The Efficiency of Frog Ventricular Muscle

Mechanical power and oxygen consumption (VO2) were measured simultaneously from isolated segments of trabecular muscle from the frog (Rana pipiens) ventricle. Power was measured using the work-loop technique, in which bundles of trabeculae were subjected to cyclic, sinusoidal length change and phasic stimulation. VO2 was measured using a polarographic O2 electrode. Both mechanical power and VO2 increased with increasing cycle frequency (0.4-0.9 Hz), with increasing muscle length and with increasing strain (= shortening, range 0-25% of resting length). Net efficiency, defined as the ratio of mechanical power output to the energy equivalent of the increase in VO2 above resting level, was independent of cycle frequency and increased from 8.1 to 13.0% with increasing muscle length, and from 0 to 13% with increasing strain, in the ranges examined. Delta efficiency, defined as the slope of the line relating mechanical power output to the energy equivalent of VO2, was 24-43%, similar to that reported from studies using intact hearts. The cost of increasing power output was greater if power was increased by increasing cycle frequency or muscle length than if it was increased by increasing strain. The results suggest that the observation that pressure-loading is more costly than volume-loading is inherent to these muscle fibres and that frog cardiac muscle is, if anything, less efficient than most skeletal muscles studied thus far.