (190 +/- 21.2 W). The workload was incremented every 6 min. The linear relationships between DAP and MSNA variables were significantly shifted downward during very mild exercise, but then shifted progressively upward as exercise intensity increased. During heavy and exhausting exercise, moreover, the DAP-MSNA relationships were also significantly shifted rightward from the resting relationship. The sensitivity of ABR control over burst incidence and total MSNA was significantly lower during very mild exercise than during rest, and the sensitivity of the burst incidence control remained lower than the resting level at all higher exercise intensities. By contrast, the sensitivity of the total MSNA control recovered to the resting level during mild and moderate exercise, and was significantly increased during heavy and exhausting exercise (versus rest). We conclude that, in humans, ABR control over MSNA is not uniform throughout a leg cycling exercise protocol in which intensity was varied from very mild to exhausting. We suggest that this non-uniformity of ABR function is one of the mechanisms by which sympathetic and cardiovascular responses are matched to the exercise intensity.

Baroreflex control of sinus node during dynamic exercise in humans: effect of muscle mechanoreflex

AIM

This study evaluated the influence of muscle mechanical afferent stimulation on the integrated arterial baroreflex control of the sinus node during dynamic exercise.

METHODS

Systolic blood pressure (SBP) and pulse interval (PI) were measured continuously and non-invasively in 15 subjects at rest and during passive cycling. The arterial baroreflex was evaluated with the cross-correlation method (xBRS) for the computation of time-domain baroreflex sensitivity on spontaneous blood pressure and PI variability. xBRS computes the greatest positive correlation between beat-to-beat SBP and PI, and when significant at P = 0.01, slope and delay are recorded as one xBRS value. Heart rate variability (HRV) was evaluated in the frequency domain.

RESULTS

Compared with rest, passive exercise resulted in a parallel increase in heart rate (67 +/- 3.2 vs. 70 +/- 3.6 beats min(-1); P < 0.05) and mean arterial pressure (87 +/- 2 vs. 95 +/- 2 mmHg; P < 0.05), and a significant decrease in xBRS (13.1 +/- 1.8 vs. 10.5 +/- 1.7 ms mmHg(-1); P < 0.01) with an apparent rightward shift in the regression line relating SBP to PI. Also low frequency power of HRV increased while high frequency power decreased (56.7 +/- 3.5 vs. 62.7 +/- 4.8 and 43.2 +/- 3.4 vs. 36.9 +/- 4.9 normalized units respectively; P < 0.05).

CONCLUSION

These data suggest that the stimulation of mechanosensitive stretch receptors is capable of modifying the integrated baroreflex control of sinus node function by decreasing the cardiac vagal outflow during exercise.

Plastic alteration of vestibulo-cardiovascular reflex induced by 2 weeks of 3-G load in conscious rats

Previous studies conducted in our laboratory have demonstrated that the vestibular system plays a significant role in controlling arterial pressure (AP) in conscious rats under conditions of transient microgravity. The vestibular system is known to be highly plastic, and on exposure to different gravitational environments, the sensitivity of the vestibular system-mediated AP response might be altered. In order to test this hypothesis, rats were maintained in a 3-G or a normal 1-G environment for 2 weeks, and the AP responses to free drop-induced microgravity were determined. In 1-G rats, the microgravity increased the AP by 37 +/- 3 mmHg; this pressor response was significantly attenuated by vestibular lesion (VL) (24 +/- 3 mmHg) or body stabilization (29 +/- 2 mmHg). Thus, the microgravity-induced pressor response was mediated by both the vestibular and nonvestibular systems; the input of the latter system was blocked by body stabilization. In the 3-G rats, the pressor responses were significantly suppressed compared to those in the corresponding 1-G rats; i.e., the AP increased by 24 +/- 2 mmHg in freely moving 3-G rats, by 10 +/- 4 mmHg in 3-G rats with VL, and by 13 +/- 4 mmHg in stabilized 3-G rats. Furthermore, there was no difference between the 1- and 3-G rats in terms of the pressor response induced by stressors such as a loud noise or an air jet. These results indicate that pre-exposure to 3-G for 2 weeks induces plasticity in both the vestibular- and nonvestibular-mediated AP responses to microgravity.

Roles of the vestibular system in controlling arterial pressure in conscious rats during a short period of microgravity

In order to evaluate the roles of the vestibular system in controlling arterial pressure (AP) during exposure to a short period of microgravity (microG), the AP was measured in conscious free-moving rats having intact vestibular systems and those having vestibular lesions (FM-Intact and FM-VL groups, respectively). During free drop-induced microG, the AP increased in the FM-Intact group; it was 38+/-4 mmHg more than the AP observed during 1G. However, the increase in AP was significantly lower in the FM-VL group (20+/-2 mmHg). Further, to examine the sudden effect of a body floating in the midair in response to the AP during exposure to muG a body stabilizer was placed on the back of rats having intact vestibular systems and those having vestibular lesions (STAB-Intact and STAB-VL groups, respectively). The increase in the AP was significantly depressed in the STAB-Intact group; when compared with that in the FM-Intact group, but the increase was still significant (27+/-2 mmHg). On the other hand, the increase in the AP was completely eliminated in the STAB-VL group (7+/-5 mmHg). These results indicate that the AP increases during exposure to muG in conscious rats, and the vestibular system and body stability are significantly involved in this response.

Short-term electroacupuncture at Zusanli resets the arterial baroreflex neural arc toward lower sympathetic nerve activity

Although electroacupuncture reduces sympathetic nerve activity (SNA) and arterial pressure (AP), the effects of electroacupuncture on the arterial baroreflex remain to be systematically analyzed. We investigated the effects of electroacupuncture of Zusanli on the arterial baroreflex using an equilibrium diagram comprised of neural and peripheral arcs. In anesthetized, vagotomized, and aortic-denervated rabbits, we isolated carotid sinuses and changed intra-carotid sinus pressure (CSP) from 40 to 160 mmHg in increments of 20 mmHg/min while recording cardiac SNA and AP. Electroacupuncture of Zusanli was applied with a pulse duration of 5 ms and a frequency of 1 Hz. An electric current 10 times the minimal threshold current required for visible muscle twitches was used and was determined to be 4.8 +/- 0.3 mA. Electroacupuncture for 8 min decreased SNA and AP (n = 6). It shifted the neural arc (i.e., CSP-SNA relationship) to lower SNA but did not affect the peripheral arc (i.e., SNA-AP relationship) (n = 8). SNA and AP at the closed-loop operating point, determined by the intersection of the neural and peripheral arcs, decreased from 100 +/- 4 to 80 +/- 9 arbitrary units and from 108 +/- 9 to 99 +/- 8 mmHg (each P < 0.005), respectively. Peroneal denervation eliminated the shift of neural arc by electroacupuncture (n = 6). Decreasing the pulse duration to <2.5 ms eliminated the effects of SNA and AP reduction. In conclusion, short-term electroacupuncture resets the neural arc to lower SNA, which moves the operating point toward lower AP and SNA under baroreflex closed-loop conditions.

Static interaction between muscle mechanoreflex and arterial baroreflex in determining efferent sympathetic nerve activity

Elucidation of the interaction between the muscle mechanoreflex and the arterial baroreflex is essential for better understanding of sympathetic regulation during exercise. We characterized the effects of these two reflexes on sympathetic nerve activity (SNA) in anesthetized rabbits (n = 7). Under open-loop baroreflex conditions, we recorded renal SNA at carotid sinus pressure (CSP) of 40, 80, 120, or 160 mmHg while passively stretching the hindlimb muscle at muscle tension (MT) of 0, 2, 4, or 6 kg. The MT-SNA relationship at CSP of 40 mmHg approximated a straight line. Increase in CSP from 40 to 120 and 160 mmHg shifted the MT-SNA relationship downward and reduced the response range (the difference between maximum and minimum SNA) to 43 +/- 10% and 19 +/- 6%, respectively (P < 0.01). The CSP-SNA relationship at MT of 0 kg approximated a sigmoid curve. Increase in MT from 0 to 2, 4, and 6 kg shifted the CSP-SNA relationship upward and extended the response range to 133 +/- 8%, 156 +/- 14%, and 178 +/- 15%, respectively (P < 0.01). A model of algebraic summation, i.e., parallel shift, with a threshold of SNA functionally reproduced the interaction of the two reflexes (y = 1.00x - 0.01; r(2) = 0.991, root mean square = 2.6% between estimated and measured SNA). In conclusion, the response ranges of SNA to baroreceptor and muscle mechanoreceptor input changed in a manner that could be explained by a parallel shift with threshold.

Resetting of the arterial baroreflex increases orthostatic sympathetic activation and prevents postural hypotension in rabbits

Since humans are under ceaseless orthostatic stress, the mechanism to maintain arterial pressure (AP) under orthostatic stress against gravitational fluid shift is of great importance. We hypothesized that (1) orthostatic stress resets the arterial baroreflex control of sympathetic nerve activity (SNA) to a higher SNA, and (2) resetting of the arterial baroreflex contributes to preventing postural hypotension. Renal SNA and AP were recorded in eight anaesthetized, vagotomized and aortic-denervated rabbits. Isolated intracarotid sinus pressure (CSP) was increased stepwise from 40 to 160 mmHg with increments of 20 mmHg (60 s for each CSP level) while the animal was placed supine and at 60 deg upright tilt. Upright tilt shifted the CSP-SNA relationship (the baroreflex neural arc) to a higher SNA, shifted the SNA-AP relationship (the baroreflex peripheral arc) to a lower AP, and consequently moved the operating point to marked high SNA while maintaining AP. A simulation study suggests that resetting in the neural arc would double the orthostatic activation of SNA and increase the operating AP in upright tilt by 10 mmHg, compared with the absence of resetting. In addition, upright tilt did not change the CSP-AP relationship (the baroreflex total arc). A simulation study suggests that although a downward shift of the peripheral arc could shift the total arc downward, resetting in the neural arc would compensate this fall and prevent the total arc from shifting downward to a lower AP. In conclusion, upright tilt increases SNA by resetting the baroreflex neural arc. This resetting may compensate for the reduced pressor responses to SNA in the peripheral cardiovascular system and contribute to preventing postural hypotension.

Dynamic Characteristics of Carotid Sinus Pressure-Nerve Activity Transduction in Rabbits

The dynamic characteristics of the baroreflex neural arc from pressure input to efferent sympathetic nerve activity (SNA) reveal derivative characteristics in the frequency range of 0.01 to 0.8 Hz (i.e., the baroreflex gain augments with increasing frequency) and high-cut characteristics in the frequency range above 0.8 Hz (i.e., the baroreflex gain decreases with increasing frequency) in rabbits. The derivative characteristics accelerate the arterial pressure regulation via the baroreflex. The high-cut characteristics preserve the baroreflex gain against pulsatile pressure by attenuating the high-frequency components less necessary for arterial pressure regulation. However, to what extent the carotid sinus baroreceptor transduction from pressure input to afferent baroreceptor nerve activity (BNA) contributes to these characteristics remains unanswered. To test the hypothesis that the carotid sinus pressure-BNA transduction partly explains the derivative characteristics but not the highcut characteristics, we examined the dynamic BNA response to pressure input in the frequency range from 0.01 to 3 Hz by using a white noise analysis in 7 anesthetized rabbits. The transfer function from pressure input to BNA showed slight derivative characteristics in the frequency range from 0.01 to 0.3 Hz with approximately a 1.7-fold increase in dynamic gain, but it showed no high-cut characteristics. In conclusion, the carotid sinus baroreceptor transduction partly explained the derivative characteristics but not the high-cut characteristics of the baroreflex neural arc. The present results suggest the importance of the central processing from BNA to efferent SNA to account for the overall dynamic characteristics of the baroreflex neural arc.