8/8/2019 )3(Magnesium

1/3

Mg Dr. Hamed Khouja

Magnesium

Physiologic Role & Function

Factors influence Mg blood concentration

Clinical significant

Method of determination

Physiologic Role & Function

Second Most abundant intracellular cation after K

~ 65% of Mg is in the bone and teeth (structural role)

~ 34% intracellular fluids (co-factor for enzymes and essential for cell functions)

~ 1% extracellular fluids

Mg in plasma:

o Makes approximately 1% of the whole body Mg which is divided into;

22-27% is bound to: Albumin

8% is bound to other globulins

1-5% is complexed with citrate, phosphate, oxalate, carbonate

65-70% is free form (ionized) (Mg 2+)

Mg is an important co-factor in reactions that require ATP as a source of

energy

It is an essential cation for many biological systems in the body

Carbohydrate metabolism

Nerve conduction

Neuromuscular contraction

Blood coagulation

It is a co-factor for over 300 enzymes in the body such as Na, K-ATPase, CK,

ALP, ACP

It is essential for maintaining the macrormolecular structure of RNA, DNA

and Ribosomes Its concentration in RBCs is 10 times than in plasma

Dietary sources for Mg include; whole grain cereals, legumes, milk, nuts,

bananas, leafy vegetables

Factors Regulating the concentration of Mg.

Regulation of circulating levels of Mg is not well understood

Effects of pH:

Decrease pH increase Mg2+ and decrease bound Mg total Mg not changed

Increase pH decrease Mg2+ and increase bound Mg total Mg not changed

Effects of Protein

1

8/8/2019 )3(Magnesium

2/3

Mg Dr. Hamed Khouja

-affects bound and tMg , does not affect Mg2+

- hyperproteinaemia-increased bound Mg, increased tMg no change in Mg2+

- hypoproteinaemia- decreased bound Mg, decreased tMg no change in Mg2+

PTH Increase tubular reabsorption of Mg

Thyroid hormones, calcitonin, GH, hyperaldosteronism Decrease renal reabsorption of MgIncrease Na+, Increase Ca. Alcohol ingestion Decrease renal reabsorption of Mg

Clinical significance

A) Hypomagnesaemia

Associated with: - Chronic diarrhea -malabsorption - hepatic cirrhosis -Pancratitis

-ulcerative colitis -Hemodialysis - - alcoholism -osmotic diuresis

-diuretic drugs -hypercalcemia -infants on high Pi drink -Insulin administration

-hyperaldosteronism -hypoparathyroidism -hyperthyroidism

Symptoms

Similar to hypocalcaemia and includes; weakness, sever neuromuscular irritability (tetany)

- hyperfiexia -Convulsion - disorientation -cardiac arrthymias

B) Hypermagnesaemia

Associated with: -Oliguria - dehydration -Addisons disease

- diabetic acidosis -Chronic renal filer -hypothyroidism

-ingestion of Mg containing gastroenteric medications (Milk of Magnesia)

Symptoms

-Depressed neuromuscular activity (sedation) -Depressed. cardiac activity

-Nausea -Drowsiness -Hypotension -Respiratory depression

-Cardiac arrest -death

Determination of Mg

Sample: fasting serum. Li heparin plasma (separate immediately after collection)

Do not use EDTA, Citrate or Oxalate tubes

Avoid hemolysis (Increase in Mg)

Avoid prolonged tourniquet application

Separate serum as soon as possible to avoid haemolysis and leaksge of Mg from

RBCs

Urine, 24h acidified with 6M HCl

Avoid metal containing container

Methods

1) Atomic absorption spectrophotormetry (AAS) [reference method]

2

8/8/2019 )3(Magnesium

3/3

Mg Dr. Hamed Khouja

Principle:

Same as in Ca determination -Sensitive, specific, easy to apply -Wave length 285.2 nm

Interference:1) Phosphate: Use Lanthanum chloride solution to remove Pi (1/50 dil of the sample

and control with lanthanum chloride solution)

2) Protein: deproteinize serum or plasma with trichloroacetic acid, centrifuge and use

the supernatant.

Urine by AAS. dilute 1/50 Lanthanum chloride solution for urine.

2) Colorimetric Methods

Mg forms coloured complexes with some dyes such as; Titan Yellow dye, Methylthymol

blue (MTB), Calmagite, Magon, Erichrome, Black T.

3) Complexometric method

8-hydroxy quinoline has high affinity for Mg

-EDTA titration

Major Precautions with methods 2&3:

In both the colorimetric and complexometric procedures,

o Ca is the major interfering substance as it also react with the Mg agents

o Ba-EGTA is used in these procedure to preferentially chelate Ca and prevents

its interference.

The linearity of the measurement is checked with a range of Mg standards because

the reaction obeys Beers Law only on a short range of Mg concentration.

AAS method for Mg is preferred to other methods because it offers better accuracy

and precision.

Normal Range by AAS

Adult serum 0.65-1.05 mmol/L

Urine 3-5 mmol/day

3