Heat released during relaxation equals force-length area in isometric contractions of rabbit papillary muscle.

It has been claimed that the mechanical performance and the related energy turnover of the left ventricle can be reliably predicted on the basis of its time-varying elastance behavior. In its most elementary form, this behavior can be mathematically described by E(t) = P(t)/[V(t)-Vd], where E is ventricular elastance, t is time, P is ventricular pressure, V is ventricular volume, and Vd is the intercept of the end-systolic pressure-volume line on the volume axis. To find out how this behavior of the ventricle as a whole is related to the properties of the myocardium, we tested the energetic prediction for the ventricle that the pressure-volume area of an isovolumic contraction equals the energy released in relaxation in experiments on isolated rabbit papillary muscle at 20 degrees C. To that end, the energy (joules) contained by the force-length area of the muscles, contracting isometrically, was compared with the heat (joules) liberated in relaxation as measured with thermopiles. Mechanical performance of the muscles was varied by altering initial muscle length and external calcium. The slope of the resulting relation between force-length area and heat liberated in relaxation (n = 26) was not significantly different from unity. Thus, the energetic prediction of the time-varying elastance model developed for the whole left ventricle was confirmed by experiments on rabbit papillary muscle at 20 degrees C.