The relationships between isometric contractions and duration of interstimulus intervals, which were changed randomly, were assessed in myocardial strips of adult (AR) and newborn rats (NR). The highest coefficients of variation of contractions amplitude were seen in AR and lowest those in preparations of 1-3 day of life NR. The "force-interval" autoregression coefficients were estimated in different Ca2+ concentrations, their differences being due, probably, to ontogenetic changes.
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