Modeling of Single Non-Inactivating Na Channels: Evidence for Two Open and Several Fast Inactivated States

¹ Institut für Physik
² Neurologische Klinik und Abteilung für Angewandte Physiologie

^* To whom correspondence should be addressed. E-mail: holger.lerche{at}uni-ulm.de.

Submitted on August 29, 2005
Revised on September 29, 2005
Accepted on 19 December 2005

	Abstract

Voltage-gated Na⁺ -channels play a fundamental role for the excitability of nerve and muscle cells. Defects in fast Na⁺ -channel inactivation can cause hereditary muscle diseases with hyper- or hypoexcitability of the sarcolemma. To explore the kinetics and gating mechanisms of non-inactivating muscle Na channels on a molecular level, we analyzed single channel currents from wild-type and five mutant Na⁺ channels. The mutations were localized in different protein regions which have been previously shown to be important for fast inactivation (D3-D4-linker, D3/S4-S5, D4/S4-S5, D4/S6) and exhibited distinct grades of defective fast inactivationwith certain levels of persistent Na⁺ currents caused by late channel reopenings. Different gating schemes were fitted to the data using hidden Markov models with a correction for time interval omission and compared statistically. For all investigated channels including the wild-type two open states were necessary to describe our data. Whereas for the wild-type one inactivated state was sufficient to fit its single channel behavior, modeling of the mutants with impaired fast inactivation revealed evidence for several inactivated states. We propose a single gating scheme with two open and three inactivated states to describe the behavior all five examined mutants that allows for a biological interpretation of the collected data - based on previous investigations in voltage-gated Na⁺ and K⁺ channels.

Key Words: Na channel, hidden Markov model, maximum likelihood, patch-clamp technique, single channel recordings