Calcium and Phosphorus

Mechanism

🦴 Calcium Homeostasis

• Cell Role: Second intracellular messenger, coupling of muscle contraction and neuronal synaptic depolarization.
• Humoral Control: Strict regulation by parathyroid hormone (PTH), calcitriol and calcitonin.
• Hypocalcemia prolongs the QT interval inducing tetany and laryngeal spasm.

🔋 Phosphorus Homeostasis

• Cell Role: Member of the ATP nucleotide for energy transfer and the DNA/RNA skeleton of genes.
• Humoral Control: Regulated by the renal factor FGF-23 through inhibition of sodium-phosphate transporters.
• Severe hypophosphatemia blocks the cellular production of ATP in erythrocytes and muscle.

Humoral regulation of calcemia

The extracellular calcium receptor (CaSR) located in the principal cells of the parathyroid gland constantly monitors plasma ionic levels. When free calcium decreases, the release of PTH is stimulated, which exerts three systemic actions:

1. Bone Effect: Stimulates bone resorption through induction of RANKL in osteoblasts, activating osteoclasts to release calcium and phosphate from hydroxyapatite.
2. Direct Renal Effect: Increases active calcium reabsorption in the distal convoluted tubule, and inhibits phosphate reabsorption in the proximal tubule (induces phosphaturia to prevent ectopic Ca×P solubility product).
3. Indirect Renal Effect: Activates renal CYP27B1 to increase calcitriol levels, facilitating coordinated intestinal absorption.

Security

Adverse Effects, Contraindications and Interactions

Excessive calcium supplementation favors the development of chronic kidney stones and constipation by reducing the peristalsis of the colonic smooth muscles. It is contraindicated in hypercalcemia, severe hypercalciuria or suspicion of digoxin toxicity (calcium and digoxin act synergistically on myocardial excitability, increasing the risk of fatal ventricular arrhythmias).

It interacts directly by chemical chelation with antibiotics such as tetracyclines and fluoroquinolones, drastically reducing the absorption of both drugs if their intake is not spaced by at least 2 hours.

Clinical

Comparison of Clinical Calcium Salts

For calcium supplementation, there are various formulations with key differences in their bioavailability:

• Calcium Carbonate: Contains the highest concentration of elemental calcium by weight (≈ 40%). However, it requires an acidic gastric environment to dissolve and ionize, so its absorption is significantly reduced in elderly patients with achlorhydria or under treatment with PPIs (omeprazole). It should be taken with food.
• Calcium Citrate: It contains a lower percentage of elemental calcium (≈ 21%), but its absorption is independent of gastric pH, being the choice in geriatric patients or those using gastric protectors.