Power output and efficiency of frog cardiac muscle

Mechanical work, power output, and metabolic energy expenditure of trabecular muscle from the heart of the frog Rana pipiens were measured. Work was measured using the work-loop technique, in which muscle length was cycled in a sinusoidal fashion, and the muscle given a twitch stimulus at the phase yielding maximum net work. The work done while shortening, lengthening, and the difference (net work) were measured. Metabolic energy expenditure was calculated from the oxygen consumed by the muscle.;Net work increased with strain to strains over 25% of the muscle's rest length. This large value likely reflects the slow cycle frequency of frog cardiac muscle. A distinct optimum strain for net work was not found.;Isometric twitch stress was maximal at 0.4-0.6 Hz twitch frequency in ventricular trabeculae, and 0.6-1.4 Hz in the atrium. The twitch duration decreased with increasing twitch frequency. Shortening and net work were maximal at 0.7 Hz cycle frequency in ventricular trabeculae, and 0.9-1.4 Hz in the atrium. The decline in work at slower frequencies was due in part to a decline in twitch force. Maximum power for the ventricle was about 5 Wkg;Shortening and lengthening work increased with increasing muscle length, while net work increased then declined. The length at which net work was maximum was 9-14% shorter than that for maximum isometric twitch force. The difference may be a consequence of the disproportionate increase in lengthening work with increasing muscle length due to progressively incomplete relaxation.;Mechanical power and metabolic energy expenditure (EO...