Acetylcholine-evoked currents in juvenile mouse muscles

Single fibres were obtained from juvenile (8-12 days old) mice by enzymatic dissociation. Whole-cell voltage-clamped soleus and interosseus fibres were jet-superfused by Cl-free solutions. Current responses to ACh (0.5-50 microM) were recorded. At high [Ach], current responses in all fibres showed a transient initial and a prolonged late phase. The reversal potential of the initial phase was about 5 mV and that for the late phase about 18 mV in soleus. No difference between the reversal potentials for the two phases was seen in interosseus. At low [Ach], only the late phase was seen in soleus. Differences in reversal potentials for the two phases were found in adult interosseus fibres denervated for 10-20 days. These differences were smaller, when the pipette was attached to the endplate region, than at the fibre ends. Evidence was presented supporting the view that the diminished difference between the two reversal potentials at the endplate, mentioned above, is due to the non-uniform voltage-clamping of the membrane in the presence of high [ACh].