Block of the cGMP-gated cation channel of catfish rod and cone photoreceptors by organic cations

Tetraalkylammonium compounds and other organic cations were used to probe the structure of the internal and external mouths of the pore of cGMP-gated cation channels from rod and cone photoreceptors. Both rod and cone channels were blocked by tetramethyl- through tetrapentylammonium from the intracellular side in a voltage-dependent fashion at millimolar to micromolar concentrations. The dissociation constant at 0 mV (K-D(0)) decreased monotonically with increasing carbon chain length from similar to 80 mM (TMA) to similar to 80 mu M (TPeA), where the dissociation constant in rod channels is similar to 50% that of cone channels. N-Methyl-D-glucamine and the buffer Tris also blocked the cone channel in a voltage-dependent fashion at millimolar concentrations, but with lower affinity than similarly sized tetraalkylammonium blockers. Block by tetrahexylammonium (THxA) was voltage-independent, suggesting that the diameter of the intracellular mouth of these channels is less than the size of THxA but larger than TPeA. The location of the binding site for intracellular blockers was similar to 40% across the voltage-drop from the intracellular side. The addition of one carbon to each of the alkyl side chains increased the binding energy by similar to 4 kJ mol(-1), consistent with hydrophobic interactions between the blocker and the pore. Cone, but not rod, channels were blocked by millimolar concentrations of extracellular TMA. The location of the extracellular binding site was similar to 13% of the voltage drop from the extracellular side. in cone channels, the two blocker binding sites flank the location of the cation binding site proposed previously.