Ursodeoxycholic acid choleresis: Relationship to biliary HCO₃^- and effects of Na⁺-H⁺ exchange inhibitors

We have recently shown that substitution of Li⁺ for perfusate Na⁺ eliminates the HCO₃^--rich choleresis produced by ursodeoxycholic acid (UDCA) in isolated perfused rat liver and that the increase in bile flow produced by both UDCA and taurocholic acid is partially inhibited by 1 mM amiloride. Although these findings are consistent with a role for Na⁺-H⁺ exchange in the choleresis produced by these bile acids, both Li⁺ substitution and amiloride affect other cellular processes, including Na⁺-K⁺-ATPase activity. We have not further explored both the relationship between UDCA-stimulated bile flow and biliary HCO₃^- secretion and the possible role of Na⁺-H⁺ exchange in this process by comparing the effects of amiloride with two of its more potent and presumably more specific analogues, 5-(N,N-dimethyl)amiloride hydrochloride (DMA) and 5-(N-ethyl-N-isopropyl)amiloride (EIA). In the absence of inhibitor, UDCA increased biliary HCO₃^- concentration ([HCO₃^-]) up to an apparent maximum of 60-70 mM, and bile flow and biliary HCO₃^- output appeared to be linearly related over a sixfold range of bile flow rates. Amiloride, DMA, and EIA each produced a concentration-dependent inhibition of UDCA-stimulated bile flow and biliary HCO₃^- output with an apparent rank order potency (EIA > DMA > amiloride) similar to that reported for inhibition of Na⁺-H⁺ exchange in other systems. None of the inhibitors significantly altered biliary UDCA output or the relationship between UDCA-induced bile flow and either biliary [HCO₃^-] or biliary HCO₃^- output. Effects of these inhibitors did not appear attributable either to nonspecific toxicity, as reflected by hepatic release of lactate dehydrogenase or K⁺, or to inhibition of hepatic Na⁺-K⁺-ATPase, measured as Na⁺-dependent uptake of ⁸⁶Rb. In contrast to their effects on UDCA choleresis, these inhibitors had little or no effect on basal bile flow, biliary [HCO₃^-], and biliary HCO₃^- output. These findings indicate that UDCA-induced but not basal bile formation is closely coupled to biliary HCO₃^- concentration and output, and they provide additional evidence that UDCA choleresis requires an intact Na⁺-H⁺ exchange mechanism.