T-tubule membrane

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. 

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. 

Depolarisation of the sarcolemma leads to depolarisation of the T-tubule membrane, which increases the activity of... 

The release channels open in response to an incompletely characterized linkage to the voltage sensor that is present in the T-tubule membrane and is known as the dihydropyridine receptor 240/245 This too, is a Ca2+ channel, which opens in response to arrival of an action potential (nerve impulse see Chapter 30) that move along the T-tubule membrane. Because the... 

However, closer analysis has shown that the mature T-tubule membranous complex is consistent with a G-PCS membrane. As will be discussed below, the structure seems to be consistent with a G-membrane in many of the published examples of mature "T-tubular networks" of mammals which can be identified as cubic membranes. The work by Ishikawa [18] allows detailed comparison with the gyroid membrane, which makes the structural assigmnent of the "T-tubule network" convincing. In many respects the structural analysis is similar to that described below of interstitial cells in an antebrachial organ of a lemur [17]. Some of the results of this analysis of the T-tubule associated gyroid membrane [4,94], are shown in Fig. 7.16. 

It appears that the immature T-tubule membrane exhibits polymorphism. In newly developed invaginations of the PM, it is apparently able to adopt other cubic membrane ultrastructures than the gyroid. The membrane morphology seen in Figs. 7.17(a) and (b) seems to be described by a P-PCS, rather than a G-PCS. These particular sections are similar to sections through a P-PCS in which two indices are zero, or close to zero. However, gyroid-like structural elements... 

Oz, M., Tchugunova, Y.B., and Dunn, S.M. (2000) Endogenous cannabinoid anandamide directly inhibits voltage-dependent Ca + fluxes in rabbit T-tubule membranes, Eur. J. Pharmacol, 404 13-20. Palazzo, E., de Novellis, V., Marabese, T, Cuomo, D., Rossi, E, Berrino, L., Rossi, E, and Maione, S. (2002) Interaction between vanilloid and glutamate receptors in the central modulation of nociception, Eur. J. Pharmacol, 439 69-75. 

Oz, M., Tchugunova, Y.B., and Dunn, S.M. (2000) Endogenous cannabinoid anandamide directly inhibits voltage-dependent Ca + fluxes in rabbit T-tubule membranes, European Journal of Pharmacology 404 13-20. 

ATP is cleaved by most sarcoplasmic reticulum preparations at low rates in the absence of calcium ions. This activity has been denoted as basal activity [20]. When calcium accumulation is initiated, ATP is rapidly hydrolyzed in a calcium-dependent activity, reaching its optimum at a calcium concentration of 10 jaM, and which is severely suppressed by the rising calcium concentrations in the interior of the vesicles [45,64,65]. The calcium-dependent activity was early characterized as the activity of an enzyme distinctly different from the calcium-independent enzyme. In contrast to the calcium-dependent ATPase, the calcium-independent enzyme is quite insensitive to thiol or amino group reagents. Conversely, the calcium-independent activity can be abolished by low concentrations of detergents which do not reduce the activity of the calcium-dependent enzyme [66]. The two enzymatic activities further differ in their nucleotide specificity and affinity, as well as in their magnesium and temperature dependences. The basal activity most likely originates from plasmalemma and T-tubules membranes [41]. 

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