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J Appl Physiol 85: 627-634, 1998;
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Vol. 85, Issue 2, 627-634, August 1998

Lactate efflux from exercising human skeletal muscle: role of intracellular PO2

Russell S. Richardson1, Elizabeth A. Noyszewski2, John S. Leigh2, and Peter D. Wagner1

1 Department of Medicine, University of California, San Diego, La Jolla, California 92093-0623; and 2 Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6021

    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References

It remains controversial whether lactate formation during progressive dynamic exercise from submaximal to maximal effort is due to muscle hypoxia. To study this question, we used direct measures of arterial and femoral venous lactate concentration, a thermodilution blood flow technique, phosphorus magnetic resonance spectroscopy (MRS), and myoglobin (Mb) saturation measured by 1H nuclear MRS in six trained subjects performing single-leg quadriceps exercise. We calculated net lactate efflux from the muscle and intracellular PO2 with subjects breathing room air and 12% O2. Data were obtained at 50, 75, 90, and 100% of quadriceps maximal O2 consumption at each fraction of inspired O2. Mb saturation was significantly lower in hypoxia than in normoxia [40 ± 3 vs. 49 ± 3% (SE)] throughout incremental exercise to maximal work rate. With the assumption of a PO2 at which 50% of Mb-binding sites are bound with O2 of 3.2 Torr, Mb-associated PO2 averaged 3.1 ± 0.3 and 2.3 ± 0.2 Torr in normoxia and hypoxia, respectively. Net blood lactate efflux was unrelated to intracellular PO2 across the range of incremental exercise to maximum (r = 0.03 and 0.07 in normoxia and hypoxia, respectively) but linearly related to O2 consumption (r = 0.97 and 0.99 in normoxia and hypoxia, respectively) with a greater slope in 12% O2. Net lactate efflux was also linearly related to intracellular pH (r = 0.94 and 0.98 in normoxia and hypoxia, respectively). These data suggest that with increasing work rate, at a given fraction of inspired O2, lactate efflux is unrelated to muscle cytoplasmic PO2, yet the efflux is higher in hypoxia. Catecholamine values from comparable studies are included and indicate that lactate efflux in hypoxia may be due to systemic rather than intracellular hypoxia.

blood flow; myoglobin; magnetic resonance spectroscopy; quadriceps; anaerobic threshold; knee-extensor exercise; diffusional conductance

    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References

IN THE FIRST QUARTER of this century, Hill et al. (17) postulated that blood lactate concentration increased with progressive muscular work because of the inadequacy of O2 supply to support the metabolic requirements of the contracting muscles. In 1964 the term "anaerobic threshold" was coined by Wasserman and McIlroy (51) to describe this concept. Since then, despite controversy surrounding the interpretation of blood lactate data (7, 14, 44), arterial lactate concentration continues to be used in experimental and clinical studies to evaluate O2 supply.

Although previous animal studies have repeatedly suggested that lactate efflux may be unrelated to the intracellular PO2 (12, 13), these studies were unable to progressively measure changes in intracellular PO2, intracellular pH, and lactate efflux in vivo over a range of exercise intensities. Consequently, the relationship between these variables during incremental exercise to maximum, the scenario in which the anaerobic threshold was defined, has not been resolved.

Proton magnetic resonance spectroscopy (MRS) to determine myoglobin (Mb) saturation (49), unlike most previous techniques that address tissue oxygenation, is noninvasive, is without deleterious effects, can be repeatedly and relatively rapidly measured, and thus is suitable for in vivo human studies (34, 46). In combination with the functionally isolated human quadriceps muscle model (2, 33, 34), in which arterial and venous blood are sampled and blood flow is measured across an exercising muscle, these techniques provide the opportunity to study the relationships between intracellular and intravascular events in a functionally isolated human muscle in vivo.

Thus the purpose of this study was to utilize this combination of techniques to elucidate the in vivo relationship among intracellular PO2, pH, and net muscle lactate efflux in human skeletal muscle during progressive incremental exercise. To evaluate the role of inspired O2 on these variables and their relationships, exercise was performed in hypoxic (12% O2) and normoxic conditions. Additionally because as catecholamine data were not collected in the main study, we present the relationship between the sympathetic response and net muscle lactate efflux in another similar group of subjects also performing knee-extensor exercise in hypoxia and normoxia.

    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Single-leg knee-extensor data collected from the subjects who performed MRS and catheter studies (group 1) have been recently published (34) and provide the basis of this study. However, in this previous report, the important issues of net muscle lactate efflux and its interaction with intracellular PO2 were neither reported nor discussed. Additionally, these data have now been grouped according to levels of O2 consumption (VO2) and not, as reported previously, by work level. This excludes all the data from one lower work level in hypoxia, because there was no match with the normoxia data in terms of VO2. This method of matching has influenced interpretation of the data.

Normoxic and hypoxic single-leg knee-extensor data [collected during exercise from a different, but comparable group of subjects who performed only catheter studies (group 2)] are used here to relate the catecholamine response to hypoxia. Although the normoxic data have been previously published (32), the hypoxic catecholamine data were neither reported nor discussed.

For all subjects, informed consent was obtained according to policies of the University of California, San Diego, and/or the University of Pennsylvania. All subjects were healthy nonsmoking men and were competitive bicycle racers, regularly riding 200-400 mi/wk.

Group 1: MRS and Catheter Studies

The physical characteristics of these six subjects were as follows (means ± SE): age = 23.3 ± 1.7 yr, height = 181.2 ± 1.7 cm, weight = 73.8 ± 2.4 kg, and maximal VO2 (VO2 max) = 4.42 ± 0.14 l/min or 60.0 ± 1.7 ml · kg-1 · min-1.

Group 2: Catheter Studies Only

The physical characteristics of these five subjects were as follows (means ± SE): age = 23.2 ± 1.9 yr, height = 178.3 ± 1.8 cm, weight = 75.1 ± 2.3 kg, and VO2 max = 4.36 ± 0.06 l/min or 58.3 ± 1.4 ml · kg-1 · min-1. There were no significant differences in any of these variables between the two subject groups.

Exercise Model

Exercise was performed on a knee-extensor ergometer constructed from nonmetallic materials to allow its use in the human physiology laboratory in San Diego and the MRS facility in Philadelphia. An illustration of the apparatus is presented elsewhere (34).

Experimental Protocol

Group 1 subjects were studied in two locations: San Diego, CA and Philadelphia, PA. In San Diego, two catheters (radial artery and left femoral vein) and a thermocouple (left femoral vein) were placed using sterile technique as previously reported (29, 34, 35) to allow sampling of arterial and venous blood in conjunction with muscle blood flow measurement by the thermodilution technique (34, 43). Two 11- to 15-min bouts of exercise were then performed: 1) left leg quadriceps exercise during room air (21% O2) breathing and 2) left leg quadriceps exercise during 12% O2 breathing. The sequence of these exercise bouts across subjects was reversed in three subjects to avoid ordering effects. For each bout, exercise work rate was increased from 25 to 50 to 75 and then to 90 and 100% of normoxic maximum work rate, with data obtained at each level, except in hypoxia, in which subjects were unable to complete the final exercise levels. Because of the prototype nature of the ergometer, a classic work rate calculation was not possible, but, on the basis of previous work in our laboratory, the maximum work rate could be equated to 80-100 W, depending on the subject. Each level of exercise intensity lasted for ~2-3 min, resulting in a total exercise time of 8-15 min. Group 2 followed this portion of the protocol exactly as it was conducted in San Diego.

The samples of arterial and venous blood were used to measure PO2, PCO2, pH, O2 saturation, and hemoglobin and were corrected to femoral venous temperature. All measurements were made on a blood-gas analyzer and a CO-oximeter (models IL-1306 and IL-282, respectively, Instrumentation Laboratories, Lexington, MA). Blood lactate concentration was determined using a blood-lactate analyzer (model 1500, Yellow Springs Instrument). Net muscle lactate efflux was calculated as the product of blood flow and venoarterial lactate concentration difference. Plasma epinephrine and norepinephrine were assayed in duplicate in group 2 subjects by the method of Kennedy and Zeigler (25). Blood O2 concentration was calculated as 1.39 × hemoglobin concentration × measured O2 saturation + 0.003 × measured PO2. Arteriovenous O2 concentration difference was calculated from the difference in radial artery and femoral vein O2 concentration. This difference was then divided by arterial concentration to give O2 extraction. Leg VO2 was calculated as the product of arteriovenous O2 concentration difference and blood flow. By use of the intracellular PO2 values measured by MRS, mean capillary PO2 (<OVL>P</OVL>cO2) and muscle O2 conductance were calculated as described previously (34, 47). Pulmonary minute ventilation, VO2, and CO2 production were calculated by a commercially available software package (Consentius Technologies, Salt Lake, UT) integrated with a mass spectrometer (model MGA 1100, Perkin-Elmer) and a Fleisch no. 3 pneumotachograph (Hans Rudolph).

Within 2 wk of the San Diego exercise study, group 1 subjects were transported to Philadelphia, where the incremental exercise was reproduced in a 2.0-T Oxford imaging magnet (Fig. 1 in Ref. 34). Spectra were collected from the muscle region below the 7-cm-diameter surface coil double tuned to 1H (85.45 MHz) and 31P (34.59 MHz) placed over the rectus femoris portion of the quadriceps group (42), ~20-25 cm proximal to the knee. For these studies, this "sensitive region" was <100 cm3 of muscle, which isolated signal detection predominantly to the rectus femoris (1). Details of the theory behind O2-sensitive Mb signals have been published previously (6, 34). The fraction of deoxy-Mb (fdeoxy-Mb) was determined by normalizing signal areas to the average signal obtained during minutes 9 and 10 of cuff ischemia at suprasystolic pressure (270 mmHg). Intramuscular O2 is depleted within 6-8 min of occlusion (49). Therefore, the plateaued signals obtained during the last 2 min of cuff occlusion represent complete deoxygenation of Mb and may be used to estimate total Mb content within the muscle. Conversion to PO2 values was then calculated from the O2-binding curve for Mb
P<SC>o</SC><SUB>2</SUB> =<SUP> f</SUP>MbO<SUB>2</SUB> ∗ MbP<SUB>50</SUB>/<SUP>f</SUP>deoxy-Mb
= (1 −<SUP> f</SUP>deoxy-Mb) ∗ P<SUB>50</SUB>/<SUP>f</SUP>deoxy-Mb
where fMbO2 is the fraction of Mb that is oxygenated and P50 is PO2 at which 50% of the Mb-binding sites are bound with O2. The temperature-dependent Mb half-saturation (P50) of 3.2 Torr was used (39) on the basis of an approximate muscle temperature of 39°C (41). Inasmuch as we report saturation and Mb-associated PO2, the latter can be recalculated for any P50. Intracellular pH was calculated from the difference in parts per million between Pi and phosphocreatine peaks with use of the formula cited by Taylor et al. (45).

Statistical Analyses

Least-squares regression, repeated-measures ANOVA (Tukey post hoc), and t-test analyses were computed using a commercially available software package (Graphpad Instat, Graphpad Software). Variables were considered significantly different when P <=  0.05. Values are means ± SE.

    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Group 1: MRS and Catheter Studies

Leg VO2, blood lactate, and intracellular PO2 and pH. In normoxia and hypoxia, the incremental exercise test resulted in a linear increase in leg blood flow and VO2 (Table 1). If normalized to an estimated muscle mass of ~2.5 kg, these variables rose to high mass-specific levels (~375 ml · min-1 · 100 g-1) corresponding with previous reports for which this human exercise model was used (33, 35). To facilitate interpretation of these data, all variables were grouped by the percentage of normoxic VO2 max achieved (Table 1). Arterial lactate concentration in hypoxia and normoxia increased with progressive exercise to similar levels at maximum; however, for a given VO2, arterial lactate concentration was greater in hypoxia (Table 1). Net muscle lactate efflux increased linearly with VO2 (r = 0.97 and 0.99 in normoxia and hypoxia, respectively), and again the slope of this relationship was significantly greater in hypoxia (P < 0.05), indicating a large increase in lactate efflux in this condition (Fig. 1, Table 1). Interestingly, if these data are presented and analyzed as lactate efflux for a relative exercise intensity for hypoxia and normoxia, the difference in lactate efflux is no longer apparent; however, this does not discount the observed difference for an absolute VO2. Before the exercise protocol was begun, there was no discernible deoxy-Mb signal, and thus intracellular PO2 was on the flat part of the Mb-O2 dissociation curve and, therefore, was not measurable. However, during unweighted knee-extensor exercise the deoxy-Mb signal rose to an average of 38% of the maximal deoxy-Mb signal (corresponding to PO2 = 7 Torr) collected during the cuff occlusion (rest, cuff, and exercise spectra are illustrated in Ref. 34). As the exercise progressed, in normoxia the deoxy-Mb signal increased rapidly (within 20 s) to ~50% of the maximum signal Mb-associated PO2 [(PMbO2) = 3.1 Torr] and maintained this value until VO2 max (Fig. 1, Table 1). During hypoxic exercise the deoxy-Mb signal also increased rapidly to ~60% of the maximum signal (PMbO2 = 2.1 Torr) and maintained this value through VO2 max (Fig. 1, Table 1). Thus, as net lactate efflux increased from ~1 to 15 mmol/min and as arterial blood lactate concentration increased from 1.4 to >4 mmol, intracellular PO2 remained unchanged at both inspired O2 levels (but 1 Torr lower in hypoxia). Intracellular pH demonstrated a strong negative linear relationship with muscle lactate efflux (Fig. 2) and the intensity of exercise, as measured by VO2 (r = 0.94 and 0.98 in normoxia and hypoxia, respectively; Table 1). Thus in hypoxia there was a significantly lower intracellular pH for a given VO2. Additionally, the slope of the relationship between intracellular pH and net muscle lactate efflux was not different in normoxia and hypoxia, but the intercept was significantly reduced in normoxia (P < 0.05).

                              
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  		Table 1.   Major physiological variables measured during incremental knee-extensor exercise in group 1 subjects


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  		Fig. 1.   Net muscle lactate efflux and intracellular PO2 as a function of O2 consumption (VO2) in normoxia and hypoxia in group 1 subjects (for lactate efflux, r = 0.97 and 0.99 in normoxia and hypoxia, respectively, P < 0.05). VO2 max, maximal VO2.


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  		Fig. 2.   Relationship between net muscle lactate efflux and intracellular pH in hypoxia and normoxia in group 1 (r = 0.94 and 0.98 in normoxia and hypoxia, respectively; P > 0.05).

Arterial O2 content, <OVL>P</OVL>cO2, and diffusional conductance of O2. In normoxic and hypoxic exercise, arterial O2 content was independent of increasing work rate and, therefore, VO2, but it was lower in hypoxia, as expected (Table 1). In hypoxia, the arterial and venous PO2 were reduced in comparison to normoxia, and thus calculated <OVL>P</OVL>cO2 was also significantly reduced (Table 1). In hypoxia and normoxia, the calculated O2 conductance increased as the intensity of work increased (Table 1). Submaximally, the calculated O2 conductance was elevated in hypoxia in comparison to normoxia; however, at VO2 max the conductances were not different (Table 1).

Leg O2 delivery, pulmonary VO2, and heart rate. Leg O2 delivery was not different in hypoxia and normoxia, despite the fall in arterial O2 content due to the elevated leg blood flow in hypoxia (Table 1). Muscle VO2 max was higher in normoxia than in hypoxia, whereas pulmonary VO2 increased to the same level in both, but the rate of increase was significantly elevated in hypoxia (Table 1). Here it is important to recognize that pulmonary VO2 is often only of marginal scientific relevance in the knee-extensor model, because the muscle mass recruited during the exercise may be somewhat overshadowed by accessory muscles recruited for stabilizing the body, especially as one approaches maximum work (33). Thus, in this scenario, pulmonary VO2 may not accurately reflect knee-extensor VO2 (30). Additionally, inasmuch as maximal exercise in this model equates to only ~50% of pulmonary VO2 max in conventional whole body exercise paradigms, this variable may (as seen here) be unaffected by hypoxia, because at maximum quadriceps work the subject still has significant whole body VO2 reserve. Heart rate was elevated in hypoxia at a given muscle VO2, but maximum heart rate was not significantly different from that in normoxia (Table 1).

Group 2: Catheter Studies Only

These subjects demonstrated physiological responses to incremental knee-extensor exercise in hypoxia and normoxia quantitatively similar to the main data set reported above. Muscle VO2 max was significantly (P < 0.05) reduced in hypoxia (1.05 ± 0.06 l/min) in comparison to normoxia (1.42 ± 0.14 l/min), and arterial lactate and arterial epinephrine concentrations were significantly elevated at any given work rate in hypoxia (Fig. 3). Maximal leg VO2 values in group 1 subjects (0.98 and 1.31 l/min, hypoxia and normoxia, respectively; Table 1) were not different from those in group 2 (above). These data in conjunction with the similarity between the subject characteristics and protocol experienced by subjects in groups 1 and 2 support the use of data from group 2 to explain the lactate concentration differences between hypoxia and normoxia in both groups.


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  		Fig. 3.   A: relationships of net muscle lactate efflux and arterial epinephrine levels to VO2 in hypoxia and normoxia in group 2 subjects. B: relationship between net muscle lactate efflux and arterial epinephrine level is independent of inspired O2 concentration.

    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References

The major finding of this study is that intracellular PO2 remains constant during graded incremental exercise in humans (50-100% of muscle VO2 max) and is unrelated to the linear fall in intracellular pH and concomitant linear rise in net muscle lactate efflux. In addition, we found that a reduction in the fraction of inspired O2 (despite the same O2 delivery at any given muscle VO2) resulted in a significant reduction in intracellular PO2, which again remained constant during graded incremental exercise. Under these hypoxic conditions, the rate of fall in intracellular pH and the rate of muscle lactate efflux, both in relation to absolute VO2, were significantly increased. With respect to the concept of the "anaerobic" threshold, these data demonstrate that, during incremental exercise, skeletal muscle cells do not become anaerobic as lactate levels suddenly rise, since intracellular PO2 is well preserved at a constant level, even at maximal exercise. Thus our data illustrate the lack of a relationship among intracellular PO2, lactate efflux, and muscle pH. However, the observation that in hypoxia intracellular PO2 and muscle VO2 max are reduced and muscle lactate efflux is accelerated leaves open the possibility that intracellular PO2 may still play a role in modulating muscle metabolism and ultimately muscle fatigue.

O2 Availability

There is considerable circumstantial evidence to support the notion that lactate production is related to inadequate O2 availability during exercise (22, 50). However, until the present study, only limited evidence has indicated that this relationship may be spurious: Jobsis and Stainsby (19) studied the oxidation of NADH/NAD+ at rest and in lactate-producing muscle and found no difference. One would expect a reduction in the members of the respiratory chain (including the NADH/NAD+ pair) to coincide with increased lactate production if lactate output were caused by O2-limited oxidative phosphorylation. With another approach, Mb cryomicrospectroscopy in dog gracilis muscle, Connett et al. (11-13) were unable to find loci with a PO2 of <2 Torr. Inasmuch as previous investigations (10) suggested that the critical PO2 (PO2 crit), below which maximal mitochondrial rate is compromised (0.1-0.5 Torr), Connett et al. (13) concluded that the elevated lactate concentration must be caused by factors other than simply O2-limited mitochondrial ATP synthesis rate. The present data support and extend these latter observations by providing in vivo data in humans and suggest that average intracellular PO2 remains above PO2 crit, even at maximal exercise in hypoxia. However, the present data differ from the findings of Connett et al. (13) in two important respects: 1) our data revealed lower mean intracellular PO2 values (3 vs. 5.5 Torr), which suggests that more loci may be in the realm of the PO2 crit, and 2) their data clearly indicate that intracellular PO2 in contracting muscle declines as exercise intensity increases (from 9 to 5.5 Torr from 50 to 100% of VO2 max), whereas our measured intracellular PO2 remained consistently low across the same relative work rates (~3 Torr). Until the present observations, there has been a general agreement that PO2 in contracting muscles declines as exercise intensity increases (16). These differences could be methodological or could reflect species differences, but because there are no comparable data in intact humans, the resolution of these differences must await further investigation.

Intracellular PO2 in Hypoxia vs. Normoxia

At each level of VO2, an elevated muscle blood flow compensated for the reduced arterial O2 content in hypoxia, and consequently O2 delivery was not different from normoxia at each work intensity. This suggests that convective delivery of O2 was not responsible for the observed difference in intracellular PO2 between hypoxia and normoxia. However, the diffusive component of O2 transport was affected. For example, at maximal exercise, the arterial PO2 was decreased from 115 Torr (normoxia) to 46 Torr (hypoxia). This resulted in an 8-Torr difference in calculated <OVL>P</OVL>cO2 between the two conditions and a significant reduction in the mean gradient from blood to tissue (i.e., <OVL>P</OVL>cO2 - Mb-associated PO2) of 27.4 Torr in hypoxia vs. 34.4 Torr in normoxia. We previously hypothesized that at maximal exercise with the same diffusional conductance (DO2) in hypoxia and normoxia the observed fall in intracellular PO2 and leg VO2 may be explained by this fall in <OVL>P</OVL>cO2 on the basis of the laws of diffusion: VO2 = DO2(<OVL>P</OVL>cO2 - PmitoO2), where PmitoO2 is mitochondrial PO2 (34).

Blood Lactate Levels

In many human studies, lactate measurements are limited to arterial or arterialized lactate by technical and ethical constraints. It is pertinent here to recognize the difficulty in interpreting the changes in arterial lactate concentration, inasmuch as this quantity is strongly influenced by catecholamine levels and lactate clearance by liver, heart, and active and inactive skeletal muscle (9, 15, 37). As performed in the present study, the benefit of measuring arterial and venous lactate concentration across a single isolated muscle group is thus apparent (12, 31). However, previous studies utilizing the knee-extensor model have demonstrated a profound effect of arterial lactate concentration itself on muscle lactate efflux (32, 37). Richardson et al. (32) found a 60% reduction in lactate efflux from the quadriceps when maximal arm exercise was superimposed on maximal single-leg knee-extensor exercise. Richter et al. (37) observed a reversal from lactate efflux to lactate uptake in the quadriceps when arm exercise was added to moderate knee-extensor exercise. In both cases, this difference was attributed to the elevation of arterial lactate concentration when the conversion from a small to large muscle mass exercise occurred. Thus it is evident that, under certain conditions, even net muscle lactate efflux (although perhaps more insightful than arterial lactate concentration alone) may not reflect only the rate of glycolysis (40) and certainly is determined by the difference in lactate output and uptake by the quadriceps. Additionally, this highlights the usefulness of the knee-extensor model (3) as a modality with which to study exercising human muscle in vivo in a scenario where local muscle physiology is not clouded by the typical systemic changes associated with conventional large muscle mass exercise (36).

Catecholamine Response

A discussion of lactate efflux from exercising muscle would not be complete without recognizing the role of catecholamines in the stimulation of glycolysis (predominantly epinephrine via cAMP) and the subsequent relation to lactate production. There is strong positive correlation among blood lactate concentration, epinephrine concentration, exercise intensity (18, 28), and arterial O2 saturation (26), and thus the role of increased sympathetic drive in the progressive increase in net muscle lactate efflux in hypoxia and normoxia should not be overlooked. Blood was not analyzed for catecholamines in group 1, but arterial and venous epinephrine levels, net muscle lactate efflux, and quadriceps VO2 during single-leg knee-extensor exercise are available for group 2 (a comparable group of equally trained subjects in hypoxia and normoxia, see METHODS and RESULTS in Ref. 32) (Fig. 3). Figure 3A illustrates the elevated arterial epinephrine and net muscle lactate efflux in hypoxia in comparison to normoxia at a given quadriceps VO2. In Fig. 3B it is evident that the level of net muscle lactate efflux is closely related to arterial epinephrine levels and that this relationship is independent of percentage of inspired O2. Previously, a similar relationship between the rate of lactate appearance and arterial epinephrine level was reported in acute and chronic hypobaric hypoxia (8). Additionally, it has been documented that beta -adrenergic blockade results in a profound reduction in arterial blood lactate concentration (~50%) during exercise at altitude; however, it should be recognized that, in this case, similar patterns of lactate production were recorded with and without blockade, indicating that a sympathoadrenal response, although important, does not entirely account for lactate changes during exercise at altitude (27). These observations, in conjunction with the present lack of evidence of a relationship between intracellular PO2 and lactate efflux, add credence to the hypothesis that increased blood lactate levels may, to some extent, be influenced by elevated sympathetic drive during exercise, more so in hypoxia, rather than by a lower intracellular PO2 per se.

Lactate Efflux and Intracellular pH

It has previously been documented that lactate formation is dependent on the physiochemical buffering of the myoplasm (4, 23, 38). Estimated lactate production rates based on changes in intracellular pH are complicated by a dependence on many factors (creatine kinase reaction rate, muscle protein buffering of protons, bicarbonate concentration, lactate efflux, and increase in metabolites with acid dissociation constant values within the physiological range). It is possible to utilize 31P-MRS to model lactate efflux on the basis of pH and creatine phosphate recovery times (G. Walter, K. Vandenborne, M. Elliot, and J. S. Leigh, unpublished observations). However, the relationship between these recovery times and lactate efflux is critically dependent on the model of lactate-proton cotransporter used. Two contrasting models are currently proposed: a saturable lactate-proton cotransporter (12, 20, 21) and the more simplistic assumption that efflux is linearly dependent on intracellular pH (5, 24, 38). Although we recognize that we cannot conclusively describe the lactate kinetics, inasmuch as intracellular and intravascular lactate concentrations are not known, by combining 31P-MRS and venous-arterial lactate and flow measurements, the present data clearly support the latter model of an apparently linear relationship between intracellular pH and net muscle lactate efflux in hypoxia and normoxia from moderate- to maximal-effort knee-extensor exercise (Fig. 2). They show no suggestion of saturation. Additionally, these data suggest that when exercise is performed in a hypoxic environment, the intracellular pH is reduced to a greater extent than in normoxia for a given muscle lactate efflux.

Summary

This investigation illustrates that in hypoxic or normoxic exercise conditions net muscle lactate efflux is independent of intracellular PO2. The former increases while the latter remains constant during progressive incremental exercise. However, in hypoxia, intracellular PO2 is systematically decreased in comparison to normoxia, whereas the changes in intracellular pH and muscle lactate efflux are accelerated. Although the latter observations indicate that a role for intracellular PO2 as a modulator of metabolism cannot be ruled out, arterial epinephrine levels are closely related to skeletal muscle lactate efflux in normoxia and hypoxia and, thus, may be a major stimulus for the observed rise in muscle lactate efflux during progressively intense exercise and for the elevated lactate efflux in hypoxia. We would postulate that it is systemic and not intracellular PO2 that increases catecholamine responses in hypoxia and, therefore, is responsible for the correspondingly higher net lactate efflux. Finally, these data support a physiological model of lactate efflux that depends on intracellular pH, since these are apparently linearly related without evidence of a transport maximum.

    ACKNOWLEDGEMENTS

The authors thank the subjects who participated in the study, Drs. Michael C. Hogan and Glenn Walter for suggestions and insight into the present topic, Drs. Bruno Grassi, David Poole, Douglas Knight, Kipp Erickson, Ravinder Reddy, and Erik Insko for invaluable technical assistance, and Harrieth Wagner, Nick Busan, Jeffrey Struthers, and Alan Bonner for expert technical assistance.

    FOOTNOTES

R. S. Richardson was funded by a fellowship from the Parker B. Francis Fellowship Foundation during this research. This study was concurrently supported by National Institutes of Health Grant HL-17731 and Regional Resource Grant RR-02305.

Address for reprint requests: R. S. Richardson, Dept. of Medicine 0623A, 9500 Gilman Dr., University of California, San Diego, La Jolla, CA 92093-0623 (E-mail: RRICHARDSON{at}UCSD.EDU).

Received 3 October 1997; accepted in final form 24 March 1998.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

1.   Ackerman, J. J. H., T. H. Grove, G. G. Wong, D. G. Gadian, and G. K. Radda. Mapping of metabolites in whole animals by 31P NMR using surface coils. Nature 283: 167-170, 1980[Medline].

2.   Andersen, P., R. P. Adams, G. Sjogaard, A. Thorbe, and B. Saltin. Dynamic knee extension as a model for study of isolated exercising muscle in humans. J. Appl. Physiol. 59: 1647-1653, 1985[Abstract/Free Full Text].

3.   Andersen, P., and B. Saltin. Maximal perfusion of skeletal muscle in man. J. Physiol. (Lond.) 366: 233-249, 1985[Abstract/Free Full Text].

4.   Bangsbo, J., P. D. Gollnick, T. E. Graham, C. Juel, B. Kiens, M. Mizuno, and B. Saltin. Anaerobic energy production and O2 deficit-debt relationship during exhaustive exercise in humans. J. Physiol. (Lond.) 422: 539-559, 1990[Abstract/Free Full Text].

5.   Bangsbo, J., L. Johansen, T. Graham, and B. Saltin. Lactate and H+ effluxes from human skeletal muscles during intense, dynamic exercise. J. Physiol. (Lond.) 462: 114-133, 1993.

6.   Bertinini, I., and C. Luchinat. NMR Paramagnetic Molecules in Biological Systems. Menlo Park, CA: Benjamin/Cummings, 1986.

7.   Brooks, G. E. Current concepts in lactate exchange. Med. Sci. Sports Exerc. 23: 895-906, 1990.

8.   Brooks, G. A., G. E. Butterfield, R. R. Wolfe, B. M. Groves, R. S. Mazzeo, J. R. Sutton, E. E. Wolfel, and J. T. Reeves. Decreased reliance on lactate during exercise after acclimatization to 4,300 m. J. Appl. Physiol. 71: 333-341, 1991[Abstract/Free Full Text].

9.   Carlson, K. I., J. C. Marker, D. A. Arnall, M. L. Terry, and H. T. Young. Epinephrine is unessential for stimulation of liver glycogenolysis during exercise. J. Appl. Physiol. 58: 544-548, 1985[Abstract/Free Full Text].

10.   Chance, B., and B. Quistorff. Study of tissue oxygen gradients by single and multiple indicators. Adv. Exp. Med. Biol. 94: 331-338, 1978.

11.   Connett, R. J., T. E. J. Gayeski, and C. R. Honig. Lactate production in a pure red muscle in absence of anoxia: mechanisms and significance. Adv. Exp. Med. Biol. 159: 327-335, 1983[Medline].

12.   Connett, R. J., T. E. J. Gayeski, and C. R. Honig. Lactate efflux is unrelated to intracellular PO2 in working red skeletal muscle in situ. J. Appl. Physiol. 61: 402-408, 1986[Abstract/Free Full Text].

13.   Connett, R. J., T. E. J. Gayeski, and C. R. Honing. Lactate accumulation in fully aerobic, working, dog gracilis muscle. Am. J. Physiol. 246 (Heart Circ. Physiol. 15): H120-H128, 1984[Abstract/Free Full Text].

14.   Davis, J. A. Anaerobic threshold: review of the concept and directions for future research. Med. Sci. Sports Exerc. 17: 6-21, 1985[Medline].

15.   Donovan, C. M., and G. Brooks. Endurance training affects lactate clearance not lactate production. Am. J. Physiol. 244 (Endocrinol. Metab. 7): E82-E92, 1983.

16.   Gladden, L. B. Lactate transport and exchange during exercise. In: Handbook of Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 12, chapt. 14, p. 614-648.

17.   Hill, A. V., C. N. H. Long, and H. Lupton. Muscular exercise, lactic acid, and the supply and utilization of oxygen. IV. The oxygen debt at the end of exercise. Proc. R. Soc. Lond. B Biol. Sci. 97: 127-137, 1924.

18.   Hughson, R. L., H. J. Green, and M. T. Sharratt. Gas exchange, blood lactate, and plasma catecholamines during incremental exercise in hypoxia and normoxia. J. Appl. Physiol. 79: 1134-1141, 1995[Abstract/Free Full Text].

19.   Jobsis, F. F., and W. N. Stainsby. Oxidation of NADH during contractions of circulated skeletal muscle. Respir. Physiol. 4: 292-300, 1968[Medline].

20.   Juel, C. Intracellular pH recovery and lactate efflux in mouse soleus muscles stimulated in vitro: the involvement of sodium/proton exchange and a lactate carrier. Acta Physiol. Scand. 132: 363-371, 1988[Medline].

21.   Juel, C. Lactate/proton co-transport in skeletal muscle: regulation and importance for pH homeostasis. Acta Physiol. Scand. 156: 369-374, 1996[Medline].

22.   Katz, A., and K. Sahlin. Regulation of lactic acid production during exercise. J. Appl. Physiol. 65: 509-518, 1988[Abstract/Free Full Text].

23.   Kemp, G. J., and G. K. Radda. Quantitative interpretation of bioenergetic data from 31P and 1H magnetic resonance spectroscopic studies of skeletal muscle: an analytical review. Magn. Reson. Q. 10: 43-63, 1994[Medline].

24.   Kemp, G. J., C. H. Thompson, A. L. Sanderson, and G. K. Radda. pH control in rat skeletal muscle during exercise, recovery, and acute respiratory acidosis. Magn. Reson. Med. 31: 103-109, 1994[Medline].

25.   Kennedy, B., and M. G. Zeigler. A more sensitive and specific radioenzymatic assay for catecholamines. Life Sci. 47: 2143-2154, 1990[Medline].

26.   Mazzeo, R. S., P. R. Bender, G. A. Brooks, G. E. Butterfield, B. M. Groves, J. R. Sutton, E. E. Wolfel, and J. T. Reeves. Arterial catecholamine responses during exercise with acute and chronic high-altitude exposure. Am. J. Physiol. 261 (Endocrinol. Metab. 24): E419-E424, 1991[Abstract/Free Full Text].

27.   Mazzeo, R. S., G. A. Brooks, G. E. Butterfield, A. Cymerman, A. C. Roberts, M. Selland, E. E. Wolfel, and J. T. Reeves. beta -Adrenergic blockade does not prevent the lactate response to exercise after acclimatization to high altitude. J. Appl. Physiol. 76: 610-615, 1994[Abstract/Free Full Text].

28.   Oyono-Enguelle, S., J. Marbach, A. Heitz, C. Ott, M. Gartner, A. Pape, J. C. Vollmer, and H. Freund. Lactate removal ability and graded exercise in humans. J. Appl. Physiol. 68: 905-911, 1990[Abstract/Free Full Text].

29.   Poole, D. C., G. A. Gaesser, M. C. Hogan, D. R. Knight, and P. D. Wagner. Pulmonary and leg VO2 during submaximal exercise: implications for muscular efficiency. J. Appl. Physiol. 72: 805-810, 1992[Abstract/Free Full Text].

30.   Poole, D. C., and R. S. Richardson. Determinants of oxygen uptake: implications for exercise testing. Sports Med. 24: 308-320, 1997[Medline].

31.   Richardson, R. S., R. L. Frank, and L. J. Haseler. Dynamic knee-extensor and cycle exercise: functional MRI of muscular activity. Int. J. Sports Med. 19: 1-6, 1998[Medline].

32.   Richardson, R. S., B. Kennedy, D. R. Knight, and P. D. Wagner. High muscle blood flows are not attenuated by recruitment of additional muscle mass. Am. J. Physiol. 269 (Heart Circ. Physiol. 38): H1545-H1552, 1995[Abstract/Free Full Text].

33.   Richardson, R. S., D. R. Knight, D. C. Poole, S. S. Kurdak, M. C. Hogan, B. Grassi, and P. D. Wagner. Determinants of maximal exercise VO2 during single leg knee extensor exercise in humans. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H1453-H1461, 1995[Abstract/Free Full Text].

34.   Richardson, R. S., E. A. Noyszeski, K. F. Kendrick, J. S. Leigh, and P. D. Wagner. Myoglobin O2 desaturation during exercise: evidence of limited O2 transport. J. Clin. Invest. 96: 1916-1926, 1995.

35.   Richardson, R. S., D. C. Poole, D. R. Knight, S. S. Kurdak, M. C. Hogan, B. Grassi, E. C. Johnson, K. Kendrick, B. K. Erickson, and P. D. Wagner. High muscle blood flow in man: Is maximal O2 extraction compromised? J. Appl. Physiol. 75: 1911-1916, 1993[Abstract/Free Full Text].

36.   Richardson, R. S., and B. Saltin. New insights into the control of human muscle blood flow and metabolism studied in vivo. Med. Sci. Sports Exerc. 30: 28-33, 1998[Medline].

37.   Richter, E. A., B. Kiens, B. Saltin, N. J. Christensen, and G. Savard. Skeletal muscle glucose uptake during dynamic exercise in humans: role of muscle mass. Am. J. Physiol. 254 (Endocrinol. Metab. 17): E555-E561, 1988[Abstract/Free Full Text].

38.   Roos, A., and W. F. Boron. Intracellular pH. Physiol. Rev. 61: 297-421, 1981.

39.   Rossi-Fanelli, A., and E. Antonini. Studies of the oxygen and carbon monoxide equilibria of human myoglobin. Arch. Biochem. Biophys. 77: 478-492, 1958[Medline].

40.   Roth, D. A., and G. A. Brooks. Lactate transport is mediated by a membrane-bound carrier in rat skeletal muscle sarcolemmal vesicles. Arch. Biochem. Biophys. 279: 386-394, 1990[Medline].

41.   Saltin, B., and L. Hermansen. Esophageal, rectal, and muscle temperature during exercise. J. Appl. Physiol. 21: 1757-1762, 1966[Free Full Text].

42.   Schnall, M. D., V. H. Subramanian, J. S. Leigh, and B. Chance. A new double-tuned probe for concurrent 1H and 31P NMR. J. Magn. Reson. 65: 122-129, 1985.

43.   Slattery, M. L., and D. R. Jacobs, Jr. The inter-relationships of physical activity, physical fitness, and body measurements. Med. Sci. Sports Exerc. 19: 564-569, 1987[Medline].

44.   Stainsby, W. N., C. Sumners, and G. M. Andrew. Plasma catecholamines and their effect on blood lactate and muscle lactate output. J. Appl. Physiol. 57: 321-325, 1985.

45.   Taylor, D. J., P. Styles, P. M. Mathews, D. A. Arnold, D. G. Gadian, P. Bore, and G. K. Radda. Energetics of human muscle: exercise-induced ATP depletion. Magn. Reson. Med. 3: 44-54, 1986[Medline].

46.   Thomas, A., and P. G. Morris. The effects of NMR exposure on living organisms. A microbial assay. Br. J. Radiol. 54: 615-621, 1981[Abstract].

47.   Wagner, P. D. Gas exchange and peripheral diffusion limitation. Med. Sci. Sports Exerc. 24: 54-58, 1992[Medline].

49.   Wang, Z., E. A. Noyszewski, and J. S. Leigh. In vivo MRS measurement of deoxymyoglobin in human forearms. Magn. Reson. Med. 14: 562-567, 1990[Medline].

50.   Wasserman, K. The anaerobic threshold measurement to evaluate exercise performance. Am. Rev. Respir. Dis. 61: 260-270, 1984.

51.   Wasserman, K., and M. B. McIlroy. Detecting the threshold of anaerobic metabolism in cardiac patients during exercise. Am. J. Cardiol. 14: 844-859, 1964[Medline].

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