Effects of intermittent exposure to hypobaria and cold on the microarchitecture and morphofunctional profile of rat gastrocnemius muscle

Intermittent hypobaric hypoxia (IHH) enhances oxygen transport, muscle metabolism, and cardiovascular health; however, the combined effects of hypobaria and cold are less well understood. Exposure to these stressful environmental factors, even intermittently, can affect skeletal muscle due to its plasticity, thereby altering its metabolic and contractile characteristics. To explore those possible effects, we exposed 64 young adult male rats divided into four (n = 16) groups to either IHH at 4500 m, cold at 4°C (ICE), both environmental stressors simultaneously (COHY), for 4 h daily or to 24 h normoxia control conditions (CTRL) over 9 or 21 days (n = 8 by group at each time point). Three regions (white, intermediate, and red) of the equatorial transverse sections of the gastrocnemius muscles were analyzed for fiber type, size, and capillary density. Short-term exposure almost did not affect the fiber muscle size or individual fiber capillarity (only slightly affecting the size of slow oxidative fibers in the red portion). However, long-term exposure led to greater changes, also in the red portion. After 3 weeks, IHH produced adaptations similar to those of cold and combined exposure, further reducing fiber cross-sectional area and increasing global and individual fiber capillarization without increasing capillary-to-fiber ratio. Together, intermittent cold and hypoxia triggered acclimatization responses that preserved muscle mass and metabolic profile by region-specific changes in fiber size and capillarization, maintaining oxygen supply without muscle wasting.