Excitation-dependent relaxation in the cardiac muscle.

Isolated trabeculae of rabbit and guinea pig atrium exposed to low-sodium solution developed after-contractions and increased diastolic tension when rhythmic steady-state stimulation was stopped. Single excitation applied during rest or at the peak of after-contraction brought the resting tension to the low, control level. Tension of isolated cat papillary muscle increased due to action of 17 mmol of caffeine applied during rest was reduced during rhythmic post-rest stimulation. Early extra-excitation potentiated relaxation of atrial muscle exposed to low-sodium solution. It is concluded that relaxing factor of cardiac muscle is activated by excitation of the cell.