α-catalytic subunits of 5′ AMP-activated protein kinase display fiber-specific expression and are upregulated by chronic low-frequency stimulation in rat muscle

AMP- activated protein kinase ( AMPK) signaling initiates adaptive changes in skeletal muscle fibers that restore homeostatic energy balance. The purpose of this investigation was to examine, in rats, the fiber- type protein expression patterns of the alpha- 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; 10 Hz, 10 h daily), 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 type IIA fibers stained for both the alpha 1- and alpha 2- isoforms progressively decreased to 63 +/- 12.2% of type 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. alpha 1 and alpha 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, alpha 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, alpha 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 alpha 1 and alpha 2 contents of the TA and coincided with the transition of faster type 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.