Inhibition of Na⁺ H⁺ exchange in the rat is associated with decreased ursodeoxycholate hypercholeresis, decreased secretion of unconjugated ursodeoxycholate, and increased ursodeoxycholate glucuronidation

In the perfused rat liver, ursodeoxycholate in high dose produces an HCO₃^--rich hypercholeresis which we have shown previously to be inhibited by replacement of perfusate Na⁺ with Li⁺ or by addition of amiloride (or amiloride analogues). In the present studies, we have determined whether such inhibition is associated with altered ursodeoxycholate biotransformation. Under control conditions, ursodeoxycholate infusion produced a 3.7-fold increase in bile flow and a 9.2-fold increase in biliary HCO₃^- output. By thin-layer chromatography, ursodeoxycholate radioactivity in bile was present in unconjugated form (15%) or as glycine or taurine amidates. Glucuronide conjugates of ursodeoxycholate accounted for <1% of biliary bile acids. Li⁺ Na⁺substitution decreased ursodeoxycholate-stimulated bile flow and HCO₃^- secretion by >90%, but decreased recovery of ursodeoxycholate and metabolites by only 25%. Amiloride or amiloride analogues decreased ursodeoxycholate-stimulated choleresis and HCO₃^- output by 38%-76%, yet did not cause decreased recovery of ursodeoxycholate and metabolites. Inhibition of the hypercholeresis was associated with a decrease in unconjugated ursodeoxycholate to <2% of total biliary bile acids, a striking increase in ursodeoxycholate glucuronides, and a reciprocal decrease in glycine and taurine amidates. With Li⁺ Na⁺ substitution, the predominant metabolites were a mixture of the 24-ester and the 3-aketal (ethereal) glucuronide (29%), and amidation with glycine appeared to be selectively inhibited; with amiloride or its analogues, only the 3-ethereal glucuronide was formed (20%-60% of biliary bile acids), and both taurine and glycine amidation were inhibited. Thus, maneuvers that decrease Na⁺ H⁺ exchange inhibit ursodeoxycholate hypercholeresis and cause replacement of unconjugated ursodeoxycholate in bile by its glucuronide. The secretion of unconjugated ursodeoxycholate, a lipophilic bile acid, appears to be necessary for hypercholeresis induced by high-dose ursodeoxycholate infusion.