AJP - Endocrinology and Metabolism, Vol 240, Issue 4 367-E372, Copyright © 1981 by American Physiological Society
Regulation of calcium homeostasis in the fetal and neonatal rat
M. L. Thomas, C. S. Anast and L. R. Forte
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.
