Ultrarapid freezing reveals that skeletal muscle caveolae are semipermanent structures.

Rat myofibers in the resting state and under imposed conditions of moderate to severe physiological, pharmacological, and mechanical stress were prepared by ultrarapid freezing and examined by freeze fracture. In rapidly frozen rat myofibers, caveolae morphology and distribution were found to be unchanged by brief or prolonged rest, brief direct electrical stimulation and concomitant contractile activity, prolonged direct electrical stimulation (to fatigue), myofiber stretch (within normal myofiber limits), or careful compressive scission. However, caveolae were greatly reduced or eliminated in number and size by severe mechanical disruption (shredding and/or tearing) of myofibers. Thus, we conclude that unlike apparently similar surface specialization in other cell types, skeletal muscle caveolae are not transient stages in a caveolae----vesicle endocytotic-exocytotic cycle, nor are they a membrane reservoir for normal stretch/contractile activity. Rather, they are (semi)permanent structures in the muscle plasma membrane with as yet undetermined function and kinetics.