The heart has the responsibility of distributing oxygen throughout the body. To accomplish this, the cardiac muscle needs energy to be able to contract and relax. It uses oxygen, and metabolizes free fatty acids and glucose to produce energy in the form of ATP. Depending on the circumstances, it may primarily use free fatty acids, glucose or the energy that it has been able to store.
Energy for Metabolism
The heart must have a continuous supply of energy because it has to move calcium into its cells, and constantly contract and relax. It contracts so it can pump blood to pick up oxygen from the lungs to deliver it throughout the body. The muscle then relaxes so its chambers can fill with blood. Every minute, it needs approximately 8 mL of oxygen for each 100 grams of muscle, says Brian Hoit, director of the echocardiography lab at University Hospitals Case Medical Center, according to "Hurst's The Heart."
ATP
Cardiac muscle uses oxygen so it can make its energy in the form of adenosine triphosphate, or ATP, as explained by Joseph Loscalzo, physician-in-chief at Brigham and Women's Hospital in "Harrison's Principles of Internal Medicine." To get ATP, it breaks down free fatty acids and glucose. The muscle cells get the free fatty acids when they circulate in the bloodstream after fat tissue has been broken down. They get their glucose from breaking down the glucose that they have previously stored in the form of glycogen.
Metabolic Steps to Produce ATP
In "Harrison's Principles of Internal Medicine," Dr. Loscalzo explains that the cardiac muscle cells break glucose down to a substance called pyruvate. The pyruvate is then sent to a different part of the cell and changed into acetyl coenzyme A. The free fatty acids are first changed to acyl coenzyme A in one part of the cell and then changed into acetyl coenzyme A in another area. All the acetyl coenzyme A then joins with a process taking place in the muscle cells, called the citric acid cycle, to produce ATP.
Free Fatty Acids vs. Glucose
Between meals, the heart primarily uses free fatty acids to make acetyl coenzyme A. Right after meals, when there is a lot of glucose in the bloodstream, it will mainly use glucose. In addition, Dr. Loscalzo writes that when the cardiac muscle has to work more or when there is less oxygen in the bloodstream, the cells will use glucose more than they use the free fatty acids. In periods of stress, there are more free fatty acids in the bloodstream, and the cells will use them over glucose.
Anaerobic Metabolism
Aerobic metabolism involves the use of oxygen; thus, low oxygen levels can interfere with the creation of ATP. When the levels are low, metabolism is done anaerobically. Cardiac muscle does this by metabolizing glucose to lactic acid. This creates ATP, but not as much as when oxygen is involved. The muscle also stores energy as creatine phosphate. This is formed from ATP and can be used to recreate ATP, as explained by Robert Murray, professor emeritus of Biochemistry at the University of Toronto in "Harper's Illustrated Biochemistry."
References
- "Harper's Illustrated Biochemistry"; Robert Murray, M.D., Ph.D., David Bender, Ph.D., Kathleen Botham, Ph.D. et al.; 2009
- "Harrison's Principles of Internal Medicine"; Anthony Fauci, M.D., Dennis Kasper, M.D., Dan Longo, M.D. et al.; 2008
- "Hurst's The Heart"; Valentin Fuster, M.D., Ph.D., Robert O'Rourke, M.D., Philip Poole-Wilson, M.D., Richard Walsh, M.D.; 2008
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