Differentiated contractile responses of gastric smooth muscle to substance P.

Muscle strips were excised from the circular and longitudinal layers of the fundus, corpus and antrum of canine stomach, and from the inner portion of the pyloric ring (inner pylorus). Substance P (SP) induced strong contractions, with the greatest sensitivity in fundus and circular corpus preparations (threshold near 10 -10 mol/l). The sensitivity to SP decreased in the sequence circular corpus - longitudinal antrum - circular antrum (threshold near 10(-7) mol/l); it re-increased towards the pylorus, and in the inner pylorus was nearly as high as in the fundus. The SP responses of fundus and longitudinal corpus were purely tonic, similar to acetylcholine (ACh) and histamine (H) responses. In circular corpus, SP induced a combined phasic-tonic response. SP induced in antrum strips purely phasic-rhythmical contractions of low frequency (similar to the H response), whereas ACh induced phasic contractions of high frequency, and in addition an increase of tone in the longitudinal antrum strips. The SP responses of the inner pylorus were not uniform; in some preparations purely tonic contractions were observed, and large phasic fluctuations of low frequency occurred in others. The phasic components of all the responses were completely suppressed by nifedipine (10(-6) mol/l). Tetrodotoxin as well as blockade of adrenoceptors, of ACh and of H receptors had no effect on the SP responses. Comparative studies with preparations from guinea-pig showed that species differences exist in the SP responses of gastric muscle.