In vitro spontaneous motility of gastric smooth muscles of the sheep.

Whole strips of muscle wall cut parallel to their corresponding longitudinal, circular or internal oblique orientations in the reticulum, rumen, omasal body, abomasal body and antrum and to the muscularis mucosae of the omasal leaves were mounted along a vertical axis and their mechanical activities recorded isometrically. The strips were perfused with Tyrode-Ringer solution at 37 degrees C bubbled with 100% O2 or a mixture of 95% O2 + 5% CO2. Spontaneous activity was observed in muscle strips from all regions but not all muscle strips were spontaneously active: markedly greater spontaneous activity and a greater sensitivity to stimulatory agents occurred with 100% O2. Spontaneous activity was unaffected by the ganglionic blocking agent hexamethonium (10(-5) M) whereas atropine (10(-6) M) had no effect, or reduced and in some cases with rumen only, abolished activity. Acetylcholine (10(-6)-10(-7) M) caused contraction of all muscle except muscularis mucosae from omasal leaves. Electrical stimulation evoked responses which included atropine-sensitive contractions, relaxations and rebound contraction especially following atropine. The experiments showed that both major muscle layers of the reticulum, rumen, omasum and abomasum in mixed-layer preparations exhibited spontaneous activity when removed from the influence of their cholinergic innervation and this included atropine-resistant diphasic contractions of the internal oblique muscle of the reticulum. The muscularis mucosae of the omasal leaves exhibited spontaneous contractile activity unaffected by cholinergic agonists and antagonists.