Appearance of the intestinal cytosolic receptor for 1,25-dihydroxyvitamin D3 during neonatal development in the rat.

During the early suckling period in the rat, active calcium uptake by the intestine is refractory to stimulation by 1,25-dihydroxyvitamin D3. To determine the role that the specific cytosolic receptor for 1,25-dihydroxyvitamin D3 plays in this refractory state, the concentration of the receptor and its binding characteristics were measured during the neonatal period. From Scatchard analysis, the concentration of receptor in the adult intestine was 563 +/- 64 fmol/mg of protein. At 28 and 21 days postpartum, receptor concentrations were 711 +/- 129 and 251 +/- 36 fmol/mg of protein, respectively. Estimates from sucrose density gradient profiles and Scatchard analyses indicated that, at 14 and 7 days postpartum, receptor concentrations were less than 50 fmol/mg of protein. Equilibrium dissociation constants were similar at all times measured and ranged from 0.38 to 0.52 nM. These results suggest that the lack of response observed in the early neonatal intestine to 1,25-dihydroxyvitamin D3 may stem, at least in part, from the relative absence of receptors for 1,25-dihydroxyvitamin D3. Furthermore, the appearance of the receptor may be the determining factor in the initiation of active calcium absorption in the intestine and its regulation.