Weak electrolyte transfer in the guinea pig jejunum: secretion of trimethoxybenzoic acid.

In isolated epithelia of guinea pig jejunum the transcellular permeation of 10(-4) M (carboxyl-14C)-3,4,5-trimethoxybenzoic acid (TMBA) in the direction blood-lumen was more than 10 times greater the transcellular permeation of 10(-4) M (carboxyl-was reduced to less than 2 by anaerobiosis or by increasing TMBA concentrations of up to 10(-2) M. Under aerobic conditions the cellular uptake of TMBA (10(-4) M) from the blood side was twice as high as that from the lumen side. In anaerobiosis the percentage of TMBA taken up into the epithelium was enhanced, when TMBA was administered on the lumen side, while the percentage was unchanged after administration on the blood side; thereby the difference in cellular TMBA concentrations was abolished. Similar results were obtained under aerobic conditions, if the TMBA concentration was increased up to 10(-2) M. The results are consistent with a three-compartment model with an intermediate compartment distinguished by a high pH as compared to that of the outer compartments and by a luminal boundary highly permeable for the ionized form of the substrate in contrast to the contraluminal boundary.