REGULATION OF SERUM CALCITRIOL BY SERUM IONIZED CALCIUM IN RATS DURING PREGNANCY AND LACTATION

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.