The action of flufenamic acid and other nonsteroidal anti-inflammatories on sulfate transport in the isolated perfused rat liver

Abstract

The influence of flufenamic acid and other nonsteroidal anti-inflammatories on sulfate transport in the liver was investigated. The experimental system was the isolated perfused rat liver. Perfusion was accomplished in an open, nonrecirculating system. The perfusion fluid was Krebs/Henseleit-bicarbonate buffer (pH 7.4), saturated with a mixture of oxygen and carbon dioxide (95:5) by means of a membrane oxygenator and heated to 37°C. Sulfate transport (equilibrium exchange) was measured by employing the multiple-indicator dilution technique with simultaneous injection (impulse input) of [ 35S]sulfate, [ 3H]sucrose (indicator for the distribution of the sinusoidal transit times), and [ 3H]water (indicator for the total aqueous space). Analysis was accomplished by means of a space-distributed variable transit time model. Flufenamic acid and other anti-inflammatories inhibited sulfate transport in the liver. For a concentration of 100 μM, the following decreasing series of potency could be established: flufenamic acid (53.4 ± 2.9%) > niflumic acid (41.1 ± 1.4%) > mefenamic acid (35.6 ± 3.3%) > piroxicam (16.6 ± 1.9%) > naproxen (13.5 ± 8.4)%) ≅ nimesulide (11.6 ± 5.8%). Inhibition of sulfate transport by flufenamic acid was clearly concentration dependent; 250 μM flufenamic acid produced more than 95% inhibition. Flufenamic acid in the range between 50 and 250 μM did not affect the mean transit times of tritiated water ( t̄ water) and [ 3H]sucrose ( t̄ suc), the same applying to all other anti-inflammatory agents (100 μM) tested in this work. This means that these agents do not affect vascular and cellular spaces, even when present at high concentrations. The ratio of the intra- to extracellular sulfate concentrations ([C] i/[C] e), generally between 0.4 and 0.5 under control conditions, was affected only by 250 μM flufenamic acid and 100 μM niflumic acid. In the first case, this phenomenon is possibly due to the high degree of transport inhibition (more than 95%), which does not allow a uniform tracer distribution over the whole cellular space during a single passage through the liver. The degree of inhibition of sulfate transport by 100 μM flufenamic acid was a function of the concentration of nontracer sulfate. With sulfate in the range between 1.2 and 25 mM, the inhibition degree increased linearly with the concentration. In the presence of flufenamic acid, the saturation curve of equilibrium exchange showed a substrate inhibition-like phenomenon, which was absent in the control curve. As inhibitors of sulfate transport in hepatocytes, flufenamic and niflumic acids are less active than in erythrocytes by a factor of 10 2. This observation is most probably indicative of structural differences between the hepatic sulfate carrier and the anion carrier of erythrocytes. It is unlikely that the action of flufenamic acid and its analogs on sulfate transport is a consequence of energy metabolism inhibition. Nimesulide is as active as flufenamic or niflumic acid in inhibiting energy metabolism but considerably less efficient as an inhibitor of sulfate transport. Our results as well as literature data reveal that the interactions of the nonsteroidal anti-inflammatories with the liver membranes and intracellular structures are ample and complex. Even at high concentrations, however, these interactions are not so intense as to change the vascular and cellular spaces.