A study of the mechanism of intestinal absorption of benzo(a)pyrene

Abstract

1. 1. Benzo(a)pyrene and other polynuclear aromatic hydrocarbons are readily absorbed from the gut when fed to rats, but little is known about the mechanism of the absorption. To study the mechanism, everted sacs of small intestine were incubated in a medium containing a lipid emulsion of benzo(a)pyrene (50–1500 μM). After incubation, the concentration of benzo(a)pyrene in the sac tissue and in the medium inside the sac was determined by spectrofluorimetric analysis of chloroform extracts. 2. 2. For incubation medium concentrations above 200 μM, accumulation of benzo(a)pyrene in the sac tissue increased exponentially with increase in incubation medium concentration. The transfer of benzo(a)pyrene from sac tissue to the inside medium was proportional to the concentration in the sac tissue. The regression lines were not greatly influenced by fasting, rat age, or sex. Comparison of incubation at 0, 23 and 42° with that at 37° showed a lowered sac accumulation at 0° and an increase in accumulation with temperature. Anaerobiosis and/or 0.1 M iodoacetate did not appreciably influence sac tissue accumulation. Benzo(a)pyrene also accumulated in strips of small and large intestine and slices of liver and kidney and epididymal fat. The feature of exponential accumulation was not limited to tissue, for adsorption isotherms and time curves for string incubated in the same media has the same forms as those obtained for the sacs. Intragastric doses of benzo(a)pyrene appeared quickly in thoracic duct lymph and reached a peak in 3–4 h, but no more than 20% of the dose was recovered in the lymph. An exponential relationship between size of intragastric dose and concentration in adipose and mammary tissue, 18 h later, was noted. 3. 3. The data are in accord with a mechanism of physical adsorption of benzo(a)pyrene to the intestinal mucosal surfaces and them passive diffusion into and through the intestinal wall. Two phases of adsorption are postulated: first, unilayer (Langmuir) adsorption, then multilayer (Polanyi) adsorption which would account for the exponential nature of the accumulation.