Aim: 

Mitochondria are central to the conversion of energy by oxidising substrates and generating the cell fuel, ATP. In contrast to earlier assumptions, it is now established that permanent or long-term exposure to severe hypoxia decreases the mitochondrial content of muscle fibres. However, less is known about hypoxia-induced adaptations in other tissues (e.g., the heart and lungs). Thus, one of the aims of our research is to investigate changes in mitochondrial respiration in the heart and lungs of rats exposed to long-term hypoxia. While current evidence suggests that ROS may be implicated in the hypoxia-induced adaptation of mitochondria, we also investigated the hypothesis that a decrease in muscle pH during hypoxia may affect mitochondrial signaling pathways.

Methods: 

To investigate the effects of acidosis on cell-signaling pathways that control mitochondrial biogenesis, L6 myotubes were incubated in DMEM at either 5 or 20% CO₂ (lower pH), and harvested after 1 or 6 h. The activation of p38 MAPK, AMPK and CAMKII was measured via Western blots performed on total fractions of cell lysates. In a separate study, humans ingested either ammonium chloride (ACID) or calcium carbonate (PLACEBO) the day before having a muscle biopsy from the vastus lateralis muscle. The RNA content of PGC-1a was determined by RT-PCR and normalised to total cDNA content determined by OliGreen. For the final study, 20 male Wistar rats were randomly assigned to either 30 days of hypoxia or 30 days of normoxia. Rats were then sacrificed and mitochondrial respiration was determined in the soleus, extensor digitorum longus, heart (right and left ventricle) and lungs (well adapted and maladapted areas), with pyruvate as a substrate.

Results: 

Results show that acidosis increased the phosphorylation of CaMK II, p38 MAPK and AMPK after 1 h of exposure. In the human study, acidosis resulted in elevated PGC-1 a mRNA at rest compared to the placebo condition (p<0.05). The effects of 30 days of hypoxia on mitochondrial respiration in rat skeletal muscle, heart and lungs are currently being analysed.

Conclusion: 

These results suggest that H⁺ accumulation does indeed affect cell signaling pathways involved in mitochondrial biogenesis. Furthermore, these results suggest that intramuscular acidosis may provide an additional mechanism that contributes to mitochondrial adaptations following long term hypoxia. This has important implications for populations who experience a chronic decrease in muscle pH (e.g., COPD and renal-disease patients).