Modification of alpha subunit of RIIA sodium channels by aconitine

The effect of aconitine (AC), an alkaloid toxin, on the electrophysiological properties of the rat brain type IIA alpha subunit expressed heterologously in the Chinese Hamster Ovarian (CHO) cell line was studied under the whole-cell patch-clamp configuration. The activation threshold of modified channels shifted by about -40 mV. As the number of depolarizations increased, the transient current at 0 mV decreased and, in proportion, the AC-modified current at -50 mV increased. This suggests a transition of channels to an AC-modified state. The rate of modification was nearly four times faster when 50 microM AC was applied internally than when applied in the bath solution. This supports the idea that the AC-binding site is located close to the cytoplasmic mouth of the channel pore. The AC-modified sodium currents inactivated completely, although with slower kinetics. The steady-state inactivation followed a simple Boltzmann function. AC-modified currents activated without a sigmoidal delay. The permeability of the NH4+ ion was enhanced such that its permeability ratio increased from an initial value of 0. 18 to 0.95 and for Cs+ it was enhanced from 0.03 to 0.07. These studies show that the AC-binding site resides at the pore region of the alpha subunit of the Na+ channel, and that the presence of beta subunit/s is not essential for AC binding.