Lactate Efflux Research Articles

Role of α1-adrenergic vasoconstriction in the regulation of skeletal muscle blood flow with advancing age

alpha(1)-Adrenergic vasoconstriction during dynamic leg exercise is diminished in younger individuals, although the extent of this exercise-induced "sympatholysis" in the elderly remains uncertain. Thus, in nine young (25 +/- 1 yr) and six older (72 +/- 2 yr) healthy volunteers, we evaluated changes in leg blood flow (ultrasound Doppler) during blood flow-adjusted intra-arterial infusion of phenylephrine (PE; a selective alpha(1)-adrenergic agonist) at rest and during knee-extensor leg exercise at 20, 40, and 60% of maximal work rate (WR(max)). To probe the potential contributors to exercise-induced changes in alpha(1)-adrenergic receptor sensitivity, exercising leg O(2) consumption (Vo(2)) and lactate efflux were also evaluated (n = 10). At rest, the PE-induced vasoconstriction (i.e., decrease in leg blood flow) was diminished in older (-37 +/- 3%) compared with young (-54 +/- 4%) subjects. During exercise, the magnitude of alpha(1)-adrenergic vasoconstriction in the active leg decreased in both groups. However, compared with young, older subjects maintained a greater vasoconstrictor response to PE at 40% WR(max) (-14 +/- 3%, older; -7 +/- 2%, young) and 60% WR(max) (-11 +/- 3%, older; -4 +/- 3%, young). It is possible that this observation may be attributed to lower absolute work rates in the older group, because, for a similar absolute work rate ( approximately 10 W) and leg Vo(2) ( approximately 0.36 l/min), vasoconstriction to PE was not different between groups (-14 +/- 3%; older; -17 +/- 5%, young). Together, these data challenge the concept of reduced sympatholysis in the elderly, suggesting instead that the inhibition of alpha(1)-adrenergic vasoconstriction in the exercising leg is associated with work performed and, therefore, more closely related to the rate of oxidative metabolism than to age per se.

Basal and stimulated lactate fluxes in primary cultures of astrocytes are differentially controlled by distinct proteins

Lactate release by astrocytes is postulated to be of importance for neuroenergetics but its regulation is poorly understood. Basigin, a chaperone protein for specific monocarboxylate transporters (MCTs), represents a putatively important regulatory element for lactate fluxes. Indeed, basigin knockdown by RNA interference in primary cultures of astrocytes partially reduced both proton-driven lactate influx and efflux. But more strikingly, enhancement of lactate efflux induced by glutamate was prevented while the effect of sodium azide was significantly reduced by treatment of cultured astrocytes with anti-basigin small interfering RNA. Enhancement of glucose utilization was unaffected under the same conditions. Basal lactate uptake and release were significantly reduced by MCT1 knockdown, even more so than with basigin knockdown, whereas glutamate-driven or sodium azide-induced enhancement of lactate release was not inhibited by either MCT1, 2, or 4 small interfering RNAs. In conclusion, MCT1 plays a pivotal role in the control of basal proton-driven lactate flux in astrocytes while basigin is only partly involved, most likely via its interaction with MCT1. In contrast, basigin appears to critically regulate the enhancement of lactate release caused by glutamate (or sodium azide) but via an effect on another unidentified transporter at least present in astrocytes in vitro.

Impact of anaerobic glycolysis and oxidative substrate selection on contractile function and mechanical efficiency during moderate severity ischemia

The role of anaerobic glycolysis and oxidative substrate selection on contractile function and mechanical efficiency during moderate severity myocardial ischemia is unclear. We hypothesize that 1) preventing anaerobic glycolysis worsens contractile function and mechanical efficiency and 2) increasing glycolysis and glucose oxidation while inhibiting free fatty acid oxidation improves contractile function during ischemia. Experiments were performed in anesthetized pigs, with regional ischemia induced by a 60% decrease in left anterior descending coronary artery blood flow for 40 min. Three groups were studied: 1) no treatment, 2) inhibition of glycolysis with iodoacetate (IAA), or 3) hyperinsulinemia and hyperglycemia (HI + HG). Glucose and free fatty acid oxidation were measured using radioisotopes and anaerobic glycolysis from net lactate efflux and myocardial lactate content. Regional contractile power was assessed from left ventricular pressure and segment length in the anterior wall. We found that preventing anaerobic glycolysis with IAA during ischemia in the absence of alterations in free fatty acid and glucose oxidation did not adversely affect contractile function or mechanical efficiency during myocardial ischemia, suggesting that anaerobic glycolysis is not essential for maintaining residual contractile function. Increasing glycolysis and glucose oxidation with HI + HG inhibited free fatty acid oxidation and improved contractile function and mechanical efficiency. In conclusion, these results show a dissociation between myocardial function and anaerobic glycolysis during moderate severity ischemia in vivo, suggesting that metabolic therapies should not be aimed at inhibiting anaerobic glycolysis per se, but rather activating insulin signaling and/or enhancing carbohydrate oxidation and/or decreasing fatty acid oxidation.

Acetazolamide prevents vacuolar myopathy in skeletal muscle of K+‐depleted rats

Acetazolamide and dichlorphenamide are carbonic anhydrase (CA) inhibitors effective in the clinical condition of hypokalemic periodic paralysis (hypoPP). Whether these drugs prevent vacuolar myopathy, which is a pathogenic factor in hypoPP, is unknown. The effects of these drugs on the efflux of lactate from skeletal muscle were also investigated. For 10 days, K(+)-depleted rats, a model of hypoPP, were administered 5.6 mg kg(-1) day(-1) of acetazolamide, dichlorphenamide or bendroflumethiazide (the last is not an inhibitor of CA). Histological analysis of vacuolar myopathy and in vitro lactate efflux measurements were performed in skeletal muscles from treated and untreated K(+)-depleted rats, and also from normokalemic rats. About three times as many vacuoles were found in the type II fibres of tibialis anterioris muscle sections from K(+)-depleted rats as were found in the same muscle from normokalemic rats. In ex vivo experiments, a higher efflux of lactate on in vitro incubation was found in muscles of K(+)-depleted rats compared with that found in muscles from normokalemic rats. After treatment of K(+)-depleted rats with acetazolamide, the numbers of vacuoles in tibialis anterioris muscle decreased to near normal values. Incubation with acetazolamide in vitro inhibited efflux of lactate from muscles of K(+)-depleted rats. In contrast, bendroflumethiazide and dichlorphenamide failed to prevent vacuolar myopathy after treatment in vivo and failed to inhibit lactate efflux in vitro. Acetazolamide prevents vacuolar myopathy in K(+)-depleted rats. This effect was associated with inhibition of lactate transport, rather than inhibition of CA.

Monocarboxylate Transporter 4 Regulates Maturation and Trafficking of CD147 to the Plasma Membrane in the Metastatic Breast Cancer Cell Line MDA-MB-231

Metastatic cancer cells increase glucose consumption and metabolism via glycolysis, producing large quantities of lactate. Recent work has shown that lactate efflux is mediated by monocarboxylate transporters (MCT), which are composed of a catalytic unit (MCT) and an accessory subunit (CD147), comprising the functional lactate transporter. CD147, an extracellular matrix metalloproteinase (MMP) inducer, is highly expressed in metastatic cancer cells. Because aerobic glycolysis is a hallmark of metastatic cancer, we examined whether increases in CD147 expression were linked to MCT expression in MDA-MB-231, a highly metastatic breast cancer cell line. MCT4 mRNA and protein expression were increased in MDA-MB-231 cells compared with cells derived from normal mammary tissue. MCT4 colocalized with CD147 in the plasma membrane and in membrane blebs shed from the cell surface. Small interfering RNA-mediated silencing of MCT4 impaired the maturation and trafficking of CD147 to the cell surface, resulting in accumulation of CD147 in the endoplasmic reticulum. Silencing MCT4 also resulted in fewer membrane blebs and decreased migration of MDA-MB-231 cells in vitro. Knockdown of CD147 resulted in loss of MCT4 in the plasma membrane and accumulation of the transporter in endolysosomes. These studies establish for the first time that increased expression of CD147 in metastatic cancer cells is coupled to the up-regulation of MCT4. The synergistic activities of the MCT/CD147 complex could facilitate migration of tumor cells by CD147-mediated MMP induction and lactate-stimulated angiogenesis and hyaluronan production. These data provide a molecular link between two hallmarks of metastatic cancer: the glycolytic switch and increased expression of CD147.

Reply to letter “Pyruvate metabolism in working human skeletal muscle” by Henderson et al.

The following is the abstract of the article discussed in the subsequent letter: The aim of this study was to determine whether the decreased muscle and blood lactate during exercise with hyperoxia (60% inspired O2) vs. room air is due to decreased muscle glycogenolysis, leading to decreased

Lactate and malignant tumors: A therapeutic target at the end stage of glycolysis

Metabolic aberrations in the form of altered flux through key metabolic pathways are primary hallmarks of many malignant tumors. Primarily the result of altered isozyme expression, these adaptations enhance the survival and proliferation of the tumor at the expense of surrounding normal tissue. Consequently, they also expose a unique set of targets for tumor destruction while sparing healthy tissues. Despite this fact, development of drugs to directly target such altered metabolic pathways of malignant tumors has been under-investigated until recently. One such target is the ultimate step of glycolysis, which, as expected, presents itself as a metabolic aberration in most malignant tumors. Termed "aerobic glycolysis" due to abnormal conversion of pyruvic acid to lactic acid even under normoxia, the altered metabolism requires these tumors to rapidly efflux lactic acid to the microenvironment in order to prevent poisoning themselves. Thus, exposed is a prime "choke-point" to target these highly malignant, frequently chemo- and radio- resistant tumors. This review will focus on current outcomes in targeting lactate efflux in such tumors using glioma as a model, an ongoing project in our laboratory for the past half-decade, as well as supporting evidence from recent studies by others on targeting this "tail-end" of glycolysis in other tumor models.

Expression of Monocarboxylate Transporter 1 (MCT1) in the Dog Intestine

In this study, the expression and distribution of monocarboxyolate transporter 1 (MCT1) along the intestines (duodenum, jejunum, ileum, cecum, colon and rectum) of dogs were investigated at both the mRNA and protein levels. The expression of MCT1 protein and its distribution were confirmed by Western blotting and immunohistochemical staining using the antibody for MCT1. We identified mRNA coding for MCT1 and a 43-kDa band of MCT1 protein in all regions from the duodenum to the rectum. Immunoreactive staining for MCT1 was also observed in epithelial cells throughout the intestines. MCT1 immunoreactivity was greater in the large intestine than in the small intestine. MCT1 protein was predominantly expressed on the basolateral membranes along intestinal epithelial cells, suggesting that MCT1 may play an important role in lactate efflux and transport of short-chain fatty acids (SCFAs) to the bloodstream across the basolateral membranes of the dog intestine.

A computational model of dynamics of pH, phosphoenergetics and glycogenolysis in mouse skeletal muscle: acidosis does not inhibit resting glycogenolysis

pH changes in muscle cell cytoplasm are a consequence of proton fluxes from biochemical reactions in the intracellular milieu and proton transport across the sarcolemma. These pH changes and the effects of pH on the biochemical reaction kinetics and equilibria influence the overall system behavior. In this study we integrated a previously published model of anaerobic glycogenolysis (Vinnakota et al. Biophys J 91: 1264-87) with an empirical model of oxidative phosphorylation in the intact muscle as an open system with proton transport, metabolic proton load computation, and proton buffering systems i.e., fixed buffers, metabolite buffering and bicarbonate buffering. The model was validated by fitting magnetic resonance spectroscopic data for time courses of PCr, Pi and pH in mouse EDL and SOL in anoxia transient experiments, and pH time course in the SOL in a PCO2 transient experiment. Our model analysis shows that proton fluxes due to ATP hydrolysis, creatine kinase, oxidative phosphorylation, carbon dioxide hydration and lactate efflux reactions are the major determinants of the pH transient; ATP hydrolysis is the main source of protons during anoxia; acidosis due to a transient increase in PCO2 does not inhibit glycogenolysis.

METABOLIC REMODELING OF MALIGNANT GLIOMAS FOR ENHANCED SENSITIZATION DURING RADIOTHERAPY

To investigate a novel method to enhance radiosensitivity of gliomas via modification of metabolite flux immediately before radiotherapy. Malignant gliomas are highly glycolytic and produce copious amounts of lactic acid, which is effluxed to the tumor microenvironment via lactate transporters. We hypothesized that inhibition of lactic acid efflux would alter glioma metabolite profiles, including those that are radioprotective. H magnetic resonance spectroscopy (MRS) was used to quantify key metabolites, including those most effective for induction of low-dose radiation-induced cell death. We inhibited lactate transport in U87-MG gliomas with alpha-cyano-4-hydroxycinnamic acid (ACCA). Flow cytometry was used to assess induction of cell death in treated cells. Cells were analyzed by MRS after ACCA treatment. Control and treated cells were subjected to low-dose irradiation, and the surviving fractions of cells were determined by clonogenic assays. MRS revealed changes to intracellular lactate on treatment with ACCA. Significant decreases in the metabolites taurine, glutamate, glutathione, alanine, and glycine were observed, along with inversion of the choline/phosphocholine profile. On exposure to low-dose radiation, ACCA-pretreated U-87MG cells underwent rapid morphological changes, which were followed by apoptotic cell death. Inhibition of lactate efflux in malignant gliomas results in alterations of glycolytic metabolism, including decreased levels of the antioxidants taurine and glutathione and enhanced radiosensitivity of ACCA-treated cells. Thus, in situ application of lactate transport inhibitors such as ACCA as a novel adjunctive therapeutic strategy against glial tumors may greatly enhance the level of radiation-induced cell killing during a combined radio- and chemotherapeutic regimen.

Sex differences in energy metabolism and performance of teleost cardiac tissue

This study examined the effects of different oxygenation levels and substrate availability on cardiac performance, metabolism, and biochemistry in sexually immature male and female rainbow trout (Oncorhynchus mykiss). Ventricle strips were electrically paced (0.5 Hz, 14 degrees C) in hyperoxic or hypoxic Ringer solution. Our results demonstrate that 1) males sustain isometric force production (F) longer than females under hyperoxia (P O2 = 640 mmHg) with exogenous glucose present; 2) contractility is not maintained under moderate (P O2 = 130 mmHg) or severe hypoxia (P O2 = 10-20 mmHg) with glucose in either sex; however, following reoxygenation, F is higher in females compared with males; and 3) female tissue has higher lactate levels, net lactate efflux, and lactate dehydrogenase activity than males, whereas males have higher glycogen, citrate synthase, and beta-hydroxy acyl-CoA dehydrogenase activities, and greater inotropic responses to exogenous glucose and octanoate. No sex differences were detected in responsiveness to epinephrine and inhibitors of glucose transport or activities of hexokinase and pyruvate kinase. We conclude that sex differences exist in rainbow trout cardiac tissue: females appear to prefer glycolysis for ATP production, whereas males have a higher capacity for aerobic and lipid metabolism.

Letter to the Editor

Letter to the Editor

An elevated sarcolemmal lactate (and proton) transport capacity is an advantage during muscle activity in healthy humans.

The etiology of muscle fatigue remains incompletely solved especially in vivo (3). The causes of muscle fatigue vary according to the type, duration and intensity of exercise, and to the physical fitness and health status of the subjects. Nevertheless, an elevated sarcolemmal lactate (and proton) transport capacity constitutes an advantage during muscle activity. During moderate-intensity exercise, efficient lactate exchanges contribute to the cell-cell lactate shuttle i.e., the delivery of lactate for its utilization as fuel by neighboring or distant active and oxidative muscles fibers or as substrate by other tissues (e.g. for neoglucogenesis by the liver) (1). During high-intensity exercise, the lactate exchange ability was positively correlated with the capacity to prolong exercise (4). Whether this correlation between the lactate exchange ability and performance was (directly or indirectly) causal or coincidental, the question is still under debate. Nevertheless, it remains that an elevated lactate exchange ability seems to constitute a protective mechanism against muscle fatigue. Since the transport of lactate across the sarcolemma is mediated mainly by the lactate-H+ cotransport via the monocarboxylate transporters MCT1 and MCT4, an elevated lactate transport capacity delays both muscle lactate accumulation and intracellular pH decrease and seems to favor muscle activity. In accordance, previous experiments have shown that metabolic alkalosis enhances the net lactate release rate from the active muscles and work capacity (2, 5) contrary to acidosis that reduces lactate efflux from muscle and exercise tolerance (5). All this makes an elevated sarcolemmal lactate (and proton) transport capacity an advantage during muscle activity.

The following letters are in response to Point:Counterpoint series “Lactic acid accumulation is an advantage/disadvantage during muscle activity”

The following letters are in response to Point:Counterpoint series “Lactic acid accumulation is an advantage/disadvantage during muscle activity” that appeared in the April issue ( J Appl Physiol 100: 1410–1414, 2006; ). To the Editor :

The Experimental Type 2 Diabetes Therapy Glycogen Phosphorylase Inhibition Can Impair Aerobic Muscle Function During Prolonged Contraction

Glycogen phosphorylase inhibition represents a promising strategy to suppress inappropriate hepatic glucose output, while muscle glycogen is a major source of fuel during contraction. Glycogen phosphorylase inhibitors (GPi) currently being investigated for the treatment of type 2 diabetes do not demonstrate hepatic versus muscle glycogen phosphorylase isoform selectivity and may therefore impair patient aerobic exercise capabilities. Skeletal muscle energy metabolism and function are not impaired by GPi during high-intensity contraction in rat skeletal muscle; however, it is unknown whether glycogen phosphorylase inhibitors would impair function during prolonged lower-intensity contraction. Utilizing a novel red cell-perfused rodent gastrocnemius-plantaris-soleus system, muscle was pretreated for 60 min with either 3 micromol/l free drug GPi (n=8) or vehicle control (n=7). During 60 min of aerobic contraction, GPi treatment resulted in approximately 35% greater fatigue. Muscle glycogen phosphorylase a form (P<0.01) and maximal activity (P<0.01) were reduced in the GPi group, and postcontraction glycogen (121.8 +/- 16.1 vs. 168.3 +/- 8.5 mmol/kg dry muscle, P<0.05) was greater. Furthermore, lower muscle lactate efflux and glucose uptake (P<0.01), yet higher muscle Vo(2), support the conclusion that carbohydrate utilization was impaired during contraction. Our data provide new confirmation that muscle glycogen plays an essential role during submaximal contraction. Given the critical role of exercise prescription in the treatment of type 2 diabetes, it will be important to monitor endurance capacity during the clinical evaluation of nonselective GPi. Alternatively, greater effort should be devoted toward the discovery of hepatic-selective GPi, hepatic-specific drug delivery strategies, and/or alternative strategies for controlling excess hepatic glucose production in type 2 diabetes.

Hyperoxia Decreases Muscle Glycogenolysis, Pyruvate and Lactate Production during Steady State Exercise

Breathing hyperoxic air decreases muscle glycogenolysis, which leads to decreased lactate contents and increases in exercise performance in humans. These findings suggest alterations in skeletal muscle carbohydrate metabolism. PURPOSE: The study had two aims 1) to determine if the decreased muscle and blood lactate found during steady state exercise with hyperoxic vs. normoxic breathing is due to decreased muscle glycogenolysis, leading to decreased pyruvate and lactate accumulations and efflux and 2) to elucidate the mechanisms behind the decreased glycogenolysis. We hypothesized that 60% O2 would decrease muscle glycogenolysis, from decreases in free ADP and AMP, resulting in decreased pyruvate and lactate contents and efflux with no change in pyruvate dehydrogenase (PDH) activity (pyruvate oxidation). METHODS: Seven active male subjects cycled for 40 min at 70% VO2peak, on two occasions while breathing 21 or 60% O2. Arterial and venous blood and blood flow measurements were obtained throughout each trial, and muscle biopsies were taken at rest and following 10, 20 and 40 min of exercise. RESULTS: During exercise, muscle glycogenolysis was reduced by 16% with hyperoxia (267 ± 19 vs. 317 ± 21 mmol glycosyl units · kg−1 dw), but pyruvate accumulation and pyruvate oxidation via PDH activity were unaffected (60%: 3.02 ±0.50 vs. 21%: 3.03 ± 0.39 mmol · kg−1·min−1). However, the reduced glycogenolysis with hyperoxia did result in a 22% reduction (p<0.05) in muscle lactate accumulation (60%: 22.6 ± 6.4 vs. 21%: 31.3 ± 8.7 mmol · kg−1 dw) and decreased blood lactate efflux and total lactate production over 40 min of cycling. We also found that hyperoxia significantly decreased PCr degradation (60%: 36.4 ± 4.0 vs. 21 %: 29.9 ± 5.1 mmol · kg−1 dw), attenuated (p<0.05) the accumulations of Cr and calculated ADPf and AMPf (AMPf: 60%: 3.55 ± 1.14 vs. 21%: 5.67 ± 1.60 umol · kg−1 dw) and reduced (p<0.05) blood epinephrine concentration by ∼44% throughout 40 min of cycling. CONCLUSION: Hyperoxia decreased muscle glycogenolysis, with no change in pyruvate oxidation via PDH, resulting in significantly less muscle and blood lactate accumulation during 40 min of cycling - a tighter metabolic match between pyruvate production and oxidation. The decreased glycogenolysis during exercise with hyperoxia appeared to be the result of attenuated accumulations of the potent allosteric effectors of glycogen phosphorylase, specfically free ADP and AMP and a decreased epinephrine concentration during exercise. Lastly, the significantly diminished rate of PCr utilization during hyperoxia suggests an increase energy provision via oxidative phosphorylation during aerobic exercise.

Hyperinsulinemia increases glucose uptake and oxidation and improves contractile function during ischemia despite no effect on fatty acid oxidation or lactate production

Ex vivo studies in rodent hearts show that increased glucose metabolism by hyperinsulinemia during ischemia improves cardiac mechanical efficiency. This protective effect has not been demonstrated in vivo, and it is unclear if it is dependent upon inhibition of fatty acid oxidation (FAO). In the present study we tested the hypothesis that stimulation of glucose uptake and oxidation by insulin infusion would improve ventricular wall motion independent of changes in FAO. Measurements were made in anesthetized open-chest swine hearts subjected to 40 mins of regional myocardial ischemia. Pigs were either treated with a high dose of i.v. insulin (art. conc of 4219 pM) initiated 50 min before ischemia, or were untreated (6 pM) (n=8/group). The insulin group received i.v. glucose to maintain arterial levels at ~6 mM. Glucose and FAO were measured with 14C-glucose and 3H-oleate, and regional myocardial external power was calculated from the LV pressure - segment length loop area as assessed by sonomicrometry. Results showed that insulin infusion increased glucose uptake and oxidation, but did not significantly effect the total lactate efflux during ischemia. Hyperinsulinemia showed a strong trend towards increased cardiac power index and myocardial efficiency. There were no significant differences in the MVO2, FAO and tissue ATP content between two groups under ischemic condition. Conclusion Enhanced glucose oxidation increased the myocardial energy efficiency and provided protective effect to the heart during ischemia. This effect was not dependent upon inhibition of FAO nor increased lactate production.

Hyperoxia decreases muscle glycogenolysis, lactate production, and lactate efflux during steady-state exercise

The aim of this study was to determine whether the decreased muscle and blood lactate during exercise with hyperoxia (60% inspired O2) vs. room air is due to decreased muscle glycogenolysis, leading to decreased pyruvate and lactate production and efflux. We measured pyruvate oxidation via PDH, muscle pyruvate and lactate accumulation, and lactate and pyruvate efflux to estimate total pyruvate and lactate production during exercise. We hypothesized that 60% O2 would decrease muscle glycogenolysis, resulting in decreased pyruvate and lactate contents, leading to decreased muscle pyruvate and lactate release with no change in PDH activity. Seven active male subjects cycled for 40 min at 70% VO2 peak on two occasions when breathing 21 or 60% O2. Arterial and femoral venous blood samples and blood flow measurements were obtained throughout exercise, and muscle biopsies were taken at rest and after 10, 20, and 40 min of exercise. Hyperoxia had no effect on leg O2 delivery, O2 uptake, or RQ during exercise. Muscle glycogenolysis was reduced by 16% with hyperoxia (267 +/- 19 vs. 317 +/- 21 mmol/kg dry wt), translating into a significant, 15% reduction in total pyruvate production over the 40-min exercise period. Decreased pyruvate production during hyperoxia had no effect on PDH activity (pyruvate oxidation) but significantly decreased lactate accumulation (60%: 22.6 +/- 6.4 vs. 21%: 31.3 +/- 8.7 mmol/kg dry wt), lactate efflux, and total lactate production over 40 min of cycling. Decreased glycogenolysis in hyperoxia was related to an approximately 44% lower epinephrine concentration and an attenuated accumulation of potent phosphorylase activators ADPf and AMPf during exercise. Greater phosphorylation potential during hyperoxia was related to a significantly diminished rate of PCr utilization. The tighter metabolic match between pyruvate production and oxidation resulted in a decrease in total lactate production and efflux over 40 min of exercise during hyperoxia.

Endothelial Na+-D-glucose Cotransporter: No Role in Insulin-mediated Glucose Uptake

A recent report indicates that the Na+-D-glucose cotransporter SGLT1 is present in capillaries of skeletal muscle and is required for insulin-mediated glucose uptake in myocytes. This result is based on the complete inhibition of insulin-mediated muscle glucose uptake by phlorizin, an inhibitor of SGLT1. Using the pump-perfused rat hind limb, we measured glucose uptake, lactate efflux, and radioactive 2-deoxyglucose uptake into individual muscles with saline (control), phlorizin, insulin, and insulin plus phlorizin, as well as with saline and insulin using normal and low Na+ perfusion buffer. Insulin-mediated glucose uptake was not inhibited after correction for phlorizin interference in the glucose assay. Lactate efflux and 2-deoxyglucose uptake by individual muscles were unaffected by phlorizin. Low Na+ buffer did not affect insulin-mediated glucose uptake, lactate efflux, or 2-deoxyglucose uptake. We conclude that endothelial SGLT1 exerts no barrier for glucose delivery to myocytes.

Nitric oxide synthase inhibition reduces the O2 cost of force development in rat hindlimb muscles pump perfused at matched convective O2 delivery

Nitric oxide (NO) is a physiological mediator of skeletal muscle function. Specifically, NO affects cellular respiration and muscle contractility; however, the reduced blood flow and convective O2 delivery that result from impaired vasodilatation when NO synthase (NOS) is inhibited in vivo have obscured past interpretations of the effects of NO. Therefore, we studied the effect of NOS inhibition in an in situ pump-perfused rat hindlimb to test the hypothesis that NOS inhibition would improve contractile and aerobic metabolic performance. Pump perfusion permitted matching of convective O2 delivery (516 +/- 16 micromol O2 min(-1) (100 g)(-1); mean +/- s.e.m.) between groups, allowing us to investigate the effects of NOS inhibition independent of this variable. Three groups were studied. The perfusate of one group was treated with both adenosine (0.01 mm) and the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME; 1 mm). Adenosine is a vasodilator that can act through both NO-dependent and -independent pathways; the NO-independent vasodilatory action of adenosine allowed us to match the perfusion rate and convective O2 delivery in this L-NAME group to those of the other groups. In the second group the perfusate was treated with adenosine only (Ado). In the third group the perfusate received no treatment and served as a control (Con). Oxygen consumption (VO2) was on average 26 and 14% lower during the contraction bout in L-NAME and Ado, respectively, versus Con. In Ado, lactate efflux was similar to Con and force was reduced in proportion to versus Con, whereas L-NAME was associated with a 32% lower lactate efflux and similar force to Con. Therefore, the lower :force development ratio in the L-NAME group demonstrates that the O2 cost of force development is reduced by NOS inhibition independent of convective O2 delivery.