Endothelins decrease the expression of aquaporins and plasma membrane water permeability in cultured rat astrocytes

Abstract

Some of the aquaporins (AQPs), a family of water channel proteins, are expressed in astrocytes. The expression of astrocytic AQPs is altered after brain insults, such as ischemia and head trauma. However, little is known about the regulation of AQP expressions. Endothelins (ETs), which are vasoconstrictor peptides, regulate several pathophysiological responses of astrocytes. In this study, the effects of ETs on AQP expressions and plasma membrane water permeability were examined in cultured rat astrocytes. Determination of AQP mRNA copy numbers revealed that AQP1 and AQP4 were expressed prominently in cultured astrocytes. ET-1 (100 nM) and Ala¹,³,¹¹,¹⁵-ET-1 (an ETB receptor agonist, 50 nM) decreased the AQP4 and AQP9 mRNA levels in cultured astrocytes, but the AQP1, -3, -5, and -8 mRNA levels remained unchanged. BQ788, an ETB receptor antagonist, reduced the effects of ET-1 on astrocytic AQP mRNAs, whereas FR139317, an ETA antagonist, had no effect. Immunoblot analyses revealed that treatment with ET-1 decreased the protein contents of AQP4 and AQP9 in both total cell lysates and plasma membrane fractions of cultured astrocytes. ET-1 and Ala¹,³,¹¹,¹⁵-ET-1 decreased hypoosmolarity-induced water influxes into cultured astrocytes. In the presence of 30 μM Hg²+, which inhibits water movement through AQP1 and AQP9, the hypoosmolarity-induced water influxes were reduced. Phloretin, an inhibitor of AQP9, did not greatly affect the water influxes. The ET-induced decreases in water influxes were obtained in the presence of Hg²+ and phloretin. These results suggest that ET is a factor that regulates AQP expressions and water permeability in astrocytes.