Inward transport of [H-3]-1-methyl-4-phenylpyridinium in rat isolated hepatocytes: Putative involvement of a P-glycoprotein transporter

1 The liver has an important role in the detoxification of organic cations from the circulation. [H-3]-1-methyl-4-phenylpyridinium ([H-3]-MPP+), a low molecular weight organic cation, is efficiently taken up and accumulated by rat hepatocytes through mechanisms partially unknown. 2 The aim of the present work was to characterize further the uptake of MPP+ by rat isolated hepatocytes. The putative interactions of a wide range of drugs, including inhibitors/substrates of P-glycoprotein, were studied. 3 The uptake of MPP+ was investigated in rat freshly isolated hepatocytes (incubated in Krebs-Henseleit medium with 200 nM [H-3]-MPP+ for 5 min) and in the rat liver in situ (perfused with Krebs-Henseleit/BSA medium with 200 nM [H-3]-MPP+ for 30 min). [H-3]-MPP+ accumulation in the cells and in tissue was determined by liquid scintillation counting. 4 Verapamil (100 mu M), quinidine (100 mu M), amiloride (1 mM), (+)-tubocurarine (100 mu M), vecuronium (45 mu M), bilirubin (200 mu M), progesterone (200 mu M), daunomycin (100 mu M), vinblastine (100 mu M), cyclosporin A (100 mu M) and cimetidine (100 mu M) had a significant inhibitory effect on the accumulation of [H-3]-MPP+ in isolated hepatocytes. Tetraethylammonium (100 mu M) had no effect. 5 In the rat perfused liver, both cyclosporin A (100 mu M) and verapamil (100 mu M) had much less marked inhibitory effects as compared to their effects on isolated hepatocytes (0% against 35% and 45% against 96% of inhibition, respectively). 6 Inhibition of alkaline phosphatase activity by increasing or decreasing the pH of the incubation medium or by the presence of vanadate (1 mM) or homoarginine (500 mu M) led to a significant increase in the accumulation of [H-3]-MPP+ in isolated hepatocytes. 7 It was concluded that, in addition to the type I organic cation hepatic transporter, [H-3]-MPP+ is taken up by rat isolated hepatocytes through P-glycoprotein, a canalicular transport system that usually excretes endobiotics and xenobiotics. We proposed that the reversal of transport through P-glycoprotein may be related to the loss of efficiency of alkaline phosphatase in isolated hepatocytes.