Phasic and tonic contractions of rabbit intestinal muscle

The effects of a single electrical stimulus upon the longitudinal and circular muscles of rabbit small intestine were investigated and the differentiation between phasic and tonic contractions was examined. A weak stimulus caused a phasic contraction and a stronger stimulus evoked both phasic and tonic contractions. The evoked phasic contraction was of an all-or-nothing nature, while the evoked tonic contraction was graded. Strength-duration curves of longitudinal and circular muscles were obts of rheobase and chronaxie. In the presence of verapamil or methoxyverapamil (D 600) the tonic contraction, but not the phasic contraction, could be evoked by a single strong stimulus, while in the presence of sodium nitroprusside the phasic contraction, but not the tonic contraction, could be provoked by a single strong stimulus. The phasic and tonic contractions differed in their strength-duration curves and drug responses.