The influence of pH on rectal absorption of sodium benzoate studied in man by rectal lumen perfusion.

The influence of pH on rectal absorption of sodium benzoate in man was studied by means of a rectal lumen perfusion method and compared with in vitro measurements on diffusional transport of sodium benzoate across an octanol/water interface. For nonbuffered solutions of benzoate in vitro, it was shown that mass flux across an octanol/water interface occurs in agreement with the pH-partition model. In vivo however, mass flux increases less with decreasing pH of unbuffered perfusate than is anticipated on the basis of the pH-partition model. Probably an alkaline flow across the rectal mucosa into the lumen is present as a physiological neutralization mechanism. In contrast, buffered solutions of benzoate show a linear relationship between mass flux and decreasing pH in vitro as well as in vivo. The effect of buffer on the concentration profile of benzoic acid is qualitatively explained. It is shown that an alkaline flow across the rectal mucosa only slightly influences absorption of benzoic acid from strongly buffered solutions in the rectal lumen. It is concluded that the use of strong buffers in rectal solutions induces a drastic effect on the pH of the boundary layer, an effect not seen for unbuffered solutions. This phenomenon does not invalidate the pH-partition hypothesis but can be explained by it.

Selective hepatobiliary transport defect for organic anions and neutral steroids in mutant rats with hereditary-conjugated hyperbilirubinemia.

Mutant rats (TM rats) with abnormal hepatic excretory function were used to study biliary transport of dibromosulfophthalein, ouabain, tributylmethyl ammonium, cholate and taurocholate. In whole animals, dibromosulfophthalein and ouabain clearance is reduced to 7 and 37% of normal, respectively, due to severely impaired excretion from liver to bile. Initial uptake rates of these agents are relatively little affected. In the isolated perfused liver preparation, dibromosulfophthalein is retained within liver and perfusion medium, and the 60-min recovery on bile is reduced to 1.5 vs 75% in normal controls. Biliary excretion of cholate, taurocholate and the quaternary ammonium cation, [14C]tributylmethyl ammonium, is not impaired. These results provide evidence for a selective defect of organic anion and neutral steroid transport in TM rats and confirm that multiple pathways exist for the hepatobiliary excretion of organic anions, neutral steroids, bile acids and cations. Bile flow in whole animals and in the isolated perfused liver is reduced to 50 and 30% of normal, respectively. This suggests that a normal function of the excretory systems for organic anions and neutral steroids is important for the maintenance of normal bile flow.