Metabolism of calcium

Homeostasis of calcium

The free calcium concentration in the extracellular liquid is maintained within narrow limits by a rigorous regulation (Homéostasie). The main actors of this control are 3 bodies: the Internal , the Kidney and the Os and 3 hormones: the Parathormone (parathyroidal hormone or PTH), the Vitamin D (more precisely the 1,25-dihydroxycholecalciférol) and the Calcitonine.

The exchange with the complexed fraction and that related to proteins, pH, the Thyroxine, the Cortisol and Growth hormone or GH are other actors of this control.

The role of the intestine

The intestinal absorption of calcium coming from the Food and gastro-intestinal secretions is influenced by the other intestinal contents and mechanisms of active and passive transport in the mucous membrane. These mechanisms of transport are stimulated by the 1,252D (which increases the synthesis of Calcium Binding Protein (CaBP) of the intestinal mucous membrane) and are inhibited by cortisol. For a contribution of 25 Mo (1g), average clear absorption is of 5 Mo. Many food modes are relatively or completely defective out of calcium. Certain food components can influence the absorption of calcium.

The role of the kidney

Put besides some losses not very important by the skin, the renal excretion counterbalances intestinal absorption normally. Small modifications of the renal handling have major effects because of considerable flows through the Néphron S: for 500 Mo filtered, 495 are reabsorbed. The tubular reabsorption of calcium is complex. It undergoes initially a reabsorption proximale passivates, parallel with that of sodium. PTH stimulates then an active tubular reabsorption. The diurétiques ones of the Anse of Henle (the Furosémide, for example) inhibit the reabsorption whereas the thiazidic stimulate it.

The role of the bone

The exchange of calcium with the bone consists of an accretion (agglomeration, increase in mass per matter contribution) and a resorption (progressive matter disappearance), these two processes being usually narrowly coupled. During the growth, there is an osseous mineral net profit. In the adult, accretion and resorption are roughly speaking balances some, the exchange of calcium being very active during the Grossesse and the Lactation. With the age, the balance moves towards the osseous mineral loss, which seriously increases at the woman postménopausée.

Control by calcium

Intracellular free Ca²⁺ is implied in the control of many functions. Specific proteins binding calcium, the Calmoduline for example, play a part in this control. Calcium is transported through the cellular membranes by specific transport systems which are inhibited by particular drugs: antagonists of the entry of calcium. These substances are used for the treatment of the cardiac arrhythmias, the angina pectoris and hypertension.

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