Comparison of efflux rates of hydrogen and lactate ions from isolated muscles in vitro

The influence of sodium bicarbonate on maximal force and rates of force development in the triceps surae and brachii during fatiguing exercise

NEW FINDINGS

What is the central question of this study? Does metabolic alkalosis in humans, induced by sodium bicarbonate, affect rates of skeletal muscle fatigue differentially in muscle groups composed predominately of slow- and fast-twitch fibres? What is the main finding and its importance? Sodium bicarbonate exhibited no effect on the fatigue profile observed between triceps surae and brachii muscle groups during and after 2 min of tetanic stimulation. For the first time in exercising humans, we have profiled the effect of sodium bicarbonate on the voluntary and involuntary contractile characteristics of muscle groups representative of predominately slow- and fast-twitch fibres. The effect of metabolic alkalosis on fibre-specific maximal force production and rates of force development (RFD) has been investigated previously in animal models, with evidence suggesting an improved capacity to develop force rapidly in fast- compared with slow-twitch muscle. We have attempted to model in vivo the fatigue profile of voluntary and involuntary maximal force and RFD in the triceps surae and brachii after sodium bicarbonate (NaHCO₃ ) ingestion. In a double-blind, three-way repeated-measures design, participants (n = 10) ingested either 0.3 g kg^-1 NaHCO₃ (ALK) or equivalent calcium carbonate (PLA) prior to 2 min of continuous (1 Hz) supramaximal stimulation (300 ms at 40 Hz) of the triceps surae or brachii, with maximal voluntary efforts (maximal voluntary torque) coupled with direct muscle stimulation also measured at baseline, 1 and 2 min. Metabolic alkalosis was achieved in both ALK trials but was not different between muscle groups. Regardless of the conditions, involuntary torque declined nearly 60% in the triceps brachii (P < 0.001) and ∼30% in the triceps surae (P < 0.001). In all trials, there was a significant decline in normalized involuntary RFD (P < 0.05). Maximal voluntary torque declined nearly 28% but was not different between conditions (P < 0.01), and although declining nearly 21% in voluntary RFD (P < 0.05) there was no difference between PLA and ALK in either muscle group (P = 0.93). Sodium bicarbonate exhibited no effect on the fatigue observed between representative fibre-type muscle groups on maximal voluntary and involuntary torque or rates of torque development during and after 2 min of tetanic stimulation.

Mechanistic Insights into the Efficacy of Sodium Bicarbonate Supplementation to Improve Athletic Performance

A large proportion of empirical research and reviews investigating the ergogenic potential of sodium bicarbonate (NaHCO₃) supplementation have focused predominately on performance outcomes and only speculate about underlying mechanisms responsible for any benefit. The aim of this review was to critically evaluate the influence of NaHCO₃ supplementation on mechanisms associated with skeletal muscle fatigue as it translates directly to exercise performance. Mechanistic links between skeletal muscle fatigue, proton accumulation (or metabolic acidosis) and NaHCO₃ supplementation have been identified to provide a more targeted, evidence-based approach to direct future research, as well as provide practitioners with a contemporary perspective on the potential applications and limitations of this supplement. The mechanisms identified have been broadly categorised under the sections ‘Whole-body Metabolism’, ‘Muscle Physiology’ and ‘Motor Pathways’, and when possible, the performance outcomes of these studies contextualized within an integrative framework of whole-body exercise where other factors such as task demand (e.g. large vs. small muscle groups), cardio-pulmonary and neural control mechanisms may outweigh any localised influence of NaHCO₃. Finally, the ‘Performance Applications’ section provides further interpretation for the practitioner founded on the mechanistic evidence provided in this review and other relevant, applied NaHCO₃ performance-related studies.

The differential effect of metabolic alkalosis on maximum force and rate of force development during repeated, high-intensity cycling

The purpose of this investigation was to assess the influence of sodium bicarbonate supplementation on maximal force production, rate of force development (RFD), and muscle recruitment during repeated bouts of high-intensity cycling. Ten male and female ( n = 10) subjects completed two fixed-cadence, high-intensity cycling trials. Each trial consisted of a series of 30-s efforts at 120% peak power output (maximum graded test) that were interspersed with 30-s recovery periods until task failure. Prior to each trial, subjects consumed 0.3 g/kg sodium bicarbonate (ALK) or placebo (PLA). Maximal voluntary contractions were performed immediately after each 30-s effort. Maximal force (Fmax) was calculated as the greatest force recorded over a 25-ms period throughout the entire contraction duration while maximal RFD (RFDmax) was calculated as the greatest 10-ms average slope throughout that same contraction. Fmax declined similarly in both the ALK and PLA conditions, with baseline values (ALK: 1,226 ± 393 N; PLA: 1,222 ± 369 N) declining nearly 295 ± 54 N [95% confidence interval (CI) = 84–508 N; P < 0.006]. RFDmax also declined in both trials; however, a differential effect persisted between the ALK and PLA conditions. A main effect of condition was observed across the performance time period, with RFDmax on average higher during ALK (ALK: 8,729 ± 1,169 N/s; PLA: 7,691 ± 1,526 N/s; mean difference between conditions 1,038 ± 451 N/s, 95% CI = 17–2,059 N/s; P < 0.048). These results demonstrate a differential effect of alkalosis on maximum force vs. maximum rate of force development during a whole body fatiguing task.

The teleost pseudobranch: a role for preconditioning of ocular blood supply?

The physiological relevance of the teleost pseudobranch as a remnant of a reduced gill arch is still unclear. Numerous hypotheses have been proposed regarding its physiological role, but direct confirmatory evidence is lacking. The close relationship by serial blood flow arrangement with the fish eye's choroid rete has sparked the idea that pseudobranchial preconditioning of blood pH may facilitate initiation of the Root effect and thus support the establishment of high oxygen tensions for retinal diffusive supply. This idea was critically tested by studies on isolated pseudobranchs in situ (Oncorhynchus mykiss), perfused with RBC/Ringer or RBC/plasma suspensions of widely varied composition (pH 7.4-8.2). Detailed analysis of inflowing as compared to effluent perfusates indicated normal aerobic metabolism expressed by a rise in Pco2 (+0.39 +/- 0.13 mmHg x +/- SD), an oxygen utilization of 25% and a high oxygen consumption of approximately 400 nmol g(-1 ) min(-1). Upon passage of the pseudobranch, pH (corrected for Haldane effect) was only slightly acidified (-0.03 to -0.10), [HCO3(-)] and [lactate] were slightly enhanced (+0.51 mmol l(-1) or 0.13 mmol l(-1), respectively). In order to test for yet unknown plasma components involved in pseudobranch function, a second series of experiments was conducted using RBC-suspensions in fresh plasma instead of Ringer, with results closely resembling those of the RBC/Ringer series. Lacking any physiologically significant correlation with the level of perfusate pH, the obtained data indicate pseudobranchial basic metabolic activity rather than pH regulatory characteristics. Also the observed absolute changes in pH are negligible in terms of pH regulation towards the Root-threshold. Accordingly, the present experiments as well as plausibility evaluation of mechanisms do not support the idea of blood pH pre-adjustment prior to entry into the choroid rete structure of the teleost eye to facilitate the Root-mediated oxygen release.

The hydrogen ion in normal metabolism: a review

The production of hydrogen ions (H+) by metabolic processes is described, with particular emphasis on glycolysis and ketogenesis. Total metabolic production of H+ is approximately 150 g day-1 but utilization closely balances production, so that intracellular and extracellular H+ production is maintained within narrow limits. H+ is generated at several sites in glycolysis but no net H+ production occurs unless the ATP formed is hydrolysed. The other main source of metabolic H+ production is ketogenesis. Here H+ accumulation depends on both the relative dominance of ketone body production over utilization and the loss of base in urine. The H+ is produced during the synthesis of 3-hydroxy-3-methylglutaryl-CoA and not because of dissociation of acetoacetic acid. Lipolysis and re-esterification of fats are additional major producers of H+, while net H+ production also occurs with pathological accumulation and incomplete combustion of other organic acids. Many metabolic systems are sensitive to the changes in pH. These effects have been examined in vivo using an ammonium chloride acidaemia model in the rat. Severe insulin resistance and impaired glucose metabolism in liver and muscle were found. One mechanism involved inhibition, by H+, of the binding of insulin to its receptors. Further mechanisms include inhibition of key glycolytic enzymes including phosphofructokinase. It is concluded that too little attention is paid to metabolic production of hydrogen ions and to their effects, in turn, on metabolism.

Buffering and H+ ion dynamics in muscle tissues

After anaerobic activity with release of large quantities of intermediary metabolic end products, further energy production critically depends on rapid elimination of H+ ions from the muscle tissues to secure key enzymatic activities. The involved processes, interactions and interrelationships of mechanisms have been analyzed on the basis of a physiological model and available experimental data. The H+ elimination from muscle tissue is a multifactorial process primarily governed by the capillary H+ transport capacitance, effected by buffering capabilities of intracellular and capillary fluids compartments, by dynamically interrelated regulation of intracellular and extracellular pH, by the magnitude and quality of convective perfusional transfer and further factors. Model calculations strongly resemble experimental data obtained in isolated perfused muscles and suggest that discrepancies between disparate literature data are attributable to experimental limitations.

Effects of exhausting exercise and catecholamines on K+ balance, acid-base status and blood respiratory properties in carp

The potential role of adrenergic mechanisms in the recovery of potassium balance and acid-base status following 5 min of exhausting exercise was studied in carp. The extracellular metabolic H+ load after exercise matched the lactate load, suggesting similar release rates of H+ and lactate from white muscle. Blockage of alpha-adrenoceptors by phentolamine or beta-adrenoceptors by propranolol neither influenced absolute magnitudes nor recovery kinetics of extracellular H+ and lactate loads. The arterial oxygen tension increased following exercise, but blood oxygen transport was not improved via a red cell beta-adrenergic response or modulation of the red cell nucleoside triphosphate content. Exercise induced an increase in extracellular [K+] which was corrected within 30-60 min of recovery. The recovery of K+ balance was not influenced by blockage of adrenergic receptors. Red cell [K+] changed only insignificantly following exercise, whereby a possible function of the red cells as a temporary depository for K+ during the extracellular hyperkalaemia could not be established. The minimal influence of catecholamines on the measured parameters during recovery from exercise was supported by an absence of change in these parameters upon adrenaline injection in resting carp.

Is the intracellular pH threshold an anaerobic threshold from the view point of intracellular events?: a brief review

Intracellular biochemical changes could be monitored noninvasivery and continuously by using nuclear magnetic resonance (NMR). In about the last decade, intracellular behavior of phosphorus compounds and pH during muscle contraction in man have been studied by 31P nuclear magnetic resonance spectroscopy (31P-MRS). During incremental load, lactic acidosis was followed by a decline in intracellular pH. 31P-MRS studies has been definitely proved that this change in intracellular pH shows the threshold behavior. Some reports discussed the intracellular pH threshold (pHT) as an anaerobic threshold (AT) from the view point of intracellular events. However, our studies revealed that pHT did not reflect the onset of lactate production. In this article, studies of intracellular pH of working muscle were reviewed in relation to an anaerobic threshold.

Lactate transport in rat sarcolemmal vesicles and intact skeletal muscle, and after muscle contraction

To determine whether it was possible to measure lactate transport rates into intact skeletal muscles, the transport of lactate (zero-trans) was determined in soleus muscle strips incubated in vitro and compared with lactate transport in sarcolemmal vesicles. In addition, the effects of muscle contractility on lactate transport were investigated in electrically-stimulated soleus muscle strips. In both the intact muscle and the sarcolemmal preparations the rates of transport were saturable, stereospecific, and inhibitable by monocarboxylates (pyruvate, alpha-cyano-4-hydroxycinnamate) and a protein modifier (N-ethylmaleimide; P < 0.05). The anion exchange inhibitor SITS had no effect on lactate uptake (P > 0.05). In both preparations lactate transport followed an inwardly directed proton gradient. Relative comparisons (%) between the preparations indicated a similar slope of increasing transport rates with increasing lactate concentrations and similar responses to a changing pH environment. These characterizations of L-lactate transport into isolated sarcolemmal vesicles and muscle strips revealed that both preparations yielded similar conclusions regarding the transmembrane movement of L-lactate. By using this more physiological muscle preparation, contractile activity, induced by electrical stimulation, did not increase lactate uptake in skeletal muscle in the post-exercise period whereas under similar conditions a marked increase in 2-deoxy-D-glucose uptake occurred (+ 47%; P < 0.05). These data suggest that the transport of glucose and lactate in contracting muscle is regulated differently. These studies also show that the incubated muscle strip preparation may be useful for studying lactate transport in an intact cell system during physiological experiments.

pH control in rat skeletal muscle during exercise, recovery from exercise, and acute respiratory acidosis

We used 31P magnetic resonance spectroscopy to compare the response of rat skeletal muscle to three kinds of proton load. During exercise (tetanic sciatic nerve stimulation), protons from lactic acid were buffered passively and consumed by net hydrolysis of phosphocreatine (PCr). During recovery from exercise, the pH-dependent efflux of protons produced by PCr resynthesis could be partially inhibited by amiloride or 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS), implicating both sodium/proton and bicarbonate/chloride exchange, but was not inhibited by simultaneous respiratory acidosis. In early recovery, up to 30% of proton efflux was mediated by lactate/proton cotransport. During acute respiratory acidosis at rest, the eventual change in muscle pH was consistent with passive buffering and was unaffected by amiloride or DIDS, implying no significant contribution of proton fluxes.

A voltage-dependent proton current in cultured human skeletal muscle myotubes

1. A voltage-dependent proton current, IH, was studied in cultured myotubes obtained from biopsies of human muscle, using whole-cell recording with the patch-clamp technique. 2. With a pHo of 8.0 and a calculated pHi of 6.3, IH was activated at voltages more depolarized than -50 mV and its conductance reached its maximum value at voltages more depolarized than +10 mV. 3. Studies of the reversal potential of IH during substitution of K+, Na+, Ca2+, Cl-, Cs+ and H+ in the extracellular solution indicated that protons were the major charge carriers of IH. 4. IH was also activated during a voltage step to +22 mV with a pHo of 7.3 and a calculated pHi of 7.3. 5. Acidification of the extracellular solution led to a shift towards depolarized voltages of the conductance-voltage relationship. 6. Stationary noise analysis of IH suggested that the elementary event underlying IH was very small with a conductance of less than 0.09 pS. 7. Extracellular application of various divalent cations blocked IH. The block by divalent cations was voltage dependent, being more efficient at hyperpolarized than at depolarized voltages. For Cd2+, the Michaelis-Menten constant (Km) for the block was 0.6 microM at -28 mV and 10.4 microM at +12 mV. 8. Ca2+ was a less efficient blocker than Cd2+ but could block IH at physiological concentrations (the Km values for the block were 0.9 mM at -38 mV and 7.3 mM at -8 mV). 9. The voltage-dependent properties of IH and its ability to be affected by pH and Ca2+ suggest that IH might be used by skeletal muscle cells to extrude protons during action potentials. 10. A model of IH activation suggests that under extreme conditions, the conductance of IH can reach 40% of its maximum value after less than ten action potentials.

The production, buffering and efflux of protons in human skeletal muscle during exercise and recovery

We show how quantitative information about proton handling in human skeletal muscle in exercise and recovery can be obtained by 31P MRS and illustrate this with data from metabolic disorders. Proton production, proton efflux and passive buffering can be distinguished by comparing changes in [phosphocreatine] and pH at the end of exercise and by calculating ATP turnover during ischaemic exercise and in the 'natural experiment' of myophosphorylase deficiency (McArdle's disease). We calculate the effective buffer capacity to be 20-30 mmol/L/pH unit (slykes), somewhat lower than published measurements made in vitro but similar to other values obtained in vivo. This analysis is applied to data from normal muscle and a variety of disease states to estimate proton efflux during recovery and ATP production during exercise: (i) proton efflux during recovery is pH-dependent, reaching a 10 mmol/L/min at pH 6.2, and is increased in some cases of mitochondrial myopathy and in hypertension; (ii) glycogenolytic ATP production during exercise can reach 25 mmol/L/min in normal muscle and correlates approximately with [Pi] at the start of aerobic exercise and throughout ischaemic exercise; (iii) oxidative ATP production can reach 20-25 mmol/L/min and (as during recovery) correlates approximately with [Pi].

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.

Lactate and potassium fluxes from human skeletal muscle during and after intense, dynamic, knee extensor exercise

This study examines lactate and K+ fluxes from muscle to blood during and after intense exercise. Ten men performed exhaustive dynamic exercise (mean load 65 W, mean duration 3.18 min) with the knee extensors of one leg. The mean lactate efflux was 15.5 (range 8.9-24.0) mmol min-1 at exhaustion, and it was linearly related to the lactate gradient. A linear relationship was also obtained if the H+ gradient was taken into account. Muscle pH decreased from 7.14 at rest to 6.71 (range 6.50-6.87) at exhaustion. At rest and during late recovery blood lactate was distributed across the erythrocyte membrane according to the membrane potential (intra-/extracellular ratio of 0.5), but during rapid lactate release this ratio decreased to 0.2. In-vitro experiments demonstrated a time constant of 1.2 min for lactate efflux from the erythrocytes. Approximately 70% of the K+ ions released from the muscle to the blood accumulated in the plasma; the rest were taken up by other tissues. However, erythrocytes were not involved as a dilution space. The small change in erythrocyte K+ concentration was due to cellular volume changes. During recovery the kinetics of K+ reuptake by the muscle were described by a very fast (less than 1 min) and a slow component (greater than 1 min): the magnitude of the former was equivalent to what had accumulated in the plasma. Individuals displayed a wide range of intramuscular lactate concentrations and pH values at exhaustion. Further, the pH changes were not as extreme as previously reported, suggesting that pH may not be the only factor involved in the fatigue process. A possible role for the potassium shifts as a limiting factor for muscle function is discussed.

Lactate and pyruvate transport is dominated by a pH gradient-sensitive carrier in rat skeletal muscle sarcolemmal vesicles

The mechanisms of lactate and pyruvate transport across the plasma membrane of rat skeletal muscle under various pH and ionic conditions were studied in skeletal muscle sarcolemmal (SL) membrane vesicles purified from 22 female Sprague-Dawley rats. Transport by SL vesicles was measured as uptake of L(+)-[U-14C] lactate and [U-14C] pyruvate. Lactate (La-) transport is pH-sensitive; stimulations to fivefold overshoot above equilibrium values were observed both directly by a proton gradient directed inward, and indirectly by a monensin- or nigericin-stimulated exchange of Na+ or K+ for H+ across the SL. Isotopic pyruvate could utilize the transporter, and demonstrated pH gradient-stimulated overshoot and cis-inhibition characteristics similar to those of lactate. Overshoot kinetics were also demonstrated by pH gradient formed by manipulation of external media at pH 5.9, 6.6, and 7.4 and intravesicular media at 6.6, 7.4, and 8.0, respectively. Carbonyl cyanide m-chlorophenylhydrazone, an H+ ionophore, was used as a "pH clamp" to return all stimulated uptake courses back to equilibrium values. Lactate uptake was depressed when internal pH was lower than external pH. These data strongly suggest that La- and H+ are either cotransported by the carrier, or transported as the undissociated HLa, and can account for the majority of the lactate uptake at pH 7.4. The mechanism does not require cotransport of either K+ or Na+. However, an inwardly directed Na+ gradient without ionophore in the absence of a pH gradient doubled La- transport; treatment with amiloride, an inhibitor of the Na+/H+ exchanger, abolished this stimulation, suggesting that this transporter may be an important coregulator of intracellular pH, and could disrupt 1:1 H+ and La- efflux stoichiometry in vivo. We conclude that the majority of La- crosses the skeletal muscle SL by a specific carrier-mediated process that is saturable at high La- concentrations, but flux is passively augmented at low intracellular pH by undissociated lactic acid. In addition, a Na+/H+ exchange mechanism was confirmed in skeletal muscle SL, does affect both lactate and proton flux, and is potentially an important coregulator of intracellular pH and thus, cellular metabolism.

The effect of pH and hypoxia on function and intracellular pH of the rat diaphragm

We studied the relationship between contractile function and intracellular pH (pHi) in the isolated rat diaphragm when superfusate PCO2 was changed during hyperoxia or hypoxia. Superfused diaphragm strips were field stimulated at 0.5 Herz, and twitch tension (TT) was recorded. The pHi was calculated from the volume distribution of a weak acid, dimethyl-oxazolidinedione. In hyperoxia, hypercapnic acidosis (pH 7.06-6.63) depressed diaphragm pHi and TT, whereas hypocapnic alkalosis (pH 7.82-8.15) increased pHi but did not significantly affect TT. TT was maximum at physiological pHi (7.06), but in hyperoxic hypercapnic muscles substantial force was still generated at pHi values as low as 6.44. Hypoxia (PO2 30-38 mm Hg) markedly reduced TT; this effect was slightly exacerbated by hypercapnia and attenuated by hypocapnia. Hypoxia lowered pHi by about 0.2 units, which was insufficient to account for the hypoxic contractile failure. Knowledge of the hyperoxic muscle TT/pHi relationship suggests that, in other contexts, caution should be exercised in attributing severe muscle fatigue or force loss to modest falls in pHi.

The effect of lactate on intracellular pH and force recovery of fatigued sartorius muscles of the frog, Rana pipiens

1. The effects of pHo (extracellular pH) and lactic acid on pHi (intracellular pH) and tetanic force were examined in frog sartorius muscle. Ion-selective microelectrodes were used to measure pHi. Tetanic force was elicited by field stimulation. Experiments were performed in HEPES-buffered solution equilibrated with 100% O2. 2. Mean pHi values (+/- S.E.M.) of unfatigued frog sartorius muscles were 7.14 +/- 0.02 and 7.05 +/- 0.09 at pHo 7.2 and 6.4, respectively. 3. A stimulation at a rate of one 100 ms tetanic contraction per second for 3 min reduced pHi to 6.21 +/- 0.09 and 6.20 +/- 0.04 at pHo 7.2 and 6.4, respectively. Meanwhile at pHo 7.2, the tetanic force (defined as the maximum force developed during a tetanus) decreased by 82.9 +/- 2.6%, the maximum rate of relaxation decreased by 92.9 +/- 0.9%, and the rate constant of the relaxation decreased by 88.5 +/- 1.6%. At pHo 6.4, the decrease in tetanic force, maximum rate of relaxation and rate constant were 90.6 +/- 1.8%, 93.8 +/- 0.5 and 87.5 +/- 2.7%, respectively. 4. The maximum rates of recovery of pHi following fatigue were 0.068 +/- 0.05 and 0.025 +/- 0.05 pH units min-1 at pHo 7.2 and 6.4, respectively. Recovery of normal tetanic force and relaxation rate was also slower at acidic pHo than at neutral pHo. 5. In the presence of 40 mmol l-1 L-lactic acid at pHo 7.2, the maximum rate of pHi recovery following fatigue was only 0.027 +/- 0.03 pH units min-1 at pHo 7.2. The presence of lactic acid also reduced the recovery of the relaxation phase, but not the recovery of tetanic force. 6. It is suggested that pHi recovery is not a limiting factor for tetanic force recovery and that the extracellular H+ inhibits tetanic force recovery by acting at a site located on the outer surface of the sarcolemma. The recovery of the relaxation phase is believed to be pHi dependent.

Lactic acidosis

An understanding of the pathophysiology of lactic acidosis is crucial in facilitating the optimal care of critically ill patients. The relevant biochemistry of lactic acidosis is reviewed, and the more controversial aspects relating to the genesis of the acidosis are highlighted. The current system of classification of lactic acidosis divides etiologies on the basis of the presence or absence of clinical signs of tissue hypoperfusion. Several types of lactic acidosis in which clinical evidence of tissue hypoperfusion is lacking demonstrate hemodynamic evidence of occult hypoperfusion. The diagnostic and therapeutic implications of this observation are discussed. Current diagnostic criteria for lactic acidosis include a pH less than 7.35 and blood lactate concentration greater than 5 to 6 mM/L. An important issue relates to the implications of lactate values that are greater than normal but below this diagnostic range. The use of the oxygen flux test may be valuable in the diagnosis of occult tissue hypoperfusion in patients with low-grade elevations in lactate levels. The current therapy for lactic acidosis involves addressing the primary cause and supportive management. The use of bicarbonate in the therapy for lactic acidosis is controversial due to potential adverse effects on cardiac function. The specifics of this controversy are outlined, and newer therapeutic alternatives are reviewed. The use of blood lactate concentration as a prognostic index may be more useful in patients with shock than without shock.

L(+)-lactate transport in perfused rat skeletal muscle: kinetic characteristics and sensitivity to pH and transport inhibitors

We have examined lactate uptake (as the rate of net muscle lactate accumulation) and unidirectional inward transport (measured by a paired-tracer dilution method) in muscle of the perfused skinned rat hindlimb. Inhibition of tracer influx (fractional uptake at 1 mM L(+)-lactate, 43.3 +/- 3.1% but only 32.9 +/- 1.8% at 50 mM lactate) suggested some competition between tracer and native forms of the carboxylate for transport. D(-)-lactate (50 mM) did not inhibit uptake of tracer L(+)-lactate. Pyruvate (25 mM), but none of five other monocarboxylates, inhibited uptake of tracer lactate, by 22% (P less than 0.01). Altering perfusate pH from 7.4 to 6.8 caused a 36% increase (P less than 0.001) in the unidirectional L(+)-lactate transport at 1 mM L(+)-lactate, whereas increasing pH to 7.7 reduced transport by 18% (P less than 0.01). Tracer lactate influx was inhibited by 500 microM 4-acetamido-4'-isothiocyanostilbene (SITS) (19%), 5 mM alpha-cyano-4-hydroxycinnamic acid (CIN) (20-30%), 1 mM amiloride (27%) and by a thiol group reagent p-chloromercuribenzenesulphonic acid (pCMBS) (26%). Overall the results indicate that at least two processes are involved in the transfer of lactate: one, saturable, with a Vmax of 0.84 mumol.min-1.g-1 and an apparent Km of 21 mM was sensitive to SITS, CIN, and a thiol group reagent; the other was non-saturable and insensitive to SITS and CIN with an apparent rate constant of 0.1 min-1.

Possible mechanisms of the anaerobic threshold. A review

The anaerobic threshold consists of a lactate threshold and a ventilatory threshold. In some conditions there may actually be 2 ventilatory thresholds. Much of the work detailing the lactate threshold is strongly based on blood lactate concentration. Since, in most cases, blood lactate concentration does not reflect production in active skeletal muscle, inferences about the metabolic state of contracting muscle will not be valid based only on blood lactate concentration measurements. Numerous possible mechanisms may be postulated as generating a lactate threshold. However, it is very difficult to design a study to influence only one variable. One may ask, does reducing F1O2 cause an earlier occurrence of a lactate threshold during progressive exercise by reducing oxygen availability at the mitochondria? By stimulating catecholamine production? By shifting more blood flow away from tissues which remove lactate from the blood? Or by some other mechanism? Processes considered essential to the generation of a lactate threshold include: (a) substrate utilisation in which the ability of contracting muscle cells to oxidise fats reaches maximal power at lactate threshold; and (b) catecholaminergic stimulation, for without the presence of catecholamines it appears a lactate threshold cannot be generated. Other mechanisms discussed which probably enhance the lactate threshold, but are not considered essential initiators are: (a) oxygen limitation; (b) motor unit recruitment order; (c) lactate removal; (d) muscle temperature receptors; (e) metabolic stimulation; and (f) a threshold of lactate efflux. Some mechanisms reviewed which may induce or contribute to a ventilatory threshold are the effects of: (a) the carotid bodies; (b) respiratory mechanics; (c) temperature; and (d) skeletal muscle receptors. It is not yet possible to determine the hierarchy of effects essential for generating a ventilatory threshold. This may indicate that the central nervous system integrates a broad range of input signals in order to generate a non-linear increase in ventilation. Evidence indicates that the occurrence of the lactate threshold and the ventilatory threshold may be dissociated; sometimes the occurrence of the lactate threshold significantly precedes the ventilatory threshold and at other times the ventilatory threshold significantly precedes the lactate threshold. It is concluded that the 2 thresholds are not subserved by the same mechanism.

Intracellular pH recovery and lactate efflux in mouse soleus muscles stimulated in vitro: the involvement of sodium/proton exchange and a lactate carrier

The intracellular pH recovery after stimulation of mouse soleus muscles in vitro was studied by means of intracellular pH-sensitive microelectrodes. The lactate efflux and the total lactate content were measured by means of an enzymic method. During electrical stimulation for 2 min in a CO2/HCO3- -buffered Ringer's solution, pHi decreased by 0.5 units. The rate of pHi-recovery was independent of external bicarbonate, but dependent on the buffer concentration. The rate of intracellular pH recovery was reduced by the lactate transport inhibitors PCMBS and cinnamate, whereas the inhibitors of inorganic anion-exchange SITS and DIDS had no effect. The Na+/H+ exchange inhibitor amiloride reduced the rate of pHi recovery. The pHi recovery was faster than the lactate efflux, which could be accounted for by an Na+/H+ exchange. A number of inhibitor compounds were used in order to discriminate between the three possible lactate efflux pathways: the monocarboxylate carrier mechanism, the inorganic anion exchange, and the molecular (non-ionic) diffusion of lactic acid. The lactate efflux was partly inhibited by cinnamate, PCMBS and phloretin, but was unaffected by DIDS and tetrathionate. These experiments demonstrate the existence of a lactate carrier in mammalian skeletal muscles. The lactate carrier is responsible for more than half of the lactate efflux after muscle activity. Both the pHi recovery studies and the lactate efflux measurements showed that, under the given conditions, the inorganic anion-exchange mechanism is not essentially involved in the recovery processes after muscle activity.

Intracellular and extracellular acid-base and electrolyte status of submerged anoxic turtles at 3 degrees C

Specimens of fresh water turtles (Chrysemys picta bellii) were acclimated to 3 degrees C and then submerged in completely anoxic water for time periods of up to 12 weeks. Blood withdrawn via indwelling arterial catheters was analysed for plasma pH, PCO2, bicarbonate concentration, [lactate], [Na+], [K+], [Ca2+] and [Mg2+], and tissue samples of skeletal muscle, liver and cardiac muscle were excised. Samples of skeletal muscle were analysed for intracellular pH (DMO), [lactate], [Na+], [K+], [Ca2+], and [Mg2+], and samples of liver and cardiac muscle for intracellular pH and [lactate] during normoxia and after 1, 2, 4, 8 and 12 weeks of anoxia. Arterial plasma pH fell from 8.0 during normoxia to lower than 7.2 concomitant with a reduction in plasma [HCO3-] after 12 weeks of anoxia due to the production of large amounts of lactic acid. The intracellular pH (pHi) of heart muscle and liver dropped in parallel or even more than plasma pH, whereas pHi in skeletal muscle changed less resulting in a delta pHi/delta pHe value of less than 0.6. Intracellular [lactate] and [Ca2+] increased considerably, but attained concentrations much smaller than those observed in the extracellular compartment. The intracellular concentrations of K+, Na+ and Mg2+ were also significantly affected, the changes, however, were small in comparison with those observed for Ca2+ and lactate concentration. The water distribution between intra- and extracellular compartments remained essentially unaffected by anoxia. It is concluded that the considerable increase in extracellular Mg2+ and Ca2+ cannot be the result of release from muscle cells and has to be attributed to release from skeleton and shell.(ABSTRACT TRUNCATED AT 250 WORDS)

Protons and anaerobiosis

During oxygen limitation in animals, glucose can be fermented via several metabolic pathways varying in energetic efficiency and leading to various end products (such as lactate, alanopine, octopine, succinate, or propionate). Because of opposite pH dependencies of proton production by fermentation and by hydrolysis of adenosine triphosphate formed in the fermentation, the total number of moles of protons generated is always two per mole of the fermentable substrate. However, two and three times more adenosine triphosphate can be turned over per mole of protons produced in succinate and propionate fermentations, respectively, than in lactate fermentation.

Branchial ion exchange and acid-base regulation after strenuous exercise in rainbow trout (Salmo gairdneri)

Specimens of rainbow trout (Salmo gairdneri) were electrically stimulated to exhausting activity in a closed water recirculation system and the changes in dorsal aortic plasma pH, PCO2, PO2, O2 content, [Na+], [Cl-], [K+], [Lactate-] and Ht were measured during a 24 h recovery period. Net transfer of H+, Na+, Cl- and ammonia between fish and environment were determined by measurement of the concentration changes in the recirculating water. Strenuous exercise resulted in a severe lactacidosis which was corrected by transient net transfer of H+ ions to the environmental water within 4 h, about 6-8 h before the lactate was metabolically removed. The net transfer of H+ ions was achieved in part by branchial HCO3-/Cl- ion exchanges, but to a larger extent by branchial exchange of H+ and/or NH4+ against Na+. The excretion of ammonia, which was considerably enhanced during the first 4 h after exercise, was at least partially due to non-ionic diffusion across the gill epithelium. The observed elevation in ammonia excretion was probably the result of an exercise-induced increase in nitrogen metabolism rather than of production of ammonia for the purpose of acid-base regulation.

Acid-base regulation in response to environmental hypercapnia in two aquatic salamanders, Siren lacertina and Amphiuma means

The partial pressure of CO2 (PCO2) in certain areas of the aquatic habitat of the salamanders Siren lacertina and Amphiuma means frequently rises to values of up to 60 mm Hg. This ambient hypercapnia occurs due to hindrance of gas exchange between water and air caused by dense water-surface vegetation. In order to investigate the acid-base regulation in response to the respiratory acidosis, which wound be expected to result from the high CO2 conductance of the amphibian skin, specimens of both species were subjected to water PCO2 of 47 mm Hg while having free access to normocapnic air in a closed water recirculation system. Arterial PCO2 rose considerably from 12 to 35 mm Hg in Siren and from 17 to 36 mm Hg in Amphiuma. The resultant fall in plasma pH remained uncompensated, whereas intracellular pH of white muscle and heart muscle of Siren were little affected owing to elevated intracellular bicarbonate concentrations. The bicarbonate accumulated in the intracellular compartments was in part produced by intracellular and extracellular nonbicarbonate buffering, and in part gained from the environment in exchange for Cl- ions. Elevated water bicarbonate concentration or bicarbonate infusion into Siren had no effect on the acid-base regulation. These data suggest that the availability of bicarbonate is not a limiting factor for extracellular compensation of increased PCO2, but that the threshold of the bicarbonate-regulating structures is simply not readjusted in hypercapnia. This type of regulation may have evolved as a result of the specific environmental conditions of these animals and may be considered as an energetically efficient way of maintaining a constant milieu for the pH-sensitive intracellular structures.

The anaerobic threshold as determined before and during lactic acidosis

During this study the relationships between venous lactate concentration and accociated changes in respiratory gas exchange were investigated. Five men performed two successive incremental exercise tests to exhaustion on an electronically braked cycle ergometer. These tests were separated by a 5 min rest period. During the initial test venous lactate concentrations showed a characteristic curvilinear increase and the anaerobic threshold (AT1) was determined conventionally. During the second test lactate concentrations were still decreasing at higher work rates than the AT1, and a second anaerobic threshold (AT2) was determined as the point where lactate concentrations again increased. The departure from linearity of the ventilatory response to both exercise tests occurred at a similar work rate, irrespective of whether venous lactate concentrations were increasing or decreasing. Carbon dioxide production was similar during the two exercise tests. The anaerobic thresholds as determined by respiratory gas analysis (ATR) were therefore similar for both tests. Results of this study indicate that changing venous lactate concentrations were not responsible for the ventilatory drive which occurred at the ATR. The venous lactate response to work at a constant rate determined within the range AT1-AT2 was also investigated. It was concluded that the lactate response to constant work rate will vary predictably at work rates falling within the AT1 to AT2 range. At AT1 no increase in venous lactate concentrations occurred, while at AT2 these increased progressively, and the test was terminated at varying times (12-15 min) due to subject exhaustion. At work rates determined from the ATR venous lactate concentrations varied according to the placement of the ATR within the AT1 AT2 range.