Further Studies on the Mechanism of the Absorption of Ion Pair Complex from the Rat Small Intestine

Absorption properties of an ion pair complex, quinine and bromthymol blue (BTB), were investigated by the rat small intestine. Although the disappearance of quinine from the in situ recirculated solution was enhanced in the presence of BTB and the converse was not applied for the anionic component, the enhancement of uptake by the everted intestine was observed by both components. By rinsing the everted intestine at the end of an incubation period, no effect of quinine on the tissue accumulation of BTB was found. Effect of counter ions on the binding to mucosal homogenates, brush borders, and bovine serum albumin (BSA) was hardly observed at all. Pretreatment of the intestine with pH 6.5 phosphate buffer for 10 min almost completely diminished the enhancing effect of BTB on the disappearance of quinine from the in situ recirculated solution. However, 15 min was enough after the pretreatment to regain such an enhancing effect of the anionic component. It is suggested that the mucosal surface of the small intestine plays an important role in the absorptive process of an ion pair complex.