Programme Contents

Influence of training intensity on adaptations in acid/base transport protein abundance and non-bicarbonate muscle buffer capacity in active men

Regulation of pH in skeletal muscle comprises intracellular buffering of hydrogen ions (H⁺) and acid/base transport across the sarcolemma (Juel, 2008). During high-intensity exercise H⁺ transport is primarily lactate-coupled through the monocarboxylate transporters (MCT)1/4, with non-lactate-coupled transport provided by the sodium/hydrogen exchanger (NHE) system. The chaperone protein basigin is essential for MCT functioning (Wilson et al., 2005). In addition, sodium-coupled bicarbonate transport proteins enhance intracellular buffering and H⁺ efflux, while the cytosolic and sarcolemmal carbonic anhydrase (CA) isozymes may enhance activity of each transport system through physical or functional interactions (Deitmer & Becker, 2013). Yet, despite their importance for muscle pH regulation, the response to exercise training of most of these acid/base transport proteins has not been investigated.

This study sought to provide the first comprehensive analysis of the acid/base transport protein response to exercise training and to test the assumption that training intensity is a key factor in provoking upregulation of these proteins and intracellular buffers in skeletal muscle. Using a two-group parallel design, 16 active men [23 (5) y, mean (SD)] undertook 4 weeks of work-matched, high-intensity interval training (HIT), 3 days per week. HIT comprised 2-min work intervals interspersed with 1 min of passive recovery, performed at either 20% (HITΔ20; n = 8) or 90% (HITΔ90; n = 8) of the difference between the lactate threshold and peak aerobic power. Muscle biopsies were taken from the vastus lateralis before and after 2 and 4 weeks of HIT, and also 6 weeks after stopping HIT to ascertain potential detraining. Protein content of MCT1, MCT4, basigin, NHE1, electrogenic sodium-bicarbonate cotransporter (NBCe)1, and CAII, CAIII, CAIV, and CAXIV were measured by quantitative western blotting. Coomassie blue staining of total protein was used as a loading control. Non-bicarbonate muscle buffer capacity (βm_{in vitro}) was measured by titration of homogenate. Data were analysed using linear mixed models and standardized effect sizes (ES) are reported as (ES; 90% confidence interval) of the between-group difference scores. Where no between-group differences were seen, pooled (ES; 90% CI) of the within-group difference scores are reported.

The first two weeks (six sessions) of HIT induced little change in any variable. After 4 weeks of HIT, MCT4 protein content only increased for HITΔ20 (ES; 90% CI: 1.06; 0.29 to 1.83), whereas in contrast, basigin content only increased for HITΔ90 (ES; 90% CI: 1.49; 0.07 to 2.91). Otherwise, training intensity did not discriminate between adaptations for all other pH-regulatory proteins, with abundance of MCT1, NHE1, NBCe1, CAII, and CAXIV increasing after 4 weeks of HIT, while there was either no change or a decrease in CAIII and CAIV abundance. There were no group differences in βm_{in vitro} (ES: 0.07; −0.64 to 0.78). Pooled βm_{in vitro} decreased by 5.7 mmol H⁺·kg dm⁻¹·pH⁻¹ after 4 weeks (ES: −0.41; −0.74 to −0.07), but this was less than the typical error of measurement (9.8 mmol H⁺·kg dm⁻¹·pH⁻¹). Detraining was evident from an almost complete loss of adaptations for all of the proteins 6 weeks after removing the stimulus of HIT. Notably, detraining was not total for those proteins showing the largest training effect, viz. MCT1 and NHE1.

In summary, measurement of the adaptation to training of a comprehensive selection of proteins involved in muscle pH regulation has been undertaken for the first time. Increased abundance of most proteins was achieved after 4 weeks of HIT, but 6 sessions over the first 2 weeks provided an insufficient stimulus. And rapid physiological detraining was evident 6 weeks after removal of the HIT stimulus. Finally, contrary to our hypothesis, an ∼40% difference in training intensity did not discriminate between adaptations for most proteins, with the exception of MCT4 and basigin.

Deitmer JW & Becker HM. (2013). Transport metabolons with carbonic anhydrases. Front Physiol 4, 291.

Juel C. (2008). Regulation of pH in human skeletal muscle: adaptations to physical activity. Acta Physiol (Oxf) 193, 17-24.

Wilson MC, Meredith D, Fox JEM, Manoharan C, Davies AJ & Halestrap AP. (2005). Basigin (CD147) is the target for organomercurial inhibition of monocarboxylate transporter isoforms 1 and 4: The ancillary protein for the insensitive MCT2 is embigin (gp70). J Biol Chem 280, 27213-27221.