Surface charge and XXXX channel saturation in bullfrog sympathetic neurons.

@entity969 @entity8594 @entity6860 @entity460 @entity21678
Currents carried by @entity8594 through @entity460 channels were recorded in the whole-cell configuration in isolated frog sympathetic neurons. The effect of surface charge on the apparent saturation of the channel with @entity8594 was examined by varying [ @entity8594 ]o and ionic strength. The current increased with [ @entity8594 ]o, and the I-V relation and the activation curve shifted to more positive voltages. The shift of activation could be described by Gouy-Chapman theory, with a surface charge density of 1 e-/140 A2, calculated from the Grahame equation. Changes in ionic strength (replacing @entity21678 with @entity969 ) shifted the activation curve as expected for a surface charge density of 1 e-/85 A2, in reasonable agreement with the value from changing [ @entity8594 ]o. The instantaneous I-V for fully activated channels also changed with ionic strength, which could be described either by a low surface charge density (less than 1 @entity6860 ), or by block by NMG with Kd approximately 300 mM (assuming no surface charge). We conclude that the channel permeation mechanism sees much less surface charge than the gating mechanism. The peak inward current saturated with an apparent Kd = 11.6 mM for @entity8594 , while the instantaneous I-V saturated with an apparent Kd = 23.5 mM at 0 mV. This discrepancy can be explained by a lower surface charge near the pore, compared to the voltage sensor. After correction for a surface charge near the pore of 1 @entity6860 , the instantaneous I-V saturated as a function of local [ @entity8594 ]o, with Kd = 65 mM. These results suggest that the channel pore does bind @entity8594 in a saturable manner, but the current-[ @entity8594 ]o relationship may be significantly affected by surface charge.
