Potassium cotransport at the rat choroid plexus.

The choroid plexuses are involved in cerebrospinal fluid (CSF) secretion and CSF K homeostasis. We examined K transport mechanisms present in the isolated rat choroid plexus that may be involved in these functions, predominantly using 86Rb as a marker for K. The study demonstrates that there are two primary uptake mechanisms. Ouabain-sensitive Na-K-adenosinetriphosphatase and bumetanide-sensitive cotransport, probably of the Na-K-2Cl form, account for 48 and 46% of uptake, respectively. Efflux studies demonstrate that the primary K efflux mechanism is also bumetanide-sensitive cotransport with the other major component probably being by K channels as it is inhibitable by barium or quinidine. Efflux via the cotransporter was not inhibited by R(+)-butylindazone, a KCl cotransport inhibitor, but it was enhanced in the presence of ouabain (P < 0.001) or increased extracellular Na concentration (P < 0.01). Furthermore, Na efflux was bumetanide sensitive (P < 0.05). In all, these data suggest that the efflux cotransporter is also of the Na-K-Cl form and that it is the same transporter as the influx mechanism operating in both directions. The evidence presented leads us to hypothesize that this cotransporter is on the apical membrane of the choroid plexus and that it may have a central role in CSF secretion and perhaps CSF K homeostasis.