Water channel proteins in rat cardiac myocyte caveolae: osmolarity-dependent reversible internalization

Abstract

We show by confocal immunofluorescence microscopy that the water channel protein aquaporin-1, not previously identified within cardiomyocytes, localizes at 20 and 37°C to rat cardiomyocyte sarcolemmal caveolar membrane and subsarcolemmal cytoplasm of primary atrial myocyte cultures, dissociated atrial and ventricular myocytes, and in situ cardiomyocytes of atrial and ventricular frozen sections. Confocal immunofluorescence 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 ventricular myocytes to physiological saline made hypertonic by adding 150 mM sucrose or 75 mM NaCl to isotonic physiological saline. This causes caveolae to close off from the interstitium and swell, while aquaporin-1 is internalized reversibly. At 4°C aquaporin-1 does not colocalize with caveolin-3. We suggest that 1) in vivo, under near-isotonic conditions, caveolae may alternate frequently between brief open and closed-off states; 2) aquaporin-1-caveolin-3 colocalization may be energy dependent; and 3) while closed off from the interstitium, each caveola transiently functions as an osmometer that experiences, monitors, and reacts to net water flow from or into the subcaveolar cytosol of the myocyte.

Selective Down-Regulation of Aquaporin-1 in Salivary Glands in Primary Sjögren's Syndrome

“…Our observation that AQP-1 is expressed on the cell membranes of myoepithelial cells that surround the acini of salivary glands has intriguing parallels in the observations of AQP expression in the cell membranes of other excitable contractile cells, including those of smooth muscle cells (Gannon et al, 2000;Shanahan et al, 1999), cardiac muscle cells (Page et al, 1998), and the sarcolemma of fast-twitch skeletal muscle fiber (Frigeri et al, 1998). AQP-1 has been shown to mediate accelerated smooth muscle volume changes in response to altered external osmolarity (Shanahan et al, 1999), suggesting that variation in cell volume mediated by AQP-1 may contribute to contraction and/or variation in tone of vascular smooth muscle (Shanahan et al, 1999).…”

On the Role of Aquaporins in Myocardial Injury

“…AQP -1, -4, and -6 seems to play different roles in myocardial infarction (MI) in mouse hearts. While the time dependent pattern of the observed up-regulated expression of AQP4 in MI coincides with that of ME and cardiac dysfunction, the expression of AQP1 and AQP6 persistently increase [4].One of the first reports of aquaporins in heart revealed that AQP1 colocalizes with Caveolin-3 at 20°C and 37°C in rats [7]. Interestingly, when rat cardiac myocytes were exposed to hypertonic media AQP1 was reversibly internalized [7].…”

“…One of the first reports of aquaporins in heart revealed that AQP1 colocalizes with Caveolin-3 at 20°C and 37°C in rats [7]. Interestingly, when rat cardiac myocytes were exposed to hypertonic media AQP1 was reversibly internalized [7].…”

“…Interestingly, when rat cardiac myocytes were exposed to hypertonic media AQP1 was reversibly internalized [7]. In addition, a more recent report showed that AQP1 cosegragates with Caveolin-1 in mice [5].…”

Molecular Mechanisms and Drug Development in Aquaporin Water Channel Diseases: Aquaporins in the Brain

“…For example, AQP5 present in lipid rafts in rat parotid glands is translocated to the apical plasma membrane upon the stimulation of acetylcholine receptors of the glands (43). In the rat cardiac myocytes, AQP1 colocalizes with caveolin under the isotonic condition (44). Upon exposure of myocytes to hypertonic medium, AQP1 is dissociated from caveolin and internalized into the intracellular compartments.…”