Independent modification of baroreceptor and exercise pressor reflex function by nitric oxide in nucleus tractus solitarius

Moderate dose of dietary nitrate improves skeletal muscle microvascular function in patients with peripheral artery disease

Peripheral artery disease (PAD) is an atherosclerotic disease characterized by compromised lower-extremity blood flow that impairs walking ability. We showed that a moderate dose of dietary nitrate in the form of beetroot juice (BRJ, 0.11 mmol/kg) can improve macrovascular function and maximal walking distance in patients with PAD. However, its impacts on the microcirculation and autonomic nervous system have not been examined. Therefore, we investigated the impacts of this dose of dietary nitrate on skeletal muscle microvascular function and autonomic nervous system function and further related these measurements to 6-min walking distance, pain-free walking distance, and exercise recovery in patients with PAD. Patients with PAD (n = 10) ingested either BRJ or placebo in a randomized crossover design. Heart rate variability, skeletal muscle microvascular function, and 6-min walking distance were performed pre- and post-BRJ and placebo. There were significant group × time interactions (P < 0.05) for skeletal muscle microvascular function, 6-min walking distance, and exercise recovery, but no changes (P > 0.05) in heart rate variability or pain-free walking distance were noted. The BRJ group demonstrated improved skeletal muscle microvascular function (∆ 22.1 ± 7.5 %·min-1), longer 6-min walking distance (Δ 37.5 ± 9.1 m), and faster recovery post-exercise (Δ -15.3 ± 4.2 s). Furthermore, changes in skeletal muscle microvascular function were positively associated with changes in 6-min walking distance (r = 0.5) and pain-free walking distance (r = 0.6). These results suggest that a moderate dose of dietary nitrate may support microvascular function, which is related to improvements in walking distance and claudication in patients with PAD.

Muscle stiffening is associated with muscle mechanoreflex-mediated cardioacceleration

PURPOSE

Although the muscle mechanoreflex is an important mediator to cardiovascular regulation during exercise, its modulation factors remain relatively unknown. Therefore, the purpose of this study was to investigate the effect of muscle stiffness on the muscle mechanoreflex.

METHODS

Participants were divided based on their median muscle stiffness (2.00 Nm/mm) into a low group (n = 15) and a high group (n = 15), and the muscle mechanoreflex was compared between the groups. After a 15-min rest in the supine position, heart rate (HR), blood pressure (BP), stroke volume (SV), and cardiac output (CO) were measured at rest for 3 min and during static passive dorsiflexion (SPD) at 20° for 1 min. Following a 15-min re-rest, muscle stiffness and passive resistive torque were evaluated in the distal end of the muscle belly of the medial gastrocnemius.

RESULTS

Peak relative changes in HR (low group: 6 ± 4% and high group: 12 ± 4%) and CO (low group: 8 ± 10% and high group: 13 ± 9%) were greater in the high group than in the low group (both, P < 0.05). A significant positive correlation was found between resistive torque during SPD and muscle stiffness and peak relative changes in HR (r = 0.51 and 0.61, both P < 0.05). However, there was no correlation between muscle elongation during SPD and peak relative changes in HR (r = - 0.23, P = 0.20).

CONCLUSION

These findings suggest that muscle stiffness may be modulatory factor of muscle mechanoreflex.

Recent advances in exercise pressor reflex function in health and disease

Autonomic alterations at the onset of exercise are critical to redistribute cardiac output towards the contracting muscles while preventing a fall in arterial pressure due to excessive vasodilation within the contracting muscles. Neural mechanisms responsible for these adjustments include central command, the exercise pressor reflex, and arterial and cardiopulmonary baroreflexes. The exercise pressor reflex evokes reflex increases in sympathetic activity to the heart and systemic vessels and decreases in parasympathetic activity to the heart, which increases blood pressure (BP), heart rate, and total peripheral resistance through vasoconstriction of systemic vessels. In this review, we discuss recent advancements in our understanding of exercise pressor reflex function in health and disease. Specifically, we discuss emerging evidence suggesting that sympathetic vasoconstrictor drive to the contracting and non-contracting skeletal muscle is differentially controlled by central command and the metaboreflex in healthy conditions. Further, we discuss evidence from animal and human studies showing that cardiovascular diseases, including hypertension, diabetes, and heart failure, lead to an altered exercise pressor reflex function. We also provide an update on the mechanisms thought to underlie this altered exercise pressor reflex function in each of these diseases. Although these mechanisms are complex, multifactorial, and dependent on the etiology of the disease, there is a clear consensus that several mechanisms are involved. Ultimately, approaches targeting these mechanisms are clinically significant as they provide alternative therapeutic strategies to prevent adverse cardiovascular events while also reducing symptoms of exercise intolerance.

Reduced neural baroreflex sensitivity is related to enhanced endothelial function in patients with end-stage liver disease

OBJECTIVES

Reduced baroreflex sensitivity (BRS) is a frequent complication in end-stage liver disease, but the underlying mechanism is unknown. We investigated the mechanical and neural components of BRS. Increased nitric oxide (NO) production has been reported in end-stage liver failure. Based on earlier experiments, we hypothesised that enhanced endothelial function might affect baroreflex function. Therefore, we explored the relation between endothelial function and the components of BRS.

MATERIALS AND METHODS

We enrolled 24 patients and 23 controls. BRS was determined by the spontaneous sequence method. Mechanical component was characterised by the distensibility coefficient (DC) of common carotid artery. Neural component was estimated as the ratio of integrated BRS and DC. Endothelial function was quantified by flow-mediated dilation (FMD) of the brachial artery.

RESULTS

Integrated BRS was reduced in patients [7.00 (5.80-9.25) vs. 11.1 (8.50-14.80) ms/mmHg]. The mechanical component was not different in the two groups, whereas neural component showed significant reduction in patients (3.54 ± 1.20 vs. 4.48 ± 1.43 ms/10^-3). FMD was higher in patients (9.81 ± 3.77 vs. 5.59 ± 1.36%). FMD and neural BRS were directly related in controls (r = 0.62), but inversely related in patients (r = -0.49).

CONCLUSIONS

Baroreflex impairment in end-stage liver disease might be explained by deterioration of the neural component, while the mechanical component appears to be preserved. Endothelial NO may enhance BRS in health; however, central endothelial overproduction of NO likely contributes to the reduction of neural component of BRS in patients awaiting liver transplantation.

Role of endothelial nitric oxide in control of peripheral vascular conductance during muscle metaboreflex activation

The muscle metaboreflex is a powerful pressor reflex induced by the activation of chemically sensitive muscle afferents as a result of metabolite accumulation. During submaximal dynamic exercise, the rise in arterial pressure is primarily due to increases in cardiac output, since there is little systemic vasoconstriction. Indeed, in normal animals, we have often shown a small, but significant, peripheral vasodilation during metaboreflex activation, which is mediated, at least in part, by release of epinephrine and activation of vascular β₂-receptors. We tested whether this vasodilation is in part due to increased release of nitric oxide caused by the rise in cardiac output eliciting endothelium-dependent flow-mediated vasodilation. The muscle metaboreflex was activated via graded reductions in hindlimb blood flow during mild exercise with and without nitric oxide synthesis blockade [N^G-nitro-l-arginine methyl ester (l-NAME); 5 mg/kg]. We assessed the role of increased cardiac output in mediating peripheral vasodilation via the slope of the relationship between the rise in nonischemic vascular conductance (conductance of all vascular beds excluding hindlimbs) vs. the rise in cardiac output. l-NAME increased mean arterial pressure at rest and during exercise. The metaboreflex-induced increases in mean arterial pressure were unaltered by l-NAME, whereas the increases in cardiac output and nonischemic vascular conductance were attenuated. However, the slope of the relationship between nonischemic vascular conductance and cardiac output was not affected by l-NAME, indicating that the rise in cardiac output did not elicit vasodilation via increased release of nitric oxide. Thus, although nitric oxide is intrinsic to the vascular tonus, endothelial-dependent flow-mediated vasodilation plays little role in the small peripheral vasodilation observed during muscle metaboreflex activation.

Exaggerated sympathetic and cardiovascular responses to stimulation of the mesencephalic locomotor region in spontaneously hypertensive rats

The sympathetic and pressor responses to exercise are exaggerated in hypertension. However, the underlying mechanisms causing this abnormality remain to be fully elucidated. Central command, a neural drive originating in higher brain centers, is known to activate cardiovascular and locomotor control circuits concomitantly. As such, it is a viable candidate for the generation of the augmented vascular response to exercise in this disease. We hypothesized that augmentations in central command function contribute to the heightened cardiovascular response to exercise in hypertension. To test this hypothesis, changes in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) in response to electrical stimulation of mesencephalic locomotor region (MLR; 20-50 μA in 10-μA steps evoking fictive locomotion), a putative component of the central command pathway, were examined in decerebrate, paralyzed normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Tibial nerve discharge during MLR stimulation significantly increased in an intensity-dependent manner in both WKY and SHR but was not different between groups. Stimulation of the MLR evoked significantly larger increases in RSNA and MAP with increasing stimulation intensity in both groups. Importantly, the increases in sympathetic and pressor responses to this fictive locomotion were significantly greater in SHR compared with WKY across all stimulation intensities (e.g., at 50 μA, ΔRSNA: WKY 153 ± 31%, SHR 287 ± 42%; ΔMAP: WKY 87 ± 9 mmHg, SHR 139 ± 7 mmHg). These findings provide the first evidence that central command may be a critical contributor to the exaggerated rise in sympathetic activity and blood pressure during exercise in hypertension.

Muscle mechanoreflex overactivity in hypertension: a role for centrally-derived nitric oxide

The cardiovascular response to exercise is abnormally large in hypertension. Over the past decade, it has become clear that the exercise pressor reflex (a peripheral feed-back mechanism originating in skeletal muscle) contributes significantly to the generation of this hyper-responsiveness. Further, it has been determined that overactivity of the mechanically (muscle mechanoreflex) and chemically (muscle metaboreflex) sensitive components of the exercise pressor reflex underpin its dysfunction. Given the recent attention in the literature, this review focuses upon the aberrant function of the muscle mechanoreflex in this disease. Evidence supporting a role for the mechanoreflex in the pathogenesis of the exaggerated cardiovascular response to physical activity is highlighted. The peripheral and central mechanisms that may be responsible for mechanoreflex overactivity in hypertension are likewise discussed. Particular attention is given to emerging evidence implicating a role for centrally-derived nitric oxide in this process.

Treatment of muscle mechanoreflex dysfunction in hypertension: effects of L-arginine dialysis in the nucleus tractus solitarii

NEW FINDINGS

What is the central question of this study? Does increasing NO production within the nucleus tractus solitarii (NTS) affect mechanoreflex function in normotensive and hypertensive rats?What is the main finding and its importance? Dialysis of 1 μm l-arginine, an NO precursor, within the NTS significantly attenuated the pressor response to muscle stretch in normotensive and hypertensive rats. In contrast, 10 μm l-arginine had no effect in normotensive animals, while increasing and decreasing the pressor and tachycardic responses to stretch, respectively, in hypertensive rats. This suggests that increasing NO within the NTS using lower doses of l-arginine can partly normalize mechanoreflex overactivity in hypertensive rats, whereas the effects of larger doses are equivocal. The blood pressure response to exercise is exaggerated in hypertension. Recent evidence suggests that an overactive skeletal muscle mechanoreflex contributes significantly to this augmented circulatory responsiveness. Sensory information from the mechanoreflex is processed within the nucleus tractus solitarii (NTS) of the medulla oblongata. Normally, endogenously produced nitric oxide within the NTS attenuates the increase in mean arterial pressure (MAP) induced by mechanoreflex stimulation. Thus, it has been suggested that decreases in NO production in the NTS underlie the generation of mechanoreflex dysfunction in hypertension. Supporting this postulate, it has been shown that blocking NO production within the NTS of normotensive rats reproduces the exaggerated pressor response elicited by mechanoreflex activation in hypertensive animals. What is not known is whether increasing NO production within the NTS of hypertensive rats mitigates mechanoreflex overactivity. In this study, the mechanoreflex was selectively activated by passively stretching hindlimb muscle before and after the dialysis of 1 and 10 μm l-arginine (an NO precursor) within the NTS of decerebrate normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). Stretch induced larger elevations in MAP in SHRs compared with WKY rats. In both groups, dialysis of 1 μm l-arginine significantly attenuated the pressor response to stretch. However, at the 10 μm dose, l-arginine had no effect on the MAP response to stretch in WKY rats, while it enhanced the response in SHRs. The data demonstrate that increasing NO availability within the NTS using lower doses of l-arginine partly normalizes mechanoreflex dysfunction in hypertension, whereas higher doses do not. The findings could prove valuable in the development of treatment options for mechanoreflex overactivity in this disease.

Neuronal nitric oxide synthase expression is lower in areas of the nucleus tractus solitarius excited by skeletal muscle reflexes in hypertensive rats

The functions of the skeletal muscle exercise pressor reflex (EPR) and its mechanically sensitive component are augmented in hypertension producing exaggerated increases in blood pressure during exercise. Afferent information from the EPR is processed in the nucleus tractus solitarius (NTS). Within the NT, nitric oxide (NO), produced via L-arginine oxidation by neuronal nitric oxide synthase (nNOS), buffers the pressor response to EPR activation. Therefore, EPR overactivity may manifest as a decrease in NO production due to reductions in nNOS. We hypothesized that nNOS protein expression is lower in the NTS of spontaneously hypertensive (SHR) compared with normotensive Wistar-Kyoto (WKY) rats. Further, we examined whether nNOS is expressed with FOS, a marker of neuronal excitation induced by EPR activation. The EPR and mechanoreflex were intermittently activated for 1 h via hindlimb static contraction or stretch, respectively. These maneuvers produced significantly greater pressor responses in SHR during the first 25 min of stimulation. Within the NTS, nNOS expression was lower from -14.9 to -13.4 bregma in SHR compared with WKY. For example, at -14.5 bregma the number of NTS nNOS-positive cells in SHR (13 ± 1) was significantly less than WKY (23 ± 2). However, the number of FOS-positive cells after muscle contraction in this area was not different (WKY = 82 ± 18; SHR = 75 ± 8). In both groups, FOS-expressing neurons were located within the same areas of the NTS as neurons containing nNOS. These findings demonstrate that nNOS protein expression is lower within NTS areas excited by skeletal muscle reflexes in hypertensive rats.

Exercise pressor reflex function following acute hemi-section of the spinal cord in cats

Cardiovascular disease is a leading cause of morbidity and mortality in patients post spinal cord injury (SCI). The prescription of exercise as a therapeutic modality for disease prevention in this population is promising. It is logical to suggest that the sooner an exercise program can begin the more benefit the patient will receive from the therapy. However, the time point after injury at which the requisite circulatory responses needed to support exercise are viable remains largely unknown. The skeletal muscle exercise pressor reflex (EPR) significantly contributes to cardiovascular control during exercise in healthy individuals. Experiments in patients with a chronic lateral hemi-section of the spinal cord (Brown-Séquard syndrome) suggest that the EPR, although blunted, is operational when examined months to years post injury. However, whether this critically important reflex remains functional immediately after lateral SCI or, in contrast, experiences a period of reduced capacity due to spinal shock has not been established. This study was designed to assess EPR function after acute lateral transection of the spinal cord. The EPR was selectively activated in seven decerebrate cats via electrically stimulated static contraction of the triceps surae muscles of each hindlimb before and after lateral hemi-section of the T₁₃–L₂ region of the spinal cord. Compared to responses prior to injury, increases in mean arterial pressure (MAP) were significantly decreased when contracting the hindlimb either ipsilateral to the lesion (MAP = 17 ± 3 mmHg before and 9 ± 2 mmHg after) or contralateral to the lesion (MAP = 22 ± 5 mmHg before and 12 ± 4 mmHg after). The heart rate (HR) response to stimulation of the EPR was largely unaffected by induction of acute SCI. The findings suggest that the EPR maintains the ability to importantly contribute to cardiovascular regulation during exercise immediately following a Brown-Séquard-like injury.

A role for nitric oxide within the nucleus tractus solitarii in the development of muscle mechanoreflex dysfunction in hypertension

Evidence suggests that the muscle mechanoreflex, a circulatory reflex that raises blood pressure and heart rate (HR) upon activation of mechanically sensitive afferent fibres in skeletal muscle, is overactive in hypertension. However, the mechanisms underlying this abnormal reflex function have yet to be identified. Sensory input from the mechanoreflex is processed within the nucleus tractus solitarii (NTS) in the medulla oblongata. Within the NTS, the enzymatic activity of nitric oxide synthase produces nitric oxide (NO). This centrally derived NO has been shown to modulate muscle reflex activity and serves as a viable candidate for mediating the mechanoreflex dysfunction that develops in hypertension. We hypothesized that mechanoreflex dysfunction in hypertension is mediated by abnormal alterations in NO production in the NTS. Mechanically sensitive afferent fibres were stimulated by passively stretching hindlimb muscle before and after blocking the endogenous production of NO within the NTS via microdialysis of the NO synthase inhibitor L-NAME (1 and 5 mM) in normotensive Wistar-Kyoto rats and spontaneously hypertensive rats (SHRs). Changes in HR and mean arterial pressure in response to stretch were significantly larger in SHRs compared with Wistar-Kyoto rats prior to L-NAME dialysis. Attenuating NO production via L-NAME in normotensive rats recapitulated the exaggerated cardiovascular response to stretch observed in SHRs. Dialysing L-NAME in SHRs further accentuated the increases in HR and mean arterial pressure elicited by stretch. These findings support the contention that reductions in NO production within the NTS contribute to the generation of abnormal cardiovascular control by the skeletal muscle mechanoreflex in hypertension.

Cardiovascular regulation by skeletal muscle reflexes in health and disease

Heart rate and blood pressure are elevated at the onset and throughout the duration of dynamic or static exercise. These neurally mediated cardiovascular adjustments to physical activity are regulated, in part, by a peripheral reflex originating in contracting skeletal muscle termed the exercise pressor reflex. Mechanically sensitive and metabolically sensitive receptors activating the exercise pressor reflex are located on the unencapsulated nerve terminals of group III and group IV afferent sensory neurons, respectively. Mechanoreceptors are stimulated by the physical distortion of their receptive fields during muscle contraction and can be sensitized by the production of metabolites generated by working skeletal myocytes. The chemical by-products of muscle contraction also stimulate metaboreceptors. Once activated, group III and IV sensory impulses are transmitted to cardiovascular control centers within the brain stem where they are integrated and processed. Activation of the reflex results in an increase in efferent sympathetic nerve activity and a withdrawal of parasympathetic nerve activity. These actions result in the precise alterations in cardiovascular hemodynamics requisite to meet the metabolic demands of working skeletal muscle. Coordinated activity by this reflex is altered after the development of cardiovascular disease, generating exaggerated increases in sympathetic nerve activity, blood pressure, heart rate, and vascular resistance. The basic components and operational characteristics of the reflex, the techniques used in human and animals to study the reflex, and the emerging evidence describing the dysfunction of the reflex with the advent of cardiovascular disease are highlighted in this review.

Skeletal muscle reflex-mediated changes in sympathetic nerve activity are abnormal in spontaneously hypertensive rats

In hypertension, the blood pressure response to exercise is exaggerated. We demonstrated previously that this heightened pressor response to physical activity is mediated by an overactive skeletal muscle exercise pressor reflex (EPR), with important contributions from its metaboreflex and mechanoreflex components. However, the mechanisms driving the abnormal blood pressure response to EPR activation are largely unknown. Recent evidence in humans suggests that the muscle metaboreflex partially mediates the enhanced EPR-induced pressor response via abnormally large changes in sympathetic nerve activity (SNA). Whether the muscle mechanoreflex induces similarly exaggerated alterations in SNA in hypertension remains unknown, as does the role of the mechanoreceptors mediating muscle reflex activity. To address these issues, the EPR was selectively activated by electrically inducing hindlimb muscle contraction in decerebrate normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Stimulation of the EPR evoked significantly larger increases in mean arterial pressure (MAP) and renal SNA (RSNA) in SHR compared with WKY (ΔRSNA from baseline: 140 ± 11 vs. 48 ± 8%). The mechanoreflex was stimulated by stretching hindlimb muscle which likewise elicited significantly greater elevations in MAP and RSNA in SHR than WKY (ΔRSNA from baseline: 105 ± 11 vs. 35 ± 7%). Blockade of mechanoreceptors in muscle with gadolinium significantly attenuated the MAP and RSNA responses to contraction and stretch in SHR. These data suggest that 1) the exaggerated pressor response to activation of the EPR and muscle mechanoreflex in hypertension is mediated by abnormally large reflex-induced augmentations in SNA and 2) this accentuated sympathetic responsiveness is evoked, in part, by stimulation of muscle mechanoreceptors.

Heart rate variability - a historical perspective

Heart rate variability (HRV), the beat-to-beat variation in either heart rate or the duration of the R-R interval - the heart period, has become a popular clinical and investigational tool. The temporal fluctuations in heart rate exhibit a marked synchrony with respiration (increasing during inspiration and decreasing during expiration - the so called respiratory sinus arrhythmia, RSA) and are widely believed to reflect changes in cardiac autonomic regulation. Although the exact contributions of the parasympathetic and the sympathetic divisions of the autonomic nervous system to this variability are controversial and remain the subject of active investigation and debate, a number of time and frequency domain techniques have been developed to provide insight into cardiac autonomic regulation in both health and disease. It is the purpose of this essay to provide an historical overview of the evolution in the concept of HRV. Briefly, pulse rate was first measured by ancient Greek physicians and scientists. However, it was not until the invention of the "Physician's Pulse Watch" (a watch with a second hand that could be stopped) in 1707 that changes in pulse rate could be accurately assessed. The Rev. Stephen Hales (1733) was the first to note that pulse varied with respiration and in 1847 Carl Ludwig was the first to record RSA. With the measurement of the ECG (1895) and advent of digital signal processing techniques in the 1960s, investigation of HRV and its relationship to health and disease has exploded. This essay will conclude with a brief description of time domain, frequency domain, and non-linear dynamic analysis techniques (and their limitations) that are commonly used to measure HRV.

Nitric oxide reduces vagal baroreflex sensitivity in the late gestation fetus

Goals to understand the etiology of essential hypertension have proposed that this problem arises, in part, because of changes within brainstem circuits involved in arterial blood pressure (ABP) control. It has been suggested that nitric oxide (NO) exerts inhibitory influences on the integration of afferent discharge from the arterial baroreceptors. This study tested the hypothesis that the inhibitory influence of NO on the arterial baroreflex is present in fetal life. Fetal baroreflex sensitivity was calculated in fetal sheep, before and during the NO-clamp; a technique that permits NO synthase (NOS) blockade with l-NAME while maintaining basal cardiovascular function with sodium nitroprusside. Under halothane anesthesia, five fetal sheep at 0.8 gestation were instrumented with vascular catheters. Five days later, fetuses received a range of bolus doses of phenylephrine (5-75 microg I.A.) in randomized order either during saline or treatment with the NO clamp. Basal fetal ABP and heart rate before (50 +/- 4 mm Hg, 170 +/- 3 bpm) or during (51 +/- 4 mm Hg, 173 +/- 3 bpm) the NO-clamp were similar. The gradient of the pulse interval-ABP relationship was nearly doubled during NOS blockade (14.2 =/- 2.5 versus 7.8 +/- 1.6 ms/mm Hg). The data provide in vivo evidence that NO attenuates the sensitivity of the cardiac baroreflex during fetal life.

Nitric oxide impairs baroreflex gain during acute psychological stress

Psychological stress can suppress baroreflex function, but the mechanism has not been fully elucidated. Nitric oxide in the brain and in the adrenal cortex, as well as plasma glucocorticoids, increases during stress and has been shown to suppress reflex gain in unstressed animals. Therefore, the purpose of this study was to test the hypothesis that stress, caused by exposure to a novel environment, decreases baroreflex gain in rabbits through the actions of nitric oxide to increase corticosterone release. Baroreflex control of heart rate and plasma corticosterone levels was quantified before and after blockade of nitric oxide synthase (NOS) with N(omega)-nitro-L-arginine (L-NNA; 20 mg/kg iv) in conscious rabbits exposed to a novel environment and in the same rabbits once they had been conditioned to the environment. Stress significantly reduced baroreflex gain from -23.4 +/- 2 to -12.2 +/- 1.6 beats x min(-1) x mmHg(-1) (P < 0.05) and increased plasma corticosterone levels from 5.4 +/- 0.7 to 15.5 +/- 5.0 ng/ml (P < 0.05). NOS blockade increased gain in stressed animals (to -27.2 +/- 5.4 beats x min(-1) x mmHg(-1), P < 0.05) but did not alter gain in unstressed rabbits (-26.8 +/- 4.9 beats x min(-1) x mmHg(-1)) such that gain was equalized between the two states. NOS blockade increased plasma corticosterone levels in unstressed animals (to 14.3 +/- 2.1 ng/ml, P < 0.05) but failed to significantly alter levels in stressed rabbits (14.0 +/- 3.9 ng/ml). In conclusion, psychological stress may act via nitric oxide, independently of increases in corticosterone, to decrease baroreflex gain.

The mammalian exercise pressor reflex in health and disease

The exercise pressor reflex (a peripheral neural reflex originating in skeletal muscle) contributes significantly to the regulation of the cardiovascular system during exercise. Exercise-induced signals that comprise the afferent arm of the reflex are generated by activation of mechanically (muscle mechanoreflex) and chemically sensitive (muscle metaboreflex) skeletal muscle receptors. Activation of these receptors and their associated afferent fibres reflexively adjusts sympathetic and parasympathetic nerve activity during exercise. In heart failure, the cardiovascular response to exercise is augmented. Owing to the peripheral skeletal myopathy that develops in heart failure (e.g. muscle atrophy, decreased peripheral blood flow, fibre-type transformation and reduced oxidative capacity), the exercise pressor reflex has been implicated as a possible mechanism by which the cardiovascular response to physical activity is exaggerated in this disease. Accumulating evidence supports this conclusion. This review therefore focuses on the role of the exercise pressor reflex in regulating the cardiovascular system during exercise in both health and disease. Updates on our current understanding of the exercise pressor reflex neural pathway as well as experimental models used to study this reflex are presented. In addition, special emphasis is placed on the changes in exercise pressor reflex activity that develop in heart failure, including the contributions of the muscle mechanoreflex and metaboreflex to this pressor reflex dysfunction.