The energy cost of relaxation in control and hypertrophic rabbit papillary muscles

Abstract

The energy cost of the onset and relaxation phases of cardiac isometric contractions has been investigated by ergometer controlled-length releases occurring at different times during the contraction cycle, to test the hypothesis that the energy cost of relaxation is normally small. Energy flux has been measured myothermically in 20 or 30 contractions of rabbit papillary muscles. The ergometer releases took place after different delays, starting during the latency period and incrementing in 50 ms steps, until eventually, releases were occurring late into the relaxation phase. The release step was kept constant and of a magnitude sufficient to prevent significant redevelopment of active stress at any release interval. The rate of release was several times greater than the maximum shortening velocity of the papillary muscle preparations. The heat production in each train of contractions was measured, but in order to estimate the total energy output, the elastic energy in the muscle-lever system which was removed by the ergometer release had to be added to the heat. This was estimated by integration of the stress-strain relationship found for each muscle. In normal animals the contraction peak, at 27°C and a 1.0 Hz stimulus rate, was located between the 215 and 265 ms release times, at which point the total energy flux was estimated to be 80%–90% of that measured in a normal isometric contraction. Additional experiments were performed in a group of volume-overloaded hearts and the data were compared with results from sham-operated controls. In hypertrophic muscles the contraction peak was located between 265 and 315 ms, and at such times the energy flux was about 75% of that measured in unreleased contractions. These results imply that 10% to 25% of the energy measured in a twitch contraction is used by processes occurring after the contraction peak. Some possible physiological reasons for this are considered.