Calcitonin mRNA expression and plasma calciotropic hormones in fetal lambs

The object of this study was to measure basal levels of calciotropic hormones during gestation in the ovine fetus. Thyroid glands and plasma samples were collected from 49 Ile de France x (Limousine x Romanov) fetuses (range, 80 to 145 d). Total thyroidal RNA was extracted for calcitonin (CT) mRNA analysis. Plasma immunoreactive concentrations of CT, parathyroid hormone (PTH), parathyroid hormone-related peptide (PTHrP), and 1,25-dihydroxy-vitamin D [1,25-(OH)2D] were assayed. Plasma Ca and P concentrations were measured by atomic absorption spectrophotometry and colorimetry, respectively. The ratio of thyroidal weight/body weight decreased twice between Days 80 and 145 of gestation. Simultaneously, the CT mRNA rate increased but did not significantly vary after 89 d of gestation. Plasma CT concentration increased between 80 and 89 d and then did not significantly vary until Day 145. We could not detect PTH in ovine fetuses plasma; PTHrP concentration did not vary significantly between Days 80 and 145. The concentration of 1,25-(OH)2D decreased in plasma essentially between Days 110 and 145. In conclusion, between Days 80 and 145 of gestation, CT, PTHrP, and 1,25-(OH)2D concentrations can be easily measured in plasma from ovine fetuses, but plasma PTH was undetectable. In basal conditions, no relationship could be demonstrated between any hormonal concentrations in the fetal plasma and that of Ca or P.