AMPK signaling initiates adaptive changes in skeletal muscle fibers that restore homeostatic energy balance. The purpose of this investigation was to examine, in rat, the fiber type protein expression patterns of the -catalytic subunit isoforms in various skeletal muscles, and changes in their respective contents within the tibialis anterior (TA) after chronic low-frequency electrical stimulation (CLFS:10Hz,10hr/d), applied for 4±1.2 or 25±4.8 days. Immunocytochemical staining of soleus (SOL) and medial gastrocnemius (MG) showed that 86±4.1 to 97±1.4% of IIA fibers stained for the 1- and 2-isoforms, which progressively decreased to 63±12.2% of IID/X and 9±2.4% of IIB fibers. 39±11.4% of IID/X and 83±7.9% of IIB fibers expressed only the 2 isoform in the MG, much of which was localized within nuclei; 1 and 2 contents, assessed by immunoblot, were lowest in the white gastrocnemius (WG:80% myosin heavy chain(MHC)IIb;20% MHCIId/x). Compared with the WG, 1 content was 1.6±0.08(P<0.001) and 1.8±0.04(P<0.0001) fold greater in the red gastrocnemius (RG:13%-MHCIIa) and SOL (21%-MHCIIa), respectively, and increased in proportion to MHCIIa content. Similarly, 2 content was 1.4±0.10(P<0.02) and 1.5±0.07(P<0.001) fold greater in RG and SOL compared with WG. CLFS induced 1.43±0.13 (P<0.007) and 1.33±0.08(P<0.009) fold increases in the 1 and 2 contents of the TA and coincided with the transition of faster IIB and IID/X fibers toward IIA fibers. These findings indicate that fiber types differ with regard to their capacity for AMPK signaling, and that this potential is increased by CLFS.