Energetic changes of myocardium as an adaptation to chronic hemodynamic overload and thyroid gland activity.

Pressure-overload cardiac hypertrophy and hypothyroidism were shown to be associated with a decreased maximum shortening velocity of the myocardium. To investigate the nature of these intrinsic myocardial changes, we studied the energetic consequences in left ventricular papillary muscles of the rat by using standard HILL planar vacuum-deposited antimony-bismuth thermopiles. To evaluate the economy of isometric force generation and maintenance, we analyzed the ratio of liberated heat and developed tension or developed tension-time integral in twitches and experimentally induced tetanic contractions. Hypothyroidism was induced by treatment with propylthiouracil (PTU), and hypertension by operative narrowing of the left renal artery of rats according to Goldblatt (GOP). In the myocardium of hypothyroid as well as hypertensive rats, initial heat per peak twitch tension and total activity-related heat per tension-time integral were significantly reduced compared to controls. In tetanic contractions, total activity-related heat per tension-time integral was also decreased in PTU and GOP myocardium when compared to controls. Thus, the economy of force generation and maintenance is improved in the myocardium of the experimental animals. The data is interpreted in terms of altered cross-bridge cycling rates which are shown to be associated with changes in the myosin isoenzyme pattern. The intrinsic changes of the myocardium due to pressure-overload hypertrophy and hypothyroidism are considered to be adaptive rather than pathologic reactions of the myocardium.