The modulatory site for the action of gadolinium on surface charges and channel gating.

The gadolinium (Gd3+)-induced shift of potential dependence and modulated gating of Na and K channels were analyzed. In a previous investigation, we explained the shift in terms of pure screening (no binding) of fixed surface charges and the modulation by binding to modulatory sites on the channels. In the present paper, we have extended this model by including effects on the charge density of Gd3+ binding to the modulatory sites. From fitting the extended model to experimental data, the charge density was estimated to be -0.6 e nm-2, and the Gd(3+)-induced charge change to be +0.15 e nm-2, and the maximal scaling factor to be 7.5 for both Na and K channels. Intrinsic KD values for binding to the K and Na channels were estimated to be 140 and 380 mM, respectively. Estimations of the extracellular charge density, from primary structures of cloned channels, were found to be in agreement with estimations based on the present model. The modulatory site was suggested to be located at the cluster of negatively charged residues between the fifth transmembrane segment (S5) and the pore-forming region for both Na and K channels. These suggestions imply several testable predictions about different K channels.