A Study of the "Staircase" in Ventricular Muscle and Its Relationship to the Inotropic Activity of Certain Drugs

Tension duration curves recorded during isometric contractions of toad ventricular strips were studied under a variety of conditions. Changes recorded in these curves during inotropic responses could be divided into two classes: (a) those in which the time required to reach peak tension (t_p) was decreased and the twitch duration shortened. (b) those in which t_p was either unaltered or increased and the twitch duration unchanged. The first class is represented by excess Ca⁺⁺, strophanthin, and the "staircase" phenomenon while the second is represented by noradrenaline, "amine-like" substances liberated by tyramine, cocaine, and reserpine, and reduced K⁺. The results suggest that Ca⁺⁺ is a common factor underlying the inotropic response recorded both during the staircase and during glycoside activity. The involvement of amines in the staircase was not demonstrated.