Potassium Contractures in the Ureter of the Guinea‐pig

Summary The electrical and mechanical responses of the guinea‐pig ureter to raised extracellular potassium ion concentrations hove been studied. The response consisted of an initial series of rapid contractions associated with action potentials followed by the development of maintained tension associated with maintained depolarization. Relaxation and repolarization occurred when normal potassium ion concentration was restored. The threshold potassium ion concentration for the development of maintained tension varied between 25 and 50 mM. The amplitude of the maintained tension could be immediately and reversibly changed by varying the external calcium ion concentration, though the calcium ion concentration at which these changes appeared varied considerably between preparations. Absence or reduction of the initial contractions in low calcium ion solutions was always accompanied by absence or abnormality of the action potential. It is suggested that the maintained tension depends on an increased influx of extracellular calcium ions during the potassium ion‐induced depolarization. Although this mechanism is not excluded as a possible basis for contraction initiated by the depolarization of the action potential, it is likely that a calcium ions “store” of unknown nature is involved in the normal excitation‐contraction coupling process.