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Articles in PresS, published online ahead of print February 8, 2002
J Appl Physiol, 10.1152/jap.01152.2001
Submitted on November 21, 2001
Accepted on January 8, 2002
1-adrenergic constriction than gastrocnemius arterioles
1 Departments of Veterinary Biomedical Sciences and Medical Physiology, University of Missouri, Columbia, MO, USA
* To whom correspondence should be addressed. E-mail: laughlinm{at}missouri.edu.
The sympathetic nervous system (SNS) 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 (2A's) were isolated from WG, RG, and DIA of rats and reactivity examined in vitro. Results reveal that DIA 2A's constrict less to norepinephrine (NE)(10-9 - 10-4 M) than 2A's from RG and WG, which exhibited similar NE-induced constrictions. This difference was not endothelium-dependent since responses of denuded 2A's were similar to those of intact arterioles. The blunted NE-induced constrictor response of DIA 2A''s 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 - 10-4 M), and did not respond to selective 2-receptor stimulation with UK 14,304. and 3) Endothelin-1 produced similar constriction in 2A's 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 2A's compared to 2A's from gastrocnemius muscle. Differences in 1-adrenergic constrictor responsiveness among arterioles in skeletal muscle may contribute to non-uniform muscle blood flow responses observed during exercise and serve to maintain blood flow to DIA during exercise-induced increases in sympathetic nerve activity.
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