Mechanical responses to catecholamines in isolated strips of the guinea-pig stomach muscle

Effects of catecholamines were investigated on isolated strips of the guinea-pig stomach. In the circular muscles, the relaxation and biphasic response caused by adrenaline and noradrenaline were suppressed partly by phentolamine and also partly by propranolol. The relaxation by catecholamines in the longitudinal muscles was inhibited mainly by propranolol. Thus, there are probably two types of relaxation. Alpha relaxation was more dominant in the circular muscles and beta relaxation in the longitudinal muscles. Contractions in both muscles were mediated through alpha-effects. The circular muscle is more sensitive to the alpha effect and the longitudinal muscle to the beta effect. In the presence of propranolol, isoprenaline. Field stimulation which excited intramural nerves resulted in contraction followed by relaxation in both circular and longitudinal muscles. The inhibition of the electrically evoked contractions by adrenaline and noradrenaline in both circular and longitudinal muscles was abolished by phentolamine but the electrically evoked relaxations were affected little.