Relationship between ejection phase indices of performance and myocardial functions during the development of pressure overload hypertrophy

The present study examines the temporal relationship between performance of the hypertrophied nonfalling rabbit heart and the contractility of muscles isolated from these same hearts. Ejection phase indices of ventricular function were determined cineradiographically during the development of hypertrophy. Isolated papillary muscle function was examined an average of 8.6 (early), 40.1 (middle), and 97.5 (late) days after banding of the pulmonary artery. Active tension development at Lmax (F) was depressed by 65% in the muscles examined early and by 43% in the middle group. By the late group, F was comparable to control levels. Early depression and a return to normal function were also observed for peak dF/dt and velocity of shortening at Lmax. Time to peak tension was unchanged at 8.6 days and increased at the middle and late time points. Both percentage of shortening and mean normalized velocity of shortening, determined cineradiographically in the intact heart, were depressed immediately following surgery, gradually returned toward "normal function" by the third week, and plateaued at a stable level of performance, which was maintained thereafter. The similarity of in vivo function of the hypertrophied and control hearts, despite the profound differences observed in the myograph, indicate that "ejection phase indices," such as fiber shortening rate, are poor indicators of intrinsic function during the development of hypertrophy. Moreover, these results demonstrate the extent to which intrinsic functional deficits may be overcome in the whole heart and suggest the presence of mechanisms such as enhanced sympathetic nervous activity, which contribute to the maintenance of this normal basal ventricular function.