Effects of Na⁺ replacement and amiloride on ursodeoxycholic acid-stimulated choleresis and biliary bicarbonate secretion

In these studies, we have tested the hypothesis that bile acid-dependent bile formation is attributable, in part, to the stimulation of active bicarbonate secretion and have further explored the cellular mechanism(s) possibly involved in this process using the isolated perfused rat liver. Under control conditions, ursodeoxycholic acid (UDCA) infusion (3 μmol/min x 20 min) produced a 3.7-fold increase in bile flow and a 7.4-fold increase in HCO₃^- output. Amiloride (an inhibitor of Na⁺-H⁺ exchange) decreased UDCA-stimulated bile flow by 20.6% and decreased biliary HCO₃^- output by 24.9% but increased biliary UDCA output by 42.9%. Thus amiloride decreased UDCA choleretic efficiency (μl UDCA-stimulated bile/μmol UDCA output) by 45% and UDCA-stimulated increase in HCO₃^- output per unit UDCA secreted by 48%. Substitution of Li⁺ for Na⁺ in perfusate virtually abolished (>95% decrease) both the UDCA choleresis and increase in biliary HCO₃^- output but modestly decreased (39.6%) biliary bile acid output. Li⁺ substitution thus decreased UDCA choleretic efficiency by 98% and the UDCA-stimulated increase in HCO₃^- output by 96%. Amiloride had no effect and Li⁺ substitution produced a modest decrease in basal bile flow (26.0%) and HCO₃^- output (33.5%). Neither amiloride nor Li⁺ substitution significantly affected UDCA uptake by cultured hepatocytes or by perfused liver. Amiloride (1 mM) also decreased taurocholate (TC)-stimulated choleresis by 48.5%, biliary TC output by 7.2%, and the choleretic efficiency of TC by 45%. These data collectively suggest that the UDCA choleresis is attributable in part to stimulation of HCO₃^- secretion via a mechanism requiring intact Na⁺-H⁺ exchange, and that intact Na⁺-H⁺ exchange may also be required for the choleresis produced by physiological bile acids such as TC.