ALTERNATE DISPOSITION OF TETRADS IN PERIPHERAL COUPLINGS OF SKELETAL-MUSCLE

The sarcoplasmic reticulum forms junctions with the surface membrane (peripheral couplings), which are structurally and functionally equivalent to the junctions between sarcoplasmic reticulum and transverse tubules (triads and dyads). Feet (ryanodine receptors, or sarcoplasmic reticulum calcium release channels) are disposed in arrays in the junctional sarcoplasmic reticulum membrane. Tetrads (groups of four dihydropyridine receptors, each called a unit) are disposed in ordered arrays in junctional domains of transverse tubules and surface membrane. We measured three parameters of tetrad arrays in peripheral couplings from three different species: (1) the centre-to-centre distances between tetrads (intertetrad spacing); (2) the angle between lines joining tetrad units and those joining the centres of tetrads (skew angle); (3) the centre-to-centre distance between tetrad units (intratetrad spacing). These measurements are compared with those predicted from models of feet and tetrad arrays. Intratetrad spacings and skew angles are consistent with an interaction of tetrads with alternate feet and with a location of tetrad units over feet subunits. The slightly larger size of the intratetrad spacing relative to the distance between feet subunits indicates that tetrads may be larger than feet, despite the fact that the molecular weight of DHPRs is less than that of feet subunits. This is offered as a possible explanation for the association of tetrads with alternate feet. Arrays of tetrads tend to be incomplete in images from freeze-fractures, due to lack of some of the units composing the tetrads. Since it is not possible to establish which units are actually absent and which are missing from the images due to fracturing problems, the average ratio between DHPRs and feet cannot be accurately determined.