title: Biophysics of the developing heart.
abstract: 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.
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D005318:Fetal Heart / Q000502:physiology*; D006321:Heart / Q000502:physiology*; D066298:In Vitro Techniques; D009200:Myocardial Contraction; D012756:Sheep