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

To better understand training-induced adaptations in skeletal muscle pH regulation, this study 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 titration technique (βm_{in vitro}) over the same period. Sixteen active men cycled for 7 × 2 min at ∼80% of peak aerobic power, interspersed with 1 min rest. Compared to the first 9 h post-exercise, 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 (NHE)1 protein content was lower 9 h post-HIIE (∼0.8-fold) compared to every other post-exercise timepoint, but NHE1 mRNA expression was 2.2 to 2.9-fold greater 24–72 h post-HIIE, compared to the first 24 h post-HIIE. Furthermore, we determined the intra-subject, inter-sample variability (11.5%) of βm_{in vitro} for resting samples taken on consecutive days to be greater than the typical training effect (mean 6%; 95% CL ± 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, while the temporal resolution provided by single timepoints 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 βm_{in vitro} assay for measuring true changes in βm.

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