Intestinal maturation: in vivo magnesium transport.

We investigated with an in vivo single pass perfusion technique net transport of magnesium in the jejunum, ileum and colon segments of suckling, weanling and adolescent rats. The concentration of magnesium in the isotonic sodium chloride solution perfused through the intestinal segments was either below (0 and 0.5 mmoles/liter) or above (1, 2.5 and 5 mmoles/liter) serum ionized magnesium. The relationship between net transport rates (mumoles/h per g dry weight) and luminal magnesium concentration was liner in all three intestinal segments of the suckling rats. In the weanling and adolescent rats, the relationship between net transport and luminal magnesium concentration was linear in the small intestinal segments; however, a change to a saturable, curvilinear relationship was noted in the colon segments. Rates of transport were significantly greater in all segments of suckling rats compared to corresponding segments of the adolescent rats, when luminal magnesium concentration was 2.5 and 5 mmoles/liter. Net transport rates in the colonic segments at all ages were either equal to or greater than rates in the small intestinal segment at the same age period. The change in the relationship between transport rates and luminal magnesium in the colonic segments suggested that the mode of magnesium transport change at the time of weaning from mainly a passive mechanism in the suckling period to mainly a carrier mediated mechanism in the adolescent rats. This is the first demonstration of developmental characteristics of magnesium transport in animals.