Mechanism of Staircase and Contracture in Ventricular Muscle

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Abstract
Analyses carried out in frog ventricular muscle showed, in connection with the staircase phenomenon, that loss of 3 m.eq. K out of 106.8 m.eq. internal Na+K increases the tension from 0-100%. The loss of internal Na has the same effect on the tension. Change in the internal ion concn. solely through loss or gain of water has no effect on the tension. Measurements showed that digitalis and related substances act by blocking the re-entry of K, thereby decreasing the internal ion content. As the internal ion content decreases excessively, contracture occurs, which, however, can be delayed or speeded up by decrease or increase of external K. This effect of the external K is exerted through the membrane potential. The resting potential thus exerts a static action on the contractile protein which results in relaxation in a milieu which otherwise would favor contraction. There is evidence that the resting potential exerts this action by the upkeep of a low internal pH.

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