Augmented sympathetic tone alters muscle metabolism with exercise: lack of evidence for functional sympatholysis

Neural Control of Vascular Function in Skeletal Muscle

The sympathetic nervous system represents a fundamental homeostatic system that exerts considerable control over blood pressure and the distribution of blood flow. This process has been referred to as neurovascular control. Overall, the concept of neurovascular control includes the following elements: efferent postganglionic sympathetic nerve activity, neurotransmitter release, and the end organ response. Each of these elements reflects multiple levels of control that, in turn, affect complex patterns of change in vascular contractile state. Primarily, this review discusses several of these control layers that combine to produce the integrative physiology of reflex vascular control observed in skeletal muscle. Beginning with three reflexes that provide somewhat dissimilar vascular patterns of response despite similar changes in efferent sympathetic nerve activity, namely, the baroreflex, chemoreflex, and muscle metaboreflex, the article discusses the anatomical and physiological bases of postganglionic sympathetic discharge patterns and recruitment, neurotransmitter release and management, and details of regional variations of receptor density and responses within the microvascular bed. Challenges are addressed regarding the fundamentals of measurement and how conclusions from one response or vascular segment should not be used as an indication of neurovascular control as a generalized physiological dogma. Whereas the bulk of the article focuses on the vasoconstrictor function of sympathetic neurovascular integration, attention is also given to the issues of sympathetic vasodilation as well as the impact of chronic changes in sympathetic activation and innervation on vascular health. © 2016 American Physiological Society.

Chronic heart failure and exercise intolerance: the hemodynamic paradox

Heart failure represents a major source of morbidity and mortality in industrialized nations. As the leading hospital discharge diagnosis in the United States in patients over the age of 65, it is also associated with substantial economic costs. While the acute symptoms of volume overload frequently precipitate inpatient admission, it is the symptoms of chronic heart failure, including fatigue, exercise intolerance and exertional dyspnea, that impact quality of life. Over the last two decades, research into the enzymatic, histologic and neurohumoral alterations seen with heart failure have revealed that hemodynamic derangements do not necessarily correlate with symptoms. This “hemodynamic paradox” is explained by alterations in the skeletal musculature that occur in response to hemodynamic derangements. Importantly, gender specific effects appear to modify both disease pathophysiology and response to therapy. The following review will discuss our current understanding of the systemic effects of heart failure before examining how exercise training and cardiac resynchronization therapy may impact disease course.

Modelflow estimates of cardiac output compared with Doppler ultrasound during acute changes in vascular resistance in women

We compared Modelflow (MF) estimates of cardiac stroke volume (SV) from the finger pressure-pulse waveform (Finometer) with pulsed Doppler ultrasound (DU) of the ascending aorta during acute changes in total peripheral resistance (TPR) in the supine and head-up-tilt (HUT) postures. Twenty-four women were tested during intravenous infusion of 0.005 or 0.01 microg kg(-1) min(-1) isoprenaline, 10 or 50 ng kg(-1) min(-1) noradrenaline and 0.3 mg sublingual nitroglycerine. Responses to static hand-grip exercise (SHG), graded lower body negative pressure (LBNP, from 20 to 45 mmHg) and 45 deg HUT were evaluated on separate days. Bland-Altman analysis indicated that SV(MF) yielded lower estimates than SV(DU) during infusion of 0.01 microg kg(-1) min(-1) isoprenaline (SV(MF) 92.7 +/- 15.5 versus SV(DU) 104.3 +/- 22.9 ml, P = 0.03) and SHG (SV(MF) 78.8 +/- 12.0 versus SV(DU) 106.1 +/- 28.5 ml, P < 0.01), while larger estimates were recorded with SV(MF) during 45 mmHg LBNP (SV(MF) 52.6 +/- 10.7 versus SV(DU) 46.2 +/- 14.5 ml, P = 0.04) and HUT (SV(MF) 59.3 +/- 13.6 versus SV(DU) 45.2 +/- 11.3 ml, P < 0.01). Linear regression analysis revealed a relationship (r(2) = 0.41, P < 0.01) between the change in TPR from baseline and the between-methods discrepancy in SV measurements. This relationship held up under all of the experimental protocols (regression for fixed effects, P = 0.46). These results revealed a discrepancy in MF estimates of SV, in comparison with those measured by DU, during acute changes in TPR.

Effect of graded leg cycling on postischaemic forearm blood flow in healthy subjects

This study assessed in healthy subjects, the effect of leg cycling on the forearm vascular responses to ischaemia to confirm previous results showing that exercise-induced sympathetic activation during leg cycling reduced postischaemic forearm hyperaemia. Seven young healthy subjects performed two bouts of cycling exercises at 50% and 80% of their maximal aerobic capacity (Ex(50), Ex(80) respectively) during which forearm arterial blood flow was successively occluded for 40, 90 and 180 s. Control forearm blood flow (FBF) and postischaemic forearm blood flow (pi-FBF) measured at the release of arterial occlusions were assessed using plethysmography. Digital arterial pressure was continuously monitored allowing calculation of control and postischaemic forearm conductance (FC and pi-FC respectively). At rest, pi-FBF increased with the duration of ischaemia (5 +/- 1, 19 +/- 3, 29 +/- 3, 31 +/- 4 ml min(-1) 100 ml(-1) after 0, 40, 90 and 180 s of ischaemia respectively). During Ex(50), FBF and pi-FBF did not change significantly although pi-FC was significantly reduced (Deltapi-FC = -39%, -33%, -27% for 40, 90, 180 s of ischaemia respectively). During Ex(80), there was a further dramatic decrease in pi-FC (-53%, -66%, -62% from rest) and pi-FBF were largely blunted (13 +/- 4 versus 19 +/- 3, 14 +/- 4 versus 29 +/- 3, 17 +/- 5 versus 31 +/- 4 ml min(-1) 100 ml(-1)). These results demonstrated that forearm responses to ischaemia depended on leg activities. It was suggested that exercise-induced sympathetic activation may have interfered on local vasodilatation because of ischaemia.

Forearm blood flow follows work rate during submaximal dynamic forearm exercise independent of sex

To test the hypothesis that sex influences forearm blood flow (FBF) during exercise, 15 women and 16 men of similar age [women 24.3 +/- 4.0 (SD) vs. men 24.9 +/- 4.5 yr] but different forearm muscle strength (women 290.7 +/- 44.4 vs. men 509.6 +/- 97.8 N; P < 0.05) performed dynamic handgrip exercise as the same absolute workload was increased in a ramp function (0.25 W/min). Task failure was defined as the inability to maintain contraction rate. Blood pressure and FBF were measured on separate arms during exercise by auscultation and Doppler ultrasound, respectively. Muscle strength was positively correlated with endurance time (r = 0.72, P < 0.01) such that women had a shorter time to task failure than men (450.5 +/- 113.0 vs. 831.3 +/- 272.9 s; P < 0.05). However, the percentage of maximal handgrip strength achieved at task failure was similar between sexes (14% maximum voluntary contraction). FBF was similar between women and men throughout exercise and at task failure (women 13.6 +/- 5.3 vs. men 14.5 +/- 4.9 ml.min(-1).100 ml(-1)). Mean arterial pressure was lower in women at rest and during exercise; thus calculated forearm vascular conductance (FVC) was higher in women during exercise but similar between sexes at task failure (women 0.13 +/- 0.05 vs. men 0.11 +/- 0.04 ml.min(-1).100 ml(-1).mmHg(-1)). In conclusion, the similar FBF during exercise was achieved by a higher FVC in the presence of a lower MAP in women than men. Still, FBF remained coupled to work rate (and presumably metabolic demand) during exercise irrespective of sex.

Muscle reflex control of sympathetic nerve activity in heart failure: the role of exercise conditioning

Muscle reflex control of sympathetic nerve activity has been an area of considerable investigation. During exercise, the capacity of the peripheral vasculature to dilate far exceeds the maximal attainable levels of cardiac output. The activation of sympathetic nervous system and engagement of the myogenic reflex serve as the controlling influence between the heart and the muscle vasculature to maintain blood pressure (BP). Two basic theories of neural control have evolved. The first termed "central command", suggests that a volitional signal emanating from central motor areas leads to increased sympathetic activation during exercise. According to the second theory the stimulation of mechanical and chemical afferents in exercising muscle lead to engagement of the "exercise pressor reflex". Some earlier studies suggested that group III muscle afferent fibers are predominantly mechanically sensitive whereas unmyelinated group IV muscle afferents respond to chemical stimuli. In recent years new evidence is emerging which challenges the concept of functional differentiation of muscle afferents as well as the classic description of muscle "mechano" and "metabo" receptors. Studies measuring concentrations of interstitial substances during exercise suggest that K(+) and phosphate, but not H(+) and lactate, may be important muscle afferent stimulants. The role of adenosine as a muscle afferent stimulant remains an area of debate. There is strong evidence that sympathetic vasoconstriction due to muscle reflex engagement plays an important role in restricting blood flow to the exercising muscle. In heart failure (HF), exercise leads to premature fatigue and accumulation of muscle metabolites resulting in a greater degree of muscle reflex engagement and in the process further decreasing the muscle blood flow. Conditioning leads to an increased ability of the muscle to maintain aerobic metabolism, lower interstitial accumulation of metabolites, less muscle reflex engagement and a smaller sympathetic response. Beneficial effects of physical conditioning may be mediated by a direct reduction of muscle metaboreflex activity or via reduction of metabolic signals activating these receptors. In this review, we will discuss concepts of flow and reflex engagement in normal human subjects and then contrast these findings with those seen in heart failure (HF). We will then examine the effects of exercise conditioning on these parameters in normal subjects and those with congestive heart failure (CHF).

A perspective on the muscle reflex: implications for congestive heart failure

In this review we examine the exercise pressor reflex in health and disease. The role of metabolic and mechanical stimulation of thin fiber muscle afferents is discussed. The role ATP and lactic acid play in stimulating and sensitizing these afferents is examined. The role played by purinergic receptors subdivision 2, subtype X, vanilloid receptor subtype 1, and acid-sensing ion channels in mediating the effects of ATP and H+ are discussed. Muscle reflex activation in heart failure is then examined. Data supporting the concept that the metaboreflex is attenuated and that the mechanoreflex is accentuated are presented. The role the muscle mechanoreflex plays in evoking renal vasoconstriction is also described.

How Does Cardiac Resynchronization Therapy Improve Exercise Capacity in Chronic Heart Failure?

BACKGROUND

Studies have shown that neither ejection fraction nor hemodynamic abnormalities during exercise in chronic heart failure (HF) correlate with symptoms of fatigue and exhaustion. The concept that exercise limitation in patients with chronic HF is due to abnormal hemodynamics during exercise has been revised to acknowledge that the skeletal myopathy of chronic HF contributes significantly to exercise dysfunction in heart failure. Why then does cardiac resynchronization therapy (CRT), a therapy that improves abnormalities of cardiac function, such as cardiac output and ejection fraction, produce a consistent, measurable, irrefutable increase in exercise capacity?

METHODS AND RESULTS

In this review I will (1) review the mechanisms of exercise dysfunction in chronic HF, with special attention to the concept of "coordinated adaptation"; (2) analyze the effects of CRT on autonomic dysfunction in HF; and (3) propose a unifying hypothesis to understand how a therapy that improves cardiac function can improve exercise dysfunction attributable to a skeletal myopathy. Specifically, I will review evidence that CRT improves exercise capacity by attenuating the chronic sympathetic activation of HF.

CONCLUSION

The decrease in sympathetic activation, and perhaps inflammation, during CRT likely reverses many features of the skeletal myopathy, leading to improved exercise capacity.

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

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

Integrative control of the skeletal muscle microcirculation in the maintenance of arterial pressure during exercise

Skeletal muscle blood flow and vascular conductance are influenced by numerous factors that can be divided into two general categories: central cardiovascular control mechanisms and local vascular control mechanisms. Central cardiovascular control mechanisms are thought to be designed primarily for the maintenance of arterial pressure and central cardiovascular homeostasis, whereas local vascular control mechanisms are thought to be designed primarily for the maintenance of muscle homeostasis. To support the high metabolic rates that can be generated during muscle contraction, skeletal muscle has a tremendous capacity to vasodilate and increase oxygen and nutrient delivery. During whole body dynamic exercise at maximal oxygen consumption (V̇o2 max), the skeletal muscle receives 85–90% of cardiac output. Yet despite receiving such a large fraction of cardiac output during high-intensity exercise, a vasodilator reserve remains with the potential to produce further elevations in skeletal muscle vascular conductance and blood flow. However, because maximal cardiac output is reached during exercise at V̇o2 max, further elevations in muscle vascular conductance would produce a fall in arterial pressure. Therefore, limits on muscle perfusion must be imposed during whole body exercise to prevent such drops in pressure. Effective arterial pressure control in response to a potentially hypotensive challenge during high-intensity exercise occurs primarily through reflex-mediated increases in sympathetic nerve activity, which are capable of modulating vasomotor tone of the skeletal muscle resistance vasculature. Thus skeletal muscle vascular conductance and perfusion are primarily mediated by local factors at rest and during exercise, but other centrally mediated control systems are superimposed on the dominant local control mechanisms to provide an integrated regulation of both arterial pressure and skeletal muscle vascular conductance and perfusion during whole body dynamic exercise.

Neural control of muscle blood flow during exercise

Activation of skeletal muscle fibers by somatic nerves results in vasodilation and functional hyperemia. Sympathetic nerve activity is integral to vasoconstriction and the maintenance of arterial blood pressure. Thus the interaction between somatic and sympathetic neuroeffector pathways underlies blood flow control to skeletal muscle during exercise. Muscle blood flow increases in proportion to the intensity of activity despite concomitant increases in sympathetic neural discharge to the active muscles, indicating a reduced responsiveness to sympathetic activation. However, increased sympathetic nerve activity can restrict blood flow to active muscles to maintain arterial blood pressure. In this brief review, we highlight recent advances in our understanding of the neural control of the circulation in exercising muscle by focusing on two main topics: 1) the role of motor unit recruitment and muscle fiber activation in generating vasodilator signals and 2) the nature of interaction between sympathetic vasoconstriction and functional vasodilation that occurs throughout the resistance network. Understanding how these control systems interact to govern muscle blood flow during exercise leads to a clear set of specific aims for future research.

Autonomic and vascular responses to reduced limb perfusion

The purpose of this study was to examine hemodynamic responses to graded muscle reflex engagement in human subjects. We studied seven healthy human volunteers [24 +/- 2 (SE) yr old; 4 men, 3 women] performing rhythmic handgrip exercise [40% maximal voluntary contraction (MVC)] during ambient and positive pressure exercise (+10 to +50 mmHg in 10-mmHg increments every minute). Muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP), and mean blood velocity were recorded. Plasma lactate, hydrogen ion concentration, and oxyhemoglobin saturation were measured from venous blood. Ischemic exercise resulted in a greater rise in both MSNA and MAP vs. nonischemic exercise. These heightened autonomic responses were noted at +40 and +50 mmHg. Each level of positive pressure was associated with an immediate fall in flow velocity and forearm perfusion pressure. However, during each minute, perfusion pressure increased progressively. For positive pressure of +10 to +40 mmHg, this was associated with restoration of flow velocity. However, at +50 mmHg, flow was not restored. This inability to restore flow was seen at a time when the muscle reflex was clearly engaged (increased MSNA). We believe that these findings are consistent with the hypothesis that before the muscle reflex is clearly engaged, flow to muscle is enhanced by a process that raises perfusion pressure. Once the muscle reflex is clearly engaged and MSNA is augmented, flow to muscle is no longer restored by a similar rise in perfusion pressure, suggesting that active vasoconstriction within muscle is occurring at +50 mmHg.

Bicuculline and strychnine suppress the mesencephalic locomotor region-induced inhibition of group III muscle afferent input to the dorsal horn

We examined the effect of iontophoretic application of bicuculline methiodide and strychnine hydrochloride on the mesencephalic locomotor region (MLR)-induced inhibition of dorsal horn cells in paralyzed cats. The activity of 60 dorsal horn cells was recorded extracellularly in laminae I, II, V-VII of spinal segments L7-S1. Each of the cells was shown to receive group III muscle afferent input as demonstrated by their responses to electrical stimulation of the tibial nerve (mean latency and threshold of activation: 20.1+/-6.4 ms and 15.2+/-1.4 times motor threshold, respectively). Electrical stimulation of the MLR suppressed transmission in group III muscle afferent pathways to dorsal horn cells. Specifically the average number of impulses generated by the dorsal horn neurons in response to a single pulse applied to the tibial nerve was decreased by 78+/-2.8% (n=60) during the MLR stimulation. Iontophoretic application (10-50 nA) of bicuculline and strychnine (5-10 mM) suppressed the MLR-induced inhibition of transmission of group III afferent input to laminae I and II cells by 69+/-5% (n=10) and 29+/-7% (n=7), respectively. Likewise, bicuculline and strychnine suppressed the MLR-induced inhibition of transmission of group III afferent input to lamina V cells by 59+/-13% (n=14) and 39+/-11% (n=10), respectively. Our findings raise the possibility that GABA and glycine release onto dorsal horn neurons in the spinal cord may play an important role in the suppression by central motor command of thin fiber muscle afferent-reflex pathways.

Rapid blunting of sympathetic vasoconstriction in the human forearm at the onset of exercise

The purpose of this study was to test the hypothesis that sympathetic vasoconstriction is rapidly blunted at the onset of forearm exercise. Nine healthy subjects performed 5 min of moderate dynamic forearm handgrip exercise during -60 mmHg lower body negative pressure (LBNP) vs. without (control). Beat-by-beat forearm blood flow (Doppler ultrasound), arterial blood pressure (finger photoplethysmograph), and heart rate were collected. LBNP elevated resting heart rate by approximately 45%. Mean arterial blood pressure was not significantly changed (P = 0.196), but diastolic blood pressure was elevated by approximately 10% and pulse pressure was reduced by approximately 20%. At rest, there was a 30% reduction in forearm vascular conductance (FVC) during LBNP (P = 0.004). The initial rapid increase in FVC with exercise onset reached a plateau between 10 and 20 s of 126.6 +/- 4.1 ml. min(-1). 100 mmHg(-1) in control vs. only 101.6 +/- 4.1 ml. min(-1). 100 mmHg(-1) in LBNP (main effect of condition, P = 0.003). This difference was quickly abolished during the second, slower phase of adaptation in forearm vascular tone to steady state. These data are consistent with a rapid onset of functional sympatholysis, in which local substances released with the onset of muscle contractions impair sympathetic neural vasoconstrictor effectiveness.

Carotid baroreflex pressor responses at rest and during exercise: cardiac output vs. regional vasoconstriction

The arterial baroreflex mediates changes in arterial pressure via reflex changes in cardiac output (CO) and regional vascular conductance, and the relative roles may change between rest and exercise and across workloads. Therefore, we quantified the contribution of CO and regional vascular conductances to carotid baroreflex-mediated increases in mean arterial pressure (MAP) at rest and during mild to heavy treadmill exercise (3.2 kph; 6.4 kph, 10% grade; and 8 kph, 15% grade). Dogs (n = 8) were chronically instrumented to measure changes in MAP, CO, hindlimb vascular conductance, and renal vascular conductance in response to bilateral carotid occlusion (BCO). At rest and at each workload, BCO caused similar increases in MAP (average 35 +/- 2 mmHg). In response to BCO, neither at rest nor at any workload were there significant increases in CO; therefore, the pressor response occurred via peripheral vasoconstriction. At rest, 10.7 +/- 1.4% of the rise in MAP was due to vasoconstriction in the hindlimb, whereas 4.0 +/- 0.7% was due to renal vasoconstriction. Linear regression analysis revealed that, with increasing workloads, relative contributions of the hindlimb increased and those of the kidney decreased. At the highest workload, the decrease in hindlimb vascular conductance contributed 24.3 +/- 3.4% to the pressor response, whereas the renal contribution decreased to only 1.6 +/- 0.3%. We conclude that the pressor response during BCO was mediated solely by peripheral vasoconstriction. As workload increases, a progressively larger fraction of the pressor response is mediated via vasoconstriction in active skeletal muscle and the contribution of vasoconstriction in inactive beds (e.g., renal) becomes progressively smaller.

Plasma lactate concentration and muscle blood flow during dynamic exercise with negative-pressure breathing

This study assessed the hypothesis that increasing cardiac filling pressure (CFP) would enhance contracting muscle blood flow (MBF) by stretching cardiopulmonary baroreceptors and attenuate the increase in plasma lactate concentration ([Lac(-)](p)) during dynamic exercise. Continuous negative-pressure breathing (CNPB) (-15 cmH(2)O) was used to increase the CFP by accelerating the venous return to the heart. In the first series of experiments, 10 men performed a graded exercise seated on a cycle ergometer with (N1) and without CNPB (C1). The increase in [Lac(-)](p) for N1 was attenuated at 60%, 90%, and 100% of maximal exercise intensity compared with that in C1 (P < 0.001). Also, the increases in mean arterial pressure (MAP) and plasma catecholamine concentrations were attenuated in N1 compared with those in C1 throughout the graded exercise (P < 0.05). However, heart rate and pulse pressure were not significantly influenced by CNPB. Second, we studied the impact of CNPB on forearm MBF during a rhythmic handgrip exercise in 5 of the 10 subjects. Forearm MBF was measured immediately after cessation of the exercise by venous occlusion plethysmography at rest, 30%, 50%, and 70% of maximal work load (WL(max)) with (N2) and without CNPB (C2). Forearm MBF and vascular conductance for both trials increased with the increase in intensity, but forearm skin blood flow measured by laser-Doppler flowmetry remained unchanged. MBF and vascular conductance in N2, however, increased more than in C2 at every intensity (P < 0.01) except for MBF at 70% WL(max), whereas the increase in MAP for N2 was attenuated compared with that in C2 (P < 0.05). Thus augmented active muscle vasodilation occurred in N2 with a lower increase in MAP compared with that in C2. These findings suggest that the stretch of intrathoracic baroreceptors, such as cardiopulmonary mechanoreceptors, by CNPB increased MBF by suppressing sympathetic nerve activity. The attenuation of the increase in [Lac(-)](p) might be caused, at least partially, by the increased MBF.

Effects of forearm bier block with bretylium on the hemodynamic and metabolic responses to handgrip

We tested the hypothesis that a reduction in sympathetic tone to exercising forearm muscle would increase blood flow, reduce muscle acidosis, and attenuate reflex responses. Subjects performed a progressive, four-stage rhythmic handgrip protocol before and after forearm bier block with bretylium as forearm blood flow (Doppler) and metabolic (venous effluent metabolite concentration and (31)P-NMR indexes) and autonomic reflex responses (heart rate, blood pressure, and sympathetic nerve traffic) were measured. Bretylium inhibits the release of norepinephrine at the neurovascular junction. Bier block increased blood flow as well as oxygen consumption in the exercising forearm (P < 0.03 and P < 0.02, respectively). However, despite this increase in flow, venous K(+) release and H(+) release were both increased during exercise (P < 0.002 for both indexes). Additionally, minimal muscle pH measured during the first minute of recovery with NMR was lower after bier block (6.41 +/- 0.08 vs. 6.20 +/- 0.06; P < 0.036, simple effects). Meanwhile, reflex effects were unaffected by the bretylium bier block. The results support the conclusion that sympathetic stimulation to muscle during exercise not only limits muscle blood flow but also appears to limit anaerobiosis and H(+) release, presumably through a preferential recruitment of oxidative fibers.

Upright posture reduces forearm blood flow early in exercise

The hypothesis that upright posture could modulate forearm blood flow (FBF) early in exercise was tested in six subjects. Both single (2-s duration) and repeated (1-s work/2-s rest cadence for 12 contractions) handgrip contractions (12 kg) were performed in the supine and 70 degrees head-up tilt (HUT) positions. The arm was maintained at heart level to diminish myogenic effects. Baseline brachial artery diameters were assessed at rest in each position. Brachial artery mean blood velocity (MBV; Doppler) and mean arterial pressure (MAP) (Finapres) were measured continuously to calculate FBF and vascular conductance. MAP was not changed with posture. Antecubital venous pressure (Pv) was reduced in HUT (4.55 +/- 1.3 mmHg) compared with supine (11.3 +/- 1.9 mmHg) (P < 0.01). For the repeated contractions, total excess FBF (TEF) was reduced in the HUT position compared with supine (P < 0.02). With the single contractions, peak FBF, peak vascular conductance, and TEF during 30 s after release of the contraction were reduced in the HUT position compared with supine (P < 0.01). Sympathetic blockade augmented the FBF response to a single contraction in HUT (P < 0.05) and tended to increase this response while supine (P = 0.08). However, sympathetic blockade did not attenuate the effect of HUT on peak FBF and TEF after the single contractions. Raising the arm above heart level while supine, to diminish Pv, resulted in FBF dynamics that were similar to those observed during HUT. Alternatively, lowering the arm while in HUT to restore Pv to supine levels restored the peak FBF and vascular conductance responses, but not TEF response, after a single contraction. It was concluded that upright posture diminishes the hyperemic response early in exercise. The data demonstrate that sympathetic constriction restrains the hyperemic response to a single contraction but does not modulate the postural reduction in postcontraction hyperemia. Therefore, the attenuated blood flow response in the HUT posture was largely related to factors associated with diminished venous pressures and not sympathetic vasoconstriction.

Maintained exercise pressor response in heart failure

The impact of forearm blood flow limitation on muscle reflex (metaboreflex) activation during exercise was examined in 10 heart failure (HF) (NYHA class III and IV) and 9 control (Ctl) subjects. Rhythmic handgrip contractions (25% maximal voluntary contraction, 30 contractions/min) were performed over 5 min under conditions of ambient pressure or with +50 mmHg positive pressure about the exercising forearm. Mean arterial blood pressure (MAP) and venous effluent hemoglobin (Hb) O2 saturation, lactate and H+ concentrations ([La] and [H+], respectively) were measured at baseline and during exercise. For ambient contractions, the increase (Delta) in MAP by end exercise (DeltaMAP; i.e., the exercise pressor response) was the same in both groups (10.1 +/- 1.2 vs. 7.33 +/- 1.3 mmHg, HF vs. Ctl, respectively) despite larger Delta[La] and Delta[H+] for the HF group (P < 0.05). With ischemic exercise, the DeltaMAP for HF (21.7 +/- 2.7 mmHg) exceeded that of Ctl subjects (12.2 +/- 2.8 mmHg) (P < 0.0001). Also, for HF, Delta[La] (2.94 +/- 0.4 mmol) and Delta[H+] (24.8 +/- 2.7 nmol) in the ischemic trial were greater than in Ctl (1.63 +/- 0.4 mmol and 15.3 +/- 2.8 nmol; [La] and [H+], respectively) (P < 0.02). Hb O2 saturation was reduced in Ctl from approximately 43% in the ambient trial to approximately 27% with ischemia (P < 0.0001). O2 extraction was maximized under ambient exercise conditions for HF but not for Ctl. Despite progressive increases in blood perfusion pressure over the course of ischemic exercise, no improvement in Hb O2 saturation or muscle metabolism was observed in either group. These data suggest that muscle reflex activation of the pressor response is intact in HF subjects but the resulting improvement in perfusion pressure does not appear to enhance muscle oxidative metabolism or muscle blood flow, possibly because of associated increases in sympathetic vasoconstriction of active skeletal muscle.

Head-down-tilt bed rest alters forearm vasodilator and vasoconstrictor responses

To test the hypothesis that head-down-tilt bed rest (HDBR) for 14 days alters vascular reactivity to vasodilatory and vasoconstrictor stimuli, the reactive hyperemic forearm blood flow (RHBF, measured by venous occlusion plethysmography) and mean arterial pressure (MAP, measured by Finapres) responses after 10 min of circulatory arrest were measured in a control trial (n = 20) and when sympathetic discharge was increased by a cold pressor test (RHBF + cold pressor test; n = 10). Vascular conductance (VC) was calculated (VC = RHBF/MAP). In the control trial, peak RHBF at 5 s after circulatory arrest (34.1 +/- 2.5 vs. 48.9 +/- 4.3 ml . 100 ml-1 . min-1) and VC (0.34 +/- 0.02 vs. 0.53 +/- 0.05 ml . 100 ml-1 . min-1 . mmHg-1) were reduced in the post- compared with the pre-HDBR tests (P < 0. 05). Total excess RHBF over 3 min was diminished in the post- compared with the pre-HDBR trial (84.8 vs. 117 ml/100 ml, P < 0.002). The ability of the cold pressor test to lower forearm blood flow was less in the post- than in the pre-HDBR test (P < 0.05), despite similar increases in MAP. These data suggest that regulation of vascular dilation and the interaction between dilatory and constrictor influences were altered with bed rest.