HEPATIC-UPTAKE OF HIPPURATE - A MULTIPLE-INDICATOR DILUTION, PERFUSED-RAT-LIVER STUDY

The hepatic transport of hippuric acid (HA), a glycine-conjugated meta bolite of benzoic acid that exhibits only modest plasma albumin bindin g (binding association constant of 2.1 X 10(3) M-1), was studied in th e single-pass perfused rat liver (12 ml/min), using the multiple indic ator dilution (MID) technique. The venous recovery of [H-3]HA on porta l venous injection of a MID dose containing a mixture of a set of none liminated reference indicators and [H-3]HA revealed a survival fractio n of unity, corroborating the lack of disappearance of bulk HA from pl asma. When the outflow recovery was fitted to the barrier-limited mode l of Goresky et al. (C. A. Goresky, G. G. Bach, and B. E. Nadeau. J. C lin. Invest. 52: 991-1009, 1973), the derived influx (PinS) and efflux (PoutS) permeability-surface area products were found to be dependent on the concentration of HA (1-930 mu M); PinS and PoutS were similar to 3.5 times the plasma flow rate at low HA concentration, but decreas ed with increasing HA concentration. All values, however, greatly exce eded the expected contribution from passive diffusion, because the equ ilibrium distribution ratio of chloroform to buffer for HA was extreme ly low (0.0001 at pH 7.4). The tissue equilibrium partition coefficien t (P-in/P-out, or ratio of influx to efflux rate constants, k(1)/k(-1) ) was less than unity and decreased with concentration. The optimized apparent Michaelis-Menten constant and maximal velocity were 182 +/- 6 0 mu M and 12 +/- 4 nmol . s(-1) . g(-1), respectively, for influx and 390 +/- 190 mu M and 29 +/- 13 nmol . s(-1) . g(-1), respectively, fo r efflux. In the presence of L-lactate (20 mM), however, PinS for the uptake of HA (174 +/- 3 mu M) was reduced. Benzoic acid (10-873 mu M) was also effective in reducing hepatic uptake of HA (5.3 +/- 0.9 mu M) . These interactions suggest that MCT2, the monocarboxylate transporte r that mediates the hepatic uptake of lactate and other monocarboxylic acids, may be involved in HA transport.