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¹ School of Physiotherapy and Exercise Science & Heart Foundation Research Centre, Griffith University, Gold Coast, Queensland 9726, Australia

The aim of this study was to quantify the fraction of cross-bridges that cycle during a cardiac twitch. Measurements of the energetics of contracting left ventricular mouse papillary muscle were made in vitro (27°C) using the myothermic technique. Enthalpy output was partitioned into force-dependent and force-independent components using 2,3-butanedione monoxime (BDM) to selectively inhibit cross-bridge cycling. For isometric contractions and a contraction frequency of 2 Hz the net enthalpy output was 5.7 ± 0.8 mJ g^–1 twitch^–1 and initial enthalpy output was 2.3 ± 0.3 mJ g^–1 twitch^–1 (n = 11). Assuming that low concentrations of BDM did not affect Ca²⁺ cycling, force-independent enthalpy output was 18.6 ± 1.9% (n = 7) of the initial enthalpy output. Enthalpy output decreased with increased contraction frequency but was independent of shortening velocity. On the basis of these values, it was calculated that the twitch energetics were consistent with ATP splitting by half the cross-bridges and the pumping of one Ca²⁺ into the sarcoplasmic reticulum for every three cross-bridge cycles. The simplest interpretation is that half the cross-bridges completed one ATP-splitting cycle in each twitch. The lack of influence of shortening velocity on energy cost supports the idea that the amount of energy to be used is determined early in a twitch and is not greatly influenced by events that occur during the contraction.

(Received 8 January 2006; accepted after revision 17 February 2006; first published online 23 February 2006)
Corresponding author C. J. Barclay: School of Physiotherapy and Exercise Science, Griffith University, Gold Coast, PMB50 Gold Coast Mail Centre, Queensland 9726, Australia. Email: c.barclay{at}griffith.edu.au

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