Cationic membrane conductances induced by intracellularly elevated cAMP and Ca2+: measurements with ion-selective...

Adenosine 3',5'-cyclic monophosphate (cAMP) and CaCl2 were injected by a fast and quantitative pressure injection technique into voltage-clamped, identified Helix neurons. Intracellular elevation of cAMP as well as of Ca2+ activated an inward current (IcAMP and IN). To identify the ionic fluxes during IcAMP and IN changes in [Na+]i, [K+]o, [H+]i, and [Cl-]i were measured with ion-selective microelectrodes (ISMs). Near resting potential, Na+ was the main carrier of IcAMP. K+, and less effectively Ca2+, could substitute for Na+ in carrying IcAMP. H+ and Cl- were excluded as current carriers for IcAMP by means of ISMs. Simultaneous to this action, cAMP decreased a K+ conductance. This decrease was associated with a reduction of the K+ efflux activated by long-lasting depolarizing voltage steps, as directly measured with ISMs located near the external membrane surface. The nearly compensatory increase and decrease of two membrane conductances in the same neuron left the cell input resistance unchanged despite the considerable depolarizing action of intracellularly elevated cAMP. IN was also of nonspecific nature. However, our findings indicate less selectivity for the Ca2+-activated nonspecific channels. Large cations such as choline, TEA, and Tris passed nearly as well as Na+ through the channels. Measurements with ISMs showed that [H+]i and [Cl-]i were unchanged during IN. IN was largest in bursting pacemaker neurons compared with other cells of similar size. It was found to be essential for the burst production in these cells. IcAMP, on the other hand, might be involved in the presynaptic facilitatory action of cAMP, which as yet was attributed solely to a reduction of a K+ conductance.