ABSTRACT
ATP is the high-energy phosphate-containing compound directly used for excitation and contraction in muscle cells. Cleavage of the terminal phosphate (a phosphoryl bond) by ATPases [ATP → ADP + inorganic phosphate (Pi)] releases chemical energy that is converted into the work for contraction, ion pumping, and macromolecular synthesis. Because the amount of ATP in the heart is small (~10 mM, enough for only a few beats) compared with demand (as much as 10,000 times greater), the myocardial cell must continually resynthesize ATP to maintain normal cardiac pump function and cellular viability. Thus the rates of ATP utilization and re-synthesis are very large.
