Effect of training and detraining on monocarboxylate transporter (MCT) 1 and MCT4 in Thoroughbred horses

Metabolomic responses to high-intensity interval exercise in equine skeletal muscle: effects of rest interval duration

High-intensity interval training has attracted considerable attention as a time-efficient strategy for inducing physiological adaptations, but the underlying mechanisms have yet to be elucidated. By using metabolomics techniques, we investigated changes in the metabolic network responses in Thoroughbred horses to high-intensity interval exercise performed with two distinct (15 min or 2 min) rest intervals. The peak plasma lactate level was higher during high-intensity exercise with a 2 min rest duration than that with a 15 min rest duration (24.5±6.8 versus 13.3±2.7 mmol l-1). The arterial oxygen saturation was lower at the end of all exercise sessions with a 2 min rest duration than that with a 15 min rest duration. Metabolomic analysis of skeletal muscle revealed marked changes in metabolite concentrations in the first and third bouts of the 15 min rest interval conditions. In contrast, there were no metabolite concentrations or pathways that significantly changed during the third bout of exercise performed with a 2 min rest interval. Our findings suggest that the activity of each energy production system is not necessarily reflected by apparent changes in metabolite concentrations, potentially due in part to a better match between metabolite flux into and out of the pathway and cycle, as well as between metabolite production and disposal. This study provides evidence that changes in metabolite concentrations vary greatly depending on the number of repetitions and the length of rest periods between exercises, even if the exercises themselves are identical.

Physiological and skeletal muscle responses to high-intensity interval exercise in Thoroughbred horses

Introduction

The purpose of this study was to determine whether acute high-intensity interval exercise or sprint interval exercise induces greater physiological and skeletal muscle responses compared to moderate-intensity continuous exercise in horses.

Methods

In a randomized crossover design, eight trained Thoroughbred horses performed three treadmill exercise protocols consisting of moderate-intensity continuous exercise (6 min at 70% VO2max; MICT), high-intensity interval exercise (6 × 30 s at 100% VO2max; HIIT), and sprint interval exercise (6 × 15 s at 120% VO2max; SIT). Arterial blood samples were collected to measure blood gas variables and plasma lactate concentration. Biopsy samples were obtained from the gluteus medius muscle before, immediately after, 4 h, and 24 h after exercise for biochemical analysis, western blotting and real-time RT-PCR. Effects of time and exercise protocol were analyzed using mixed models (p < 0.05).

Results

Heart rate and plasma lactate concentration at the end of exercise were higher in HIIT and SIT than those in MICT (heart rate, HIIT vs. MICT, p = 0.0005; SIT vs. MICT, p = 0.0015; lactate, HIIT vs. MICT, p = 0.0014; SIT vs. MICT, p = 0.0003). Arterial O2 saturation and arterial pH in HIIT and SIT were lower compared with MICT (SaO2, HIIT vs. MICT, p = 0.0035; SIT vs. MICT, p = 0.0265; pH, HIIT vs. MICT, p = 0.0011; SIT vs. MICT, p = 0.0023). Muscle glycogen content decreased significantly in HIIT (p = 0.0004) and SIT (p = 0.0016) immediately after exercise, but not in MICT (p = 0.19). Phosphorylation of AMP-activated protein kinase (AMPK) in HIIT showed a significant increase immediately after exercise (p = 0.014), but the increase was not significant in MICT (p = 0.13) and SIT (p = 0.39). At 4 h after exercise, peroxisome proliferator-activated receptor γ co-activator-1α mRNA increased in HIIT (p = 0.0027) and SIT (p = 0.0019) and vascular endothelial growth factor mRNA increased in SIT (p = 0.0002).

Discussion

Despite an equal run distance, HIIT and SIT cause more severe arterial hypoxemia and lactic acidosis compared with MICT. In addition, HIIT activates the AMPK signaling cascade, and HIIT and SIT elevate mitochondrial biogenesis and angiogenesis, whereas MICT did not induce any significant changes to these signaling pathways.

Inhibition of monocarboxylate transporters (MCT) 1 and 4 reduces exercise capacity in mice

The concept of lactate shuttle is widely accepted in exercise physiology. Lactate transport is mediated by monocarboxylate transporters (MCT), which enable cells to take up and release lactate. However, the role of lactate during exercise has not yet been fully elucidated. In this study, we investigated the effects of lactate transport inhibition on exercise capacity and metabolism in mice. Here, we demonstrated that MCT1 inhibition by α-cyano-4-hydroxycinnamate administration (4-CIN, 200 mg/g of body weight) reduced the treadmill running duration at 20 m/min. The administration of 4-CIN increased the blood lactate concentration immediately after exercise. With matched exercise duration, the muscle lactate concentration was higher while muscle glycogen content was lower in 4-CIN-administered mice. Further, we showed that MCT4 inhibition by bindarit administration (50 mg/kg of body weight) reduced the treadmill running duration at 40 m/min. Bindarit administration increased the muscle lactate but did not alter the blood lactate and glucose concentrations, as well as muscle glycogen content, immediately after exercise. A negative correlation was observed between exercise duration at 40 m/min and muscle lactate concentration immediately after exercise. Our results suggest that lactate transport via MCT1 and MCT4 plays a pivotal role in sustaining exercise.

Effects of endurance training on metabolic enzyme activity and transporter protein levels in the skeletal muscles of orchiectomized mice

This study investigated whether endurance training attenuates orchiectomy (ORX)-induced metabolic alterations. At 7 days of recovery after sham operation or ORX surgery, the mice were randomized to remain sedentary or undergo 5 weeks of treadmill running training (15-20 m/min, 60 min, 5 days/week). ORX decreased glycogen concentration in the gastrocnemius muscle, enhanced phosphofructokinase activity in the plantaris muscle, and decreased lactate dehydrogenase activity in the plantaris and soleus muscles. Mitochondrial enzyme activities and protein content in the plantaris and soleus muscles were also decreased after ORX, but preserved, in part, by endurance training. In the treadmill running test (15 m/min, 60 min) after 4 weeks of training, orchiectomized sedentary mice showed impaired exercise performance, which was restored by endurance training. Thus, endurance training could be a potential therapeutic strategy to prevent the hypoandrogenism-induced decline in muscle mitochondrial content and physical performance.

Effect of post-exercise muscle cooling on PGC-1α and VEGF mRNA expression in Thoroughbreds

Besides preventing exertional heat illness, muscle cooling can be a potential strategy to enhance exercise-training induced adaptations. This study aimed to examine the effects of post-exercise cooling on the mRNA expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and vascular endothelial growth factor (VEGF) in Thoroughbred skeletal muscle. Five Thoroughbred horses performed treadmill running until their pulmonary artery temperature reached 42 °C, followed by walking on the treadmill with no additional cooling (CONT) or muscle cooling with a shower using the tap water (26 °C, 0.4 l/s; COOL), for 30 min. Muscle biopsies were obtained before (PRE) and 3 h after exercise (3 Hr-REC) from the gluteus medius muscle. PGC-1α mRNA expression was elevated 3 h after exercise in both the CONT (PRE vs 3 Hr-REC: 1.0±0.1 vs 5.0±0.8, P<0.01) and COOL (PRE vs 3 Hr-REC: 1.1±0.3 vs 6.6±0.9, P<0.01) conditions; however, there was no difference between the two conditions at 3 h after exercise (P=0.17). VEGF mRNA expression was elevated 3 h after exercise in COOL (PRE vs 3 Hr-REC: 1.0±0.2 vs 2.2±0.2, P<0.05) but not in CONT (PRE vs 3 Hr-REC: 1.0±0.1 vs 1.8±0.3, P=0.08). VEGF mRNA expression at 3 h after exercise was significantly negatively correlated with rectal temperature at the end of the 30-min cooling period (r = -0.65, P<0.05). Our results suggest that the decline in body temperature after exercise may lead to greater expression of the key angiogenic gene in Thoroughbred horses.

Cost of transport, but not gluteus medius and red blood cells monocarboxylate-transporters density differentiated Brazilian Sport Horses at two performance levels

Cost of transport (COT) and monocarboxylate transporters (MCTs) could affect the ability to perform fast actions during a jumping discipline. This study aimed to compare the COT and evaluate the MCT1, MCT4, and their auxiliary protein CD147 content in the gluteus medius and RBCs of Brazilian sport horses (BH), a breed developed for jumping competitions, with low-level (LL) or intermediate-level (IL) jumping capacities. The physiological difference between the horses was assessed by an incremental jump test (IJT), in which the cost of lactate (COT_LAC) and heart rate (COT_HR) of running were determined for each animal by the ratio between each variable and the running speed. Western blotting was performed on muscle and RBC membranes to quantify MCT1, MCT4, and CD147. IL showed lower COT_LAC and COT_HR than LL at all jumping heights. The amount of MCT1, MCT4, and CD147 found in muscle and RBCs were not dependent on performance level. Muscle MCT4 and MCT1 were correlated positively with CD147. We conclude that the relatively small differences between performances did not relevantly influence MCT expression in BH. While MCT analyses are inaccessible for most trainers and veterinarians, the cost of transport measurements is a feasible and sensitive tool to distinguish intermediate and low-level jumping horses.

Moderate-intensity training in hypoxia improves exercise performance and glycolytic capacity of skeletal muscle in horses

We investigated whether moderate-intensity training of horses in moderate hypoxia for 4 weeks elicits greater adaptations in exercise performance, aerobic capacity, and glycolytic/oxidative metabolism in skeletal muscle compared to normoxic training. In a randomized crossover study design, seven untrained Thoroughbred horses (5.9 ± 1.1 years, 508 ± 9 kg) completed 4 weeks (3 sessions/week) of two training protocols consisting of 3-min cantering at 70% of maximal oxygen consumption ( V ˙ O 2 max ) in hypoxia (HYP; FI O2  = 14.7%) and normoxia (NOR; FI O2  = 21.0%) with a 4-month washout period. Normoxic incremental exercise tests (IET) were conducted before and after training. Biopsy samples were obtained from the middle gluteal muscle before IET and monocarboxylate transporter (MCT) protein expression and glycolytic/mitochondrial enzyme activities were analyzed. Data were analyzed using mixed models (p < 0.05). Running speed was 7.9 ± 0.2 m/s in both groups and arterial oxygen saturation during training in NOR and HYP were 92.9 ± 0.9% and 75.7 ± 3.9%, respectively. Run time in HYP (+9.7%) and V ˙ O 2 max in both groups (NOR, +6.4%; HYP, +4.3%) at IET increased after 4 weeks of training. However, cardiac output, arterial-mixed venous O2 difference, and hemoglobin concentration at exhaustion were unchanged in both conditions. While MCT1 protein and citrate synthase activity did not increase in both conditions after training, MCT4 protein (+13%), and phosphofructokinase activity (+42%) increased only in HYP. In conclusion, 4 weeks of moderate-intensity hypoxic training improves exercise performance and glycolytic capacity of skeletal muscle in horses.

Lactate administration does not affect denervation-induced loss of mitochondrial content and muscle mass in mice

Lactate is considered to be a signaling molecule that induces mitochondrial adaptation and muscle hypertrophy. The purpose of this study was to examine whether lactate administration attenuates denervation-induced loss of mitochondrial content and muscle mass. Eight-week-old male Institute of Cancer Research mice underwent unilateral sciatic nerve transection surgery. The contralateral hindlimb served as a sham-operated control. From the day of surgery, mice were injected intraperitoneally with PBS or sodium lactate (equivalent to 1 g·kg-1 body weight) once daily for 9 days. After 10 days of denervation, gastrocnemius muscle weight decreased to a similar extent in both the PBS- and lactate-injected groups. Denervation significantly decreased mitochondrial enzyme activity, protein content, and MCT4 protein content in the gastrocnemius muscle. However, lactate administration did not have any significant effects. The current observations suggest that daily lactate administration for 9 days does not affect denervation-induced loss of mitochondrial content and muscle mass.

Short-term hypoxic training increases monocarboxylate transporter 4 and phosphofructokinase activity in Thoroughbreds

The aim of this study was to investigate effects of short-term hypoxic training on lactate metabolism in the gluteus medius muscle of Thoroughbreds. Using crossover design (3 months washout), eight Thoroughbred horses were trained for 2 weeks in normoxia (FI O2  = 21%) and hypoxia (FI O2  = 18%) each. They ran at 95% maximal oxygen consumption (V̇O2max ) on a treadmill inclined at 6% for 2 min (3 days/week) measured under normoxia. Before and after each training period, all horses were subjected to an incremental exercise test (IET) under normoxia. Following the 2-week trainings, V̇O2max in IET increased significantly under both oxygen conditions. The exercise duration in IET increased significantly only after hypoxic training. The monocarboxylate transporter (MCT) 1 protein levels remained unchanged after training under both oxygen conditions, whereas MCT4 protein levels increased significantly after training in hypoxia but not after training in normoxia. Phosphofructokinase activity increased significantly only after hypoxic training, whereas cytochrome c oxidase activity increased significantly only after normoxic training. Our results suggest that hypoxic training efficiently enhances glycolytic capacity and levels of the lactate transporter protein MCT4, which facilitates lactate efflux from the skeletal muscle.

Oral Lactate Administration Additively Enhances Endurance Training-Induced Increase in Cytochrome C Oxidase Activity in Mouse Soleus Muscle

We tested the hypothesis that oral lactate supplementation increases mitochondrial enzyme activity given the potential role of lactate for inducing mitochondrial biogenesis. In this study, mice were assigned to a saline-ingested sedentary group (S+S; n = 8), a lactate-ingested sedentary group (L+S; n = 9), a saline-ingested training group (S+T; n = 8), and a lactate-ingested training group (L+T; n = 8). Mice in the S+S and S+T groups received saline, whereas mice in the L+S and L+T groups received sodium lactate (equivalent to 5 g/kg of body weight) via oral gavage 5 days a week for 4 weeks. At 30 min after the ingestion, mice in the S+T and L+T groups performed endurance training (treadmill running, 20 m/min, 30 min, 5 days/week). At 30 min after lactate ingestion, the blood lactate level reached peak value (5.8 ± 0.4 mmol/L) in the L+S group. Immediately after the exercise, blood lactate level was significantly higher in the L+T group (9.3 ± 0.9 mmol/L) than in the S+T group (2.7 ± 0.3 mmol/L) (p < 0.01). Following a 4-week training period, a main effect of endurance training was observed in maximal citrate synthase (CS) (p < 0.01; S+T: 117 ± 3% relative to S+S, L+T: 110 ± 3%) and cytochrome c oxidase (COX) activities (p < 0.01; S+T: 126 ± 4%, L+T: 121 ± 4%) in the plantaris muscle. Similarly, there was a main effect of endurance training in maximal CS (p < 0.01; S+T: 105 ± 3%, L+T: 115 ± 2%) and COX activities (p < 0.01; S+T: 113 ± 3%, L+T: 122 ± 3%) in the soleus muscle. In addition, a main effect of oral lactate ingestion was found in maximal COX activity in the soleus (p < 0.05; L+S: 109 ± 3%, L+T: 122 ± 3%) and heart muscles (p < 0.05; L+S: 107 ± 3%, L+T: 107 ± 2.0%), but not in the plantaris muscle. Our results suggest that lactate supplementation may be beneficial for increasing mitochondrial enzyme activity in oxidative phenotype muscle.

Differential Expression of Monocarboxylate Transporter 1 and Ancillary Protein CD147 in Red Blood Cells of Show Jumping Horses

We compare the expression levels of the lactate transporter complex consisting of the lactate transporter, monocarboxylate transporter 1 (MCT1), and its ancillary protein, cluster of differentiation 147 (CD147), in the membranes of red blood cells (RBCs) from two breeds of jumping horses and associate the expression levels of these proteins with their jumping ability. The expression levels of MCT1 and CD147 proteins on the membranes of RBCs collected from 30 show jumping horses of two different breeds were quantified: the Brazilian Sport Horses (n = 17) and the European Warmbloods (n = 13). The levels of MCT1 and CD147 in the RBC membranes were measured by Western blot using horse-specific antibodies. Statistical analyses included unpaired Student t-test and Chi-squared test. According to the expression levels of MCT1 and CD147 proteins, 88% of the Brazilian Sport Horses were categorized as high lactate transporters and the remaining 12% as low lactate transporters. The opposite was found for the European Warmbloods, where most animals (77%) were classified as low lactate transporters and the remaining animals (23%) were classified as high lactate transporters. Brazilian Sport Horses express statistically significantly higher levels of CD147 and MCT1 than European Warmbloods. The classification of horses considering the expression of proteins involved in the ability to transport lactate through the complex MCT1-CD147 seems to be breed dependent, with horses that are able to jump higher obstacles showing lower expression of the MCT1-CD147 complex in their RBCs.

Effects of lactate administration on mitochondrial enzyme activity and monocarboxylate transporters in mouse skeletal muscle

Growing evidence shows that lactate is not merely an intermediate metabolite, but also a potential signaling molecule. However, whether daily lactate administration induces physiological adaptations in skeletal muscle remains to be elucidated. In this study, we first investigated the effects of daily lactate administration (equivalent to 1 g/kg of body weight) for 3 weeks on mitochondrial adaptations in skeletal muscle. We demonstrated that 3-week lactate administration increased mitochondrial enzyme activity (citrate synthase, 3-hydroxyacyl CoA dehydrogenase, and cytochrome c oxidase) in the plantaris muscle, but not in the soleus muscle. MCT1 and MCT4 protein contents were not different after 3-week lactate administration. Next, we examined whether lactate administration enhances training-induced adaptations in skeletal muscle. Lactate administration prior to endurance exercise training (treadmill running, 20 m/min, 60 min/day), which increased blood lactate concentration during exercise, enhanced training-induced mitochondrial enzyme activity in the skeletal muscle after 3 weeks. MCT protein content and blood lactate removal were not different after 3-week lactate administration with exercise training compared to exercise training alone. In a single bout experiment, lactate administration did not change the phosphorylation state of AMPK, ACC, p38 MAPK, and CaMKII in skeletal muscle. Our results suggest that lactate can be a key factor for exercise-induced mitochondrial adaptations, and that the efficacy of high-intensity training is, at least partly, attributed to elevated blood lactate concentration.

Equine skeletal muscle adaptations to exercise and training: evidence of differential regulation of autophagosomal and mitochondrial components

BACKGROUND

A single bout of exercise induces changes in gene expression in skeletal muscle. Regular exercise results in an adaptive response involving changes in muscle architecture and biochemistry, and is an effective way to manage and prevent common human diseases such as obesity, cardiovascular disorders and type II diabetes. However, the biomolecular mechanisms underlying such responses still need to be fully elucidated. Here we performed a transcriptome-wide analysis of skeletal muscle tissue in a large cohort of untrained Thoroughbred horses (n = 51) before and after a bout of high-intensity exercise and again after an extended period of training. We hypothesized that regular high-intensity exercise training primes the transcriptome for the demands of high-intensity exercise.

RESULTS

An extensive set of genes was observed to be significantly differentially regulated in response to a single bout of high-intensity exercise in the untrained cohort (3241 genes) and following multiple bouts of high-intensity exercise training over a six-month period (3405 genes). Approximately one-third of these genes (1025) and several biological processes related to energy metabolism were common to both the exercise and training responses. We then developed a novel network-based computational analysis pipeline to test the hypothesis that these transcriptional changes also influence the contextual molecular interactome and its dynamics in response to exercise and training. The contextual network analysis identified several important hub genes, including the autophagosomal-related gene GABARAPL1, and dynamic functional modules, including those enriched for mitochondrial respiratory chain complexes I and V, that were differentially regulated and had their putative interactions 're-wired' in the exercise and/or training responses.

CONCLUSION

Here we have generated for the first time, a comprehensive set of genes that are differentially expressed in Thoroughbred skeletal muscle in response to both exercise and training. These data indicate that consecutive bouts of high-intensity exercise result in a priming of the skeletal muscle transcriptome for the demands of the next exercise bout. Furthermore, this may also lead to an extensive 're-wiring' of the molecular interactome in both exercise and training and include key genes and functional modules related to autophagy and the mitochondrion.

Studies on exercise physiology of the racehorse performed in Japan during the period from the 1930s to the 1970s: respiration and heart rate during exercise and the effect of exercise on blood characteristics

After publication of the epic report on equine exercise physiology by Matsuba and Shimamura in 1933, papers on exercise physiology of the racehorse in Japan began appearing in scientific journals and increased in number. In 1944, respiration during exercise at a walk, trot, and canter was measured by recording expiratory sounds with a microphone attached near the nostril. Respiratory frequency during cantering was synchronized with stride frequency, and expiratory sounds were found to occur during the stance phase of the trailing forelimb. Development of a radiotelemetry system in 1964 for electrocardiogram recording enabled the first recording of an equine electrocardiogram during field exercise that included fast galloping and calculation of heart rate (HR) during exercise. During low intensity exercise including walking, trotting, cantering and extended cantering, HR increased from 45 beat/min during pre-exercise to 150 beat/min at an extended canter. HR increased to 200 beat/min or more in most horses during 100 m of high-intensity sprint galloping. When blood lactate was measured after 3 days of draft work in 12 warhorses in 1934, no increase in blood lactate was found. The erythrocyte sedimentation rate (ESR) was decreased by intense exercise and also decreased as training increased. It was suggested that measuring changes in ESR and body weight in relation to training might become useful as a screening index of training, condition, and fatigue. This evaluation method was named the “ESR-body weight method.”

Skeletal muscle adaptations and muscle genomics of performance horses

Skeletal muscles in horses are characterised by specific adaptations, which are the result of the natural evolution of the horse as a grazing animal, centuries of selective breeding and the adaptability of this tissue in response to training. These adaptations include an increased muscle mass relative to body weight, a great locomotor efficiency based upon an admirable muscle-tendon architectural design and an adaptable fibre-type composition with intrinsic shortening velocities greater than would be predicted from an animal of comparable body size. Furthermore, equine skeletal muscles have a high mitochondrial volume that permits a higher whole animal aerobic capacity, as well as large intramuscular stores of energy substrates (glycogen in particular). Finally, high buffer and lactate transport capacities preserve muscles against fatigue during anaerobic exercise. Many of these adaptations can improve with training. The publication of the equine genome sequence in 2009 has provided a major advance towards an improved understanding of equine muscle physiology. Equine muscle genomics studies have revealed a number of genes associated with elite physical performance and have also identified changes in structural and metabolic genes following exercise and training. Genes involved in muscle growth, muscle contraction and specific metabolic pathways have been found to be functionally relevant for the early performance evaluation of elite athletic horses. The candidate genes discussed in this review are important for a healthy individual to improve performance. However, muscle performance limiting conditions are widespread in horses and many of these conditions are also genetically influenced.

Effect of AMPK activation on monocarboxylate transporter (MCT)1 and MCT4 in denervated muscle

It is now evident that exercise training leads to increases in monocarboxylate transporter (MCT)1 and MCT4, but little is known about the mechanisms of coupling muscle contraction with these changes. The aim of this study was to investigate the effect of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) induced activation of AMP-activated protein kinase (AMPK) on MCT1, MCT4, and GLUT4 in denervated muscle. Protein levels of MCT4 and GLUT4 after 10 days of denervation were significantly decreased in mice gastrocnemius muscle, while MCT1 protein levels were not altered. AICAR treatment for 10 days significantly increased MCT4, and GLUT4 protein levels in innervated muscle as shown in previous studies. We found that the MCT1 protein level was also increased in AICAR treated innervated muscle. AICAR treatment prevented the decline in MCT4 and GLUT4 protein levels in denervated muscle. Thus, the current study suggests that MCT1 and MCT4 protein expression in muscles, as well as GLUT4, may be regulated by AMPK-mediated signal pathways, and AMPK activation can prevent denervation-induced decline in MCT4 protein.

Effects of high-intensity training on lipid metabolism in Thoroughbreds

OBJECTIVE

To investigate the effects of high-intensity training (HIT) on carbohydrate and fat metabolism in Thoroughbreds.

ANIMALS

12 Thoroughbreds (3 to 4 years old; 6 males and 6 females).

PROCEDURES

Horses performed HIT for 18 weeks. They ran at 90% or 110% of maximal oxygen consumption ((V)O(2max)) for 3 minutes (5 d/wk) and were subjected to incremental exercise testing (IET) before and after training. Blood samples were collected during IET, and muscle samples were obtained from the gluteus medius muscle immediately after IET. Phosphofructokinase, citrate synthase, and β-3-hydroxyacyl CoA dehydrogenase (β-HAD) activities were measured to determine glycolytic and oxidative capacities. Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and fatty acid translocase (FAT/CD36) protein contents were detected via western blotting. Metabolome analysis was performed via capillary electrophoresis-electrospray ionization mass spectrometry to measure substrate concentrations related to carbohydrate metabolism.

RESULTS

Peak speed during IET and (V)O(2max) increased after HIT. Activities of citrate synthase and β-HAD increased after HIT, whereas phosphofructokinase activity remained unchanged. The PGC-1α and FAT/CD36 protein contents increased after HIT, but plasma lactate concentration and the respiratory exchange ratio decreased after HIT. The plasma free fatty acid concentration increased after HIT, whereas the glucose concentration was not altered. Fructose 1,6-diphosphate, phosphoenolpyruvate, and pyruvate concentrations decreased after HIT.

CONCLUSIONS AND CLINICAL RELEVANCE

HIT caused an increase in oxidative capacity in equine muscle, which suggested that there was a decreased reliance on carbohydrate utilization and a concomitant shift toward fatty acid utilization during intensive exercise.

Expression of monocarboxylate transporter (MCT) 1 and MCT4 in overloaded mice plantaris muscle

A number of studies have shown that changes in muscle contractile activity regulate the expression of monocarboxylate transporters (MCTs) in the skeletal muscle. The aim of this study was to investigate the effect of functional overload on MCT1 and MCT4 protein expression. Plantaris muscles were functionally overloaded for 15 days by ablation of the synergistic muscles. MCT1 and MCT4 mRNA abundance increased by 160-161% (p < 0.01) and 265-325% (p < 0.05), respectively, after 1-3 days of functional overload. MCT1 and MCT4 protein expression increased by 92 and 61%, respectively, after 12 days of functional overload (p < 0.05). AMP-activated protein kinase (AMPK) phosphorylation status [phospho-AMPK (Thr172)/total AMPK] was significantly elevated after 3-9 days of functional overload. Plasma testosterone concentration was elevated after 12 days of functional overload, while blood lactate concentration was not altered. Thus, the current study demonstrated that heavy mechanical loading induces increase in MCT1 and MCT4 protein expression in the muscles with increase in AMPK phosphorylation status and plasma testosterone concentration.