The present study was undertaken to assess whether the protein and mRNA expression levels of the glial water channel aquaporin-4 (AQP4) undergo downregulation and whether there is a subcellular redistribution of AQP4 protein in rat brain in response to systemic hyponatremia and brain edema. Systemic hyponatremia was induced for 4 or 48 h by combined administration of hypotonic dextrose i.p. and 8-deamino-arginine vasopressin (dDAVP) s.c. Semiquantitative immunoblotting of membrane enriched fractions showed significantly increased immunoreactivity to 164 +/- 12% (n = 6) and 153 +/- 12% (n = 6) of control levels in brain after 4 or 48 h of systemic hyponatremia, respectively. Similarly, immunoblots of cerebellar samples revealed an increase in AQP4 immunoreactivity to 136 +/- 6% (n = 6) and 218 +/- 44% (n = 6) of control levels, after 4 or 48 h of hyponatremia. In contrast, AQP4 mRNA levels were unchanged after 4 h of severe hyponatremia (104 +/- 14% of control levels; n = 17), indicating that there are no changes in AQP4 expression in response to systemic hypoosmolarity. Immunocytochemistry and high-resolution immunogold electron microscopy revealed highly polarized labeling of AQP4 in astrocyte end-feet surrounding capillaries and forming the glia limitans, This pattern of labeling was not changed whereas an increased labeling intensity of AQP4 could be observed in response to hyponatremia. In conclusion, hyponatremia causes a pronounced and rapid increase in AQP4 immunoreactivity that is not accompanied by any increase in AQP4 mRNA expression. The increased AQP4 immunosignal may reflect secondary conformational modifications of AQP4 protein.