Effects of amitraz, several opiate derivatives and anticholinergic agents on intestinal transit in ponies

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Abstract
Amitraz, atropine, glycopyrrolate and morphine (but not its derivatives, meperidine [pethidine], oxymorphone and butorphanol) produced similar effects on clinically observed intestinal functions and significantly prolonged intestinal transit, as assessed with polyethylene glycol (PEG). However, their mechanisms of action, particularly on intestinal transport (fluid movement), may differ widely. Loperamide appeared to exert a preferential action on intestinal transport rather than motility, as there was no significant delay in PEG transit. The action of amitraz in perturbing smooth muscle coordination and enhancing absorption involved, at least partially, an .alpha.2 adrenergic component susceptible to yohimbine. Naloxone, an opiate antagonist, did not influence amitraz responses but induced the return of intestinal sounds and defecation after a refractory period in morphine-dosed ponies. The results suggest potential clinical applications in horses for loperamide in diarrhoea therapy, for .alpha.2 adrenergic antagonists in obstruction and ileus and, possibly, of amitraz-like agents in reducing fluid losses in diarrhoea.