The cardiac, or heart, muscle is responsible for generating the pressures that help circulate blood throughout the body. In the cardiac muscle, proteins are needed for muscle contraction; such proteins include the filaments, troponin, CPK-MB and LDH. These proteins have forms that are unique to the cardiac muscle, and therefore can help identify problems associated with the heart.
The Filaments
Actin and myosin are the two protein filaments directly involved with cardiac muscle contraction. Actin is also called the thin filament, whereas myosin is sometimes called the thick filament. The myosin filaments have extensions, or arms, that grip onto and pull the actin filaments, thereby causing the two filaments to slide against each other in opposite directions; this generates the force needed for a muscle's contraction.
Troponin
Troponin is another protein directly involved in muscle contraction because its action allows myosin filaments to grip on to actin filaments. The myosin arms can only grip onto certain areas of the actin filaments, called active sites; by exposing the active sites on the actin filaments, troponin permits the attachment of myosin arms.
CPK-MB
CPK-MB, or creatine phosphokinase, is a protein used by the cardiace muscle to transfer available energy into stored energy. The energy contained in ATP -- the energy currency of the cell, is transferred to another molecule for later use. CPK-MB is unique to the cardiac muscle and consequently is used as a marker for certain diseases that affect the cardiac muscle. This is because, when the cardiac muscle dies or is dying, as in a heart attack, it releases its contents into the blood.
LDH
LDH, or lactate dehydrogenase, is a protein that is found in different forms throughout the various cells of the body; the form of LDH that is specific to cardiac muscle cells may be used to verify the presence of heart disease. LDH is mostly active when oxygen in cardiac muscle cells is low or depleted; it helps the cardiac muscle cells generate ATP in the absence of oxygen.
References
- "Harrison's Principles of Internal Medicine"; Dennis L. Kasper M.D., et al.; 2005
- MedlinePlus: Creatine Phosphokinase Test
- MedlinePlus: Lactate Dehydrogenase Test
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