Desensitization onset and recovery at the potassium-depolarized frog neuromuscular junction are voltage sensitive

The influence of voltage on the time-course of desensitization onset and recovery has been studied at the frog neuromuscular junction. The activation-desensitization sequence was determined from carbachol- induced end-plate currents in potassium-depolarized fibers voltage- clamped either to -40 mV or +40 mV. The time-course of both desensitization onset and recovery developed exponentially, with onset occurring more rapidly than recovery. Desensitization onset was voltage dependent, the onset time constant being 8.3 +/- 1.3 s (11 fibers) at - 40 mV and 19.3 +/- 3.4 s (15 fibers) at +40 mV. Recovery from desensitization was also influenced by voltage. The extent of recovery after 2 min was 80.4 +/- 6.3% in those fibers voltage-clamped to -40 mV and 57.4 +/- 3.6% in those fibers voltage-clamped to +40 mV. The voltage dependence of desenistization onset and recovery did not result from a difference in ability to control voltage at these two levels of membrane potential. These results demonstrate that in the potassium- depolarized preparation the processes controlling both desensitization onset and recovery of sensitivity from the desensitivity from the desensitized state are influenced by membrane voltage.