The chronic stirnulation of predominantly fast-twitch mammalian skeleldl muscle causes a transformation to physiological characteristics of slow-twitch skeletal muscle. Here, we report the effects of chronic stimulation on the protein components of the sarcoplasmic reticulum and transverse tubular membranes which are directly involved in excitation-contraction coupling. Comparison of protein composition of microsomal fractions from control and chronically stimulated muscle was performed by immunoblot analysis and also by staining with Coomassie blue or the cationic carbocyanine dye Stains-all. Consistent with previous experiments, a greatly reduced density was observed for the fast-twitch isozyme of Ca2+-ATPase, while the expression of the slow-twitch Ca2+-ATPase was found to be greatly enhanced. Components of the sarcolemma (Na'/K+-ATPase, dystrophin-glycoprotein complex) and the free sarcoplasmic reticulum (Ca2+-binding protein sarcalumenin and a 53-kDa glycoprotein) were not affected by chronic stimulation. The relative abundance of calsequestrin was slightly reduced in transformed skeletal muscle. However, the expression of the ryanodine receptor/Ca2+ -release channel from junctional sarcoplasmic reticulum and the transverse tubular dihydropyridine-sensitive Ca2 + channel, as well as two junctional sarcoplasmic reticulum proteins of 90 kDa and 94 kDa, was greatly suppressed in transformed muscle. Thus, the expression of the major protein components of the triad junction involved in excitation-contraction coupling is suppressed, while the expression of other muscle membrane proteins is not affected in chronically stimulated muscle.In excitation-contraction (EC) coupling of skeletal muscle, Ca2 + release from the sarcoplasmic reticulum (SR) is initiated and coupled to the depolarization of the transverse tubular membrane system [I, 21. The ryanodine receptor (RyR) of rabbit skeletal muscle has been purified and shown to be identical to the Ca2+-release channel of the SR [3-51. The 'foot' structure at the junction between the SR and the transverse tubules is composed of the Ca2 '-release channel and has been proposed to interact with the transverse tubule tetrad [6]. The receptor for the 1,4-dihydropyridine class of Ca2+ blockers (DHPR) functions in skeletal muscle, not only as a Ca2+ channel, but also as the voltage sensor in EC coupling [7,8], and is probably the primary component of the transverse tubule tetrad. The subunit composition of the DHPR from rabbit skeletal muscle has been analyzed [9, 101 and each of the subunits have been cloned [8,11-131. Other components found in the triad fraction of skeletal muscle are thc junctional sarcoplasmic reticulum (JSR) proteins of apparent 90 kDa and 94 kDa [14], which could play a regulatory role in EC

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