Forskolin interaction with voltage-dependent K channels in Helix is not mediated by cyclic nucleotides

The effects of the adenylate cyclase activator forskolin upon voltage-activated K(V) currents were investigated in Helix nerve cells. Forskolin induced a fast concentration-dependent inactivation of K(V) currents without modifying the activation kinetics. The induced fast inactivation did not result from a speeding up of the voltage-controlled slow inactivation; it could not be mimicked by intracellular injection of cAMP and it was not potentiated by inhibition of the phosphodiesterase. The forskolin induced K current relaxation can be simulated by a model assuming that the drug binds reversibly to open K channels with a dissociation constant of 16.4 microM.