The sources of energy for cardiac muscle function were described in Chapter 10. Although the major source of energy for the formation of ATP is oxidative phosphoryla-tion, glycolysis can briefly compensate for a transient lack of aerobic production of ATP when a portion of the heart receives too little oxygen, as during brief coronary artery occlusion.
Oxidative phosphorylation in the heart can use either carbohydrates or fatty acids as metabolic substrates. The formation of ATP depends on a steady supply of oxygen via coronary blood flow. Oxygen delivery by coronary blood flow is, therefore, the most important determinant of an adequate supply of ATP for the mechanical, electrical, and metabolic energy needs of cardiac cells. Furthermore, cardiac oxygen consumption is an accurate measure of the use of energy by the heart. (Coronary blood flow is discussed in Chapter 17.)
As in skeletal muscle, ATP in cardiac muscle is in near equilibrium with phosphocreatine. The presence of phos-phocreatine adds to the storage capacity of high-energy phosphate and speeds its transport from mitochondria to actomyosin crossbridges.
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