Objective      To study the effect of chronic electrical stimulation with lower physiological frequency (CESLPF) on expression of myosin heavy chain (MHC) isoforms in differentiated C2C12 cells.       Methods      Three days after a C2C12 cell differentiation model was induced by 2% horse serum (HS)

expression level of MHC isoforms in differentiated C2C12 cells treated with calcineurin (CaN) inhibitor cytosporin-A (CsA

8 μmol) or calcium/calmodulin-dependent protein kinase (CaMK) inhibitor KN93 (6 μmol) alone or in combination with CESLPF (10+40 Hz

1.5 h/d for 2 d) was measured by RT-PCR and Western blotting

respectively. Activity of CaN was detected by colorimetry.       Results      CsA and KN93 could inhibit the expression of MHC I mRNA and protein in C2C12 cell differentiation model in vitro. The expression level of MHC Ⅰ mRNA and protein was higher in control group than in CsA and KN93 treatment group (0.79±0.04vs0.62±0.03 and 0.69±0.05

0.91±0.05vs0.36±0.01 and 0.66±0.02

P0.05). The expression level of MHC Ⅱb mRNA was lower in control group than in CsA and KN93 treatment group (0.53±0.01vs0.63±0.02 and 0.75±0.03

P0.05). CESLPF was able to reverse the expression of MHC isoforms (P0.05). CsA could suppress while CESLPF could increase the CaN activity which was still significantly lower than its normal level (P0.05).       Conclusion      CaN and CaMK are the important regulatory enzymes for the transformation of MHC Ⅱ to MHC Ⅰ. CESLPF can promote the transformation of MHC Ⅱ to MHC Ⅰ through CaN-dependent and CaN-independent pathway. Our results provide evidence for the application of CESLPF in treatment of obstructive sleep apnea syndrome.