Indirect inhibitory effect of succinylcholine on acetylcholine-activated channel activities and its modulation by external Ca2+ in mouse skeletal muscles.

1. The effect of extracellular calcium on single acetylcholine (ACh)-activated channel activities when desensitizing concentrations of succinylcholine (SuCh) were applied to the surrounding endplate membrane was investigated by the cell-attached patch-clamp technique at endplates of single skeletal muscle (flexor digitorum brevis) fibres of adult mice. 2. Bath-applied SuCh (0.1-3 microM, in 2.5 mM Ca2+) increased in a concentration-dependent manner the mean open time of ACh-activated channel currents recorded at membrane potentials which cancelled the SuCh-induced depolarizations. 3. In the presence of 0.5 and 2.5 mM external Ca2+, SuCh (3 microM) applied outside the patch pipette prolonged the mean open time of ACh-activated channel currents in a time-dependent manner (by 45% and 52%, respectively), and simultaneously significantly decreased the single channel conductance (by 14% and 10%, respectively). These SuCh-induced effects did not occur in a nominally Ca(2+)-free extracellular medium. 4. Under the same conditions, SuCh (3 microM) augmented the time-dependent decline in the opening frequency of ACh-activated channel currents obtained in nominally Ca(2+)-free medium. 5. These results suggest that external calcium ions act to modulate nicotinic ACh receptor channel activity, and accelerate desensitization of the receptor.