Intestinal maturation: Calcium transport by basolateral membranes

The overall characteristics of calcium transport across the intestine have been defined using in vivo perfusion techniques and in vitro everted gut sacs. However, calcium transport represents three separate processes: entry at the brush border membranes, movement across the cytoplasm, and exit at the basolateral membranes (BLM). Studies in adult animals indicated that the active step in calcium transport is located at the BLM. The present studies describe for the first time the matu-rational aspects of calcium transport across the BLM of the enterocyte. We utilized a percoll density gradient to prepare enriched BLMs from suckling and adolescent rats. Calcium uptake into BLMs represented mainly transport into the intravesicular space. Calcium transport in both age groups was driven by ATP; the calcium transport in the presence and absence of ATP was significantly greater in suckling rats compared to adolescent rats. Kinetics of calcium uptake calculated from uptake values in the presence of ATP minus no ATP conditions showed a Km of 0.06 ± 0.01 and 0.03 ± 0.01 u.M for adolescent and suckling rats respectively (p < 0.05). Vmax was 1.5 ± 0.1 and 0.8 ± 0.08 nmol/mg protein/min for adolescent and suckling rats respectively (p < 0.01). Calcium/sodium exchange mechanisms was also present in both age groups. However, the magnitude of sodium-dependent calcium exchange was smaller in suckling rats compared to adolescent rats. These data suggest that calcium exit at the BLMs of enterocytes of suckling and adolescent rats occurs by an ATP-dependent and a calcium/sodium exchange mechanism. Both mechanisms exhibit maturational changes.