Ion movement through gramicidin A channels. Single-channel measurements at very high potentials.

The patch-clamp technique of Mueller (1975, Ann. N.Y. Acad. Sci., 274:247-264) and Neher and Sakmann (1976, Nature (Lond.), 260:799-802) was modified to be suitable for single-channel measurements in lipid bilayers at potentials up to 500 mV. This method was used to study gramicidin A single-channel current-voltage characteristics. It was found that the sublinear current-voltage behavior normally observed at low permeant ion concentrations and rather low potentials (V less than or equal to 200 mV) continues to be seen all the way up to 500 mV. This phenomenon is characteristic of the low permeant ion situation in which the channel is far from saturation, and implies that the overall rate constant for association between ion and channel is very weakly, if at all, voltage dependent. The magnitude of the single channel currents at 500 mV is consistent with the notion that the aqueous convergence conductance is a significant factor in determining the permeability characteristics of the gramicidin A channel.