Mechanism of intestinal fatty acid uptake in the rat: the role of an acidic microclimate.

1. Micellar solubilization of lipolytic products is an important step in lipid absorption. However, micelles are not absorbed intact; dissociation of lipolytic products from bile salt micelles must occur. The dissociation of micelles has been postulated to occur in an acidic microclimate. 2. The effect of an acidic microclimate on the uptake of micellar fatty acid was examined in the rat intestine. We reported that the presence of a lower pH microclimate is associated with a higher fatty acid uptake, suggesting that a lower pH enhances fatty acid uptakes from the micelles. 3. Fatty acid uptake from solutions containing a constant amount of bile salt (10 mM) and varying amounts of fatty acid (3.3-26.4 mM) revealed a saturation phenomenon which reflects the fatty acid carrying capacity of a 10 mM-taurocholate solution. 4. There was a linear relationship between fatty acid uptake and fatty acid concentration when the micellar solutions contained a constant ratio of fatty acid and taurocholate (1.32). 5. Our results indicate that the fatty acid carrying capacity of the micelle and the number of micelles in the solution are both important determinants for the amount of fatty acids delivered to the microclimate. The amount of fatty acids derived from the dissociation of micelles within the microclimate determines fatty acid uptake by the intestine.