INWARD TRANSPORT OF [H-3] 1-METHYL-4-PHENYLPYRIDINIUM IN RAT ISOLATEDHEPATOCYTES - PUTATIVE INVOLVEMENT OF A P-GLYCOPROTEIN TRANSPORTER

1 The liver has an important role in the detoxification of organic cat ions from the circulation. [H-3]-1-methyl-4-phenylpyridinium ([H-3]-MP P(+)), a low molecular weight organic cation, is efficiently taken up and accumulated by rat hepatocytes through mechanisms partially unknow n. 2 The aim of the present work was to characterize further the uptak e of MPP(+) by rat isolated hepatocytes. The putative interactions of a wide range of drugs, including inhibitors/substrates of P-glycoprote in, were studied. 3 The uptake of MPP(+) was investigated in rat fresh ly 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 liq uid 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 (10 0 mu M) had a significant inhibitory effect on the accumulation of [H- 3]-MPP(+) in isolated hepatocytes. Tetraethylammonium (100 mu M) had n o effect. 5 In the rat perfused liver, both cyclosporin A (100 mu M) a nd verapamil (100 mu M) had much less marked inhibitory effects as com pared to their effects on isolated hepatocytes (0% against 35% and 45% against 96% of inhibition, respectively). 6 Inhibition of alkaline ph osphatase activity by increasing or decreasing the pH of the incubatio n 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 t ype I organic cation hepatic transporter, [H-3]-MPP(+) is taken up by rat isolated hepatocytes through P-glycoprotein, a canalicular transpo rt system that usually excretes endobiotics and xenobiotics. We propos ed that the reversal of transport through P-glycoprotein may be relate d to the loss of efficiency of alkaline phosphatase in isolated hepato cytes.