Control of intrinsic reticulo-ruminal motility in the vagotomized sheep.

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Abstract
The intrinsic motility of the reticulo-rumen was studied by electromyography in 19 sheep subjected to bilateral thoracic vagotomy and maintained by intragastric infusion of a complete liquid diet. The influences of distension, temperature and tactile and chemical stimuli on the intrinsic reticulo-ruminal motility were investigated. The level of electrical discharge in the reticulum and rumen in the 1st 3 days after vagotomy was increased progressively with distension without giving rise to the large group discharges characteristic of the long-term vagotomized sheep and was reduced by atropine (0.1-1 mg kg-1) but not by hexamethonium (2 mg kg-1). In the long-term vagotomized animals, the frequency of the large group discharges over the reticulo-rumen varied with the degree of reticulo-ruminal distension. The discharges were absent below a threshold rumen volume; above the threshold they increased progressively with volume until a maximal rate of 6 to 7 regular discharges per minute was established at large ruminal volumes. The discharges were abolished by atropine (0.1-1 mg kg-1) or hexamethonium (2 mg kg-1). With the rumen volume below the threshold, in all areas of the reticulo-rumen localized distension stimulated local discharge only and did not induce large group discharge. Replacement of rumen contents with an equal volume of 0.2 M -acetic, -propionic or -butyric acid buffered to pH 4.0 rapidly abolished the large group discharges over the entire reticulo-rumen. Replacement of rumen contents with an equal volume of 0.9% NaCl at 30.degree. C immediately abolished the large group discharges; at temperatures between 35.degree. and 43.degree. C this had no effect. Gentle tactile stimulation increased local discharge in the reticulum and cranial dorsal sac but not in other areas of the rumen and did not affect large group discharge in any region. The intrinsic reticulo-ruminal motility of chronically vagotomized sheep is apparently principally regulated by the degree of reticulo-ruminal distension. Like the CNS-controlled motility of the vagus-intact sheep it is inhibited by high concentrations of volatile fatty acids. Local control mechanisms, therefore, may interact with central control in the overall regulation of motility in vagus-intact sheep.