Block of sodium channels by internal mono- and divalent guanidinium analogues. Modulation by sodium ion concentration

We have investigated the block of squid axon sodium channels by mono- and divalent guanidinium analogues. The action of these compounds on steady state sodium currents was independent of the presence or absence of the normal inactivation process. Block by both mono- and divalent analogues was voltage-dependent, but was a steeper function of potential for divalent molecules. The voltage-dependence could not, in general, be reproduced by a simple model based on Boltzmann's equation. Inhibition of steady state currents by guanidinium ions with 50 mM internal sodium was reasonably well described by a 1:1 drug/channel binding function. Increasing the internal sodium ion concentration increased both the degree and voltage-dependence of current inhibition. This is in sharp contrast to the decrease in inactivation caused by internal sodium. Changes in the external sodium concentration had very little effect on drug block. These results are consistent with a model of the sodium channel as a multi-ion pore. Only a small increase in block can be produced by increased internal sodium in a three-barrier two-site model, but a four-barrier three-site model can reproduce these experimental findings. The implications of these results for physical models of inactivation are discussed.