Disorders of Magnesium Metabolism

Magnesium Biological Importance & Function

Intracellular Role: The second most common intracellular cation after potassium.
Enzymatic Cofactor: Required for >300 enzymatic reactions involving ATP, including glycolysis and oxidative phosphorylation.
Cellular Mechanics:
- Critical for the Na+/K+-ATPase pump.
- Essential for synthesis of RNA, DNA, and protein.
Neuromuscular & Cardiac Regulation:
- Stabilizes membranes and facilitates nerve conduction.
- Regulates calcium movement into smooth muscle, affecting cardiac contractile strength and vascular tone.
- Low concentrations accelerate the transmission of nerve impulses.

Homeostasis & Distribution:

Body Distribution:
- Bone: 50%–60% of total body Mg.
- Soft Tissues: 40%–50%.
- Extracellular Fluid: <1% (meaning plasma levels do not accurately reflect total body stores).
Regulation: Unlike many other minerals, Mg is not under direct hormonal control.
Key Drivers: Balance is determined by GI absorption, renal excretion, and physiological demands (pregnancy, lactation, growth).

Species-Specific Absorption:

Ruminants:
- Main Site: Rumen (via active transport).
- Efficiency: Relatively low (35% of intake).
- Susceptibility: More prone to deficiency due to anatomic and physiologic differences.
Non-ruminants:
- Main Site: Small intestine.
- Efficiency: Higher than ruminants (70% of intake).

Hypomagnesemia (Magnesium Deficiency):

Contributing Factors: Chronic dietary deficiency, acute illness, anorexia, lactation, and stress (e.g., transport).
High-Risk Groups: Lactating beef cattle on winter pastures.
Clinical Signs:
- Neuromuscular: Muscle weakness, fasciculations (twitching), seizures, ataxia, and belligerent behavior.
- Cardiac: Ventricular arrhythmias.
- Critical: Coma and sudden death.

Hypermagnesemia (Magnesium Excess)

Cause: Primary cause is oversupplementation.
Clinical Signs: Can lead to secondary hypocalcemia, weakness, cardiac arrhythmia, and death.

Diagnostics:

Serum Testing:
- Total Magnesium (tMg): Divided into ionized, protein-bound, and chelated forms.
- Ionized Magnesium (iMg2+): The physiologically active (free) fraction. It must be measured directly and cannot be accurately calculated from tMg.
Urine Assessment:
- Fractional Clearance: Uses spot samples of urine and serum to assess renal excretion without timed collections.
The "Gold Standard": IV Magnesium Retention Test. Deficient animals retain a high proportion of an administered dose, while healthy animals excrete most of it.

Therapeutic Uses in Horses:

Cathartic: High dosages used to treat large-intestinal impactions.
Neurological: Lower dosages shown to decrease trigeminal-mediated head shaking.
Behavioral: Anecdotally used as a calming agent.

Hypermagnesemia

Hypomagnesemic Tetany in Cattle and Sheep

Magnesium as an Equine Dietary Supplement

Subclinical Hypomagnesemia in Critically Ill Animals

Disorders of Magnesium Metabolism

Magnesium Biological Importance & Function

Intracellular Role: The second most common intracellular cation after potassium.

Enzymatic Cofactor: Required for >300 enzymatic reactions involving ATP, including glycolysis and oxidative phosphorylation.

Cellular Mechanics:

Critical for the Na+/K+-ATPase pump.

Essential for synthesis of RNA, DNA, and protein.

Neuromuscular & Cardiac Regulation:

Stabilizes membranes and facilitates nerve conduction.

Regulates calcium movement into smooth muscle, affecting cardiac contractile strength and vascular tone.

Low concentrations accelerate the transmission of nerve impulses.

Homeostasis & Distribution:

Body Distribution:

Bone: 50%–60% of total body Mg.

Soft Tissues: 40%–50%.

Extracellular Fluid: <1% (meaning plasma levels do not accurately reflect total body stores).

Regulation: Unlike many other minerals, Mg is not under direct hormonal control.

Key Drivers: Balance is determined by GI absorption, renal excretion, and physiological demands (pregnancy, lactation, growth).

Species-Specific Absorption:

Ruminants:

Main Site: Rumen (via active transport).

Efficiency: Relatively low (35% of intake).

Susceptibility: More prone to deficiency due to anatomic and physiologic differences.

Non-ruminants:

Main Site: Small intestine.

Efficiency: Higher than ruminants (70% of intake).

Hypomagnesemia (Magnesium Deficiency):

Contributing Factors: Chronic dietary deficiency, acute illness, anorexia, lactation, and stress (e.g., transport).

High-Risk Groups: Lactating beef cattle on winter pastures.

Clinical Signs:

Neuromuscular: Muscle weakness, fasciculations (twitching), seizures, ataxia, and belligerent behavior.

Cardiac: Ventricular arrhythmias.

Critical: Coma and sudden death.

Hypermagnesemia (Magnesium Excess)

Cause: Primary cause is oversupplementation.

Clinical Signs: Can lead to secondary hypocalcemia, weakness, cardiac arrhythmia, and death.

Diagnostics:

Serum Testing:

Total Magnesium (tMg): Divided into ionized, protein-bound, and chelated forms.

Ionized Magnesium (iMg2+): The physiologically active (free) fraction. It must be measured directly and cannot be accurately calculated from tMg.

Urine Assessment:

Fractional Clearance: Uses spot samples of urine and serum to assess renal excretion without timed collections.

The "Gold Standard": IV Magnesium Retention Test. Deficient animals retain a high proportion of an administered dose, while healthy animals excrete most of it.

Therapeutic Uses in Horses:

Cathartic: High dosages used to treat large-intestinal impactions.

Neurological: Lower dosages shown to decrease trigeminal-mediated head shaking.

Behavioral: Anecdotally used as a calming agent.