Activity fronts in fed dogs: effect of a beta-adrenergic agonist.

The effect of the beta-adrenergic agonist isoproterenol on intestinal myoelectric activities was studied in fed dogs. A 15-min isoproterenol infusion (0.5 micrograms X kg-1 X min-1) initiated activity fronts after a meal in 14 of 16 experiments. The phase III motor activity was of similar duration and migrated aborally at the same rate as spontaneous fronts occurring in the interdigestive period; however, the activity fronts usually originated in the midjejunum and seldom migrated to the terminal ileum. The isoproterenol-induced fronts were blocked by pretreatment with propranolol (1 mg/kg). Activity fronts were not induced by sodium nitroprusside (10 micrograms X kg-1 X min-1), even though it inhibited myoelectric activity and induced hypotension and tachycardia to a degree similar to that caused by isoproterenol. The effects of isoproterenol were not blocked by vagotomy. Longer isoproterenol infusions of 2 h produced one but not more activity fronts and infusions in the fasted state did not induce premature fronts. These results do not support the hypothesis that the effect of isoproterenol is due to a neural reflex, and further studies are necessary to determine which of several potential mechanisms is responsible for this effect.