Potassium efflux of squid giant axons in low external divalent cation

¹ Department of Pharmacology, University of Cincinnati College of Medicine Cincinnati, Ohio, and the Marine Biological Laboratory, Woods Hole, Massachusetts

In squid (Loligo pealii) giant axons, potassium efflux was measured after soak-loading during washout, and membrane potentials were monitored using an intracellular pipette electrode. As external divalent cation is lowered down to 1/10 of normal, there is little change of resting potassium efflux or resting membrane potential (slight hyperpolarization), despite a tendency of the axon to fire repetitively. Below this concentration, efflux rises as much as eight times the normal 265 pmole·cm^–2·sec^–1 and potential falls (from 55 as far as 32 mv), inexcitability occurring at about 1/100 of normal external divalent cation concentration.