Ionic currents in cultured mouse neuroblastoma cells under voltage‐clamp conditions.

Abstract

Ionic currents in differentiated cells of mouse neuroblastoma clone N1E-115 were studied under voltage-clamp conditions. Depolarizing voltage steps from a holding potential of -85 mV to levels more positive than -40 mV produced fast transient inward currents followed by delayed outward currents. The fast inward current was carried by Na+; it was blocked by tetrodotoxin and was absent in Na+-free solutions. Its kinetic behavior resembled that of the Na+ current in squid giant axon. A mean value of 85 mmhos/cm2 was found for the maximum Na+ conductance (.hivin.GNa). The delayed outward current was carried primarily by K+; it was blocked by externally applied tetraethylammonium (TEA, 15 mM) and had a reversal potential (mean -71 mV) close to the theoretical K+ equilibrium potential. Its instantaneous I-V curve was linear. By analogy with the formulation of Hodgkin et Huxley, the outward current can be described by IK = .hivin.GKn2(V - EK) where .hivin.GK = 12 mmhos/cm2. During prolonged depolarizations the delayed outward current declined. This decline, which occurs in 2 phases, represents a partial inactivation of the K+ conductance. A weak inward current with slow activation and inactivation kinetics appeared in Na+-free solution containing 10 mM Ca2+. It was activated at a membrane potential of -55 mV and reached its maximum at -20 mV with a time to peak of about 10 ms. This current was tetrodotoxin-resistant, reversibly blocked by Co2+ (5 mM) and may be carried by Ca2+. An increase in the external divalent cation concentration resulted in a parallel shift of the steady-state I-V curve along the voltage axis in positive direction. The activation of delayed outward currents may not depend on Ca2+ influx. Apparently separate voltage-dependent Na+, K+ and Ca2+ channels exist in the differentiated neuroblastoma membrane with kinetic and pharmacological properties similar to those observed in non-mammalian preparations.