Single voltage-dependent K+ and Cl− channels in cultured rat astrocytes

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Abstract
The kinetic reactions of a voltage-dependent K+ channel, which constituted about 14% of all the recorded K+ channels in the membrane of cultured rat astrocytes were studied in detail. A scheme of one open and three closed states is necessary to describe the kinetic reactions of this channel. The channel contributes little to the resting membrane potential. Its steady state open probability (Po) is 0.06 at −70 mV. When the cell is depolarized to 0 mV, Po approaches 1. This represents a 17-fold increase. Such channels could contribute to the potassium clearance by enhancing the effect of "spatial buffering." Additionally, single anion-selective channels with very high conductances were found in inside-out patches in approximately 15% of all recorded channels in the membrane of rat astrocytes. Channel openings are characterized by more than one conductance level; the main level showed a mean conductance of 400 pS. These channels are divided into two groups. Approximately 90% of the recorded chloride channels showed a strong voltage dependency of their current fluctuations. Within a relatively small potential range (±15 mV) the channels have a high probability of being in the active state. After a voltage jump to varying testing potentials in the range of ±20 to ±50 mV the channels continued to be in the active state for some time and then closed to a shut state. If the testing potential persisted, the channels were not able to leave this shut state. The active state could only be reached again if the membrane potential was reset close to zero for some time. The time course of the current relaxation was measured by ensemble averaging of single channel current fluctuations. When at the end of a testing potential the voltage was set back to zero, the channel remained in the shut state for some time before it reached the open state again. The voltage dependence of this recovery period was analyzed as well but is not shown in this paper. The reaction indicates a nonstationary process as the open probability is time dependent, and for better differentiation I will call these channels nonstationary chloride channels. A subgroup of 10% of all recorded chloride channels showed no voltage-dependent kinetic reactions. I will denote them as stationary chloride channels. Both types of Cl− channels are mainly permeable to anions but showed a slight permeability to cations. An idea of the role of these channels at this state must be highly speculative. The possibilities include a cell to cell transfer of material or a regulation of the internal or external ion environment. In the latter case, they could provide an uptake mechanism for potassium ions in addition to the spatial buffer currents.