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The Journal of Experimental Biology 206, 2725-2732 (2003)
doi: 10.1242/jeb.00480

Initial mechanical efficiency of isolated cardiac muscle

C. J. Barclay^1,*, C. Widén² and L. J. Mellors¹

¹ Department of Physiology, Monash University, Clayton, Victoria 3800, Australia
² School of Physiotherapy and Exercise Science, Griffith University, Gold Coast, Queensland 9726, Australia

^* Author for correspondence (e-mail: c.barclay{at}griffith.edu.au)

Accepted 7 May 2003

The aim of this study was to determine whether the initial mechanical efficiency (ratio of work output to initial metabolic cost) of isolated cardiac muscle is over 60%, as has been reported previously, or whether it is approximately 30%, as suggested by an estimate based on the well-established net mechanical efficiency (ratio of work output to total, suprabasal energy cost) of 15%. Determination of initial efficiency required separation of the enthalpy output (i.e. heat + work) into initial and recovery components. The former corresponds to energy produced by reactions that use high-energy phosphates and the latter to energy produced in the regeneration of high-energy phosphates. The two components were separated mathematically. Experiments were performed in vitro (30°C) using preparations dissected from rat left ventricular papillary muscles (N=13). Muscle work output and heat production were measured during a series of 40 contractions using a contraction protocol designed to mimic in vivo papillary muscle activity. Net mechanical efficiency was 13.3±0.7%. The total enthalpy output was 2.16 times greater than the initial enthalpy output, so that initial mechanical efficiency was 28.1±1.2%.

Key words: muscle energetics, heat production, efficiency, cardiac muscle

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