Physiologie du calcium et des phosphates

Abstract

Calcium and phosphate homeostasis is subjected to three challenges. The first one consists in accumulating and maintaining an important stock of calcium and phosphates which confers bones their mechanical resistance. Independently of nutritional constraints, this objective is achieved by: (1) the vitamin D (calcitriol)-dependent increase of calcium and phosphates intestinal absorption; (2) the capability of parathormone (PTH) to reduce calciuria; (3) the coordinated effect of both hormones to stimulate bone turnover and to exert, in physiological conditions, a predominant anabolic effect and mineral accretion. The second challenge consists in maintaining stable both extracellular calcium and the cellular stock of phosphates. Ionized calcium is a vital determinant of neuromuscular excitability and coagulation. A calcium sensing receptor (CaSR) detects even small variations of extracellular calcium and modulates both renal calcium excretion and PTH secretion. In turn, PTH mobilizes calcium from bones and stimulates the synthesis of calcitriol. The preservation of the cellular stock of phosphates is crucial for phosphorylation and the synthesis of ATP. Global effect of calcitriol results in a positive phosphates balance, while an excess of phosphates stimulates both PTH and Fibroblast Growth Factor-23 (FGF-23) secretion. Both hormones induce a marked phosphaturia whereas FGF-23 only inhibits the synthesis of calcitriol. The last homeostatic challenge is to produce mineral accretion in bones while simultaneously preventing calcium-phosphates complexes deposits in soft tissues. This capability, based on FGF-23 activity and interaction with the co-receptor Khloto, appears to be connected with senescence and could partly determine life longevity.