Length-dependent inotropic changes in cardiac muscle.

1. Length-dependent changes of the inotropic state of rabbit atrial strips were studied using inotropic interventions. Increases in extracellular calcium and adrenaline were used as positive inotropic agents, whereas acetylcholine and verapamil were the negative inotropic agents. Length-tension curves were normalized with respect to maximal developed tension and length. 2. The expected length-tension relations were obtained when plotted in a conventional manner. However, when length-tension curves were normalized with respect to maximal developed tension and length, a length-dependent activation was indicated by the separation of the curves. 3. These results suggest that length-dependent activation altered the inotropic state of cardiac muscle when length changed.