Question

Describe and/or diagram the mechanism that regulates intracellular Ca+2 during excitation-contraction coupling in cardiomyocytes.

Answer 1

Excitation–contraction coupling is the process by which a muscular action potential in the muscle fiber causes the myofibrils to contract.

Excitation–contraction coupling occurs when depolarization of skeletal muscle cell results in a muscle action potential, which spreads across the cell surface and into the muscle fiber's network of T-tubules, thereby depolarizing the inner portion of the muscle fiber. Depolarization of the inner portions activates dihydropyridine receptors in the terminal cisternae, which are in close proximity to ryanodine receptors in the adjacent sarcoplasmic reticulum. The activated dihydropyridine receptors physically interact with ryanodine receptors to activate them via foot processes (involving conformational changes that allosterically activates the ryanodine receptors). As the ryanodine receptors open, Ca2+is released from the sarcoplasmic reticulum into the local junctional space, which then diffuses into the bulk cytoplasm to cause a calcium spark

The Ca2+ released into the cytosol binds to Troponin C by the actin filaments, to allow crossbridge cycling, producing force and, in some situations, motion. The sarco/endoplasmic reticulum calcium-ATPase (SERCA) actively pumps Ca2+back into the sarcoplasmic reticulum. As Ca2+declines back to resting levels, the force declines and relaxation occurs.