Effect of mild essential hypertension on control of forearm blood flow during exercise in the heat

Six essential hypertensive (resting mean arterial pressure, MAP greater than 110 mmHg) and eight normotensive (resting MAP less than 95 mmHg) men, aged 30-58 yr, were tested during 1 h of dynamic leg exercise in the heat. Environmental conditions were fixed at 38 degrees C dry-bulb temperature and 28 degrees C wet-bulb temperature; exercise intensity was preset to approximate 40% of each subject's maximal aerobic capacity (actual range 38-43%). Forearm blood flow (FBF) was measured by impedance plethysmography. The intergroup difference in arterial pressure was maintained but not increased or decreased during exercise in the heat. FBF increased in both groups, but the increase was significantly less for the hypertensive subjects. FBF showed a significant linear correlation (different from 0) with core temperature in seven of eight control subjects but in none of the hypertensive subjects. The magnitude of FBF increase was inversely proportional to resting MAP (r = -0.89). It was concluded that essential hypertensive subjects respond to exercise in the heat with a diminished FBF response related to an alteration in control relative to central (core temperature) influences. This may be due to an imbalance between thermal and nonthermal (baroreflex) mechanisms controlling cutaneous blood flow.     

Effect of hypovolemia on forearm vascular resistance control during exercise in the heat.

To determine the influence of hypovolemia on the control of forearm vascular resistance (FVR) during dynamic exercise, we studied five physically active men during 60 min of supine cycle ergometer exercise bouts at 35 degrees C in control (normovolemic) and hypovolemic conditions. Hypovolemia was achieved by 3 days of diuretic administration and resulted in an average decrease in plasma volume of 15.9%. Relative to normovolemia, hypovolemia caused an attenuation of the progressive rise in forearm blood flow (P less than 0.05) and an increase in heart rate (P less than 0.05) during exercise. Because mean arterial blood pressure during hypovolemic exercise was well maintained, the attenuation of forearm blood flow was due entirely to a relative increase in FVR. At the onset of dynamic exercise, FVR was increased significantly in control and hypovolemic conditions by 13.2 and 27.1 units, respectively. The increase in FVR was significantly different between control and hypovolemic conditions as well. We attributed the increased vasoconstrictor bias during hypovolemia to cardiopulmonary baroreceptor unloading and/or an increased sensitivity to cardiopulmonary baroreceptor unloading. We concluded that reduced blood flow to the periphery during exercise in the hypovolemic condition was caused entirely by an increase in vascular resistance, thereby preserving arterial blood pressure and adequate perfusion to the organs requiring increased flow.     

Effect of luteal phase elevation in core temperature on forearm blood flow during exercise

Kolka, Margaret A., and Lou A. Stephenson. Effect ofluteal phase elevation in core temperature on forearm blood flow duringexercise. J. Appl. Physiol. 82(4):1079-1083, 1997.Forearm blood flow (FBF) as an index of skinblood flow in the forearm was measured in five healthy women by venousocclusion plethysmography during leg exercise at 80% peak aerobicpower and ambient temperature of 35°C (relative humidity 22%;dew-point temperature 10°C). Resting esophagealtemperature (T_es) was 0.3 ± 0.1°C higher in the midluteal than in the early follicular phase ofthe menstrual cycle (P < 0.05).Resting FBF was not different between menstrual cycle phases. TheT_es threshold for onset of skinvasodilation was higher (37.4 ± 0.2°C) in midluteal than inearly follicular phase (37.0 ± 0.1°C; P < 0.05). The slope of the FBF toT_es relationship was not different between menstrual cycle phases (14.0 ± 4.2 ml · 100 ml¹ · min¹ · °C¹for early follicular and 16.3 ± 3.2 ml · 100 ml¹ · min¹ · °C¹for midluteal phase). Plateau FBF was higher during exercise inmidluteal (14.6 ± 2.2 ml · 100 ml¹ · min¹ · °C¹)compared with early follicular phase (10.9 ± 2.4 ml · 100 ml¹ · min¹ · °C¹;P < 0.05). The attenuation of theincrease in FBF to T_es occurred when T_es was 0.6°C higher andat higher FBF in midluteal than in early follicular experiments(P < 0.05). In summary, the FBF response is different during exercise in the two menstrual cycle phasesstudied. After the attenuation of the increase in FBF and whileT_es was still increasing, thegreater FBF in the midluteal phase may have been due to the effects ofincreased endogenous reproductive endocrines on the cutaneousvasculature.

     

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)     

Non-thermal influences on the control of skin blood flow have minimal effects on heat transfer during exercise

During exercise, circulatory reflexes ensure that the cardiac output is sufficiently elevated to meet the oxygen delivery requirements of the contracting skeletal muscles and the heat delivery requirements of the body to the skin. The latter requirements are met by increasing skin blood flow. These increases are largely driven by elevations in the body temperatures, although non-thermal effects on the control of skin blood flow occur in certain conditions. These effects are largely the consequence of high and/or low baroreflex stimulation. Even in the face of such non-thermal effects, which occur during exercise in the heat, the body's requirements for heat transfer from core to skin are largely met by the increased skin blood flow. Thus, non-thermal effects on the control of skin blood flow are relatively unimportant in the body's overall regulatory response to exercise.     

Regional distribution of blood flow during mild dynamic leg exercise in the baboon

Five chair-restrained baboons were trained with operant techniques and a food reward to perform dynamic leg exercise. Cardiac output and blood flows to most tissues were determined by radioactive microsphere distribution. After 2 min of exercise mean arterial blood pressure had increased by 11 +/- 3% (SE), heart rate by 34 +/- 7%, cardiac output by 50 +/- 12%, and O2 consumption by 157 +/- 17%. The blood flow to exercising leg muscle increased by 585 +/- 338% and to the myocardium by 35 +/- 19%. Blood flow to torso and limb skin fell by 38 +/- 4 and 38 +/- 6%, respectively, and similar reductions occurred in adipose tissue blood flow. Nonworking skeletal muscle blood flow decreased by 30 +/- 10%. Renal blood flow was lowered by 16 +/-2%. The lower visceral organs had more variable responses, but when grouped together total splanchnic blood flow fell by 21 +/- 9%. Blood flow to the brain was unchanged with exercise, whereas spinal cord perfusion increased 23 +/- 3%. Thus during short dynamic exercise baboons redistributed blood flow away from skin, fat, nonworking muscles, and visceral organs to supply the needs of exercising muscles. Our data suggest the baboon is a useful animal model for investigating vascular responses of tissues, such as torso skin, adipose, individual visceral organs, and the spinal cord, that cannot be examined in humans.     

Sympathetic control of the forearm blood flow in man during brief isometric contractions

Experiments were performed to assess the possible neurally mediated constriction in active skeletal muscle during isometric hand-grip contractions. Forearm blood flow was measured by venous occlusion plethysmography on 5 volunteers who exerted a series of repeated contractions of 4 s duration every 12 s at 60% of their maximum strength of fatigue. The blood flows increased initially, but then remained constant at 20-24 ml X min(-1) X 100 ml(-1) throughout the exercise even though mean arterial blood pressure reached 21-23 kPa (160-170 mm Hg). When the same exercise was performed after arterial infusion of phentolamine, forearm blood flow increased steadily to near maximal levels of 38.7 +/- 1.4 ml X min(-1) X 100 ml(-1). Venous catecholamines, principally norepinephrine, increased throughout exercise, reaching peak values of 983 +/- 258 pg X ml(-1) at fatigue. Of the vasoactive substances measured, the concentration of K+ and osmolarity in venous plasma also increased initially and reached a steady-state during the exercise but ATP increased steadily throughout the exercise. These data indicate a continually increasing alpha-adrenergic constriction to the vascular beds in active muscles in the human forearm during isometric exercise, that is only partially counteracted by vasoactive metabolites.     

Effect of blood volume on forearm venous and cardiac stroke volume during exercise

Five healthy men exercised at 65-70% of maximum O2 uptake (VO2 max) for 30 min in an ambient temperature of 30 degrees C. Duplicate experiments were conducted at three levels of plasma volume:control, hypovolemia, in which blood volume (BV) was reduced an average of 490 ml (9.7%) with diuretics, and hypervolemia, in which BV was increased an average of 440 ml (7.8%) by infusing an isotonic solution containing 5% human serum albumin. Marked venoconstriction occurred during exercise in all conditions and persisted despite large increases in deep body temperature. The degree of venoconstriction was similar during control and hypervolemic conditions, but was potentiated during hypovolemia. The observed venoconstriction appeared to consist of two components: an early one related to autonomic adjustments at the onset of exercise, and a later one possibly related to progressive decreases in cardiac filling. Heart rate, cardiac stroke volume (SV), and cardiac output during exercise were significantly affected by changes in BV. During hypovolemia the average differences from control values were 10 beats X min-1, -14 ml, and -2.2 l X min-1, respectively; during hypervolemia the differences from control were -7 X min-1, 10 ml, and 1.0 l X min-1, respectively. The pattern of SV over the course of exercise indicates that pooling of blood in veins may be quantitatively more important than plasma water loss in reducing cardiac filling pressure in the heat.     

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.     

Leg exercise conditioning increases peak forearm blood flow.

To examine whether forearm vascular adaptations could occur after upright-leg exercise training, the reactive hyperemic blood flow after 10 min of forearm circulatory arrest (RHBF10) was studied. RHBF10 was examined in seven subjects before, at 2 wk, and after the completion of 4 wk of bicycle ergometer training. Maximal O2 consumption (VO2max) for leg ergometer work increased 13% (P less than 0.05) over 4 wk. Over that period of time RHBF10 in the forearm increased 50% (P less than 0.05), with a reciprocal drop in minimum vascular resistance. Resting heart rate decreased 15% (P less than 0.05) during the same period. Changes in RHBF10 and VO2max were noted after 2 wk of training. Mean arterial pressure did not change. We conclude that vascular adaptations can occur in the forearm muscle beds, even though the training regimen is designed to condition the lower extremities.     

Age, fitness, and regional blood flow during exercise in the heat

Ho, C. W., J. L. Beard, P. A. Farrell, C. T. Minson, and W. L. Kenney. Age, fitness, and regional blood flow during exercisein the heat. J. Appl. Physiol. 82(4):1126-1135, 1997.During dynamic exercise in warm environments,the requisite increase in skin blood flow (SkBF) is supported by anincrease in cardiac output (c) and decreases insplanchnic (SBF) and renal blood flows (RBF). To examine interactionsbetween age and fitness in determining this integrated response, 24 men, i.e., 6 younger fit (YF), 6 younger sedentary (YS), 6 older fit (OF), and 6 older sedentary (OS) rested for 50 min, thenexercised at 35 and 60% maximalO₂ consumption(O_{2 max}) at36°C ambient temperature. YF had a significantly higherc and SkBF than any other group during exercise,but fitness level had no significant effect on any measured variable inthe older men. At 60%O_{2 max}, younger subjects had significantly greater decreases in SBF and RBF than theolder men, regardless of fitness level. Total flow redirected fromthese two vascular beds (SBF + RBF) followed YF >> YS > OF > OS. A rigorous 4-wk endurance training programincreased exercise SkBF in OS, but SBF and RBF were unchanged.Under these conditions, older men distribute cdifferently to regional circulations, i.e., smaller increases in SkBFand smaller decreases in SBF and RBF. In younger subjects, the higherSkBF associated with a higher fitness level is a function of both ahigher c and a greater redistribution of flow fromsplanchnic and renal circulations, but the attenuated splanchnic andrenal vasoconstriction in older men does not appear to change withenhanced aerobic fitness.

     

Effects of caffeine on blood pressure, heart rate, and forearm blood flow during dynamic leg exercise

This study examined the acute effects of caffeine on thecardiovascular system during dynamic leg exercise. Ten trained,caffeine-naive cyclists (7 women and 3 men) were studied at rest andduring bicycle ergometry before and after the ingestion of 6 mg/kgcaffeine or 6 mg/kg fructose (placebo) with 250 ml of water. Afterconsumption of caffeine or placebo, subjects either rested for 100 min(rest protocol) or rested for 45 min followed by 55 min of cycleergometry at 65% of maximal oxygen consumption (exercise protocol).Measurement of mean arterial pressure (MAP), forearm blood flow (FBF),heart rate, skin temperature, and rectal temperature and calculation offorearm vascular conductance (FVC) were made at baseline and at 20-minintervals. Plasma ANG II was measured at baseline and at 60 minpostingestion in the two exercise protocols. Before exercise, caffeineincreased both systolic blood pressure (17%) and MAP (11%) withoutaffecting FBF or FVC. During dynamic exercise, caffeine attenuated theincrease in FBF (53%) and FVC (50%) and accentuated exercise-inducedincreases in ANG II (44%). Systolic blood pressure and MAP were alsohigher during exercise plus caffeine; however, these increases weresecondary to the effects of caffeine on resting bloodpressure. No significant differences were observed inheart rate, skin temperature, or rectal temperature. These findingsindicate that caffeine can alter the cardiovascular response to dynamicexercise in a manner that may modify regional blood flow andconductance.

     

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)     

Failure of prostaglandins to modulate the time course of blood flow during dynamic forearm exercise in humans

Shoemaker, J. K., H. L. Naylor, Z. I. Pozeg, and R. L. Hughson. Failure of prostaglandins to modulate the time course ofblood flow during dynamic forearm exercise in humans.J. Appl. Physiol. 81(4):1516-1521, 1996.The time course and magnitude of increases inbrachial artery mean blood velocity (MBV; pulsed Doppler), diameter(D; echo Doppler), mean perfusionpressure (MPP; Finapres), shear rate ( = 8 ^· MBV/D), andforearm blood flow (FBF = MBV ^· r²)were assessed to investigate the effect that prostaglandins (PGs) haveon the hyperemic response on going from rest to rhythmic exercise inhumans. While supine, eight healthy men performed 5 min of dynamichandgrip exercise by alternately raising and lowering a 4.4-kg weight(~10% maximal voluntary contraction) with a work-to-rest cycle of1:1 (s/s). When the exercise was performed with the arm positionedbelow the heart, the rate of increase in MBV and wasfaster compared with the same exercise performed above the heart.Ibuprofen (Ibu; 1,200 mg/day, to reduce PG-induced vasodilation) andplacebo were administered orally for 2 days before two separate testingsessions in a double-blind manner. Resting heart rate was reduced inIbu (52 ± 3 beats/min) compared with placebo (57 ± 3 beats/min)(P < 0.05) without change to MPP.With placebo, D increased in both armpositions from ~4.3 mm at rest to ~4.5 mm at 5 min of exercise(P < 0.05). This response was notaltered with Ibu (P > 0.05). Ibudid not alter the time course of MBV or forearm blood flow(P > 0.05) in either arm position. The was significantly greater in Ibu vs. placebo at 30 and 40 s of above the heart exercise and for all time points after 25 sof below the heart exercise (P < 0.05). Because PG inhibition altered the time course of at the brachial artery, but not FBF, it was concludedthat PGs are not essential in regulating the blood flow responses todynamic exercise in humans.