Developmental kinetics of colonic calcium transport

Young animals absorb and retain more calcium (Ca) than their older counterparts. Our previous studies on small intestinal calcium transport indicated an evolving pattern with a change in the kinetics of calcium uptake. Whether the lower intestine also mirrors and participates in calcium transport is not known. Therefore, the unidirectional uptake of ⁴⁵Ca was determined in everted colonic sacs of suckling, weanling and adolescent rats using ³H dextran as a marker of adherent mucosal volume. These measurements were carried out over a 0.5-30 mM Ca²⁺ concentrations. Results indicate an evolving pattern of intestinal calcium transport with different kinetic characteristics emerging as the animals matured. The mediated component of transport became more pronounced with increasing age. Actual Km and J(max) values for mediated colonic ⁴⁵Ca uptake showed an evolving pattern. A steady rise in both Km and J(max) was noted with increasing age. Transport at higher concentrations (10-30 mM) was non-saturable. The transition to a more saturable process occurred around the time of weaning. These findings suggest that colonic calcium transport is characterized by a maturation pattern that starts with a predominately passive system during infancy and changes to a saturable mechanism during maturation.