<<') 1995 International Society for Neurochemistry Choroid Plexus Potassium Cotransport: Modulation by Osmotic Stress and External Potassium

Abstract: The choroid plexuses are involved in CSF se-cretion and CSF K homeostasis. This study examines the potential role of K cotransport in these two processes using isolated rat lateral ventricle choroid plexuses. Bu-metanide-sensitive 86Rb influx and efflux were measured to assess the response of K cotransport to changes in media osmolality and K concentration. Alterations in os-molality had no effect on K uptake (in the presence or absence of Bumetanide). However, the efflux rate con-stant for Kwas 0.29- 0.02, 0.44-- 0.04, and 0.84- 0.06 min ' in 240, 300, and 424 mOsm/kg solutions, respec-tively (p < 0.001). This increase in efflux with osmolality, an opposite effect to that found in many cells, was solely due to enhanced K cotransport. The increased cotrans-port may be involved in limiting brain shrinkage during hyperosmotic stress if the cotransporter is present on the apical membrane. The rate of Bumetanide-sensitive efflux was unaffected by changes in external [K]. However, the rate of K uptake (measured on return to normal [K] media) was reduced gradually by exposure to low [K]. It was 21 + 1, 19-!- 3, 13 ± 2, and 6 + 1 nmol/mg/min after 0, 10, 30, and 60-min exposure to 1 mM K. Sixty minutes of exposure to 1 mM [K] abolished the Bumetanide-sensi-tive K uptake present in plexuses exposed continually to normal media. This modulation of K cotransport by exter-nal [K] may be important in CSF K homeostasis by lim-iting K loss from the CSF if CSF [K] is low. Key Words: CSF-Choroid plexus-Potassium cotransport-Potas-sium homeostasis-86Rb-Volume regulation. J. Neurochem. 64, 2747-2754 (1995). The choroid plexuses, situated in the lateral, third, and fourth ventricles, form part of the blood-CSF bar-rier. They are the main site of CSF production and they are also involved in the regulation of CSF ionic composition (reviewed in Bradbury, 1979). For exam-ple, the concentration of potassium ( [ K]) in the CSF is maintained constant during fluctuations in plasm