Nonuniform changes in arteriolar myogenic tone within skeletal muscle following hindlimb unweighting

Department of Veterinary Biomedical Sciences and Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri 65211 Hindlimb unweighting (HLU) has been shown to alter myogenic tone distinctly in arterioles isolated from skeletal muscles composed predominantly of fast-twitch (white gastrocnemius) compared with slow-twitch (soleus) fibers. Based on these findings, we hypothesized that HLU would alter myogenic tone differently in arterioles isolated from distinct fiber-type regions within a single skeletal muscle. We further hypothesized that alterations in myogenic tone would be associated with alterations in voltage-gated Ca 2+ channel current (VGCC) density of arteriolar smooth muscle. After 14 days of HLU or weight bearing (control), first-order arterioles were isolated from both fast-twitch and mixed fiber-type regions of the gastrocnemius muscle, cannulated, and pressurized at 90 cmH 2 O. Mixed gastrocnemius arterioles of HLU rats demonstrated increased spontaneous tone [43 ± 5% (HLU) vs. 27 ± 4% (control) of possible constriction] and an approximately twofold enhanced myogenic response when exposed to step changes in intraluminal pressure (10-130 cmH 2 O) compared with control rats. In contrast, fast-twitch gastrocnemius arterioles of HLU rats demonstrated similar levels of spontaneous tone [6 ± 2% (HLU) vs. 6 ± 2% (control)] and myogenic reactivity to control rats. Neither KCl-induced contractile responses (10-50 mM KCl) nor VGCC density was significantly different between mixed gastrocnemius arterioles of HLU and control rats. These results suggest that HLU produces diverse adaptations in myogenic reactivity of arterioles isolated from different fiber-type regions of a single skeletal muscle. Furthermore, alterations in myogenic responses were not attributable to altered VGCC density. microgravity; vascular smooth muscle; calcium channel