Conversely, ERK1/2 activation was reduced in P12 quadriceps muscle obtained from smPit1−/−; smPit2−/− mice, and AMPK phosphorylation was increased (Fig. 4D), consistent with increased energy-stress and reduced muscle ATP production previously described by us6 in the setting of hypophosphatemia. Here, MAPK3 is linked to hypophosphatemia.