Fiche de révision : Calcium Homeostasis and Regulation

📋 Course Outline

1. Calcium functions and body distribution
2. Calcium pools and bone exchange dynamics
3. Calcium regulation by PTH, calcitriol and calcitonin
4. Vitamin D metabolism and activation steps
5. Phosphate regulation and renal handling
6. Parathyroid gland structure and PTH secretion control
7. Calcitonin and PTHrP roles in calcium homeostasis
8. Calcium homeostasis in birds and long-term deficits
9. Hypocalcaemia clinical cases and treatment
10. Calcium disorders and urolithiasis mechanisms

📖 1. Calcium functions and body distribution

🔑 Key Concepts & Definitions

• Calcium : Calcium is an essential ion used for muscle contraction, neuronal activity, exocytosis, coagulation, cell communication, and bone structure.
• Hydroxyapatite crystals : Hydroxyapatite crystals are bone mineral made of calcium, phosphate, and water that provide structural integrity.
• Intracellular calcium : Intracellular calcium is the pool inside cells, typically bound to proteins or stored in organelles like mitochondria and ER.
• Extracellular fluid calcium pool : The extracellular fluid (ECF) calcium pool controls blood calcium levels and supports rapid regulation.
• Blood calcium forms : Blood calcium exists in three forms: protein-bound, anion-complexed, and free ionised Ca2+.

📝 Essential Points

• About 99% of body calcium is stored in bone.
• Free ionised Ca2+ is 50% of blood calcium, while albumin-bound calcium is 40%.
• Calcium complexed to anions (e.g., carbonate, citrate) accounts for 10% of blood calcium.
• Intracellular calcium is kept low and is often protein-bound or sequestered in mitochondria/ER.
• Bone calcium is split into a stable mineral store and a more rapidly exchangeable soluble portion.

💡 Memory Hook

Bone holds ~99%; blood splits 40/10/50 into albumin/anion/free Ca2+.

📖 2. Calcium pools and bone exchange dynamics

🔑 Key Concepts & Definitions

• Readily exchangeable bone calcium : Readily exchangeable bone calcium is the small fraction (0.1–1%) that can rapidly exchange with the soluble bone portion.
• Slowly exchangeable bone calcium : Slowly exchangeable bone calcium is the large stable pool (~99%) that exchanges slowly via bone remodelling.
• Rapid exchange in bone fluid : Rapid exchange in bone fluid is movement of Ca2+ between the soluble bone portion and plasma when regulation is needed quickly.
• Stable bone pool : The stable bone pool is the slowly exchanging mineral store that buffers long-term calcium availability.
• Canaliculi between osteoblasts and osteocytes : Canaliculi are microchannels where soluble bone calcium resides and can be mobilised toward blood.

📝 Essential Points

• Rapid exchange involves Ca2+ movement in the soluble portion of bone that contains crystals plus soluble calcium.
• The rapidly exchangeable portion is located in canaliculi between osteoblasts and osteocytes.
• When plasma Ca2+ is low, PTH stimulates pumps in osteocytes to transport Ca2+ from rapid bone fluid to plasma.
• When plasma Ca2+ is low, PTH also increases osteoclast activity for the slow pool, with Ca2+ transport via a PTH-independent process.
• When plasma Ca2+ is high, the regulation reverses (opposite direction of transport and mobilisation).

💡 Memory Hook

Low Ca2+ → rapid Ca2+ via PTH pumps + slow Ca2+ via osteoclasts; high Ca2+ → opposite.

📖 3. Calcium regulation by PTH, calcitriol and calcitonin

🔑 Key Concepts & Definitions

• Parathyroid hormone (PTH) : PTH is a hormone that raises ECF calcium concentration by acting on bone, kidney, and indirectly the GI tract.
• Calcitriol : Calcitriol is the active vitamin D form that increases intestinal absorption of calcium.
• Calcitonin : Calcitonin is a hormone that lowers calcium and phosphate by reducing bone resorption and increasing renal excretion.
• PTH-sensitive receptors (CaSR) : CaSR are calcium-sensing receptors on chief cell membranes that detect ECF [Ca2+] and control PTH release.
• Negative feedback control : Negative feedback control is the mechanism where changes in calcium levels adjust hormone secretion to restore normal ECF [Ca2+].

📝 Essential Points

• PTH secretion responds within minutes to sudden changes in ECF [Ca2+].
• PTH acts directly on bone and kidney and indirectly on the GI tract.
• Calcitriol increases calcium uptake from the small intestine by promoting transcellular transport.
• Calcitonin decreases ion movement from bone pools to the ECF and inhibits osteoclasts.
• Calcitonin increases renal excretion of both calcium and phosphate.

💡 Memory Hook

PTH raises Ca2+; calcitriol boosts gut absorption; calcitonin brakes bone resorption and increases urinary loss.

📖 4. Vitamin D metabolism and activation steps

🔑 Key Concepts & Definitions

• Vitamin D2 : Vitamin D2 is a vitamin D form synthesised by plants.
• Vitamin D3 : Vitamin D3 is a vitamin D form synthesised by animals.
• 25-hydroxylase : 25-hydroxylase is the liver enzyme that adds the first hydroxyl group to vitamin D.
• 24-hydroxylase : 24-hydroxylase is the kidney enzyme that adds a second hydroxyl group when calcium levels are normal.
• 1α-hydroxylase : 1α-hydroxylase is the kidney enzyme stimulated by low calcium via PTH to produce active calcitriol.

📝 Essential Points

• Vitamin D activation requires adding hydroxyl (OH) groups.
• Step 1 in the liver: 25-hydroxylase converts vitamin D to calcidiol (25-hydroxylated form), which is inactive storage.
• Step 2 in the kidney: when calcium levels are normal, 24-hydroxylase produces 24,25(OH)2-vitamin D, which is inactive and excreted.
• When calcium levels are low, PTH stimulates 1α-hydroxylase to produce calcitriol (1,25(OH)2-vitamin D).
• Calcitriol increases transcellular calcium absorption by upregulating apical Ca2+ channels, basolateral pumps, and calbindin-D9K.

💡 Memory Hook

Liver makes calcidiol; kidney decides: normal Ca2+ → 24-hydroxylase off-ramp, low Ca2+ + PTH → 1α-hydroxylase to calcitriol.

📖 5. Phosphate regulation and renal handling

🔑 Key Concepts & Definitions

• Phosphate : Phosphate is an inorganic blood component that buffers pH and supports bone/teeth structure, membranes, DNA synthesis, and ATP.
• Renal phosphate regulation : Renal phosphate regulation is the kidney-based control of phosphate reabsorption and excretion to match dietary intake and body needs.
• Renal threshold : The renal threshold is the filtered phosphate level above which the kidney cannot reabsorb all phosphate, leading to urinary loss.
• PTH effect on phosphate : PTH effect on phosphate is the hormone-driven reduction of renal phosphate reabsorption to increase phosphate excretion.
• Na/P transporters : Na/P transporters are renal transport proteins whose activity is decreased by PTH to lower phosphate reabsorption.

📝 Essential Points

• Phosphate is mainly regulated in the kidney.
• Renal tubules reabsorb 80–90% of filtered phosphate.
• If dietary phosphate intake increases beyond the renal threshold, phosphate is excreted in urine.
• PTH decreases renal reabsorption of phosphate, increasing phosphate excretion.
• PTH decreases Na/P transporters to reduce phosphate uptake.

💡 Memory Hook

PTH is the phosphate “release switch”: less reabsorption via Na/P transporter down → more urinary phosphate.

📖 6. Parathyroid gland structure and PTH secretion control

🔑 Key Concepts & Definitions

• Parathyroid gland : The parathyroid gland is the main organ controlling central calcium and phosphate metabolism.
• Chief or principal cells : Chief cells are the parathyroid cell type that secretes PTH when ECF [Ca2+] is low.
• Dark active cells : Dark active cells are the parathyroid cells with higher PTH secretory activity.
• Oxyphil cells : Oxyphil cells are a parathyroid cell type that is absent in most veterinary species but present in humans.
• PTH secretion by exocytosis : PTH secretion by exocytosis is the rapid release mechanism used by chief cells to respond to calcium sensing.

📝 Essential Points

• The parathyroid gland consists of four small nodules associated with the thyroid gland.
• Darkly staining active chief cells secrete PTH, while lighter staining inactive cells increase with age.
• PTH is secreted by exocytosis.
• PTH is rapidly metabolised by the liver and kidneys.
• ECF [Ca2+] is detected by CaSR on chief cell membranes, and normal [Ca2+] yields a moderate PTH level.

💡 Memory Hook

CaSR on chief cells: normal Ca2+ → moderate PTH; sudden Ca2+ shifts → minutes-fast PTH change.

📖 7. Calcitonin and PTHrP roles in calcium homeostasis

🔑 Key Concepts & Definitions

• Calcitonin : Calcitonin is a thyroid hormone that reduces calcium and phosphate by limiting bone resorption and increasing renal excretion.
• PTHrP : PTHrP is a PTH-like peptide produced in many tissues that binds the PTH receptor and can also bind other receptors.
• Parafollicular cells (C-cells) : Parafollicular cells, also called C-cells, are the thyroid cells that produce calcitonin.
• PTH receptor binding : PTH receptor binding is the shared receptor interaction that allows PTHrP to mimic some PTH effects.
• No negative feedback hypercalcaemia : No negative feedback hypercalcaemia describes how PTHrP-driven calcium elevation can persist because the usual feedback control is absent.

📝 Essential Points

• Calcitonin is produced by parafollicular (C) cells in the thyroid gland.
• Calcitonin reduces movement of ions from bone pools to the ECF and inhibits osteoclasts.
• Calcitonin increases renal excretion of calcium and phosphate.
• PTHrP binds the same receptor as PTH (a G-protein coupled receptor) so it can produce similar effects.
• Some cancers produce PTHrP causing hypercalcaemia paraneoplastic syndrome without negative feedback, and serum PTHrP can aid differential diagnosis.

💡 Memory Hook

Calcitonin = thyroid brake; PTHrP = PTH-like signal from tissues/cancers, and it can run without feedback → hypercalcaemia.

📖 8. Calcium homeostasis in birds and long-term deficits

🔑 Key Concepts & Definitions

• Eggshell calcium carbonate : Eggshell calcium carbonate is the dominant eggshell mineral, providing most of the calcium used for shell formation.
• Medullary bone : Medullary bone is a calcium storage tissue that accumulates before laying to supply calcium for eggshell production.
• PTH and calcitriol buffering : PTH and vitamin D (calcitriol) are key hormones that help correct calcium and phosphate balance when intake is low.
• Hypocalcaemia clinical signs : Hypocalcaemia clinical signs are neuromuscular problems such as inappetence, ataxia, paresis, and tetany.
• Stable vs exchangeable bone pools : Stable and exchangeable bone pools provide different rates of calcium release to buffer ECF calcium over time.

📝 Essential Points

• Eggshell is 97% calcium carbonate and is produced in the shell gland.
• Yolk and egg white contain high phosphate, so phosphate availability is high in eggs.
• Eggshells account for 10% of skeletal calcium.
• Medullary bone accumulates in pullets before egg-laying to increase the available bone calcium pool.
• If calcium loss from ECF exceeds what bone pools can supply, hypocalcaemia develops with inappetence, ataxia, paresis, and tetany.

💡 Memory Hook

Birds: pre-load medullary bone; shell needs CaCO3 (97%) and draws ~10% of skeletal calcium.

📖 9. Hypocalcaemia clinical cases and treatment

🔑 Key Concepts & Definitions

• Hypoparathyroidism : Hypoparathyroidism is a condition where inadequate PTH leads to low calcium levels in the ECF.
• Iatrogenic hypoparathyroidism : Iatrogenic hypoparathyroidism is hypoparathyroidism caused by surgical removal of parathyroid tissue during thyroid surgery.
• Intact-PTH assay : An intact-PTH assay measures circulating PTH to help interpret hypocalcaemia causes.
• Intravenous calcium gluconate : Intravenous calcium gluconate is an acute treatment that rapidly raises calcium levels in hypocalcaemia.
• Intravenous calcium borogluconate : Intravenous calcium borogluconate is used to treat hypocalcaemia in cattle (milk fever).

📝 Essential Points

• Cornelius had convulsions and muscle tremors with bradycardia and an irregular rhythm after thyroidectomy.
• Hypoparathyroidism is indicated when calcium is low and intact PTH is absent/low.
• The most common cause described is iatrogenic removal of parathyroid tissue during surgery for hyperthyroidism.
• Treatment for Cornelius started with IV calcium gluconate, then moved to oral calcium plus vitamin D3.
• In cattle (milk fever), treatment is IV calcium borogluconate and prevention involves maintaining ideal BCS (2.3–3/5) and restricting calcium.

💡 Memory Hook

Low Ca2+ + low/absent intact PTH → hypoparathyroidism; treat fast with IV calcium, then support with oral calcium/vitamin D3.

📖 10. Calcium disorders and urolithiasis mechanisms

🔑 Key Concepts & Definitions

• Urolithiasis : Urolithiasis is the presence of calculi (uroliths) in the ureters, bladder, or urethra.
• Calculi composition : Calculi are polycrystalline structures formed from precipitated urinary solutes and proteins.
• Excess dietary calcium : Excess dietary calcium is a risk factor proposed for formation of some urinary crystals.
• Acidic urine : Acidic urine is proposed to contribute to formation of certain types of calcium-related crystals.
• Calcium oxalate uroliths : Calcium oxalate uroliths are a calcium crystal type and are described as the second most common after struvite.

📝 Essential Points

• Uroliths can obstruct the ureters, bladder, or urethra and cause urinary straining and haematuria.
• Uroliths are polycrystalline structures made from precipitated urinary solutes/proteins.
• Excess dietary calcium combined with acidic urine is thought to contribute to formation of some crystal types.
• Calcium oxalate is the second most common urolith type after struvite.
• Calcium carbonate is another calcium crystal type mentioned as forming uroliths.

💡 Memory Hook

Urolithiasis = precipitated solutes/proteins; risk: excess Ca + acidic urine → calcium oxalate (2nd after struvite).

📊 Synthesis Tables

Birds vs general buffering of calcium

⚠️ Common Pitfalls & Confusions

1. Mixing up blood calcium fractions: only free ionised Ca2+ is 50%, while albumin-bound is 40% and anion-complexed is 10%.
2. Assuming all bone calcium is equally available: only 0.1–1% is readily exchangeable while ~99% is slowly exchangeable.
3. Confusing vitamin D activation steps: calcidiol is inactive storage from liver 25-hydroxylation, while calcitriol is active from kidney 1α-hydroxylation.
4. Thinking calcitonin raises calcium: it lowers calcium and phosphate by inhibiting osteoclasts and increasing renal excretion.
5. For hypocalcaemia, forgetting the diagnostic logic: low calcium plus low/absent intact PTH points to hypoparathyroidism rather than just “low diet”.
6. Overgeneralising milk fever as simple dietary deficiency: the described mechanism is failure to meet sudden calcium demand, even with high late-pregnancy calcium diets.

✅ Exam Checklist

1. List major calcium functions and identify where ~99% of body calcium is stored.
2. Describe the three blood calcium forms and their approximate percentages.
3. Explain the two bone calcium pools (readily vs slowly exchangeable) and where rapid exchange occurs.
4. State how low vs high plasma calcium changes PTH-driven bone transport for rapid and slow pools.
5. Describe how PTH, calcitriol, and calcitonin act on bone, kidney, and GI tract to regulate ECF calcium.
6. Reconstruct vitamin D activation: vitamin D2 vs D3, liver 25-hydroxylase, kidney 24-hydroxylase vs PTH-stimulated 1α-hydroxylase.
7. Explain calcitriol’s mechanism of increasing intestinal calcium absorption via nuclear VDR and specific transport proteins.
8. Summarise phosphate regulation: kidney as main site, 80–90% reabsorption, renal threshold, and PTH decreasing Na/P transporters.
9. Describe parathyroid gland structure (four nodules) and cell types, including which cells secrete PTH and how CaSR controls secretion.
10. Explain calcitonin’s source (C-cells) and effects on bone resorption and renal excretion.
11. Explain PTHrP: receptor binding similarity to PTH, extra receptor diversity, and the paraneoplastic hypercalcaemia mechanism without negative feedback.
12. For birds, state eggshell calcium carbonate composition, yolk/egg phosphate context, and the role of medullary bone before laying.
13. For long-term calcium deficits, link excessive ECF calcium loss to hypocalcaemia signs (inappetence, ataxia, paresis, tetany).
14. Use the Cornelius case logic to interpret hypocalcaemia with intact-PTH assay results and state the treatment sequence (IV calcium gluconate then oral calcium + vitamin D3).