The adrenergic receptors in the control of reticulo-ruminal myoelectrical activity in sheep.

The involvement of adrenoreceptors in the control of reticulo-ruminal (R-R) motility was electromyographically studied in five conscious adult ewes. Intravenous (i.v.) administration of the beta 2-agonist ritodrine (10 or 20 micrograms/kg.min for 15 min) or alpha 1-blocker prazosin (20 micrograms/kg.min for 30 min) or the beta 1- and beta-blocker acebutolol and propranolol, respectively, (30 micrograms/kg.min for 30 min) had no significant effect on R-R phasic myoelectrical grouped discharges (PMGD). Administration of the alpha 1-agonist phenylephrine (4 micrograms/kg.min i.v. for 15 min) significantly increased for 25 min both the frequency of R-R PMGD and the percentage of occurrence of secondary ruminal PMGD and provoked rumination. Pretreatment of animals with prazosin prevented these R-R responses to phenylephrine. Administration of the alpha 2-agonist naphazoline (2.5 micrograms/kg.min i.v. for 15 min) stopped the reticular PMGD for 1 hour, after an initial increase in their frequency, and greatly decreased the frequency of ruminal PMGD. When animals were pretreated with the alpha 2-blocker yohimbine (20 micrograms/kg.min i.v. for 30 min), naphazoline failed to produce any effect on R-R PMGD. Yohimbine, given either alone or followed by naphazoline, abolished secondary ruminal PMGD for about 2 hours. The beta 1-agonist (+)-dobutamine (30 micrograms/kg.min i.v. for 15 min) provoked a pause in R-R PMGD, which reappeared soon after the end of drug administration. Pretreatment of ewes with acebutolol or propranolol prevented dobutamine from producing its effects on R-R PMGD.(ABSTRACT TRUNCATED AT 250 WORDS)