Calcium transport by plasma membranes of enterocytes during development: Role of 1,25-(OH)₂ vitamin D₃

Calcium transport across the intestinal enterocytes represents an entry process at the brush border membranes and an ATP-dependent exit process located at the basolateral membranes. Both processes exhibit developmental changes. The present studies were designed to define the role of vitamin D in calcium transport during maturation. Brush border and basolateral membranes from vitamin D-deficient suckling and adolescent rats were used to study calcium entry and exit. 1,25-(OH)₂ vitamin D₃ administration enhanced calcium entry at the brush border membranes of suckling and adolescent rats. The increase in calcium uptake in both age groups was secondary to an increase in maximal transport capacity (V_max) rather than in K_m. In suckling rat brush border membranes, l,25-(OH)₂ vitamin D₃ treatment increased the V_max from 1.0 ± 0.1 to 1.8 ± 0.2 nmol/mg protein/7 s (p < 0.01), whereas in adolescent rats, V_max increased from 1.5 ± 0.1 to 2.5 ± 0.3 nmol/mg protein/7 s (p < 0.01). K_m values were not altered. Similarly, l,25-(OH)₂ vitamin D₃ administration enhanced ATP-dependent calcium exit at the basolateral membranes of both suckling and adolescent rats. Vmax of ATP-dependent calcium uptake by basolateral membranes of suckling rats increased from 0.5 ± 0.05 to 0.81 ± 0.06 nmol/mg protein/20 s (p < 0.01) whereas in adolescent rats, Vmax increased from 0.3 ± 0.03 to 0.6 ± 0.04 nmol/mg protein/ 20 s (p < 0.001). Km values were not altered. The current studies indicate that 1,25-(OH)₂ vitamin D₃ stimulates calcium entry and exit across the enterocytes during maturation.