Forskolin has been used to stimulate adenylyl cyclase. However, we found that forskolin inhibited voltage-sensitive Ca2+ channels (VSCCs) in a cyclic AMP (cAMP)-independent manner in PC12 cells. Ca2+ influx induced by membrane depolarization with 70 mM K+ was inhibited when cells were preincubated with 10 microM forskolin. Almost maximum inhibitory effect on Ca2+ influx without any significant increase in cellular cAMP level was observed in PC12 cells exposed to forskolin for 1 min. In addition, the forskolin effect on Ca2+ influx was not affected by the presence of 2',5'-dideoxyadenosine, an inhibitor of adenylyl cyclase that reduces dramatically forskolin-induced cAMP production. 1,9-Dideoxyforskolin, an inactive analogue of forskolin, also inhibited approximately 80% of Ca2+ influx induced by 70 mM K+ without any increase in cAMP. The data suggest that forskolin and its analogue inhibit VSCCs in PC12 cells and that the inhibition is independent of cAMP generation.