Uniformity of the Contractile Response of Ventricular Muscle in High-Calcium Tyrode's Solution

Abstract

AMONG several conditions related to the sequence and rate of stimulation which affect the contractile response of cardiac muscle, prematurity of an excitation, interposed in a regular sequence, results in a beat of reduced tension or shortening. The weakness of the premature beat is unrelated to factors of electrical excitability, time-course or amplitude of the trans-membrane action potential or pre-contractile length of the muscle^1,2. The possibility that developed tension or shortening may vary with the transient level of intraccllular calcium (or calcium-containing ‘activator’) has been experimentally supported^3,4. Unlike the various potentiating effects, such as the ascending limb of staircase, post-extrasystolic potentiation and post-stimulation potentiation, however, the weakness of a premature beat does not clearly follow from such a mechanism without additional assumptions which would predict a low intracellular calcium in the time-interval immediately following a contraction. The residual effect of any contraction on the succeeding one has thus been seen as consisting of separate positive (potentiation) and negative (restitution) components with different time courses of decay⁵.