Freeze-fracture analysis of plasma membrane cholesterol in fast- and slow-twitch muscles

Abstract

We studied cholesterol in mammalian muscle plasma membranes by analyzing complex formation with the sterol-specific ligand digitonin. Freeze-fracture of muscles treated with digitonin showed patches of tubular complexes that were convex on P faces and concave on E faces. We found a consistent variation in the extent of digitonin complex formation in different muscles according to the predominant fiber type. The slow-twitch soleus had a significantly greater proportion of surface area taken up by digitonin complexes than the predominantly fast-twitch extensor digitorum longus and anterior sternomastoid. This suggests that slow-twitch muscle fibers have a higher concentration of cholesterol in their plasma membrane, consistent with biochemical studies that have shown they have a greater total cholesterol content. A difference in membrane cholesterol could account for functional differences between fast- and slow-twitch muscle plasma membranes.