Vol. 92, Issue 3, 1145-1151, March 2002
Nonuniform changes in arteriolar myogenic tone within
skeletal muscle following hindlimb unweighting
Cristine L.
Heaps and
Douglas K.
Bowles
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 Ca2+ 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 cmH2O. 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 cmH2O)
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