Effect of intraluminal pH on cholesterol and oleic acid absorption from micellar solutions in the rat.

We have studied the effects of intraluminal pH on micellar solubilization and on absorption rate of oleic acid (OA) and cholesterol in proximal small bowel segments and the total small bowel in rats. In addition, pH effect on fecal excretion of [3H]cholesterol was studied over a period of 4 days after duodenal administration of cholesterol in solutions at different pH using beta-sitosterol as a nonabsorbable marker. Rates of absorption of OA and cholesterol were, respectively, 1.8 and 1.9 times higher at pH 5.5 in both proximal bowel segments as compared with pH 6.5 (P less than 0.001). Similar observations were made when the whole small bowel was perfused. At pH 5.5, 1.8 times more OA was absorbed and 1.5 times more cholesterol than at pH 6.5 (P less than 0.001). Also, fecal output of radioisotope following administration of pH 5.5 solution was only 47% of the excretion using the pH 6.5 solution. Excretion of beta-sitosterol, which serves as a nonabsorbable marker, was not affected by pH differences. In vitro measurements of micellar solubility at the two pH levels by ultracentrifugation showed that an increase of one pH unit resulted in a decreased number of particles in the oil phase (oleic acid and cholesterol in the emulsified particles in pH 6.5 solution is 37 and 34%, respectively, of that in the pH 5.5 solution) due to increased solubilization into the micellar phase. Measurements of monomer activities of OA and cholesterol using a polyethylene disk technique showed a significantly higher activity at the lower pH.(ABSTRACT TRUNCATED AT 250 WORDS)