Distribution of the aquaporin CHIP in secretory and resorptive epithelia and capillary endothelia.

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Abstract
The existence of water-selective channels has been postulated to explain the high water permeability of erythrocytes and certain epithelial cells. The aquaporin CHIP (channel-forming integral membrane protein of 28 kDa), a molecular water channel, is abundant in erythrocytes and water-permeable segments of the nephron. To determine whether CHIP may mediate transmembrane water movement in other water-permeable epithelia, membranes of multiple organs were studied by immunoblotting, immunohistochemistry, and immunoelectron microscopy using affinity-purified anti-CHIP IgG. The apical membrane of the choroid plexus epithelium was densely stained, implying a role for CHIP in the secretion of cerebrospinal fluid. In the eye, CHIP was abundant in apical and basolateral domains of ciliary epithelium, the site of aqueous humor secretion, and also in lens epithelium and corneal endothelium. CHIP was detected in membranes of hepatic bile ducts and water-resorptive epithelium of gall bladder, suggesting a role in bile secretion and concentration. CHIP was not detected in glandular epithelium of mammary, salivary, or lacrimal glands, suggesting the existence of other water-channel isoforms. CHIP was also not detected within the epithelium of the gastrointestinal mucosa. CHIP was abundant in membranes of intestinal lacteals and continuous capillaries in diverse tissues, including cardiac and skeletal muscle, thus providing a molecular explanation for the known water permeability of certain lymphatics and capillary beds. These studies underscore the hypothesis that CHIP plays a major role in transcellular water movement throughout the body.