Role of intracellular calcium in the phasic and tonic contractions of stomach smooth muscles.

The double sucrose gap is used to study the role of Ca-ions in the electromechanical coupling in the phasic and tonic contractions in the different parts of cat stomach. Extracellular Ca2+ is of primary importance in the activation of the tonic contractions. The regulation of the muscle tone under the action of external stimuli is achieved through gradual changes in the Ca-permeability of the cell membrane. Of primary significance for the activation of the phasic contractions is Ca which is released from the intracellular stores. The Ca2+-influx during the action potential results in additional modelation of the intensity of phasic contractions as well as to a re-charging of the intracellular Ca-depots. It is assumed that the differences in the mechanisms of activation of phasic and tonic contractions result not only from the specificities of membrane control but also from definite differences in the organization of the intracellular systems responsible for these contractions.