Amino acid inhibition of bile acid uptake by isolated rat hepatocytes: relationship to dissipation of transmembrane Na+ gradient

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Abstract
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 [median inhibitory concentration], .apprx. 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 .cntdot. mg protein-1 .cntdot. min-1 but did not significantly affect taurocholate Km (42 .+-. 7 vs. 35 .+-. 7 .mu.M). 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 2-fold elevation of intracellular Na concentration as determined by the steady-state uptake of 22Na. Na+-dependent amino acids probably 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.