Effects of xylazine on canine coronary artery vascular rings.
OBJECTIVE
To determine the effects of xylazine on canine coronary artery smooth muscle tone.


METHODS
Hearts of 26 healthy dogs.


METHODS
Dogs were anesthetized with pentobarbital, and vascular rings of various diameters were prepared from the epicardial coronary arteries. Vascular rings were placed in tissue baths to which xylazine was added (cumulative concentrations ranging from 10(-10) to 10(-4) M), and changes in vascular ring tension were continuously recorded. Effects of the nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L NAME; 5 mM), the alpha1-adrenoceptor antagonist prazosin (10 mM), and the alpha2-adrenoceptor antagonist atipamezole (10 mM) on xylazine-induced changes in vascular ring tension were determined. Results were expressed as percentage of maximal contraction for each vascular ring preparation.


RESULTS
Xylazine induced vasoconstriction of small (< 500-microm-diameter) and medium (500- to 1,000-microm-diameter) vascular rings but not of large (> 1,000-microm-diameter) rings. For large vascular rings, L-NAME, atipamezole, and prazosin did not significantly affect the contractile response to xylazine. For small vascular rings, the contractile response following addition of xylazine to rings treated with L-NAME was not significantly different from the contractile response following addition of xylazine to control rings, except at a xylazine concentration of 10(-6) M. Xylazine-induced vasoconstriction of small vascular rings was blocked by atipamezole, but the addition of prazosin had no effect on xylazine-induced vasoconstriction.


CONCLUSIONS
Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation of alpha2adrenoceptors.
Teng
Bunyen
B
Department of Pharmacology and Toxicology, Brody School of Medicine, East Carolina University, Greenville, NC 27834, USA.
Muir
William W
WW
D000704:Analysis of Variance; D000818:Animals; D003331:Coronary Vessels / Q000187:drug effects*; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D007093:Imidazoles / Q000378:metabolism; D009129:Muscle Tonus / Q000187:drug effects; D011224:Prazosin / Q000378:metabolism; D014661:Vasoconstriction / Q000187:drug effects*; D014991:Xylazine / Q000378:metabolism / Q000494:pharmacology*
D007093:Imidazoles; C050701:atipamezole; D014991:Xylazine; D011224:Prazosin