Bile Acid Uptake via the Human Apical Sodium-Bile Acid Cotransporter Is Electrogenic

Abstract

Intestinal absorption of bile acids depends on a sodium-bile acid cotransport protein in the apical membrane of the ileal epithelial cell. Transport is Na ؉ -dependent, but the Na ؉ -bile acid stoichiometry and electrogenicity of transport are not known. Studies in whole intestine, isolated cells, and ileal membrane vesicles have been unable to resolve this issue because transport currents are small and can be obscured by other ionic conductances and transport proteins present in these membranes. In this study, the human apical sodium-bile acid transporter was expressed in stably transfected Chinese hamster ovary cells that lack other bile acid transporters. The Na ؉ -dependent transport of a fluorescent bile acid analog, chenodeoxycholyl-N⑀-nitrobenzoxadiazol-lysine, was monitored by fluorescence microscopy in single, voltage-clamped cells. Bile acid movement was bidirectional and voltage-dependent with negative intracellular voltage-stimulating influx. A 3-fold reduction in extracellular Na؉ produced a negative 52 mV shift of the flux-voltage relationship, consistent with a 2:1 Na ؉ :bile acid coupling stoichiometry. No Na ؉ -or voltage-dependent uptake was observed in nontransfected Chinese hamster ovary cells. These results indicate that the cotransport of bile acids and Na ؉ by human apical sodium-bile acid transporter is electrogenic and bidirectional and is best explained by a 2:1 Na ؉ :bile acid coupling stoichiometry. These results suggest that membrane potential may regulate bile acid transport rates under physiological and pathophysiological conditions.

The solute carrier family SLC10: more than a family of bile acid transporters regarding function and phylogenetic relationships

“…As two sodium ions are translocated together with one bile acid molecule via NTCP and ASBT, this transport is electrogenic Weinman 1997;Weinman et al 1998). As reported for other sodium-coupled cotransporters and exchangers (Nicoll et al 1996;Jung 2001;Murtazina et al 2001), negatively charged amino acids at the outer surface of the carrier protein are potential binding sites for cationic sodium ions.…”

“…Similar to NTCP/Ntcp, ASBT/Asbt transports conjugated bile acids with high affinity in a sodium-dependent manner (Wong et al 1994Craddock et al 1998). This transport is electrogenic and shows a 2:1 Na + /bile acid coupling stoichiometry (Weinman et al 1998). In contrast to the basolateral localisation of Ntcp, Asbt is highly expressed in the apical brush border membrane of enterocytes of the terminal ileum (Shneider et al 1995).…”

Biophysical Properties of ClC-3 Differentiate It from Swelling-activated Chloride Channels in Chinese Hamster Ovary-K1 Cells

“…First, as detailed above, the ClC-3 currents had very specific biophysical and pharmacological properties. We never observed this channel activity either in untransfected CHO-K1 cells, CHO-K1 cells transfected with GFP alone, or CHO-K1 cells transfected with another membrane protein, such as the apical sodium bile acid cotransporter (18). In contrast, the currents associated with pICln and P-glycoprotein can occasionally be seen in untransfected cells and can always be induced by cell swelling in untransfected cells.…”

Apical Sodium Dependent Bile Acid Transporter (ASBT, SLC10A2): A Potential Prodrug Target

“…The transporter is electrogenic, coupled with sodium in a 2:1 sodium: bile acid stoichiometry. hASBT presumably functions as a monomer, although some evidence suggests the existence of a dimer as well 32 . hASBT is expressed at high levels in the terminal ileum, renal proximal tubules and biliary epithelium 33,34 .…”