Excitability of isolated hearts from rats during postnatal development

INTRODUCTION: After birth, cardiac myocytes undergo substantial growth and differentiation that affect their stimulation threshold. Cells from younger animals have a higher stimulation threshold than cells from adults. The aim of this work was to compare the excitability of isolated hearts of rats during postnatal development by measuring the stimulation threshold at several pulse durations. METHODS AND RESULTS: Stimulation threshold of isolated hearts were measured at eight different pulse durations between 0.1 to 20 msec. For each heart, a strength-duration curve was constructed and data were fitted using both Weiss-Lapicque and Blair models. Analysis of variance showed significant age-dependent differences in both rheobase field (E(reob)) and chronaxie (c). E(reob) decreased while c increased during development (E(reob) was 0.21, 0.16, 0.13, 0.10, and 0.09 V/cm and c was 2.0, 2.2, 2.3, 2.7, and 3.2 msec for rats aged 1, 2, 4, 8, and 20 weeks, respectively). There was a decrease in the threshold field with heart weight between 0.1 and 0.7 g, whereas the threshold was almost constant in the range from 0.7 to 2.0 g. Estimation of the energy density needed to defibrillate the heart was performed and appeared to be higher for younger than for adult animals. CONCLUSION: Hearts from younger animals have higher stimulation threshold than those from adults. This probably is due to changes in the cellular threshold as a result of maturation. The smaller excitability of younger hearts can have consequences with regard to the energy levels required for younger patients.