Choroid plexus potassium cotransport: modulation by osmotic stress and external potassium.

The choroid plexuses are involved in CSF secretion and CSF K homeostasis. This study examines the potential role of K cotransport in these two processes using isolated rat lateral ventricle choroid plexuses. Bumetanide-sensitive 86Rb influx and efflux were measured to assess the response of K cotransport to changes in media osmolality and K concentration. Alterations in osmolality had no effect on K uptake (in the presence or absence of bumetanide). However, the efflux rate constant for K was 0.29 +/- 0.02, 0.44 +/- 0.04, and 0.84 +/- 0.06 min-1 in 240, 300, and 424 mOsm/kg solutions, respectively (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 cotransport 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-sensitive K uptake present in plexuses exposed continually to normal media. This modulation of K cotransport by external [K] may be important in CSF K homeostasis by limiting K loss from the CSF if CSF [K] is low.