FGFs appear unlikely to contribute to altered sodium channel gating in CIM. ( A) FGF12 and 13 levels in muscle membrane samples from control and CIM muscle. As positive controls, tissues or expression vectors known to produce the protein of interest were used: for the pan-NaV 1.x Ab, skeletal muscle; for FGF13, a lysate of 293 cells transfected with an FGF13 expression vector; for FGF12, brain membranes. (B) FGF13 was expressed in both control and CIM muscle, such that the relative ratio of FGF13: sodium channel did not change in CIM. In contrast, FGF12B was expressed at low levels in control muscle, but increased in CIM muscle, such that the ratio of FGF12B: sodium channel significantly increased. Asterisk indicates P < 0.05. ( C) Cross-section of control and CIM medial gastrocnemius muscle stained for FGF12, dystrophin, and overlay reveals that most FGF12 staining is in myonuclei and nuclei of cells in the interstitial space.