TENSION AND SHORTENING VELOCITY OF RABBIT PAPILLARY MUSCLE UNDER INERTIA LOADING AT HIGH BEAT RATES
Shortening and mechanical work were investigated in rabbit papillary muscle contracting under inertial loads at high beat rates. As long as the beat rates were below 100/min, an increase in rates always augumented the contraction under any inertial loads. The dynamic work for the muscle to exert motion to the inertia lever attained to the maximum at the equivalent mass of around 150 g and stayed nearly constant with further increase in the equivalent mass. At the beat rate of 150/min, the dynamic work showed the peak when the muscle contracted under the equivalent mass of about 150 g. Further increase in beat rates up to 200/min decreased the dynamic work under the given equivalent mass and reduced the value of the optimum equivalent mass under which the muscle performed the maximum dynamic work. The increase in beat rates showed the positive inotropic action on the mechanical work if the inertial loads were small, but under considerable amounts of inertia loading the inotropic action was turned to negative.