Cation-anion gap and choleretic properties of rat bile

To investigate whether bile contains choleretic anion(s) other than bile salts (BS) that could account for the observed cation-anion gap in bile, the choleretic properties of bile were investigated in the rat. Infusion of bile increased bile flow significantly more than did taurocholate (TC) (P less than 0.005). By contrast, TC increased BS excretion more substantially (P less than 0.01). This effect was dose dependent for both bile and TC. The choleretic principle had a molecular weight of less than 1,000 as estimated by ultrafiltration of bile. Infusion of bile that had been chromatographed on BioBeads SM-2 still elicited choleresis, whereas bile that had been chromatographed on Dowex 1 x 50W did not. Administration of bile in vivo did not affect Na+-K+-stimulated ATPase activity in liver plasma membrane fractions. These results suggest that bile contains anion(s) other than 3-hydroxy-BS, which increase bile flow in a dose-dependent fashion without affecting the permeability of the biliary tree. This putative choleretic appears to be anionic in nature, heat stable, and has an apparent molecular weight of less than 1,000. This finding suggested that bile salt-independent bile flow partly depends on the excretion of a currently undefined anion.