Initial mechanical efficiency of isolated cardiac muscle

Abstract

Two groups of biochemical reactions underlie muscle contraction: those that consume high-energy phosphates, called initial reactions, and those that regenerate high-energy phosphates, called recovery reactions. Muscular efficiency is the ratio of mechanical work produced to the metabolic energy consumed in the production of that work. The energy consumption term can either incorporate just the initial energy costs, giving the initial mechanical efficiency (eI), or can encompass the net energy cost (the energetic equivalent of the oxygen consumed), giving the net mechanical efficiency (eN). eI is of interest because it provides insights into the fundamental mechanism of energy conversion by myosin cross-bridges.The efficiency of mechanical work generation by crossbridges in cardiac muscle is poorly established because it is difficult to experimentally separate the initial and recovery energy costs. The kinetics of recovery metabolism in cardiac muscle are so rapid that even the energy used within the time course of a single twitch includes a significant recovery metabolism component (Gibbs et al., 1967;. Peterson and Alpert (1991) subtracted the presumed recovery heat component from the energy output recorded during isotonic shortening of rabbit papillary muscles and concluded that the maximum eI in rabbit papillary muscles was 65%. This value is high compared with an estimate based on reported values of eN. eN of isolated cardiac muscle is typically~1 5% (Gibbs et al., 1967;Syme, 1994;. The magnitude of the net metabolic cost of a series of contractions is typically twice that of initial metabolism (e.g. ) so eI should bẽ 2-fold greater than eN; that is, about 30%. Both the approaches described above for determining eI contain elements of uncertainty. For example, Peterson and Alpert (1991) implicitly assumed that the energy output associated with shortening was synchronous with shortening, but, at least in isometric contractions, a substantial fraction of the initial energy output associated with a single twitch appears late in the contraction, during force relaxation . To estimate eI from eN, it must be assumed that the ratio of energy output from recovery processes (R) to energy output from initial processes (I) is the same in isometric contractions and contractions with shortening because the R:I ratio in cardiac muscle has only been measured using isometric contractions . Although it seems reasonable to assume that the R:I ratio is independent of contraction type, the only published comparison of the R:I ratio in isometric and working contractions, which was made using mouse skeletal muscle (Woledge and Yin, 1989), revealed that the ratio was greater in shortening contractions (1.25) than in isometric contractions (1.0). It seems unlikely that such an effect could underlie the combination of eI=65% and eN=15% in cardiac muscle, because this would require the R:I ratio in working The aim of this study was to determine whether the initial mechanical efficiency (ratio of work output to initial metabolic cost) ...

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