Ultrastructure of isolated sarcolemma from dog and rabbit myocardium. Comparison to intact tissue.

We compared the morphology of cardiac sarcolemmal membranes isolated from dog and rabbit hearts with the sarcolemma in intact cells, using freeze-fracture and thin-section electron microscopy. In addition, we estimated the sidedness of the isolated sarcolemma based on its freeze-fracture morphology and biochemical determinations of sialic acid content and Na,K-ATPase activity. The bilayer in isolated membranes is similar, in its morphology, to intact membrane. The isolated sarcolemmal vesicles have the same density of intramembrane particles per micron2 as the intact sarcolemma (approximately 2800/micron2). The particle counts in isolated sarcolemma were very homogeneous, with the peak in particle density curve the same as found for intact cells. In the intact myocardium, both sarcolemmal and transverse tubular membranes have the same density of intramembrane particles. Thin-section morphology of the isolated sarcolemma shows an intact surface coat, whereas portions of the external lamina are absent. Both the biochemical and morphological data indicate that there is a substantial fraction of inside-out and right side-out vesicles in this preparation. Considering the approximations inherent in both morphological and biochemical approaches, we find the qualitative agreement of the estimations of vesicle orientation noteworthy.