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

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Abstract
5′-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 α-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 α1- and α2-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 α2 isoform in the MG, much of which was localized within nuclei. α1 and α2 contents, assessed by immunoblot, were lowest in the white gastrocnemius [WG; 80% myosin heavy chain (MHC) IIb; 20% MHCIId/x]. Compared with the WG, α1 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, α2 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 α1 and α2 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.

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