Blocking of the gramicidin channel by divalent cations

Abstract

The conductance of the gramicidin channel in the presence of alkali ions is strongly reduced when divalent cations such as Ca++ or Ba++ are added to the aqueous solutions in concentrations between 0.1 and 1m. Under the same conditions, carrier-mediated alkali ion transport is not affected by Ca++ and Ba++. Different divalent cations, differ considerably in their blocking action on the gramicidin channel; the effect of Mg++ or Zn++ is much smaller than that of Ca++ and Ba++. Besides reducing the single-channel conductance, the blocking ions also change the current-voltage characteristic of the channel from a nearly, linear to a strongly saturating behavior. These observations suggest that Ca++ or Ba++ (which are not permeable themselves) bind to a site at or near the channel mouth, thereby reducing the rate by which permeable ions enter and leave the channel. The blocking effect is analyzed in terms of the potential energy profile of the permeable ion in the channel. The saturating current-voltage characteristic may be explained by the assumption that in the presence of the blocking ion the passage over the entrance barrier is rate-limiting and, at the same time, only weakly voltage-dependent.