Structure-function studies of canine cardiac sarcolemmal membranes. II. Structural organization of the sarcolemmal membrane as determined by electron microscopy and lamellar X-ray diffraction

Abstract

The morphological and ultrastructural properties of highly purified canine cardiac sarcolemmal vesicles, prepared by a modification (Colvin, R.A., Ashavaid, T.F. and Herbette, L.G. (1985) Biochim. Biophys. Acta 812, 601–608) of the method of Jones et al. (Jones L.R., Madlock, S.W. and Besch, H.R. (1980) J. Biol. Chem. 255, 9971–9980), were examined by several techniques. Thin-section electron microscopy showed predominantly intact unilamellar vesicles with little staining beyond the lipid bilayer boundaries. Freeze-fracture electron microscopy demonstrated that the majority of particles are approx. 90 Å diameter and present at a density of 780 ± 190 μm −2 (±S.D.). If it is assumed that some of these particles represent the (Na + + K +)-ATPase, the finding that they are largely confined to the convex fracture face suggests a predominant right-side-out orientation of these sarcolemmal vesicles that is consistent with biochemical assays. The sarcolemmal membrane width measured by electron microscopy (unhydrated membrane width of 50–70 Å) is consistent with the unit cell dimensions of 56–77 Å determined by lamellar X-ray diffraction (hydrated membrane width). A unit cell dimension of 56–62 Å was also found by X-ray diffraction for sarcolemmal lipids extracted from these preparations, indicating that the isolated sarcolemmal preparations do not contain a significant surface coat (glycocalyx). As both cardiac and skeletal sarcoplasmic reticulum membranes have a 80–100 Å membrane width, these findings demonstrate that the purified sarcolemmal membrane is structurally distinct from both cardiac and skeletal sarcoplasmic reticulum. In contrast to the protein-rich skeletal sarcoplasmic reticulum membrane, which contains a single essential protein responsible for the regulation of cytosolic Ca 2+ concentration, the sarcolemma is a lipid-rich membrane that contains a variety of proteins associated with many regulatory functions served by this membrane in cardiac muscle.