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Articles in PresS, published online ahead of print October 22, 2001
J Appl Physiol, 10.1152/jap.01031.2000
Submitted on October 19, 2000
Accepted on October 16, 2001
* To whom correspondence should be addressed. E-mail: heapsc{at}missouri.edu.
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 Ca2+ channel current (VGCC) density of arteriolar smooth muscle. After 14 days of either HLU or weight-bearing control (CTL), first-order arterioles were isolated from both fast-twitch and mixed fiber-type regions of the gastrocnemius muscle, cannulated and pressurized at 90 cmH2O. Mixed gastrocnemius arterioles of HLU rats demonstrated increased spontaneous tone (HLU, 43±5 vs. CTL, 27±4% of possible constriction) and an approximate 2-fold enhanced myogenic response when exposed to step changes in intraluminal pressure (10-130 cmH2O) compared with similar arterioles from CTL rats. In contrast, fast-twitch gastrocnemius arterioles demonstrated similar levels of spontaneous tone (HLU, 6±2 vs. CTL, 6±2%) and myogenic reactivity in HLU and CTL rats. Neither KCl-induced contractile responses (10-50 mM KCl) nor VGCC density was significantly different between mixed gastrocnemius arterioles of HLU and CTL 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.
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