Structural and functional remodeling of skeletal muscle microvasculature is induced by simulated microgravity

Hindlimb unloading of rats results in a diminished ability of skeletal musc le arterioles to constrict in vitro and elevate vascular resistance in vivo . The purpose of the present study was to determine whether alterations in the mechanical environment (i.e., reduced fluid pressure and blood flow) of the vasculature in hindlimb skeletal muscles from 2-wk hindlimb-unloaded ( HU) rats induces a structural remodeling of arterial microvessels that may account for these observations. Transverse cross sections were used to dete rmine media cross-sectional area (CSA), wall thickness, outer perimeter, nu mber of media nuclei, and vessel luminal diameter of feed arteries and firs t-order (1A) arterioles from soleus and the superficial portion of gastrocn emius muscles. Endothelium-dependent dilation (ACh) was also determined. Me dia CSA of resistance arteries was diminished by hindlimb unloading as a re sult of decreased media thickness (gastrocnemius muscle) or reduced vessel diameter (soleus muscle). ACh-induced dilation was diminished by 2 wk of hi ndlimb unloading in soleus 1A arterioles, but not in gastrocnemius 1A arter ioles. These results indicate that structural remodeling and functional ada ptations of the arterial microvasculature occur in skeletal muscles of the HU rat; the data suggest that these alterations may be induced by reduction s in transmural pressure (gastrocnemius muscle) and wall shear stress (sole us muscle).