Biological Functions of Calcium
The vast majority of calcium acts as a structural component of bone tissue. The remaining one percent circulates in the extracellular fluid as dissolved Ca2+ ions or bound to plasma proteins. This small pool performs vital tasks:
- Initiates muscle contraction.
- Regulates cell membrane permeability to potassium ions and sodium conductance.
- Controls the activity of various ion pumps.
- Ensures hormone secretion.
- Participates in the blood coagulation cascade.
- Serves as a crucial second messenger for intracellular signal transduction.
Main Regulators of Homeostasis
Calcium and phosphate homeostasis is regulated by three main hormones acting on target organs:
| Hormone | Site of Synthesis | Mechanism and Effects |
|---|---|---|
| Parathyroid hormone (PTH) | Parathyroid glands | Secreted in response to low Ca2+. Activates osteoclasts via osteoblast adenylate cyclase signaling (bone resorption). Enhances renal calcium reabsorption and inhibits phosphate reabsorption. |
| Calcitriol | Kidneys | Increases intestinal calcium and phosphate absorption. Regulates nuclear transcription. |
| Calcitonin | Thyroid C-cells | Released during hypercalcemia. Suppresses osteoclasts (inhibits bone resorption) and increases urinary calcium excretion. |
Vitamin D Metabolism
The synthesis of the active hormone, calcitriol, involves several sequential steps across multiple organs:
- Precursor formation: Cholesterol is converted into 7-dehydrocholesterol (containing a double bond between carbon atoms 7 and 8 in ring B).
- Cutaneous step (photolysis): Ultraviolet radiation in the epidermis non-enzymatically cleaves the bond between carbons 9 and 10, producing cholecalciferol (vitamin D3).
- Hepatic step: The enzyme 25-hydroxylase adds an OH group to carbon 25, yielding the transport form, calcidiol.
- Renal step: In the mitochondria of proximal tubules, 1-alpha-hydroxylase synthesizes the active form, calcitriol (1,25-dihydroxycholecalciferol).
Clinical Aspects: Hyper- and Hypoparathyroidism
In primary hyperparathyroidism (e.g., due to an adenoma), excess PTH causes massive bone resorption. This leads to hypercalcemia, which decreases neuromuscular excitability, causing muscle weakness and intestinal atony (constipation). Excess calcium is filtered into the urine (hypercalciuria), which, combined with phosphaturia, promotes kidney stone formation (renal colic).
In hypoparathyroidism (following parathyroid gland removal or damage), PTH deficiency causes a drop in blood calcium levels, sharply increasing neuromuscular excitability. Patients develop muscle cramps, tetany, and potentially life-threatening laryngospasm.
Vitamin D Disorders and Rickets
Calcitriol binds to the intracellular VDR receptor, enters the nucleus, and induces the synthesis of calbindin, a protein required for intestinal calcium absorption. Disruptions at any stage of this pathway lead to rickets or osteomalacia.
Major causes of deficiency:
- Dietary insufficiency or inadequate sun exposure.
- Intestinal malabsorption.
- Liver disease (impaired 25-hydroxylation).
- Kidney disease (impaired 1-alpha-hydroxylation).
- Genetic defects in VDR receptors.