Potassium efflux from infant and adult rat choroid plexuses: effects of CSF anion substitution, N-ethylmaleimide and Cl transport inhibitors.

To analyze interdependent transport of K and Cl, we investigated Rb (K) efflux from in vitro choroid plexus (CP) in isotonic artificial CSF (aCSF) medium containing anions or agents that alter KCl transport. Lateral ventricle CP was loaded with 86Rb for release to enable calculation of the efflux rate coefficient, k. With Cl as the main anion in control aCSF, the k value for 86Rb (K) in CP of 1 week infant rats (0.177 min-1) was 19% lower than in adults (0.218 min-1) (P < 0.005). Replacing CSF Cl with NO3 or SCN, respectively, reduced k for K in infant CP by 73% and 43%; similar anion selectivity was observed in adult tissues (P < 0.05). N-Ethylmaleimide (NEM), which stimulates KCI cotransport, significantly enhanced K efflux in infants and adults. In adult CP, the KCl cotransport inhibitor, furosemide (1 mM), decreased K efflux by 23% or 65%, respectively, when aCSF had Cl or NO3 as the main anion. In infant rat CP, 0.1 mM bumetanide (another KCl cotransport inhibitor), reduced k for K by 65%, whereas the Cl channel blocker diphenylamine carboxylate (1 mM) did not significantly alter K efflux. The collective findings for rat CP indicate a substantial component of K efflux that is associated with Cl concentration and the Cl transport protein sensitive to loop diuretics and NEM. The Cl-dependent K efflux is present in infants.