Abstract
The model proposed for external TEA block of Shaker K+ channels predicts a proportional relationship between TEA sensitivity and calculated electrical distance derived from measurements of voltage dependence of TEA block. In the present study, we examined this relationship for the A-type K+ current (IA) of Helix aspersa in neuronal somata using the whole-cell patch-clamp technique. External TEA inhibited IA with strong voltage dependence, such that the TEA dissociation constant was increased at depolarized test potentials. The half-inhibition constant (V0.5) for TEA block was approximately 21 mM at 0 mV, and V0.5 increased to approximately 67 mM at 50 mV. The calculated electrical distance for TEA block suggested that TEA traversed 65% of the way into the membrane electrical field. TEA also caused significant shifts in the voltage-dependence of A-type K+ channel gating. For example, at TEA concentrations below that required to fully suppress delayed outward currents, TEA caused depolarizing shifts in the voltage-dependence of A-type channel activation, steady-state inactivation, time for removal of inactivation, and slowed channel activation kinetics. Taken together, these observations suggest that TEA biased the local field potential near voltage-sensing domains of A-type K+ channels, causing the transmembrane electrical field to be relatively hyperpolarized in the presence of TEA. In summary, the calculated electrical distance of TEA block of A-type K+ channels in H. aspersa neurons is unprecedented among other K+ channels. This raises concerns about the conventional interpretation of this value. Furthermore, the voltage-dependent properties of IA are modified by TEA at concentrations previously used to isolate delayed rectifier potassium channels (IKDR) selectively. This lack of specificity has important implications for recent, as well as future studies of IA in H. aspersa and possibly other snail neurons.
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