Cyclic AMP enhances agonist-induced Ca2+ entry into endothelial cells by activation of potassium channels and membrane hyperpolarization

The mechanism underlying cyclic AMP (cAMP)-mediated amplification of agonist-induced Ca2+ responses in endothelial cells was investigated in pig endothelial cells. Forskolin, adenosine and isoprenaline, as well as the membrane-permeant cAMP analogue dibutyryl cAMP, enhanced bradykinin-induced rises in intracellular free Ca2+ as well as bradykinin-induced Mn2+ entry. These agents were also found to hyperpolarize endothelial cells without increasing intracellular Ca2+ by itself, i.e. in the absence of bradykinin. Both amplification of bradykinin effects and the hyperpolarizing action was blocked by the protein kinase inhibitor H-8. The involvement of K+ channels in the hyperpolarizing effects of forskolin was consequently studied in perforated outside-out vesicles. Two different types of K+ channels were recorded, one of which had a large conductance (170 pS) and was activated by forskolin. We suggest that stimulation of endothelial adenylate cyclase results in activation of large-conductance K+ channels and consequently in membrane hyperpolarization, which in turn enhances bradykinin-induced entry of Ca2+ by increasing its electrochemical gradient.