Activation of three types of membrane currents by various divalent cations in identified molluscan pacemaker neurons

Abstract

We investigated membrane currents activated by intracellular divalent cations in two types of molluscan pacemaker neurons. A fast and quantitative pressure injection technique was used to apply Ca2+ and other divalent cations. Ca2+ was most effective in activating a nonspecific cation current and two types of K+ currents found in these cells. One type of outward current was quickly activated following injections with increasing effectiveness for divalent cations of ionic radii that were closer to the radius of Ca2+ (Ca2+ > Cd2+ > Hg2+ > Mn2+ > Zn2+ > Co2+ > Ni2+ > Pb2+ > Sr2+ > Mg2+ > Ba2+). The other type of outward current was activated with a delay by Ca2+ > Sr2+ > Hg2+ > Pb2+. Mg2+, Ba2+, Zn2+, Cd2+, Mn2+, Co2+, and Ni2+ were ineffective in concentrations up to 5 mM. Comparison with properties of Ca2+-sensitive proteins related to the binding of divalent cations suggests that a Ca2+-binding protein of the calmodulin/troponin C type is involved in Ca2+-dependent activation of the fast-activated type of K+ current. The sequence obtained for the slowly activated type is compatible with the effectiveness of different divalent cations in activating protein kinase C. The nonspecific cation current was activated by Ca2+ > Hg2+ > Ba2+ > Pb2+ > Sr2+ a sequence unlike sequences for known Ca2+-binding proteins. © 1989, Rockefeller University Press., All rights reserved.