Biophysics of the developing heart. II. The interaction of the force-interval relationship with inotropic state and muscle length (preload).

The interaction of the force-interval relationship, muscle length (ventricular volume), and inotropy was evaluated in isolated fetal myocardial preparations and in the in utero chronically instrumented fetal lamb. In the isolated muscle, 93 to 141 days' gestation, a change in muscle length strengthened all contractions equally (multiplicatively) but produced no other changes in either the quantitative or the qualitative characteristics of the force-interval relationship; it was significantly altered by isoproterenol. Similarly, in the intact fetal lamb the qualitative and quantitative features of the relationship did not depend on ventricular volume, e.g., postextrasystolic potentiation was constant over a wide range of volumes. But isoproterenol produced a decrease in postextrasystolic potentiation. These results demonstrate that the force-interval relationship satisfies two major criteria for an index of cardiac contractility and suggest that the relationship may provide a basis for the evaluation of changes in contractility with development and birth.