Two ion-selecting filters in the calcium channel of the somatic membrane of mollusc neurons

Summary

The slow inward current carried by Na⁺ through potential-dependent calcium channels in conditions when divalent cations were removed from the extracellular solution by EDTA has been investigated on isolated internally perfused neurons of the snailHelix pomatia. The calcium channels also acquire the capability to pass monovalent cations if other calcium-binding substances are added to the extracellular solution. Based on these facts the conclusion is made that the immediate reason for the modification of the channel selectivity is the absence of divalent cations in the extracellular medium. All potential-dependent characteristics of the modified calcium channel are shifted by 60 to 70 mV in the hyperpolarizing direction compared with those of the original calcium channel. The series of relative permeabilities for modified calcium channels towards monovalent cations ($P_{Na^ + } :P_{Li^ + } :P_{N_2 H_5 ^ + } :P_{NH_3 OH^ + }$=1.0∶0.8∶0.55∶0.21) is close to that of common “fast” sodium channels ($P_{Na^ + } :P_{Li^ + } :P_{N_2 H_5 ^ + } :P_{NH_3 OH^ + }$=1.0∶1.04∶0.44∶0.21). The induced sodium current decreases immediately when the concentration of divalent cations in the extracellular solution is elevated. This decrease is not potential dependent and can be approximated by Langmuir's isotherm with dissociation constants pK_Ca∶pK_Sr∶pK_Ba∶pK_Mg=6.6∶5.5∶4.8∶4.2. The conclusion is drawn that the calcium channels in the somatic membrane have two ion-selecting filters with different functions —an external one consisting, probably, of several carboxylic groups which bind divalent cations in a highly specific manner and determine the impermeability of the channel to monovalent cations in physiological conditions, and the channel ion-selecting filter including a single carboxylic group normally determining the channel selectivity for different divalent cations.