Dietary calcium deficiency increases Ca²⁺ uptake and Ca ²⁺ extrusion mechanisms in chick enterocytes

Ca²⁺ uptake and Ca²⁺ extrusion mechanisms were studied in enterocytes with different degree of differentiation from chicks adapted to a low Ca²⁺ diet as compared to animals fed a normal diet. Chicks adapted to a low Ca²⁺ diet presented hypocalcemia, hypophosphatemia and increased serum 1,25(OH)₂D₃ and Ca²⁺ absorption. Low Ca²⁺ diet increased the alkaline phosphatase (AP) activity, independently of the cellular maturation, but it did not alter γ-glutamyl-transpeptidase activity. Ca²⁺ uptake, Ca²⁺-ATPase and Na⁺/Ca²⁺ exchanger activities and expressions were increased by the mineral-deficient diet either in mature or immature enterocytes. Western blots analysis showS that vitamin D receptor (VDR) expression was much higher in crypt cells than in mature cells. Low Ca²⁺ diet decreased the number of vitamin D receptor units in both kinds of cells. In conclusion, changes in Ca²⁺ uptake and Ca ²⁺ extrusion mechanisms in the enterocytes by a low Ca²⁺ diet appear to be a result of enhanced serum levels of 1,25(OH) ₂D₃, which would promote cellular differentiation producing cells more efficient to express vitamin D dependent genes required for Ca²⁺ absorption.