Maternal and fetal parathyroid hormone responsiveness in pregnant primates.

Maternal and fetal parathyroidd hormone (PTH) responsiveness to hypocalcemia induced by EDTA infusion (50 mg/kg over 2 h) was studied in rhesus monkeys in late pregnancy. Although baseline serum total calcium (Ca) levels in the fetus exceeded those in the mother (4.83 +/- 0.13 vs. 4.28 +/- 0.15 meq/liter; P < 0.001), PTH values were not significantly different (5.62 +/- 0.37 vs. 6.18 +/- 0.33 muleq/ml; P > 0.05). EDTA infusion directly to five fetuses produced significant hypocalcemia (maximal decline averaging 19 +/- 2%) and PTH response (maximal increase averaging 46 +/- 5%). In contrast, in four control studies involving fetal saline infusion, there were no significant changes in fetal Ca or PTH levels. Four maternal control infusions produced no significant changes in either Ca or PTH levels. A comparison of maternal and fetal PTH responses indicated considerable similarity, although fetal PTH levels tended to return to baseline somewhat more gradually after cessation of the hypocalcemic stimulus than did maternal levels. These studies indicate that fetal PTH secretion, both baseline and in response to hypocalcemia, is quantitatively similar to that of the adult, and thus, the fetal parathyroid does not appear to be suppressed by the relative hypercalcemia of late fetal life.