A simple and rapid method for the determination of the concentrations of magnesium, sodium, potassium and sodium, potassium pumps in human skeletal muscle

1. For the diagnosis of electrolyte disorders, data on skeletal muscle composition are often valuable, but rarely available. We have therefore developed a simple and rapid needle biopsy procedure for the determination of the concentrations of K⁺, Na⁺, Mg²⁺ and Na⁺, K⁺-pumps in muscle.

2. Using a Bergström needle, biopsies weighing around 25 mg were taken from the vastus lateralis muscle of 18 normal subjects (aged 44–86 years) and extracted with trichloroacetic acid (TCA). The concentrations of K⁺, Na⁺ and Mg²⁺ were 90.7 ± 1.8, 31.9 ± 1.6 and 9.5 ± 0.2 μmol/g wet wt., respectively (means ± se).

3. The TCA extraction gave the same values as digestion with 65% HNO₃ or 35% H₂O₂, could be used over the weight range 10–55 mg and showed a Mg²⁺ recovery of 101.7%.

4. The concentration of Na⁺, K⁺-pumps was quantified as the total capacity for [³H]ouabain binding. In vastus lateralis biopsies obtained from six normal subjects the mean value was 258 ± 16 pmol/g wet wt.

5. Comparison of the concentrations of K⁺, Mg²⁺ and [³H]ouabain-binding sites in samples obtained post mortem showed modest variation among different muscles with varying fibre composition.

6. Measurements of the concentrations of K⁺, Na⁺, Mg²⁺ and Na⁺, K⁺-pumps in duplicate biopsies of the vastus lateralis yield values which seem representative for the total pool of skeletal muscle fibres and can be performed within a few hours.

7. Measurements on patients receiving chronic diuretic treatment demonstrated that these procedures allow the detection of reduced levels of K⁺, Mg²⁺ and Na⁺, K⁺-pumps in skeletal muscle, as well as a partial recovery after oral Mg²⁺ supplementation.