The influence of age and particle number on absorption of polystyrene particles from the rat gut.

The bioavailability of polystyrene particles (1 microns) labelled with FITC (3M Company, Düsseldorf) was tested in rats after enteral administration. Since macromolecules and particles are preferentially transported in the lymph, the number of particles was counted in the lymph of the thoracic duct over a 6 h period. Uptake in young rats (6-8 wk) was compared with that in 5 and 9 mo animals. Young animals absorbed only 87 particles whereas a marked increase in the uptake of particles was observed in 5 mo animals (up to 775) but there was a decrease to 518 particles in older animals (9 mo). This number of particles is the total number of the entire output of the thoracic duct lymph over a 6 h period. In individual animals this number showed a considerable fluctuation over time. The volume of the collected lymph fluid was relatively constant (3.5 +/- 0.5 ml) in all animals. The bioavailability of the particles in the lymph was also influenced by the applied dose of particles. After intraduodenal administration of 3.7 x 10(5) particles only 18 particles could be found in the lymph. Increasing the dose to 3.7 x 10(7) particles raised the number of particles in the lymph to 116. The highest dose of 3.7 x 10(9) was correlated with the greatest absorption, 775 particles being found in the lymph. The uptake of particles from the gut thus depends on different factors including the age of the animal and the number of applied particles.