Forskolin Activates Adenylate Cyclase Activity and Inhibits Mitosis in In Vitro in Pig Epidermis

The novel adenylate cyclase activator forskolin caused rapid and high intracellular accumulation of cyclic AMP in a floating skin (epidermal) slice system. Increased cAMP levels were also detected in the media. Addition of a phosphodiesterase inhibitor to forskolin-containing medium caused only a slight increase in the intracellular cAMP level and forskolin itself did not inhibit phosphodiesterase activity. Ka of forskolin for epidermal adenylate cyclase was about 2–3 × 10-5 M. This forskolin activation was rapidly reversed after washing. The forskolin stimulation (Ka 5 × 10-5 M) was also found when tested with an epidermal membrane preparation which contained the catalytic unit of adenylate cyclase but lacked either the GTP or receptor stimulation. With the epidermal slice system, the combination of forskolin and epinephrine (or histamine) stimulated adenylate cyclase synergistically. The data suggest that forskolin activates not only the catalytic unit but also the nucleotide regulatory protein or the receptor-regulatory protein complex of the adenylate cyclase system. The cAMP accumulation caused by forskolin produced a dose-dependent mitotic inhibition of epidermal cells in an in vitro outgrowth system. This inhibitory effect was reversible 48 h after washing out the forskolin