Muscle blood flow and muscle metabolism during exercise and heat stress

The effect of heat stress on blood flow and metabolism in an exercising leg was studied in seven subjects walking uphill (12-17%) at 5 km/h on a treadmill for 90 min or until exhaustion. The first 30 min of exercise were performed in a cool environment (18-21 degrees C); then subjects moved to an adjacent room at 40 degrees C and continued to exercise at the same speed and inclination for a further 60 min or to exhaustion, whichever occurred first. The rate of O2 consumption, 2.6 l/min (1.8-3.3) (average from cool and hot conditions), corresponded to 55-77% of their individual maximums. In the cool environment a steady state was reached at 30 min. When the subjects were shifted to the hot room, the core temperature and heart rate started to rise and reached values greater than 39 degrees C and near-maximal values, respectively, at the termination of the exercise. The leg blood flow (thermodilution method), femoral arteriovenous O2 difference, and consequently leg O2 consumption were unchanged in the hot compared with the cool condition. There was no increase in release of lactate and no reduction in glucose and free net fatty acid uptake in the exercising leg in the heat. Furthermore, the rate of glycogen utilization in the gastrocnemius muscle was not elevated in the hot environment. There was a tendency for cardiac output to increase in the heat (mean 15.2 to 18.4 l/min), which may have contributed to the increase in skin circulation, together with a possible further reduction in flow to other vascular beds, because muscle blood flow was not reduced.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ocular blood flow decreases during passive heat stress in resting humans

Background

Heat stress induces various physiological changes and so could influence ocular circulation. This study examined the effect of heat stress on ocular blood flow.

Findings

Ocular blood flow, end-tidal carbon dioxide (P_ETCO₂) and blood pressure were measured for 12 healthy subjects wearing water-perfused tube-lined suits under two conditions of water circulation: (1) at 35°C (normothermia) for 30 min and (2) at 50°C for 90 min (passive heat stress). The blood-flow velocities in the superior temporal retinal arteriole (STRA), superior nasal retinal arteriole (SNRA), and the retinal and choroidal vessels (RCV) were measured using laser-speckle flowgraphy. Blood flow in the STRA and SNRA was calculated from the integral of a cross-sectional map of blood velocity. P_ETCO₂ was clamped at the normothermia level by adding 5% CO₂ to the inspired gas. Passive heat stress had no effect on the subjects’ blood pressures. The blood-flow velocity in the RCV was significantly lower after 30, 60 and 90 min of passive heat stress than the normothermic level, with a peak decrease of 18 ± 3% (mean ± SE) at 90 min. Blood flow in the STRA and SNRA decreased significantly after 90 min of passive heat stress conditions, with peak decreases of 14 ± 3% and 14 ± 4%, respectively.

Conclusion

The findings of this study suggest that passive heat stress decreases ocular blood flow irrespective of the blood pressure or arterial partial pressure of CO₂.     

Cutaneous blood flow during exercise is higher in endurance-trained humans.

This study determined whether cutaneous blood flow during exercise is different in endurance-trained (Tr) compared with untrained (Untr) subjects. Ten Tr and ten Untr men (62.4 +/- 1.7 and 44.2 +/- 1.8 ml. kg(-1). min(-1), respectively; P < 0.05) underwent three 20-min cycling-exercise bouts at 50, 70, and 90% peak oxygen uptake in this order, with 30 min rest in between. The environmental conditions were neutral ( approximately 23-24 degrees C, 50% relative humidity, front and back fans at 2.5 m/s). Because of technical difficulties, only seven Tr and seven Untr subjects completed all forearm blood flow and laser-Doppler cutaneous blood flow (CBF) measurements. Albeit similar at rest, at the end of all three exercise bouts, forearm blood flow was approximately 40% higher in Tr compared with Untr subjects (50%: 4.64 +/- 0.50 vs. 3. 17 +/- 0.20, 70%: 6.17 +/- 0.61 vs. 4.41 +/- 0.37, 90%: 6.77 +/- 0. 62 vs. 5.01 +/- 0.37 ml. 100 ml(-1). min(-1), respectively; n = 7; all P < 0.05). CBF was also higher in Tr compared with Untr subjects at all relative intensities (n = 7; all P < 0.05). However, esophageal temperature was not different in Tr compared with Untr subjects at the end of any of the aforementioned exercise bouts (50%: 37.8 +/- 0.1 vs. 37.9 +/- 0.1 degrees C, 70%: 38.1 +/- 0.1 vs. 38.1 +/- 0.1 degrees C, and 90%: 38.8 +/- 0.1 vs. 38.6 +/- 0.1 degrees C, respectively). We conclude that a higher CBF may allow Tr subjects to achieve an esophageal temperature similar to that of Untr, despite their higher metabolic rates and thus higher heat production rates, during exercise at 50-90% peak oxygen uptake.     

Cerebral blood flow during static exercise in humans

Cerebral blood flow (CBF) was determined in humans at rest and during four consecutive unilateral static contractions of the knee extensors. Each contraction was maintained for 3 min 15 s with the subjects in a semisupine position. The contractions corresponded to 8, 16, 24, and 32% of the maximal voluntary contraction (MVC) and utilized alternate legs. CBF (measured by the 133Xe clearance technique) was expressed by a noncompartmental flow index (ISI). Heart rate and mean arterial pressure increased from resting values of 73 (55-80) beats/min and 88 (74-104) mmHg to 106 (86-138) beats/min and 124 (102-146) mmHg, respectively (P less than 0.0005), during the contraction at 32% MVC. Arterial PCO2 and central venous pressure did not change. Corrected to the average resting PCO2, CBF during control was 55 (35-73) ml.100 g-1.min-1 and remained constant during contractions. Cerebral vascular resistance increased from 1.5 (1.0-2.2) to 2.4 (1.4-3.0) mmHg. 100 g.min.ml-1 (P less than 0.025) at 32% of MVC. There was no difference in CBF between the two hemispheres at rest or during exercise. In contrast to dynamic leg exercise, static leg exercise is not associated with an increase in global CBF when measured by the 133Xe clearance technique.     

Muscle metabolism during exercise in the heat in unacclimatized and acclimatized humans

The effect of heat acclimatization on aerobic exercise tolerance in the heat and on subsequent sprint exercise performance was investigated. Before (UN) and after (ACC) 8 days of heat acclimatization, 10 male subjects performed a heat-exercise test (HET) consisting of 6 h of intermittent submaximal [50% of the maximal O2 uptake] exercise in the heat (39.7 degrees C dB, 31.0% relative humidity). A 45-s maximal cycle ride was performed before (sprint 1) and after (sprint 2) each HET. Mean muscle glycogen use during the HET was lower following acclimatization [ACC = 28.6 +/- 6.4 (SE) and UN = 57.4 +/- 5.1 mmol/kg; P less than 0.05]. No differences were noted between the UN and ACC trials with respect to blood glucose, lactate (LA), or respiratory exchange ratio. During the UN trial only, total work output during sprint 2 was reduced compared with sprint 1 (24.01 +/- 0.80 vs. 21.56 +/- 1.18 kJ; P less than 0.05). This reduction in sprint performance was associated with an attenuated fall in muscle pH following sprint 2 (6.86 vs. 6.67, P less than 0.05) and a reduced accumulation of LA in the blood. These data indicate that heat acclimatization produced a shift in fuel selection during submaximal exercise in the heat. The observed sparing of muscle glycogen may be associated with the enhanced ability to perform highly intense exercise following prolonged exertion in the heat.     

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)     

Laser-Doppler and plethysmographic skin blood flow during exercise and during acute heat stress in the sauna

Forearm skin blood flow was measured in six male subjects by laser-Doppler flowmetry (LDF) and venous occlusion plethysmography (VOP) during constant-load (125-200 W) upright bicycle exercise in a warm environment (X + SD, ta 34.6 +/- 0.2 degrees C) and during a 15 min sauna bath (ta 69.0 +/- 2.8 degrees C). During the sauna test the LDF values correlated well with the VOP measurements in the initial phase of active cutaneous vasodilation, after which the LDF values almost leveled off in spite of a steady increase in VOP measurements. During the exercise the mean VOP and LDF values rose in parallel with each other to steady state levels. The relationship between the results of the two methods proved to be nonlinear. It was concluded that different parameters were measured by VOP and LDF. The latter measured mainly the integrated velocity of blood flow in the outermost cutaneous tissue, and this velocity seemed to be partly dependent on the level of the arterial inflow (VOP), but also on the prevailing pressure-flow and pressure-volume relations in the cutaneous vascular bed.     

Muscle blood flow patterns during exercise in partially curarized rats

We studied the distribution of blood flow within and among muscles of partially curarized (40-100 micrograms/kg body wt) rats during preexercise and at 1 min of low-speed treadmill exercise (15 m/min). Glycogen loss in the deep red muscles and parts of muscles was significantly reduced in the curarized animals during exercise, indicating the fibers in these muscles were recruited to a lesser extent and/or had lower metabolisms than fibers in the same muscles of control rats. However, elevations in blood flow in the red muscles of the curarized rats were as great or greater than those in the control rats. Thus reduced recruitment and/or metabolism of the deep red muscle fibers of the curarized animals was not accompanied by reduced blood flow. These findings suggest a dissociation between red fiber metabolism and blood flow in the curarized rats during the 1st min of slow treadmill exercise and indicate that release of vasodilator substances or local physical factors associated with muscle fiber activity are not solely responsible for the initial hyperemia during exercise.     

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.     

Cerebral blood flow during submaximal and maximal dynamic exercise in humans

Cerebral blood flow (CBF) in humans was measured at rest and during dynamic exercise on a cycle ergometer corresponding to 56% (range 27-85) of maximal O2 uptake (VO2max). Exercise bouts were performed by 16 male and female subjects, lasted 15 min each, and were carried out in a semisupine position. CBF (133Xe clearance) was expressed as the initial slope index (ISI) and as the first compartment flow (F1). CBF at rest [ISI, 58 (range 45-73); F1, 76 (range 55-98) ml.100 g-1.min-1] increased during exercise [ISI to 79 (57-94) and F1 to 118 (75-164) ml.100 g-1.min-1, P less than 0.01]. CBF did not differ significantly between work loads from 32 (24-33) to 86% (74-96) of VO2max (n = 10). During exercise, mean arterial pressure increased from 84 (60-100) to 101 (78-124) Torr (P less than 0.01) and PCO2 remained unchanged [5.1 (4.6-5.6) vs. 5.4 (4.4-6.3) kPa, n = 6]. These results demonstrate a median increase of 31% (0-87) in CBF by ISI and a median increase of 58% (0-133) in CBF by F1 during dynamic exercise in humans.     

Skeletal muscle blood flow and flow heterogeneity during dynamic and isometric exercise in humans

The effects of dynamic and intermittent isometric knee extension exercises on skeletal muscle blood flow and flow heterogeneity were studied in seven healthy endurance-trained men. Regional muscle blood flow was measured using positron emission tomography (PET) and an [(15)O]H(2)O tracer, and electromyographic (EMG) activity was recorded in the quadriceps femoris (QF) muscle during submaximal intermittent isometric and dynamic exercises. QF blood flow was 61% (P = 0.002) higher during dynamic exercise. Interestingly, flow heterogeneity was 13% (P = 0.024) lower during dynamic compared with intermittent isometric exercise. EMG activity was significantly higher (P < 0.001) during dynamic exercise, and the change in EMG activity from isometric to dynamic exercise was tightly related to the change in blood flow in the vastus lateralis muscle (r = 0.98, P < 0.001) but not in the rectus femoris muscle (r = -0.09, P = 0.942). In conclusion, dynamic exercise causes higher and less heterogeneous blood flow than intermittent isometric exercise at the same exercise intensity. These responses are, at least partly, related to the increased EMG activity.     

Muscle blood flow and fiber activity in partially curarized rats during exercise

We previously reported that low doses of d-tubocurarine attenuated glycogen loss in red muscles of rats during treadmill walking but that the initial hyperemia in the muscles was normal. The present studies were performed to 1) determine with electromyography (EMG) whether red muscle fiber activity is reduced in walking, curarized rats and 2) study muscle blood flow and glycogen loss during running with different doses of curare (dose response). At 0.5 min of treadmill walking (15 m/min), integrated EMG in vastus intermedius (VI) muscle was reduced by an average of 18% in curarized (60 micrograms/kg) rats, although blood flow (measured with microspheres) was the same as in saline control rats. Comparison of blood flows and glycogen loss in quadriceps muscles at 1 min of treadmill running (30 m/min) with different curare doses (20-60 micrograms/kg) demonstrated that red muscle glycogen loss was inversely related to curare dose but that blood flows in the same muscles were unaffected by curare. These findings provide support for our previous conclusion that at the initiation of low to moderate treadmill exercise, red muscle blood flow is not proportional to the activity or metabolism of the muscle fibers.     

Regulation of subcutaneous adipose tissue blood flow during exercise in humans

Regulation of subcutaneous adipose tissue blood flow (ATBF) remains poorly elucidated in humans, especially during exercise. In the present study we tested the role of adenosine in the regulation of ATBF adjacent to active and inactive thigh muscles during intermittent isometric knee-extension exercise (1 s contraction followed by 2 s rest with workloads of 50, 100, and 150 N) in six healthy young women. ATBF was measured using positron emission tomography (PET) without and with unspecific adenosine receptor inhibitor theophylline infused intravenously. Adipose regions were localized from fused PET and magnetic resonance images. Blood flow in subcutaneous adipose tissue adjacent to active muscle increased from rest (1.0 ± 0.3 ml·100 g(-1)·min(-1)) to exercise (P < 0.001) and along with increasing exercise intensity (50 N = 4.1 ± 1.4, 100 N = 5.4 ± 1.8, and 150 N = 6.9 ± 3.0 ml·100 g(-1)·min(-1), P = 0.03 for the increase). In contrast, ATBF adjacent to inactive muscle remained at resting levels with all intensities (～1.0 ± 0.5 ml·100 g(-1)·min(-1)). During exercise theophylline prevented the increase in ATBF adjacent to active muscle especially during the highest exercise intensity (50 N = 4.3 ± 1.8 ml·100 g(-1)·min(-1), 100 N = 4.0 ± 1.5 ml·100 g(-1)·min(-1), and 150 N = 4.9 ± 1.8 ml·100 g(-1)·min(-1), P = 0.06 for an overall effect) but had no effect on blood flow adjacent to inactive muscle or adipose blood flow in resting contralateral leg. In conclusion, we report in the present study that 1) blood flow in subcutaneous adipose tissue of the leg is increased from rest to exercise in an exercise intensity-dependent manner, but only in the vicinity of working muscle, and 2) adenosine receptor antagonism attenuates this blood flow enhancement at the highest exercise intensities.     

Regional cerebral artery mean flow velocity and blood flow during dynamic exercise in humans.

Transcranial Doppler ultrasound-determined middle (MCA) and anterior (ACA) cerebral artery mean flow velocities (Vmean) and pulsatility indexes (PI) were measured during "no-load" [21, 60, and 102 revolutions/min (rpm)] and loaded cycling (30, 60, and 149 W) at approximately 60 rpm. At rest Vmean MCA was 51 (36-55) cm/s (median and range; n = 10) and Vmean ACA was 41 (36-49) cm/s (n = 7; P < 0.05). With no load on the cycle Vmean MCA increased 4 (2-36), 10 (0-47), and 27% (4-58) (P < 0.05) at the three pedaling frequencies, respectively; arterial PCO2 (PaCO2) remained constant. During loaded cycling the increases were 19 (6-42), 25 (2-45), and 32% (12-67) (P < 0.01), respectively, with only a minimal change in PaCO2. No significant changes were observed in Vmean ACA. Changes in Vmean MCA were similar to those recorded by the initial slope index (ISI) of the 133Xe clearance method (n = 11), which in turn were smaller than increases recorded by the fast-compartment flow. PI ACA followed PI MCA during no-load as well as loaded exercise and increased with work rate, perhaps reflecting an increase in pulse pressure from 56 (48-63) mmHg at rest to 109 (88-123) mmHg at 149 W (P < 0.01). Data demonstrate a graded increase in regional cerebral perfusion during dynamic exercise corresponding to the MCA territory.     

Effects of exercise and heat stress on regional blood flow in pregnant sheep

Radioactive microspheres were used to measure cardiac output and blood flow to most major tissues, including those in the pregnant uterus, in late-pregnant ewes at rest and during treadmill exercise (approximately 3-fold increase in metabolic rate for 30 min) in thermoneutral (TN) (dry bulb temperature (Tdb) = 13 degrees C, wet bulb temperature (Twb) = 10 degrees C) and mildly hot (MH) (Tdb = 40 degrees C, Twb = 27 degrees C) environments. Exercise caused major increases in blood flow to respiratory muscles, nonrespiratory limb muscles, and adipose tissue, and flow was decreased to some gastrointestinal tissues, spleen, pancreas, and to placental and nonplacental tissues in the pregnant uterus. Heat exposure had relatively little effect on these exercise-induced changes, except that flow was further increased in the respiratory muscles. Results are compared with those of a similar study on nonpregnant sheep in which changes in muscle, skin, and visceral flows during exercise were attenuated by heat exposure. It is suggested that redistribution of blood flow from the pregnant uterus, which in resting ewes took 22% of cardiac output, is a significant buffer against the potentially deleterious effects of combined exercise and heat stress on blood flow to exercising muscles and thermoregulatory tissues.     

Middle cerebral artery flow velocity and blood flow during exercise and muscle ischemia in humans.

Changes in middle cerebral artery flow velocity (Vmean), measured by transcranial Doppler ultrasound, were used to determine whether increases in mean arterial pressure (MAP) or brain activation enhance cerebral perfusion during exercise. We also evaluated the role of "central command," mechanoreceptors, and/or muscle "metaboreceptors" on cerebral perfusion. Ten healthy subjects performed two levels of dynamic exercise corresponding to a heart rate of 110 (range 89-134) and 148 (129-170) beats/min, respectively, and exhaustive one-legged static knee extension. Measurements were continued during 2-2.5 min of muscle ischemia. MAP increased similarly during static [114 (102-133) mmHg] and heavy dynamic exercise [121 (104-136) mmHg] and increased during muscle ischemia after dynamic exercise. During heavy dynamic exercise, Vmean increased 24% (10-47%; P less than 0.01) over approximately 3 min despite constant arterial carbon dioxide tension. In contrast, static exercise with a higher rate of perceived exertion [18 (13-20) vs. 15 (12-18) units; P less than 0.01] was associated with no significant change in Vmean. Muscle ischemia after exercise was not associated with an elevation in Vmean, and it did not provoke an increase in Vmean after static exercise. Changes in Vmean during exercise were similar to those recorded with the initial slope index of the 133Xe clearance method. The data show that middle cerebral artery mean flow velocity reflects changes in cerebral perfusion during exercise. Furthermore, they support the hypothesis that cerebral perfusion during exercise reflects an increase in brain activation that is independent of MAP, central command, and muscle metaboreceptors but is likely to depend on influence of mechanoreceptors.