Mechanism of pH effect on oleic acid and cholesterol absorption in the rat

Previous experiments [K. Chijiiwa and W. G. Linscheer, Am. J. Physiol. 246 (Gastrointest. Liver Physiol. 9): G492-G499, 1984] have shown higher rates of absorption of oleic acid (OA) and cholesterol (CH) from micellar solutions perfused through segments of small bowel at pH 5.5 than at pH 6.5. Both solutions contained equal amounts of these lipids in addition to sodium taurocholate (30 mM). It was hypothesized that there may be two reasons to explain this observation. First, the micelles of one of the two solutions (pH 6.5) became depleted of these lipids during the perfusion procedure, while the micelles of the other solution (pH 5.5) were kept saturated with these lipids by the presence of emulsified particles in the perfusate. Second, the pH difference resulted in a much lower ratio of protonated vs. ionized OA in the pH 6.5 solution. Most investigators assume that primarily protonated fatty acid (FA) is absorbed and very little ionized FA is absorbed. The purposes of the present study are to evaluate further the factor of lipid saturation of the micelles by comparison of rates of absorption from two partially lipid-depleted micellar solutions containing equal amounts of OA and CH, in which the micelles of one solution were much more lipid depleted than the micelles of the other. This was caused by the pH difference. The second purpose is to evaluate the effect of the pH-related difference in protonate vs. ionized OA on absorption of OA from nonmicellar aqueous solutions.(ABSTRACT TRUNCATED AT 250 WORDS)