Effects of xylazine on canine coronary artery vascular rings

Objective-To determine the effects of xylazine on canine coronary artery smooth muscle tone. Sample Population-Hearts of 26 healthy dogs. Procedure-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 N-G-nitro-L-arginine methyl ester (L-NAME; 5mM), the (alpha(1)-adrenoceptor antagonist prazosin (10mM), and the alpha(2)-adrenoceptor antagonist atipamezole (10mM) 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-mum-diameter) and medium (500- to 1,000-mum-diameter) vascular rings but not of large (> 1,000-mum-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 and Clinical Relevance-Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation Of alpha(2)adrenoceptors.