Active calcium transport by intestinal endoplasmic reticulum during maturation

Calcium uptake by intestinal endoplasmic reticulum was determined during maturation in the rat. Calcium uptake was enhanced severalfold by the presence of ATP in suckling, weanling, and adolescent rats. Uptake values were higher during early life and decreased gradually with age. Calcium uptake represented transport into the intravesicular space of the microsomes as evident by marked decrease in the uptake at 0°C compared with values at 25°C and by rapid release of intravesicular calcium by the ionophore A23187. Calcium uptake was dependent on magnesium and media pH and was inhibited by vanadate. Sodium oxalate enhanced calcium uptake. Oligomycin and sodium azide did not inhibit calcium uptake by microsomes, suggesting that calcium uptake represents a property of the microsomes rather than mitochondrial uptake. Initial rate uptake was linear up to 30 s. Maximal uptake occurred at pH 7.2. Kinetic studies revealed a high-affinity, high-capacity system in microsomes from suckling rats (V(max) 2.26 ± 0.2 nmol · mg protein^-1 · 15 s^-1 and K(m) 0.56 ± 0.01 μM) compared with a low-capacity system in microsomes from adolescent rats (V(max) 0.72 ± 0.1 nmol · mg protein^-1 · 15 s^-1 and K(m) 0.69 ± 0.02 μM). These findings suggest that the endoplasmic reticulum of the enterocyte may play a major role in regulating intestinal cytosolic calcium homeostasis during early development.