Regulatory mechanism of aquaporin expression in cerebral microvessels : its pathophysiological significance

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To clarify the regulatory mechanism of water transport between blood and brain, we have identified water channel aquaporin (AQP) subtypes expressed in the rat cerebral microvessels. mRNA and protein of AQP1 and AQP4 were present in the microvessels prepared from rat cerebral cortex. AQP1 mRNA was expressed abundantly as kidney but the AQP1 mRNA level was 1/100 of the kidney. AQP4 protein was detected as 30 kDa band with higher molecular weight bands consisted of its oligomer and smear band hardly solubilized form by western blot AQP1 was detected as a 28 kDa band and its level was less than 1/10 of that in the kidney. AQP4 was detected at the cell membrane of the microvessel, but AQP1 protein was hardly detectable by immunohistochemistry. The expression of AQP1 was decreased by the addition of conditioned medium of astrocytes in the primary culture of the microvessels as well as in the immortalized cell line of the rat microvessels, GP8. The expression of AQP1 increased under hypoxic condition.The regulation of AQP1 expression by astrocyte-derived factor and its increase by hypoxia suggest that AQP1 is a key molecule involved in the generation of cytotoxic edema.