Dequalinium has recently been reported to block CNGA1 and CNGA2 channels expressed in Xenopus laevis1,2. We have recently shown that dequalinium also blocks currents through the rat olfactory CNGA2 channel expressed in HEK293 cells. Cytoplasmic dequalinium interacts with a binding site that is about 1/3 to 1/2 of the way across the membrane electric field (from the intracellular end of the channel), with an IC50 of approximately 1.3 μM at a Vm of +60 mV3. Neutralization of the negatively-charged pore-lining glutamate acid (E342Q) led to a profound decrease in the voltage-dependence of block by cytoplasmic dequalinium3. Dequalinium remained an effective channel blocker of the mutant channel with an IC50 of approximately 1.9 μM at a Vm of +60 mV3. Our additional experiments suggest that block is more effective when saturating cAMP, rather than saturating cGMP, is used as the activating ligand. As the channel spends about 300-fold more time in the closed state in the presence of saturating cAMP compared to that in the presence of saturating cGMP4, this suggests that dequalinium reduces currents by interacting predominantly with the closed channel. We now also report that extracellular dequalinium seemed to more effectively block the wild-type CNGA2 channel than did cytoplasmic application. The E342Q mutation significantly increased the IC50 of external dequalinium by about 10-fold, from 350 nM for wild-type to about 3 μM for mutant at a Vm of –60 mV. These results indicate that E342 contributes to the inhibition caused by this presumed pore-blocker for cytoplasmic application and possibly also forms a binding site for extracellular application.
1. Rosenbaum et al. 2003. J. Gen. Physiol., 121:37-47.
2. Rosenbaum et al. 2004. J. Gen. Physiol., 123:295-304.
3. Qu et al. 2004. Proc. Aus. Soc. Biophys. (in press).
4. Gordon et al. 1995. Neuron, 14:857-864.