Key result
Formamide yields gramicidin A K+ blocking and unblocking rates of ~10^7 s-1.
Why the study?
Blocking events in single-channel currents occur over a wide range of times, but rapid kinetics of very brief blocking events are difficult to characterize with conventional single-channel recordings.
Demonstrates a method combining open-channel noise measurements with model fits to estimate very fast channel blocking kinetics.
Enables sub-10 μs channel block analysis; leaves open utility in cardiac electrophysiology pending validation.
Blocking events in currents through biological ion channels occur over a wide range of characteristic times. The interruptions in single-channel currents from blocking events may be characterized by the direct measurement of gap durations or by analyzing open-channel current histograms, provided that the events are not much shorter than the time resolution of single-channel recordings (approximately 10 microseconds). Here we present a method for the characterization of channel block on a much faster time scale by combining open-channel noise measurements with subsequent model fits according to a theoretical approach (Frehland, E. 1978. Biophysical Chemistry. 8:255-265). Although the bandwidth limitations in open-channel noise experiments are the same as in conventional single-channel experiments, from the dependence of the mean current and the spectral density of the noise on the concentration of the blocking agent, kinetics of very brief blocking events can be estimated. As an example we have analyzed the open-channel noise of K+ currents through the gramicidin A channel in the presence of various concentrations of formamide, a weak blocker, at neutral pH. We estimate the blocking and unblocking rates to be approximately 10(7)s-1 at 1 M formamide and discuss possible mechanisms for the blocking process.
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Heinemann et al. (1988) studied this question. Formamide was evaluated on Blocking and unblocking rates. Using open-channel noise measurements, the blocking and unblocking rates of K+ currents through the gramicidin A channel were estimated to be approximately 10(7) s-1 at 1 M formamide.
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