Mechanism of action of pentagastrin and acetylcholine on the longitudinal muscle of the canine antrum.

1. Electrical and mechanical activities of the longtitudinal muscle of the dog antrum were recorded with the double sucrose‐gap technique. 2. The muscle exhibited spontaneous action potentials which consisted of a spike‐like potential which, after a brief and partial repolarization, was followed by a negative‐going, plateau‐type potential. In 97% of the preparations, no tension changes were produced by spontaneous action potentials. 3. Tetrodotoxin, atropine, alpha‐ and beta‐adrenoceptor antagonists, and H1 and H2 receptor blocking agents had no effect on the action potential. It was concluded that the action potential was myogenic in origin. 4. The mean frequency of the action potential at 37+/‐ 0.5 degrees C was 1.0/min+/‐0.06 (s.e. of mean, n=92) and the mean duration 7.1+/‐0.2 sec (s.e. of mean, n=11). 5. Steady depolarizing current increased whereas hyperpolarizing current decreased the frequency of the action potential. 6. Length‐tension relations were studied. In twelve strips, the average resting, passive, tension at LO was 570 mg. The active force of isometric contraction produced by acetylcholine increased with strip length up to a maximum, then decreased wtih further increased in length. There were no mechanical responses to pentagastrin. 7. Pentagastrin had two sites of action. On smooth muscle, it increased the frequency of the action potential in a dose dependent fashion. Threshold concentraions ranged from 2X10‐14 to 10‐11M. The ED50 was 2X10‐10M. The maximum response, 5.4/min, was reached at 10‐8M. Pentagastrin also released acetylcholine from intramural cholinergic nerves. 8. Pentagastrin reduced the amplitude and duration of the action potential.