FORCE-VELOCITY AND POWER-LOAD CURVES IN RAT SKINNED CARDIAC MYOCYTES

1. This study utilized a skinned myocyte preparation with low end comp liance to examine force-velocity and power-load curves at 12 degrees C in myocytes from rat hearts. 2. In maximally activated myocyte prepar ations, shortening velocities appeared to remain constant during load clamps in which shortening took place over a sarcomere length range of similar to 2.30-2.00 mu m. These results suggest that previously repo rted curvilinear length traces during load clamps of multicellular pre parations were due in part to extracellular viscoelastic structures th at give rise to restoring forces during myocardial shortening. 3. Duri ng submaximal Ca2+ activations, the velocity of shortening at low load s slowed and the time course of shortening became curvilinear, i.e. ve locity progressively slowed as shortening continued. This result impli es that cross-bridge cycling kinetics are slower at low levels of acti vation and that an internal load arises during shortening of submaxima lly activated myocytes, perhaps due to slowly detaching cross-bridges. 4. Reduced levels of activator Ca2+ also reduced maximal power output and increased the relative load at which power output was optimal. Fo r a given absolute load, the shift has the effect of maintaining power output near the optimum level despite reductions in cross-bridge numb er and force generating capability at lower levels of Ca2+.