EFFECTS OF EPINEPHRINE ON THE MECHANICAL WORK DONE BY RABBIT CARDIAC MUSCLE UNDER INERTIA LOADING
The mechanical work done by isolated rabbit papillary muscle was investigated under constant load but different degrees of inertia. When the muscle was stimulated at a beat rate of 6/min in the standard medium solution, the work done by muscle to impart the velocity to the lever (work for the velocity factor) was much smaller than the work done by muscle to lift the load by a certain distance (work for the length factor). For the muscle treated with 2×l0-6 g/ml epinephrine, the total mechanical work increased with an increase in the moment of the inertia lever showing the maximum at the equivalent mass of about 150-300 g. The increase in the mechanical work was mainly due to the increase in the work for the velocity factor rather than in the work for the length factor, and the former attained to 22-68％ of the total work under the optimum equivalent mass. The result suggests that the muscle liberates extra mechanical energy when the shortening is restricted by the inertia.