Response of infant and adult rat choroid plexus potassium transporters to increased extracellular potassium

The choroid plexus (CP) has an important role in regulating ion concentrations in the cerebrospinal fluid (CSF). Maintenance of potassium concentration [( K]) in brain extracellular fluids lower than in serum is critical to proper neuronal function. We studied K (86Rb) transport by the isolated, lateral CP from infant and mature rats to determine the nature and development of CSF [K] regulation. Comparison of transport characteristics in media with 3-11 mM [K] showed that the adult CP increased uptake in a stepwise fashion with each 2 mM increase in [K], up to a 90% increase with 9 mM [K] over control (3 mM [K]). In contrast, 3-day CP increased uptake in 5 mM, but could not increase K uptake further with higher [K]. One-week tissues showed an intermediate response. Kinetic analysis of the ouabain-inhibitable component suggested that the immature tissues may express a different isoform of the catalytic (alpha) subunit of the Na,K-ATPase. These data may explain the previously established inability of neonatal rats to regulate CSF [K] when serum [K] is elevated, and they indicate that active K transport by the CP cells plays an integral role in CSF [K] regulation.