Na+-Ca2+ exchange in limb muscles of dystrophic (C57 BL/6J dy2J/dy2J) mice.

Isometric tension of Na+-withdrawal contractures, membrane potential and intracellular Na+ activity (aiNa) have been measured in vitro under conditions which modify the activity of the Na+-Ca2+ exchanger of dystrophic limb muscles of (dy2J/dy2J) C57 BL/6J mice. In dystrophic soleus (Sol) and extensor digitorum longus (EDL) muscles exposed to Na+-free medium, there was a decrease in aiNa 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 aiNa 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 aiNa compared with that in normal muscle, may decrease the efficacy of the exchange.