Periodic responses in squid axon membrane exposed intracellularly and extracellularly to solutions containing a single species of salt.

A periodic membrane potential change was found to occur in squid giant axons which were internally and externally perfused with solutions of an identical composition and were hyperpolarized by passing a sustained inward current. The solution contained Co2+ or Mn2+ as the sole cation species at a concentration of 1-10 mM. The amplitude of the response was roughly 100 mV. The current intensity and the ion concentration had large effects on the response. The voltage-clamp technique revealed an N-shaped I-V characteristic of the membrane system. The membrane emf of the resting and excited states was almost the same but the membrane conductance was increased in the excited states. The response was suppressed with 4-aminopyridine reversibly but unchanged with tetrodotoxin or D-600. Those unusual ionic conditions did not deprive axons of their ability to produce ordinary action potentials in physiological solutions. The experimental conditions employed and the results obtained were very close to those for some of the artificial membrane models. Applicability of the physico-chemical theories developed for these models is discussed.