Permeability of the squid axon membrane to several organic molecules

¹ National Institutes of Health, Bethesda, Maryland

The permeability of the squid axon membrane to choline, guanidine, thiourea, urea, sucrose, starch, and cesium was investigated by use of radiotracers. In the resting state, the time constant for the loss of intracellular radioactive (free) choline, guanidine, and cesium was of the same order of magnitude as that for labeled sodium or potassium ions. Choline ion was found to bind rapidly with the constituents of the axon. In sea water containing labeled choline, there was accumulation of radioactivity by the axon. On repetitive stimulation, guanidine and cesium efflux was markedly increased; the choline efflux was slightly increased. Labeled thiourea and urea moved across the membrane far more readily than the univalent cations. Permeation of labeled sucrose and starch was very slow.