Stereological analysis of freeze-fracture subfractions from skeletal muscle. I. Relative intrinsic protein. II. Relative lipid content and protein-to-lipid ratio.

Standard microsomal subfractions from biological tissues are not homogeneous but mixtures of membranes derived from the various cellular organelles. In the case of skeletal muscle, freeze-fracture replicas show both smooth concave faces and concave faces densely populated with 90-A particles. Stereological sampling techniques have been applied to such replicas and the relative surface area of sarcoplasmic reticulum (SR) membrane calculated. Expressions are derived that estimate the relative fraction of SR intrinsic protein and lipid as a function of the relative surface area. Although most of the protein in our subfraction is SR protein, a significant amount of lipid is non-SR lipid. The effect of this on measurements of the protein-to-lipid ratio is discussed.