Feet, bridges, and pillars in triad junctions of mammalian skeletal muscle: their possible relationship to calcium buffers in terminal cisternae and T-tubules and to excitation-contraction coupling

Abstract

The structure of the triad junction was examined in thin sections of mammalian fast-twitch skeletal muscle. The aims of the experiments were twofold: first, to examine relationships between the contents of the junctional gap and the terminal cisternae that could be significant in excitation-contraction coupling and, second, to look for structures in the transverse tubules that could support a calcium buffer system. Procedures known to stabilize cytoskeletal elements were used in an attempt to retain the original structure. "Feet," "pillars" and "bridges" were often seen side by side in the same junction. In one such junction, the average center-to-center spacing between four bridges was 30.9 +/- 1.7 nm and between five foot-like structures was 29.2 +/- 1.4 nm. The subunit structure of the feet could be seen in many sections. The lumen of the terminal cisternae was filled with a tetragonal network of calsequestrin which formed parallel strands near the junctional membrane, in register with the feet. The strands overlay the area occupied by "rods" seen in freeze-fracture replicas of terminal cisterna membrane. The contents of the transverse tubules were aggregated into bands, or "tethers," which extended across the short axis of the tubule at regular intervals of about 30 nm. The tethers consisted of flattened discs, stacked across the long axis of the tubule, aligned with the junctional feet. Lanthanum staining of the tethers indicated cationic binding sites that could buffer luminal calcium ion concentration in the vicinity of the voltage sensor for contraction. It is suggested (i) that the control of calcium concentration near the voltage sensor is necessary for normal activation, (ii) that feet, pillars and bridges are different images of a spanning structure, and (iii) that the regular alignment of tethers, feet and calsequestrin is functionally significant in excitation-contraction coupling.