Mechanisms of harmol sulfate biliary excretion in mice and rats

Previous experiments demonstrated that the biliary excretion of harmol sulfate (HS) was mediated by Bcrp, and not by Mrp2 or P-glycoprotein in mice. However, recent reports suggested that species differences in hepatic canalicular transport mechanisms for a given substrate exist between mice and rats. In the present study, biliary excretion of HS was examined in perfused livers from mice and rats in the absence or presence of the P-glycoprotein and Bcrp inhibitor, GF120918. As expected, in mouse liver perfusions the biliary excretion of HS was decreased ~3.5-fold by GF120918, consistent with previous reports of Bcrp-mediated HS biliary excretion. However, despite sufficient hepatic unbound concentrations of GF120918 to achieve extensive inhibition of Bcrp, the biliary excretion of HS was not decreased significantly in rats (50 ± 12 vs. 41 ± 6 %). In summary, biliary excretion of HS was mediated by a GF120918-sensitive mechanism in mice, previously elucidated as Bcrp. In contrast the pathway responsible for HS biliary excretion in rats was not impaired by GF120918. Thus, transport mechanism(s) responsible for harmol sulfate biliary excretion appear to differ between mice and rats. 0% B, 4.1–6.0 min linear gradient to 0% B, 3.1–4 min hold at 0% B; flow rate = 0.65 mL/min; 1–5 min directed to mass spectrometer) and were detected in positive ion mode using multiple reaction monitoring: harmol: 199 → 131 m/z, HS: 279 → 199 m/z, cimetidine: 253 → 117 m/z. Analytes in mouse samples were eluted from an Aquasil C18 column (2.1 × 50 mm, d p = 5 µm, Thermo Electron Corporation, Waltham, MA) using a mobile phase gradient (A: 0.05% formic acid, B: 0.05% formic acid in methanol; 0–0.75 min hold at 0% B, 0.75–2 min linear gradient to 70% B, 2–3.5 min hold at 70% B, 3.5–3.6 min linear gradient to 0% B, 3.6–4 min hold at 0% B; flow rate = 0.75 mL/min; 0.8–4 min directed to mass spectrometer) and were detected in positive ion mode: harmol: 199 → 131 m/z, cimetidine: 253 → 117 m/z, or negative ion mode: HS: 277 → 197 m/z, cimetidine: 251 → 157 m/z.