[The effect of stimulation frequency on the speed of isometric myocardial relaxation in mammals].

The relaxation of tension of the rabbit and rat heart right ventricle papillary muscles was estimated by the characteristic time t30. An increase in the stimulation rate as well as an increase in extracellular concentration of Ca ions decelerated the relaxation in the rabbit myocardium, whereas in the rat myocardium a shortening of interimpulse interval accelerated isometric relaxation. Constant administration of reserpine and beta-blocking agents did not alter the effects. The mechanism of the stimulation rate effect on the velocity of relaxation are discussed.