The significance of the interrelationship between the intensity of the contractile state and the velocity of relaxation in adapting cardiac muscle to function at high work loads

The relationship between the amplitude of contraction and the velocity of relaxation has been studied, using papillary muscles isolated from rats and from guinea-pigs. Under isotonic conditions and with varying preloads an increase in the amplitude of contraction is followed by a proportionate increase in the velocity of relaxation. This same relationship was observed in experiments performed under isotonic conditions with different afterloads, provided that the contractile force did not exceed 60% of the maximal isometric value. This correlation disappeared at high work loads under isometric conditions, when the resting length exceeded a critical value. A positive correlation between developed ventricular pressures and the velocity of relaxation was also found in anaesthetized dogs and rats, when the developed pressure increased in response to an increased resistance in the outflow tract.