Selective inhibition of potassium conductance in node of Ranvier with a photoaffinity label derived from tetraethylammonium

Abstract

BIOCHEMICAL investigation of membrane components involved in ion-specific conductance changes is difficult because of the lack of a functional assay for the components after membrane solubilisation. Proteins thought to participate in conductivity changes during synaptic transmission¹ have been isolated using a binding assay and affinity chromatography^2,3, because of their high affinity for a variety of ligands; but the connection between such receptor proteins and postulated ion channels remains to be shown. To isolate molecules forming sodium channels of the axon membrane^4–7, tetrodotoxin was used as ligand (binding assay K_d=3 nM). The most efficient known blocking agent of the change in potassium conductance in nerve and muscle known is tetraethylammonium (TEA) but its affinity is too low (K_d=0.4mM (ref. 8)) for either of the above techniques. We have attempted to label radioactively those molecules associated with the voltage-dependent potassium conductance changes covalently in situ with the aim of recovering that radioactivity after solubilisation using classical fractionation procedures. We suggest that it is possible to block selectively and irreversibly potassium conductance in myelinated nerve fibres using a photoaffinity lable. This method may open the way for the isolation of the TEA receptor of excitable membranes, proposed as part of the potassium channel (if not the channel itself).