Physiology of Contraction

Contraction of myocytes begins with electrical depolarization of the sarcolemma, resulting in an influx of calcium into the cell through channels in the T tubules 5).
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This initial calcium entry stimulates the rapid release of large amounts of calcium from the sarcoplasmic reticulum into the cell cytosol. The calcium then binds to the calcium-binding troponin subunit(troponin C) on the actin filaments of the sarcomere, resulting in a conformatiobal change in the tropnin-tropomyosin complex. This change facilitates the actin-myosin interaction, which results in cellular contradiction.

As the wave of depolarization passes, the calcium is rapidly and actively resequstered in the sarcoplasmic reticulum, where it is stored by various proteins, including calsequestrin, until the next wave of depolarization occurs. Calcium is also extruded from the cytosol by various calcium pumps in the sarcolemma. The force of myocyte contraction can be regulated by the amount of free calcium released into the cell by the sarcoplasmic reticulum: more calcium allows for greater actin-myosin interaction, producing a stronger contraction. The energy for myocyte contraction is derived from adenosine triphosphate(ATP), which is generated by oxidative phosphorylation of adenosine diphosphate (ADP) in the abundant mitochondria of the cell. ATP is required both for calcium influx and for force generation by actin-myosin interaction. During contraction, ATP promotes dissociation of myosin from actin, thereby permitting the sliding of thick filaments past thin filaments as the sarcomere shortens. Under normal circumstances, fatty acids are the preferred energy source, although glucose can also be used as a substrate. These substrates must be constantly delivered to the heart through the blood stream because there are minimal energy stores in the heart itself .

Myocardial metabolism is aerobic and, thus, requires a constant supply of oxygen. Under anaerobic conditions, glycolysis and lactate may serve as a source of ATP, although in insufficient quantities the working heart.

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