Voltage-dependent slowing of K channel closing kinetics by Rb+.
We have studied the effect of Rb+ on K channel closing kinetics in toadfish pancreatic islet cells. These channels are voltage dependent, activating at voltages positive to -10 mV. The channels also inactivate upon prolonged depolarizations, and the inactivation time course is best fit by the sum of two exponentials. Instantaneous current-voltage relationships show that external Rb+ enters the channel as easily as K+, but carries less current. In the voltage range from -140 to -50 mV, the closing time course of the channels can be fit with a single exponential. When Rb+ is present in the external solution the channels close more slowly. The magnitude of this Rb+ effect is voltage dependent, decreasing at more negative voltages. Similarly, when the internal solution contains Rb+ instead of K+ the closing time constants are increased. The effect of internal Rb+ is also voltage dependent; at voltages positive to -80 mV the closing time constant in internal Rb+ is slower than in K+, whereas at more negative voltages the difference is negligible. With internal Rb+, the relationship between the closing time constant and voltage is best fit with two exponential components, suggesting the presence of two distinct voltage-dependent processes. The results are discussed in terms of a model of the K channel with two internal binding sites, and we conclude that Rb+ produces its effects on channel gating by binding to a site in the pore.
Sala
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Department of Biophysics, University of Maryland School of Medicine, Baltimore 21201.
Matteson
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D000818:Animals; D001692:Biological Transport; D002478:Cells, Cultured; D005399:Fishes / Q000378:metabolism*; D015640:Ion Channel Gating; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D015221:Potassium Channels / Q000378:metabolism*; D012413:Rubidium / Q000378:metabolism*
D015221:Potassium Channels; D012413:Rubidium