Na⁺-Ca²⁺ EXCHANGE IN LIMB MUSCLES OF DYSTROPHIC (C57 BL/6J dy^2J/dy^2J) MICE

¹ Laboratory of General Physiology, E.N.V., University of Nantes, CP 3013, F-44087 Nantes Cedex 03, France

Isometric tension of Na⁺-withdrawal contractures, membrane potential and intracellular Na⁺ activity (aⁱ_Na) have been measured in vitro under conditions which modify the activity of the Na⁺-Ca²⁺ exchanger of dystrophic limb muscles of (dy^2J/dy^2J) C57 BL/6J mice. In dystrophic soleus (Sol) and extensor digitorum longus (EDL) muscles exposed to Na⁺-free medium, there was a decrease in aⁱ_Na associated with a hyperpolarization of the membrane and the generation of a transient contracture. By contrast, exposing normal or denervated EDL muscles to Na⁺-free solution induced no change in aⁱ_Na nor in the resting tension. This study shows that Na⁺-Ca²⁺ exchange is still present in dystrophic Sol muscle. However from thermodynamic considerations, it could be suggested that the higher resting aⁱ_Na compared with that in normal muscle, may decrease the efficacy of the exchange.

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