Abstract
Intrafiber Na +-activity, ( a Na) i, was measured in vivo in single muscle fibers in the gastrocnemius and soleus muscles of normal and dystrophic ( dy 2J dy 2J ) C57BL 6J mice by means of double-barrel Na +-selective microelectrodes. Values of ( a Na) i in normal gastrocnemius and soleus muscles were 9.8 ± 1.0 m M ( x ± SE; N = 15 ) and 13.7 ± 1 m M ( N = 19), respectively. Values of ( a Na) i in dystrophic gastrocnemius and soleus muscles were 18.7 ± 1.3 m M ( N = 10) and 26.2 ± 3.3 m M ( N = 15), respectively. The soleus muscle of both normal and dystrophic mice was “Na + loaded” by an intermittent tetanus delivered via the sciatic nerve. Muscles from both normal and dystrophic mice could extrude the Na + load at similar rates. The results suggest that in the dystrophic mice the elevated ( a Na) i is not due to inability of the Na + pump to respond to a Na + load; rather, it may be due to an alteration in the affinity of the pump for Na +, resulting in a different “set point” for ( a Na) i.
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