Na+ and Ca2+ cooperatively down regulate the A-type potassium currents in Helix neurons

Abstract

Modulatory actions of Na+, Tris+, Li+ and Ca2+ on A-type potassium currents were investigated in identified neurons of the snail, Helix pomatia L. A-currents were isolated under spike threshold voltages by use of the double pulse method in combination with computer techniques. To estimate the action of Na+ on A-type potassium currents the normal physiological solution was changed to Na-free (mannitol) medium which increased the amplitude of the A-currents at -30 mV membrane potential by 20.5% without significant modulation of the time-dependent inactivation. When Tris-HCl or LiCl substituted for NaCl, the amplitude of the A-currents decreased by 27.5 or 32.5%, respectively. Li+ did not modify the time constant of decay of the A-currents, but Tris+ decreased it by 15.8%. There was an increase of the amplitude of the A-currents in the Na-, Ca-free (Tris) medium relative to the Na-free (Tris) solution by 27.7% and a decrease of the time constant of decay by 25.3%. It is concluded that A-currents are down regulated by Na+ and Ca2+ under physiological circumstances, but the modulatory action of Ca2+ is more complex and influences the amplitude, time-dependent inactivation and potential-dependent inactivation characteristics of IA. Among monovalent cations, the A-current attenuation increases in the sequence of Na+ < Tris+ < Li+. All identified neurons react in a similar way but with different sensitivities.