Central and peripheral β‐adrenergic influences on reticulo‐rumen and upper‐gut myoelectrical activity in sheep

The effects of intravenous (i.v.) and intracerebroventricular (i.e.v.) administration of β‐adrenoceptor agonists were evaluated on the reticulo‐rumen and upper‐gut myoelectrical activity in six ewes chronically fitted with intraparietal electrodes and a cannula in a lateral ventricle of the brain. Intravenous infusion of the β 1 agonist dobutamine (30 μg/kg/min for 15 min) reduced the frequency of reticulo‐ruminal and abomasal contractions and stimulated duodeno‐jejunal motility, inducing a Phase III on the jejunum. These effects were reproduced by i.c.v. dobutamine at a dose of 10 μg/kg. Intravenous infusion of the β 2 agonist ritodrine (15 μg/kg/min for 15 min) selectively inhibited antral and duodenal motility. Ritodrine i.c.v. (15 μg/kg) did not affect forestomach or gastrointestinal motility. The mixed β 1 , β 2 agonist isoprenaline infused i.v. (0.6 μg/kg/min for 15 min) reproduced the effects of i.v. dobutamine, except at the antro‐duodenal level which was strongly inhibited. The effects of i.v. dobutamine were antagonized by i.v. or i.c.v. acebutolol, a specific β 1 antagonist. The effects of i.v. ritodrine were blocked by i.v. but not i.c.v. administration of propranolol, a mixed β 1 , β 2 antagonist. These data indicate that the stimulation of central β 1 adrenoceptors inhibits forestomach and antral motility and stimulates duodeno‐jejunal motility. Stimulation of peripheral β 2 adrenoceptors selectively inhibits duodeno‐jejunal motility.