Hepatic transport of bile acid and effect of conjugation

Biliary transport of bile salts was investigated by measuring: 1) biliary transport maxima values (Tm) for different conjugated bile salts; and 2) biliary excretion of unconjugated bile salts relative to their conjugates under the continuous i.v. infusion of various unconjugated bile salts. The order of Tm values found in the rat of both sexes was tauro (and glyco) ursodeoxycholate (TUDC, GUDC), tauro alpha- and beta-muricholate (T alpha-MC, T beta-MC) > taurocholate(TC) > taurochenodeoxycholate (TCDC), while in female hamsters it was TC > TCDC > TUDC. The differences in the Tm order between rats and hamsters cast doubt on the currently proposed view that the apparent Tm values of bile salts are primarily determined by their physical-chemical properties (detergent property in particular). The biliary excretion of unconjugated bile salts was most efficient with ursocholate (UC) and alpha-MC followed by beta-MC, with UDC (and probably 7 ketolithocholate) being the least efficient for excretion. Thus, while for some bile salts such as cholate and UC, the amidation is not a prerequisite to their efficient excretion, for other bile salts such as UDC, the amidation is an excellent mechanism for facilitating the biliary excretion. In an attempt to explain the above order for the efficacy of the biliary excretion of unconjugated bile salts on the basis of their physical-chemical properties, we must remember that unlike rats, the biliary excretion of dehydrocholate and cholate in dogs is more limited than their respective taurine conjugates.(ABSTRACT TRUNCATED AT 250 WORDS)