The diterpene forskolin is widely known for its ability to directly activate adenylyl cyclase and consequently increase intracellular cAMP. In cardiac cells, one result is a cAMP-mediated increase in the L-type Ca2(+)-channel current (ICa). However, forskolin was also shown recently to affect a number of ionic channels in noncardiac cells by mechanisms that do not involve activation of adenylyl cyclase. The present study reveals such an effect of forskolin on cardiac Ca2+ channels. Indeed, under appropriate conditions, forskolin was found to cause an inhibition of ICa. Although the stimulation of adenylyl cyclase and ICa requires micromolar concentrations of forskolin, the inhibitory effect of forskolin was observed in the nanomolar range of concentrations, i.e., 2-3 orders of magnitude lower. This high affinity forskolin inhibition of ICa was observed when ICa was previously enhanced via a cAMP-dependent pathway, but not when ICa was at its basal level or when the current was elevated by the dihydropyridine Bay K 8644. The inhibitory effect occurred at a site of action remote from adenylyl cyclase, because forskolin similarly inhibited ICa that had been previously elevated by isoprenaline (a beta-adrenergic agonist) or directly by intracellular perfusion with cAMP. Under these conditions, forskolin was inhibitory when applied to either side of the cell membrane, but only in its lipid-soluble form. The inhibitory effect of forskolin appeared to be independent of membrane potential and was not accompanied by a change in the time constants of ICa activation and inactivation. This may indicate that forskolin mainly reduces the number of functional Ca2+ channels without changing the gating of individual channels. However, the reduction in ICa amplitude was not equally distributed among the different exponential components that constitute ICa, which suggests that forskolin also modifies the resting state of the channels. This novel high affinity forskolin inhibition of ICa may take place at some step in the pathway between cAMP and Ca2+ channel phosphorylation and/or at Ca2+ channels only after they have been phosphorylated.