All cellular processes require ATP as a primary energy source. The heart requires ATP for the function of membrane transport systems (e.g., Na+/K+-ATPase) as well as for sarcomere contraction and relaxation, which involve myosin ATPase and ATP-dependent transport of calcium by the sarcoplasmic reticulum. Therefore, increasing the mechanical activity of the heart by increasing heart rate and contractility increases myocardial metabolism (click here for more information).
Cellular ATP pools depend on the balance between ATP utilization and ATP production. The heart has an absolute requirement for aerobic production of ATP to maintain adequate ATP concentrations because anaerobic capacity is limited in the heart. Cellular ATP levels will fall if there is insufficient O2 available to produce ATP aerobically, or if there is an increase in ATP utilization (increased ATP hydrolysis) that is not matched by a parallel increase in ATP synthesis..
The heart can use a variety of substrates to oxidatively regenerate ATP depending upon availability. In the postabsorptive state several hours after a meal, the heart utilizes fatty acids (60-70%) and carbohydrates (~30%). Following a high carbohydrate meal, the heart can adapt itself to utilize carbohydrates (primarily glucose) almost exclusively. Lactate can be used in place of glucose, and becomes a very important substrate during exercise. The heart can also utilize amino acids and ketones instead of fatty acids. Ketone bodies (e.g., acetoacetate) are particularly important in diabetic acidosis. During ischemia and hypoxia, the coronary circulation is unable to deliver metabolic substrates to the heart to support aerobic metabolism. Under these conditions, the heart is able to utilize glycogen (a storage form of carbohydrate) as a substrate for anaerobic production of ATP and the formation of lactic acid. However, the amount of ATP that the heart is able to produce by this pathway is very small compared to the amount of ATP that can be produced via aerobic metabolism. Furthermore, the heart has a limited supply of glycogen, which is rapidly depleted under severely hypoxic conditions.