Sensitivity of bile acid transport by organic anion-transporting polypeptides to intracellular pH.
We investigated the influence of intracellular pH (pHi) on [14C]-glycocholate (GC) uptake by human hepatoblastoma HepG2 cells that express sodium-independent (mainly OATP-A and OATP-8), but not sodium-dependent, GC transporters. Replacement of extracellular sodium by choline (Chol) stimulated GC uptake but did not affect GC efflux from loaded cells. Amiloride or NaCl replacement by tetraethylammonium chloride (TeACl) or sucrose also increased GC uptake. All stimulating circumstances decreased pHi. By contrast, adding to the medium ammonium or imidazole, which increased pHi, had no effect on GC uptake. In Chinese hamster ovary (CHO) cells expressing rat Oatp1, acidification of pHi had the opposite effect on GC uptake, that is, this was reduced. Changes in extracellular pH (pHo) between 7.40 and 7.00 had no effect on GC uptake at pHi 7.30 or 7.45 when pHo<pHi. However, GC uptake was inhibited at pHo 7.40 and 7.80 when pHo>pHi. Inhibition was not proportional to the pHo-pHi difference. Intracellular acidification decreased V(max), but had no effect on K(m). In sum, sodium-independent GC transport can be affected by intracellular acidification, possibly due both to modifications in the driving forces and to the particular response to protonation of carrier proteins involved in this process.
Marin
Jose J G
JJ
Department of Physiology and Pharmacology, Faculty of Pharmacy, University of Salamanca, Campus Miguel Unamuno, EID-S09, 37007- Salamanca, Spain. jjgmarin@usal.es
Mangas
Daniel
D
Martinez-Diez
Maria C
MC
El-Mir
Mohamad Y
MY
Briz
Oscar
O
Serrano
Maria A
MA
D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D016466:CHO Cells / Q000378:metabolism; D002250:Carbon Radioisotopes; D002794:Choline; D006224:Cricetinae; D003470:Culture Media; D006000:Glycocholic Acid / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D027361:Organic Anion Transporters / Q000737:chemistry / Q000378:metabolism*; D029362:Organic Anion Transporters, Sodium-Independent / Q000235:genetics / Q000378:metabolism; D012965:Sodium Chloride; D014407:Tumor Cells, Cultured
D001647:Bile Acids and Salts; D002250:Carbon Radioisotopes; D003470:Culture Media; D027361:Organic Anion Transporters; D029362:Organic Anion Transporters, Sodium-Independent; C421102:Slco1a1 protein, rat; D012965:Sodium Chloride; D006000:Glycocholic Acid; D002794:Choline