Role of divalent cations in excitation of squid giant axons

Abstract

The effects of divalent cations on resting potential and excitability of intracellularly perfused squid giant axons were studied. Reduction of the external divalent cation concentration produced a reversible increase in intracellular negativity. Although excitability could be maintained in axons immersed in univalent cation-free external media, it was not possible to demonstrate excitability in media free of divalent cations. A significant increase in Ca ion efflux during excitation was demonstrated through radiotracer techniques. Intracellularly applied Ca ion resulted in a reversible suppression of action potential amplitude and a lowering of membrane resistance. The effects of anesthetic agents and non-electrolytes on excitability in external media free of univalent cations were qualitatively similar to effects in sodium-containing media. The significance of these findings and implications for excitation theory are discussed.