Cardiovascular and humoral responses to sustained muscle metaboreflex activation in humans

Mechanisms mediating muscle metaboreflex control of cardiac output during exercise: Impaired regulation in heart failure

The ability to increase cardiac output during dynamic exercise is paramount for the ability to maintain workload performance. Reflex control of the cardiovascular system during exercise is complex and multifaceted involving multiple feedforward and feedback systems. One major reflex thought to mediate the autonomic adjustments to exercise is termed the muscle metaboreflex and is activated via afferent neurons within active skeletal muscle which respond to the accumulation of interstitial metabolites during exercise when blood flow and O2 delivery are insufficient to meet metabolic demands. This is one of the most powerful cardiovascular reflexes capable of eliciting profound increases in sympathetic nerve activity, arterial blood pressure, central blood volume mobilization, heart rate and cardiac output. This review summarizes the mechanisms meditating muscle metaboreflex-induced increases in cardiac output. Although much has been learned from studies using anaesthetized and/or decerebrate animals, we focus on studies in conscious animals and humans performing volitional exercise. We discuss the separate and interrelated roles of heart rate, ventricular contractility, ventricular preload and ventricular-vascular coupling as well as the interaction with other cardiovascular reflexes which modify muscle metaboreflex control of cardiac output. We discuss how these mechanisms may be altered in subjects with heart failure with reduced ejection fraction and offer suggestions for future studies.

The exercise pressor reflex: An update

The exercise pressor reflex is a feedback mechanism engaged upon stimulation of mechano- and metabosensitive skeletal muscle afferents. Activation of these afferents elicits a reflex increase in heart rate, blood pressure, and ventilation in an intensity-dependent manner. Consequently, the exercise pressor reflex has been postulated to be one of the principal mediators of the cardiorespiratory responses to exercise. In this updated review, we will discuss classical and recent advancements in our understating of the exercise pressor reflex function in both human and animal models. Particular attention will be paid to the afferent mechanisms and pathways involved during its activation, its effects on different target organs, its potential role in the abnormal cardiovascular response to exercise in diseased states, and the impact of age and biological sex on these responses. Finally, we will highlight some unanswered questions in the literature that may inspire future investigations in the field.

Modulation of muscle metaboreceptor activation upon sweating and cutaneous vascular responses to rising core temperature in humans

The present study investigated the role of muscle metaboreceptor activation on human thermoregulation by measuring core temperature thresholds and slopes for sweating and cutaneous vascular responses during passive heating associated with central and peripheral mechanisms. Six male and eight female subjects inserted their lower legs into hot water (43°C) while wearing a water perfusion suit on the upper body (34°C). One minute after immersion, an isometric handgrip exercise--40% of maximum voluntary contraction-was conducted for 1.5 min in both control and experimental conditions, while postexercise occlusion was performed in the experimental condition only for 9 min. The postexercise forearm occlusion during passive heating consistently stimulated muscle metaboreceptors, as implicated by significantly elevated mean arterial blood pressure throughout the experimental period (P <0.05). Stimulation of the forearm muscle metaboreceptors increased sweating and cutaneous vascular responses during passive heating, and was associated with significant reductions in esophageal temperature threshold of sweating and cutaneous vasodilation (Δ threshold, sweating: 0.33 ± 0.05 and 0.16 ± 0.04°C, cutaneous vascular conductance: 0.38 ± 0.08 and 0.16 ± 0.05°C for control and experimental groups, respectively, P < 0.05). The slopes of these responses were not different between the conditions. These results suggest that muscle metaboreceptor activation in the forearm accelerates sweating and cutaneous vasodilation during passive heating associated with a reduction in core temperature thresholds and may be related to central mechanisms controlling heat loss responses.

Sweating responses and the muscle metaboreflex under mildly hyperthermic conditions in sprinters and distance runners

To investigate the effects of different training methods on nonthermal sweating during activation of the muscle metaboreflex, we compared sweating responses during postexercise muscle occlusion in endurance runners, sprinters, and untrained men under mild hyperthermia (ambient temperature, 35°C; relative humidity, 50%). Ten endurance runners, nine sprinters, and ten untrained men (maximal oxygen uptakes: 57.5 ± 1.5, 49.3 ± 1.5, and 36.6 ± 1.6 ml·kg(-1)·min(-1), respectively; P < 0.05) performed an isometric handgrip exercise at 40% maximal voluntary contraction for 2 min, and then a pressure of 280 mmHg was applied to the forearm to occlude blood circulation for 2 min. The Δ change in mean arterial blood pressure between the resting level and the occlusion was significantly higher in sprinters than in untrained men (32.2 ± 4.4 vs. 17.3 ± 2.6 mmHg, respectively; P < 0.05); however, no difference was observed between distance runners and untrained men. The Δ mean sweating rate (averaged value of the forehead, chest, forearm, and thigh) during the occlusion was significantly higher in distance runners than in sprinters and untrained men (0.38 ± 0.07, 0.19 ± 0.03, and 0.11 ± 0.04 mg·cm(-2)·min(-1), respectively; P < 0.05) and did not differ between sprinters and untrained men. Our results suggest that the specificity of training modalities influences the sweating response during activation of the muscle metaboreflex. In addition, these results imply that a greater activation of the muscle metaboreflex does not cause a greater sweating response in sprinters.

Muscle metaboreflex activation by static exercise dilates pupil in humans

We examined a hypothesis that static exercise and activation of sympathetic activation by metabolically sensitive skeletal muscle afferents (metaboreflex) influence the sympathetic nervous activity modulating pupil diameter. Nine subjects performed 2 min isometric handgrip exercise at 30% maximal voluntary contraction, which was followed by either 2 min of postexercise muscle ischemia (PEMI) in the forearm or no PEMI (control trial). The pupil diameter and mean blood pressure (MAP) increased significantly from rest during exercise in PEMI and control trials (5 ± 1 and 7 ± 1% in diameter; 13 ± 2 and 12 ± 2 mmHg in MAP, respectively). These increases in the diameter and MAP were maintained during PEMI (7 ± 2% and 9 ± 2 mmHg) but not during the recovery period in the control trial (3 ± 2% and 1 ± 2 mmHg). These results demonstrate that static handgrip exercise increases the pupil diameter, and this increase is partly due to the activation of metaboreflex in humans.

Individual differences in the heart rate response to activation of the muscle metaboreflex in humans

We tested the hypotheses that the heart rate (HR) response to muscle metaboreflex activation induced by postexercise muscle ischemia (PEMI) varies considerably among subjects and that individual differences in the HR response are associated with differences in cardiac autonomic tone and/or arterial baroreflex function during PEMI. Fifty-one healthy subjects (36 men and 15 women) performed a 1-min isometric handgrip exercise at 50% maximal voluntary contraction, which was followed by a 3.5-min period of imposed PEMI. We estimated cardiac autonomic tone using spectral analysis of beat-to-beat variation in the R-R interval (RRI). In addition, the sensitivity of the arterial baroreflex control of HR (BRS) was evaluated using transfer function analysis of systolic arterial pressure (SAP) and RRI. Although the mean RRI during the PEMI and subsequent recovery period did not differ from the resting value, the variance among the individual differences in RRI between the rest and PEMI periods was significantly greater than between the rest and recovery periods. The changes in RRI elicited by PEMI correlated significantly with changes in the spectral power of the RRI variability in the high-frequency range and the BRS. By contrast, no significant correlation was observed between changes in RRI and changes in mean arterial pressure or the power of the RRI variability in the low-frequency range. This suggests that, in humans, the HR response to PEMI-induced activation of muscle metaboreflex varies considerably from individual to individual and that these differences reflect changes in cardiac parasympathetic tone and spontaneous BRS during PEMI.

Progressive muscle metaboreflex activation gradually decreases spontaneous heart rate baroreflex sensitivity during dynamic exercise

Ischemia of active skeletal muscle elicits a pressor response termed the muscle metaboreflex. We tested the hypothesis that in normal dogs during dynamic exercise, graded muscle metaboreflex activation (MMA) would progressively attenuate spontaneous heart rate baroreflex sensitivity (SBRS). The animals were chronically instrumented to measure heart rate (HR), cardiac output (CO), mean and systolic arterial pressure (MAP and SAP), and left ventricular systolic pressures (LVSP) at rest and during mild or moderate treadmill exercise before and during progressive MMA [via graded reductions of hindlimb blood flow (HLBF)]. SBRS [slopes of the linear relationships (LRs) between HR and LVSP or SAP during spontaneous sequences of > or =3 consecutive beats when HR changed inversely vs. pressure] decreased during mild exercise from the resting values (-5.56 +/- 0.86 vs. -2.67 +/- 0.50 beats.min(-1).mmHg(-1), P <0.05), and in addition, these LRs were shifted upward. Progressive MMA gradually and linearly increased MAP, CO, and HR; linearly decreased SBRS; and shifted LRs upward and rightward to higher HR and pressures denoting baroreflex resetting. Moderate exercise caused a substantial reduction in SBRS (-1.57 +/- 0.38 beats.min(-1).mmHg(-1), P <0.05) and both an upward and rightward resetting. Gradual MMA at this higher workload also caused significant progressive increases in MAP, CO, and HR and progressive decreases in SBRS, and the LRs were shifted to higher MAP and HR. Our results demonstrate that gradual MMA during mild and moderate dynamic exercise progressively decreases SBRS. In addition, baroreflex control of HR is progressively reset to higher blood pressure and HR in proportion to the extent of MMA.

Reduced central blood volume and cardiac output and increased vascular resistance during static handgrip exercise in postural tachycardia syndrome

Postural tachycardia syndrome (POTS) is characterized by exercise intolerance and sympathoactivation. To examine whether abnormal cardiac output and central blood volume changes occur during exercise in POTS, we studied 29 patients with POTS (17-29 yr) and 12 healthy subjects (18-27 yr) using impedance and venous occlusion plethysmography to assess regional blood volumes and flows during supine static handgrip to evoke the exercise pressor reflex. POTS was subgrouped into normal and low-flow groups based on calf blood flow. We examined autonomic effects with variability techniques. During handgrip, systolic blood pressure increased from 112 +/- 4 to 139 +/- 9 mmHg in control, from 119 +/- 6 to 143 +/- 9 in normal-flow POTS, but only from 117 +/- 4 to 128 +/- 6 in low-flow POTS. Heart rate increased from 63 +/- 6 to 82 +/- 4 beats/min in control, 76 +/- 3 to 92 +/- 6 beats/min in normal-flow POTS, and 88 +/- 4 to 100 +/- 6 beats/min in low-flow POTS. Heart rate variability and coherence markedly decreased in low-flow POTS, indicating uncoupling of baroreflex heart rate regulation. The increase in central blood volume with handgrip was absent in low-flow POTS and blunted in normal-flow POTS associated with abnormal splanchnic emptying. Cardiac output increased in control, was unchanged in low-flow POTS, and was attenuated in normal-flow POTS. Total peripheral resistance was increased compared with control in all POTS. The exercise pressor reflex was attenuated in low-flow POTS. While increased cardiac output and central blood volume characterizes controls, increased peripheral resistance with blunted or eliminated in central blood volume increments characterizes POTS and may contribute to exercise intolerance.

Modulation of the spontaneous beat-to-beat fluctuations in peripheral vascular resistance during activation of muscle metaboreflex

Continuous measurement of leg blood flow (LBF) using Doppler ultrasound with simultaneous noninvasive mean arterial blood pressure (MAP) measurement permits beat-to-beat estimates of leg vascular resistance (LVR) in humans. We tested the hypothesis that the beat-to-beat fluctuations in LVR and the dynamic relationship between MAP and LVR are modulated by the activation of muscle metaboreflex. Twelve healthy subjects performed a 1-min isometric handgrip exercise at 50% maximal voluntary contraction, which was followed by a period of imposed postexercise muscle ischemia (PEMI). We then employed transfer function analysis to examine the dynamic relationships between MAP and LBF and between MAP and LVR, both at rest (control) and during PEMI. We found the following. 1) The spectral power for LBF and LVR in low-frequency (∼0.03–0.15 Hz) range significantly increased from control during PEMI without a significant change in the high-frequency (∼0.15–0.35 Hz) power. 2) During PEMI, the transfer function gains for MAP-LBF and MAP-LVR relationships in the low-frequency (∼0.05–0.15 Hz) range were significantly increased during PEMI (vs. control) but were unchanged in the high-frequency (∼0.2–0.3 Hz) range. 3) The phases for MAP-LBF and MAP-LVR relationships were not different during control and PEMI. The phase for MAP-LVR relationship revealed that changes in MAP were followed by directionally similar changes in LVR, which is consistent with the characteristics of intrinsic vascular regulatory mechanisms such as the myogenic response of the resistance arteries. We suggest that, in humans, modulation of the dynamic MAP-LVR relationship during activation of the muscle metaboreflex reflects complex interactions between intrinsic vascular regulatory mechanisms and sympathetic vascular regulation.

Muscle metaboreflex attenuates spontaneous heart rate baroreflex sensitivity during dynamic exercise

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

Changes in regional blood volume and blood flow during static handgrip

Increased blood pressure (BP) and heart rate during exercise characterizes the exercise pressor reflex. When evoked by static handgrip, mechanoreceptors and metaboreceptors produce regional changes in blood volume and blood flow, which are incompletely characterized in humans. We studied 16 healthy subjects aged 20-27 yr using segmental impedance plethysmography validated against dye dilution and venous occlusion plethysmography to noninvasively measure changes in regional blood volumes and blood flows. Static handgrip while in supine position was performed for 2 min without postexercise ischemia. Measurements of heart rate and BP variability and coherence analyses were used to examine baroreflex-mediated autonomic effects. During handgrip exercise, systolic BP increased from 120 +/- 10 to 148 +/- 14 mmHg, whereas heart rate increased from 60 +/- 8 to 82 +/- 12 beats/min. Heart rate variability decreased, whereas BP variability increased, and transfer function amplitude was reduced from 18 +/- 2 to 8 +/- 2 ms/mmHg at low frequencies of approximately 0.1 Hz. This was associated with marked reduction of coherence between BP and heart rate (from 0.76 +/- 0.10 to 0.26 +/- 0.05) indicative of uncoupling of heart rate regulation by the baroreflex. Cardiac output increased by approximately 18% with a 4.5% increase in central blood volume and an 8.5% increase in total peripheral resistance, suggesting increased cardiac preload and contractility. Splanchnic blood volume decreased reciprocally with smaller decreases in pelvic and leg volumes, increased splanchnic, pelvic and calf peripheral resistance, and evidence for splanchnic venoconstriction. We conclude that the exercise pressor reflex is associated with reduced baroreflex cardiovagal regulation and driven by increased cardiac output related to enhanced preload, cardiac contractility, and splanchnic blood mobilization.

Renal sympathetic and circulatory responses to activation of the exercise pressor reflex in rats

We investigated the role played by the exercise pressor reflex in sympathetic regulation of the renal circulation in rats. In mid-collicular decerebrate rats, mean arterial pressure (MAP), heart rate (HR), left renal cortical blood flow (RCBF) and left renal sympathetic nerve activity (RSNA) were recorded before and during 30 s of static contraction of the left triceps surae muscles evoked by electrical stimulation of the tibial nerve, which activates both metabo- and mechanosensitive muscle afferents, and during 30 s of passive stretch of the left Achilles tendon, which selectively activates mechanosensitive muscle afferents. Static contraction (n = 17, +344 +/- 34 g developed tension) significantly (P < 0.05) increased MAP (+14 +/- 3 mmHg), HR (+6 +/- 1 beats min(-1)) and RSNA (n = 11, +19 +/- 5%) and significantly decreased renal cortical vascular conductance (RCVC, n = 11, -11 +/- 2%). Passive stretch (n = 20, +378 +/- 11 g) also significantly increased MAP (+11 +/- 2 mmHg), HR (+7 +/- 2 beats min(-1)) and RSNA (n = 15, +14 +/- 4%) and significantly decreased RCVC (n = 11, -12 +/- 3%). RCBF showed no significant changes during static contraction or passive stretch. Renal denervation abolished the decrease in RCVC during contraction (n = 12) or stretch (n = 13). These data indicate that both the exercise pressor reflex and its mechanically sensitive component, the muscle mechanoreflex, induced renal cortical vasoconstriction through sympathetic activation in rats.

Effects of rhythmic muscle compression on cardiovascular responses and muscle oxygenation at rest and during dynamic exercise

We examined the way in which the duration of rhythmic muscle compressions affects cardiovascular responses and muscle oxygenation at rest and during dynamic exercise. We measured the mean arterial pressure (MAP), heart rate (HR) and oxygenation of the vastus lateralis muscle (by near-infrared spectroscopy) in eight healthy male subjects at rest and during supine bicycle exercise (50 and 100 W at 60 r.p.m.) while applying pulsed muscle compressions at 1000 ms intervals. Compression pressure and durations were 150 mmHg and 300, 600, 900 and 1000 ms (1000 ms being static continuous compression), respectively. During exercise, the pulsed leg compression was synchronized to each thigh extensor muscle contraction. The observed changes in muscle oxygenation were dependent on compression duration (increased at 300 ms, no change at 600 ms and decreased at 900 or 1000 ms) and were different from those seen at rest (increases at < 1000 ms and decrease at 1000 ms). This suggests that the effects of external pulsed muscle compression may have a duration threshold below which muscle pumping counteracts the obstruction to flow caused by the compression, and that the threshold is set at a shorter compression duration during exercise than at rest. Although HR and MAP did not change during pulsed compression at rest, during exercise they both increased progressively as compression duration increased. Thus, while exercising, the increased MAP and HR seen during the compression could be due to the combination and interaction of mechanical effects and the muscle mechanoreflex and/or metaboreflex.

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

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

Attenuated arterial baroreflex buffering of muscle metaboreflex in heart failure

Previous studies have shown that heart failure (HF) or sinoaortic denervation (SAD) alters the strength and mechanisms of the muscle metaboreflex during dynamic exercise. However, it is still unknown to what extent SAD may modify the muscle metaboreflex in HF. Therefore, we quantified the contribution of cardiac output (CO) and peripheral vasoconstriction to metaboreflex-mediated increases in mean arterial blood pressure (MAP) in conscious, chronically instrumented dogs before and after induction of HF in both barointact and SAD conditions during mild and moderate exercise. The muscle metaboreflex was activated via partial reductions in hindlimb blood flow. After SAD, the metaboreflex pressor responses were significantly higher with respect to the barointact condition despite lower CO responses. The pressor response was significantly lower in HF after SAD but still higher than that of HF in the barointact condition. During control experiments in the barointact condition, total vascular conductance summed from all beds except the hindlimbs did not change with muscle metaboreflex activation, whereas in the SAD condition both before and after induction of HF significant vasoconstriction occurred. We conclude that SAD substantially increased the contribution of peripheral vasoconstriction to metaboreflex-induced increases in MAP, whereas in HF SAD did not markedly alter the patterns of the reflex responses, likely reflecting that in HF the ability of the arterial baroreflex to buffer metaboreflex responses is impaired.

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

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

Impaired muscle metaboreflex-induced increases in ventricular function in heart failure

We investigated to what extent heart failure alters the ability of the muscle metaboreflex to improve ventricular function. Dogs were chronically instrumented to monitor mean arterial pressure (MAP), cardiac output (CO), heart rate (HR), stroke volume (SV), and central venous pressure (CVP) at rest and during mild treadmill exercise (3.2 km/h) before and during reductions in hindlimb blood flow imposed to activate the muscle metaboreflex. These control experiments were repeated at constant heart rate (ventricular pacing 225 beats/min) and at constant heart rate coupled with a beta-adrenergic blockade (atenolol, 2 mg/kg iv) in normal animals and in the same animals after the induction of heart failure (HF, induced via rapid ventricular pacing). In control experiments in normal animals, metaboreflex activation caused tachycardia with no change in SV, resulting in large increases in CO and MAP. At constant HR, large increases in CO still occurred via significant increases in SV. Inasmuch as CVP did not change in this setting and that beta-adrenergic blockade abolished the reflex increase in SV at constant HR, this increase in SV likely reflects increased ventricular contractility. In contrast, after the induction of HF, much smaller increases in CO occurred with metaboreflex activation because, although increases in HR still occurred, SV decreased thereby limiting any increase in CO. At constant HR, no increase in CO occurred with metaboreflex activation even though CVP increased significantly. After beta-adrenergic blockade, CO and SV decreased with metaboreflex activation. We conclude that in HF, the ability of the muscle metaboreflex to increase ventricular function via both increases in contractility as well as increases in filling pressure are markedly impaired.

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

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

Sweating responses during activation of the muscle metaboreflex in humans is altered by time of day

AIM

The aim of the present study was to test for a time-of-day effect on sweating responses to activation of the muscle metaboreflex.

METHODS

Eight male subjects each participated in two exercise sessions, one in the morning and one in the evening. Within each session there were two 60-s bouts of isometric handgrip (IHG) exercise at 50% maximal voluntary contraction. Prior to IHG, whole body warming by a water-perfused suit initiated mild sweating. The first bout of IHG exercise began at 06.00 hours (am) and 18.00 hours (pm). Blood circulation to the forearm was occluded for 120 s, beginning 5 s before the end of the second bout of IHG to activate the muscle metaboreflex.

RESULTS

During both bouts of exercise, sweating rate (SR) on both the chest and right forearm significantly increased from the pre-exercise period in both am and pm sessions. SR rapidly decreased during first minute of recovery after the first bout of IHG exercise. However, during post-exercise ischaemia (PEI) after the second bout of IHG exercise, SR was maintained significantly above the pre-exercise level only in the pm session. The increases in SR on the chest and right forearm during PEI were significantly greater in the pm, than in the am, session. However, SR of the palm was not maintained during PEI.

CONCLUSIONS

We conclude that under mild hyperthermic conditions, the sweating response in non-glabrous skin to activation of the muscle metaboreflex exhibits a time-of-day effect.

Modulation of arterial baroreflex dynamic response during muscle metaboreflex activation in humans

We aimed to investigate the interaction between the arterial baroreflex and muscle metaboreflexes (as reflected by alterations in the dynamic responses shown by muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP) and heart rate (HR)) in humans. In nine healthy subjects (eight male, one female) who performed a sustained 1 min handgrip exercise at 50 % maximal voluntary contraction followed by forearm occlusion, a 5 s period of neck pressure (NP) (30 and 50 mmHg) or neck suction (NS)(-30 and -60 mmHg) was used to evaluate carotid baroreflex function at rest (CON) and during post-exercise muscle ischaemia (PEMI). In PEMI (as compared with CON): (a) the augmentations in MSNA and MAP elicited by 50 mmHg NP were both greater; (b) MSNA seemed to be suppressed by NS for a shorter period, (c) the decrease in MAP elicited by NS was smaller, and (d) MAP recovered to its initial level more quickly after NS. However, the HR responses to NS and NP were not different between PEMI and CON. These results suggest that during muscle metaboreflex activation, the dynamic arterial baroreflex response is modulated, as exemplified by the augmentation of the MSNA response to arterial baroreflex unloading (i.e. NP) and the reduction in the suppression of MSNA induced by baroreceptor stimulation (i.e. NS).

Sweating response in physically trained men to sustained handgrip exercise in mildly hyperthermic conditions

To investigate the effects of physical training on heat loss response to sustained handgrip exercise (non-thermal factors), we compared the sweating response during isometric handgrip exercise to mild hyperthermia in physically trained and untrained subjects. Seven trained and untrained male subjects (maximal oxygen uptake 62.7 +/- 2.4 and 42.7 +/- 1.6 mL kg-1 min-1, respectively, P < 0.05) performed isometric handgrip exercises at 20, 35 and 50% maximal voluntary contraction (MVC) for 60 s. The study was conducted in a climatic chamber with a regulated ambient temperature of 35 degrees C and relative humidity of 50% to induce sweating response at rest by rising skin temperature without a marked change in internal temperature. Sublingual and mean skin temperatures (thermal factors) in both trained and untrained groups were essentially constant throughout all exercise intensities. Changes in heart rate, mean arterial blood pressure, and rating of perceived exertion with increased exercise intensity were similar in both groups. Sweating rate (SR) on the limbs (mean value of forearm and thigh) was significantly greater in the trained group than in the untrained group at 50% MVC (P < 0.05). In addition, the slopes of the relationship between increased SR and exercise intensity (% MVC) on the trunk (chest) and limbs were significantly greater in the trained group than in the untrained group (P < 0.05). Our results suggest that the sweating response caused by non-thermal factors against a background of changing thermal factors was enhanced by physical training. It is also thought that the enhanced sweating response may be especially evident on the limbs than on the trunk, such as improvement of sweating response associated with thermal factors.

Effects of rhythmic muscle compression on arterial blood pressure at rest and during dynamic exercise in humans

This study was designed to examine the hypothesis that a rhythmic mechanical compression of muscles would affect systemic blood pressure regulation at rest and during dynamic exercise in humans. We measured the changes in mean arterial pressure (MAP) occurring (a) at rest with pulsed (350 ms pulses at 50 pulses min(-1)) or static compression (50 and 100 mmHg) of leg muscles with or without upper thigh occlusion, and (b) during 12-min supine bicycle exercise (75 W, 50 r.p.m.) with or without pulsed compression (50, 100, 150 mmHg) of the legs in synchrony with the thigh extensor muscle contraction. At rest with thigh occlusion, MAP increased by 4-8 mmHg during static leg compression, and by 5-9 mmHg during pulsed leg compression. This suggests that at rest pulsed leg compression elicits a reflex pressor response of similar magnitude to that evoked by static compression. During dynamic exercise without leg compression, MAP (having risen initially) gradually declined, but imposition of graded pulsed leg compression prevented this decline, the MAP values being significantly higher than those recorded without pulsed leg compression by 7-10 mmHg. These results suggest that the rhythmic increase in intramuscular pressure that occurs during dynamic exercise evokes a pressor response in humans.

Static contraction causes a reflex-induced release of arginine vasopressin in anesthetized cats

We tested the hypothesis that brief static contraction of the triceps surae muscle causes reflex-induced increases in plasma arginine vasopressin (AVP) in anesthetized cats. Arterial blood samples, for measurement of plasma AVP, were taken before and after 30 s of electrically stimulated static contraction performed at a low intensity (<20% of maximal; n = 5), a high intensity (>70% of maximal; n = 7), and a high intensity after denervation of the triceps surae (n = 5). The low intensity contraction protocol was repeated during alpha-adrenergic blockade (n = 7) to minimize potential baroreflex-induced inhibition of AVP release. Passive stretch of the triceps surae was conducted (n = 5) to determine effects of muscle mechanoreceptor stimulation on the release of AVP. Low intensity contraction had no effect on plasma AVP. During alpha-adrenergic blockade, this same contraction intensity caused this peptide to increase from 12.8 +/- 2.1 to 17.7 +/- 2.6 pg/ml. High intensity contraction caused an increase in AVP (13.2 +/- 3.5 to 26.1 +/- 6.6 pg/ml) that was abolished by denervation (14.4 +/- 3. 7 vs. 17.1 +/- 6.6 pg/ml). Passive stretch had no effect on plasma AVP. These findings suggest that brief static contraction causes increases in plasma AVP that are reflex in nature, intensity dependent, opposed by the arterial baroreflex, and probably unrelated to muscle mechanoreceptor activation.

Heart failure alters the strength and mechanisms of the muscle metaboreflex

We hypothesized that excessive sympathoactivation observed during strenuous exercise in subjects with heart failure (HF) may result from tonic activation of the muscle metaboreflex (MMR) via hypoperfusion of active skeletal muscle. We studied MMR responses in dogs during treadmill exercise by graded reduction of terminal aortic blood flow (TAQ) before and after induction of HF by rapid ventricular pacing. At a low workload, in both control and HF experiments, large decreases in TAQ were required to elicit the MMR pressor response. During control experiments, this pressor response resulted from increased cardiac output (CO), whereas in HF CO did not increase; thus the pressor response was solely due to peripheral vasoconstriction. In HF, MMR activation also induced higher plasma levels of vasopressin, norepinephrine (NE), and renin. At a higher workload, in control experiments any reduction of TAQ elicited MMR pressor responses. In HF, before any vascular occlusion, TAQ was already below MMR control threshold levels and reductions in TAQ again did not result in higher CO; thus SAP increased via peripheral vasoconstriction. NE rose markedly, indicating intense sympathetic activation. We conclude that in HF, the MMR is likely tonically active at moderate workloads and contributes to the tonic sympathoactivation.

Human cardiovascular and humoral responses to moderate muscle activation during dynamic exercise

We examined the hypothesis that activation of the muscle metaboreflex during dynamic exercise would augment influences tending to cause a rise in arginine vasopressin, plasma renin activity, and catecholamines during dynamic exercise in humans. Ten healthy adults performed 30 min of supine cycle ergometer exercise at approximately 50% of peak oxygen consumption with or without moderate muscle metaboreflex activation by application of 35 mmHg lower body positive pressure (LBPP). Application of LBPP during the first 15 or last 15 min of exercise increased mean arterial blood pressure, plasma lactate concentration, and minute ventilation, indicating an activation of the muscle metaboreflex. These changes were rapidly reversed when LBPP was removed. During exercise at this intensity, LBPP augmented the release of arginine vasopressin and catecholamines but not of plasma renin activity. These results suggest that, although in humans hormonal responses are induced by moderate activation of the muscle metaboreflex during dynamic exercise, the thresholds for these responses may not be uniform among the various glands and hormones.

Near-infrared monitoring of tissue oxygenation during application of lower body pressure at rest and during dynamical exercise in humans

During the application of a wide range of graded lower body pressures (LBP) (-50 to 50 mmHg), we examined how (1) the tissue oxygenation in the lower and upper parts of the body changes at rest, and (2) how tissue oxygenation changes in the lower extremities during dynamical leg exercise. We used near-infrared spectroscopy (NIRS) to measure the changes induced by LBP in total Hb content and Hb oxygenation in seven subjects. At rest, total Hb increased and Hb oxygenation decreased in the thigh muscles during -25 and -50 mmHg LBP, while both decreased during +25 and +50 mmHg LBP. However, in the forearm muscles during graded LBP, the pattern of change in total Hb was the reverse of that in the thigh. Measurements from the forehead showed changes only during +50 mmHg LBP. These results demonstrated that the pattern of change in total Hb and Hb oxygenation differed between upper and lower parts with graded LBP at rest. During dynamical leg exercise, total Hb and Hb oxygenation in the thigh muscles decreased during stepwise increases in LBP above -25 mmHg, Hb oxygenation decreasing markedly during +50 mmHg LBP. These results suggest that during dynamical exercise (i) LBP at +25 mmHg or more causes a graded decline in blood volume and/or flow in the thigh muscles, and (ii) especially at +50 mmHg LBP, the O2 content may decrease markedly in active muscles. Our results suggest that NIRS can be used to monitor in a non-invasive and continuous fashion the changes in oxygenation occurring in human skeletal muscles and head during the graded changes in blood flow and/or volume caused by changes in external pressure and secondary reflexes both at rest and during dynamical exercise.

Modulation of the thermoregulatory sweating response to mild hyperthermia during activation of the muscle metaboreflex in humans

1. To investigate the effect of the muscle metaboreflex on the thermoregulatory sweating response in humans, eight healthy male subjects performed sustained isometric handgrip exercise in an environmental chamber (35 C and 50 % relative humidity) at 30 or 45 % maximal voluntary contraction (MVC), at the end of which the blood circulation to the forearm was occluded for 120 s. The environmental conditions were such as to produce sweating by increase in skin temperature without a marked change in oesophageal temperature. 2. During circulatory occlusion after handgrip exercise at 30 % MVC for 120 s or at 45 % MVC for 60 s, the sweating rate (SR) on the chest and forearm (hairy regions), and the mean arterial blood pressure were significantly above baseline values (P < 0.05). There were no changes from baseline values in the oesophageal temperature, mean skin temperature, or SR on the palm (hairless regions). 3. During the occlusion after handgrip exercise at 30 % MVC for 60 s and during the occlusion alone, none of the measured parameters differed from baseline values. 4. It is concluded that, under mildly hyperthermic conditions, the thermoregulatory sweating response on the hairy regions is modulated by afferent signals from muscle metaboreceptors.

Effects of posture on cardiovascular responses to lower body positive pressure at rest and during dynamic exercise

We tested the hypothesis that cardiovascular responses to lower body positive pressure (LBPP) would be dependent on the posture of the subject and also on the background condition (rest or exercise). We measured heart rate (HR), mean arterial blood pressure (MAP), and cardiac stroke volume in eight subjects at rest and during cycle ergometer exercise (76 +/- 3 W) with and without LBPP (25, 50, and 75 mmHg) in the supine and upright positions. At rest, the increase in MAP was proportional to the increase in LBPP and was greater in the supine (6 +/- 2, 15 +/- 3, and 26 +/- 3 mmHg) than in the upright (2 +/- 3, 9 +/- 3, and 17 +/- 3 mmHg) position. During dynamic exercise, the increases in MAP evoked by 25, 50, and 75 mmHg LBPP were greater in the supine (13 +/- 2, 28 +/- 3, and 40 +/- 3 mmHg) than in the upright (7 +/- 3, 12 +/- 3, and 25 +/- 3 mmHg) position. We conclude that the systemic pressure response to LBPP is clearly dependent on the body position, with the larger pressure responses being associated with the supine position both at rest and during dynamic leg exercise.