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 degrees C, 30 degrees C, 25 degrees C and 15 degrees C.
Myotonia developed at 37 degrees C, 30 degrees C and 25 degrees C, but not at 15 degrees C, indicating that at a temperature between 25 degrees C and 15 degrees 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 degrees C. 5. The
myotonia which developed at 37 degrees C could be significantly reduced by exposure to 1 x 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 degrees C, exposure to 7.5 mmol/l K+ at 37 degrees C or exposure to 1 x 10(-4) mol/l
ouabain at 37 degrees 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.