L19.2 Magnesium

Question 1

What is the EC: IC distribution of Mg, which organs is it found, where source of food does it come from

Answer 1

99% found in bone, muscle and soft tissue.
IC > EC, but not much bioavailable.

Mg comes from green leafy vegetables, nuts and seeds.

Question 2

How does Mg enter and exit cells. Which organ controls the serum Mg, which excretes the most Mg

Answer 2

Attached to active carrier proteins which strip off the water (Mg binds to so tightly can’t go through channels) and deliver via intracellular transport mechanisms.

Kidney excretion in urine controls serum Mg. Most Mg excretion is in faeces

Question 3

Where along the renal tubule is Mg reabsorbed and what percent, how much overall excreted out of majority filtered by glomeruli

Answer 3

-5% excreted in the urine

All reabsorption
PCT: 10-20%
TAL of LoH: 60-70%
DCT: 10%

Question 4

What are the main ways that Mg is reabsorbed along renal tubule

Answer 4

1.Paracellular pathway
?? driven by electrical gradient generated by Na+/H20 absorption
- PCT

2. Proteins forming a channel for active reabsorption of Mg in the lumen (through paracellular pathway)
   - TAL
3. Active transcellular transport mechanism (TRPM6)
   - DCT

Question 5

How is Mg assessed

Answer 5

1. serum Mg (not completely reflective of total Mg
2. RBC Mg: RBC turnover slow: so can be low even if serum Mg normal
3. Mg retention test: after oral load then
4. 24 hour excretion (needs 24hrs bc of circadian rhythm of excretion (spot test not good looking for excessive excretion).
5. Isotope analysis: search

Question 6

What are symptoms of Hypomagnesaemia - common and typically mild (5)

Answer 6

• Weakness/fatigue
• Fasciculations/cramps –> Tetany/ carpopedal spasm
• Progressive Numbness, Paresthesiae
• Seizures
• Arrhythmias

Question 7

What are the causes of hypomagnesaemia (4)

Answer 7

1. decreased dietary intake
2. GI malabsorption and loss
3. Endocrine - hyperaldosteronism, DM, SIADH, hungry bone syndrome
4. Renal loss through
   - congenital, acquired or drug induced kidney damage

Question 8

What are 3 drugs which are nephrotoxic.

What is treatment for hypomagnesaemia

Answer 8

Aminoglycoside, amphotericin B, omeprazole

Treatment: oral replacement or IV magnesium sulphate.

Question 9

What are the symptoms of Hypermagnesaemia (5)

Answer 9

• Hypotension, cutaneous flushing
• Nausea, vomiting

At higher concentrations: may lead to
- neuromuscular dysfunction: ranging from drowsiness to resp despression, SOB.

• Hypotonia, areflexia and coma in severe cases
• Cardiac abnormalities (arrhythmia)

Question 10

What are the causes of Hypermagnesaemia (not common) (4)

Answer 10

1. Advanced CKD: when compensatory mechanisms start to become inadequate
2. Excessive oral administration of magnesium salt or magnesium containing drugs (laxatives)
3. Hypothyroidism/ addisons disease
4. congential familial hypocalcuric hypercalcaemia