WATER CHANNEL PROTEINS IN RAT CARDIAC MYOCYTE CAVEOLAE - OSMOLARITY-DEPENDENT REVERSIBLE INTERNALIZATION

We show by confocal immunofluorescence microscopy that the water chann el protein aquaporin-1, not previously identified within cardiomyocyte s, localizes at 20 and 37 degrees C to rat cardiomyocyte sarcolemmal c aveolar membrane and subsarcolemmal cytoplasm of primary atrial myocyt e cultures, dissociated atrial and ventricular myocytes, and in situ c ardiomyocytes of atrial and ventricular frozen sections. Confocal immu nofluorescence microscopy shows that the normal in situ colocalization of the quasi-muscle-specific caveolar coating protein caveolin-3 with aquaporin-1 is reversibly disrupted by exposing in situ atrial or ven tricular myocytes to physiological saline made hypertonic by adding 15 0 mM sucrose or 75 mM NaCl to isotonic physiological saline. This caus es caveolae to close off from the interstitium and swell, while aquapo rin-1 is internalized reversibly. At 4 degrees C aquaporin-1 does not colocalize with caveolin-3. We suggest that 1) in vivo, under near-iso tonic conditions, caveolae may alternate frequently between brief open and closed-off states; 2) aquaporin-1-caveolin-3 colocalization may b e energy dependent; and 3) while closed off from the interstitium, eac h caveola transiently functions as an osmometer that experiences, moni tors, and reacts to net mater flow from or into the subcaveolar cytoso l of the myocyte.