Age dependence of post-pause contractions in the ventricular myocardium of rats

The maximal post-pause contraction has been measured in random bred albino rats by varying the contraction interval preceding this event. The amplitude of the maximal post-pause contraction as well as the duration of the pause interval corresponding to this contraction depend on the driving frequency applied just prior to this pause. The amplitude increases whereas the duration of the appropriate interval shortens when the driving frequency is increased. Both parameters are also influenced by the age of the tested animals; they increase from the 6th to the 15th week of life if the driving frequency is low. The effects are explained by the assumption that the circulation of the activator calcium is transferred from extracellular more to intracellular storage sites during maturation of the myocardium.