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Abstract:
More than 99 p. 100 of calcium and 90 p. 100 of phosphorus are located in the bone tissue in the form of hydroxy-apatite crystals. Plasma ionized calcium is very tightly regulated, in order to stabilize the membranes of excitable cells, such as nerve and muscle. The PTH-1,25 (OH)2 D axis, triggered by very small variations in plasma ionized calcium (negative feedback) is the major control system of intestinal absorption, movements between bone cells and extracellular fluid, and tubular reabsorption of calcium by the kidney. Thus, usual variations in dietary intake induced no detectable change in plasma calcium and only small variations in 24 h-urinary calcium excretion rate. Plasma phosphate fluctuates over 24h-periods, and homeostatic mechanisms seem to control the concentration and/or content of phosphate in soft tissue cells (phosphate is necessary to ATP-synthesis and phosphorylation of intermediate metabolites). The PTH-1,25 (OH)2D axis which is not triggered by variations in plasma phosphate, does not control tightly intestinal absorption, movements between bone cells and extracellular fluid, or tubular reabsorption of phosphate by the kidney. However, the phosphate reabsorption in the proximal tubule of the kidney is rigidly adjusted to the phosphate available in the digestive lumen and the phosphate needs of soft tissue cells, probably via as yet undetermined factor(s). Thus, usual variations in dietary phosphate intake influence notably the plasma concentration and 24 h-urinary excretion rate.