Effects of extracellular Ca and Ca-channel blockers on A-currents in snail brain neurons.

A study was made to characterize the extracellular Ca sensitivity of the A-currents in identified neurons (LPa2, 3) of the snail, Helix pomatia L. The characteristics and voltage-dependence of activation and inactivation of isolated A-currents were examined under voltage clamp in normal, low Ca and high Ca solutions. Modification of the extracellular Ca concentration between 0 and 28 mM caused a dose-dependent attenuation of the amplitude of A-currents and alteration of the current kinetics. Anorganic Ca-channel blockers (Ni, Cd, La) also decreased the amplitude of A-currents in a dose-dependent reversible manner, but the organic Ca-channel blocker verapamil did not influence the A-current amplitude. An analysis of the Hill plots of Ca and Ni dose-inhibition data showed similar values for the Hill constants (nH) with 1.3 and 1.2 respectively. Lowering the potassium concentration in the saline increased the amplitude of A-currents, while Na-deficiency slightly decreased the amplitude of A-currents. Tetraethylammonium (TEA) modulated the amplitude and kinetics of A-currents and 4-aminopyridine (4-AP) blocked all components of the A-currents.