Specific features in the electrical and contractile activities of the gastro-intestinal sphincters

Muscle strips (2/20 mm) are isolated from cat gastro-intestinal sphincters (GIS): lower esophageal sphincter (LES), pyloric sphincter (PS), ileo-caecal sphincter (ICS) and from the fundus. The activity of the four fibres is recorded simultaneously under isometric conditions in a bath. In another series of experiments the electrical and contractile activity of GIS was traced by the sucrose gap method. LES manifests only tonic changes which are associated only with changes in the membrane potential (MP); PS - rhythmic spike-dependent phasic contractions, ICS - in one part of the preparations both tonic and phasic contractions. The tonic changes in LES and ICS are sensitive both to nitroprusside-Na (10-6M) and to Verapamil (10-6 to 5X10-6 M). GIS sensitivity to external Ca2+ is different: maximum contractions in LES and ICS occur at Ca=2.5 mM, in PS at Ca=1.5mM. Ba2+ and Sr2+ (at CA=0) increase the sensitivity of LES and ICS to polarizing currents. After several stimulations there appear spontaneous spike-dependent phasic contractions in LES. TEA (5 mM) induces spike-dependent phasic contractions in LES and ICS. In PS the stimulating effect of TEA is manifested at very low concentrations (0.05-0.4 mM), while higher concentrations inhibit the electrical and contractile activity.