Calcium and Phosphate Handling

Ca & PO4 Basics - The Dynamic Duo

• Calcium ($Ca^{2+}$):
  • Total plasma: 9-11 mg/dL (2.2-2.7 mmol/L).
  • Forms: Ionized (~50%, active), albumin-bound (~40%), complexed (citrate, phosphate, ~10%).
  • Key Functions: Bone/teeth mineralization, muscle contraction, nerve impulse, blood coagulation.
  • Major Regulators: Parathyroid Hormone (PTH), Vitamin D, Calcitonin.
• Phosphate ($PO_4^{3-}$):
  • Plasma: 2.5-4.5 mg/dL (0.8-1.45 mmol/L).
  • Key Functions: Bone/teeth structure, energy (ATP), DNA/RNA, acid-base buffer.
  • Major Regulators: PTH, Vitamin D, Fibroblast Growth Factor 23 (FGF-23).
• Key Interplay:
  • Often reciprocal: PTH $\uparrow Ca^{2+}$ reabsorption, $\downarrow PO_4^{3-}$ reabsorption in kidney.
  • $Ca \times PO_4$ product: If > 55 mg²/dL², risk of ectopic calcification.

⭐ Ionized calcium (~50% of total) is the biologically active form and its levels are meticulously controlled.

Calcium's Kidney Journey - Tight Leash Calcium

• Overall: ~99% filtered $Ca^{2+}$ reabsorbed.
• Segments & % Reabsorption:
  • PCT: 60-70% (mainly paracellular, solvent drag).
  • TAL: 20-25% (paracellular via claudins; transcellular).
  • DCT: 5-10% (active, transcellular; main regulatory site).
    • Apical: TRPV5 channels.
    • Intracellular: Calbindin-D28K.
    • Basolateral: PMCA ($Ca^{2+}$-ATPase), NCX1 ($Na^{+}/Ca^{2+}$ exchanger).
  • CD: <1%.
• Key Regulators (DCT/TAL):
  • ↑ Reabsorption:
    • PTH (↑ TRPV5, calbindin, PMCA).
    • Vitamin D (Calcitriol) (↑ TRPV5, calbindin).
    • Thiazide diuretics (📌 Thia-HIDE $Ca^{2+}$: ↑ reabsorption).
    • Hypocalcemia, Alkalosis.
  • ↓ Reabsorption:
    • Hypercalcemia (via CaSR).
    • Loop diuretics (↓ TAL paracellular transport).
    • Acidosis.
    • Calcitonin (minor effect).

⭐ Thiazide diuretics enhance calcium reabsorption in the DCT, beneficial for preventing recurrent calcium kidney stones.

Phosphate's Path - Phosphorus Freeway

• Primary Reabsorption Sites: Proximal Convoluted Tubule (PCT) (~80%), Distal Convoluted Tubule (DCT).
• PCT Mechanisms:
  • Apical Na⁺-Phosphate cotransporters: NaPi-IIa, NaPi-IIc.
  • Secondary active transport, dependent on Na⁺/K⁺-ATPase.
  • Process is saturable (Tm-limited).
• Key Regulators:
  • PTH (Parathyroid Hormone): ↓ Phosphate reabsorption (internalizes NaPi-IIa/c in PCT) → phosphaturia.
  • FGF-23 (Fibroblast Growth Factor 23): ↓ Phosphate reabsorption (↓ NaPi-IIa/c expression in PCT) → phosphaturia.
  • Vitamin D (Active - Calcitriol): ↑ Phosphate reabsorption (stimulates NaPi-IIa/c expression).
  • Dietary Phosphate: ↑ intake → ↓ reabsorption; ↓ intake → ↑ reabsorption.
  • Insulin: ↑ reabsorption.
  • Acidosis: ↓ reabsorption; Alkalosis: ↑ reabsorption.
• Excretion: Primarily renal; matches net intestinal absorption.

⭐ PTH and FGF-23 are the principal phosphaturic hormones, significantly decreasing renal phosphate reabsorption, primarily in the PCT.

Hormonal Harmony - The Calcium Commandos

Three main hormones precisely control renal Ca²⁺ & PO₄³⁻ levels: PTH, Vitamin D, and Calcitonin. Their interplay is crucial.

• Parathyroid Hormone (PTH): Secreted by parathyroid glands.
  • Kidney Actions:
    • ↑ Ca²⁺ reabsorption (mainly DCT, TAL).
    • ↓ PO₄³⁻ reabsorption (PCT) → phosphaturia.
    • ↑ $1\alpha$-hydroxylase activity → ↑ active Vitamin D ($1,25(OH)_2D_3$).
• Vitamin D (Calcitriol - $1,25(OH)_2D_3$): Steroid hormone, activated in kidney.
  • Kidney Actions: ↑ Ca²⁺ reabsorption & ↑ PO₄³⁻ reabsorption (mainly PCT, DCT).
• Calcitonin: Secreted by thyroid C-cells (parafollicular cells).
  • Kidney Actions: Weakly ↓ Ca²⁺ & ↓ PO₄³⁻ reabsorption.
  • 📌 Calcitonin tones down serum Ca²⁺.

⭐ PTH's major effect on phosphate is to cause phosphaturia by inhibiting Na⁺/PO₄³⁻ cotransporters in the PCT.

High‑Yield Points - ⚡ Biggest Takeaways

• PTH ↑ Ca²⁺ reabsorption (DCT, TAL) & ↓ PO₄³⁻ reabsorption (PCT).
• Vitamin D (Calcitriol) ↑ Ca²⁺ & PO₄³⁻ reabsorption across PCT & DCT.
• FGF-23 ↓ PO₄³⁻ reabsorption (PCT) & ↓ 1α-hydroxylase activity (↓ calcitriol).
• PCT reabsorbs the majority: ~65% Ca²⁺ & ~80% PO₄³⁻.
• TAL reabsorbs ~25% Ca²⁺ paracellularly, driven by Na⁺-K⁺-2Cl⁻ cotransporter.
• DCT fine-tunes Ca²⁺ reabsorption (~8%), regulated by PTH & Vitamin D.