Elevation of a calcium fraction tightly bound to serum proteins in maternal blood at delivery and in cord blood from newborn infants.

The intimate mechanisms for calcium transport and accumulation in bone cells are complex. We have previously observed that calcium tightly bound to serum proteins is taken up by bone cells preferentially over ionized calcium. To explore the significance of this phenomenon as a potential mechanism for calcium transport in vivo, we studied the distribution of calcium in sera from normal individuals, pregnant women, cord blood from newborn infants, and patients with disorders of calcium homeostasis. Tightly bound calcium was defined as the fraction that remained bound to serum proteins after six consecutive ultrafiltration cycles. Cord blood from newborn infants showed striking elevations of total serum calcium, 11.2 +/- 0.3 mg/dl (+/- SEM), and of the fraction tightly bound to serum proteins, 25.1% +/- 1.7%, compared to normal control values of 9.5 +/- 0.1 mg/dl, and 18.9% +/- 1.1%, respectively (p less than 0.01). Maternal blood at delivery also exhibited an increase of the tightly bound calcium fraction, 27.1% +/- 0.7% (p less than 0.01), but normal total serum calcium concentration, 9.4 +/- 0.2 mg/dl. There was no correlation between the simultaneous fractional concentrations of tightly bound calcium in the maternal and fetal blood compartments (r = -0.03; p = 0.92; n = 12 sets of paired samples). The high concentrations of calcium tightly bound to serum proteins in cord blood suggests that this fraction could play a role in the mineralization of fetal bone.