I1iii: Outline the distribution, clearance & physiological functions of Mg2+ in the body

Defintion

Cation; chemical element. Atomic no. 12

Quantify

• Major intracellular cation
  • ECF7 – 1.0mmol/L
  • ICF 5 – 20 mmol/L
• Body total ~24g

Functions

CO-Factor

• • >300 enzyme reactions
  • Na/K/ATPase
  • Generation of cAMP

Neuromuscular

• “Antagonises Ca²⁺” (ACh,NA from nerve terminals)
• ↓transmitter release
• ↓nerve & muscle membrane excitability
• Inhibits E-C coupling in skeletal & cardiac m.
• Relaxes smooth m.

CVS

• • Depresses myocardial contractility
  • Inhibits catecholamine release
  • Systemic & coronary VD
  • Anti-arrhythmic:
    • ↓rate impulse formation from SA Node
    • Prolongs SA conduction
    • Prolongs AV node refractory period

Nervous System

• ↓ACh release @ NMT
• Blocks NMDA receptor
• Anticonvulsant
• Reverses cerebral vasospasm

MSK

• • Terminates contraction
  • Initiates skeletal m. relaxation
  • ↑↑↑Mg²⁺ = muscle weakness

Resp

• Bronchodilation

GU

• • Powerful tocolysis
  • Mild diuresis

HAEM

• ↓platelet activity

Control of Regulation

• Plasma Mg²⁺
  • PTH
  • ALDOSTERONE

Regulation

• Balance: intestinal absorption v. renal excretion

Absorption

• SI →paracellular by electrochem. gradient & solvent drag
• LI → (small amount)

Excretion

• • Circadian rhythm (mainly at night)
  • Freely filtered by kidneys (= eGFR x Plasma Mg²⁺)
  • 95% reabsorbed: TAL > PCT > Distal Tubules
    • This is why loop diuretics cause ↓Mg²⁺
  • 3 – 5% excreted
  • Reabs. regulated by plasma Mg²⁺
  • ↑Aldosterone = ↑Mg²⁺ excretion
  • ↓Mg²⁺ = ↑PTH = releases Mg²⁺ from bone