Potassium and caffeine contractures in limb muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice.

The strength of contractures, produced by 15 to 146 mM [K]0 (as L-glutamate), was measured in isolated small bundles of muscle fibers from the fast-twitch extensor digitorum longus and from the slow-twitch soleus of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice. The analysis of the relation between the maximal amplitude of the contracture vs the membrane potential and the time constant of relaxation of the K-contractures has shown that dystrophy induced an attenuation of the differences between fast- and slow-twitch muscles. The repriming of K-contractures was more affected by changes in [Ca]0 in normal soleus than in normal extensor digitorum longus and this difference was unaffected by dystrophy. For both types of muscles, the ability of caffeine to produce contractures was reduced in dystrophic muscle and this modification was not related to a change in the fiber typing.