Biophysics of the developing heart

The force-interval relationship was evaluated in the developing heart of the lamb. The qualitative characteristics of the relationship were the same in isolated muscle (from 93 to 141 days' gestation to 1 year old) as in the chronically instrumented in utero fetus (122 to 141 days' gestation): (1) contractility, e.g., the maximum rate of rise of force, increased monotonically from a small value immediately after a contraction to a plateau, and (2) postextrasystolic potentiation was present in all preparations. In the intact animal, postextrasystolic potentiation depended on the basic pacing interval, t0, and the timing of the extrasystole, t1: when t0 was held constant and t1 was increased, potentiation decreased; when t1 was held constant and t0 was increased, potentiation increased. The qualitative characteristics of the relationship, and so the underlying myocardial basis, were unchanged over the developmental period studied.