Miniature end-plate currents (mepcs) and acetylcholine induced current noise were recorded from the cutaneous pectoris muscle of the frog with the voltage-clamp technique. Analysis of current noise was used to estimate mean single channel current and the mean lifetime of an open channel. Adding Ni2+ or Ca2+ to the bathing solution reduced the amplitude of the mepcs. Ten mM-Ni2+ decreased the amplitude 64%, while raising Ca2+ from 2 to 10 mM decreased the amplitude 35%. The decreased amplitude of the mepc in Ni2+ and increased Ca2+ can be explained by a decrease in single channel current. Ten mM-Ni2+ decreased mean single channel current 64% while raising Ca2+ from 2 to 10 mM decreased single channel current 28%. The decrease in single channel current was due to a decrease in the driving potential and single channel conductance. Ten mM-Ni2+ and Ca2+ shifted the reversal potential for the mepc about 10 mV negative from the control value of -4.6 mV; at the same time single channel conductance was decreased 59% in Ni2+ and 18% in increased Ca2+. In contrast to the similar direction of effects of Ni2+ and Ca2+ on mepc amplitude, reversal potential, and single channel conductance, Ni2+ and Ca2+ had different effects on mepc time course. Ten mM-Ni2+ increased the time constant of mepc decay 80% while raising Ca2+ from 2 to 10 mM decreased the time constant of decay 17%. Ni2+ and Ca2+ also had different effects on single channel lifetimes. Ten mM-Ni2+ increased channel lifetime about 50%, while raising Ca2+ from 2 to 10 mMdid not significantly affect channel lifetime. These results suggest that changes in single channel lifetime and conductance due to ionic influences are not necessarily tightly coupled. The results also suggest that the effects of both Ni2+ and Ca2+ on channel lifetime cannot be accounted for in terms of a simple surface potential hypothesis.
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