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.

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