Intracellular pH alterations induced by tacrine in a rat liver biliary epithelial cell line

The effects of tacrine (THA) on intracellular pH (pH i ) were examined in a rat liver biliary epithelial cell line (RLEC) in HEPES‐buffered medium. pH i was recorded using the pH‐sensitive fluoroprobe, carboxy‐SNARF‐1 (carboxy‐seminaphtorhodafluor). In the steady state, short‐term exposures to THA resulted in alkalinization and re‐acidification at 0.1 and 0.25 m M . Following a 24 h‐treatment, no significant difference in pH i could be detected at 0.1 and 0.25 m M THA, whereas at 0.05 m M , pH i was slightly more acid (7.17±0.02, n =16 versus 7.21±0.02, n =24 [control]). In control and short‐term treated cells, intracellular intrinsic buffering power (β i ) increased roughly linearly as pH i decreased. This dependence was not seen following long‐term treatment. In all cases, β i was increased by THA (by 1.6 to 3.5 fold). Following an acid load (induced by 20 m M NH 4 Cl removal), pH i recovery in RLEC relied upon Na + /H + exchange. A short‐term treatment (0.25 m M THA) did not affect total acid extrusion. In contrast, a 24 h‐treatment with 0.05 m M THA reduced it (by ∼36% at a pH i of 6.73) while at 0.25 m M , a large increase was detected (by ∼109% at a pH i of 6.75). In Na + ‐free medium, THA (0.25 m M ) still induced an alkalinization in the steady state. Following an acid load, THA stimulated a Na + ‐independent acid efflux in a dose‐dependent manner, inhibitable by α‐cyano‐4‐hydroxy cinnamate (CHC, 4 m M ) but not by quercetin (0.125 m M ). In conclusion, this work demonstrates that THA affects pH i in RLEC, through a decrease in Na + /H + exchange and an increase in β i . Stimulation of a CHC‐inhibitable, Na + ‐independent acid efflux is also detected.British Journal of Pharmacology (1999) 128 , 1673–1682; doi: 10.1038/sj.bjp.0702972