['Gap junction channels provide for direct electrical coupling between cells, and play an important role in homeostasis and electrical coupling. One of the proteins that form gap junctions, Connexin40 (Cx40), shows restricted expression in the body, and is found in blood vessels and in the atrium and conduction system of the heart. We have investigated whether gap junction channels formed of Cx40 are modulated by protein-kinase-A-mediated phosphorylation.', 'A communication-deficient human hepatoma cell line (SKHep1) was stably transfected with human Cx40 cDNA and the properties of Cx40 gap junctions channels and their modulation by cAMP were analyzed using immunocytochemistry, Western blotting, dual patch clamp, and dye coupling.', 'Administration of 1 mM 8-Br-cAMP resulted in a mobility shift of Cx40 protein on western blot and increased macroscopic gap junctional conductance between cell pairs by 46.2 +/- 12.0% (mean +/- S.E.M., n = 8). Under control conditions, single channel experiments revealed three single channel conductances around 30, 80 and 120 pS. When cAMP was added, channel conductances of 46 and 120 pS were observed. In monolayers, cAMP also increased the permeability of Cx40 gap junction channels for Lucifer Yellow by 58%.'] Are human connexin40 gap junction channels modulated by cAMP?