The effect of intracellular cAMP injections on stationary membrane conductance and voltage- and time-dependent ionic currents in identified snail neurons.

3'5'-cAMP injected iontophoretically into identified neurons of the snail, Helix pomatia, induced depolarization of the membrane. Clamping the membrane potential revealed the appearance of a simultaneous inward transmembrane current ('cAMP-current') followed by a weaker outward current. External application of theophylline increased the amplitude of cAMP-current. Imidazole had an opposite effect on this current. Tolbutamide and lowering of temperature largely reduced its rate of rise and, correspondingly, its amplitude. Simultaneous removal of Na+ ions from external solution and addition of Cd2+ ions resulted in complete disappearance of the inward cAMP-current. Analysis of current-voltage characteristics of the cAMP-current at varying external concentrations of Na+, K+, Cl and Ca2+-ions has shown that the cAMP-current is due to an increase of membrane conductance to Na+, K+ and Ca2+ ions; a late component of the cAMP-current is associated with an increase of potassium conductance of the neuronal membrane induced, probably by Ca2+, influx. Besides the induction of stationary currents, cAMP injection also decreased the voltage- and time-dependent calcium currents reducing the maximum calcium conductance. After the end of injection, calcium currents restored their initial value in 1-2 min. An analogous decrease of the calcium current could be evoked by prolonged external application of theophylline. Possible mechanisms of intracellular effects of cAMP on electrical characteristics of the neuronal membrane are discussed.