Drug‐induced myotonia in human intercostal muscle

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Abstract
The myotonia‐inducing effects of furosemide and clofibrate, two widely used pharmaceutical agents, were investigated in excised human external intercostal muscle. The effects of anthracene‐9‐carboxylic acid (9‐AC), a well‐known myotonia‐producing chemical, were also tested for comparison. In the presence of these drugs the electrical threshold was lowered, and a constant current pulse produced multiple spiking. Short trains of direct stimuli were often followed by after‐activity, and this caused a myotonia‐like prolongation of muscle contraction. Voltage‐clamp experiments showed that 0.05 mM anthracene‐9‐carboxylic acid, 1 mM furosemide, and 1 mM clofibrate decreased the chloride conductance of the muscle fiber membrane to 14, 18, and 40%, respectively, of the normal value, and the myotonia‐inducing potency of the 3 drugs was correlated with the decreased chloride conductance. The potassium currents were not affected by these compounds.