Calf blood flow and posture: Doppler ultrasound measurements during and after exercise.

To investigate the joint effects of body posture and calf muscle pump, the calf blood flow of eight healthy volunteers was measured with pulsed Doppler equipment during and after 3 min of rhythmic exercise on a calf ergometer in the supine, sitting, and standing postures. Muscle contractions seriously impeded calf blood flow. Consequently, blood flow occurred mainly between contractions and reached a plateau that lasted at least the final 100 s of each exercise series. After exercise the blood flow decreased much faster in the sitting and standing postures than in the supine posture. There was no difference in blood flow between various postures during the same submaximal exercise. However, subjects in the standing posture were able to perform exercise with a higher load than in the supine posture, and blood flow in the standing posture could become twice as high as in the supine posture. We conclude that calf blood flow is regulated according to needs; available perfusion pressure determined maximal blood flow and exercise; and compared with the supine posture, the standing posture and calf muscle pump increase the perfusion pressure.     

Exercise hyperemia and vasoconstrictor responses in humans with cystic fibrosis.

ATP released from circulating erythrocytes is a potential signal regulating muscle blood flow during exercise (exercise hyperemia), and intravascular ATP appears to blunt sympathetic vasoconstriction during exercise. Erythrocytes from patients with cystic fibrosis (CF) do not release ATP. The goal of the present study was to determine whether increases in forearm blood flow during exercise are blunted in CF patients and whether CF patients exhibit greater vasoconstrictor responsiveness during exercise. Nine control subjects and 10 CF patients who were free of other disease complications (approximately 96% O2 saturation) performed incremental rhythmic forearm exercise at 5, 10, and 15% of maximum handgrip strength for 21 min (7 min at each workload). We used a cold pressor test to evoke sympathetic vasoconstriction under resting conditions and at each exercise workload. As a control, subjects performed a second exercise bout without the cold pressor test. Continuous brachial artery blood velocity was monitored beat-to-beat, and vessel diameter was assessed by Doppler ultrasound. Artery diameter, as well as blood pressure, heart rate, and O2 saturation, was measured at steady-state exercise and at 1 min into the cold pressor stimulus. Blood pressure and heart rate responses to the forearm exercise and each cold pressor test were similar in both groups (P > 0.05). Contrary to our hypothesis, forearm blood flow (P = 0.91) and forearm vascular conductance (P = 0.82) were similar at rest and at each level of exercise between CF patients and controls. Additionally, there was no difference in the degree of sympathetic vasoconstriction between groups at rest and at each level of exercise (P = 0.22). Our results suggest that ATP released from the deformation of erythrocytes is not an obligatory signal for exercise hyperemia in human skeletal muscle.     

Effects of midodrine on exercise-induced hypotension and blood pressure recovery in autonomic failure.

We tested the hypothesis that the oral alpha1-adrenergic agonist, midodrine, would limit the fall in arterial pressure observed during exercise in patients with pure autonomic failure (PAF). Fourteen subjects with PAF underwent a stand test, incremental supine cycling exercise (25, 50, and 75 W), and ischemic calf exercise, before (control) and 1 h after ingesting 10 mg midodrine. Heart rate (ECG), beat-to-beat blood pressure (MAP, arterial catheter), cardiac output (Q, open-circuit acetylene breathing), forearm blood flow (FBF, Doppler ultrasound), and calf blood flow (CBF, venous occlusion plethysmography) were measured. The fall in MAP after standing for 2 min was similar ( approximately 60 mmHg; P = 0.62). Supine MAP immediately before cycling was greater after midodrine (124 +/- 6 vs 117 +/- 6 mmHg; P < 0.03), but cycling caused a workload-dependent hypotension (P < 0.001), whereas increases in Q were modest but similar. Midodrine increased MAP and total peripheral resistance (TPR) during exercise (P < 0.04), but the exercise-induced fall in MAP and TPR were similar during control and midodrine (P = 0.27 and 0.14). FBF during cycling was not significantly reduced by midodrine (P > 0.2). By contrast, recovery of MAP after cycling was faster (P < 0.04) after midodrine ( approximately 25 mmHg higher after 5 min). Ischemic calf exercise evoked similar peak CBF in both trials, but midodrine reduced the hyperemic response over 5 min of recovery (P < 0.02). We conclude midodrine improves blood pressure and TPR during exercise and dramatically improves the recovery of MAP after exercise.     

Regional blood flow during exercise in humans measured by near-infrared spectroscopy and indocyanine green.

Using near-infrared spectroscopy (NIRS) and the tracer indocyanine green (ICG), we quantified blood flow in calf muscle and around the Achilles tendon during plantar flexion (1-9 W). For comparison, blood flow in calf muscle was determined by dye dilution in combination with magnetic resonance imaging measures of muscle volume, and, for the peritendon region, blood flow was measured by (133)Xe washout. From rest to a peak load of 9 W, NIRS-ICG blood flow in calf muscle increased from 2.4+/-0.2 to 74+/-5 ml x 100 ml tissue(-1) x min(-1), similar to that measured by reverse dye (77+/-6 ml x 100 ml tissue(-1) x min(-1)). Achilles peritendon blood flow measured by NIRS-ICG rose with exercise from 2.2+/-0.5 to 15.1+/-0.2 ml x 100 ml(-1) x min(-1), which was similar to that determined by (133)Xe washout (2.0+/-0.6 to 14.6+/-0.3 ml x 100 ml tissue(-1) x min(-1)). This is the first study using NIRS and ICG to quantify regional tissue blood flow during exercise in humans. Due to its high spatial and temporal resolution, the technique may be useful for determining regional blood flow distribution and regulation during exercise in humans.     

Cerebral hemodynamic response to maximal exercise

The response of central and cerebral hemodynamics to a stepwise increase in dynamic exercise until failure was studied in healthy young men. Each subject was examined using Doppler ultrasound assessment of blood flow in the middle cerebral artery (MCA), Doppler echocardiography, and spiroergometry. Hemodynamic parameters were recorded before the study and during the last several seconds of each step of the dynamic exercise. The central hemodynamic and energy exchange parameters exhibited typical changes with increasing exercise intensity. The peak systolic blood flow velocity in the MCA increased only in response to exercise of a moderate intensity (1 W/kg body weight, 45% of the maximal oxygen uptake); the further increase in exercise intensity did not affect the blood flow velocity. The cerebral vascular resistance index at the last step of the exercise was 24% higher than at rest. The increase in the MCA resistance index during the exercise was moderately correlated with the increase in the pulse pressure and systolic blood pressure, whereas the increase in blood pressure was not related to the increase in the peak systolic blood flow velocity in the MCA in response to an exercise intensity at which the oxygen uptake was higher than 45% of its maximal value. The differences between the responses of central and cerebral hemodynamics to the stepwise increase in exercise intensity reflect the phenomenon of cerebral hemodynamic autoregulation.     

Vascular adaptation to 4 wk of deconditioning by unilateral lower limb suspension

Physical inactivity or deconditioning is an independent risk factor for atherosclerosis and cardiovascular disease. In contrast to exercise, the vascular changes that occur as a result of deconditioning have not been characterized. We used 4 wk of unilateral lower limb suspension (ULLS) to study arterial and venous adaptations to deconditioning. In contrast to previous studies, this model is not confounded by denervation or microgravity. Seven healthy subjects participated in the study. Arterial and venous characteristics of the legs were assessed by echo Doppler ultrasound and venous occlusion plethysmography. The diameter of the common and superficial femoral artery decreased by 12% after 4 wk of ULLS. Baseline calf blood flow, as measured by plethysmography, decreased from 2.1 +/- 0.2 to 1.6 +/- 0.2 ml.min(-1).dl tissue(-1). Both arterial diameter and calf blood flow returned to baseline values after 4 wk of recovery. There was no indication of a decrease in flow-mediated dilation of the superficial femoral artery after ULLS deconditioning. This means that functional adaptations to inactivity are not simply the inverse of adaptations to exercise. The venous pressure-volume curve is shifted downward after ULLS, without any effect on compliance. In conclusion, deconditioning by 4 wk of ULLS causes significant changes in both the arterial and the venous system.     

Reduction in extracellular muscle volume increases heart rate and blood pressure response to isometric exercise

To investigate the effect of local dehydration on heart rate and blood pressure during static exercise, six healthy male subjects performed exercise of the calf muscles with different extracellular volumes of the working muscles. Exercise consisted of 5 min of static calf muscle contractions at about 10% of maximal voluntary contraction. The body position during exercise was identical in all tests, i.e. supine with the knee joint 90 degrees flexed. During a 25-min pre-exercise period three different protocols were employed to manipulate the calf volume. In test A the subjects rested in the exercise position; in test B the body position was the same as in A but calf volumes were increased by venous congestion [cuffs inflated to 10.67 kPa (80 mmHg)]; in test C the calf volumes were decreased by lifting the calves about 40 cm above heart level with the subjects supine. To clamp the changed calf volumes in tests B and C, cuffs were inflated to 300 mmHg 5 min before the onset of exercise. This occlusion was maintained for 1 min after the termination of exercise. Compared to tests A and B, the reduced volume of test C led to significant increases in heart rate and blood pressure during exercise. Oxygen uptake did not exceed resting levels in tests B and C until the cuffs were deflated, indicating that only calf muscles contributed to the neurogenic peripheral drive. It is concluded that extracellular muscle volume plays a significant role in adjusting heart rate and blood pressure during static exercise.     

Effect of lower limb exercise on forearm vascular function: contribution of nitric oxide

We examined vascular function in an inactive muscle bed, the forearm, during lower limb exercise and determined the contribution of endothelium-derived nitric oxide (NO) to the hyperemic response. Eight young males were randomized to participate in two studies, each consisting of two bouts of lower limb exercise, separated by a 30-min recovery. Peak forearm blood flow (PFBF) and mean blood flow (MFBF) were continuously recorded at baseline and during exercise using continuous high-resolution vascular ultrasound and Doppler flow velocity measurement. During one session, the brachial artery was cannulated to allow continuous infusion of saline or N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthase. The alternate session was performed to control for possible effects of repeated exercise. At 60, 100, and 160 W, L-NMMA significantly decreased both PFBF and MFBF compared with the saline infusion. These results suggest that systemic production of NO occurs during exercise in resting vessel beds, which do not feed metabolically active tissue. This finding provides a plausible explanation for the antiatherogenic benefits of exercise.     

The oxygen delivery response to acute hypoxia during incremental knee extension exercise differs in active and trained males

Background

It is well known that hypoxic exercise in healthy individuals increases limb blood flow, leg oxygen extraction and limb vascular conductance during knee extension exercise. However, the effect of hypoxia on cardiac output, and total vascular conductance is less clear. Furthermore, the oxygen delivery response to hypoxic exercise in well trained individuals is not well known. Therefore our aim was to determine the cardiac output (Doppler echocardiography), vascular conductance, limb blood flow (Doppler echocardiography) and muscle oxygenation response during hypoxic knee extension in normally active and endurance-trained males.

Methods

Ten normally active and nine endurance-trained males (VO_2max = 46.1 and 65.5 mL/kg/min, respectively) performed 2 leg knee extension at 25, 50, 75 and 100% of their maximum intensity in both normoxic and hypoxic conditions (FIO₂ = 15%; randomized order). Results were analyzed with a 2-way mixed model ANOVA (group × intensity).

Results

The main finding was that in normally active individuals hypoxic sub-maximal exercise (25 – 75% of maximum intensity) brought about a 3 fold increase in limb blood flow but decreased stroke volume compared to normoxia. In the trained group there were no significant changes in stroke volume, cardiac output and limb blood flow at sub-maximal intensities (compared to normoxia). During maximal intensity hypoxic exercise limb blood flow increased approximately 300 mL/min compared to maximal intensity normoxic exercise.

Conclusion

Cardiorespiratory fitness likely influences the oxygen delivery response to hypoxic exercise both at a systemic and limb level. The increase in limb blood flow during maximal exercise in hypoxia (both active and trained individuals) suggests a hypoxic stimulus that is not present in normoxic conditions.

     

Differential effects of aging on limb blood flow in humans

Aging appears to attenuate leg blood flow during exercise; in contrast, such data are scant and do not support this contention in the arm. Therefore, to determine whether aging has differing effects on blood flow in the arm and leg, eight young (22 +/- 6 yr) and six old (71 +/- 15 yr) subjects separately performed dynamic knee extensor [0, 3, 6, 9 W; 20, 40, 60% maximal work rate (WRmax)] and handgrip exercise (3, 6, 9 kg at 0.5 Hz; 20, 40, 60% WRmax). Arterial diameter, blood velocity (Doppler ultrasound), and arterial blood pressure (radial tonometry) were measured simultaneously at each of the submaximal workloads. Quadriceps muscle mass was smaller in the old (1.6 +/- 0.1 kg) than the young (2.1 +/- 0.2 kg). When normalized for this difference in muscle mass, resting seated blood flow was similar in young and old subjects (young, 115 +/- 28; old, 114 +/- 39 ml x g(-1) x min(-1)). During exercise, blood flow and vascular conductance were attenuated in the old whether expressed in absolute terms for a given absolute workload or more appropriately expressed as blood flow per unit muscle mass at a given relative exercise intensity (young, 1,523 +/- 329; old, 1,340 +/- 157 ml x kg(-1) x min(-1) at 40% WRmax). In contrast, aging did not affect forearm muscle mass or attenuate rest or exercise blood flow or vascular conductance in the arm. In conclusion, aging induces limb-specific alterations in exercise blood flow regulation. These alterations result in reductions in leg blood flow during exercise but do not impact forearm blood flow.     

Measurement of forearm blood flow by venous occlusion plethysmography: influence of hand blood flow during sustained and intermittent isometric exercise

The requirement for using an arterial occlusion cuff at the wrist when measuring forearm blood flows by plethysmography was tested on a total of 8 subjects at rest and during and after sustained and intermittent isometric exercise. The contribution of the venous effluent from the hand to the forearm flow during exercise was challenged by immersing the arm in water at 20, 34, and 40 degrees C. Occlusion of the circulation to the hand reduced the blood flow through the resting forearm at all water temperatures. There was an inverse relationship between the temperature of the water and the proportion in the reduction of forearm blood flow upon inflation of the wrist-cuff, ranging from 45 to 19% at 20 degrees to 40 degrees C, respectively. However, during sustained isometric exercise at 10% of the subjects maximum voluntary contraction (MVC) there was no reduction in the measured forearm flow when an arterial occlusion cuff was inflated aroung the wrist. Similarly, there was no alteration in the blood flow measured 2 s after each of a series of intermittent isometric contractions exerted at 20% or 60% MVC for 2 s whether or not circulation to the hand was occluded nor of the post-exercise hyperemia following 1 min of sustained contraction at 40% MVC. These results indicate that a wrist-cuff is not required for accurate measurement of forearm blood flows during or after isometric exercise.     

Differential control of forearm and calf vascular resistance during one-leg exercise

The purpose of this study was to determine whether blood flow (BF) and vascular resistance (VR) are controlled differently in the nonactive arm and leg during submaximal rhythmic exercise. In eight healthy men we simultaneously measured BF to the forearm and calf (venous occlusion plethysmography) and arterial blood pressure (sphygmomanometry) and calculated whole limb VR before (control) and during 3 min of cycling with the contralateral leg at 38, 56, and 75% of peak one-leg O2 uptake (VO2). During the initial phase of exercise (0-1.5 min) at all work loads, BF increased and VR decreased in the forearm (P less than 0.05), whereas calf BF and VR remained at control levels. Thereafter, BF decreased and VR increased in parallel and progressive fashion in both limbs. At end exercise, forearm BF and VR were not different from control values (P greater than 0.05); however, in the calf, BF tended to be lower (P less than 0.05 at 75% peak VO2 only) and VR was higher (23 +/- 9, 44 +/- 14, and 88 +/- 23% above control at 38, 56, and 75% of peak VO2, respectively, all P less than 0.05). In a second series of studies, forearm and calf skin blood flow (laser-Doppler velocimetry) and arterial pressure were measured during the same levels of exercise in six of the subjects. Compared with control, skin BF was unchanged and VR was increased (P less than 0.05) in the forearm by end exercise at all work loads, whereas calf skin BF increased (P less than 0.05) and VR decreased (P less than 0.05). The present findings indicate that skeletal muscle and skin VR are controlled differently in the nonactive forearm and calf during the initial phase of rhythmic exercise with the contralateral leg. Skeletal muscle vasodilation occurs in the forearm but not in the calf; forearm skin vasoconstricts, whereas calf skin vasodilates. Finally, during exercise a time-dependent vasoconstriction occurs in the skeletal muscle of both limbs.     

Muscle contraction-blood flow interactions during upright knee extension exercise in humans.

To test for evidence of a muscle pump effect during steady-state upright submaximal knee extension exercise, seven male subjects performed seven discontinuous, incremental exercise stages (3 min/stage) at 40 contractions/min, at work rates ranging to 60-75% peak aerobic work rate. Cardiac cycle-averaged muscle blood flow (MBF) responses and contraction-averaged blood flow responses were calculated from continuous Doppler sonography of the femoral artery. Net contribution of the muscle pump was estimated by the difference between mean exercise blood flow (MBFM) and early recovery blood flow (MBFR). MBFM rose in proportion with increases in power output with no significant difference between the two methods of calculating MBF. For stages 1 and 5, MBFM was greater than MBFR; for all others, MBFM was similar to MBFR. For the lighter work rates (stages 1-4), there was no significant difference between exercise and early recovery mean arterial pressure (MAP). During stages 5-7, MAP was significantly higher during exercise and fell significantly early in recovery. From these results we conclude that 1) at the lightest work rate, the muscle pump had a net positive effect on MBFM, 2) during steady-state moderate exercise (stages 2-4) the net effect of rhythmic muscle contraction was neutral (i.e., the impedance due to muscle contraction was exactly offset by the potential enhancement during relaxation), and 3) at the three higher work rates tested (stages 5-7), any enhancement to flow during relaxation was insufficient to fully compensate for the contraction-induced impedance to muscle perfusion. This necessitated a higher MAP to achieve the MBFM.     

Effect of heat stress on muscle blood flow during dynamic handgrip exercise

During exercise in a hot environment, blood flow in the exercising muscles may be reduced in favour of the cutaneous circulation. The aim of our study was to examine whether an acute heat exposure (65-70 degrees C) in sauna conditions reduces the blood flow in forearm muscles during handgrip exercise in comparison to tests at thermoneutrality (25 degrees C). Nine healthy men performed dynamic handgrip exercise of the right hand by rhythmically squeezing a water-filled rubber tube at 13% (light), and at 34% (moderate) of maximal voluntary contraction. The left arm served as a control. The muscle blood flow was estimated as the difference in plethysmographic blood flow between the exercising and the control forearm. Skin blood flow was estimated by laser Doppler flowmetry in both forearms. Oesophageal temperature averaged 36.92 (SEM 0.08) degrees C at thermoneutrality, and 37.74 (SEM 0.07) degrees C (P less than 0.01) at the end of the heat stress. The corresponding values for heart rate were 58 (SEM 2) and 99 (SEM 5) beats.min-1 (P less than 0.01), respectively. At 25 degrees C, handgrip exercise increased blood flow in the exercising forearm above the control forearm by 6.0 (SEM 0.8) ml.100 ml-1.min-1 during light exercise, and by 17.9 (SEM 2.5) ml.100 ml-1.min-1 during moderate exercise. In the heat, the increases were significantly higher: 12.5 (SEM 2.2) ml.100 ml-1.min-1 at the light exercise level (P less than 0.01), and 32.2 (SEM 5.9) ml.100 ml-1.min-1 (P less than 0.05) at the moderate exercise level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of the cerebral hemodynamic response to rhythmic handgrip.

The response of the cerebral circulation to exercise has been studied with transcranial Doppler ultrasound (TCD) because this modality provides continuous measurements of blood velocity and is well suited for the exercise environment. The use of TCD as an index of cerebral blood flow, however, requires the assumption that the diameter of the insonated vessel is constant. Here, we examine this assumption for rhythmic handgrip using a spectral index designed to measure trends in vessel flow. Nineteen normal subjects were studied during 5 min of volitional maximum rhythmic right handgrip at 1 Hz. TCD velocities from both middle arteries (left and right), blood pressure, and end-tidal PCO(2) were recorded every 10 s. A spectral weighted sum was also calculated as a flow index (FI). Averages were computed from the last 2 min of handgrip. Relative changes in velocity, FI, and pressure were calculated. The validity of FI was tested by comparing the change in diameter derived from equations relating flow and diameter. Mean blood pressure increased 23.8 +/- 17.8% (SD), and velocity increased 13.3 +/- 9.8% (left) and 9.6 +/- 8.3% (right). Although the mean change in FI was small [2.0 +/- 18. 2% (left) and 4.7 +/- 29.7% (right)], the variation was high: some subjects showed a significant increase in FI and others a significant decrease. Diameter estimates from two equations relating flow and luminal area were not significantly different. Decreases in FI were associated with estimated diameter decreases of 10%. Our data suggest that the cerebral blood flow (CBF) response to rhythmic handgrip is heterogeneous and that middle cerebral artery flow can decrease in some subjects, in agreement with prior studies using the Kety-Schmidt technique. We speculate that the velocity increase is due to sympathetically mediated vasoconstriction rather than a ubiquitous flow increase. Our data suggest that the use of ordinary TCD velocities to interpret the CBF response during exercise may be invalid.     

Skin blood flow during incremental exercise in a thermoneutral and a hot dry environment

Eight physically fit men performed two incremental bicycle ergometer tests, one in an ambient temperature of 25 degrees C and the other at 40 degrees C. Oesophageal temperature (Tes) increased continuously throughout the tests up to 38.0 and 38.3 degrees C, respectively. In both environments, forearm blood flow (plethysmography) was linearly related to Tes above the Tes threshold for vasodilation, but at the heaviest work loads this relationship was clearly attenuated and therefore indicated skin vasoconstriction, which tended to be more pronounced at 25 degrees C. During recovery at 25 degrees C, in some subjects the forearm blood flow increased above the levels observed at the end of the graded exercise in spite of a decreasing Tes. Skin blood flow, measured by laser Doppler flow meter at the shoulder, was quantitatively different but, on average, seemed to reveal the same response pattern as the forearm blood flow. In spite of the higher level of skin blood flow in the heat, blood lactate accumulation did not differ between the two environments. The present results suggest that there is competition between skin vasoconstriction and vasodilation at heavy work rates, the former having precedence in a thermoneutral environment to increase muscle perfusion. During short-term graded exercise in a hot environment, skin vasoconstriction with other circulatory adjustments seems to be able to maintain adequate muscle perfusion at heavy work levels, but probably not during maximum exercise.     

Walking at moderate speed with heel-less shoes increases calf blood flow

This study was designed to examine the physiological and biochemical effects of wearing heel-less shoes over a wide range of walking speeds. Six male students wearing alternately regular shoes and heel-less shoes walked at the constant speeds of 60, 80, 100 and 120 m/min for 10 min on a treadmill at 0% grade. The average heart rate was higher during heel-less shoe trials than when subjects walked in regular shoes at each speed, but differences were not significant. The calf blood flow showed its highest mean value at 80 m/min when subjects walked in heel-less shoes, and at 100 m/min when they walked in regular shoes. However, at walking speeds higher than these, calf blood flow decreased for wearers of both types of shoes. The calf blood flow after 80 m/min was higher when walking in heel-less rather than regular shoes. Blood lactate concentration after walking in heel-less shoes at 120 m/min was significantly higher than basal level, but after walking in regular shoes it was unchanged from the level before walking. Noradrenaline concentration at 120 m/min while walking in heel-less shoes was significantly higher than while walking in regular shoes. In conclusion, walking exercise in heel-less shoes induced an increase of the calf blood flow at a moderate speed, and increased glycogen metabolism and noradrenaline secretion at a faster speed.     

Endothelin-1-mediated vasoconstriction at rest and during dynamic exercise in healthy humans

It is now generally accepted that alpha-adrenoreceptor-mediated vasoconstriction is attenuated during exercise, but the efficacy of nonadrenergic vasoconstrictor pathways during exercise remains unclear. Thus, in eight young (23 +/- 1 yr), healthy volunteers, we contrasted changes in leg blood flow (ultrasound Doppler) before and during intra-arterial infusion of the alpha(1)-adrenoreceptor agonist phenylephrine (PE) with that of the nonadrenergic endothelin A (ET(A))/ET(B) receptor agonist ET-1. Heart rate, arterial blood pressure, common femoral artery diameter, and mean blood velocity were measured at rest and during knee-extensor exercise at 20%, 40%, and 60% of maximal work rate (WR(max)). Drug infusion rates were adjusted for blood flow to maintain comparable doses across all subjects and conditions. At rest, PE infusion (8 ng x ml(-1) x min(-1)) provoked a rapid and significant decrease in leg blood flow (-51 +/- 3%) within 2.5 min. Resting ET-1 infusion (40 pg x ml(-1) x min(-1)) significantly decreased leg blood flow within 5 min, reaching a maximal vasoconstriction (-34 +/- 3%) after 25-30 min of continuous infusion. Compared with rest, an exercise intensity-dependent attenuation to PE-mediated vasoconstriction was observed (-18 +/- 5%, -7 +/- 2%, and -1 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). Vasoconstriction in response to ET-1 was also blunted in an exercise intensity-dependent manner (-13 +/- 3%, -7 +/- 4%, and 2 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). These findings support a significant contribution of ET-1 and alpha-adrenergic receptors in the regulation of skeletal muscle blood flow in the human leg at rest and suggest a similar, intensity-dependent "lysis" of peripheral ET and alpha-adrenergic vasoconstriction during dynamic exercise.     

Acute effects of dry immersion upon blood supply of calf muscles during upright exercise

Healthy males performed upright exercise before and after three days of dry immersion. During exercise, calf blood flow, heart rate, and oxygen consumption were measured. Immersion resulted in increased heart rate and oxygen ventilatory equivalent, and decreased calf blood flow. Implications for the alterations in blood flow dynamics are discussed.     

Adenosine release into venous plasma during free flow exercise

We measured adenosine release into venous plasma as an index of interstitial adenosine concentration during free flow exercise hyperemia. Isolated, blood-perfused dog calf muscles were stimulated at 6 Hz for 10 min at free flow. Plasma samples were collected before, during, and after the exercise period for analysis of plasma adenosine concentration [( ADO]) by HPLC. Adenosine release (Rado) was calculated as plasma flow times venous-arterial [ADO] difference. Rado (nmole/min/100 g) went from -0.1 +/- 0.1 at rest to 6.6 +/- 4.6 during 6-Hz exercise. Isoproterenol infusion, which caused an increase in blood flow equivalent to 6-Hz exercise, did not result in increased Rado. Infusion of the 5'-nucleotidase inhibitor, alpha, beta, methylene adenosine 5'-diphosphate (AOPCP) did not prevent the increase in Rado during exercise. These results support the hypothesis that interstitial adenosine concentration increases during sustained free flow twitch exercise and that this results in increased release of adenosine into venous plasma.