The purpose of the present study was to determine whether hindlimb unloading of rats alters vasoconstrictor and myogenic responsiveness of skeletal muscle arterioles. After either 2 wk of hindlimb unloading (HU) or cage control (C), second-order arterioles were isolated from the white portion of gastrocnemius (WG; C: n = 9, HU: n = 10) or soleus (Sol; C: n = 9, HU: n = 10) muscles and cannulated with two micropipettes connected to reservoir systems for in vitro study. Intraluminal pressure was set at 60 cmH₂O. The arterioles were exposed to step changes in intraluminal pressure ranging from 20 to 140 cmH₂O to determine myogenic responsiveness and to KCl (10-100 mM) and norepinephrine (10⁹-10⁴ M) to determine vasoconstrictor responsiveness. Although maximal diameter of WG arterioles was not different between C (185 ± 12 µm) and HU (191 ± 14 µm) rats, WG arterioles from HU rats developed less spontaneous tone (C: 33 ± 5%, HU 20 ±3%), were unable to maintain myogenic tone at pressures from 140 to 100 cmH₂O, and were less sensitive to the vasoconstrictor effects of KCl and norepinephrine (as indicated by a higher agonist concentration that produced 50% of maximal vasoconstrictor response). In contrast, maximal diameter of Sol arterioles from HU rats (117 ± 12 µm) was smaller than that in C rats (148 ± 14 µm). However, the development of spontaneous tone (C: 30 ± 4%, HU: 36 ± 5%), myogenic activity, and the responsiveness to vasoconstrictor agonists were not different between Sol arterioles from C and HU rats. These results indicate that hindlimb unloading diminishes the myogenic autoregulatory and contractile responsiveness of arterioles from muscle composed of type IIB fibers and suggest that the compromised ability to elevate vascular resistance after exposure to microgravity may be related to these vascular alterations. In addition, hindlimb unloading appears to induce vascular remodeling of arterioles from muscle composed of type I fibers, as indicated by the decrease in maximal diameter of arterioles from Sol muscle.

autoregulation; blood flow; hemodynamics; hindlimb unweighting; morphology; suspension

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