MULTIPLE RESPONSES TO ELECTRICAL-FIELD STIMULATION IN CIRCULAR MUSCLE OF CANINE GASTRIC CORPUS

Innervation of circular muscle of the canine stomach studied in vitro was investigated by subjecting muscle strips to electrical field stimulation. Strips were cut from the lesser curvature of the gastric corpus and stimulated with 10-s trains of 0.5-ms pulses at 0.5-20 Hz, 40 V. Most responses were classified into 1 of 3 types. In general, field stimulation tended to elicit sequences of varying magnitudes of transient on-contraction, on-relaxation, off-relaxation, off-contraction. Responses were abolished by tetrodotoxin. On-contraction was almost abolished by atropine plus desensitization by 5-hydroxytryptamine (5-HT) or substance P. On-relaxation and off-relaxation were not affected by adrenergic blockade, methysergide, apamin, or 4-aminopyridine. ATP usually caused contraction and slightly diminished relaxation to field stimulation. Vasoactive intestinal polypeptide (VIP) had little effect on tone and response to field stimulation. Relaxation disappeared after scorpion venom treatment. This probably resulted from depletion of the transmitter which mediates relaxation. Off-contraction was reduced by atropine, desensitization by serotonin or substance, cromoglycate, indomethacin or ATP, but was not affected by adrenergic blockade, hexamethonium, methysergide, mepyramine, or VIP. Innervation of gastric corpus circular muscle included excitatory cholinergic and both excitatory and inhibitory noncholinergic, nonadrenergic innervation. The responses of circular muscle to field stimulation in vitro were drastically different from those obtained previously in vivo, suggesting damage or altered inputs to circular muscle when strips of circular muscle are studied.