Objective    To study the effect of chronic electrical stimulus at a low physiological frequency on the expression of myosin heavy chain (MHC) in genioglossus of rabbits with chronic hypoxia.     Methods     Twenty-four adult rabbits

weighing 2.3±0.1kg

were randomized into control group (A)，chronic hypoxia group (B)，10 Hz electrical stimulus group (C)

and 10+40 Hz electrical stimulus group (D). Rabbits in groups C and D received electrical stimulus of genioglossus at a frequency of 10 Hz and 10+40 Hz

respectively

while those in group B received no electrical stimulus

after they were placed in a hypobaric hypoxia cabin

8 h a day for 5 weeks

with free access to food and water. Expression of MHC in genioglossus of rabbits in the 4 groups was detected by SDSPAGE and semiquantitative inverse transcription RT-PCR

respectively.     Results     The expression level of MHCⅠand MHCⅠmRNA was significantly lower while that of MHCⅡa was significantly higher in group B than in group A [(0.69±0.07)% vs (17.48±4.00)%，(0.69±0.07)% vs (1.05±0.06)%

and (79.89±5.09)% vs (73.33±4.17)%

P0.05]. The expression level of MHCⅠwas significantly higher in groups C and D than in group B [(18.78±3.49)% vs (11.37±4.27)%

and (79.89±5.09)% vs (73.33±4.17)%

P0.05]

while that of MHCⅡa was significantly lower in groups C and D than in group B [(67.90±5.66)% vs (79.89±5.09)%， (79.89±5.09)% vs (73.33±4.17)%， P0.05]. The expression level of MHC I mRNA was significantly higher in groups C and D than in group B [（0.79±0.07)% vs (0.69±0.07)%，(0.97±0.08)% vs (0.69±0.07)%

P0.05]. No significant difference was found in expression of MHCⅡa mRNA between groups C

D and B.     Conclusion    MHCⅡa in genioglossus of rabbits with chronic hypoxia is transformed to MHC I

namely from rapid to slow transformation

after chronic electrical stimulus，especially at a low physiological frequency.