The time courses of intracellular free calcium and related electrical effects after injection of CaCl2 into neurons of the snail, Helix pomatia.

Controlled quantities of 100 mM aqueous CaCl2 solutions were pressure injected into voltage-clamped neurons with a resolution of 10(-11) 1. Ca2+-selective microelectrodes monitored the time course of changes in [Ca2+]i. At a membrane potential of -50 mV CaCl2 quantities in the range of 1% of the cell volume induced an inward current, associated with a conductance increase and having an equilibrium potential between -20 and +20 mV, which accompanied the rise in [Ca2+]i. An artifactual origin of the inward current by the injection procedure or by calcium screening of membrane sites could be excluded. The calcium-induced hyperpolarizing conductance, producing an outward current at -50 mV, followed the inward current and reached maximum during the late decline in [Ca2+]i. In most cases its development was separated from the inward current by an intermediate relative decrease of the membrane conductance. Neither of the two transient conductance increases showed a particular dependence on voltage. Renewed Ca2+ injection quickly decreased the calcium-induced hyperpolarizing conductance for several seconds. Ca2+ injections below 0.05% of the cell volume mostly produced pure outward currents or hyperpolarizing responses. Partial substitution of extracellular CaCl2 by NiCl2 decreased the hyperpolarizing response but not the initial inward current. The immediate effects of increased [Ca2+]i are activation of a depolarizing conductance and the partial block of the late hyperpolarizing conductance. The latter is probably produced through intermediate steps after increasing [Ca2+]i.