Evidence for vitamin D-independent active calcium absorption in newborn piglets

The role of 1,25-dihydroxycholecalciferol (calcitriol) for intestinal calcium (Ca2+) absorption was studied in newborn (< 1 week old) and weaned piglets (> 6 weeks old). In both groups, normal piglets and piglets suffering from inherited pseudo vitamin D-deficiency rickets, type I (PVDRI) were used. In this inherited disorder, renal production of calcitriol is absent. Plasma samples were assayed for calcitriol and total Ca, and dissociation constants (Kd) and maximum binding capacities (Bmax) of intestinal calcitriol receptors were determined under equilibrium conditions at 4 degrees C. Unidirectional Ca(2+)-flux rates were measured across stripped duodenal mucosae in Ussing chambers in the absence of electrochemical gradients. The plasma calcitriol concentrations of neonatal (26.5 +/- 7.1 pg/ml, n = 11; mean +/- SEM) and weaned PVDRI piglets (18.8 +/- 5.7 pg/ml, n = 8) were unphysiologically low and differed significantly from control animals (83.6 +/- 14.8 pg/ml, n = 8, and 86.9 +/- 9.6 pg/ml, n = 11, respectively). However, newborn PVDRI piglets had normal plasma Ca levels at least during the first days of life. They became hypocalcemic and developed clinical symptoms of rickets during the following weeks. In newborn PVDRI and control piglets, Bmax was significantly lower (84 +/- 28 fmol/mg protein and 127 +/- 55 fmol/mg protein, n = 9, respectively) than in weaned piglets (741 +/- 82 fmol/mg protein, n = 9, and 778 +/- 121 fmol/mg protein, n = 8, respectively). Significant net Ca(2+)-fluxes were found in both newborn PVDRI and control piglets (88.8 +/- 25.1 nmol.cm-2 x h-1, n = 6, and 86.5 +/- 10.5 nmol.cm-2 x h-1, n = 9, respectively).