Components and control of cellular energy expenditures in endothermy

Attempts to predict and to measure the energy cost of ionic homeostasis in tissue cells are briefly reviewed and attention is drawn to the importance of ion-flux--as well as ion-gradient--maintenance for the preservation of cell structure and function. The energetic consequences of acute and chronic adjustments of two primary-active transport processes, Na-K transport across plasma(sarco) lemma and Ca transport across the endo(sarco)plasmic-reticular membrane, are illustrated. The question of the contribution of protein turnover to the energy expenditure of cell maintenance is raised, but not answered. After looking back on the now well-established concepts of the "intrinsic" cellular control of energy expenditure and the so-called "extrinsic" cellular control by intracellular calcium ion activities, it is suggested that the organ may also control cell metabolism under non-ischemic conditions, through heterogeneous distribution of arterial blood and erythrocytes to cells.