We have shown that cyanidin-3-O-glucoside swelling has marked effects on the configuration of caveolae without affecting Cav localisation. Disrupting caveolae has significant effects on volume regulation and contractility in response to swelling, two processes which involve ICl,swell activation. Our data are consistent with the idea that disrupting caveolae removes essential membrane reserves so that cells swell more quickly, thereby potentiating activation of the mechanosensitive ICl,swell channel. A simple model summarising our findings is shown in Figure 8. Using electron microscopy we have quantified for the first time the effect of hyposmotic swelling on the morphology and number of caveolae in the isolated cardiac myocyte. We see a 50% reduction in the number of caveolae after 15 min exposure to hypotonic solution. This change in caveolae number reflects a decrease in the number of closed caveolae for which we cannot visualise a neck which connects with the sarcolemma. No change in the number of open caveolae was recorded. In agreement, Kohl et al. have reported an absence of closed caveolae in rabbit myocardium after 30 min swelling with 0.75T solution. In past decades there has been discussion as to whether closed subplasmalemmal vesicles of a size consistent with caveolae are truly closed caveolae, or whether they in fact represent a Synephrine population of caveolae sectioned outside the connecting neck region. Early EM studies showed that in smooth, skeletal and cardiac muscle extracellular markers penetrate virtually all subplasmalemmal vesicles of a size consistent with caveolae, suggesting that they are relatively static structures open to the extracellular space. However, Gabella reported nearly twice as many caveolae with transmission EM than with freeze fracture EM, suggesting that around half of caveolae are in the closed configuration.Moreover, increases in the number and diameter of connecting necks visible in freeze-fracture EM in response to increases in osmolarity have been reported in the atria, consistent with the idea that caveolar necks may be reversibly inserted into, and withdrawn from, the sarcolemma in response to changes in osmolarity.