MATURATIONAL DEFECT IN PASSIVE MEMBRANE PROPERTIES OF DYSTROPHIC MOUSE MUSCLE

Electrical properties, component ionic conductances, and histochemical characteristics of normal and dystrophic ( dy2J dy2J) mouse extensor digitorum longus (EDL) and soleus (SOL) muscles were studied between 1 and 6 months of age. Normal EDL and SOL membrane electrical parameters were indistinguishable at 1 month. Between 1 and 3 months, membrane resistance (Rm) in normal EDL decreased progressively to a mature value one-half that found in SOL muscles. Measurements of component conductances indicated that this decrease in Rm was due to a specific increase in membrane chloride conductance (GC1) in EDL fibers. During the same time membrane capacitance increased slightly in both EDL and SOL. Dystrophic EDL failed to develop fully the changes in membrane resistance seen in controls, showing significant deviation at all time points beyond 1 month. An abnormal high-resistance subpopulation was evident in Rm histograms of mature dystrophic EDL. Average membrane properties of dystrophic SOL appeared normal during the 6-month interval studied but significant changes in Rm histograms were found. Histochemical analysis of normal and dystrophic EDL showed that both contained almost exclusively type II fibers (> 95%); normal and dystrophic SOL contained a nearly equal mixture of type I and type II fibers. Progressive degenerative changes were seen in both dystrophic muscles with age but were more severe in the SOL. The depressed average GC1 in the dystrophic EDL was due to a subpopulation of fibers (30% of total) having a high membrane resistance. Denervation was ruled out as a cause for this subpopulation by a lack of correlation between membrane resistance and resting potential. Histochemical evidence eliminated the possibility that this subpopulation was composed of slow-twitch fibers. Data are presented indicating that the subpopulation represents a group of dystrophic EDL fibers which fail to undergo normal maturation. © 1978.