Burst properties of a supergated double-barrelled chloride ion channel

The chloride selective channel from Torpedo electroplax, ClC-0, is the prot otype of a large gene family of chloride channels that behave as functional dimers, with channel currents exhibiting two non-zero conductance levels, Each pore has the same conductance and is controlled by a subgate, and thes e have seemingly identical fast gating kinetics. However, in addition to th e two subgates there is a single slower 'supergate' which simultaneously af fects both channels. In the present paper, we consider a six state Markov m odel that is compatible with these observations and develop approximations as well as exact results for relevant properties of groupings of openings, known as bursts. Calculations with kinetic parameter values typical of ClC- 0 suggest that even simple approximations can be quite accurate. Small devi ations from the assumption of independence within the model lead to marked changes in certain predicted burst properties. This suggests that analysis of these properties may be helpful in assessing independence/non-independen ce of gating in this type of channel. Based on simulations of models of bot h independent and non-independent gating, tests using binomial distribution s can lead to false conclusions in each situation. This is made more proble matic by the difficulty of selecting an appropriate critical time in defini ng a burst empirically. (C) 2000 Published by Elsevier Science Tnc. All rig hts reserved.