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Content: Through these mechanisms, T-tubules allow heart muscle cells to contract more forcefully by synchronising calcium release throughout the cell.[1] T-tubule structure may be affected by disease, potentially contributing to heart failureand arrhythmias. Although these structures were first seen in 1897, research into T-tubule biology is ongoing. Contents
1Structure
1.1Regulators
2Function
2.1Excitation-contraction coupling
2.2Calcium control
2.3Detubulation
3History
4Clinical significance
5See also
6References
Structure[edit]
T-tubules are tubules formed from the same phospholipid bilayeras the surface membrane or sarcolemmaof skeletal or cardiac muscle cells.[1] They connect directly with the sarcolemma at one end before travelling deep within the cell, forming a network of tubules with sections running both perpendicular (transverse) to and parallel (axially) to the sarcolemma.[1] Due to this complex orientation, some refer to T-tubules as the transverse-axial tubular system.[2] The inside or lumen of the T-tubule is open at the cell surface, meaning that the T-tubule is filled with fluid containing the same constituents as the solution that surrounds the cell (the extracellular fluid). Rather than being just a passive connecting tube, the membrane that forms T-tubules is highly active, being studded with proteins including L-type calcium channels, sodium-calcium exchangers, calcium ATPases and Beta adrenoceptors. [ 1]
T-tubules are found in both atrialand ventricularcardiac muscle cells (cardiomyocytes), in which they develop in the first few weeks of life.[3] They are found in ventricular muscle cells in most species, and in atrial muscle cells from large mammals.[4] In cardiac muscle cells, T-tubules are between 20 and 450 nanometers in diameter and are usually located in regions called Z-discswhere the actin filaments anchor within the cell.[1] T-tubules within the heart are closely associated with the intracellular calcium store known as the sarcoplasmic reticulumin specific regions referred to as terminal cisternae. The association of the T-tubule with a terminal cistern is known as a diad. [ 5]
In skeletal muscle cells, T-tubules are between 20 and 40 nm in diameter and are typically located either side of the myosin strip, at the junction of overlap between the A and I bands. T-tubules in skeletal muscle are associated with two terminal cisternae, known as a triad. [ 1][6]
Regulators[edit]
The shape of the T-tubule system is produced and maintained by a variety of proteins. The protein amphiphysin-2is encoded by the gene BIN1and is responsible for forming the structure of the T-tubule and ensuring that the appropriate proteins (in particular L-type calcium channels) are located within the T-tubule membrane.[7] Junctophilin-2 is encoded by the gene JPH2and helps to form a junction between the T-tubule membrane and the sarcoplasmic reticulum, vital for excitation-contraction coupling.[5] Titin capping protein or Telethonin is encoded by the gene TCAPand helps with T-tubule development and is potentially responsible for the increasing number of T-tubules seen as muscles grow. [