Cable parameters, sodium, potassium, chloride, and water content, and potassium efflux in isolated external intercostal muscle of normal volunteers and patients with myotonia congenita

Abstract

In isolated fiber bundles of external intercostal muscle from each of 13 normal volunteers and each of 6 patients with myotonia congenita, some or all of the following were measured: concentrations of Na⁺, K⁺, and Cl^-, extracellular volume, water content, K⁺ efflux, fiber size, fiber cable parameters, and fiber resting potentials. Muscle from patients with myotonia congenita differed significantly (0.001 <P< 0.025) with respect to the following mean values (myotonia congenita vs. normal): the membrane resistance was greater (5729 vs. 2619 ω·cm²), the internal resistivity was less (75.0 vs. 123.2 ω·cm), the water content was less (788.2 vs. 808.2 ml/kg wet weight), and the mean resting potential was greater (68 vs. 61 mv). No significant differences were found with respect to the following variables: K⁺ content (73.5 vs. 66.7 mEq/kg wet weight) and the calculated intracellular K⁺ concentration (215 vs. 191 mEq/liter fiber water), fiber capacitance (5.90 vs. 5.15 μf/cm²), Na⁺ content (97.7 vs. 94.1 mEq/kg wet weight), Cl^- content (79.0 vs. 74.7 mEq/kg wet weight), mannitol extracellular volume (45.1 vs. 46.6 cc/100 g wet weight), and K⁺ efflux (23.2 vs. 21.5 moles × 10^-12 cm^-2·sec^-1). These abnormalities of skeletal muscle in human myotonia congenita are like those of skeletal muscle in goats with hereditary myotonia. We tentatively conclude that a decreased Cl^- permeability accounts for some of the abnormal electrical properties of skeletal muscle in myotonia congenita.