1. The objective of the study was to determine the effect of temperature reduction on the response of rat skeletal muscles to myotonia-inducing agents.

2. A model myotonia was induced in the muscles in vitro, using either the chloride channel blocker anthracene-9-carboxylic acid or chloride-free Krebs solution. This model is similar in its characteristics to the myotonia which occurs in autosomal recessive generalized myotonia congenita in humans.

3. Isometric twitch contractions were recorded in the muscles in Krebs solution before and after the addition of the myotonia-inducing agent. The presence of myotonia was confirmed when the half-relaxation time of the twitch contraction after the addition of the agent was significantly greater than that before its addition.

4. Recordings were made at 37°C, 30°C, 25°C and 15°C. Myotonia developed at 37°C, 30°C and 25°C, but not at 15°C, indicating that at a temperature between 25°C and 15°C, anthracene-9-carboxylic acid-induced myotonia failed to develop. This supports the results obtained in humans suffering from myotonia congenita where myotonic contractions in the adductor pollicis muscle disappeared when the muscle temperature was cooled to 20°C.

5. The myotonia which developed at 37°C could be significantly reduced by exposure to 1 × 10−4 mol/l ouabain or by elevation of the K+ concentration of the Krebs solution to 7.5 mmol/l.

6. Measurements made using microelectrodes showed that the conditions under which myotonia either did not develop or was significantly reduced, i.e. a temperature of 15°C, exposure to 7.5 mmol/l K+ at 37°C or exposure to 1 × 10−4 mol/l ouabain at 37°C, were each associated with membrane depolarization. The results are discussed in terms of a possible role for depolarization in preventing/reducing the myotonic response.

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