Relationship between hypoxemia, aging and skeletal muscle fiber type

Objective Determine if muscle fiber type is related to hypoxia and aging. Methods 55 female Sprague‐Dawley (SD) rats were subjected to intratracheal instillation of either elastase (n=30), to induce emphysema and hypoxemia, or saline (n=25), for normoxic controls , at 10 weeks of age. At either 14, 18, 22, 26, or 30 weeks of age, arterial blood gases were drawn while breathing room air. The soleus (SOL) and spinotrapezius (ST) muscles were processed using SDS‐PAGE to determine relative proportions of myosin heavy chain (MHC) isoforms. Results Although both groups initially had low PaO2, the emphysema group remained consistently hypoxemic over time, while the control group recovered by 22 weeks of age to normal values. Overall, with the exception of ST MHC type IIx proportions, 40% emphysema and 51% control, there were no differences between groups. Relative MHC proportions did change over time, with an increase in both ST and SOL MHC I and decrease in IIa, as well as an increase in ST MHC IIx. Conclusion Due to a novel rat respiratory syndrome, the control group was nearly as hypoxic as the emphysema group, especially at younger ages. This homogeneity in PaO 2 between groups clouds our ability to compare skeletal muscle development during either normoxia or hypoxia. However, compared to reference values from prior studies, we have demonstrated a substantial shift away from MHC I in animals exposed to hypoxemia at early ages, even in the, previously, primarily slow twitch, SOL muscle. NIH/NHLBI HL18292, NIH/NIDCD DC03211, Foundation for Physical Therapy, VCU AD Williams Fund