Selected Contribution: Effects of fiber composition and hindlimb unloadingon the vasodilator properties of skeletal muscle arterioles

It has been hypothesized that microgravity-induced orthostatic hypotension may result from an exaggerated vasodilatory responsiveness of arteries. The purpose of this study was to determine whether skeletal muscle arterioles exhibit enhanced vasodilation in rats after 2 wk of hindlimb unloading (HU) . First-order arterioles isolated from soleus and white gastrocnemius muscl es were tested in vitro for vasodilatory responses to isoproterenol (Iso), adenosine (Ado), and sodium nitroprusside (SNP). HU had no effect on respon ses induced by Iso but diminished maximal vasodilation to Ado and SNP in bo th muscles. In addition, vasodilatory responses in arterioles from control rats varied between muscle types. Maximal dilations induced by Iso (soleus: 42 +/- 6%; white gastrocnemius: 60 +/- 7%) and Aho (soleus: 51 +/- 8%; whi te gastrocnemius: 81 +/- 6%) were greater in arterioles from white gastrocn emius muscles. These data do not support the hypothesis that microgravlty-i nduced orthostatic hypotension results from an enhanced vasodilatory respon siveness of skeletal muscle arterioles. Furthermore, the data support the c oncept that dilatory responsiveness of arterioles varies in muscle composed of different fiber types.