Know all about : Muscle Channelopathies

Channelopathies are diseases that have as their underlying fea­ture mutations or dysregulation of ion channels. Such diseases are frequently associated with excitable cells, including muscle. In the various forms of clinical myotonia, muscle relaxation is prolonged after voluntary contraction. The molecular bases of myotonias are due to dysfunction of channels that shape the action potential. Myotonia dystrophy is caused by an autoso­mal dominant mutation that leads to overexpression of a K⁺ channel (although the mutation is not at the K⁺ channel).

A vari­ety of myotonias are associated with mutations in Na+ channels (eg: hyperkalemic periodic paralysis, paramyotonia congenita, or Na⁺ channel congenita) or CI^– channels (eg: dominant or recessive myotonia congenita}. Myasthenia, defined as abnor­mal muscle weakness or disease, can also be related to loss of ion channel function in the muscle. In congenital myasthenia, the patient has an inheritable disorder of one of a group of ion channels necessary for the transmission of neuronal signaling to muscle response.

Mutations in Ca²⁺ channels that allow for neuronal transmitter release or in the acetylcholine receptor nonspecific cation channels, important in recognition of neu­ronal transmitters, have both been shown to cause congenital myasthenia. Alterations of channel functions can also occur via autoimmune diseases, such as that observed in myasthenia gravis. In this disease, antibodies to the nicotinic acetylcho­line receptor can reduce its functional presence at the muscle membrane by up to 80%, and thus limit muscle response to neuronal transmitter release.

Channelopathies can also occur in the Ca²⁺ release channels in muscle (ryanodine receptors) that amplify the Ca²⁺ response within the cell. Such mutations can cause malignant hyper­thermia. Patients with this condition display normal muscle function under normal conditions. However, certain anesthetic agents, or in rare cases exposure to high environmental heat or strenuous exercise, can trigger an abnormal release of Ca²⁺ from the sarcoplasmic reticulum in the muscle cell, resulting in sustained muscle contraction and heat production. In severe cases, fatality can occur.