Emergence of a chloride permeability after internal perfusion with calcium in squid giant axons.

In the resting condition, the membrane of the squid giant axon is highly permeable to K+, and shows large changes in membrane potential following an increase in external or internal K-concentration. When 3-5 mM Ca2+ was applied internally for 3 min, the membrane became highly permeable to Cl- as compared to K+. The permeability ratio PCl/PK was about 0.77 in control experiments, rising to a ratio of 5.5 after Ca2+ administration. The membrane conductance also increased 6-fold over controls. The permeability ratios for other anions, Cl-, F-, methanesulfate (MS), and HEPES (PCl, PF, PMS, and PHEPES) were 1.00, 0.54, 0.23, and 0.13, respectively.