APPS November 2002 Meeting Abstract 1362

The overall efficiency of muscular contraction is the product of the efficiencies of as the combination of processes that consume high energy phosphates (called initial reactions) and those that regenerate high-energy phosphates (recovery reactions). The efficiency of the initial reactions in cardiac muscle has been reported to be over 60 %¹. In contrast, an estimate based on well-established net efficiency values (efficiency calculated incorporating both initial and recovery energy costs) suggests that 30 % is a more likely value. Thus, the aim of this study was to establish an accurate value for the efficiency of the processes that use high-energy phosphates in cardiac muscle.

Experiments were performed in vitro (30°C) using left ventricular papillary muscles (n = 16) from the hearts of adult rats. 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, defined as the ratio of work output to total, suprabasal energy cost, was 13.4 ą 0.6 %. Enthalpy output was separated into initial and recovery components mathematically. The total enthalpy output was 2.16-times greater than the initial enthalpy output so that initial mechanical efficiency, defined as the ratio of work output to initial metabolic cost, was 28.3 ą 1.3 %. This value, which is similar to that for fast-twitch skeletal muscles, can be accommodated within conventional cross-bridge schemes incorporating the known force output and power stroke amplitude of striated muscle cross-bridges. The ability of cross-bridges in cardiac muscle to convert chemical energy into mechanical energy appears similar to that of other striated muscles.