Colonic alpha 2-adrenoceptor-mediated responses in the pony.
The motor responses of the caecum and colon to stimulation of alpha 2-adrenoceptors by xylazine and detomidine at the recommended dose levels of 0.6 and 0.1 mg/kg were investigated in three ponies. The motor changes of the left ventral colon induced by continuous intra-arterial infusion of a prostaglandin (PGF2 alpha) were used to assess the relative inhibitory effects of xylazine and detomidine in a colic model. The administration of alpha 2-agonists inhibited the spiking activity on the whole of the large intestine for 20-30 min (xylazine) or 2-3 h (detomidine). However, the detomidine-induced inhibition was preceded by a short period of increased smooth muscle basal tone as indicated by strain-gauge force transducer measurements. This pattern of activity was neither reversed nor prevented by the administration of tolazoline (10 micrograms/kg/min) intra-arterially. In contrast, inhibition of the colonic phasic and tonic motor activity by alpha 2-adrenoceptor stimulation was reversed competitively by tolazoline. The intra-arterial infusion of prostaglandin F2 alpha (10 micrograms/kg/min) induced prolonged and sustained spiking activity that might be related to signs of mild colic. Detomidine, and to a lesser extent xylazine, relaxed the whole of the large intestine and this was accompanied by alleviation of the signs of visceral pain.
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D002432:Cecum / Q000187:drug effects / Q000502:physiology; D003106:Colon / Q000187:drug effects / Q000502:physiology*; D015237:Dinoprost; D005769:Gastrointestinal Motility / Q000187:drug effects*; D006736:Horses / Q000502:physiology*; D007093:Imidazoles / Q000494:pharmacology*; D011460:Prostaglandins F / Q000494:pharmacology; D014043:Tolazoline / Q000494:pharmacology; D014991:Xylazine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D007093:Imidazoles; D011460:Prostaglandins F; D014991:Xylazine; C041255:detomidine; D015237:Dinoprost; D014043:Tolazoline