Bicarbonate and fast-twitch muscle: evidence for a major role in pH regulation

The ins and outs of acid-base transport in skeletal muscle

Aalkjær and Nielsen discuss new data revealing the basis of acid–base transport in t-tubules of skeletal muscle.

NHE- and diffusion-dependent proton fluxes across the tubular system membranes of fast-twitch muscle fibers of the rat

The regulation of pH across the t-system membrane of skeletal muscle fibers is poorly understood. Using a sealed tubule preparation, Launikonis et al. reveal Na⁺/H⁺ exchange activity and characterize the properties of the diffusional and NHE proton fluxes across the t-system.

The complex membrane structure of the tubular system (t-system) in skeletal muscle fibers is open to the extracellular environment, which prevents measurements of H⁺ movement across its interface with the cytoplasm by conventional methods. Consequently, little is known about the t-system’s role in the regulation of cytoplasmic pH, which is different from extracellular pH. Here we describe a novel approach to measure H⁺-flux measurements across the t-system of fast-twitch fibers under different conditions. The approach involves loading the t-system of intact rat fast-twitch fibers with a strong pH buffer (20 mM HEPES) and pH-sensitive fluorescent probe (10 mM HPTS) before the t-system is sealed off. The pH changes in the t-system are then tracked by confocal microscopy after rapid changes in cytoplasmic ionic conditions. T-system sealing is achieved by removing the sarcolemma by microdissection (mechanical skinning), which causes the tubules to pinch off and seal tight. After this procedure, the t-system repolarizes to physiological levels and can be electrically stimulated when placed in K⁺-based solutions of cytosolic-like ionic composition. Using this approach, we show that the t-system of fast-twitch skeletal fibers displays amiloride-sensitive Na⁺/H⁺ exchange (NHE), which decreases markedly at alkaline cytosolic pH and has properties similar to that in mammalian cardiac myocytes. We observed mean values for NHE density and proton permeability coefficient of 339 pmol/m² of t-system membrane and 158 µm/s, respectively. We conclude that the cytosolic pH in intact resting muscle can be quantitatively explained with respect to extracellular pH by assuming that these values apply to the t-system membrane and the sarcolemma.

Influence of exercise on nutritional requirements

There is no consensus on the best diet for exercise, as many variables influence it. We propose an approach that is based on the total energy expenditure of exercise and the specific macro- and micronutrients used. di Prampero quantified the impact of intensity and duration on the energy cost of exercise. This can be used to determine the total energy needs and the balance of fats and carbohydrates (CHO). There are metabolic differences between sedentary and trained persons, thus the total energy intake to prevent overfeeding of sedentary persons and underfeeding athletes is important. During submaximal sustained exercise, fat oxidation (FO) plays an important role. This role is diminished and CHO's role increases as exercise intensity increases. At super-maximal exercise intensities, anaerobic glycolysis dominates. In the case of protein and micronutrients, specific recommendations are required. We propose that for submaximal exercise, the balance of CHO and fat favors fat for longer exercise and CHO for shorter exercise, while always maintaining the minimal requirements of each (CHO: 40% and fat: 30%). A case for higher protein (above 15%) as well as creatine supplementation for resistance exercise has been proposed. One may also consider increasing bicarbonate intake for exercise that relies on anaerobic glycolysis, whereas there appears to be little support for antioxidant supplementation. Insuring minimal levels of substrate will prevent exercise intolerance, while increasing some components may increase exercise tolerance.

Regulation of pH in human skeletal muscle: adaptations to physical activity

Regulation of pH in skeletal muscle is the sum of mechanisms involved in maintaining intracellular pH within the normal range. Aspects of pH regulation in human skeletal muscle have been studied with various techniques from analysis of membrane proteins, microdialysis, and the nuclear magnetic resonance technique to exercise experiments including blood sampling and muscle biopsies. The present review characterizes the cellular buffering system as well as the most important membrane transport systems involved (Na(+)/H(+) exchange, Na-bicarbonate co-transport and lactate/H(+) co-transport) and describes the contribution of each transport system in pH regulation at rest and during muscle activity. It is reported that the mechanisms involved in pH regulation can undergo adaptational changes in association with physical activity and that these changes are of functional importance.

Importance of pH regulation and lactate/H+transport capacity for work production during supramaximal exercise in humans

We examine the influence of the cytosolic and membrane-bound contents of carbonic anhydrase (CA; CAII, CAIII, CAIV, and CAXIV) and the muscle content of proteins involved in lactate and proton transport [monocarboxylate transporter (MCT) 1, MCT4, and Na+/H+exchanger 1 (NHE1)] on work capacity during supramaximal exercise. Eight healthy, sedentary subjects performed exercises at 120% of the work rate corresponding to maximal oxygen uptake (Ẇmax) until exhaustion in placebo (Con) and metabolic alkalosis (Alk) conditions. The total (Wtot) and supramaximal work performed (Wsup) was measured. Muscle biopsies were obtained before and immediately after standardized exercises (se) at 120% Ẇmaxin both conditions to determine the content of the targeted proteins, the decrease in muscle pH (ΔpHm), and the muscle lactate accumulation ([Lac]m) per joule of Wsup(ΔpHm/Wsup-seand Δ[Lac]m/Wsup-se, respectively) and the dynamic buffer capacity. In Con, Wsupwas negatively correlated with ΔpHm/Wsup-se, positively correlated with Δ[Lac]m/Wsup-seand MCT1, and tended to be positively correlated with MCT4 and NHE1. CAII + CAIII were correlated positively with ΔpHm/Wsup-seand negatively with Δ[Lac]m/Wsup-se, while CAIV was positively related to Wtot. The changes in Wsupwith Alk were correlated positively with those in dynamic buffer capacity and negatively with Wsupin Con. Performance improvement with Alk was greater in subjects having a low content of proteins involved in pH regulation and lactate/proton transport. These results show the importance of pH regulating mechanisms and lactate/proton transport on work capacity and the role of the CA to delay decrease in pHmand accumulation in [Lac]mduring supramaximal exercise in humans.

Intracellular pH regulation in isolated fast-twitch skeletal muscle from dystrophin-deficient mouse

In muscles from anaesthetized dystrophin-deficient mdx mice, exercise results in a stronger acidification and a slower intracellular pH recovery compared to control mice. We examined whether this observation could be attributed to defective H+-carriers in dystrophin-lacking muscles. Immunohistochemistry and Western blots revealed no defect in mdx muscles for the presence of the lactate-/H+co-transporter MCT4 and of the Na+/H+ antiporter NHE1, the main H+-carriers active in fast-twitch skeletal muscle after exercise. Functional tests of the H+-transporters, on isolated muscles submitted to identical flow of superfusion, were performed in conditions meant to lower intracellular pH: repetitive electrical stimulation or NH4Cl pre-pulse. These revealed no defect in intracellular pH recovery in mdx muscles. Therefore, we conclude that impaired intracellular pH regulation in anaesthetized mdx mice is not attributable to a reduced presence or activity of H+-extruders. We propose that CO2 washout might be slowed down in vivo in mdx muscles because of the defective vascular response in contracting muscles from these mice.

Extracellular carbonic anhydrase activity facilitates lactic acid transport in rat skeletal muscle fibres

1. In skeletal muscle an extracellular sarcolemmal carbonic anhydrase (CA) has been demonstrated. We speculate that this CA accelerates the interstitial CO2/HCO3- buffer system so that H+ ions can be rapidly delivered or buffered in the interstitial fluid. Because > 80 % of the lactate which crosses the sarcolemmal membrane is transported by the H+-lactate cotransporter, we examined the contributions of extracellular and intracellular CA to lactic acid transport, using ion-selective microelectrodes for measurements of intracellular pH (pHi) and fibre surface pH (pHs) in rat extensor digitorum longus (EDL) and soleus fibres. 2. Muscle fibres were exposed to 20 mM sodium lactate in the absence and presence of the CA inhibitors benzolamide (BZ), acetazolamide (AZ), chlorzolamide (CZ) and ethoxzolamide (EZ). The initial slopes (dpHs/dt, dpHi/dt) and the amplitudes (DeltapHs, DeltapHi) of pH changes were quantified. From dpHi/dt, DeltapHi and the total buffer factor (BFtot) the lactate fluxes (mM min-1) and intracellular lactate concentrations ([lactate]i) were estimated. 3. BFtot was obtained as the sum of the non-HCO3- buffer factor (BFnon-HCO3) and the HCO3- buffer factor (BFHCO3). BFnon-HCO3 was 35 +/- 4 mM pH-1 for the EDL (n = 14) and 86 /- 16 mM pH-1 for the soleus (n = 14). 4. In soleus, 10 mM cinnamate inhibited lactate influx by 44 % and efflux by 30 %; in EDL, it inhibited lactate influx by 37 % and efflux by 20 %. Cinnamate decreased [lactate]i, in soleus by 36 % and in EDL by 45 %. In soleus, 1 mM DIDS reduced lactate influx by 18 % and efflux by 16 %. In EDL, DIDS lowered the influx by 27 % but had almost no effect on efflux. DIDS reduced [lactate]i by 20 % in soleus and by 26 % in EDL. 5. BZ (0.01 mM) and AZ (0.1 mM), which inhibit only the extracellular sarcolemmal CA, led to a significant increase in dpHs/dt and pHs by about 40 %-150 % in soleus and EDL. BZ and AZ inhibited the influx and efflux of lactate by 25 %-50 % and reduced [lactate]i by about 40 %. The membrane-permeable CA inhibitors CZ (0.5 mM) and EZ (0.1 mM), which inhibit the extracellular as well as the intracellular CAs, exerted no greater effects than the poorly permeable inhibitors BZ and AZ did. 6. In soleus, 10 mM cinnamate inhibited the lactate influx by 47 %. Addition of 0.01 mM BZ led to a further inhibition by only 10 %. BZ alone reduced the influx by 37 %. 7. BZ (0.01 mM) had no influence on the Km value of the lactate transport, but led to a decrease in maximal transport rate (Vmax). In EDL, BZ reduced Vmax by 50 % and in soleus by about 25 %. 8. We conclude that the extracellular sarcolemmal CA plays an important role in lactic acid transport, while internal CA has no effect, a difference most likely attributable to the high internal vs. low extracellular BF(non-HCO3). The fact that the effects of cinnamate and BZ are not additive indicates that the two inhibitors act at distinct sites on the same transport pathway for lactic acid.

Effects of endurance training on the isocapnic buffering and hypocapnic hyperventilation phases in professional cyclists

OBJECTIVES

To evaluate the changes produced in both the isocapnic buffering and hypocapnic hyperventilation (HHV) phases of professional cyclists (n = 11) in response to endurance training, and to compare the results with those of amateur cyclists (n = 11).

METHODS

Each professional cyclist performed three laboratory exercise tests to exhaustion during the active rest (autumn: November), precompetition (winter: January), and competition (spring: May) periods of the sports season. Amateur cyclists only performed one exercise test during the competition period. The isocapnic buffering and HHV ranges were calculated during each test and defined as Vo2 and power output (W).

RESULTS

No significant differences were found in the isocapnic buffering range in each of the periods of the sports season in professional cyclists. In contrast, there was a significant reduction in the HHV range (expressed in W) during both the competition (p<0.01) and precompetition(p<0.05) periods compared with the rest period. On the other hand, a longer HHV range (p<0.01) was observed in amateur cyclists than in professional cyclists (whether this was expressed in terms of Vo2 or W).

CONCLUSIONS

No change is observed in the isocapnic buffering range of professional cyclists throughout a sports season despite a considerable increase in training loads and a significant reduction in HHV range expressed in terms of power output.

Expression of the Na(+)/H(+) exchanger isoform NHE1 in rat skeletal muscle and effect of training

The expression of the Na(+)/H(+) exchanger isoform NHE1 was quantified in homogenates of various rat skeletal muscles by means of immunoblotting, and the effect of 3 weeks of treadmill training on NHE1 expression was determined in a red (oxidative) as well as a white (glycolytic)-muscle preparation. The NHE1 antibodies recognized a glycosylated protein at 101-111 kDa. There was a positive correlation between the NHE1 expression in the muscle and percent type IIB fibres and percent type IID/X fibres, whereas the NHE1 expressions were negatively correlated to percent type I fibres and percent type I + IIA fibres. Thus the highest NHE1 expression was evident in the most glycolytic fibres. Treadmill training increased (P < 0.05) the NHE1 content by 29 and 36% in oxidative and glycolytic fibres, respectively, suggesting that training enhanced the NHE1 content of all muscle-fibre types. Therefore training may improve the capacity for pH regulation in skeletal muscle.

Carbon dioxide transport and carbonic anhydrase in blood and muscle

CO(2) produced within skeletal muscle has to leave the body finally via ventilation by the lung. To get there, CO(2) diffuses from the intracellular space into the convective transport medium blood with the two compartments, plasma and erythrocytes. Within the body, CO(2) is transported in three different forms: physically dissolved, as HCO(3)(-), or as carbamate. The relative contribution of these three forms to overall transport is changing along this elimination pathway. Thus the kinetics of the interchange have to be considered. Carbonic anhydrase accelerates the hydration/dehydration reaction between CO(2), HCO(3)(-), and H(+). In skeletal muscle, various isozymes of carbonic anhydrase are localized within erythrocytes but are also bound to the capillary wall, thus accessible to plasma; bound to the sarcolemma, thus producing catalytic activity within the interstitial space; and associated with the sarcoplasmic reticulum. In some fiber types, carbonic anhydrase is also present in the sarcoplasm. In exercising skeletal muscle, lactic acid contributes huge amounts of H(+) and by these affects the relative contribution of the three forms of CO(2). With a theoretical model, the complex interdependence of reactions and transport processes involved in CO(2) exchange was analyzed.

Cloning, tissue distribution, genomic organization, and functional characterization of NBC3, a new member of the sodium bicarbonate cotransporter family

Previous functional studies have demonstrated that muscle intracellular pH regulation is mediated by sodium-coupled bicarbonate transport, Na+/H+ exchange, and Cl-/bicarbonate exchange. We report the cloning, sequence analysis, tissue distribution, genomic organization, and functional analysis of a new member of the sodium bicarbonate cotransporter (NBC) family, NBC3, from human skeletal muscle. mNBC3 encodes a 1214-residue polypeptide with 12 putative membrane-spanning domains. The approximately 7.8-kilobase transcript is expressed uniquely in skeletal muscle and heart. The NBC3 gene (SLC4A7) spans approximately 80 kb and is composed of 25 coding exons and 24 introns that are flanked by typical splice donor and acceptor sequences. Expression of mNBC3 cRNA in Xenopus laevis oocytes demonstrated that the protein encodes a novel stilbene-insensitive 5-(N-ethyl-N-isopropyl)-amiloride-inhibitable sodium bicarbonate cotransporter.

Skeletal muscle Na+/H+ exchange in rats: pH dependency and the effect of training

Skeletal muscle Na+/H+ exchange was studied using giant sarcolemmal vesicles obtained from rat hind limb muscle. Experiments with either the 22Na tracer technique or with the Na+ sensitive fluorescent probe SBFI were conducted to determine the activity of the Na+/H+ exchanger, which was quantified from the amiloride or amiloride derivative 5-N-ethyl-N-propylamiloride (EIPA) sensitive Na+ influx. At a constant external pH of 7.4 the exchange system was close to half-activation at an internal pH of 7.2. A further activation was observed at lower internal pH values. The activity of the muscle Na+/H+ exchanger was elevated after 6 weeks of high-intensity treadmill training. In contrast, the activity of the system was unaffected by endurance training. The enhanced initial rate of amiloride-sensitive Na+/H+ exchange appears to be involved in the elevated in vivo (dynamical) buffer capacity reported for trained rats and human subjects, indicating that adaptive changes in the exchange system are of importance for pH regulation in association with high-intensity exercise.

Muscle pH regulation: role of training

In most cell types, including resting skeletal muscle fibers, internal pH (pHi) is kept constant at a relatively alkaline level. The high pHi is obtained in spite of a chronic acid load resulting from cellular metabolism and passive influx of protons driven by electrochemical forces. Regulation of pHi depends on continuous activity of membrane transport systems that mediate an outflux of H+ (or bicarbonate influx), whereby the acid load is counterbalanced. The transporters involved in muscle pH regulation at rest are the Na+/H+ exchange system as well as the Na+-dependent and Na+-independent Cl- bicarbonate transport systems. The Na+/H+ exchanger seems to be active at resting pHi levels in skeletal muscle. Therefore, pH homeostasis in skeletal muscle most likely involves an equilibrium between counter-directed H+ fluxes. A minor fraction of H+ release during intense exercise is mediated by the Na+/H+ exchanger. The capacity of this system is increased with training and hypoxia in rat skeletal muscle. The dominant acid extruding system associated with intense exercise is the lactate/H+ co-transporter. It has been demonstrated that the capacity of the lactate/H+ co-transporter of rat skeletal muscle is upregulated with training and chronic electrical stimulation, and that it is reduced upon denervation and hindlimb unweighting. Moreover, athletes can have an elevated lactate/H+ co-transport capacity, whereas the thigh muscle of spinal cord-injured individuals has a lower transport capacity than the one of healthy untrained subjects. Thus, it appears that the capacity of the lactate/H+ transporter is affected by the level of muscle activity in both rats and humans. In addition, the rate of H+ release from muscle may also be influenced by capillarization and local blood flow. Finally the resulting pH displacement during acid accumulation is determined by the cellular buffer capacity, which may also undergo adaptive changes.

Regulation of cellular pH in skeletal muscle fiber types, studied with sarcolemmal giant vesicles obtained from rat muscles

Sarcolemmal giant vesicles obtained from rat hindlimb muscles were used as a model for the study of pH regulation in skeletal muscle. The transport systems involved in the recovery from 40 mM lactate and pHi 6.5 were quantified from both flux measurements of the co-transported ions and counter-ions, and from measurements of the rate of the internal pH change. The diffusion of lactic acid plus the carrier-mediated co-transport of lactate and H+ had the highest capacity to transport protons (240 nmol H+/mg protein per min). These systems are therefore responsible for a large part of the H+ efflux in periods with a high lactate production. The capacity of the HCO(3)- - dependent systems was 47 nmol/mg per min, and the capacity of the Na+/H+ exchange system was 33 nmol/mg per min in vesicles from mixed muscles. The capacity to remove H+ by the lactate/H+ co-transport system and by the bicarbonate-dependent systems was significantly higher in vesicles from predominantly red fibers than in vesicles from white fibers, whereas the distribution of the Na+/H+ exchange system was independent of fiber type. These observations demonstrate that the pH regulation during muscle activity in red muscles is more effective than in white muscles.

Relative functional buffering capacity in 400-meter runners, long-distance runners and untrained individuals

Buffering is a factor which influences performance in short and middle-term endurance by compensating exercise acidosis. The aim of the study was to establish whether respiration parameters are a relative measure of buffering capacity and to study the influence of buffering on specific performance parameters. Three groups (each of ten subjects) with defined degrees of adaptation [untrained (UT), aerobic-trained (AeT) and elite 400-m runners (AnT) with a best time of 48.47 +/- 0.98 s] were examined in an incremental multi-stage test on the treadmill. Breath-by-breath gas analysis was performed using mass spectrometry and computer routines. Serum lactate concentrations were determined at each exercise level until subjective exhaustion. A value for the relative functional buffering capacity (relFB) was calculated using exercise metabolic parameters. Running speed at the lactate threshold was used as the starting point of buffering. The start of respiratory compensation of acidosis (RCP) was taken as the endpoint of buffering. RCP was determined at the point of decrease in end-tidal CO2 content (CO2-ET). RelFB was given in percent of buffering to running speed at RCP. Group AnT attained the same maximum performance data (maximum running speed, maximum rate of O2 consumption) as group AeT. However, these values were attained in group AnT with a significantly higher relFB (AnT: 31.0 +/- 3.2% vs. AeT: 15.7 +/- 3.9%, P < 0.0001), while a higher lactate threshold indicated a greater oxidative capacity in AeT (AeT: 3.07 +/- 0.26 m.s-1 vs. AnT: 2.68 +/- 0.22 m.s-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation of lactate and pH in human skeletal muscle after exercise by 1H NMR

We have made in vivo 1H NMR measurements of the time course of pH and lactate in human skeletal muscle after exercise. Spectra were obtained in a 4.7-T 30-cm bore Bruker Biospec spectrometer with a 2.5-cm diameter single surface coil. pH was determined from the shift of the endogenous carnosine H-C2 peak while lactate concentrations were determined by comparison with endogenous total creatine, taken to be 28.5 mM/kg wet wt. Fitting the data shows that the exponential decay of lactate (-0.094 +/- 0.014 min-1. t1/2 = 10.6 min) is slower than that of pH (-0.147 +/- 0.015 min-1, t1/2 = 4.7 min), n = 7 with two different volunteers. These values are significantly different with P less than 0.0005. Relaxation times of lactate and creatine were also measured for lactate quantitation; creatine T1, 1.23 +/- 12 s, T2, 136.2 +/- 26.4 ms (both in resting human muscle); lactate T1 (in postmortem rabbit muscle), 1.0 +/- 11 s and T2, 80 ms (in postexercise human muscle). At the end of intense exercise, the lactate level reached was 25.3 +/- 4.0 mM and the average pH drop was 1.0 pH unit. We discuss the implications of these measurements in conjunction with existing data on other sources of H+ flux, phosphocreatine resynthesis, H+ transport, and contribution of inorganic phosphate to buffering.

Na(+)-independent Cl(-)-HCO3- exchange in sarcolemmal vesicles from vascular smooth muscle

Intracellular pH (pHin) affects vascular smooth muscle function, but the mechanisms that control pHin in this tissue are not well understood. These studies were performed to determine whether sarcolemmal vesicles from bovine superior mesenteric artery (SMA) contain a Na(+)-independent Cl(-)-HCO3- exchanger and, if so, to determine its sensitivity to membrane voltage and inhibitors. 36Cl- was taken up by vesicles into an osmotically active intravesicular space. In Na(+)-free media, an outwardly or inwardly directed HCO3- gradient stimulated 36Cl- transport in the opposite direction. An outwardly directed unlabeled Cl- gradient stimulated 36Cl- uptake by a mechanism that was inhibited by external HCO3-. HCO3- or Cl- gradient-stimulated 36Cl- uptake was not due to voltage coupling between ions. In the nominal absence of HCO3-, a threefold outwardly directed OH- gradient did not affect 36Cl- uptake. Total 36Cl- uptake was stimulated by an inside-positive voltage, but the HCO3- gradient-stimulated component of 36Cl- uptake was insensitive to a change in membrane voltage. Finally, HCO3- gradient-stimulated 36Cl- uptake was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and furosemide, with 50% inhibitory concentration values equalling approximately 1.0 and 0.5 mM, respectively. These data indicate that sarcolemmal vesicles from bovine SMA contain a Na(+)-independent Cl(-)-HCO3- exchanger. This transport system is probably electroneutral and is inhibitable by DIDS and furosemide. A conductive pathway for Cl- is present in the vesicles, but Cl(-)-OH- exchange activity was not observed.