The goals of this clinical trial are:

1. to study lactate kinetic between plasma and erythrocytes during an intervallic exercise and its subsequent recovery, considering the blood pH, the genotype for the T1470A polymorphism of the SLC16A1 gene (rs1049434), and the amount of MCT1 in erythrocytes membrane;
2. to analyze the levels of MCT1 in the erythrocytes membrane according to training status and genotype for the T1470A polymorphism of SLC16A1 (rs1049434).

For this, the project will have two phases:

• In phase I, trained participants will perform one maximal incremental test and one intervallic submaximal test with a final active recovery.
• Phase II, levels of the MCT1 protein in the erythrocyte membrane will be quantified from trained and sedentary participants.

Introduction

Lactate is the end product of glycolysis and has the capacity to be transported from one cell to another, performing numerous functions. This transport is mainly through several monocarboxylate transporters (MCT). The most predominant is MCT1, which transfers lactate bidirectionally depending on the lactate concentration and pH on each side of the membrane. MCT1 is ubiquitous, being present in most human tissues like the erythrocyte membrane, indicating that these cells are capable of carrying lactate throughout the bloodstream. Some studies have described lactate kinetics between the different blood compartments (erythrocyte-plasma) during incremental and steady-state exercises, but not during intermittent exercise and/or recovery after exercise. Moreover, several studies have observed that the function of MCT1 is conditioned by the presence of a genetic variant: the T1470A polymorphism (rs1049434), which involves a substitution of aspartic acid for glutamic acid at position 490 of the protein. Data suggest that an AA (Glu/Glu) genotype for this single nucleotide polymorphism (or SNP) leads to a higher lactate transport capacity than the TT (Arg/Arg) profile. However, no study has analyzed if this SNP directly affects erythrocyte MCT1 function or content Therefore, the two general objectives of this clinical trial are:

1. to study lactate kinetic between plasma and erythrocytes during an intervallic exercise and its subsequent recovery, considering the blood pH, the genotype for the T1470A polymorphism of the SLC16A1 gene (rs1049434), and the amount of MCT1 in erythrocytes membrane;
2. to analyze the levels of MCT1 in the erythrocytes membrane according to training status and genotype for the T1470A polymorphism of SLC16A1 (rs1049434).

Procedures

The study will be divided into two phases, each of which will be primarily oriented toward achieving one of two general objectives:

• Phase I: Focused primarily on the study of lactate kinetics between blood compartments during exercise.
• Phase II: Focused primarily on the analysis of the amount of MCT1 in the erythrocyte membrane.

For both phases, volunteers will be informed and sign an informed consent form before a blood sample is taken for genotyping. Once the genotype is known, only those participants with AA or TT genotype will be selected.

The tests that will be performed throughout Phase I will be the following:

• Once participants with the genotypes AA and TT are selected, they will perform a step incremental test until exhaustion to determine the first and second lactate threshold. These thresholds will be determined by measuring lactate concentration from capillary blood samples from the earlobe.
• Finally, an intervallic exercise protocol will be performed starting with 1 min rest followed by a 5 min warm up at an intensity equal to 80% of the first lactate threshold. This is followed by 4 sets of 1 min 30 sec at an intensity 20% above the second lactate threshold, with recovery intervals of 5 min at 80% of the first lactate threshold. After the 4 sets, a final recovery of 20 min at 80% of the first lactate threshold will be performed. During this test, capillary blood samples from the earlobe and venous blood samples from the forearm will be analyzed for lactate concentration.

The procedure for the Phase II will be the following:

• Once participants with the genotype AA and TT are selected, venous blood will be extracted from their forearm at rest.
• Physical activity levels from all the participants will be quantified using the International Physical Activity Questionnaire (IPAQ).
• Analysis of the erythrocyte MCT1 protein content by flow cytometry will be performed from the venous blood samples.

Inclusion Criteria:

• BMI between 18 and 25 kg/m2
• Not smoker
• In the case of trained participants: training at least 5 hours/week, of which at least 3 must be aerobic/anaerobic endurance training. Presentation of a medical certificate attesting to their medical fitness (capability) to perform intense physical exercise.
• In the case of sedentary participants, less than 1 day of training per week or less than 2 hours of structured exercise per week.

Exclusion Criteria:

• Presenting any pathology that could impede the correct performance of the tests, or could influence the main study variables.
• In the case of the female sample, having irregular menstrual cycles, oligomenorrhea or amenorrhea.