INTRODUCTION

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HYPERVOLEMIA HAS BEEN KNOWN to be one of the most important determinants for maximal aerobic power (O_2 max) in humans (27). The hypervolemia by endurance training increases cardiac filling pressure (CFP), maximal stroke volume, and maximal cardiac output (7), leading to higher blood flow to actively contracting skeletal muscles. Although the lactate threshold is known to increase with O_2 max, equivalent to almost constant relative exercise intensity of ~60-70% O_2 max (25), the role of hypervolemia in lactate threshold has not been studied sufficiently.

There have been several studies suggesting the involvement of baroreflexes in lactate threshold (6, 21, 33). Connelly et al. (6) studied the influence of an acute increase in central venous pressure with head-out water immersion on sympathoadrenal and lactate responses to graded treadmill exercise in humans. They reported that the increase in venous return to the heart with water immersion reduced the sympathoadrenal and lactate responses to high-intensity exercise. Joyner et al. (20, 21) studied the effects of posture change on muscle blood flow (MBF) in the forearm during a graded rhythmic handgrip exercise and reported that the increase in MBF in response to exercise intensity was larger in the supine position than in the standing position. Later they reported that MBF and the O₂ consumption rate in the forearm increased more in subjects with local sympathetic nerve block by anesthetic block of the stellate ganglion than in intact subjects with less reduction in the pH of forearm venous blood during handgrip exercise (21). These findings suggest that the increase in CFP by water immersion or posture change increases MBF not only by elevated cardiac output but also by muscle vasodilation via baroreflexes, resulting in an increase in O₂ consumption and reduction in plasma lactate concentration ([Lac]_p).

However, it has been difficult to elucidate the contribution of sinoaortic and/or cardiopulmonary baroreceptors to the regulation of MBF, because the elevation in CFP usually causes sinoaortic baroreceptors as well as cardiopulmonary baroreceptors to stretch by increasing cardiac stroke volume and pulse pressure (PP) (6, 28). Continuous negative-pressure breathing (CNPB) has been used as a maneuver to stretch cardiopulmonary baroreceptors by increasing the negative pressure within the intrathoracic cavity (3, 38). CNPB increases stroke volume at rest and during exercise (3), reducing muscle sympathetic nerve activity and plasma norepinephrine levels in resting subjects (38). Recently, Nagashima et al. (28) reported that CNPB enhanced vasodilation of skin above an esophageal temperature of ~38°C during exercise in a hot environment. Because CNPB enhanced the thermal cutaneous vasodilation with significant reduction in mean arterial pressure (MAP) and with unchanged PP and heart rate (HR), they concluded that cardiopulmonary mechanoreceptors play a major role in the regulation of skin blood flow. Taken together, CNPB was expected to provide selective stretch of intrathoracic baroreceptors such as cardiopulmonary baroreceptors.

The purpose of the present study was to evaluate the effects of CNPB on MBF and [Lac]_p during dynamic exercise. Our hypotheses were that 1) the increase in CFP would attenuate the increase in [Lac]_p during dynamic exercise, 2) the increase in CFP would increase MBF during dynamic exercise, and 3) the stretch of cardiopulmonary baroreceptors would cause muscle vasodilation during dynamic exercise with concomitant reductions in MAP and sympathetic nerve activity.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The present study was approved by the Review Board on Human Experiments, Shinshu University School of Medicine. Subjects gave their written, informed consent before participating in the study.

First Series (Cycle Exercise)

Subjects. The physical characteristics of 10 male subjects (means ± SE) were: age, 27 ± 1 yr; height, 171 ± 1 cm; body weight, 66.2 ± 2.6 kg; O_2 max, 44.7 ± 2.5 ml · min¹ · kg body wt¹. O_2 max was determined in each subject before the experiment by using a cycle ergometer in an environmental chamber adjusted to an ambient temperature of 25°C and relative humidity of 60%.

Protocol. Experiments were conducted twice in each subject in an environment of 24.6 ± 1.4°C and relative humidity of 52 ± 17%: 1) exercise with CNPB (15 cmH₂O below ambient pressure; N1 trial) and 2) exercise with ambient pressure breathing (C1 trial). The order of experiments for each subject was randomized, with an interval between the trials of >1 wk.

Measurements. HR was recorded every 1 min from the trace of an electrocardiogram (Life Scope 8, Nihon Kohden, Tokyo, Japan). Systolic (SAP) and diastolic (DAP) arterial pressures were automatically measured every 1 min from the right upper arm placed at heart level, by inflating a cuff with an R-wave triggered sonometric pickup of Korotkoff's sound (STPB-780, Colin, Komaki, Japan). MAP was calculated as DAP + (SAP  DAP)/3.

Blood analysis. Four milliliters of blood were collected at the power requirements corresponding to 0%, 31.9 ± 2.0%, 60.9 ± 1.8%, 86.0 ± 0.9%, and 100% Ex_max. Blood was taken during the last 30 s of each exercise intensity period. A 1-ml aliquot of blood was used for determinations of hematocrit (microcentrifuge), hemoglobin concentration (cyanomethemoglobin), and total plasma protein concentration ([TP]_p) (refractometry). Three milliliters of aliquot were immediately centrifuged at room temperature, and the plasma was used for determination of the [Lac]_p (YSI 2300 Stat Plus, Yellow Springs, OH), sodium and potassium (480 flame photometer, Corning, Medfield, MA), and chloride (chloride analyzer 925, Corning). In 7 of 10 subjects, additional 7-ml aliquots of blood were collected (a total sampled volume of 11 ml). Four milliliters of aliquot were placed in a chilled tube (sodium EDTA 1.5 mg/ml), which was centrifuged at 4°C to determine plasma concentrations of epinephrine ([Epi]_p) and norepinephrine ([NE]_p). Three milliliters of aliquot were placed in another chilled tube (sodium EDTA trasylol) to determine plasma concentration of atrial natriuretic peptide ([ANP]_p). These samples were stored at 85°C until hormone assays were performed.

Second Series (Handgrip Exercise)

Subjects. The physical characteristics of the five male subjects were: age, 27 ± 1 yr; height, 171.8 ± 1.2 cm; body weight, 64.5 ± 3.7 kg; maximal handgrip work load (WL_max), 16.5 ± 1.7 kg. Five of ten subjects in the first series of experiments participated in the second series of experiments. To determine WL_max before the experiment, each subject was asked to perform a graded handgrip exercise until he could not maintain the rhythm of 30 contractions/min, using a pulley system that caused a 2.5-cm vertical displacement of a given weight while the weight was incremented by 2.5 kg every 1 min.

Protocol. Experiments were conducted twice in each subject in the same environment as the first series of experiments: 1) exercise with CNPB (15 cmH₂O below ambient pressure; N2 trial) and 2) exercise with ambient pressure breathing (C2 trial) as the time control of the N2 trial. Wearing only shorts and shoes, subjects entered the environmental chamber and rested in the sitting position for ~30 min while all measurement devices were applied. Subjects performed three exercise intensities: 30.0 ± 1.4%, 50.2 ± 0.8%, and 71.2 ± 0.6% of their WL_max. To compare the oxygenation state of forearm muscles during handgrip exercise with that in the lower extremities during cycle exercise, we measured venous O₂ saturation at 70% WL_max (270 CO-oximeter, Corning) in the subjects. The venous O₂ saturation decreased from 51.7 ± 5.2% at rest to 25.9 ± 4.1% at 3 min of 70% WL_max handgrip exercise, almost equal to the value reported in the femoral venous blood at 70% O_2 max during bicycle exercise (23).

Measurements of forearm MBF. Forearm MBF was measured by venous occlusion plethysmography (11). The wrist of the exercising arm was suspended from the ceiling, and a mercury-in-Silastic tube strain gauge was placed on the exercising forearm positioned above the heart level. Because forearm blood flow was reported to change by alteration of the arm position (32), the vertical level of the exercising arm in the N2 trial was carefully adjusted to that in the C2 trial. During measurement of forearm MBF, the circulation to the hand was arrested by inflation of a pneumatic wrist cuff to 300 mmHg to exclude the blood flow in the hand. At rest, forearm MBF was measured every 30 s from increasing rate of forearm volume with the Silastic-tube strain gauge by inflating a cuff around the upper arm to 60 mmHg. During exercise, forearm MBF was measured twice at the end of each minute during a 10- to 13-s pause in forearm contractions. When the wrist cuff was inflated to 300 mmHg at 2 s before the measurement of forearm MBF, subjects stopped contractions and relaxed the upper exercising limb immediately, the upper arm cuff was inflated, and forearm MBF was measured (20). Forearm vascular conductance (FVC) was calculated by dividing forearm MBF by MAP.

Other measurements. To estimate the contribution of forearm skin blood flow to forearm MBF, skin blood flow was measured at the end of each pause in the contractions by laser-Doppler flowmetry (ALF 21, Advance, Tokyo). The flow probe was located near the strain gauge. Skin blood flow was recorded continuously on a chart recorder, and the values just before the measurements of forearm MBF were adopted. The change in skin blood flow was expressed as the percent change from the resting values, which was not significantly different between the C2 and N2 trials. HR, SAP, and DAP were measured every minute as described above.

Statistics

	RESULTS

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First Series

View larger version (18K): [in this window] [in a new window]   Fig. 1.   Lactate concentration in plasma ([Lac]p) during graded cycle ergometer exercise with (, N1 trial) and without (, C1 trial) continuous negative-pressure breathing (CNPB) as a function of percent of maximal work intensity (%Exmax). Values are means ± SE for 10 subjects. ***Significant differences between trials; P < 0.001.

Table 1 shows cardiovascular variables in the N1 and C1 trials. Although SAP, DAP, and MAP in both trials increased with the increase in exercise intensities, the increases in the N1 trial were significantly attenuated with respect to those in the C1 trial at every intensity of exercise (P < 0.05) except for SAP at 100% Ex_max and DAP at 0% and 30% Ex_max. There were no significant differences in HR and PP between the trials except that HR at rest in the N1 trial increased significantly after application of CNPB (P < 0.05).

Table 2 shows the changes in plasma volume, [TP]_p, and hormonal concentrations in plasma during graded exercise in the N1 and C1 trials. Plasma volume for both trials decreased with the increase in exercise intensities, but the decrease in the N1 trial was slightly but significantly greater than that in the C1 trial at 60% and 100% Ex_max (P < 0.05). [TP]_p for both trials increased in response to exercise intensities, but the increase in the N1 trial was slightly but significantly less than that in the C1 trial at 90% Ex_max (P < 0.05). Both [Epi]_p and [NE]_p increased with exercise intensities, but the increases in the N1 trial were significantly attenuated compared with those in the C1 trial at every intensity of exercise (P < 0.05). [ANP]_p tended to increase with exercise intensities, and the increase in the N1 trial was more than that in the C1 trial with significant differences at 90% and 100% Ex_max (P < 0.05). There were no significant differences in electrolyte concentrations (sodium, potassium, and chloride) between the trials at any exercise intensity.

Second Series

View larger version (35K): [in this window] [in a new window]   Fig. 2.   Change in arterial blood pressure during handgrip exercise with (, N2 trial) and without (, C2 trial) CNPB as a function of %WLmax. Values are means ± SE for 10 subjects. SAP, systolic arterial blood pressure; DAP, diastolic arterial blood pressure; MAP, mean arterial blood pressure; WLmax, maximal workload. *, **, ***Significant differences between the C2 and N2 trials at the levels of P < 0.05, P < 0.01, and P < 0.001, respectively. Significant differences before and after CNPB at rest in the N2 trial at the level of P < 0.05.

As shown in Fig. 3, there were no significant differences in HR and PP between the trials at 30%, 50%, and 70% WL_max except for PP at rest during 70% WL_max.

View larger version (28K): [in this window] [in a new window]   Fig. 3.   Change in heart rate (HR) and pulse pressure (PP) during handgrip exercise with (, N2 trial) and without (, C2 trial) CNPB as a function of %WLmax. Values are means ± SE for 5 subjects. *Significant differences between the C2 and N2 trials: P < 0.05.

Figure 4 shows forearm MBF, FVC, and percent change in skin blood flow during handgrip exercise at 30%, 50%, and 70% WL_max. Forearm MBF at 30% WL_max in the N2 and C2 trials, respectively, increased from 1.56 ± 0.25 and 1.85 ± 0.10 ml · min¹ · 100 ml at rest to 21.26 ± 1.38 and 25.28 ± 2.27 ml · min¹ · 100 ml at the end of exercise with significant differences between the trials during 10 min of exercise (P < 0.001) except for 1, 2, and 5 min. Forearm MBF at 50% WL_max increased more rapidly and peaked at 33.03 ± 1.57 in the C2 and at 37.32 ± 3.17 ml · min¹ · 100 ml in the N2 trial at 8 min with significant differences from 3 to 8 min of exercise (P < 0.01). Forearm MBF at 70% WL_max increased most sharply compared with other exercise intensities, and maximal forearm MBF in the C2 and N2 trials were 41.38 ± 4.35 and 44.06 ± 4.84 ml · min¹ · 100 ml at the end of exercise, respectively, but without any significant differences between the trials (P = 0.09). There were no significant differences in the change in skin blood flow during exercise between the C2 and N2 trials.

View larger version (24K): [in this window] [in a new window]   Fig. 4.   Changes in forearm blood flow (FBF), forearm vascular conductance (FVC), and skin blood flow (SkBF) during handgrip exercise in the C2 () and N2 trials (). *, **, ***Significant differences between the trials: P < 0.05, P < 0.01, and P < 0.001, respectively. Significant differences before and after CNPB at rest in the N2 trial: P < 0.05.

Similar to the forearm MBF response, FVC in all trials increased gradually after the beginning of exercise, and the increasing rates were elevated with exercise intensities. The effect of CNPB was more prominent in FVC than that in forearm MBF. FVC at 30% WL_max in the N2 and C2 trials, respectively, increased from 0.018 ± 0.003 and 0.022 ± 0.001 at rest to 0.228 ± 0.007 and 0.287 ± 0.020 units by the end of exercise with significant differences between the trials from 1 to 10 min of exercise (P < 0.001). FVC at 50% WL_max increased to the maximal value of 0.395 ± 0.038 at 8 min in the N2 trial and 0.407 ± 0.030 units at 10 min in the C2 trial with significant differences from 3 to 9 min (P < 0.001). The maximal FVC at 70% WL_max were attained at 5 min: 0.427 ± 0.041 in the N2 trial and 0.399 ± 0.043 units in the C2 trial, with significant differences between the trials throughout exercise (P < 0.001).

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The major findings of the present study were that the increased CFP by CNPB 1) significantly attenuated the increase in [Lac]_p during dynamic exercise on a cycle ergometer, 2) enhanced the increase in forearm MBF during the rhythmic handgrip exercise, and 3) caused more muscle vasodilation with the reductions in arterial pressures and sympathetic nerve activity.

The maneuver of CNPB was used to increase venous return to the heart. Bjurstedt et al. (3) reported that cardiac output and stroke volume increased by ~15% of the baseline value during 15 cmH₂O of CNPB at rest and during exercise of 50% O_2 max in humans. In the present study, we confirmed that CNPB increased the end-diastolic left ventricular volume, measured with the echo Doppler ultrasound-imaging system at rest by ~10%. In addition, we also confirmed that the [ANP]_p in the N1 trial increased more than that in the C1 trial at 90% and 100% Ex_max, suggesting that the atrial wall during exercise was distended more in the N trials by CNPB (30). These findings support our idea that CFP increased by CNPB at rest and during dynamic exercise in the present study.

Because the systemic cardiovascular demands elicited in the two series of experiments were very different, the handgrip exercise would not reflect the MBF regulation in the lower extremities during cycling exercise. However, the aim of the handgrip experiment was to evaluate qualitatively the effects of increased CFP by CNPB on the muscle vasculature per se. We confirmed that O₂ saturation in the forearm venous blood was 26% at 70% WL_max, almost equal to the value reported in the femoral vein during cycling exercise at 70% O_2 max (23). Moreover, Bjurstedt et al. (3) reported that the relative increment of cardiac stroke volume by 15 cmH₂O of CNPB was not different between rest and exercise, suggesting that the relative contribution of stretched baroreceptors to systemic vasodilation was similar between the two series of experiments. This idea may be supported by the findings in the present study that the reduction in MAP by CNPB in the second experiment was similar to that in the first experiment. From these findings, it was suggested that the local oxygenation state in the forearm muscle in the second experiment reproduced that in the lower extremities in the first experiment and that relative effects of CNPB on muscular vasculature were not different between the experiments.

In the first series of experiments, the increase in MAP during exercise was significantly attenuated by CNPB despite the possible increase in cardiac output, suggesting that the increase in systemic vascular conductance by CNPB was relatively larger than that in cardiac output. This idea was supported by the second series of experiments, which showed that the increase in FVC during handgrip exercise was greater in the N2 trial than that in the C2 trial (Fig. 4). As shown in Table 2, the change in plasma catecholamine concentrations in the N1 trial was attenuated compared with those in the C1 trial, suggesting that sympathetic nerve activity should be attenuated by CNPB. Thus muscle vasodilation was caused by suppression of the vasoconstrictive effect of sympathetic nerve activity and/or by the vasodilatory effect of the enhanced ANP secretion (30).

Although the increase in MAP was significantly attenuated by CNPB, neither HR nor PP was altered by CNPB during exercise in either series of experiments. Johnson et al. (18) and Rowell (31) reported that forearm blood flow decreased in parallel with the reduction in right atrial pressure by lower body negative pressure (LBNP) above 20 mmHg, accompanied by a slight reduction in splanchnic blood flow but not by any significant changes in HR, MAP, and PP. In contrast, below 20 mmHg of LBNP, MAP decreased with LBNP, causing an increase in HR, a decrease in PP, and a more pronounced reduction in splanchnic blood flow. The findings obtained by Johnson et al. (18) and Rowell (31) suggest that vasoconstriction of the forearm is caused by an unloading of cardiopulmonary mechanoreceptors above 20 mmHg of LBNP, with unchanged HR and PP. Abboud et al. (1) examined the effect of neck suction on forearm and splanchnic vasoconstrictions induced by 40 mmHg of LBNP in supine subjects. They reported that the splanchnic vasoconstriction was attenuated by neck suction, whereas the forearm vasoconstriction was not altered. They concluded that forearm blood flow was regulated by cardiopulmonary baroreceptors rather than by carotid baroreceptors. Recently, Mack et al. (24) obtained similar findings in exercising subjects and suggested that cardiopulmonary baroreflex control of forearm blood flow was preserved during mild dynamic exercise. Again, because HR did not decrease and PP did not increase during CNPB in the present study, the muscle vasodilation was unlikely to be caused by stretch of arterial baroreceptors but more likely by cardiopulmonary mechanoreceptors.

One would speculate that sinoaortic baroreceptors would also be stretched by the decrease in intrathoracic pressure by CNPB, acting against the influence of the reduced MAP on carotid baroreceptors. In the present study, MAP decreased by ~10 mmHg in the N trials compared with that in the C trials. Bjurstedt et al. (3) reported that central venous pressure decreased by ~4-8 mmHg during CNPB of 15 cmH₂O despite a 150-ml increase in central blood volume at rest and during exercise, caused by the reduction in the intrathoracic pressure. Considering that pulmonary vascular compliance is 0.217 ml · kg¹ · mmHg (10), the decrease in central venous pressure was attenuated by 10 mmHg due to the increased central blood volume, assuming that the body weight of the subject is 66 kg. From these findings, we assumed that the negativity of intrathoracic pressure is approximately 14 to 18 mmHg, almost equal to 15 cmH₂O of the airway pressure. If so, the transmural pressure of the aortic arch in the N trials would be only ~5 mmHg higher than that in the C trials by subtraction of the 15 mmHg decrease in the intrathoracic pressure from the 10 mmHg decrease in MAP. It has been reported that the operating range and the sensitivity of baroreflex in dogs are similar in carotid and sinoaortic baroreceptors (2), or the sensitivity is much less in sinoaortic baroreceptors (15). In addition, Guz et al. (14) reported that sinoaortic baroreceptors had minimal blood pressure effects in humans. These findings suggest that the reduction of MAP was not caused by the stretch of sinoaortic baroreceptors overcoming the effect of unloading of carotid baroreceptors but by cardiopulmonary baroreceptors.

The increase in [Lac]_p during graded exercise was significantly attenuated in the N1 trial compared with that in the C1 trial (Fig. 1). One possible explanation for the attenuated increase in [Lac]_p by CNPB is that the removal of lactate ion by active muscles themselves and/or other tissues, such as liver, kidney, heart, or nonactive muscles (4, 5), may be accelerated by increased blood flows to each organ.

The recent studies using tracers suggest that lactate is preferentially used as fuel in oxidative muscle fibers of the active skeletal muscles (4, 5, 9, 19, 37), suggesting that the active muscles are major sites of lactate uptake as well as production (5, 9, 19, 37). A few studies have shown that the increased MBF accelerates the lactate uptake in the contracting muscle (9, 41). Gladden et al. (9) examined the effects of MBF on lactate uptake in the contracting muscles by perfusing resting and electrically stimulated canine muscles with varying lactate concentrations and flow rates of perfusate. Their findings suggested that lactate uptake increased with lactate concentration and flow rate of perfusate. Watt et al. (41) also examined the effects of perfusion rate on lactate uptake in the rat hindlimb muscles and obtained similar results.

Thomas et al. (39) compared the effects of lumbar sympathetic nerve stimulation on femoral blood flow during electrical stimulation between the oxidative muscle (the soleus) and the glycolytic muscle (the gastrocnemius-plantaris) in rats. They reported that sympathetic stimulation reduced the blood flow during contractions of the oxidative muscles but did not reduce the flow in glycolytic muscles, suggesting that blood flow in the oxidative muscles is more influenced by sympathetic nerve activity than that in the glycolytic muscles. Moreover, it has been reported that the oxidative fibers in skeletal muscles are abundant with monocarboxylate transporter 1, which is highly related to lactate uptake in skeletal muscles (22, 26). These findings suggest that the attenuation of sympathetic nerve activity by CNPB enhances vasodilation in the oxidative fibers in skeletal muscles during dynamic exercise, accelerating lactate uptake and attenuating the increase in [Lac]_p.

Although there have been few studies on the relationship between the lactate clearance rate and blood flow in splanchnic organs, the liver and kidney are known to uptake plasma lactate and produce gluconeogenesis from it (4, 5). In the present study, plasma volume in the N1 trial decreased more than that in the C1 trial at 60% and 100% Ex_max (Table 2), whereas the increase in [TP]_p in the N1 trial was less than that in the C1 trial at 90% Ex_max, suggesting that plasma protein moved from intra- to extravascular space more in the N1 trial than in the C1 trial. Because the permeability for plasma protein movement between the two spaces is higher in the splanchnic area (the liver and kidney) than in other areas (the muscle and skin; Ref. 13), the greater shift of plasma protein to extravascular space may be caused by an increase in splanchnic blood flow by CNPB. However, considering that cardiopulmonary baroreceptors were reported to exert a major influence on the neural control of skeletal muscle resistance in response to changes in central venous pressure and only a minimal influence on the splanchnic circulation in humans (1, 18, 31), and, moreover, that the major lactate uptake site may be the contracting muscles rather than other tissues during exercise (4, 19), the increased splanchnic blood flow may have a minor effect on lactate uptake.

Another possible explanation for the attenuated increase in [Lac]_p in the N1 trial is the reduction in lactate production in the contracting muscles. The lactate production is likely to depend on the balance between the rates of glycolysis and oxidative phosphorylation in the mitochondria. Higher rates in glycolysis than in oxidative phosphorylation may facilitate lactate production. In contrast, higher rates of phosphorylation than of glycolysis may reduce lactate production.

There have been several studies showing the involvement of epinephrine in increasing lactate production by accelerating glycolysis during exercise (6, 8, 35). Stainsby et al. (35) examined whether intravenous infusion of epinephrine and norepinephrine, together or alone, would increase lactate release from electrically stimulated muscles in the lower extremities of anesthetized dogs and reported that lactate output increased with epinephrine concentration in the femoral arterial blood. However, it might be difficult to compare the findings of their study with those of the present study because their findings were obtained during muscle contraction by electrical stimulation in anesthetized dogs, whereas our findings were obtained during voluntary muscle contraction in exercising humans. Moreover, [Epi]_p in their study was approximately five- to eightfold higher than that in the present study. The vasodilation in the active muscles induced by the higher [Epi]_p through ₂-adrenergic receptors may overcome the vasoconstrictive effect by norepinephrine through ₁-adrenergic receptors. Thus their results were obtained under much different experimental conditions from ours.

Febbraio et al. (8) studied the effects of increased epinephrine on intramuscular glucose metabolism by intravenous infusion of epinephrine during exercise and reported that the infusion increased glycogen utilization and lactate accumulation in active muscles after 40 min of exercise at 70% O_2 max. However, they reported that the infusion showed no significant influence on [Lac]_p after 10 min of exercise when [Epi]_p was ~130 pg/ml higher in the infusion trial than that in the noninfusion trial. In the present study, the difference in [Lac]_p between the trials appeared at least 9 min after the beginning of exercise at the lower intensity of exercise at 60% Ex_max, and at a smaller difference in [Epi]_p (26 pg/ml at 60% Ex_max, than in the previous study; Ref. 8). In addition, [NE]_p in the N1 trial was significantly reduced compared with that in the C1 trial, whereas [NE]_p in the previous study (8) was not significantly different between the infusion and noninfusion trials. Thus [Epi]_p in the present study was much lower than that in the previous studies, suggesting that the reduction in [Lac]_p by CNPB was caused by increased MBF due to attenuated sympathetic nervous outflow rather than glycolytic effects of epinephrine.

The lactate production in the contracting muscles may be also increased by a mass effect of pyruvate (16) when pyruvate production exceeds pyruvate oxidation through glycolysis accelerated by other activators than [Epi]_p: Ca²⁺, ADP, AMP, and Pi concentrations in cytosol (16, 34, 36). However, Wasserman et al. (40) reported that [Lac]_p and the ratio of [Lac]_p to plasma pyruvate concentration started to increase almost at the same exercise intensity and that the increase in plasma pyruvate concentration was delayed and occurred at higher exercise intensity, during graded exercise on a cycle ergometer. They concluded that the increase in lactate concentration in plasma was caused not by a mass effect of pyruvate but by the oxygenation state of contracting muscles.

We observed that by CNPB the increase in [Lac]_p was attenuated during graded cycle exercise (Fig. 1), and blood flow in the actively contracting muscles was increased during graded handgrip exercise (Fig. 4). A close relationship among the [Lac]_p, oxygenation state, and MBF has been suggested in several studies (21, 33). Joyner et al. (21) reported that the reduction in pH in forearm venous blood during handgrip exercise was attenuated and O₂ consumption rate was increased when MBF was increased by posture change from standing to supine or by local anesthetic block on sympathetic nerves to the forearm. Shoemaker et al. (33) studied the effects of the LBNP on the oxygenation state of the forearm during graded handgrip exercise. They reported that the reductions in creatinine phosphate concentration and pH in the active muscles were enhanced when brachial arterial blood flow was reduced by LBNP, which accompanied larger increased lactate concentrations and more reduced O₂ saturation of hemoglobin in the forearm venous blood. In addition, Hogan et al. (17) assessed the effect of O₂ delivery, decreased by reduction of MBF or arterial O₂ pressure, on O₂ consumption rate in the electrically stimulated dog gastrocnemius muscle. They reported that the O₂ consumption rate was reduced with the reduction in O₂ delivery irrespective of ischemia or hypoxemia. They also reported that the O₂ consumption rate was closely correlated with change in any of the proposed regulators of mitochondrial respiration; ADP, P_i, and ATP/(ADP × Pi), measured with a ³¹P-NMR study. These findings suggest that the increased O₂ delivery improves the oxygenation state in the active muscles, which may result in reduced lactate production by increasing oxidative phosphorylation.

In summary, in the present study, the increase in CFP by CNPB attenuated the increase in [Lac]_p during dynamic exercise on the cycle ergometer and increased blood flow to actively contracting muscles during handgrip exercise. The stretch of intrathoracic baroreceptors such as cardiopulmonary baroreceptors caused muscle vasodilation during dynamic exercise with concomitant reductions in MAP and sympathetic nerve activity, resulting in enhanced contracting MBF. The attenuation of the increase in [Lac]_p might be at least partially explained by the increased MBF.