A. Boass et al., REGULATION OF SERUM CALCITRIOL BY SERUM IONIZED CALCIUM IN RATS DURING PREGNANCY AND LACTATION, Journal of bone and mineral research, 12(6), 1997, pp. 909-914
Serum calcitriol concentrations in rats follow a biphasic pattern duri
ng reproduction, with elevated levels during late pregnancy, a decline
after parturition, and a rise to even higher levels during peak lacta
tion. We have previously shown that serum calcitriol in rats at peak l
actation correlates significantly with, and appears to be regulated by
, serum ionized Ca (Ca2+), with parathyroid hormone (PTH) serving a pe
rmissive role. We have extended this study by determining if serum cal
citriol also correlates with serum Ca2+ during late pregnancy, when ca
lcitriol levels are clearly elevated, and during early lactation, when
only modest increases in serum calcitriol are observed. Analyses of d
ata combined from nonmated, 21-day pregnant (P), and 1-day lactating r
ats (L) revealed a significant regression (p < 0.001) of calcitriol on
Ca2+, but a nonsignificant regression (p = 0.34) of calcitriol on ser
um PTH. An even stronger correlation (p < 0.001) between calcitriol an
d Ca2+ was found for the combined data for 5-, 8-, and 14-day L rats.
The partial correlation coefficient for calcitriol versus Ca2+, with P
TH as the independent variable, was highly significant (p < 0.01) for
the data from both combined groups. However, the coefficient for calci
triol versus PTH, with Ca2+ as the independent variable, was not signi
ficant (p > 0.05). Fetal weights (uterus and contents) correlated sign
ificantly with both maternal calcitriol and Ca2+ concentrations (p < 0
.01), but not with maternal PTH levels. Litter weights for 14-day-old
pups likewise correlated significantly with maternal calcitriol and Ca
2+ (p < 0.001). We conclude that hypocalcemia, induced by the demands
for Ca for fetal calcification and milk production, appears to be a co
ntrolling factor in serum calcitriol elevation in late pregnancy and t
hroughout lactation, whereas PTH may be important for calcitriol synth
esis without playing a direct regulatory role.