Filipin-sterol complexes in the membranes of cardiac muscle

Filipin-cholesterol complexes which are visible in freeze-fracture preparations were used as a probe to explore special domains in the sarcoplasmic reticulum (SR), the plasmalemma, and other membranes of mouse and finch cardiac muscle. The question was especially raised as to whether or not a predilection of filipin-cholesterol complexes for the junctional sarcoplasmic reticulum could be established in cardiac muscle as it was found to exist in skeletal muscle. No such predilection was discovered. Filipin-cholesterol complexes were observed mainly in the plasmalemma, in the sarcoplasmic reticulum, and in mitochondria of cardiac muscle cells. In the SR they were very rare. In the sarcolemma they occurred in patches while, sometimes, they densely populated the entire cell surface.