Further observations on the uptake and effects of phosphonates in pet-fused rat liver studied by P-31-NMR

We examined the route of uptake of 2-aminoethylphosphonate (NEthPo) and of phenylphosphonate (PhePo; 10 mM each) in perfused liver by P-31-NMR. Uptake of NEthPo was concentrative. The rate of uptake was reduced to 21 +/- 2% ( n = 3; all percentages refer to control rates) by substituting choline for Na+, and to 21 +/- 4% (n = 3), 32 +/- 6% (n = 5) acid 70 +/- 5% (n = 3) by replacing Cl- by gluconate, SO42- Or NO3-, respectively. Taurine (20 mM) re duced NEthPo uptake to 38 +/- 6% (n = 3). The data are consistent with upta ke of NEthPo by the Na+-coupled Cl--dependent beta-amino acid transporter. A small fraction of NEthPo was incorporated into phospholipid. PhePo uptake evolved over 1 h towards levels of the membrane-permeant volume marker dim ethyl methylphosphonate. Uptake depended on H+, and was inhibited by 4,4'-d iisothiocyanato-stilbene-2,2'-disulphonic acid (100 mu M), bumetanide and f urosemide (1 mM each) and alpha-cyano-4-OH-cinnamic acid (5 mM) to 31 +/- 4 % (n = 4), 28 +/- 4% (n = 4), 27 +/- 5% (n = 6) and 40 +/- 7% (n =4), respe ctively. These characteristics of PhePo uptake are reminiscent of H+-couple d monocarboxylate transport. The monocarboxylates, lactate and acetate (20 mM), and the substrate analogue, phenylalanine (20 mM), were not inhibitory , while benzoic acid (20 mM) slightly inhibited (to 82 +/- 5%; n = 4) PhePo uptake. The tested phosphonates (10 mM) did not significantly affect hepat ic extraction of [H-3]-cholate or [H-3]-taurocholate (25 mu M each; 1:3 bil e salt:albumin). The monocarboxylate analogue, PhePo (10 mM), did not signi ficantly interfere with disposal of lactate (0.3-5 mM). Copyright (C) 1999 John Wiley & Sons, Ltd.