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 10–4M), and changes in vascular ring
tension were continuously recorded. Effects of the
nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L-NAME;
5mM), the α1-adrenoceptor antagonist prazosin
(10mM), and the α2-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-µm-diameter) and medium (500- to 1,000-µmdiameter)
vascular rings but not of large (> 1,000-µmdiameter)
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 10–6M. 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 α2adrenoceptors.(
Am J Vet Res 2004;65:431–435)