Muscle blood flow and muscle metabolism during exercise and heat stress B. NIELSEN, August Krogh
G. SAVARD, E. A. RICHTER, M. HARGREAVES, AND B. SALTIN Copenhagen 0, Denmark Institute, University of Copenhagen, DK-2100
E. A. RICHTER, M. HARGREAVES, flow and muscle metabolism during exercise and heat stress. J. Appl. Physiol. 69(3): 1040-1046, 1990.-The effect of heat stress on blood flow and metabolism in an exercising leg was studied in seven subjects walking uphill (12-17s) 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°C); then subjects moved to an adjacent room at 40°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 5% 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 >39”C and near-maximal values, respectively, at the termination of the exercise. The leg blood flow (thermsdilution method), femoral arteriovenous 02 difference, and consequently leg On 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. We conclude that a flow limitation to the exercising muscle, and subsequent altered metabolism, is not what makes it impossible to continue the exercise in the heat in the present circumstances. Instead, it may be that the high core temperature has an effect on the central nervous system in reducing mental drive for motor performance. NIELSEN, EL, G. SAVARD, AND EL SALTIN. Muscle blood
circulation;
core temperature;
oxygen consumption
DUIXING EXERCISE in hot conditions the increased demand for blood flow to the skin for thermoregulatory heat transport is met by a redistribution of cardiac output, resulting in a reduction in splanchnic and renal blood flow (reviewed in Refs. 15 and 16). The problem that remains to be clarified is whether the blood supply to the working muscles is reduced, as has been postulated previously (2O), and whether a reduced muscle perfusion contributes then, by altered muscle metabolism, to the reduction in work output and to the fatigue that limits performance in hot environments (e.g., Refs. 7 and 9). In a recent study we found that muscle blood flow was not reduced by heat stress imposed by a water-perfused suit during three types of exercise: one-legged knee ex1040
0161-7567/90
$1.50
Copyright
tension and two-legged seated or upright bicycling (23). The water-perfused suit creates highly artificial conditions. For example, sweat evaporation cannot take place, and skin temperature is clamped at levels higher than those reached in natural environments. Therefore, in the present experiments subjects walked uphill to the point of exhaustion in a hot room maintained at 40°C an environment in which the evaporation of sweat could contribute significantly to thermoregulation. We studied the effect of this heat stress and the gradually increasing core temperature on muscle blood flow and metabolic events in the working muscles. The following questions were addressed: 1) is limb blood flow reduced in the heat, and if so, is it this reduction in perfusion that limits performance in hot environments? 2) is it the resulting higher muscle temperature, interfering with chemical processes in the muscle, that leads to a faster depletion of energy stores (1 l)? and/or 3) are other effects of high core temperature (e.g., effects on central nervous function) contributing to the reduced work capacity? METHODS
Seven male subjects [age 25 yr (22-31), weight 75 kg (63-93), height 181 cm (177-195), maximal O2 uptake . WO 2 rnax)4.03 l/ min (3.46-4.90)] took part in the study. Each was fully informed of all procedures and risks involved before giving his consent to participate. The study was approved by the Copenhagen Ethics committee. Before the actual experiment the Vo2max of each subject for treadmill walking was determined. After a lomin warm-up on a level surface, walking at 5 km/h, the inclination was increased to 10% for 5 min and thereafter by 2.5% every 2 min until exhaustion. Heart rate (HR) and O2 consumption (Vo2) were measured at each step and continuously for the last 4-5 min. On a separate day, capacity of the subjects for exercising at 40°C environmental temperature was tested. An uphill walking speed that elicited -65% Vozmax was selected so that their core temperature surpassed 39°C and their HR reached near-maximal values at the end of a 60-min exercise period. Procedure. The subjects arrived in the morning after an overnight fast and lay down on a bed. Catheters were then placed in the femoral vein (proximal to vena saphena in 4 subjects and retrograde in 3 subjects), the right brachial vein, and the left brachial artery. After catheterization, six skin thermocouples were placed on
0 1990 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (192.236.036.029) on August 14, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
MUSCLE
BLOOD
FLOW
DURING
the chest, abdomen, shoulder, low back, forearm, and front of the thigh. Esophageal (core) temperature (T,,) was measured by a thermocouple placed in the esophagus at the level of the heart. In one subject tympanic temmeasurements were taken instead of perature &np) esophageal. Radiolabeled serum albumin ( 13’1-RISA;
G. SAVARD, E. A. RICHTER, M. HARGREAVES, AND B. SALTIN Copenhagen 0, Denmark Institute, University of Copenhagen, DK-2100
E. A. RICHTER, M. HARGREAVES, flow and muscle metabolism during exercise and heat stress. J. Appl. Physiol. 69(3): 1040-1046, 1990.-The effect of heat stress on blood flow and metabolism in an exercising leg was studied in seven subjects walking uphill (12-17s) 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°C); then subjects moved to an adjacent room at 40°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 5% 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 >39”C and near-maximal values, respectively, at the termination of the exercise. The leg blood flow (thermsdilution method), femoral arteriovenous 02 difference, and consequently leg On 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. We conclude that a flow limitation to the exercising muscle, and subsequent altered metabolism, is not what makes it impossible to continue the exercise in the heat in the present circumstances. Instead, it may be that the high core temperature has an effect on the central nervous system in reducing mental drive for motor performance. NIELSEN, EL, G. SAVARD, AND EL SALTIN. Muscle blood
circulation;
core temperature;
oxygen consumption
DUIXING EXERCISE in hot conditions the increased demand for blood flow to the skin for thermoregulatory heat transport is met by a redistribution of cardiac output, resulting in a reduction in splanchnic and renal blood flow (reviewed in Refs. 15 and 16). The problem that remains to be clarified is whether the blood supply to the working muscles is reduced, as has been postulated previously (2O), and whether a reduced muscle perfusion contributes then, by altered muscle metabolism, to the reduction in work output and to the fatigue that limits performance in hot environments (e.g., Refs. 7 and 9). In a recent study we found that muscle blood flow was not reduced by heat stress imposed by a water-perfused suit during three types of exercise: one-legged knee ex1040
0161-7567/90
$1.50
Copyright
tension and two-legged seated or upright bicycling (23). The water-perfused suit creates highly artificial conditions. For example, sweat evaporation cannot take place, and skin temperature is clamped at levels higher than those reached in natural environments. Therefore, in the present experiments subjects walked uphill to the point of exhaustion in a hot room maintained at 40°C an environment in which the evaporation of sweat could contribute significantly to thermoregulation. We studied the effect of this heat stress and the gradually increasing core temperature on muscle blood flow and metabolic events in the working muscles. The following questions were addressed: 1) is limb blood flow reduced in the heat, and if so, is it this reduction in perfusion that limits performance in hot environments? 2) is it the resulting higher muscle temperature, interfering with chemical processes in the muscle, that leads to a faster depletion of energy stores (1 l)? and/or 3) are other effects of high core temperature (e.g., effects on central nervous function) contributing to the reduced work capacity? METHODS
Seven male subjects [age 25 yr (22-31), weight 75 kg (63-93), height 181 cm (177-195), maximal O2 uptake . WO 2 rnax)4.03 l/ min (3.46-4.90)] took part in the study. Each was fully informed of all procedures and risks involved before giving his consent to participate. The study was approved by the Copenhagen Ethics committee. Before the actual experiment the Vo2max of each subject for treadmill walking was determined. After a lomin warm-up on a level surface, walking at 5 km/h, the inclination was increased to 10% for 5 min and thereafter by 2.5% every 2 min until exhaustion. Heart rate (HR) and O2 consumption (Vo2) were measured at each step and continuously for the last 4-5 min. On a separate day, capacity of the subjects for exercising at 40°C environmental temperature was tested. An uphill walking speed that elicited -65% Vozmax was selected so that their core temperature surpassed 39°C and their HR reached near-maximal values at the end of a 60-min exercise period. Procedure. The subjects arrived in the morning after an overnight fast and lay down on a bed. Catheters were then placed in the femoral vein (proximal to vena saphena in 4 subjects and retrograde in 3 subjects), the right brachial vein, and the left brachial artery. After catheterization, six skin thermocouples were placed on
0 1990 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (192.236.036.029) on August 14, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
MUSCLE
BLOOD
FLOW
DURING
the chest, abdomen, shoulder, low back, forearm, and front of the thigh. Esophageal (core) temperature (T,,) was measured by a thermocouple placed in the esophagus at the level of the heart. In one subject tympanic temmeasurements were taken instead of perature &np) esophageal. Radiolabeled serum albumin ( 13’1-RISA;
