Excitationcontraction coupling

The importance of calcium ions in heart muscle function was first discovered by Sydney Ringer in 1883; his name has since been used to describe the saline solutions that maintain frog tissues in isolation from the body. More recently, transient rises in intracellular calcium ion concentrations have been observed in heart muscle just as in skeletal muscle.

Heart muscle contains DHP receptors (voltage-gated calcium channels) in the plasma membrane and ryanodine receptors (calcium-release channels) in the sarcoplasmic reticular membrane, just as does skeletal muscle. However, the molecular nature of the two channels and their structural arrangement is somewhat different; the DHP receptors do not form tetrads in contact with the ryanodine receptor feet.

The calcium transient in heart muscle is probably derived from two sources. First, there is entry of calcium ions via the voltage-gated calcium channels that open during the action potential. Second, these calcium ions act as a trigger for the release of calcium from the sarcoplasmic reticulum by opening the calcium release channels; this process is called calcium-induced calcium release.

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