Regulation of calcium homeostasis in the fetal and neonatal rat

Circulating levels of calcium (Ca) and immunoreactive parathyroid hormone (IPTH), and the renal cyclic AMP responses to PTH, calcitonin (CT), and vasopressin (VP) were measured in fetal and neonatal rats. Serum Ca increased from a mean value of 9.1 mg/dl on the 19th day of gestation to 10.9 on day 20. Circulating IPTH decreased from 875 pg/ml to 213. Serum Ca declined rapidly after birth to a nadir of 7.6 by 3 h and IPTH increased to 2,006 pg/ml, indicating that fetal and newborn parathyroids are capable of responding appropriately to changes in circulating Ca. Renal responsiveness to hormones was assessed in vitro in the presence of methylisobutylxanthine, a phosphodiesterase inhibitor. The tissue cyclic AMP response to PTH and CT (15- to 18-fold over basal) was greatest at gestational days 18 and 19, progressively declined throughout the remainder of gestation, and remained low during the first 24 h after birth (6- to 7-fold). Renal cyclic AMP response to VP remained consistently low throughout this period. The depressed renal cyclic AMP response to PTH at the time of birth may contribute to the hypocalcemia found in newborn rats.