Cyclic AMP stimulates K⁺ channel activity in mesophyll cells of Vicia faba L.

Whole-cell patch-clamp recordings from Vicia faba mesophyll protoplasts reveal that outward K+ current is increased in a dose-dependent fashion by intracellular application of cAMP. The enhancement of the outward current by cAMP is specific and it cannot be mimicked by a series of nucleotides that includes AMP, cGMP, and GMP. The enhancement is evoked by micromolar concentrations of cAMP in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine. PKI or Walsh inhibitor, a specific peptide inhibitor of cAMP-dependent protein kinase (PKA), inhibits the outward K⁺ current. Adenosine 3′,5′-phosphothioate, a competitive inhibitor of PKA, has a similar effect. Conversely, the catalytic subunit of PKA (cAMP independent) from bovine brain enhances the magnitude of the outward K⁺ current in the absence of added cAMP. Our results indicate that cAMP modulates K⁺ channel activity in mesophyll cells and suggest that this modulation occurs through a cAMP-regulated protein kinase.