T-tubule

FIGURE 17.11 The structure of a skeletal muscle cell, showing the mauuer iu which t-tubules enable the sarcolemmal membrane to contact the ends of each myofibril iu the muscle fiber. The foot structure is shown iu the box. 

The trigger for all musele eontraetion is an increase in Ca eoneentration in the vicinity of the muscle fibers of skeletal muscle or the myocytes of cardiac and smooth muscle. In all these cases, this increase in Ca is due to the flow of Ca through calcium channels (Figure 17.24). A muscle contraction ends when the Ca concentration is reduced by specific calcium pumps (such as the SR Ca -ATPase, Chapter 10). The sarcoplasmic reticulum, t-tubule, and sarcolemmal membranes all contain Ca channels. As we shall see, the Ca channels of the SR function together with the t-tubules in a remarkable coupled process. 

FIGURE 17.25 The structures of nifedipine and ryanodine. Nifedipine binds with high affinity to the Ca" -release channels of t-tubules. Ryanodine binds with high affinity to the Ca" channels of SR terminal cisternae. 

FIGURE 17.26 The o i-subuiiit of the t-tubule Ca" chainiel/DHP receptor contains six peptide segments that may associate to form the Ca" channel. This Ca" channel polypeptide is homologous with the voltage-sensitive Na channel of neuronal tissue. 

The central canal (CC), radial canals (RC), and peripheral vestibules (PV) are indicated, (d) The relationship between the foot structures, t-tubule, terminal cisternae, and muscle fiber. (Photo courtesy of Sidney Fleischer, Vanderbilt University)... 

So how do the foot structures effect the release of Ca from the terminal cisternae of the SR The feet that join the t-tubules and the terminal cis-ternae of the SR are approximately 16 nm thick. The feet apparently function by first sensing either a voltage-dependent conformation change (skeletal mus-... 

The structure of heart myocytes is different from that of skeletal muscle fibers. Heart myocytes are approximately 50 to 100 p,m long and 10 to 20 p,m in diameter. The t-tubules found in heart tissue have a fivefold larger diameter than those of skeletal muscle. The number of t-tubules found in cardiac muscle differs from species to species. Terminal cisternae of mammalian cardiac muscle can associate with other cellular elements to form dyads as well as triads. The association of terminal cisternae with the sarcolemma membrane in a dyad structure is called a peripheral coupling. The terminal cisternae may also form dyad structures with t-tubules that are called internal couplings (Figure 17.31). As with skeletal muscle, foot structures form the connection between the terminal cisternae and t-tubule membranes. 

In higher animals, large percentages of the terminal cisternae of cardiac muscle are not associated with t-tubules at all. For SR of this type, Ca release must occur by a different mechanism from that found in skeletal muscle. In this case, it appears that Ca leaking through sarcolemmal Ca channels can trigger the release of even more Ca from the SR. This latter process is called Ca -induced Ca release (abbreviated CICR). 

The myocytes of smooth muscle are approximately 100 to 500 p,m in length and only 2 to 6 p,m in diameter. Smooth muscle contains very few t-tubules and much less SR than skeletal muscle. The Ca that stimulates contraction in smooth muscle cells is predominantly extracellular in origin. This Ca enters the cell through Ca channels in the sarcolemmal membrane that can be opened by electrical stimulation, or by the binding of hormones or drugs. The contraction response time of smooth muscle cells is very slow compared with that of skeletal and cardiac muscle. 

DICR (depolarization-induced Ca2+ release) is Ca2+ release triggered by depolarization of the sarcolemma. In skeletal muscle, conformational change in the voltage sensor (a 1S subunit of the dihydropyridine receptor) in the T-tubule is directly transmitted to the... 

Ryanodine Receptor. Figure 1 Three-dimensional architecture of the RyR1 by cryo-electron microscopy, (a), top view (from the T-tubule) (b), bottom view (from the SR lumen) (c), side view (parallel to the SR membrane). The binding sites of FKBP12, apo-CaM and Ca -CaM are indicated in the side view. Courtesy of Dr. M. Samso (modified from Samso etal. (2005) Nat Struct Mol Biol 12 539-544). 

In E-C coupling in the heart, the RyR2 channel releases Ca2+ from the SR on depolarization of the plasma membrane or T-tubules by CICR mechanisms. The... 

T-tubule is a transverse invagination of the plasma membrane, which occurs at the specified sites characteristic to animal species and organs, i.e. at the Z-line in cardiac ventricle muscle and non-mammalian vertebrate skeletal muscle and at the A-I junction in mammalian skeletal muscle. It is absent in all avian cardiac cells, all cardiac conduction cells, many mammalian atrial cells and most smooth muscle cells. T-tubule serves as an inward conduit for the action potential. 

Muscle contraction is initiated by a signal from a motor nerve. This triggers an action potential, which is propagated along the muscle plasma membrane to the T-tubule system and the sarcotubular reticulum, where a sudden large electrically excited release of Ca " into the cytosol occurs. Accessory proteins closely associated with actin (troponins T, I, and C) together with tropomyosin mediate the Ca -dependent motor command within the sarcomere. Other accessory proteins (titin, nebulin, myomesin, etc.) serve to provide the myofibril with both stability... 

According to Schneider and Chandler (1973), depolarization of the T-tubules affects sensors which open Ca " channels in the SR. The sensors are modified Ca channels which act as voltage sensors (Tanabe et al., 1987). The signal from the sensor reaches the SR and opens the Ca channels with the release of Ca to the myoplasm. The Ca channels in the SR system are opened by micromolar [Ca ], mM [ATP], and caffeine but are inhibited by Mg (Smith et al., 1986 Rosseau et al., 1988). The channels are closed in resting muscle and are opened when the voltage sensor is activated. 

In the sarcoplasm of resting muscle, the concentration of Ca + is 10 to 10 mol/L. The resting state is achieved because Ca + is pumped into the sarcoplasmic reticulum through the action of an active transport system, called the Ca + ATPase (Figure 49-8), initiating relaxation. The sarcoplasmic reticulum is a network of fine membranous sacs. Inside the sarcoplasmic reticulum, Ca + is bound to a specific Ca -binding protein designated calsequestrin. The sarcomere is surrounded by an excitable membrane (the T tubule system) composed of transverse (T) channels closely associated with the sarcoplasmic reticulum. 

When the sarcolemma is excited by a nerve impulse, the signal is transmitted into the T tubule system and a release channel in the nearby sarcoplasmic reticulum opens, releasing Ca + from the sarcoplasmic reticulum into the sarcoplasm. The concentration of Ca in the sarcoplasm rises rapidly to 10 mol/L. The Ca -binding sites on TpC in the thin filament are quickly occupied by Ca +. The TpC-4Ca + interacts with Tpl and TpT to alter their interaction with tropomyosin. Accordingly, tropomyosin moves out of the way or alters the conformation of F-actin so that the myosin head-ADP-P (Figure 49-6) can interact with F-actin to start the contraction cycle. 

Figure 49-8. Diagram of the relationships among the sarcolemma (plasma membrane), a T tubule, and two cisternae of the sarcoplasmic reticulum of skeletal muscle (not to scale). The T tubule extends inward from the sarcolemma. A wave of depolarization, initiated by a nerve impulse, is transmitted from the sarcolemma down the T tubule. It is then conveyed to the Ca release channel (ryanodine receptor), perhaps by interaction between it and the dihydropyridine receptor (slow Ca voltage channel), which are shown in close proximity. Release of Ca from the Ca release channel into the cytosol initiates contraction. Subsequently, Ca is pumped back into the cisternae of the sarcoplasmic reticulum by the Ca ATPase (Ca pump) and stored there, in part bound to calsequestrin.

Small T tubules. 3. Large T tubules. 3. Generally rudimentary T tubules. 

For reasons that are not yet clear, skeletal muscle transverse (T)-tubule membranes contain 50-100-fold more high affinity DHP receptors than any other source yet identified [43,45]. Transverse tubule membranes contain 30-70 pmol/mg protein of DHP receptors that bind [ H]PN 200-100 with a of 0.1-0.2nM. The strategy utilized for the purification of L-type channels was similar to that used for the purification of other high affinity ligand binding proteins, and its success was predicted from the prior use of such an approach for the purification of other ion channels [54,55]. Thus the L-type channels were purified as high affinity DHP receptors, with the anticipation that the purified component(s) would constitute functional Ca channels. 

Using the reconstitution approaches described above, we have demonstrated that phosphorylation of the skeletal muscle Ca channels by PKC results in activation of the channels [108], In the fluo 3-containing liposomes, channels phosphorylated by PKC exhibited a two-fold increase in the rate and extent of Ca " influx [108], Using the lipid bilayer-T-tubule membrane reconstitution system we are currently analyzing the effects of PKC-catalyzed phosphorylation at the single channel level [133], The demonstration that these channels undergo phosphorylation as a result of activation of PKC in intact skeletal muscle cells has not yet been achieved. 

Other studies have demonstrated that the skeletal muscle ai peptide can be phosphorylated in T-tubule membranes by a multifunctional Ca " /calmodulin (CaM)-dependent protein kinase [111], Phosphorylation occurs on the i subunit to an extent of 2 mol phosphate/mol subunit and on the /i subunit to an extent of 0.7-1 mol phosphate/mol channel [108,111]. Phosphorylation catalyzed by the CaM-kinase on the ai subunit is additive to that caused by PKA and occurs on distinct sites [111]. So far, however, we have not observed any functional consequences of phosphorylation of the skeletal muscle Ca channels by the CaM-kinase. 

The skeletal muscle Ca channels also can be phosphorylated in vitro by a protein kinase endogenous to the T-tubule membranes [111,115]. This kinase is neither Ca - nor cyclic nucleotide-dependent [115], and is interesting in that it phosphorylates primarily the P subunit while the ai subunit is a poor substrate. However, the amount of this kinase that co-purifies with the T-tubule membranes is variable, and consequently, very few studies have been performed. So far, only low levels of phosphorylation have been obtained (no more than 0.2 mol phosphate/ mol P subunit) and no functional effects of this phosphorylation have been observed in reconstitution studies. 

All types of muscle require calcium for contraction. In skeletal muscle, Ca++ ions are stored within an extensive membranous network referred to as the sarcoplasmic reticulum. This network is found throughout the muscle fiber and surrounds each myofibril. Furthermore, segments of the sarcoplasmic reticulum lie adjacent to each T tubule that, with a segment of sarcoplasmic reticulum on either side of it, is referred to as a triad. As the action potential is transmitted along the T tubule, it stimulates the release of Ca++ ions from the sarcoplasmic reticulum. The only source of calcium for skeletal muscle contraction is the sarcoplasmic reticulum. 

Calcium couples muscle membrane excitation to filament contraction. Important work has focused on the proteins present in the T-tubule/SR junction. One protein, an integral component of the T-tubular membrane, is a form of L-type, dihydropyridine-sensitive, voltage-dependent calcium channel. Another, the ryanodine receptor (RyR), is a large protein associated with the SR membrane in the triad that may couple the conformational changes in the Ca2+ channel protein induced by T-tubular depolarization to the Ca2+ release from the SR (Fig. 43-6). 

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