Age-dependent changes in gastrointestinal transport and retention of particulate manganese oxide in the rat

Translocation of inhaled particulates from the nasopharynx and upper tracheobronchial area to the gastrointestinal tract is a major route of exposure for particles with a mass median diameter of greater than 1 micron. Previous studies in this laboratory with particulate Mn3O4 have shown that preweanling rats have substantially higher tissue Mn concentrations than similarly treated adults, indicating possible differences in uptake or elimination or both. This study was conducted to evaluate changes in gastrointestinal movement and retention of particulate Mn3O4 in the preweanling and weaned rat. 85Sr-labeled microspheres were used to evaluate gastrointestinal transit rate (TR), while particulate Mn3O4 was used to evaluate particulate retention at selected ages. The results show that stomach retention time in the preweanling is at least twice that of the postweanling (90 min versus 42 min). In general, intestinal TR was not different in any of the ages evaluated, while transit time increased as intestinal length increased. Analysis of the Mn data demonstrated that the preweanling rat had a two-component retention curve with half-times of between 2 and 6 h for the short component and of between 24 and 26 h for the long component. In the postweanling rat, only one component was identified, with a half-time of between 2 and 5 h.