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1-
adrenergic constriction than gastrocnemius arterioles
Departments of Veterinary Biomedical Sciences and Medical Physiology, University of Missouri, Columbia, Missouri 65211
The sympathetic nervous
system has greater influence on vascular resistance in low-oxidative,
fast-twitch skeletal muscle than in high-oxidative skeletal muscle
(17). The purpose of this study was to test the hypothesis
that arterioles isolated from low-oxidative, fast-twitch skeletal
muscle [the white portion of gastrocnemius (WG)] possess greater
responsiveness to adrenergic constriction than arterioles isolated from
high-oxidative skeletal muscle [red portion of the gastrocnemius
muscle (RG) and diaphragm (Dia)]. Second-order arterioles (2As) were
isolated from WG, RG, and Dia of rats and reactivity examined in vitro.
Results reveal that Dia 2As constrict less to norepinephrine (NE)
(10
9 to 10
4 M) than 2As from RG and WG,
which exhibited similar NE-induced constrictions. This difference was
not endothelium dependent, because responses of denuded 2As were
similar to those of intact arterioles. The blunted NE-induced
constrictor response of Dia 2As appears to be the result of differences
in
1-receptor effects because 1) arterioles
from Dia also responded less to selective
1-receptor
stimulation with phenylephrine than RG and WG arterioles; 2)
arterioles from Dia, RG, and WG dilated similarly to isoproterenol (10
9 to 10
4 M) and did not respond to
selective
2-receptor stimulation with UK-14304; and
3) endothelin-1 produced similar constriction in 2As from
Dia, RG, and WG. We conclude that differences in oxidative capacity
and/or fiber type composition of muscle tissue do not explain different
NE responsiveness of Dia 2As compared with 2As from gastrocnemius
muscle. Differences in
1-adrenergic constrictor responsiveness among arterioles in skeletal muscle may contribute to
nonuniform muscle blood flow responses observed during exercise and
serve to maintain blood flow to Dia during exercise-induced increases
in sympathetic nerve activity.
endothelium; endothelial-derived factors; norepinephrine; isoproterenol; phenylephrine; UK-14304; endothelin-1
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