Effects of alpha(2)-adrenergic drugs on small intestinal motility in the horse: An in vitro study

The effects of selective a2-agonists (xylazine, detomidine and medetomidine) and antagonists (yohim- bine and atipamezole) on in vitro small intestine motility in the horse were evaluated. Samples of equine jejunum were placed in isolated organ baths and drug-induced modifications of motility were measured by means of an isotonic transducer. All tested a2-agonists dose-dependently reduced both spontaneous and electrically-evoked phasic contractions. Conversely, a2-antagonists were ineffective when tested alone, and showed a heterogeneous and dose-independent ability to inhibit agonist activity. In particular, the antagonism exerted by higher concentrations of both yohimbine and atipamezole against a2-agonists was weaker than when lower concentrations were used. The data are indicative of the presence of both pre- and post-synaptic a2-adrenoceptors with inhibitory activity on equine jejunum motility, and sup- port a possible therapeutic utility of these drugs in horse intestinal disorders associated with hypermotility.