Distinct protein and mRNA kinetics of skeletal muscle proton transporters following exercise can influence interpretation of adaptations to training.

What is the central question of this study? Following a training intervention, how is the interpretation of adaptations in skeletal muscle H+ transporters influenced by biopsy timing in the context of individual protein and mRNA kinetics after the final exercise bout? What is the main finding and its importance? We show that distinct postexercise protein and mRNA kinetics for monocarboxylate transporter 1/4 and sodium-hydrogen exchanger 1 indicate that timing of a single end-point biopsy after a training intervention can influence the inferences made. Furthermore, we found the intrasubject, intersample variability of the muscle buffer capacity titration assay to be greater than the typical training effect. In order to gain a better understanding of training-induced adaptations in skeletal muscle pH regulation, in this study we measured protein and mRNA kinetics of proton (H+ ) transporters for 72 h following a bout of high-intensity interval exercise (HIIE), conducted after 4 weeks of similar training. We also assayed muscle buffer capacity (βm) by a titration technique (βmin vitro ) over the same period. Sixteen active men cycled for seven bouts of 2 min at ∼80% of peak aerobic power, interspersed with 1 min rest. Compared with the first 9 h postexercise, monocarboxylate transporter (MCT) 1 protein content was ∼1.3-fold greater 24-72 h post-HIIE, whereas there was no such change in MCT4 protein content. Conversely, MCT1 and MCT4 mRNA expression progressively decreased 9-72 h post-HIIE. Sodium-hydrogen exchanger 1 (NHE1) protein content was lower 9 h post-HIIE (∼0.8-fold) compared with every other postexercise time point, but NHE1 mRNA expression was 2.2 to 2.9-fold greater 24-72 h post-HIIE, compared with the first 24 h post-HIIE. Furthermore, we determined the intrasubject, intersample variability (11.5%) of βmin vitro for resting samples taken on consecutive days to be greater than the typical training effect (mean 6%; 95% confidence limits ±4%). In conclusion, the delay in steady-state protein turnover should inform biopsy timing in studies investigating the response to training of the H+ transport proteins, whereas the temporal resolution provided by single time points has been shown to be of limited epistemological value for their corresponding mRNA expression. Finally, our data cast doubt on the ecological validity of the βmin vitro assay for measuring true changes in βm.