Sodium-dependent antiporters in choroid plexus epithelial cultures from rabbit

The mechanism of recovery from an acid load in primary cultures of rabbit choroid plexus epithelium (CPE) was examined, with emphasis on Na(+)-dependent antiports. Cells were incubated in saline solutions buffered to pH 7.38 with either HEPES or HCO3- plus 95% O2/5% CO2. Intracellular pH (pHi) was determined from the steady-state distribution of [14C]benzoate. Recovery after acidification with NH4Cl was rapid (t1/2 = 5 min) and was dependent on external Na+ (EC50 = 12 mM). Hexamethyleneamiloride and ethylisopropylamiloride, potent inhibitors of the Na+/H+ antiport, blocked 80% of recovery when [Na+] was 5 mM with IC50 values of 100 nM. However, neither drug blocked recovery in normal [Na+]. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of Cl-/HCO3- antiports, blocked recovery of pHi in a dose-related fashion in the presence of bicarbonate, but not in the presence of HEPES. No inhibition occurred with benzamil, an amiloride congener with high affinity for the Na+ channel, nor with dimethylbenzamil, an inhibitor of Na+/Ca2+ exchange. The carbonic anhydrase inhibitor acetazolamide also did not alter recovery from acidification. In CPE that had been pH-clamped with nigericin and KCl, the initial rate of 22Na+ uptake was very rapid (227 pmol/micrograms of DNA/min at pH 6.2), was dependent on external [Na+] with an EC50 value of 8 mM, and was inversely related to the pH of the medium. The maximal inhibition of 22Na+ uptake by hexamethyleneamiloride was 60% with an IC50 value of 76 nM.(ABSTRACT TRUNCATED AT 250 WORDS)