Ontogenesis of the rabbit intestinal receptor for 1,25-dihydroxyvitamin D3--evidence for increased receptor content during late suckling and lactating periods.

The 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) intestinal receptor was not detected in term fetal rabbits. This receptor was present at 2 weeks postpartum and its concentration reached a maximum at 4 weeks of age, and declined to adult levels by 10 weeks postpartum. The 1,25(OH)2D3 intestinal receptor concentration was elevated at 2 weeks postpartum in lactating rabbits, but returned to normal adult concentrations by 4 weeks postpartum. In rabbits of various ages, only minor changes in the equilibrium dissociation constant of this receptor were observed. These data indicate that increasing the small intestine 1,25(OH)2D3 receptor concentration is one mechanism by which the rabbit adapts to periods of increased calcium demand.