FGFR1 protein is up-regulated in muscle fibers during disuse atrophy. FGFR1 was immunolocalized in sections of gastrocnemius muscle obtained from control mice or mice suspended for 7 days. In comparison to the control gastrocnemius (A), FGFR1 protein, as depicted by the brown precipitate (black arrows), was increased in atrophying muscle (B). In sections of the gastrocnemius isolated from suspended mice, the area of the FGFR1-positive muscle fibers was significantly greater than the FGFR1 negative fibers (C). The area of the FGFR1 positive myofibers in the atrophying muscle was the same as the area of control myofibers (1688 mm2). This data suggests that FGFR1 acts to inhibit fiber atrophy associated with disuse. ** indicates p < .01; SEM indicated; bar represents 50 mm.