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As the T-tubules are the primary location for excitation-contraction coupling, the ion channels and proteins involved in this process are concentrated here - there are 3 times as many L-type calcium channels located within the T-tubule membrane compared to the rest of the sarcolemma. Furthermore, beta adrenoceptors are also highly concentrated in the T-tubular membrane,[13]and their stimulation increases calcium release from the sarcoplasmic reticulum. [ 14]
Calcium control[edit]
As the space within the lumen of the T-tubule is continuous with the space that surrounds the cell (the extracellular space), ion concentrations between the two are very similar. However, due to the importance of the ions within the T-tubules (particularly calcium in cardiac muscle), it is very important that these concentrations remain relatively constant. As the T-tubules are very thin, they essentially trap the ions. This is important as, regardless of the ion concentrations elsewhere in the cell, T-tubules still have enough calcium ions to permit muscle contraction. Therefore, even if the concentration of calcium outside the cell falls (hypocalcaemia), the concentration of calcium within the  T-tubule remains relatively constant, allowing cardiac contraction to continue. [ 5]
As well as T-tubules being a site for calcium entry into the cell, they are also a site for calcium removal. This is important as it means that calcium levels within the cell can be tightly controlled in a small area (i.e. between the T-tubule and sarcoplasmic reticulum, known as local control).[15] Proteins such as the sodium-calcium exchanger and the sarcolemmal ATPase are located mainly in the T-tubule membrane.[5] The sodium-calcium exchanger passively removes one calcium ion from the cell in exchange for three sodium ions. As a passive process it can therefore allow calcium to flow into or out of the cell depending on the combination of the relative concentrations of these ions and the voltage across the cell membrane (the electrochemical gradient).[8] The calcium ATPase removes calcium from the cell actively, using energy derived from  adenosine triphosphate(ATP). [