Myocardial adaptation to stress from the viewpoint of adaptation and development

Myocardial adaptation to stress and development includes reorganization of subcellular systems. Using a myothermal method, changes in the contractile protein system were investigated across species (rat, rabbit, human myocardium) and in consequence of hemodynamic (volume overload human, pressure overload rabbit myocardium) or hormonal stresses (hypothyroid rat, hyperthyroid rabbit myocardium). Mechanical and myothermal measurements were performed in isometrically contracting right or left ventricular muscle strips and the force-time integral of the individual crossbridge cycle was calculated from heat and force data. Within species, crossbridge force-time integral increased by 85% from control human to volume overload human myocardium. Crossbridge force-time integral increased by 100% from control to hypothyroid rat myocardium. In rabbit myocardium, crossbridge force-time integral increased by 164% in pressure overload and decreased by 47% in hyperthyroid compared to control myocardium. Across species, crossbridge force-time integral was smallest in control rat myocardium (0.16 +/- 0.01 pNs) and increased in the order: control rat < hyperthyroid rabbit < hypothyroid rat, control rabbit < control human < pressure overload rabbit < volume overload human myocardium (0.96 +/- 0.01 pNs). Within and across species, crossbridge force-time integral was positively correlated with time to peak tension (r = 0.86; p < 0.05) and negatively correlated with maximum rate of tension rise (r = -0.85; p < 0.05) and maximum rate of tension fall (r = -0.78; p < 0.05). Furthermore, there were significant correlations between crossbridge force-time integral and total activity related heat (r = -0.81; p < 0.05) as well as total activity related heat per tension-time integral (r = -0.89; p < 0.005). Thus, the close relationship between crossbridge force-time integral and myocardial function within and across species demonstrates that alterations of crossbridge force-time integral reflect an important mechanism of subcellular adaptation to stress from a mechanical point of view. Moreover, alterations of the crossbridge force-time integral have pronounced effects on energy consumption in the different types of myocardium.