Myocardial cell energetics.

Energy transformation at the main energy consuming processes of the myocardium takes place with high efficiency, i.e., with relatively small differences between the free energy level provided and the free energy level required for the two coupled processes. Thus, the free energy of ATP is only moderately higher than that of various ATP dependent processes. Under energy deficiency caused by hypoxia, free energy of ATP can drop to a level that critically affects subsequent steps. Detailed evaluation of cell energetics was carried out with the following approach: Cell shortening, oxygen consumption and intracellular calcium transients of isolated rat cardiomyocytes which were investigated under the influence of inotropic interventions. Increased extracellular Ca2+ and isoproterenol reduced the economy of contraction (contraction amplitude/VO2), whereas Ca-sensitizing agent EMD 57033 did not. This seems to be a consequence of the increased costs of ion cycling under the effect of Ca2+ and isoproterenol. Our current investigation suggests that alterations of ion transport processes and crossbridge kinetics have substantial impact on myocardial energetics.