Active intestinal transport of methyl red isomers

Transport across the wall of rat jejunum of two isomers of methyl red has been studied. These isomers, meta-methyl red (MMR) and para-methyl red (PMR), are absorbed against a concentration gradient. Uptake consists of three components: rapid adsorption on to the mucosa, a steady uptake proportional to lumen concentration and a reflux which increases exponentially to a limiting value. A substantial part of the uptake is stored in the mucosa, and some of it is metabolized to colourless derivatives. If methyl red is washed out of the lumen after the mucosa has been loaded, there is some reflux into the lumen, but most of the stored methyl red passses into the secretion. The rate of this transport on to the serosal surface is not noticeably diminished until the adverse concentration gradient exceeds 13-14:1. Once the methyl red has been washed out of the lumen there is no further metabolism of methyl red to colourless derivatives. Shifting the lumen pH from 7.4 to 6.4 whilst keeping the tissue fluid pH constant increases the rate of uptake from the lumen by a factor of only 1.68 although the concentration of unionized methyl red is increased approximately 10-fold. It seems that at pH 7.4 about 90% of the uptake is of the ionized form, and that at pH 6.4 this percentage falls to 50. It is concluded that the methyl reds form fresh examples of foreign organic compounds which can be transported actively by the small intestine and that they can be taken up from the lumen into the mucosa in both ionized and unionized states.