Subunits of the triadic junction in fast skeletal muscle as revealed by freeze-fracture

Freeze-fracture analysis of triadic junctions utilizing an abbreviated glycerination schedule reveals regularly arranged intramembranous subunits within junctional portions of sarcoplasmic reticulum terminal cisterns. The subunits appear as round to square 20-nm elevations of the junctional terminal cistern E-face; the complementary P-face is characterized by a concentration of small particles, the detailed arrangement of which is less obvious. The E-face subunits may contain a pore, and their spatial arrangement corresponds to the tetragonal array of dimples or feet previously described within the junctional gap. No corresponding subunits have been detected within T-tubular membranes. Stereo techniques and complementary replica analysis have been utilized for detailed study of the subunits as well as associated features of the fracture faces of both T-tubular and terminal cistern junctional and nonjunctional membranes. The new findings are discussed in relation to structural models which might explain the mechanisms of the triadic junction involved in excitation-contraction coupling.