Movement of radioactive tracers across squid axon membrane

¹ Laboratory of Neurophysiology, National Institute of Neurological Diseases and Blindness, National Institutes of Health, Bethesda, Maryland

Radioactive tracers of potassium, sodium, water, chlorine, sulfate, phosphate and calcium were injected into squid giant axons and the movement of these tracers across the surface membrane was traced either by the standard isotope technique or by the method of gamma-ray spectrometry. The time constants of loss of these tracers through the axon at rest were found to be approximately 8 hours for K⁴², approximately 3 hours for Na²⁴, 0.7–2 minutes for tritiated water, 25–55 hours for Cl³⁶, 50–85 hours for radioactive sulfate, approximately 50 hours (at the onset) for radioactive phosphate and 20–30 minutes (at the onset) for Ca⁴⁵. The time constants for the last two tracers were found to increase during experiments, suggesting chemical binding of these tracers with the substance of the membrane and the axoplasm. The effects of repetitive stimulation, potassium depolarization, calcium and magnesium deficiency, etc., upon the movement of the tracers were investigated. Expressed in terms of Krogh's permeability, the ratio of the sodium permeability to the potassium permeability varied under different experimental conditions within the range of 2:1–1:5. The existence of negative fixed charges in the squid axon excitable membrane complex was suggested.