Figure 4.

MR antagonism reduces myofiber size after acute muscle injury. (A) Bar graphs showing the average size of centrally nucleated tibialis anterior myofibers at 4 (n = 15 Cre−, and 13 MRcko) (n = 9 untreated, and 8 spironolactone), 7 (n = 12 Cre−, and 14 MRcko) (n = 8 untreated, and 10 spironolactone), and 14 (n = 11 Cre−, and 13 MRcko) (n = 12 untreated, and 13 spironolactone) days post-injury in MRcko versus Cre− mouse littermates (left panel) and MR antagonist-treated and untreated wild-type littermate mice (right panel). The data are presented as mean ± SEM. ^**p ≤ 0.01. (B) The distribution of centrally nucleated myofiber percentages was broken down by size in line graphs into small myofibers 75–600 μm² and larger myofibers 600–3,600 μm². The centrally nucleated myofiber size was analyzed at 4 (n = 15 Cre−, and 13 MRcko) (n = 9 untreated, and 8 spironolactone), 7 (n = 12 Cre−, and 14 MRcko) (n = 8 untreated, and 10 spironolactone), and 14 (n = 11 Cre−, and 13 MRcko) (n = 12 untreated, and 13 spironolactone) days post-injury. For size analysis, the following numbers of centrally nucleated myofibers were analyzed for each group and days post-injury: 2,653 ± 105 Cre− and 2,584 ± 161 MRcko at 4 days post-injury, 2,810 ± 166 Cre− and 2,818 ± 181 MRcko at 7 days post-injury, 3,103 ± 157 Cre− and 3,173 ± 193 MRcko at 14 days post-injury, 2,466 ± 167 untreated and 2,697 ± 167 spironolactone treated at 4 days post-injury, 2,465 ± 165 untreated and 2,629 ± 130 spironolactone treated at 7 days post-injury, and 2,730 ± 159 untreated and 3,019 ± 157 spironolactone treated at 14 days post-injury. (C) Bar graph showing representative myogenic factor and myofiber growth genes assessed in spironolactone treated (n = 5) and untreated (n = 5) littermates at 7 days post-injury. Beta-actin levels are used as a normalization control for each sample and fold-changes are normalized to the same untreated control. Mstn, myostatin; Igf1, insulin-like growth factor 1; Myog, myogenin; Myod1, myoblast determination protein 1. The data are presented as mean ± SEM. All data were analyzed using a Student’s t-test, ^*p ≤ 0.05 and ^**p ≤ 0.01 for the bins of fiber-sizes shown by the bracket.