Is there a shortening-heat component in mammalian cardiac muscle contraction?
It has been suggested that there is a shortening-heat component that is an extra liberation of heat on shortening above that due to the external work, which contributes to the total energy expenditure of the beating heart. The presence of a shortening heat component was studied in isolated papillary muscles from the right ventricle of rabbits killed by cervical dislocation. At the onset of a contraction, muscles were shortened from various initial lengths through fixed distances at near maximum velocity before being allowed to develop force at the new length; the heat production accompanying such contractions was measured. The measured heat was compared with heat values predicted from previously established heat-stress curves obtained by using either preshortening or latency release methods. There was no shortening-related increment in heat output per contraction when comparison was made to a control heat-stress curve, obtained using the latency release method. An increase in heat production of 10% was observed with long shortening distances when comparison was made to a control heat-stress curve obtained by preshortening the muscles; however, this difference is most likely due to an underestimate of the magnitude of the activation heat component in these control heat-stress curves. An increase in isometric heat production due to maintained stretch per se was observed. The present data indicate that it is unlikely that there is a significant shortening heat component when cardiac muscle shortens. The absence of such a metabolic component may account for the rapid fall off in total enthalpy output in isotonic contractions at low to medium afterloads when compared with the skeletal muscle data.