COMPARED EFFECTS OF SYNTHETIC AND NATURAL BILE-ACID SURFACTANTS ON XENOBIOTIC ABSORPTION .1. STUDIES WITH POLYSORBATE AND TAUROCHOLATE IN RAT COLON

Some expected differences between synthetic and natural bile acid surfactants relative to their influences on xenobiotic absorption are briefly outlined on the basis of literature data. Then, experimental work is presented which shows that these differences exist and that they can be even more relevant than suspected. Absorption tests were developed in rat colon in situ with polysorbate (synthetic) and sodium taurocholate (natural) surfactants, using a homologous series of phenylalkylcarboxylic acids as test compounds. At the critical micelle concentration (CMC), the two previously reported actions of synthetic surfactants on xenobiotic absorption (i.e. the increase in absorbing membrane polarity, and the nullification of the resistance of the aqueous boundary layer adjacent to the membrane to solute penetration) were exerted ad libitum by polysorbate, while taurocholate completely lacked the latter action (it is even possible that the opposite exists, i.e., an increase of the aqueous layer resistance to solute diffusion), although it apparently produces an increase in membrane polarity. At supramicellar concentrations (SMC), the solubilising action of taurocholate was almost negligible as compared with that of polysorbate. It is concluded that, as far as colonic absorption is concerned, polysorbate and taurocholate behave as different biophysical species, which could lead to substantially dissimilar in vivo effects.