RAT ATRIAL MYOCYTE PLASMALEMMAL CAVEOLAE INSITU - REVERSIBLE EXPERIMENTAL INCREASES IN CAVEOLAR SIZE AND IN SURFACE-DENSITY OF CAVEOLAR NECKS

The structure, size, and surface density of the conspicuous flask-shap ed structures called caveolae that are located under the plasma membra ne of cardiac myocytes in intact rat atria were studied by electron mi croscopy after physiological perturbations designed to examine whether caveolae and/or their necks are fixed or mobile and whether the caveo lar lumen is always open or can close off from the interstitial space. We showed that, in stretched and unstretched atria, horseradish perox idase could enter or be washed out of caveolae at 37-degrees, 18-degre es, and 4-degrees-C, but this finding does not rule out that caveolae and/or their necks can cycle rapidly between states closed and open to the interstitial space. Electron microscopy of thin sections revealed that exposure of atria at 37-degrees or 18-degrees-C to physiological salt solutions made hypertonic by adding 150 mM sucrose or mannitol r esulted in a striking enlargement of caveolar profiles within 1 to 5 m inutes. Caveolar enlargement was rapidly reversible on return to contr ol saline. After freeze fracture of atria exposed to these hypertonic solutions, quantitative analysis of electron micrographs of the fractu re faces revealed statistically significant increases in cross-section al diameter of cross-fractured caveolar necks and in mean number of ca veolar necks penetrating per unit area of plasmalemmal fracture face. These results suggest that atrial myocyte caveolae are dynamic structu res whose necks may be reversibly inserted into and withdrawn from the plasmalemma, possibly (but not necessarily) corresponding to states i n which caveolae are, respectively, open and closed to the interstitia l spaces.