Cardiac basal and activation metabolism.

Cardiac basal metabolism is the rate of energy expenditure of the quiescent myocardium. It is species dependent and increases with pre-load. It has small contributions from membrane-bound cation pumps. The contribution of protein metabolism remains open to question. Calculations show that mitochondrial proton pumping may account for a large fraction of the cardiac basal metabolism. Nevertheless this component remains essentially ill-understood. Cardiac activation metabolism is the supra-basal rate of energy expenditure associated with those processes that activate contraction. In isolated muscle preparations it is typically measured as the rate of heat production or oxygen consumption of a muscle, pre-shortened to a length where active force production is negligible, although it is also estimated by pharmacological intervention. In whole-heart studies it is indexed by the supra-basal rate of oxygen consumption of the empty, beating but non-working heart. Activation metabolism underwrites electrical excitation (the ECG) and excitation-contraction coupling (the cycling of calcium ions). It is increased by agents that increase contractility; it probably increases with pre-load, via the phenomenon of length-dependent activation. The basal and activation components each account for one-quarter to one-third of the total energy expenditure of the heart under normal conditions.