Modulation of Parathyroid Cell Function by Calcium Ion in Health and Uremia

The calcium-sensing receptor: a key factor in the pathogenesis of secondary hyperparathyroidism

Serum calcium levels are regulated by the action of parathyroid hormone (PTH). Major drivers of PTH hypersecretion and parathyroid cell proliferation are the hypocalcemia and hyperphosphatemia that develop in chronic kidney disease patients with secondary hyperparathyroidism (SHPT) as a result of low calcitriol levels and decreased kidney function. Increased PTH production in response to systemic hypocalcemia is mediated by the calcium-sensing receptor (CaR). Furthermore, as SHPT progresses, reduced expression of CaRs and vitamin D receptors (VDRs) in hyperplastic parathyroid glands may limit the ability of calcium and calcitriol to regulate PTH secretion. Current treatment for SHPT includes the administration of vitamin D sterols and phosphate binders. Treatment with vitamin D is initially effective, but efficacy often wanes with further disease progression. The actions of vitamin D sterols are undermined by reduced expression of VDRs in the parathyroid gland. Furthermore, the calcemic and phosphatemic actions of vitamin D mean that it has the potential to exacerbate abnormal mineral metabolism, resulting in the formation of vascular calcifications. Effective new treatments for SHPT that have a positive impact on mineral metabolism are clearly needed. Recent research shows that drugs that selectively target the CaR, calcimimetics, have the potential to meet these requirements.

Management of secondary hyperparathyroidism of dialysis patients

Hyperphosphatemia, vitamin D deficiency, and resulted hypocalcemia have been regarded as classical pathogeneses of secondary hyperparathyroidism. These factors have been treated by the administration of phosphorus binder and vitamin D derivatives. However, these therapies have not brought about a successful result for the prevention and treatment of secondary hyperparathyroidism. The reason could be mainly attributed to the hypercalcemia that results from the administration of calcium salts as a phosphorus binder and the calcemic action of vitamin D. To prevent hypercalcemia, non-calcium containing phosphorus binder (sevelamer hydrochloride) and vitamin D analogues, which suppress PTH secretion with minimum calcemic action, have been developed. These new vitamin D analogues include 19-nor-1-alpha, 25-dihydroxyvitamin D2 (paricalcitol), 1-alpha-hydroxyvitamin D2 (doxercalciferol), 22oxa-calcitriol (maxacalcitol) and F6-calcitriol (falecalcitriol). Furthermore, calcimimetics that stimulate calcium-sensing receptor of parathyroid cells as calcium and suppress PTH secretion are now under clinical trial. Percutaneous direct injection therapy of vitamin D, vitamin D analogue or calcimimetics into parathyroid gland has also been reported. The combination of these new strategies is expected to effectively and safely suppresses secondary hyperparathyroidism that has been resistant to conventional medical treatments.