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Calcium homeostasis

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"calcium homeostasis" AND regulation

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Calcium Homeostasis

Calcium homeostasis is the set of integrated physiological mechanisms that maintain the extracellular fluid (ECF) Ca2+ concentration within a tight normal range (~8.5-10.5 mg/dL or ~2.1-2.6 mmol/L). Three organ systems (bone, intestine, kidney) and three principal hormones (PTH, vitamin D, calcitonin) work in concert to achieve this.

1. Body Calcium Distribution

  • Total body calcium: ~1000-1300 g
  • 99.3% in bone and teeth as hydroxyapatite crystal [Ca₁₀(PO₄)₆(OH)₂]
  • 0.6% in soft tissues (intracellular)
  • 0.1% in ECF, including 0.03% in plasma
  • Intracellular free Ca2+ is ~100 nM vs. extracellular ~1 mM - a 10,000-fold gradient is maintained

Fractions of Plasma Calcium

Fraction% of TotalNotes
Protein-bound (mainly albumin)~40%Not ultrafilterable
Complexed (citrate, phosphate)~10%Not ionized
Ionized (free) Ca2+~50%Biologically active; regulated
The ionized fraction is what the parathyroid glands sense and regulate.

2. The Overall Calcium Balance (Daily Fluxes)

Ca2+ homeostasis diagram showing intestinal absorption/secretion, renal filtration/reabsorption/excretion, and bone deposition/resorption with PTH and vitamin D effects
Fig. Ca2+ homeostasis in an adult eating 1000 mg/day. - Costanzo Physiology, 7th ed., p. 453
In a healthy adult eating 1000 mg/day of calcium:
  • Intestine: 350 mg absorbed (stimulated by 1,25-dihydroxycholecalciferol), 150 mg secreted back in digestive juices. Net absorption = 200 mg/day; 800 mg excreted in feces.
  • Bone: Continuous remodeling with ~500 mg/day released (resorption) and ~500 mg/day deposited - net zero in healthy adults.
  • Kidney: ~10 g filtered daily; ~9.8 g reabsorbed; ~200 mg excreted in urine (equal to net intestinal absorption, maintaining balance).

3. Parathyroid Hormone (PTH) - The Primary Regulator

Structure

  • Synthesized by chief cells of the 4 parathyroid glands
  • 84-amino acid single-chain polypeptide; biologic activity in N-terminal 34 amino acids
  • Precursors: preproPTH (115 aa) → proPTH (90 aa) → PTH (84 aa)

Regulation of PTH Secretion - The Calcium-Sensing Receptor (CaSR)

PTH secretion is inversely related to plasma ionized Ca2+:
Graph showing PTH secretion rate vs. total plasma Ca2+: maximal secretion at low Ca2+ (~7.5 mg/dL), suppressed at normal/high Ca2+
Fig. PTH secretion vs. plasma Ca2+ concentration. - Costanzo Physiology, 7th ed., p. 454
Mechanism:
  • Parathyroid chief cells express the calcium-sensing receptor (CaSR), a Gq-coupled GPCR
  • When ECF Ca2+ is high: Ca2+ binds CaSR → activates phospholipase C → ↑ IP₃/Ca2+ → inhibits PTH secretion
  • When ECF Ca2+ is low: reduced CaSR activation → stimulates PTH secretion within seconds
  • PTH response to falling Ca2+ is remarkably fast; the faster the fall, the greater the PTH secretory response

Actions of PTH on Target Organs

Bone

PTH acts in two phases:
  1. Rapid phase (minutes-hours): Activates existing osteocytes and osteoblasts of the osteocytic membrane system - an interconnected network that separates bone fluid from ECF. PTH activates the osteocytic pump, pulling Ca2+ from bone fluid into ECF. Called osteolysis - occurs without matrix resorption.
  2. Slow phase (days-weeks): Proliferation of osteoclasts → increased bone resorption (matrix + mineral). Also mobilizes phosphate from bone.

Kidney

PTH has two renal effects:
  1. Proximal tubule: Inhibits Na⁺-phosphate cotransport → phosphaturia (urinary phosphate excretion). This is functionally critical: the phosphate released from bone would otherwise complex ECF Ca2+ and blunt the calcium rise. By excreting phosphate, PTH "allows" ionized Ca2+ to increase.
  2. Distal convoluted tubule: Stimulates Ca2+ reabsorption, directly reducing urinary calcium loss.

Intestine (Indirect)

PTH has no direct effect on the intestine. Instead, PTH stimulates renal 1α-hydroxylase, converting 25-hydroxycholecalciferol → 1,25-dihydroxycholecalciferol (calcitriol), which then directly stimulates intestinal Ca2+ absorption.

PTH's Net Effect

↓ Ca2+ → ↑ PTH → ↑ bone resorption + ↑ renal Ca2+ reabsorption + ↓ phosphaturia + ↑ vitamin D activation → ↑ plasma Ca2+

4. Vitamin D (Calcitriol)

Synthesis and Activation

  1. Skin: UV-B converts 7-dehydrocholesterol → cholecalciferol (D3)
  2. Liver: 25-hydroxylase converts D3 → 25-hydroxycholecalciferol [25(OH)D3] (storage form; measured for vitamin D status)
  3. Kidney: 1α-hydroxylase converts 25(OH)D3 → 1,25-dihydroxycholecalciferol [1,25(OH)₂D3] = calcitriol (active form)
    • This step is stimulated by PTH, hypocalcemia, and hypophosphatemia
    • Inhibited by calcitriol itself (negative feedback) and by FGF-23

Actions of Calcitriol

  • Intestine: Major target. Stimulates synthesis of calcium-binding proteins (e.g., calbindin) → markedly increases Ca2+ and phosphate absorption (both active and passive transport)
  • Bone: At physiologic levels, promotes bone mineralization. At pharmacologic/excess levels, enhances PTH-mediated bone resorption
  • Kidney: Mildly increases Ca2+ and phosphate reabsorption
  • Parathyroid: Directly suppresses PTH gene transcription (negative feedback loop)

5. Calcitonin

  • Secreted by parafollicular C cells of the thyroid gland
  • 32-amino acid peptide
  • Released in response to elevated plasma Ca2+ (opposite trigger to PTH)

Actions

  • Bone: Directly inhibits osteoclast activity → decreases bone resorption → lowers plasma Ca2+
    • Rapid effect: reduces osteoclastic absorption
    • Prolonged effect: decreases formation of new osteoclasts
  • Kidney: Minor effects opposing PTH (↑ Ca2+ excretion)

Physiologic Role - Limited in Adults

Calcitonin has a weak and transient effect on plasma Ca2+ in adult humans for two reasons:
  1. Any drop in Ca2+ from calcitonin powerfully stimulates PTH, which overrides the calcitonin effect
  2. Daily calcium flux rates are small relative to the 1 g total ECF calcium
Calcitonin is more relevant in children (rapid bone remodeling) and in conditions like Paget disease (accelerated osteoclast activity).

6. The Calcium-Sensing Receptor (CaSR) - Central Sensor

The CaSR is a class C GPCR expressed on:
  • Parathyroid chief cells (primary regulator of PTH secretion)
  • Thyroid C cells (controls calcitonin secretion)
  • Renal tubular cells (directly regulates tubular Ca2+ handling)
  • Bone cells and intestinal epithelium
Loss-of-function mutations → Familial Hypocalciuric Hypercalcemia (FHH) - CaSR set-point shifted, PTH not suppressed at normal Ca2+ levels Gain-of-function mutations → Autosomal Dominant Hypocalcemia (ADH) - CaSR oversensitive, excess PTH suppression

7. FGF-23 and Phosphatonins (Indirect Role)

Fibroblast growth factor 23 (FGF-23), secreted by osteocytes, primarily regulates phosphate and vitamin D homeostasis:
  • Inhibits renal 1α-hydroxylase → ↓ calcitriol production → indirectly lowers intestinal Ca2+ absorption
  • Stimulates renal phosphate excretion (phosphaturia)
  • FGF-23 does not directly modify ECF calcium

8. Three Lines of Defense Against Calcium Perturbation

LineMechanismSpeed
1stExchange of calcium from amorphous bone salts (CaHPO4) - the "exchangeable pool"Seconds to minutes
2ndPTH feedback via CaSR - bone resorption, renal Ca2+ retention, ↑ calcitriolMinutes to hours
3rdChanges in vitamin D synthesis and intestinal absorptionHours to days

9. Pathophysiology Summary

DisorderMechanismCa2+PO4PTH
Primary hyperparathyroidismPTH-secreting adenoma↑↓↑
HypoparathyroidismPTH deficiency (post-surgical, autoimmune)↓↑↓
PseudohypoparathyroidismTarget organ resistance to PTH (Gs mutation)↓↑↑↑
Secondary hyperparathyroidismChronic kidney disease → ↓ calcitriol → ↓ Ca2+ → ↑ PTH↓↑↑
Hypercalcemia of malignancyPTHrP from tumor mimics PTH; bone metastases↑↓↓
Vitamin D toxicityExcess calcitriol → ↑ intestinal absorption + bone resorption↑↑↓
The classic mnemonic for primary hyperparathyroidism is "stones, bones, and groans": kidney stones (hypercalciuria), bone resorption, and constipation/abdominal pain.
Sources:
  • Costanzo Physiology, 7th Edition, pp. 453-456 (Overall Ca2+ homeostasis, PTH, Vitamin D)
  • Guyton and Hall Textbook of Medical Physiology, pp. 989-992 (PTH bone effects, calcitonin, Ca2+ control summary)
  • Brenner and Rector's The Kidney, pp. 796-798 (Whole-body Ca2+ homeostasis, PTH-Vitamin D system)
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