The water channel aquaporin-8 is mainly intracellular in rat hepatocytes, and its plasma membrane insertion is stimulated by cyclic AMP

We previously found that water transport across hepatocyte plasma membranes occurs mainly via a nonchannel mediated pathway. Recently, it has been rep orted that mRNA for the water channel, aquaporin-8 (AQP8), is present in he patocytes. To further explore this issue, we studied protein expression, su bcellular localization, and regulation of AQP8 in rat hepatocytes. By subce llular fractionation and immunoblot analysis, we detected an N-glycosylated band of similar to 34 kDa corresponding to AQP8 in hepatocyte plasma and i ntracellular microsomal membranes. Confocal immunofluorescence microscopy f or AQP8 in cultured hepatocytes showed a predominant intracellular vesicula r localization. Dibutyryl cAMP (Bt(2)cAMP) stimulated the redistribution of AQP8 to plasma membranes. Bt,cAMP also significantly increased hepatocyte membrane water permeability, an effect that was prevented by the water chan nel blocker dimethyl sulfoxide. The microtubule blocker colchicine but not its inactive analog lumicolchicine inhibited the Bt(2)cAMP effect on both A QP8 redistribution to cell surface and hepatocyte membrane water permeabili ty. Our data suggest that in rat hepatocytes AQP8 is localized largely in i ntracellular vesicles and can be redistributed to plasma membranes via a mi crotubule-depending, cAMP-stimulated mechanism. These studies also suggest that aquaporins contribute to water transport in cAMP-stimulated hepatocyte s, a process that could be relevant to regulated hepatocyte bile secretion.