Table of Contents
At rest, the intracellular concentration of free calcium ions (Ca2+) is kept at 0.1 μM. During excitation, a transient rise of up to 10 μM elicits contraction in muscle cells (electromechanical coupling) and secretion in glandular cells (electrosecretory coupling).
The cellular content of Ca2+ is in equilibrium with the extracellular Ca2+ concentration (approx. 1000 μM), as is the plasma protein- bound fraction of calcium in blood. Ca2+ may crystallize with phosphate to form hydroxyapatite, the mineral of bone.
Osteoclasts are phagocytes that mobilize Ca2+ by resorption of bone. Slight changes in extracellular Ca2+ concentration can alter organ function: thus, excitability of skeletal muscle increases markedly as Ca2+ is lowered (e.g., in hyperventilation tetany).
Three hormones are available to the body for maintaining a constant extracellular Ca2+ concentration:
- Vitamin D
Vitamin D hormone
Vitamin D hormone is derived from vitamin D (cholecalciferol). Vitamin D can also be produced in the body; it is formed in the skin from dehydrocholesterol during irradiation with UV light. When there is lack of solar radiation, dietary intake becomes essential, cod liver oil being a rich source.
Metabolically active vitamin D hormone results from two successive hydroxylations: in the liver at position 25 ( calcifediol) and in the kidney at position 1 (calcitriol = vit. D hormone). 1-Hydroxylation depends on the level of calcium homeostasis and is stimulated by parathormone and a fall in plasma levels of Ca2+ or phosphate.
Vit. D hormone promotes enteral absorption and renal reabsorption of Ca2+ and phosphate. As a result of the increased Ca2+ and phosphate concentration in blood, there is an increased tendency for these ions to be deposited in bone in the form of hydroxyapatite crystals. In vit. D deficiency, bone mineralization is inadequate (rickets, osteomalacia).
Therapeutic use of Vitamin D
Therapeutic use aims at replacement. Mostly, vit. D is given:
- in liver disease calcifediol may be indicated
- in renal disease calcitriol
Effectiveness, as well as rate of onset and cessation of action, increase in the order vit. D. < 25-OH-vit. D < 1,25-di-OH-vit. D.
Overdosage may induce hypercalcemia with deposits of calcium salts in tissues (particularly in kidney and blood vessels): calcinosis.
The polypeptide parathormone is released from the parathyroid glands when plasma Ca2+ level falls.
- It stimulates osteoclasts to increase bone resorption; in the kidneys
- it promotes calcium reabsorption, while phosphate excretion is enhanced
As blood phosphate concentration diminishes, the tendency of calcium to precipitate as bone mineral decreases. By stimulating the formation of vit. D hormone, parathormone has an indirect effect on the enteral uptake of Ca2+ and phosphate. In parathormone deficiency, vitamin D can be used as a substitute that, unlike parathormone, is effective orally.
The polypeptide calcitonin is secreted by thyroid C-cells during imminent hypercalcemia. It lowers plasma Ca2+ levels by inhibiting osteoclast activity.
Therapeutic use – Its uses include hypercalcemia and osteoporosis. Remarkably, calcitonin injection may produce a sustained analgesic effect that is not restricted to bone pain.
Hypercalcemia can be treated by:
- administering 0.9% NaCl solution plus furosemide (if necessary) to increase renal excretion
- the osteoclast inhibitors calcitonin, plicamycin, or clodronate (a bisphosphonate) to lower bone calcium mobilization
- the Ca2+ chelators EDTA sodium or sodium citrate