Muscle Na+-K+-ATPase response during 16 h of heavy intermittent cycle exercise

Digoxin and exercise effects on skeletal muscle Na+,K+-ATPase isoform gene expression in healthy humans

In muscle, digoxin inhibits Na+,K+-ATPase (NKA) whereas acute exercise can increase NKA gene expression, consistent with training-induced increased NKA content. We investigated whether oral digoxin increased NKA isoform mRNA expression (qPCR) in muscle at rest, during and post-exercise in 10 healthy adults, who received digoxin (DIG, 0.25 mg per day) or placebo (CON) for 14 days, in a randomised, double-blind and cross-over design. Muscle was biopsied at rest, after cycling 20 min (10 min each at 33%, then 67%V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_2}{\mathrm{peak}}}}$ ), then to fatigue at 90%V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_2}{\mathrm{peak}}}}$ and 3 h post-exercise. No differences were found between DIG and CON for NKA α1-3 or β1-3 isoform mRNA. Both α1 (354%, P = 0.001) and β3 mRNA (P = 0.008) were increased 3 h post-exercise, with α2 and β1-2 mRNA unchanged, whilst α3 mRNA declined at fatigue (-43%, P = 0.045). In resting muscle, total β mRNA (∑(β1+β2+β3)) increased in DIG (60%, P = 0.025) and also when transcripts for each isoform were normalised to CON then either summed (P = 0.030) or pooled (n = 30, P = 0.034). In contrast, total α mRNA (∑(α1+α2+α3), P = 0.348), normalised then summed (P = 0.332), or pooled transcripts (n = 30, P = 0.717) did not differ with DIG. At rest, NKA α1-2 and β1-2 protein abundances were unchanged by DIG. Post-exercise, α1 and β1-2 proteins were unchanged, but α2 declined at 3 h (19%, P = 0.020). In conclusion, digoxin did not modify gene expression of individual NKA isoforms at rest or with exercise, indicating NKA gene expression was maintained consistent with protein abundances. However, elevated resting muscle total β mRNA with digoxin suggests a possible underlying β gene-stimulatory effect. HIGHLIGHTS: What is the central question of this study? Na+,K+-ATPase (NKA) in muscle is important for Na+/K+ homeostasis. We investigated whether the NKA-inhibitor digoxin stimulates increased NKA gene expression in muscle and exacerbates NKA gene responses to exercise in healthy adults. What is the main finding and its importance? Digoxin did not modify exercise effects on muscle NKA α1-3 and β1-3 gene transcripts, which comprised increased post-exercise α1 and β3 mRNA and reduced α3 mRNA during exercise. However, in resting muscle, digoxin increased NKA total β isoform mRNA expression. Despite inhibitory-digoxin or acute exercise stressors, NKA gene regulation in muscle is consistent with the maintenance of NKA protein contents.

A century of exercise physiology: effects of muscle contraction and exercise on skeletal muscle Na+,K+-ATPase, Na+ and K+ ions, and on plasma K+ concentration-historical developments

This historical review traces key discoveries regarding K+ and Na+ ions in skeletal muscle at rest and with exercise, including contents and concentrations, Na+,K+-ATPase (NKA) and exercise effects on plasma [K+] in humans. Following initial measures in 1896 of muscle contents in various species, including humans, electrical stimulation of animal muscle showed K+ loss and gains in Na+, Cl- and H20, then subsequently bidirectional muscle K+ and Na+ fluxes. After NKA discovery in 1957, methods were developed to quantify muscle NKA activity via rates of ATP hydrolysis, Na+/K+ radioisotope fluxes, [3H]-ouabain binding and phosphatase activity. Since then, it became clear that NKA plays a central role in Na+/K+ homeostasis and that NKA content and activity are regulated by muscle contractions and numerous hormones. During intense exercise in humans, muscle intracellular [K+] falls by 21 mM (range - 13 to - 39 mM), interstitial [K+] increases to 12-13 mM, and plasma [K+] rises to 6-8 mM, whilst post-exercise plasma [K+] falls rapidly, reflecting increased muscle NKA activity. Contractions were shown to increase NKA activity in proportion to activation frequency in animal intact muscle preparations. In human muscle, [3H]-ouabain-binding content fully quantifies NKA content, whilst the method mainly detects α2 isoforms in rats. Acute or chronic exercise affects human muscle K+, NKA content, activity, isoforms and phospholemman (FXYD1). Numerous hormones, pharmacological and dietary interventions, altered acid-base or redox states, exercise training and physical inactivity modulate plasma [K+] during exercise. Finally, historical research approaches largely excluded female participants and typically used very small sample sizes.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Molecular stressors underlying exercise training-induced improvements in K+ regulation during exercise and Na+ ,K+ -ATPase adaptation in human skeletal muscle

Despite substantial progress made towards a better understanding of the importance of skeletal muscle K⁺ regulation for human physical function and its association with several disease states (eg type-II diabetes and hypertension), the molecular basis underpinning adaptations in K⁺ regulation to various stimuli, including exercise training, remains inadequately explored in humans. In this review, the molecular mechanisms essential for enhancing skeletal muscle K⁺ regulation and its key determinants, including Na⁺ ,K⁺ -ATPase function and expression, by exercise training are examined. Special attention is paid to the following molecular stressors and signaling proteins: oxygenation, redox balance, hypoxia, reactive oxygen species, antioxidant function, Na⁺ ,K⁺ , and Ca²⁺ concentrations, anaerobic ATP turnover, AMPK, lactate, and mRNA expression. On this basis, an update on the effects of different types of exercise training on K⁺ regulation in humans is provided, focusing on recent discoveries about the muscle fibre-type-dependent regulation of Na⁺ ,K⁺ -ATPase-isoform expression. Furthermore, with special emphasis on blood-flow-restricted exercise as an exemplary model to modulate the key molecular mechanisms identified, it is discussed how training interventions may be designed to maximize improvements in K⁺ regulation in humans. The novel insights gained from this review may help us to better understand how exercise training and other strategies, such as pharmacological interventions, may be best designed to enhance K⁺ regulation and thus the physical function in humans.

Carbohydrate supplementation stabilises plasma sodium during training with high intensity

BACKGROUND

Investigations of the effect of beverages containing carbohydrates, only, on the sodium and fluid balance during intermittent exercise of high intensity are rare. Therefore, we compared the effects of water and carbohydrate supplementation on plasma, blood volume, and electrolyte shifts during intermittent exercise.

METHODS

Ten male subjects performed an intermittent exercise test twice. In one trial, tap water (4 ml/kg/15 min) was consumed (Plac trial). In the other trial, the same amount of water supplemented with maltodextrin to achieve a 9.1 % carbohydrate solution (CHO trial) was ingested. Training schedule: warm-up at 50 % for 15 min. Afterwards, power changed between 100 % of the maximum power from a previous incremental test minus 10 and 10 W for each 30 s. Venous blood was sampled to measure electrolytes, osmolality, [protein], hct, [Lactate], [glucose], [Hb] and catecholamines. Hydration status was evaluated by BIA before and after exercise.

RESULTS

After beverage ingestion [glucose] was significantly higher in CHO until the end of the trial. Starting with similar resting values, osmolality increased significantly more during CHO (p = 0.002). PV decreased by 5 % under both conditions, but recovered partly during exercise under Plac (p = 0.002). [Na+] and [Cl(-)] decreased with Plac during exercise (both p < 0.001) but remained constant during exercise with CHO.

CONCLUSIONS

Sole carbohydrate supplementation seems to stabilise plasma [Na+]. This cannot be explained simply by a cotransport of glucose and [Na+], because that should lead to a recovery of the blood and plasma volume under CHO. In contrast, this was found during exercise with Plac.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Quantification of Na+,K+ pumps and their transport rate in skeletal muscle: functional significance

During excitation, muscle cells gain Na⁺ and lose K⁺, leading to a rise in extracellular K⁺ ([K⁺]_o), depolarization, and loss of excitability. Recent studies support the idea that these events are important causes of muscle fatigue and that full use of the Na⁺,K⁺-ATPase (also known as the Na⁺,K⁺ pump) is often essential for adequate clearance of extracellular K⁺. As a result of their electrogenic action, Na⁺,K⁺ pumps also help reverse depolarization arising during excitation, hyperkalemia, and anoxia, or from cell damage resulting from exercise, rhabdomyolysis, or muscle diseases. The ability to evaluate Na⁺,K⁺-pump function and the capacity of the Na⁺,K⁺ pumps to fill these needs require quantification of the total content of Na⁺,K⁺ pumps in skeletal muscle. Inhibition of Na⁺,K⁺-pump activity, or a decrease in their content, reduces muscle contractility. Conversely, stimulation of the Na⁺,K⁺-pump transport rate or increasing the content of Na⁺,K⁺ pumps enhances muscle excitability and contractility. Measurements of [³H]ouabain binding to skeletal muscle in vivo or in vitro have enabled the reproducible quantification of the total content of Na⁺,K⁺ pumps in molar units in various animal species, and in both healthy people and individuals with various diseases. In contrast, measurements of 3-O-methylfluorescein phosphatase activity associated with the Na⁺,K⁺-ATPase may show inconsistent results. Measurements of Na⁺ and K⁺ fluxes in intact isolated muscles show that, after Na⁺ loading or intense excitation, all the Na⁺,K⁺ pumps are functional, allowing calculation of the maximum Na⁺,K⁺-pumping capacity, expressed in molar units/g muscle/min. The activity and content of Na⁺,K⁺ pumps are regulated by exercise, inactivity, K⁺ deficiency, fasting, age, and several hormones and pharmaceuticals. Studies on the α-subunit isoforms of the Na⁺,K⁺-ATPase have detected a relative increase in their number in response to exercise and the glucocorticoid dexamethasone but have not involved their quantification in molar units. Determination of ATPase activity in homogenates and plasma membranes obtained from muscle has shown ouabain-suppressible stimulatory effects of Na⁺ and K⁺.

Influence of chronic and acute spinal cord injury on skeletal muscle Na+-K+-ATPase and phospholemman expression in humans

Na(+)-K(+)-ATPase is an integral membrane protein crucial for the maintenance of ion homeostasis and skeletal muscle contractibility. Skeletal muscle Na(+)-K(+)-ATPase content displays remarkable plasticity in response to long-term increase in physiological demand, such as exercise training. However, the adaptations in Na(+)-K(+)-ATPase function in response to a suddenly decreased and/or habitually low level of physical activity, especially after a spinal cord injury (SCI), are incompletely known. We tested the hypothesis that skeletal muscle content of Na(+)-K(+)-ATPase and the associated regulatory proteins from the FXYD family is altered in SCI patients in a manner dependent on the severity of the spinal cord lesion and postinjury level of physical activity. Three different groups were studied: 1) six subjects with chronic complete cervical SCI, 2) seven subjects with acute, complete cervical SCI, and 3) six subjects with acute, incomplete cervical SCI. The individuals in groups 2 and 3 were studied at months 1, 3, and 12 postinjury, whereas individuals with chronic SCI were compared with an able-bodied control group. Chronic complete SCI was associated with a marked decrease in [(3)H]ouabain binding site concentration in skeletal muscle as well as reduced protein content of the α(1)-, α(2)-, and β(1)-subunit of the Na(+)-K(+)-ATPase. In line with this finding, expression of the Na(+)-K(+)-ATPase α(1)- and α(2)-subunits progressively decreased during the first year after complete but not after incomplete SCI. The expression of the regulatory protein phospholemman (PLM or FXYD1) was attenuated after complete, but not incomplete, cervical SCI. In contrast, FXYD5 was substantially upregulated in patients with complete SCI. In conclusion, the severity of the spinal cord lesion and the level of postinjury physical activity in patients with SCI are important factors controlling the expression of Na(+)-K(+)-ATPase and its regulatory proteins PLM and FXYD5.

Effect of exercise and training on phospholemman phosphorylation in human skeletal muscle

Phospholemman (PLM, FXYD1) is a partner protein and regulator of the Na(+)-K(+)-ATPase (Na(+)-K(+) pump). We explored the impact of acute and short-term training exercise on PLM physiology in human skeletal muscle. A group of moderately trained males (n = 8) performed a 1-h acute bout of exercise by utilizing a one-legged cycling protocol. Muscle biopsies were taken from vastus lateralis at 0 and 63 min (non-exercised leg) and 30 and 60 min (exercised leg). In a group of sedentary males (n = 9), we determined the effect of a 10-day intense aerobic cycle training on Na(+)-K(+)-ATPase subunit expression, PLM phosphorylation, and total PLM expression as well as PLM phosphorylation in response to acute exercise (1 h at ∼72% Vo(2peak)). Biopsies were taken at rest, immediately following, and 3 h after an acute exercise bout before and at the conclusion of the 10-day training study. PLM phosphorylation was increased both at Ser(63) and Ser(68) immediately after acute exercise (75%, P < 0.05, and 30%, P < 0.05, respectively). Short-term training had no adaptive effect on PLM phosphorylation at Ser(63) and Ser(68), nor was the total amount of PLM altered posttraining. The protein expressions of α(1)-, α(2)-,and β(1)-subunits of Na(+)-K(+)-ATPase were increased after training (113%, P < 0.05, 49%, P < 0.05, and 27%, P < 0.05, respectively). Whereas an acute bout of exercise increased the phosphorylation of PKCα/βII on Thr(638/641) pre- and posttraining, phosphorylation of PKCζ/λ on Thr(403/410) was increased in response to acute exercise only after the 10-day training. In conclusion, we show that only acute exercise, and not short-term training, increases phosphorylation of PLM on Ser(63) and Ser(68), and data from one-legged cycling indicate that this effect of exercise on PLM phosphorylation is not due to systemic factors. Our results provide evidence that phosphorylation of PLM may play a role in the acute regulation of the Na(+)-K(+)-ATPase response to exercise.

Exercise-induced regulation of muscular Na+-K+ pump, FXYD1, and NHE1 mRNA and protein expression: importance of training status, intensity, and muscle type

It is investigated if exercise-induced mRNA changes cause similar protein expression changes of Na(+)-K(+) pump isoforms (α(1), α(2), β(1), β(2)), FXYD1, and Na(+)/K(+) exchanger (NHE1) in rat skeletal muscle. Expression was evaluated (n = 8 per group) in soleus and extensor digutorum longus after 1 day, 3 days, and 3 wk (5 sessions/wk) of either sprint (4 × 3-min sprint + 1-min rest) or endurance (20 min) running. Two hours after exercise on day 1, no change in protein expression was apparent in either training group or muscle, whereas sprint exercise increased the mRNA of soleus α(2) (4.9 ± 0.8-fold; P < 0.05), β(2) (13.2 ± 4.4-fold; P < 0.001), and NHE1 (12.0 ± 3.1-fold; P < 0.01). Two hours after sprint exercise, protein expression normalized to control samples was higher on day 3 than day 1 for soleus α(1) (41 ± 18% increase vs. 15 ± 8% reduction; P < 0.05), α(2) (64 ± 35% increase vs. 37 ± 12% reduction; P < 0.05), β(1) (17 ± 21% increase vs. 14 ± 29% reduction; P < 0.05), and FXYD1 (35 ± 16% increase vs. 13 ± 10% reduction; P < 0.05). In contrast, on day 3, soleus α(1) (0.1 ± 0.1-fold; P < 0.001), α(2) (0.2 ± 0.1-fold; P < 0.001), β(1) (0.4 ± 0.1-fold; P < 0.05), and β(2)-mRNA (2.9 ± 1.7-fold; P < 0.001) expression was lower than after exercise on day 1. After 3 wk of training, no change in protein expression relative to control existed. In conclusion, increased expression of Na(+)-K(+) pump subunits, FXYD1 and NHE1 after 3 days exercise training does not appear to be an effect of increased constitutive mRNA levels. Importantly, sprint exercise can reduce mRNA expression concomitant with increased protein expression.

Cellular assessment of muscle in COPD: case studies of two males

The objective of this paper is to provide an overview of the recent developments in muscle physiology and biochemistry in general, and with respect to chronic obstructive pulmonary disease (COPD) specifically. As a way of illustration, we have presented data on the remodeling that occurs in vastus lateralis in two patients with COPD (COPD #1, forced expiratory volume in one second/forced vital capacity [FEV₁/FVC] = 63%; COPD #2, FEV₁/FVC = 41%) exhibiting differences in muscle wasting as compared to healthy controls (CON; FEV₁/FVC = 111 ± 2.2%, n = 4). Type I fibers percentages were lower in both COPD #1 (16.7) and COPD #2 (24.9) compared to CON (57.3 ± 5.2). Cross sectional area of the type I fibers of the patients ranged between 65%–68% of CON and for the type II subtypes (IIA, IIAX, IIX) between 74% and 89% (COPD #1) and 17%–32% (COPD #2). A lower number of capillary contacts were observed for all fiber types in COPD #1 but not COPD #2. Lower concentrations of adenosine triphosphate (ATP) (24%–26%) and phosphocreatine (18%–20%), but not lactate occurred in COPD. In contrast to COPD #1, who displayed normal glucose transporter content, GLUT1 and GLUT4 were only 71% and 54%, respectively of CON in COPD #2. Lower monocarboxylate contents were found for MCT1 in both COPD #1 (63%) and COPD #2 (41%) and for MCT4 (78%) in COPD #1. Maximal oxidative enzyme activities (V_max) for COPD #2 ranged between 37% (succinic dehydrogenase) and 70% (cytochrome C oxidase) of CON. For the cytosolic enzymes, V_max ranged between 89% (hexokinase) to 31% (pyruvate kinase) of CON. Depressions were also observed in V_max of the Na⁺-K⁺-ATPase for COPD #1 (66% of CON) but not COPD #2 (92% of CON) while V_max of the Ca²⁺-ATPase was near normal in COPD #1 (84% CON). It is concluded that disturbances can occur in muscle to a wide range of excitation, contraction and metabolic processes in COPD.

Dissociation between changes in muscle Na+-K+-ATPase isoform abundance and activity with consecutive days of exercise and recovery

The early plasticity of vastus lateralis Na(+)-K(+)-ATPase to the abrupt onset of prolonged submaximal cycling was studied in 12 untrained participants (Vo(2 peak) 44.8 +/- 2.0 ml x kg(-1) x min(-1), mean +/- SE) using a 6-day protocol (3 days of exercise plus 3 days of recovery). Tissue samples were extracted prior to (Pre) and following exercise (Post) on day 1 (E1) and day 3 (E3) and on each day of recovery (R1, R2, R3) and analyzed for changes in maximal protein (beta(max)) (vanadate-facilitated [(3)H]ouabain binding), alpha- and beta-isoform concentration (quantitative immunoblotting) and maximal Na(+)-K(+)-ATPase activity (V(max)) (3-O-methylfluorescein K(+)-stimulated phosphatase assay). For beta(max) (pmol/g wet wt), an increase (P < 0.05) of 11.8% was observed at R1 compared with E1-Pre (340 +/- 14 vs 304 +/- 17). For the alpha-isoforms alpha(1), alpha(2), and alpha(3), increases (P < 0.05) of 46, 42, and 31% were observed at R1, respectively. For the beta-isoform, beta(1) and beta(2) increased (P < 0.05) by 19 and 28% at R1, whereas beta(3) increased (P < 0.05) by 18% at R2. With the exception of alpha(2) and alpha(3), the increases in the isoforms persisted at R3. Exercise resulted in an average decrease (P < 0.05) in V(max) by 14.3%. No differences were observed in V(max) at E1 - Pre and E3 - Pre or between R1, R2, and R3. It is concluded that 3 days of prolonged exercise is a powerful stimulus for the rapid upregulation of the Na(+)-K(+)-ATPase subunit isoforms. Contrary to our hypothesis, the increase in subunit expression is not accompanied by increases in the maximal catalytic activity.

Acute responses in muscle mitochondrial and cytosolic enzyme activities during heavy intermittent exercise

To examine the effects of repetitive bouts of heavy exercise on the maximal activities of enzymes representative of the major metabolic pathways and segments, 13 untrained volunteers [peak aerobic power (V̇o2 peak) = 44.3 ± 2.3 ml·kg−1·min−1] cycled at ∼91% V̇o2 peak for 6 min once per hour for 16 h. Maximal enzyme activities ( Vmax, mol·kg−1·protein·h−1) were measured in homogenates from tissue extracted from the vastus lateralis before and after exercise at repetitions 1 (R1), 2 (R2), 9 (R9), and 16 (R16). For the mitochondrial enzymes, exercise resulted in reductions ( P < 0.05) in cytochrome- c oxidase (COX, 14.6%), near significant reductions in malate dehydrogenase (4.06%; P = 0.06) and succinic dehydrogenase (4.82%; P = 0.09), near significant increases in β-hydroxyacyl-CoA dehydrogenase (4.94%; P = 0.08), and no change in citrate synthase (CS, 2.88%; P = 0.37). For the cytosolic enzymes, exercise reduced ( P < 0.05) Vmax in hexokinase (Hex, 4.4%), creatine phosphokinase (9.0%), total phosphorylase (13.5%), phosphofructokinase (16.6%), pyruvate kinase (PK, 14.1%) and lactate dehydrogenase (10.7%). Repetition-dependent reductions ( P < 0.05) in Vmax were observed for CS (R1, R2 > R16), COX (R1, R2 > R16), Hex (1R, 2R > R16), and PK (R9 > R16). It is concluded that heavy exercise results in transient reductions in a wide range of enzymes involved in different metabolic functions and that in the case of selected enzymes, multiple repetitions of the exercise reduce average Vmax.

Rapid upregulation of GLUT-4 and MCT-4 expression during 16 h of heavy intermittent cycle exercise

In this study, we have investigated the hypothesis that an exercise protocol designed to repeatedly induce a large dependence on carbohydrate and large increases in glycolytic flux rate would result in rapid increases in the principal glucose and lactate transporters in working muscle, glucose transporter (GLUT)-4 and monocarboxylate transporter (MCT)4, respectively, and in activity of hexokinase (Hex), the enzyme used to phosphorylate glucose. Transporter abundance and Hex activity were assessed in homogenates by Western blotting and quantitative chemiluminescence and fluorometric techniques, respectively, in samples of tissue obtained from the vastus lateralis in 12 untrained volunteers [peak aerobic power (V̇o2peak) = 44.3 ± 2.3 ml·kg−1·min−1] before cycle exercise at repetitions 1 (R1), 2 (R2), 9 (R9), and 16 (R16). The 16 repetitions of the exercise were performed for 6 min at ∼90% V̇o2peak, once per hour. Compared with R1, GLUT-4 increased ( P < 0.05) by 28% at R2 and remained elevated ( P < 0.05) at R9 and R16. For MCT-4, increases ( P < 0.05) of 24% were first observed at R9 and persisted at R16. No changes were observed in GLUT-1 and MCT-1 or in Hex activity. The ∼17- to 24-fold increase ( P < 0.05) in muscle lactate observed at R1 and R2 was reduced ( P < 0.05) to an 11-fold increase at R9 and R16. It is concluded that an exercise protocol designed to strain muscle carbohydrate reserves and to result in large increases in lactic acid results in a rapid upregulation of both GLUT-4 and MCT-4.