Open channel noise. II. A test for coupling between current fluctuations and conformational transitions in the acetylcholine receptor.

The first paper of this series demonstrated that the open-channel currents in the acetylcholine receptors in cultured rat muscle show fluctuations on a time scale of approximately 1 ms. In this paper the hypothesis is tested that these fluctuations are coupled to the gating mechanism that opens and closes the channel. Such a coupling could arise if the channel current and the energy barrier for gating transitions both showed fluctuations having a common origin such as a motion of part of the receptor molecule. A test for coupled fluctuations is made by averaging approximately 1,000 channel opening or closing transitions to search for the small relaxation in the current that is predicted. At a resolution of approximately 1% of the single-channel current amplitude, no such relaxation is observed. It is concluded that any coupled fluctuations are small; fluctuations in the energy barrier for the open-closed conformational transition must be smaller than about 0.3 kT.