Amino acid inhibition of bile acid uptake by isolated rat hepatocytes: relationship to dissipation of transmembrane Na+ gradient.
The effects of amino acids on bile acid uptake were studied in isolated rat hepatocytes. The Na+-dependent amino acid L-alanine inhibited [14C]taurocholate uptake in a nonlinear fashion (IC50, approximately 7 mM). Kinetic studies showed that alanine (30 mM) reduced the Vmax for taurocholate uptake from 1.7 +/- 0.1 to 1.1 +/- 0.1 nmol . mg protein-1 . min-1 but did not significantly affect taurocholate Km (42 +/- 7 vs. 35 +/- 7 microM). Taurocholate uptake was also inhibited by alpha-methylaminoisobutyric acid (which shares a common Na+-dependent transport pathway with alanine but is not metabolized) and by L-glutamine (undergoes Na+-dependent hepatic uptake via a carrier distinct from that for alanine). In contrast, the Na+-independent amino acid 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid had no effect on hepatocyte bile acid uptake. Alanine induced a twofold elevation of intracellular sodium concentration as determined by the steady-state uptake of 22Na. These findings suggest that Na+-dependent amino acids noncompetitively inhibit hepatocyte taurocholate uptake by dissipating the transmembrane Na+ gradient and thereby reduce the driving forces for Na+-coupled bile acid entry. Dissipation of the Na+ gradient by substrates that undergo Na+-dependent hepatic transport may represent a novel mechanism of bile secretory failure.
D000409:Alanine / Q000494:pharmacology; D000596:Amino Acids / Q000494:pharmacology*; D000598:Amino Acids, Cyclic; D000621:Aminoisobutyric Acids / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D005973:Glutamine / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013656:Taurocholic Acid / Q000378:metabolism*
D000596:Amino Acids; D000598:Amino Acids, Cyclic; D000621:Aminoisobutyric Acids; D005973:Glutamine; C006504:2-aminobicyclo(2,2,1)heptane-2-carboxylic acid; C017911:2-(methylamino)isobutyric acid; D013656:Taurocholic Acid; D012964:Sodium; D000409:Alanine