1.4 protein levels are decreased in muscles from mouse models of SMA. (A) Immunoblot analysis using muscle lysate from P2, P5, P9, and P21 wild type mice. Nav1.4 protein levels increase during postnatal muscle development and form the predominant sodium channel expressed in mature skeletal muscle. GAPDH served as a loading control (N = 3). (B) Representative immunoblot with quantification, showing a decrease in levels of sodium channel Nav1.4 and Nav1.5 in P5 Smn-/-;SMN2 hindlimb muscle compared with controls (N = 3). (C) Quantification of immunoblot analyses in P21 Smn2B/- and control hindlimb muscles revealed a decrease in Nav1.4 levels. Early in postnatal muscle development, the Nav1.5 sodium channel isoform is the most predominant. In P21 Smn2B/- mice, the protein levels of Nav1.5 are higher than in controls (N = 3). (D) The protein level of the Nav1.4 positive regulator, NF1, is not altered in muscles from P21 Smn2B/- mice. Similarly, no change was detected in the protein levels of the Nav1.4 repressor ZEB. (E) Expression of sodium channel Nav1.4 in control sham and denervated samples 1 and 7 days post-denervation was assessed by immunoblot (N = 3). A decrease in the levels of Nav1.4 in muscle was noted at 7 days post-denervation. *, P < 0.05; **, P < 0.01.