23B13: Exam Report

Outline the distribution, clearance and physiologic functions of magnesium in the body

36% of candidates passed this question.

This question was best answered under the headings distribution, clearance and physiologic functions.

Distribution involved intracellular vs extracellular concentrations, the spread amongst organ systems and state of ionisation and protein binding. Clearance of magnesium required an accurate description of its renal filtration and sites and proportion of reabsorption and secretion along the nephron.

The regulatory factors and factors that influence this clearance should also be outlined. This included; Mg plasma concentrations, other cations, ECF volume and PTH.

Physiologic functions should cover its role as a cofactor of metabolism and enzyme systems with some examples, the role and mechanism in the musculoskeletal system as a calcium antagonist and inhibitory action in the nervous system including the action against Ach, nerves and NMDA activity.

I1iii / 23B13: 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