CYCLIC-AMP STIMULATES K-CELLS OF VICIA-FABA-K( CHANNEL ACTIVITY IN MESOPHYLL)

Whole-cell patch-clamp recordings from Vicia faba mesophyll protoplast s reveal that outward K+ current is increased in a dose-dependent fash ion by intracellular application of cAMP. The enhancement of the outwa rd current by cAMP is specific and it cannot be mimicked by a series o f nucleotides that includes AMP, cGMP, and GMP. The enhancement is evo ked by micromolar concentrations of cAMP in the presence of the phosph odiesterase inhibitor 3-isobutyl-1-methyl-xanthine. PKI or Walsh inhib itor, a specific peptide inhibitor of cAMP-dependent protein kinase (P KA), 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 mesop hyll cells and suggest that this modulation occurs through a cAMP-regu lated protein kinase.