Selective blockade of the responses of reticuloruminal muscle to 5-HT in sheep

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Abstract
The effects of i.v. administration of 5-HT [5-hydroxytryptamine] on the patterns of reticulo-rumen contractions were studied in conscious sheep. Contractions were measured by strain gauges applied to the external wall of the reticulum and the ventral and/or dorsal sac of the rumen. The responses to 5-HT comprised a short-lived contraction followed by a sustained increase in muscle tone and a concomitant inhibition of the extrinsic reticulo-rumen contractions. The corresponding blockades produced by atropine, 5-HT antagonists and chemical sympathectomy suggest involvement of a peripheral cholinergic mechanism in the initial contractile response and a central adrenergic mechanism in the reflex inhibition of extrinsic reticulo-rumen contractions.