Distribution of lactate between red blood cells and plasma after exercise

Quantification of the energy expenditure during training exercises in Standardbred trotters

An appropriate energy feeding management that ensures the optimal dietary energy supply according to the energy expenditure (EE) is a crucial component for the horse's performance. The main purpose of this study was to determine the EE during four specific exercises used in the training of Standardbred trotters (promenade, jogging, parcours and interval work-outs). A total of six Standardbred geldings performed four different testing situations on a track. The intensity (expressed in percentage of the maximal velocity over 500 m, i.e. v500) and volume (distance and duration) of the testing situations were determined according to practices reported by French trainers. Promenade and jogging included only an exercise phase, whereas parcours and interval situations also included a warm-up and a recovery phase. Oxygen uptake (VO2), carbon dioxide production (VCO2) and heart rate (HR) were continuously recorded from 2 min before the beginning through to the end of the testing situations, using a portable respiratory gas analyser. Blood lactate levels and rectal temperature were determined before and immediately after the exercise phase of each testing situations. EE of the different phases (warm-up, exercise and recovery) and EE of the entire testing situations (EETOTAL) were calculated from VO2 measurements and the O2 caloric equivalent. Interval and parcours situations induced higher physiological responses than promenade and jogging situations, particularly in terms of VO2peak, VCO2peak and HRpeak. The highest blood lactate concentration (6 mmol/l) was measured after the interval exercise, and respiratory exchange ratios ⩾1 were observed only for the parcours situation. The EE of exercise phase varied from 0.49 to 1.79 kJ/min per kg for promenade and parcours situations. The EE of warm-up and recovery phases did not differ between parcours and interval situations, and was estimated at 1.04 and 0.57 kJ/min per kg BW, respectively. On average, the warm-up and the recovery phases contributed to 38% and 19% of the EETOTAL. For promenade, jogging, parcours and interval situations, EETOTAL was evaluated at 12 618, 11 119, 13 698 and 18 119 kJ, respectively.

Assessment of glucose disposal with the hyperglycaemic clamp technique during low intensity exercise in Warmblood horses

REASONS FOR PERFORMING STUDY

The quantity of glucose disposal during exercise (walk and trot) compared to rest by use of the hyperglycaemic clamp technique has not been reported previously and has relevance to nutritional requirements.

HYPOTHESIS

Exercise (walk and trot) significantly increases glucose disposal compared to rest.

METHODS

Seven healthy Dutch Warmblood mares, all in dioestrus, mean ± s.d. age 11.6 ± 2.4 years and weighing 569 ± 40 kg were fasted for 12 h prior to a hyperglycaemic clamp at rest (maintaining a steady state of the blood glucose concentration during 30 min), walk (10 min, 1.5 m/s), trot (20 min, 4.4 m/s), walk (10 min, 1.5 m/s) and rest again (maintaining a steady state during 30 min). Plasma glucose concentrations were measured every 5 min. The mean rate of glucose disposal was calculated by corrections for glucose loss via the glucose space and urine. A one-way ANOVA with post hoc Bonferroni was performed.

RESULTS

The mean ± s.d. rate of glucose disposal was 15.0 ± 2.1 at first rest, 25.1 ± 6.2 at first walk, 37.4 ± 9.1 at trot, 33.0 ± 13.1 at second walk and 18.7 ± 4.6 µmol/kg bwt/min at second rest. Values at trot and at second walk differed significantly from values at first rest, whereas values at both rests were similar as well as at first rest and at first walk.

CONCLUSIONS

Mean rate of glucose disposal of Warmblood horses increased 2.5 times during trot compared to basal.

POTENTIAL RELEVANCE

The hyperglycaemic clamp technique is an attractive nonisotope method to assess the rate of glucose disposal in exercising horses.

Monocarboxylate transporters (MCT) as lactate carriers in equine muscle and red blood cells

REASONS FOR PERFORMING STUDY

Monocarboxylate transporters (MCT) facilitate the transport of lactate across membranes. In red blood cells (RBC) the transport activity varies interindividually due to differences in the amount of an ancillary protein CD147. Similar variations in muscles could have a great influence on lactate accumulation during exercise.

OBJECTIVES

To study the expression of MCT isoforms and CD147 in the middle gluteal muscle.

METHODS

Venous blood and muscle biopsy samples were taken from 14 Standardbred horses. Lactate transport activity in RBC and the amounts of MCT1, 2, 4 and CD147 were measured.

RESULTS

In muscle MCT1, MCT4 and CD147 were found. Amount of MCT1 was variable and not dependent on age or training. Expression of MCT4 increased with age and correlated positively with CD147. CD147 in muscle correlated with that in RBC. MCT4 in muscle and CD147/MCT1 in RBC were higher in race fit than in moderately trained horses.

CONCLUSIONS

MCT isoform profile in equine muscle is similar to that in man. The correlation between CD147 in muscle and RBC supports the view that lactate transport activity in muscles may vary interindividually as with RBC.

POTENTIAL RELEVANCE

A larger number of horses need to be analysed to confirm the relationship of CD147 in muscle and RBC; and to allow the use the lactate transport activity in RBC as an indicator of the respective activity in muscles.

Changes in arterial, mixed venous and intraerythrocytic concentrations of ions in supramaximally exercising horses

REASONS FOR PERFORMING STUDY

Horses experience major perturbations in acid-base balance during supramaximal exercise. Ion movement in and out of erythrocytes (RBCs) is believed to be important in maintaining acid-base balance but it is unclear as to the extent to which this happens, nor how it affects single measurements of ion concentrations in arterial and venous blood.

OBJECTIVES

To clarify the role RBCs play in mitigating perturbations in acid-base balance during high speed exercise in horses, and to describe associated differences in arterial (a) and mixed venous (v) concentrations of key ions.

METHODS

Six exercise-trained Thoroughbreds galloped to fatigue at speeds calculated to have an oxygen demand that was 115% of the VO2max. Blood samples (a and v) were collected pre-exercise, during warm-up, at fatigue, and immediately post exercise. Packed cell volume (PCV), pH, PCO2, and plasma concentrations of bicarbonate (HCOP3-), chloride (Cl-), sodium (Na+), potassium (K+), and lactate (Lac-) and strong ion difference (SID) were determined, and RBC concentrations of Lac- and electrolytes calculated for each sample. Data were analysed using a 2-way ANOVA for repeated measures testing for effects of sampling time and site (P<0.05).

RESULTS

Plasma and RBC [Cl-] were increased with hypercapnoea and acidaemia. [HCO3-]v was greater than pre-exercise values at fatigue, although [HCO3l]a was lower. Hyperkalaemia and decreased RBC [K+] were evident at fatigue, as was an increased RBC [Na+]. Plasma [K+] started to decrease as soon as exercise ceased and Na+ began to move back onto RBCs in exchange for K+. Concentrations of all measures of Lac- rose from fatigue to post exercise. The SID decreased with exercise and was higher in v at fatigue and post exercise, reflecting the decrease in pH.

CONCLUSIONS

RBCs act as a repository for lactate, and therefore the increase in PCV facilitates the maintenance of the muscle to plasma Lac- diffusion gradient during exercise.

POTENTIAL RELEVANCE

This serves to keep intramuscular [Lac-] lower than it would otherwise be and, because of the link between Lac- accumulation, pH decrease and the onset of fatigue, may help delay the onset of fatigue.

MCT1 and CD147 gene polymorphisms in Standardbred horses

REASONS FOR PERFORMING STUDY

Transport of lactate across membranes is facilitated by proton-monocarboxylate transporters (MCT). The most widely distributed isoform is MCT1, which needs an ancillary protein CD147. Studies on erythrocytes have shown that high activity of MCT1 is inherited as the dominant allele and that activity is regulated through CD147. Mutations of human MCT1 have been described that appear to impair lactate transport in muscles and cause exertional rhabdomyolysis. There are no reports of this potential relationship in the horse.

OBJECTIVES

To obtain sequences of equine MCT1 and CD147 to examine differences between horses with high and low lactate transport activity in their erythrocytes.

METHODS

Muscle biopsy samples were taken from 3 healthy Standardbred horses and from 7 horses which according to the owners had signs of myopathy after intense exercise. DNA and RNA were isolated and PCR analysis and sequencing performed.

RESULTS

Currently, PCR fragments covering 100% of MCT1 and 70% of CD147 coding region are retained and sequence analysis has demonstrated one single nucleotide polymorphism (SNP) in the C-terminal area of MCT1 and one SNP in the extracellular domain of CD147. Both cause an amino acid change. The SNPs found are not related to lactate transport activity in erythrocytes or signs of myopathy.

CONCLUSIONS

More samples need to be analysed to make conclusions on the significance of the polymorphisms found. Furthermore, full sequence coverage of the coding region of CD147 is needed.

POTENTIAL RELEVANCE

The molecular probes produced could be used as tools to study gene regulation of lactate transport.

The concept of maximal lactate steady state: a bridge between biochemistry, physiology and sport science

The maximal lactate steady state (MLSS) is defined as the highest blood lactate concentration (MLSSc) and work load (MLSSw) that can be maintained over time without a continual blood lactate accumulation. A close relationship between endurance sport performance and MLSSw has been reported and the average velocity over a marathon is just below MLSSw. This work rate delineates the low- to high-intensity exercises at which carbohydrates contribute more than 50% of the total energy need and at which the fuel mix switches (crosses over) from predominantly fat to predominantly carbohydrate. The rate of metabolic adenosine triphosphate (ATP) turnover increases as a direct function of metabolic power output and the blood lactate at MLSS represents the highest point in the equilibrium between lactate appearance and disappearance both being equal to the lactate turnover. However, MLSSc has been reported to demonstrate a great variability between individuals (from 2-8 mmol/L) in capillary blood and not to be related to MLSSw. The fate of enhanced lactate clearance in trained individuals has been attributed primarily to oxidation in active muscle and gluconeogenesis in liver. The transport of lactate into and out of the cells is facilitated by monocarboxylate transporters (MCTs) which are transmembrane proteins and which are significantly improved by training. Endurance training increases the expression of MCT1 with intervariable effects on MCT4. The relationship between the concentration of the two MCTs and the performance parameters (i.e. the maximal distance run in 20 minutes) in elite athletes has not yet been reported. However, lactate exchange and removal indirectly estimated with velocity constants of the individual blood lactate recovery has been reported to be related to time to exhaustion at maximal oxygen uptake.

Distribution of monocarboxylate transporter isoforms MCT1, MCT2 and MCT4 in porcine muscles

AIM

Monocarboxylate transporters (MCT), which cotransport lactate anions and protons across cell membranes, are important for regulation of muscle pH. We measured amounts of MCT1, MCT2 and MCT4 by immunoblotting in five different porcine muscles, to study MCT-isoform distribution both in oxidative and highly glycolytic muscles.

METHODS

Samples from the longissimus dorsi, gluteus superficialis, semimembranosus, infraspinatus and masseter were taken from 18 slaughtered pigs.

RESULTS

Oxidative capacity, estimated on the basis of the activities of lactate dehydrogenase (LDH), citrate synthase (CS) and 3-OH-acyl-CoA dehydrogenase (HAD), was highest in the infraspinatus and masseter, and was very low in the gluteus, semimembranosus and longissimus dorsi. In all muscles, the amount of MCT1 was small but variable. The amount of MCT2 was more abundant in the glycolytic than in the oxidative muscles, while MCT4 was found in equal amounts in all muscles. MCT2, but not MCT4, correlated negatively with CS and HAD.

CONCLUSIONS

The results together with measured concentrations of lactate suggest that MCT2 may function as the housekeeping lactate transporter, preventing acidification especially in highly glycolytic muscles in which the capacity to oxidize lactate is low. The results also support the view that, as in other species, MCT4 would be important at high lactate concentrations that occur during stress.

Induction of heat shock protein 72 mRNA in skeletal muscle by exercise and training

In response to stress, cells synthesise heat shock proteins (HSP) to maintain protein homeostasis. To study whether exercise and training induce expression of HSP72 in the middle gluteal muscle, 10 Finnhorses performed a submaximal 60 min exercise test on a treadmill. Test A was performed after 3 months of training, and the other two tests 2 (B) and 5 (C) weeks later. Blood samples were taken during and after the tests, and biopsy samples before, immediately after and 23 h after each test. HSP72 mRNA was analysed using a digoxigenin-labelled probe. Blood lactate concentration in the 3 tests varied between 7.2 and 10.2 mmol/l. Training increased HSP72 mRNA, as indicated by increases in samples taken at rest (A<B<C). Exercise also tended to increase HSP72 mRNA transiently but, 23 h later, values had returned to pre-exercise levels. HSP72 mRNA was expressed in all muscle fibres. After exercise, HSP72 mRNA correlated positively with the peak concentration of blood lactate, but not with indicators of energy status. Therefore, acidosis rather than energy depletion was the major inducer of HSP72 expression after moderate intensity exercise. Because HSP72 may protect cells against stress, knowledge about their expression may help in planning optimal trainng regimes.

Lactate transport in red blood cells by monocarboxylate transporters

The lactate transport activity of red blood cells (RBC) varies widely among different species; in equine RBC, the activity of the main lactate carrier, H+-monocarboxylate co-transporter (MCT), is distributed bimodally. The influence of lactate transport activity is measurable in vivo; after maximal exercise, the RBC lactate concentration in horses with high (HT) lactate transport activity is higher than in those with low (LT) activity. To study the expression of MCT in HT and LT horses, blood samples were taken from 10 horses at rest and after submaximal exercise. Blood and plasma lactate concentrations, lactate and pyruvate transport activities and the amounts of MCT1, MCT2 and MCT4 were measured. After exercise, RBC lactate concentration was higher in HT (n = 5) than in LT (n = 5) horses. At lactate concentrations of 0.25-30 mmol/l and at a pyruvate concentration of 1 mmol/l, transport activity was higher in HT horses. At a lactate concentration of 0.1 mmol/l, transport was similar. In Western blots, the signals for MCT1 and MCT2 were similar in both groups. The amount of CD147, a chaperone necessary for the activity of MCT1, was lower in LT horses. We suggest that MCT2 transports lactate at low concentrations, while MCT1 is needed at higher concentrations. MCT1 may be less active in LT horses and, therefore, during exercise their capacity to take up lactate is low. Further studies are needed to show whether the differences in lactate influx in RBC affect the function of erythrocytes or the performance capacity of horses.

Age-related changes and inheritance of lactate transport activity in red blood cells

In red blood cell membranes, the activity of the main lactate carrier, H+-monocarboxylate co-transporter (MCT), varies interindividually and its distribution is bimodal. To show the repeatability of MCT activity, 2 to 5 blood samples were taken, at an interval of approximately 1 year, from 51 Standardbred horses, age 2 weeks-8 years, for a total of 128 observations. The horses could be divided into low (LT) and high (HT) lactate transport activity groups. Age significantly affected (P<0.05) MCT activity such that activity was highest in foals, reached a nadir at 2-3 years, and tended to increase again thereafter. Interindividual variation was not sufficiently high to allow a horse to switch from the LT-group to the HT-group, or vice versa. When MCT activity from 4 sires, 15 dams and their 52 offspring was analysed, the data showed that MCT activity is heritable and supported the hypothesis that low MCT activity was caused by a recessive allele in a single autosomal locus. Because MCT activity affects RBC lactate concentrations, the phenomenon may be physiologically significant.

Lactate-transport activity in RBCs of trained and untrained individuals from four racing species

In red blood cells (RBC) of horses, both lactate-transport activity and lactate accumulation during races vary interindividually. To study whether similar variation in lactate transport is apparent also in RBCs of other racing species, blood samples were collected from 21 reindeer, 40 horses, 31 humans, and 38 dogs. Total lactate-transport activity was measured at 10 and 30 mM concentrations, and the roles of the monocarboxylate-transporter (MCT) and the inorganic anion-exchange transporter (band-3 protein) were studied with inhibitors. In the reindeer and in one-third of the horses, lactate transport was low and mediated mainly by band-3 protein and nonionic diffusion. In the humans, dogs, and the remaining two-thirds of the horses, lactate transport was high and MCT was the main transporter. No correlation existed between MCT activity and the athleticism of the species. In the horses and humans, training had no effect on lactate transport, but in the reindeer and sled dogs, training increased total lactate transport. These results show that among the racing species studied, only in horses was the distribution of lactate-transport activity bimodal, and the possible connection between RBC lactate and performance capacity, especially in this species, warrants further studies.

Factors affecting accumulation of lactate in red blood cells

In horses, both the post exercise distribution of lactate between plasma and red blood cells (RBC) and the activity of lactate transporters on the RBC membrane vary widely between individuals. In this study, we investigated the effects of pH, time and temperature on lactate distribution in vitro, and compared the in vitro activity of lactate transporters with the accumulation of lactate into RBC in vivo. To accomplish this, we took venous blood samples at rest and after trotting races. The post exercise accumulation of lactate into RBC was shown to depend on the activity of lactate transporters. The results, in vitro, also indicate that pH, incubation time and temperature influence the activity of lactate transporters and the accumulation of lactate into RBC, underscoring the fact that in practice it is important to standardise the measurement conditions of lactate. These results support the view that whole blood lactate concentrations should be measured in estimating the accumulation of lactate from exercising muscles into the blood, because the effect of blood pH, temperature, time to centrifugation of the sample and also interindividual variation in lactate transport into RBC are therefore minimised.

Fluid, electrolyte, and acid-base responses to exercise in racehorses

During both high-intensity and short-distance exercise, the high rate of expended energy is met by anaerobic oxidation of glucose to lactic acid; this is the main cause of metabolic acidosis observed during racing. In addition, plasma volume decreases because water moves from the vasculature to the intracellular and interstitial spaces at the onset of intense exercise. These fluid shifts, together with active ion-exchange between blood and tissue, cause marked changes in electrolyte concentrations. This article reviews the mechanisms of acid-base disturbances, fluid shifts, and electrolyte changes, and discusses related areas such as buffer capacity, lactic acid distribution, and the effects of training. The influences of health, dietary cation-anion balance, supplements, and medication such as creatine, sodium bicarbonate, and furosemide are emphasized.

Interindividual variation in total and carrier-mediated lactate influx into red blood cells

To study in standardbred horses interindividual variation in the influx of lactate into red blood cells, venous blood samples were collected from 89 horses from 2 wk to 9 yr of age. For 62 horses, the rate of influx was normally distributed with a mean rate of 4.09 nmol.mg protein-1.min-1 at a lactate concentration of 10 mM, and the respective value for the other 27 horses was 0.58 nmol.mg protein-1.min-1. At 30 mM of lactate, the rates were 8.71 and 1.97 nmol.mg protein-1.min-1, respectively. This bimodal distribution was independent of age. In horses with high transport activity, the monocarboxylate transporter (MCT) appears to be the major carrier, whereas, in those with low transport activity, no activity of the MCT could be detected. The band 3 protein may account for 18-39% of transport activity. With all age groups combined, the transport activity tended to be higher in mares than in stallions. Lactate transport into red blood cells seems thus to be an inherent property in which participation of various transporters varies interindividually.