Intracellular dnalent cations and plateau duration of squid giant axons treated with tetraethylammonium

Abstract

Squid giant axons were intracellularly perfused with a solution containing 100 mM K+ and 5 mM tetraethylammonium (TEA) to obtain action potentials with a long-lasting plateau. Effects of intracellular divalent cations on the plateau of the action potentials were examined by introducing 0.1-10 mM Ca2+, Sr2+, Mg2+, Ba2+, or Mn2+ into the perfusate under these conditions. Ca2+, Sr2+ and Mn2+ were found to shorten the plateau duration drastically. Upon removal of the divalent cations in the perfusate, there was a recovery in the plateau duration. The recovery was imperfect in the case of Ca2+, nearly perfect in the case of Sr2+, and perfect in the case of Mn2+. Voltage-clamp experiments carried out on axons intracellularly perfused with the control (i.e., divalent cation free) perfusate revealed the existence of a late inward current through the membrane. Intracellularly administered Mn2+ (1-3 mM) blocked this late inward current and decreased the membrane conductance during the late period of voltage clamping. Intracellular Mg2+ (1-10 mM) prolonged the action potential plateau produced by TEA reversibly in some axons; however, this effect was masked by an opposing (i.e., irreversibly plateau-shortening) effect in most axons. Ba2+ applied intracellularly prolonged the action potential duration regardless of the presence or absence of TEA in the perfusate. We emphasize that the effects of divalent cations applied internally are very similar to those observed when these cations are applied externally, except that the effective concentration is far lower in the case of internal application. It is suggested that the primary site of action of divalent cations might be on or near the internal surface of the axonal membrane. © 1977, PHYSIOLOGICAL SOCIETY OF JAPAN. All rights reserved.