Background: The set point for parathyroid hormone (PTH) secretion is increased in patients with primary hyperparathyroidism, possibly because of receptor defect(s). A decreased expression of calcium receptor (CaR) messenger ribonucleic acid (mRNA) and protein and a decreased expression of the putative calcium-sensing CAS (gp330/megalin) protein have been demonstrated in parathyroid adenomas.
Methods: Expression of CAS mRNA was studied in matched pairs of adenomas and adenoma-associated biopsy specimens from normal parathyroid glands from 15 patients with sporadic primary hyperparathyroidism. Cryostat sections were hybridized with an oligonucleotide complementary to CAS mRNA, rinsed, air dried, and exposed to x-ray film for semiquantification of radioactivity.
Results: Expression of CAS mRNA in the adenomas was lowered significantly to 25% (median; range 9% to 80%) of that of the corresponding biopsy specimens of normal parathyroid glands. No correlation was seen between CAS mRNA in the adenoma and preoperative serum calcium levels, PTH level, or weight of the adenoma. The levels of CAS mRNA were significantly lower than those observed previously for CaR mRNA. There was no significant correlation between the levels of CAS and CaR mRNA.
Conclusions: Lowered levels of receptors sensing extracellular calcium (CaR and CAS) probably contribute to the increased set point for PTH secretion in primary hyperparathyroidism.