Physiological factors determining cardiac energy expenditure

Abstract

The factors determining the energy flux in cardiac muscle are outlined. The suggestion is made that the initial energy liberated during a contraction can be explained in terms of three major ATPases:-the Na⁺-K⁺ and Ca⁺⁺ion transport ATPases and the actin-activated myosin ATPase whilst recovery metabolism relates primarily to the process of oxidative phosphorylation restoring the levels of high energy phosphates. It is further suggested that under normal in vivo conditions there is considerable temporal overlap of the initial and recovery metabolisms. Total energy flux can be divided into basal and activity-related metabolisms. The difficulty of measuring the former is described and its biochemical basis is considered. Active energy flux per minute depends upon 4 major factors namely; (i) heart rate, (ii) end-diastolic volume, (iii) contractile state and (iv) afterload. The active enthalpy per beat can be subdivided into at least 3 distinct components; (i) an activation term that relates to Ca⁺⁺ release and retrieval, (ii) a work term and (iii) a stress-dependent term. The likely physiological magnitudes of these components are estimated from myothermic data and the way these values change with afterload and preload is shown. Enthalpy: load curves are discussed and the way these curves change with contractility is outlined. A brief mention is made of some of the other mechanical parameters that can be shown to predict myocardial oxygen consumption and attention is paid to a recently introduced index (pressure-volume-area). The reason why this latter index is successful is explained and the relationship between enthalpy: PVA and enthalpy: load curves is discussed. Differences between mechanical and thermodynamic efficiency definitions are explained. If allowances are made for recovery metabolism plus the basal and activation processes it is calculated that the actomyosin mechanical efficiency may exceed 50% whereas the actomyosin thermodynamic efficiency possibly just reaches 40%. There is a brief discussion of current muscle models, particularly those related to the heart, and it is concluded that although it is possible to predict cardiac energy expenditure with a fair degree of accuracy on the basis of [resent experimental knowledge real progress will only be made when our limited knowledge of (a) the cardiac activation cycle and (b) the cross-bridge mechanism is improved.