L. Kordylewski et al., RAT ATRIAL MYOCYTE PLASMALEMMAL CAVEOLAE INSITU - REVERSIBLE EXPERIMENTAL INCREASES IN CAVEOLAR SIZE AND IN SURFACE-DENSITY OF CAVEOLAR NECKS, Circulation research, 73(1), 1993, pp. 135-146
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res whose necks may be reversibly inserted into and withdrawn from the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.