Relationship between Ca-Dependent Change of Serum PTHand Extracellular Ca²⁺-Sensing Receptor Expression in Parathyroid Adenoma

Abnormal PTH secretion and cell growth in hyperparathyroid tissues are accompanied with reduced expression of Ca²⁺-sensing receptor (CaR) which plays a key role in Ca-regulated PTH release. In this study, we examined the receptor expression in parathyroid adenomas using specific anti-CaR antibody and investigated relationship between CaR expression in adenomatous tissues and parameters of Ca-dependent change of serum PTH. The results show a considerable variation in the number of CaR positive cells among the adenomatous tissues. Expression of the receptor protein was not related to set-point error but was more reduced in the patients with more elevated minimum or baseline levels of serum PTH. CaR expression was severely reduced in the patients with highly elevated maximum serum PTH, while the receptor expression was also decreased in some patients with normal maximum serum PTH. Baseline level / maximum level ratio of serum PTH was increased in these patients. In conclusion, reduced CaR expression is related to abnormality in three parameters of PTH secretion (minimum serum PTH, maximum serum PTH, and baseline level / maximum level ratio of serum PTH) and may contribute to hypersecretion from parathyroid adenomas.