Responses of group III and IV muscle afferents to distension of the peripheral vascular bed

Acute effects of static balance exercise combined with different levels of blood flow restriction on motor performance fatigue as well as physiological and perceptual responses in young healthy males and females

PURPOSE

This study investigated the acute effects of a static balance exercise combined with different blood flow restriction (BFR) pressures on motor performance fatigue development and recovery as well as physiological and perceptual responses during exercise in males and females.

METHODS

Twenty-four recreational active males (n = 13) and females (n = 11) performed static balance exercise on a BOSU ball (3 sets of 60 s with 30 s rest in-between) on three separate (> 3 days) laboratory visits with three different BFR pressures (80% arterial occlusion pressure [AOP], 40%AOP, 30 mmHg [SHAM]) in random order. During exercise, activity of various leg muscles, vastus lateralis muscle oxygenation, and ratings of effort and pain perception were recorded. Maximal squat jump height was measured before, immediately after, 1, 2, 4, and 8 min after exercise to quantify motor performance fatigue development and recovery.

RESULTS

Quadriceps muscle activity as well as ratings of effort and pain were highest, while muscle oxygenation was lowest in the 80%AOP compared to the 40%AOP and SHAM condition, with no differences in postural sway between conditions. Squat jump height declined after exercise with the highest reduction in the 80%AOP (- 16.4 ± 5.2%) followed by the 40%AOP (- 9.1 ± 3.2%), and SHAM condition (- 5.4 ± 3.3%). Motor performance fatigue was not different after 1 min and 2 min of recovery in 40% AOP and 80% AOP compared to SHAM, respectively.

CONCLUSION

Static balance exercise combined with a high BFR pressure induced the largest changes in physiological and perceptual responses, without affecting balance performance. Although motor performance fatigue was increased by BFR, it may not lead to long-term impairments in maximal performance.

The role of clinical neurophysiology in the definition and assessment of fatigue and fatigability

Though a common symptom, fatigue is difficult to define and investigate, occurs in a wide variety of neurological and systemic disorders, with differing pathological causes. It is also often accompanied by a psychological component. As a symptom of long-term COVID-19 it has gained more attention. In this review, we begin by differentiating fatigue, a perception, from fatigability, quantifiable through biomarkers. Central and peripheral nervous system and muscle disorders associated with these are summarised. We provide a comprehensive and objective framework to help identify potential causes of fatigue and fatigability in a given disease condition. It also considers the effectiveness of neurophysiological tests as objective biomarkers for its assessment. Among these, twitch interpolation, motor cortex stimulation, electroencephalography and magnetencephalography, and readiness potentials will be described for the assessment of central fatigability, and surface and needle electromyography (EMG), single fibre EMG and nerve conduction studies for the assessment of peripheral fatigability. The purpose of this review is to guide clinicians in how to approach fatigue, and fatigability, and to suggest that neurophysiological tests may allow an understanding of their origin and interactions. In this way, their differing types and origins, and hence their possible differing treatments, may also be defined more clearly.

Control of Breathing

Substantial advances have been made recently into the discovery of fundamental mechanisms underlying the neural control of breathing and even some inroads into translating these findings to treating breathing disorders. Here, we review several of these advances, starting with an appreciation of the importance of V̇A:V̇CO2:PaCO2 relationships, then summarizing our current understanding of the mechanisms and neural pathways for central rhythm generation, chemoreception, exercise hyperpnea, plasticity, and sleep-state effects on ventilatory control. We apply these fundamental principles to consider the pathophysiology of ventilatory control attending hypersensitized chemoreception in select cardiorespiratory diseases, the pathogenesis of sleep-disordered breathing, and the exertional hyperventilation and dyspnea associated with aging and chronic diseases. These examples underscore the critical importance that many ventilatory control issues play in disease pathogenesis, diagnosis, and treatment.

Increased brain 1H-MRS glutamate and lactate signals following maximal aerobic capacity exercise in young healthy males: an exploratory study

Physical exercise involves increased neuronal activity of many brain structures, but 1H-MRS investigations on the effects of human brain glutamate (Glu) concentrations on acute exercise have been sparse. Previous studies consistently found increases in brain lactate (Lac) concentrations following graded exercise up to 85% of the predicted maximal heart rate. However, the reported effects on brain concentrations of glutamine and glutamate were not consistent. This study aimed to determine the effect of acute intense graded maximal exercise on 1H-MRS signals related to concentrations of Glu, glutamate+glutamine (Glx), and Lac. Young adult males were randomly divided into two groups and subjected to 1H-MRS when resting (NE) or shortly after cessation of the intense graded exercise intended to pass the anaerobic threshold (E). 1H-MRS spectra were acquired from the large voxel encompassing the occipito-parietal cortex only once. Estimates of Glu, Glx, and Lac concentrations were calculated in institutional units by normalizing to a spectroscopic signal originating from creatine-containing compounds (Cr). Concentrations of Glu, Glx, and Lac were respectively 11%, 12.6%, and 48.5% higher in E than in NE (p < 0.001). The increased brain Lac signal in the exercising group indicated that in our experiment, vigorous exercise resulted in passing the anaerobic threshold and lactate apparently entered the brain. Concomitantly glutamate-related resonance signals from the vicinity of the occipito-parietal cortex were significantly increased; physiological mechanisms underlying these phenomena require further study. Future studies should evaluate whether the normalization rate of these concentrations is a marker of general physical fitness.

Effect of Cyclooxygenase Inhibition on Peripheral Venous Distension Reflex in Healthy Humans

BACKGROUND

Peripheral venous distension evokes a pressor reflex (venous distension reflex). Afferent group III and IV nerves innervating veins are suggested as the afferent arm of the venous distension reflex. Prostaglandins stimulate/sensitize group III/IV nerves. We hypothesized that inhibition of prostaglandin synthesis by local cyclooxygenase blockade would attenuate the muscle sympathetic nerve activity (MSNA) and blood pressure responses to venous distension.

METHODS

Nineteen healthy volunteers (age, 27±5 years) participated in the study with 2 visits. To induce venous distension, a volume of solution (saline alone or 9 mg ketorolac tromethamine in saline) was infused into the vein in the antecubital fossa of an arterially occluded forearm. During the procedure, beat-by-beat heart rate, blood pressure and MSNA were recorded simultaneously. The vein size was measured with ultrasound.

RESULTS

In both visits, the venous distension procedure significantly increased blood pressure, heart rate, and MSNA (all, P<0.05). The increase in mean arterial pressure and MSNA in the ketorolac visit was significantly lower than in the control visit (∆ mean arterial pressure, 7.0±6.2 versus 13.8±7.7 mm Hg; ∆MSNA, 6.0±7.1 versus 14.8±7.7 bursts/min; both, P<0.05). The increase in vein size induced by the infusion was not different between visits.

CONCLUSIONS

The presented data show that cyclooxygenase blockade attenuates the responses in MSNA and blood pressure to peripheral venous distension reflex. The results suggest that cyclooxygenase products play a key role in evoking afferent activation responsible for the venous distension reflex.

Muscle stretching induces the mechanoreflex response in human arterial blood pressure

The muscle mechanoreflex has been considered to make a small contribution to the cardiovascular response to exercise in healthy humans because no pressor response has been observed during stimulation of mechanosensitive receptors, such as static passive stretching, during many human studies. There is room for rethinking this consideration since the pressor response to upper limb exercise is greater than that to lower limb exercise. We examined whether static passive stretching of the forearm muscles causes a muscle mechanoreflex-induced pressor response in humans. Eighteen healthy men were recruited for this study. After a 15-min rest period in the supine position with a neutral (0°) wrist joint angle, all participants completed static passive stretching of the forearm for 60 s at four different intensities: minimal painful passive stretching (PPS), moderate-intensity passive stretching (MPS), low-intensity passive stretching (LPS), and no load (NL). During the procedure, beat-to-beat arterial blood pressure was measured using finger photoplethysmography. The force generated between the passively stretched hand and the experimenter's hands was recorded using a force transducer. Mean arterial pressure (MAP) during PPS and MPS significantly increased from baseline during the last 40 s (P < 0.05). MAP was significantly greater at 50 s and 60 s, depending on the intensity. MPS induced a greater peak response in MAP than lower intensities (P < 0.05). None of the subjects reported pain during the MPS and LPS trials. Static passive stimulation of the forearm is an effective method of isolating the muscle mechanoareflex-induced pressor response in humans.NEW & NOTEWORTHY The muscle mechanoreflex was considered to have a small contribution to cardiovascular regulation during exercise in healthy humans. In contrast, the results of this study indicate that static stretching of the forearm induces a pressor response in healthy humans and suggest that the mechanoreflex explicitly induces the pressor response during exercise in humans. The methods applied are useful for evaluating the pressor response to the mechanoreflex regardless of health, aging, and disease.

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.

Physiological Responses to Acute Cycling With Blood Flow Restriction

Aerobic exercise with blood flow restriction (BFR) can improve muscular function and aerobic capacity. However, the extent to which cuff pressure influences acute physiological responses to aerobic exercise with BFR is not well documented. We compared blood flow, tissue oxygenation, and neuromuscular responses to acute cycling with and without BFR. Ten participants completed four intermittent cycling (6 × 2 min) conditions: low-load cycling (LL), low-load cycling with BFR at 60% of limb occlusion pressure (BFR60), low-load cycling with BFR at 80% of limb occlusion pressure (BFR80), and high-load cycling (HL). Tissue oxygenation, cardiorespiratory, metabolic, and perceptual responses were assessed during cycling and blood flow was measured during recovery periods. Pre- to post-exercise changes in knee extensor function were also assessed. BFR60 and BFR80 reduced blood flow (~33 and ~ 50%, respectively) and tissue saturation index (~5 and ~15%, respectively) when compared to LL (all p < 0.05). BFR60 resulted in lower VO₂, heart rate, ventilation, and perceived exertion compared to HL (all p < 0.05), whereas BFR80 resulted in similar heart rates and exertion to HL (both p > 0.05). BFR60 and BFR80 elicited greater pain compared to LL and HL (all p < 0.05). After exercise, knee extensor torque decreased by ~18 and 40% for BFR60 and BFR80, respectively (both p < 0.05), and was compromised mostly through peripheral mechanisms. Cycling with BFR increased metabolic stress, decreased blood flow, and impaired neuromuscular function. However, only BFR60 did so without causing very severe pain (>8 on pain intensity scale). Cycling with BFR at moderate pressure may serve as a potential alternative to traditional high-intensity aerobic exercise.

The physiology and pathophysiology of exercise hyperpnea

In health, the near-eucapnic, highly efficient hyperpnea during mild-to-moderate intensity exercise is driven by three obligatory contributions, namely, feedforward central command from supra-medullary locomotor centers, feedback from limb muscle afferents, and respiratory CO₂ exchange (V̇CO₂). Inhibiting each of these stimuli during exercise elicits a reduction in hyperpnea even in the continuing presence of the other major stimuli. However, the relative contribution of each stimulus to the hyperpnea remains unknown as does the means by which V̇CO2 is sensed. Mediation of the hyperventilatory response to exercise in health is attributed to the multiple feedback and feedforward stimuli resulting from muscle fatigue. In patients with COPD, diaphragm EMG amplitude and its relation to ventilatory output are used to decipher mechanisms underlying the patients' abnormal ventilatory responses, dynamic lung hyperinflation and dyspnea during exercise. Key contributions to these exercise-limiting responses across the spectrum of COPD severity include high dead space ventilation, an excessive neural drive to breathe and highly fatigable limb muscles, together with mechanical constraints on ventilation. Major controversies concerning control of exercise hyperpnea are discussed along with the need for innovative research to uncover the link of metabolism to breathing in health and disease.

Sympathetic activation due to limb venous distension is preserved during muscle metaboreceptor stimulation

Venous saline infusions in an arterially occluded forearm evoke reflex increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP) in humans (venous distension reflex). It is unclear if the inputs from metabolically sensitive skeletal muscle afferents (i.e., muscle metaboreflex) would modify the venous distension reflex. We hypothesized that muscle metaboreceptor stimulation might augment the venous distension reflex. BP (Finapres), heart rate (ECG), and MSNA (microneurography) were assessed in 18 young healthy subjects. In trial A, saline (5% forearm volume) was infused into the veins of an arterially occluded arm (nonhandgrip trial). In trial B, subjects performed 2-min static handgrip followed by postexercise circulatory occlusion (PECO) of the arm. During PECO, saline was infused into the veins of the arm (handgrip trial). In trial A, the infusion increased MSNA and BP as expected (both P < 0.001). In trial B, handgrip significantly raised MSNA, BP, and venous lactic acid concentrations. Venous saline infusion during PECO further raised MSNA and BP (both P < 0.001). The changes in MSNA (Δ8.6 ± 1.5 to Δ10.6 ± 1.8 bursts/min, P = 0.258) and mean arterial pressure (P = 0.844) evoked by the infusion during PECO were not significantly different from those in the nonhandgrip trial. These observations indicate that venous distension reflex responses are preserved during sympathetic activation mediated by the muscle metaboreflex.

Impact of whole body passive heat stress and arterial shear rate modification on radial artery function in young men

We sought to determine how whole body heating acutely influences radial artery function, characterized using flow-mediated dilation (FMD) and low-flow-mediated constriction (L-FMC), and the mechanistic role of shear rate modification on radial artery functional characteristics during heating. Eleven young healthy men underwent whole body heating (water-perfused suit) sufficient to raise the core temperature by +1°C. Trials were repeated with (heat + WC) and without (heat) the application of a wrist cuff located distal to the radial artery examined, known to prevent increases in mean and anterograde shear rates but increase retrograde shear rate. Radial artery characteristics were assessed throughout each trial, with FMD and L-FMC assessed before and upon reaching the target core temperature. Heat markedly increased radial artery mean and anterograde shear rates, along with radial artery diameter and blood flow (P < 0.05). Heat + WC abolished the heat-induced increase in mean and anterograde shear rates (P > 0.05) but markedly increased retrograde shear rate (P < 0.05). Concomitantly, increases in radial artery diameter and blood flow were decreased (heat + WC vs. heat, P < 0.05). Heat attenuated FMD (8.6 ± 1.2% vs. 2.2 ± 1.4%, P < 0.05), whereas no change in FMD was observed in heat + WC (7.8 ± 1.2% vs. 10.8 ± 1.2%, P > 0.05). In contrast, L-FMC was not different in either trial (P > 0.05). In summary, acute whole body heating markedly elevates radial artery shear rate, diameter, and blood flow and diminishes FMD. However, marked radial artery vasodilation and diminished FMD are absent when these shear rate changes are prevented. Shear rate modifications underpin the radial artery response to acute whole body heat stress, but further endothelium-dependent vasodilation (FMD) is attenuated likely as the vasodilatory range limit is approached.NEW & NOTEWORTHY We observed that acute whole body heating elevates radial artery shear rate, diameter, and blood flow. This results in a diminished flow-meditated dilatation (FMD) but does not change low-flow-mediated constriction (L-FMC). Preventing shear rate changes during whole body heating reduces radial artery vasodilation and reverses FMD reductions but has no effect on L-FMC. These findings indicate that shear rate changes underpin conduit artery responses to acute whole body heat stress, but further endothelium-dependent flow-mediated vasodilation is attenuated as the vasodilatory range limit is approached.

Lower-limb venous distension reflex and orthostatic tolerance in young healthy humans

Earlier reports suggest that limb venous distension evokes reflex increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP) (i.e., venous distension reflex). Our recent report also shows that suction of arterially occluded limb evokes venous distension reflex. We postulate that the venous distension reflex contributes to autonomic responses to orthostatic stress. In this study, we hypothesized that orthostatic tolerance would be linked to the MSNA response seen with lower limb suction. Fifteen healthy subjects were tested in the supine position. Negative pressure (-100 mmHg) was applied on an arterially occluded lower limb for 2 min. MSNA from the peroneal nerve in the limb not exposed to suction, ECG, and BP (Finometer) was recorded throughout the study. Limb occlusion without suction was used as a control trial. In a separate visit, the individual's orthostatic tolerance was assessed using a graded lower body negative pressure (LBNP) tolerance test. Mean arterial BP and MSNA (18.6 ± 1.9 to 23.6 ± 2.0 bursts/min) significantly (both P < 0.05) increased during limb suction. Orthostatic tolerance index positively correlated (R = 0.636, P = 0.011) with the MSNA response seen with suction during occlusion. Since the venous distension reflex strength correlates with the level of orthostatic tolerance, we speculate that lower-limb venous distension reflex engagement increases the sympathetic responses during orthostatic challenge and serves to maintain BP with postural stress.

The role of carotid bodies in the generation of active inspiratory and expiratory responses to exercise in rats

NEW FINDINGS

What is the central question of this study? What is the carotid bodies' contribution to active inspiratory and expiratory response to exercise? What is the main finding and its importance? Removal of the carotid bodies reduced the active inspiratory and expiratory responses of diaphragm and abdominal internal oblique muscles, respectively, to high-intensity, but not to low-intensity, exercise in rats. Removal of the carotid bodies increased P aC O 2 and decreased arterial pH in response to high-intensity exercise. The carotid bodies contribute to the inspiratory and expiratory adjustments to high-intensity exercise in rats.

ABSTRACT

Exercise involves the interaction of several physiological processes, in which adjustments in pulmonary ventilation occur in response to increased O₂ consumption, CO₂ production and altered acid-base equilibrium. The peripheral chemoreceptors (carotid bodies; CBs) are sensitive to changes in the chemical composition of arterial blood, and their activation induces active inspiratory and expiratory responses. Herein, we tested the hypothesis that the CBs contribute to the active inspiratory and expiratory responses to exercise in rats. We performed electromyographic recordings of the diaphragm (Dia_EMG ) and abdominal internal oblique (Abd_EMG ) muscles in rats before and after bilateral removal of the CBs (CBX) during constant-load low-intensity and high-intensity progressive treadmill exercise. We also collected arterial blood samples for gaseous and pH analyses. Similar increases in Dia_EMG frequency in both experimental conditions (before and after CBX) during low-intensity exercise were observed, without significant changes in the Dia_EMG amplitude. During high-intensity exercise, lower responses of both Dia_EMG frequency and Dia_EMG amplitude were observed in rats after CBX. The Abd_EMG phasic active expiratory response was not significant either before or after CBX during low-intensity exercise. However, CBX reduced the phasic active expiratory responses during high-intensity exercise. The blunted responses of inspiratory and expiratory adjustments to high-intensity exercise after CBX were associated with higher P aC O 2 levels and lower arterial pH values. Our data show that in rats the CBs do not participate in the inspiratory and expiratory responses to low-intensity exercise, but are involved in the respiratory compensation against the metabolic acidosis induced by high-intensity exercise.

Carotid chemoreflex and muscle metaboreflex interact to the regulation of ventilation in patients with heart failure with reduced ejection fraction

Synergism among reflexes probably contributes to exercise hyperventilation in patients with heart failure with reduced ejection fraction (HFrEF). Thus, we investigated whether the carotid chemoreflex and the muscle metaboreflex interact to the regulation of ventilation ( V ˙ E ) in HFrEF. Ten patients accomplished 4-min cycling at 60% peak workload and then recovered for 2 min under either: (a) 21% O2 inhalation (tonic carotid chemoreflex activity) with legs' circulation free (inactive muscle metaboreflex); (b) 100% O2 inhalation (suppressed carotid chemoreflex activity) with legs' circulation occluded (muscle metaboreflex activation); (c) 21% O2 inhalation (tonic carotid chemoreflex activity) with legs' circulation occluded (muscle metaboreflex activation); or (d) 100% O2 inhalation (suppressed carotid chemoreflex activity) with legs' circulation free (inactive muscle metaboreflex) as control. V ˙ E , tidal volume (VT ) and respiratory frequency (fR ) were similar between each separated reflex (protocols a and b) and control (protocol d). Calculated sum of separated reflexes effects was similar to control. Oppositely, V ˙ E (mean ± SEM: Δ vs. control = 2.46 ± 1.07 L/min, p = .05) and fR (Δ = 2.47 ± 0.77 cycles/min, p = .02) increased versus control when both reflexes were simultaneously active (protocol c). Therefore, the carotid chemoreflex and the muscle metaboreflex interacted to V ˙ E regulation in a fR -dependent manner in patients with HFrEF. If this interaction operates during exercise, it can have some contribution to the HFrEF exercise hyperventilation.

Effect of fatigue-related group III/IV afferent firing on intracortical inhibition and facilitation in hand muscles

Fatiguing exercise causes a reduction in motor drive to the muscle. Group III/IV muscle afferent firing is thought to contribute to this process; however, the effect on corticospinal and intracortical networks is poorly understood. In two experiments, participants performed sustained maximal isometric finger abductions of the first dorsal interosseous (FDI) muscle, with postexercise blood flow occlusion (OCC) to maintain the firing of group III/IV afferents or without occlusion (control; CON). Before and after exercise, single- and paired-pulse transcranial magnetic stimulation (TMS) tested motor evoked potentials (MEPs), intracortical facilitation [ICF (12 ms)], and short-interval intracortical inhibition [SICI₂ (2 ms), SICI₃ (3 ms)]. Ulnar nerve stimulation elicited maximal M waves (M_MAX). For experiment 1 (n = 16 participants), TMS intensities were 70% and 120% of resting motor threshold (RMT) for the conditioning and MEP stimuli, respectively. For experiment 2 (n = 16 participants), the MEP was maintained at 1 mV before and after exercise and the conditioning stimulus individualized. In experiment 1, MEP/M_MAX was reduced after exercise (~48%, P = 0.007) but was not different between conditions. No changes occurred in ICF or SICI. In experiment 2, MEP/M_MAX increased (~27%, P = 0.027) and less inhibition (SICI₂: ~21%, P = 0.021) occurred after exercise for both conditions, whereas ICF decreased for CON only (~28%, P = 0.006). MEPs and SICI₂ were modulated by fatiguing contractions but not by group III/IV afferent firing, whereas sustained afferent firing appeared to counteract postexercise reductions in ICF in FDI. The findings do not support the idea that actions of group III/IV afferents on motor cortical networks contribute to the reduction in voluntary activation observed in other studies.NEW & NOTEWORTHY This is the first study to investigate, in human hand muscles, the action of fatigue-related group III/IV muscle afferent firing on intracortical facilitation and inhibition. In fatigued and nonexercised hand muscles, intracortical inhibition is reduced after exercise but is not modulated differently by the firing of group III/IV afferents. However, facilitation is maintained for the fatigued muscle when group III/IV afferents fire, but these results are unlikely to explain the reduction in voluntary activation observed in other studies.

Muscle sympathetic nerve activity and hemodynamic responses to venous distension: does sex play a role?

Peripheral venous distension mechanically stimulates type III/IV sensory fibers in veins and evokes pressor and sympathoexcitatory reflex responses in humans. As young women have reduced venous compliance and impaired sympathetic transduction, we tested the hypothesis that pressor and sympathoexcitatory responses to venous distension may be attenuated in women compared with men. Mean arterial pressure (photoplethysmography), heart rate (HR), stroke volume (SV; Modelflow), cardiac output (CO = HR × SV), muscle sympathetic nerve activity (MSNA), femoral artery blood flow, and femoral artery conductance (Doppler ultrasound) were quantified in eight men (27 ± 4 yr) and nine women (28 ± 4 yr) before [control (CON)], during (INF), and immediately after (post-INF) a local infusion of saline [5% of the total forearm volume (30 ml/min); the infusion time was 2 ± 1 and 1 ± 1 min ( P = 0.0001) for men and women, respectively] through a retrograde catheter inserted into an antecubital vein, to which venous drainage and arterial supply had been occluded. Mean arterial pressure increased during and after infusion in both groups (vs. the CON group, P < 0.05), but women showed a smaller pressor response in the post-INF period (Δ+7.2 ± 2.0 vs. Δ+18.3 ± 3.9 mmHg in men, P = 0.019). MSNA increased and femoral artery conductance decreased similarly in both groups (vs. the CON group, P < 0.05) at post-INF. Although HR changes were similar, increases in SV (Δ+20.4 ± 8.6 vs. Δ+2.6 ± 2.7 ml, P = 0.05) and CO (Δ+0.84 ± 0.17 vs. Δ+0.34 ± 0.10 l/min, P = 0.024) were greater in men compared with women. Therefore, venous distension evokes a smaller pressor response in young women due to attenuated cardiac adjustments rather than reduced venous compliance or sympathetic transduction. NEW & NOTEWORTHY We found that the pressor response to venous distension was attenuated in young women compared with age-matched men. This was due to attenuated cardiac adjustments rather than reduced venous compliance, sympathetic activation, or impaired transduction and vascular control. Collectively, these findings suggest that an attenuated venous distension reflex could be involved in orthostatic intolerance in young women.

Impact of Blood Flow Restriction Exercise on Muscle Fatigue Development and Recovery

PURPOSE

The present study was designed to provide mechanistic insight into the time course and etiology of muscle fatigue development and recovery during and after low-intensity exercise when it is combined with blood flow restriction (BFR).

METHODS

Seventeen resistance-trained males completed four sets of low-intensity isotonic resistance exercise under two experimental conditions: knee extension exercise (i) with BFR and (ii) without BFR (CON). Neuromuscular tests were performed before, during (immediately after each set of knee extension exercise), and 1, 2, 4, and 8 min after each experimental condition. Maximal voluntary torque, quadriceps twitch torque in response to paired electrical stimuli at 10 Hz (PS10) and 100 Hz (PS100), PS10·PS100 ratio as an index of low-frequency fatigue, and voluntary activation were measured under isometric conditions. Perceptual and EMG data were recorded during each exercise condition.

RESULTS

After the first set of exercise, BFR induced significantly greater reductions in maximal voluntary torque, PS100, and PS10·PS100 ratio compared with CON. These parameters progressively declined throughout the BFR protocol but recovered substantially within 2 min postexercise when blood flow was restored. Neither a progressive decline in the course of the exercise protocol nor a substantial recovery of these parameters occurred during and after CON. Only at exercise termination, voluntary activation differed significantly between BFR and CON with greater reductions during BFR.

CONCLUSION

At the early stage of exercise, BFR exacerbated the development of muscle fatigue mainly due to a pronounced impairment in contractile function. Despite the high level of muscle fatigue during BFR exercise, the effect of BFR on muscle fatigue was diminished after 2 min of reperfusion, suggesting that BFR has a strong but short-lasting effect on neuromuscular function.

Sex differences in diaphragmatic fatigue: the cardiovascular response to inspiratory resistance

KEY POINTS

Diaphragmatic fatigue (DF) elicits a sympathetically mediated metaboreflex resulting in increased heart rate, blood pressure and limb vascular resistance. Women may be more resistant to DF compared to men, and therefore it was hypothesised that women would experience an attenuated inspiratory muscle metaboreflex during inspiratory pressure-threshold loading (PTL) performed to task failure. At the time of PTL task failure, the severity of DF was not different between sexes; however, inspiratory muscle endurance time was significantly longer in women than in men. For a given cumulative diaphragmatic force output, the severity of DF was less in women than in men. Women exhibited a blunted cardiovascular response to inspiratory resistance (i.e. metaboreflex) that may have implications for exercise tolerance.

ABSTRACT

Diaphragmatic fatigue (DF) elicits reflexive increases in sympathetic vasomotor outflow (i.e. metaboreflex). There is some evidence suggesting women may be more resistant to DF compared to men, and therefore may experience an attenuated inspiratory muscle metaboreflex. To this end, we sought to examine the cardiovascular response to inspiratory resistance in healthy young men (n = 9, age = 24 ± 3 years) and women (n = 9, age = 24 ± 3 years). Subjects performed isocapnic inspiratory pressure-threshold loading (PTL, 60% maximal inspiratory mouth pressure) to task failure. Diaphragmatic fatigue was assessed by measuring transdiaphragmatic twitch pressure (P_di,tw ) using cervical magnetic stimulation. Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat throughout PTL via photoplethysmography, and low-frequency systolic pressure (LF_SBP ; a surrogate for sympathetic vasomotor tone) calculated from arterial waveforms using power spectrum analysis. At PTL task failure, the degree of DF was similar between sexes (∼23% reduction in P_di,tw ; P = 0.33). However, time to task failure was significantly longer in women than in men (27 ± 11 vs. 16 ± 11 min, respectively; P = 0.02). Women exhibited less of an increase in HR (13 ± 8 vs. 19 ± 12 bpm; P = 0.02) and MAP (10 ± 8 vs. 14 ± 9 mmHg; P = 0.01), and significantly lower LF_SBP (23 ± 11 vs. 34 ± 8 mmHg² ; P = 0.04) during PTL compared to men. An attenuation of the inspiratory muscle metaboreflex may influence limb and respiratory muscle haemodynamics with implications for exercise performance.

Hyperadditive ventilatory response arising from interaction between the carotid chemoreflex and the muscle mechanoreflex in healthy humans

Physical exercise potentiates the carotid chemoreflex control of ventilation (VE). Hyperadditive neural interactions may partially mediate the potentiation. However, some neural interactions remain incompletely explored. As the potentiation occurs even during low-intensity exercise, we tested the hypothesis that the carotid chemoreflex and the muscle mechanoreflex could interact in a hyperadditive fashion. Fourteen young healthy subjects inhaled randomly, in separate visits, 12% O₂ to stimulate the carotid chemoreflex and 21% O₂ as control. A rebreathing circuit maintained isocapnia. During gases administration, subjects either remained at rest (i.e., normoxic and hypoxic rest) or the muscle mechanoreflex was stimulated via passive knee movement (i.e., normoxic and hypoxic movement). Surface muscle electrical activity did not increase during the passive movement, confirming the absence of active contractions. Hypoxic rest and normoxic movement similarly increased VE [change (mean ± SE) = 1.24 ± 0.72 vs. 0.73 ± 0.43 l/min, respectively; P = 0.46], but hypoxic rest only increased tidal volume (Vt), and normoxic movement only increased breathing frequency (BF). Hypoxic movement induced greater VE and mean inspiratory flow (Vt/Ti) increase than the sum of hypoxic rest and normoxic movement isolated responses (VE change: hypoxic movement = 3.72 ± 0.81 l/min vs. sum = 1.96 ± 0.83 l/min, P = 0.01; Vt/Ti change: hypoxic movement = 0.13 ± 0.03 l/s vs. sum = 0.06 ± 0.03 l/s, P = 0.02). Moreover, hypoxic movement increased both Vt and BF. Collectively, the results indicate that the carotid chemoreflex and the muscle mechanoreflex interacted, mediating a hyperadditive ventilatory response in healthy humans. NEW & NOTEWORTHY The main finding of this study was that concomitant carotid chemoreflex and muscle mechanoreflex stimulation provoked greater ventilation increase than the sum of ventilation increase induced by stimulation of each reflex in isolation, which, consequently, supports that the carotid chemoreflex and the muscle mechanoreflex interacted, mediating a hyperadditive ventilatory response in healthy humans.

Peripheral Mechanisms of Ischemic Myalgia

Musculoskeletal pain due to ischemia is present in a variety of clinical conditions including peripheral vascular disease (PVD), sickle cell disease (SCD), complex regional pain syndrome (CRPS), and even fibromyalgia (FM). The clinical features associated with deep tissue ischemia are unique because although the subjective description of pain is common to other forms of myalgia, patients with ischemic muscle pain often respond poorly to conventional analgesic therapies. Moreover, these patients also display increased cardiovascular responses to muscle contraction, which often leads to exercise intolerance or exacerbation of underlying cardiovascular conditions. This suggests that the mechanisms of myalgia development and the role of altered cardiovascular function under conditions of ischemia may be distinct compared to other injuries/diseases of the muscles. It is widely accepted that group III and IV muscle afferents play an important role in the development of pain due to ischemia. These same muscle afferents also form the sensory component of the exercise pressor reflex (EPR), which is the increase in heart rate and blood pressure (BP) experienced after muscle contraction. Studies suggest that afferent sensitization after ischemia depends on interactions between purinergic (P2X and P2Y) receptors, transient receptor potential (TRP) channels, and acid sensing ion channels (ASICs) in individual populations of peripheral sensory neurons. Specific alterations in primary afferent function through these receptor mechanisms correlate with increased pain related behaviors and altered EPRs. Recent evidence suggests that factors within the muscles during ischemic conditions including upregulation of growth factors and cytokines, and microvascular changes may be linked to the overexpression of these different receptor molecules in the dorsal root ganglia (DRG) that in turn modulate pain and sympathetic reflexes. In this review article, we will discuss the peripheral mechanisms involved in the development of ischemic myalgia and the role that primary sensory neurons play in EPR modulation.

How important is the CO2 chemoreflex for the control of breathing? Environmental and evolutionary considerations

Haldane and Priestley (1905) discovered that the ventilatory control system is highly sensitive to CO₂. This "CO₂ chemoreflex" has been interpreted to dominate control of resting arterial PCO₂/pH (P_aCO₂/pH_a) by monitoring P_aCO₂/pH_a and altering ventilation through negative feedback. However, P_aCO₂/pH_a varies little in mammals as ventilation tightly couples to metabolic demands, which may minimize chemoreflex control of P_aCO₂. The purpose of this synthesis is to (1) interpret data from experimental models with meager CO₂ chemoreflexes to infer their role in ventilatory control of steady-state P_aCO₂, and (2) identify physiological causes of respiratory acidosis occurring normally across vertebrate classes. Interestingly, multiple rodent and amphibian models with minimal/absent CO₂ chemoreflexes exhibit normal ventilation, gas exchange, and P_aCO₂/pH_a. The chemoreflex, therefore, plays at most a minor role in ventilatory control at rest; however, the chemoreflex may be critical for recovering P_aCO₂ following acute respiratory acidosis induced by breath-holding and activity in many ectothermic vertebrates. An apparently small role for CO₂ feedback in the genesis of normal breathing contradicts the prevailing view that central CO₂/pH chemoreceptors increased in importance throughout vertebrate evolution. Since the CO₂ chemoreflex contributes minimally to resting ventilation, these CO₂ chemoreceptors may have instead decreased importance throughout tetrapod evolution, particularly with the onset and refinement of neural innovations that improved the matching of ventilation to tissue metabolic demands. This distinct and elusive "metabolic ventilatory drive" likely underlies steady-state P_aCO₂ in air-breathers. Uncovering the mechanisms and evolution of the metabolic ventilatory drive presents a challenge to clinically-oriented and comparative respiratory physiologists alike.

Influence of menopause status and age on integrated central and peripheral hemodynamic responses to subsystolic cuffing during submaximal exercise

Although pathophysiological links between postmenopause and healthy aging remain unclear, both factors are associated with increased blood pressure and sympathetic nerve activity (SNA) in women. Activation of polymodal musculoskeletal neural afferents originating within adventia of venules modulates SNA and blood pressure control during exercise in healthy adults. We hypothesized transient subsystolic regional circulatory occlusion (RCO) during exercise sensitizes these afferents leading to augmented systemic vascular resistance (SVR)-mediated increased mean arterial pressure (MAP) in postmenopause vs. premenopause. Normotensive women in premenopause or postmenopause (n = 14 and 14; ages: 30 ± 9 and 55 ± 7 yr, respectively; P < 0.01) performed: 1) peak exercise testing and 2) fixed-load cycling at 30% peak workload (48 ± 11 and 38 ± 6 W, respectively; P < 0.01), whereby the initial 3 min were control exercise without RCO (CTL), thereafter including 2 min of bilateral-thigh RCO to 20, 40, 60, 80, or 100 mmHg (randomized), with 2 min deflation between RCO. Both MAP (17 ± 4 vs. 4 ± 4%, P = 0.02) and SVR (16 ± 8 vs. -3 ± 8%, P = 0.04) increased at 80 mmHg from CTL in postmenopause vs. premenopause, respectively. However, cardiac index was similar in postmenopause vs. premenopause at 80 mmHg from CTL (1 ± 6 vs. 7 ± 6%, respectively; P = 0.15). There was no continuous effect of aging in MAP (P = 0.12), SVR (P = 0.07), or cardiac index (P = 0.18) models. These data suggest transient locomotor subsystolic RCO sensitizes musculoskeletal afferents, which provoke increased SVR to generate augmented MAP during exercise in postmenopause. These observations provide a novel approach for understanding the age-independent variability in exercise blood pressure control across the normotensive adult pre- to postmenopause spectrum.

Peripheral venous distension elicits a blood pressure raising reflex in young and middle-aged adults

Distension of peripheral veins in humans elicits a pressor and sympathoexcitatory response that is mediated through group III/IV skeletal muscle afferents. There is some evidence that autonomic reflexes mediated by these sensory fibers are blunted with increasing age, yet to date the venous distension reflex has only been studied in young adults. Therefore, we tested the hypothesis that the venous distension reflex would be attenuated in middle-aged compared with young adults. Nineteen young (14 men/5 women, 25 ± 1 yr) and 13 middle-aged (9 men/4 women, 50 ± 2 yr) healthy normotensive participants underwent venous distension via saline infusion through a retrograde intravenous catheter in an antecubital vein during limb occlusion. Beat-by-beat blood pressure, muscle sympathetic nerve activity (MSNA), and model flow-derived cardiac output (Q), and total peripheral resistance (TPR) were recorded throughout the trial. Mean arterial pressure (MAP) increased during the venous distension in both young (baseline 83 ± 2, peak 94 ± 3 mmHg; P < 0.05) and middle-aged adults (baseline 88 ± 2, peak 103 ± 3 mmHg; P < 0.05). MSNA also increased in both groups [young: baseline 886 ± 143, peak 1,961 ± 242 arbitrary units (AU)/min; middle-aged: baseline 1,164 ± 225, peak 2,515 ± 404 AU/min; both P < 0.05]. TPR (P < 0.001), but not Q (P = 0.76), increased during the trial. However, the observed increases in blood pressure, MSNA, and TPR were similar between young and middle-aged adults. Additionally, no correlation was found between age and the response to venous distension (all P > 0.05). These findings suggest that peripheral venous distension elicits a pressor and sympathetic response in middle-aged adults similar to the response observed in young adults.

Central and Peripheral Fatigue During Resistance Exercise - A Critical Review

Resistance exercise is a popular form of conditioning for numerous sport disciplines, and recently different modes of strength training are being evaluated for health benefits. Resistance exercise differs significantly in nature, and several variables determine the direction and range of adaptive changes that occur in the muscular and skeletal system of the body. Some modes of resistance training can also be effective in stimulating the cardiovascular system. These variables include exercise selection (general, specific, single or multi joint, dynamic, explosive), type of resistance (free weights, variable resistance, isokinetics), order of exercise (upper and lower body or push and pull exercises), and most of all the training load which includes intensity expressed as % of 1RM, number of repetitions, number of sets and the rest interval between sets. Manipulating these variables allows for specific adaptive changes which may include gains in muscle mass, muscle strength or muscle endurance. It has been well established that during resistance exercise fatigue occurs, regardless of the volume and intensity of work applied. The peripheral mechanisms of fatigue have been studied and explained in more detail than those related to the CNS. This review is an attempt to bring together the latest knowledge regarding fatigue, both peripheral and central, during resistance exercise. The authors of this review concentrated on physiological and biochemical mechanisms underlying fatigue in exercises performed with maximal intensity, as well as those performed to exhaustion with numerous repetitions and submaximal load.

Limb suction evoked during arterial occlusion causes systemic sympathetic activity in humans

Venous saline infusions in an arterially occluded forearm evokes reflex increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP). We hypothesized that the application of suction to the human limbs would activate this venous distension reflex and raise sympathetic outflow. We placed airtight pressure tanks and applied 100 mmHg negative pressure to an arterially occluded limb (occlusion and suction, O&S) to induce tissue deformation without fluid translocation. BP, heart rate (HR), and MSNA were assessed in 19 healthy subjects during 2 min of arm or leg O&S. Occlusion without suction served as a control. During a separate visit, saline (5% forearm volume) was infused into veins of the arterially occluded arm (n = 13). The O&S increased limb circumference, MSNA burst rate (arm: Δ6.7 ± 0.7; leg: Δ6.8 ± 0.7 bursts/min), and total activity (arm: Δ199 ± 14; leg: Δ172 ± 22 units/min) and BP (arm: Δ4.3 ± 0.3; leg: Δ9.4 ± 1.4 mmHg) from the baseline. The MSNA and BP responses during arm O&S correlated with those during leg O&S. Occlusion alone had no effect on MSNA and BP. MSNA (r = 0.607) responses during arm O&S correlated with those evoked by the saline infusion into the arm. These correlations suggest that sympathetic activation during limb O&S is likely, at least partially, to be evoked via the venous distension reflex. These data suggest that suction of an occluded limb evokes sympathetic activation and that the limb venous distension reflex exists in arms and legs of normal humans.

Hindlimb venous distention evokes a pressor reflex in decerebrated rats

The distention of small vessels caused by an increase in blood flow to dynamically exercising muscles has been proposed as a stimulus that activates the thin fiber (groups III and IV) afferents evoking the exercise pressor reflex. This theory has been supported by evidence obtained from both humans and animals. In decerebrated unanesthetized rats with either freely perfused femoral arteries or arteries that were ligated 3 days before the experiment, we attempted to provide evidence in support of this theory by measuring arterial pressure, heart rate, and renal sympathetic nerve discharge while retrogradely injecting Ringer's solution in increasing volumes into the femoral vein just as it excited the triceps surae muscles. We found that the pressor response to injection was directly proportional to the volume injected. Retrograde injection of volumes up to and including 1 mL had no significant effect on either heart rate or renal sympathetic nerve activity. Cyclooxygenase blockade with indomethacin attenuated the reflex pressor response to retrograde injection in both groups of rats. In contrast, gadolinium, which blocks mechanogated channels, attenuated the reflex pressor response to retrograde injection in the “ligated rats,” but had no effect on the response in “freely perfused” rats. Our findings are consistent with the possibility that distension of small vessels within exercising skeletal muscle can serve as a stimulus to the thin fiber afferents evoking the exercise pressor reflex.

In decerebrated unanesthetized rats with either freely perfused femoral arteries or arteries that were ligated 3 days before the experiment, we attempted to provide evidence in support of this theory by measuring arterial pressure, heart rate, and renal sympathetic nerve discharge while retrogradely injecting Ringer's solution in increasing volumes into the femoral vein just as it excited the triceps surae muscles. Our findings are consistent with the possibility that distension of small vessels within exercising skeletal muscle can serve as a stimulus to the thin fiber afferents evoking the exercise pressor reflex.

Neural regulation of cardiovascular response to exercise: role of central command and peripheral afferents

During dynamic exercise, mechanisms controlling the cardiovascular apparatus operate to provide adequate oxygen to fulfill metabolic demand of exercising muscles and to guarantee metabolic end-products washout. Moreover, arterial blood pressure is regulated to maintain adequate perfusion of the vital organs without excessive pressure variations. The autonomic nervous system adjustments are characterized by a parasympathetic withdrawal and a sympathetic activation. In this review, we briefly summarize neural reflexes operating during dynamic exercise. The main focus of the present review will be on the central command, the arterial baroreflex and chemoreflex, and the exercise pressure reflex. The regulation and integration of these reflexes operating during dynamic exercise and their possible role in the pathophysiology of some cardiovascular diseases are also discussed.

Influence of the metaboreflex on arterial blood pressure in heart failure patients

BACKGROUND

Feedback from active locomotor muscles contributes to the exercise pressor response in healthy humans, and is thought to be more prominent in heart failure (HF). The purpose of this study was to examine the influence of metaboreflex stimulation on arterial pressure in HF.

METHODS

Eleven HF patients (51 ± 5 years, New York Heart Association Class I/II, left ventricular ejection fraction 32 ± 3%) and 11 controls (42 ± 3 years) were recruited. Participants completed two exercise sessions on separate days: (1) symptom limited graded exercise test; and (2) constant work rate cycling (60% peak oxygen consumption,V˙O2) for 4 minutes with 2 minutes passive recovery. Recovery was randomized to normal or locomotor muscle regional circulatory occlusion (RCO). Mean arterial pressure (MAP), systolic pressure (SBP), diastolic pressure, heart rate (HR) and V˙O2 were measured at rest, end-exercise and recovery. O2 pulse (V˙O2/HR) and the rate pressure product (RPP = HR × SBP) were calculated.

RESULTS

In response to RCO, mean arterial pressure and SBP increased in HF compared with CTLs (6.8 ± 5.8% vs -3.0 ± 7.8%, P < .01 and 3.4 ± 6.4% vs -12.7 ± 10.4%, P < .01, respectively), with no difference in diastolic pressure (P = .61). HF patients had a smaller reduction in HR and RPP, but also displayed a larger decrease in O2 pulse consequent to locomotor metaboreflex stimulation (P < .05, for all).

CONCLUSION

RCO resulted in a markedly increased pressor response in HF relative to controls, due primarily to an increase of SBP and attenuated cardiac recovery as noted by the persistent elevation in HR.

Control of breathing during exercise

During exercise by healthy mammals, alveolar ventilation and alveolar-capillary diffusion increase in proportion to the increase in metabolic rate to prevent PaCO2 from increasing and PaO2 from decreasing. There is no known mechanism capable of directly sensing the rate of gas exchange in the muscles or the lungs; thus, for over a century there has been intense interest in elucidating how respiratory neurons adjust their output to variables which can not be directly monitored. Several hypotheses have been tested and supportive data were obtained, but for each hypothesis, there are contradictory data or reasons to question the validity of each hypothesis. Herein, we report a critique of the major hypotheses which has led to the following conclusions. First, a single stimulus or combination of stimuli that convincingly and entirely explains the hyperpnea has not been identified. Second, the coupling of the hyperpnea to metabolic rate is not causal but is due to of these variables each resulting from a common factor which link the circulatory and ventilatory responses to exercise. Third, stimuli postulated to act at pulmonary or cardiac receptors or carotid and intracranial chemoreceptors are not primary mediators of the hyperpnea. Fourth, stimuli originating in exercising limbs and conveyed to the brain by spinal afferents contribute to the exercise hyperpnea. Fifth, the hyperventilation during heavy exercise is not primarily due to lactacidosis stimulation of carotid chemoreceptors. Finally, since volitional exercise requires activation of the CNS, neural feed-forward (central command) mediation of the exercise hyperpnea seems intuitive and is supported by data from several studies. However, there is no compelling evidence to accept this concept as an indisputable fact.

Tracking pulmonary gas exchange by breathing control during exercise: role of muscle blood flow

Populations of group III and IV muscle afferent fibres located in the adventitia of the small vessels appear to respond to the level of venular distension and to recruitment of the vascular bed within the skeletal muscles. The CNS could thus be informed on the level of muscle hyperaemia when the metabolic rate varies. As a result, the magnitude and kinetics of the change in peripheral gas exchange – which translates into pulmonary gas exchange – can be sensed. We present the view that the respiratory control system uses these sources of information of vascular origin, among the numerous inputs produced by exercise, as a marker of the metabolic strain imposed on the circulatory and the ventilatory systems, resulting in an apparent matching between pulmonary gas exchange and alveolar ventilation.

Ventilatory control in infants, children, and adults with bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD), or chronic lung disease of prematurity, occurs in ~30% of preterm infants (15,000 per year) and is associated with a clinical history of mechanical ventilation and/or high inspired oxygen at birth. Here, we describe changes in ventilatory control that exist in patients with BPD, including alterations in chemoreceptor function, respiratory muscle function, and suprapontine control. Because dysfunction in ventilatory control frequently revealed when O2 supply and CO2 elimination are challenged, we provide this information in the context of four important metabolic stressors: stresses: exercise, sleep, hypoxia, and lung disease, with a primary focus on studies of human infants, children, and adults. As a secondary goal, we also identify three key areas of future research and describe the benefits and challenges of longitudinal human studies using well-defined patient cohorts.

Distension of central great vein decreases sympathetic outflow in humans

Classic canine studies suggest that central great vein distension evokes an autonomic reflex tachycardia (Bainbridge reflex). It is unclear whether central venous distension in humans is a necessary and sufficient stimulus to evoke a reflex increase in heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA). Prior work from our laboratory suggests that limb venous distension evokes a reflex increase in BP and MSNA in humans. We hypothesized that in humans, compared with the limb venous distension, inferior vena cava (IVC) distension would evoke a less prominent increase in HR and MSNA. IVC distension (monitored with ultrasonography) was induced by two methods: 1) head-down tilt (HDT, N = 13); and 2) lower-body positive pressure (LBPP, N = 10). Two minutes of HDT induced IVC distension (Δ2.6 ± 0.2 mm, P < 0.001, ~27% in cross-sectional area), slightly increased mean BP (Δ2.3 ± 0.7 mmHg, P = 0.005), decreased MSNA (Δ5.2 ± 0.8 bursts/min, P < 0.001, N = 10), and did not alter HR (P = 0.37). LBPP induced similar IVC distension, increased BP (Δ2.0 ± 0.7 mmHg, P < 0.01), and did not alter HR (P = 0.34). Thus central venous distension leads to a rapid increase in BP and a subsequent fall in MSNA. Central venous distension does not evoke either bradycardia or tachycardia in humans. The absence of a baroreflex-mediated bradycardia suggests that the Bainbridge reflex is engaged. Clearly, this reflex differs from the powerful sympathoexcitation peripheral venous distension reflex described in humans.

Muscle metaboreflex and autonomic regulation of heart rate in humans

We elucidated the autonomic mechanisms whereby heart rate (HR) is regulated by the muscle metaboreflex. Eight male participants (22 ± 3 years) performed three exercise protocols: (1) enhanced metaboreflex activation with partial flow restriction (bi-lateral thigh cuff inflation) during leg cycling exercise, (2) isolated muscle metaboreflex activation (post-exercise ischaemia; PEI) following leg cycling exercise, (3) isometric handgrip followed by PEI. Trials were undertaken under control (no drug), β1-adrenergic blockade (metoprolol) and parasympathetic blockade (glycopyrrolate) conditions. HR increased with partial flow restriction during leg cycling in the control condition (11 ± 2 beats min(-1); P < 0.05). The magnitude of this increase in HR was similar with parasympathetic blockade (11 ± 2 beats min(-1)), but attenuated with β-adrenergic blockade (4 ± 1 beats min(-1); P < 0.05 vs. control and parasympathetic blockade). During PEI following leg cycling exercise, HR remained similarly elevated above rest under all conditions (11 ± 2, 13 ± 3 and 9 ± 4 beats min(-1), for control, β-adrenergic and parasympathetic blockade; P > 0.05 between conditions). During PEI following handgrip, HR was similarly elevated from rest under control and parasympathetic blockade (4 ± 1 vs. 4 ± 2 beats min(-1); P > 0.05 between conditions) conditions, but attenuated with β-adrenergic blockade (0.2 ± 1 beats min(-1); P > 0.05 vs. rest). Thus muscle metaboreflex activation-mediated increases in HR are principally attributable to increased cardiac sympathetic activity, and only following exercise with a large muscle mass (PEI following leg cycling) is there a contribution from the partial withdrawal of cardiac parasympathetic tone.

New perspectives concerning feedback influences on cardiorespiratory control during rhythmic exercise and on exercise performance

The cardioaccelerator and ventilatory responses to rhythmic exercise in the human are commonly viewed as being mediated predominantly via feedforward 'central command' mechanisms, with contributions from locomotor muscle afferents to the sympathetically mediated pressor response. We have assessed the relative contributions of three types of feedback afferents on the cardiorespiratory response to voluntary, rhythmic exercise by inhibiting their normal 'tonic' activity in healthy animals and humans and in chronic heart failure. Transient inhibition of the carotid chemoreceptors during moderate intensity exercise reduced muscle sympathetic nerve activity (MSNA) and increased limb vascular conductance and blood flow; and reducing the normal level of respiratory muscle work during heavier intensity exercise increased limb vascular conductance and blood flow. These cardiorespiratory effects were prevented via ganglionic blockade and were enhanced in chronic heart failure and in hypoxia. Blockade of μ opioid sensitive locomotor muscle afferents, with preservation of central motor output via intrathecal fentanyl: (a) reduced the mean arterial blood pressure (MAP), heart rate and ventilatory responses to all steady state exercise intensities; and (b) during sustained high intensity exercise, reduced O(2) transport, increased central motor output and end-exercise muscle fatigue and reduced endurance performance. We propose that these three afferent reflexes - probably acting in concert with feedforward central command - contribute significantly to preserving O(2) transport to locomotor and to respiratory muscles during exercise. Locomotor muscle afferents also appear to provide feedback concerning the metabolic state of the muscle to influence central motor output, thereby limiting peripheral fatigue development.

Limb venous distension evokes sympathetic activation via stimulation of the limb afferents in humans

We have recently shown that a saline infusion in the veins of an arterially occluded human forearm evokes a systemic response with increases in muscle sympathetic nerve activity (MSNA) and blood pressure. In this report, we examined whether this response was a reflex that was due to venous distension. Blood pressure (Finometer), heart rate, and MSNA (microneurography) were assessed in 14 young healthy subjects. In the saline trial (n = 14), 5% forearm volume normal saline was infused in an arterially occluded arm. To block afferents in the limb, 90 mg of lidocaine were added to the same volume of saline in six subjects during a separate visit. To examine whether interstitial perfusion of normal saline alone induced the responses, the same volume of albumin solution (5% concentration) was infused in 11 subjects in separate studies. Lidocaine abolished the MSNA and blood pressure responses seen with saline infusion. Moreover, compared with the saline infusion, an albumin infusion induced a larger (MSNA: Δ14.3 ± 2.7 vs. Δ8.5 ± 1.3 bursts/min, P < 0.01) and more sustained MSNA and blood pressure responses. These data suggest that venous distension activates afferent nerves and evokes a powerful systemic sympathoexcitatory reflex. We posit that the venous distension plays an important role in evoking the autonomic adjustments seen with postural stress in human subjects.

Effect of muscle metaboreflex activation on spontaneous cardiac baroreflex sensitivity during exercise in humans

We sought to determine whether the activation of metabolically sensitive skeletal muscle afferents (muscle metaboreflex) is a potential mechanism for the decrease in spontaneous cardiac baroreflex sensitivity (cBRS) during exercise in humans. In protocol 1, 15 male subjects (22 ± 1 years) performed steady-state leg cycling at low (26 ± 4 W) and moderate workloads (105 ± 7 W), under free-flow conditions and with partial flow restriction (bilateral thigh cuff inflation at 100 mmHg) to evoke muscle metaboreflex activation during exercise. In protocol 2, rhythmic handgrip exercise at 35% maximum voluntary contraction was performed with progressive upper arm cuff inflation (0, 80, 100 and 120 mmHg) to elicit graded metaboreflex activation. Both protocols were followed by post-exercise ischaemia (PEI) to isolate the muscle metaboreflex. Leg cycling-induced increases in HR and mean BP were augmented by partial flow restriction (P < 0.05 vs. free flow), while HR and mean BP both remained elevated during PEI (P < 0.05 vs. rest). Leg cycling evoked an intensity-dependent decrease in cBRS (16 ± 2, 7 ± 1 and 2 ± 0.2 ms mmHg(-1) at rest, low and moderate workloads, respectively; P < 0.05), which was further reduced with partial flow restriction (by -2.6 ± 0.8 and -0.4 ± 0.1 ms mmHg(-1) at low and moderate workloads). cBRS remained suppressed during PEI following leg cycling with partial flow restriction (4 ± 1 ms mmHg(-1); P < 0.05 vs. rest). cBRS was unchanged during handgrip under free-flow conditions, handgrip with partial flow restriction and PEI following handgrip (P > 0.05 vs. rest). These data indicate that the activation of metabolically sensitive skeletal muscle afferents (muscle metaboreflex) decreases cardiac baroreflex responsiveness during leg cycling exercise in humans.

Sympathetic and cardiovascular responses to venous distension in an occluded limb

We recently showed that a fixed volume (i.e., 40 ml) of saline infused into the venous circulation of an arterially occluded vascular bed increases muscle sympathetic nerve activity (MSNA) and blood pressure. In the present report, we hypothesized that the volume and rate of infusion would influence the magnitude of the sympathetic response. Blood pressure, heart rate, and MSNA were assessed in 13 young healthy subjects during forearm saline infusions (arrested circulation). The effects of different volumes of saline (i.e., 2%, 3%, 4%, or 5% forearm volume at 30 ml/min) and different rates of infusion (i.e., 5% forearm volume at 10, 20, or 30 ml/min) were evaluated. MSNA and blood pressure responses were linked with the infusion volume. Infusion of 5% of forearm volume evoked greater MSNA responses than did infusion of 2% of forearm volume (Δ11.6 ± 1.9 vs. Δ3.1 ± 1.8 bursts/min and Δ332 ± 105 vs. Δ38 ± 32 units/min, all P < 0.05). Moreover, greater MSNA responses were evoked by saline infusion at 30 ml/min than 10 ml/min (P < 0.05). Sonographic measurements confirmed that the saline infusions induced forearm venous distension. The results suggest that volume and rate of saline infusion are important factors in evoking sympathetic activation. We postulate that venous distension contributes to cardiovascular autonomic adjustment in humans.

Venous blood gas and metabolite response to low-intensity muscle contractions with external limb compression

The effect of low-intensity resistance exercise with external limb compression (100 [EC100] and 160 [EC160] mm Hg) on limb blood flow and venous blood gas-metabolite response was investigated and compared with that of high-intensity resistance exercise (no external compression). Unilateral elbow flexion muscle contractions were performed at 20% (75 repetitions, 4 sets, 30-second rest intervals) and 70% of 1-repetition maximum (1-RM; 3 sets, each set was until failure, 3-minute rest intervals). Precontraction brachial arterial blood flow (Doppler ultrasound) was reduced with EC100 or EC160 (56% and 39% of baseline value, respectively) compared with no external compression (control). At 20% 1-RM, brachial arterial blood flow increased after contractions performed with EC160 (190%), but not with the others. Decreases in venous oxygen partial pressure (P(v)O(2)) and venous oxygen saturation (S(v)O(2)) were greater during EC100 and EC160 than control (mean [SE]: P(v)O(2), 28 [3] vs 26 [2] vs 33 [2] mm Hg; S(v)O(2), 41% [5%] vs 34% [4%] vs 52% [5%], respectively). Changes in venous pH (pH(v)), venous carbon dioxide partial pressure (P(v)CO(2)), and venous lactate concentration ([L(-)](v)) were greater with EC160 than EC100 and/or control (pH(v), 7.19 [0.01] vs 7.25 [0.01] vs 7.27 [0.02]; P(v)CO(2), 72 [3] vs 64 [2] vs 60 [3] mm Hg; [L(-)](v), 5.4 [0.6] vs 3.7 [0.4] vs 3.0 [0.4] mmol/L, respectively). Seventy percent 1-RM contractions resulted in greater changes in pH(v) (7.14 [0.02]), P(v)CO(2) (91 [5] mm Hg), and [L(-)](v) (7.0 [0.5] mmol/L) than EC100 and EC160, but P(v)O(2) (30 [4] mm Hg) and S(v)O(2) (40% [3%]) were similar. In conclusion, changes in pH(v), P(v)CO(2), and [L(-)](v), but not in P(v)O(2) and S(v)O(2), are sensitive to changes in relative, "internal" intensity of low-intensity muscle contractions caused by reduced blood flow (EC160) or high-intensity muscle contractions. Given the magnitude of the changes in pH(v), P(v)CO(2), and [L(-)](v), it appears plausible that they may be involved in stimulating the observed increase in muscle activation via group III and IV afferents.

Respiratory effects of changing the volume load imposed on the peripheral venous system

This study was designed to determine if acute distension of the hindlimb venous circulation stimulates breathing, thereby contributing to the respiratory responses to rapid changes in total blood volume. In 10 spontaneously breathing anesthetized sheep, we withdrew 15 ml kg(-1) of blood from a femoral vein over approximately 1-2 min. We then compared the respiratory effects of infusing this venous blood back into the femoral veins across two conditions: the inferior vena cava (IVC) was either unobstructed or obstructed by a balloon-tipped catheter. We found that when blood was withdrawn and blood volume decreased, an absolute increase in breathing often occurred, but more importantly that a relative hyperventilation was always observed. When this blood was re-infused into the animal, effectively increasing blood volume, the respiratory response depended upon whether or not the IVC was open or obstructed. With the IVC unobstructed, a relative hypoventilation occurred, accompanied by an increase in alveolar PCO(2). In contrast, when the venous blood was re-infused and the IVC was obstructed, ventilation increased significantly, and the response was often hypocapnic. These results indicate that increasing the volume load in the venous circulation increases breathing, and that the transduction mechanism is contained within the peripheral venous system. Further, the respiratory drive from this sensory mechanism is subject to modulation via changes in the circulatory status, most likely within the arterial side.

Control of breathing during acute change in cardiac preload in a patient with partial cardiopulmonary bypass

We recently had the opportunity to investigate the ventilatory effects of changing the rate of venous return to the heart (and thus pulmonary gas exchange) in a patient equipped with a venous-arterial oxygenated shunt (extracorporeal membrane oxygenation (ECMO) support). The presence of the ECMO support provided a condition wherein venous return to the right heart could be increased or decreased while maintaining total aortic blood flow and arterial blood pressure (ABP) constant. The patient, who had received a heart transplant 12 years ago, was admitted for acute cardiac failure related to graft rejection. The clinical symptomatology was that of right heart failure. We studied the patient on the 4th day of ECMO support, while she was breathing spontaneously. The blood flow diverted through the ECMO system represented 2/3 of the total aortic flow (4 l min(-1)). With these ECMO settings, the baseline level of ventilation was low (3.89+/-0.99 l min(-1)), but PET(CO2) was not elevated (37+/-2 mmHg). When Pa(CO2) in the blood coming from the ECMO was increased, no stimulatory effect on ventilation was observed. However, when the diversion of the venous return to the ECMO was stopped then restored, minute ventilation respectively increased then decreased by more than twofold with opposite changes in PET(CO2). These maneuvers were associated with large changes in the size of the right atrium and ventricle and of the left atrium. This observation suggests that the change in venous return affects breathing by encoding some of the consequences of the changes in cardiac preload. The possible sites of mediation are discussed.

Effects of digoxin on muscle reflexes in normal humans

Blockade of the skeletal muscle Na(+)-K(+)-ATPase pump by digoxin could result in a more marked hyperkaliema during a forearm exercise, which in turn could stimulate the mechano- and metaboreceptors. In a randomized, double-blinded, placebo-controlled, and cross-over-design study, we measured mean blood pressure (MBP), heart rate (HR), ventilation (V(E)), oxygen saturation (SpO(2)), muscle sympathetic nerve activity (MSNA), venous plasma potassium and lactic acid during dynamic handgrip exercises, and local circulatory arrest in 11 healthy subjects. Digoxin enhanced MBP during exercise but not during the post-handgrip ischemia and had no effect on HR, V(E), SpO(2), and MSNA. Venous plasma potassium and lactic acid were also not affected by digoxin-induced skeletal muscle Na(+)-K(+)-ATPase blockade. We conclude that digoxin increased MBP during dynamic exercise in healthy humans, independently of changes in potassium and lactic acid. A modest direct sensitization of the muscle mechanoreceptors is unlikely and other mechanisms, independent of muscle reflexes and related to the inotropic effects of digoxin, might be implicated.