THE EXPRESSION OF PARATHYROID-HORMONE AND PARATHYROID HORMONE-RELATEDPROTEIN IN DEVELOPING RAT PARATHYROID-GLANDS

Secretion of parathyroid hormone-related protein (PTHrP) by sheep feta l parathyroid glands is reported to be an important factor in the main tenance of a placental calcium pump. The aim of the present study was to determine whether the developing rat parathyroid glands express PTH rP or parathyroid hormone (PTH), or both. Hybridisation histochemistry was used to detect transcription of PTHrP and PTH in serial paraffin sections through the 12 . 5- and 13 . 5-day rat embryo parathyroid anl age, as well as in sections through the 17 . 5-day embryonic and adult parathyroid glands. Results show strong expression of PTH in the 13 . 5-day embryonic parathyroid anlage, as well as in the parathyroid gla nd of the 17 . 5-day embryo and adult. Transcription of the PTHrP gene was not detected. The more sensitive technique of reverse transcripti on PCR was then performed. The pharyngeal region of 11 . 5-, 12 . 5- a nd 13 . 5-day rat embryos was dissected out and, at each stage, RNA wa s extracted from these tissues, as well as pooled tissues from the res t of the embryo. RNA that had been extracted from adult thyroid/parath yroid tissue was also tested. After reverse transcription, the resulti ng cDNAs were amplified by PCR (50 cycles) using specific PTH and PTHr P primers. The results show an abundance of PTH mRNA, specific to the pharyngeal region of the 13 . 5-day embryo, as well as to adult thyroi d/parathyroid tissue. PTHrP expression was detected at very low levels in both parathyroid and extraparathyroid tissues. The presence of imm unoreactive PTHrP and immunoreactive PTH in the pharyngeal region and rest of the body of 12 . 5- and 13 . 5-day rat embryos was assessed by specific RIAs. Whilst immunoreactive PTHrP was not detected in any of the tissues assayed, immunoreactive PTH detected only was in the phar yngeal region of the 13 . 5-day embryo. This confirms the results obta ined from the gene expression studies. We conclude then that, in the d eveloping rat embryo, PTH rather than PTHrP is more likely to play a r ole in calcium regulation. This is in contrast with the reported situa tion in the sheep, suggests that fundamental species differences in fe tal calcium regulation exist in mammals.