Dominant role of cAMP in regulation of microvessel permeability

We reported previously that increasing cAMP levels in endothelial cells att enuated ATP-induced increases in hydraulic conductivity (L-p), and that the activation of cGMP-dependent pathways was a necessary step to increase L-p in response to inflammatory mediators. The aim of the present study was to evaluate the role of basal levels of cAMP in microvessel permeability unde r resting conditions and to evaluate the cross talk between cAMP- and cGMP- dependent signaling mechanisms in regulation of microvessel permeability un der stimulated conditions, using individually perfused microvessels from fr og and rat mesenteries. We found that reducing cAMP levels by inhibition of adenylate cyclase or inhibiting cAMP-dependent protein kinase through the use of H-89 increased basal L-p in both frog and rat mesenteric venular mic rovessels. We also found that 8-bromocAMP (8-BrcAMP, 0.2 and 2 mM) was suff icient to attenuate or abolish the increases in L-p due to exposure of frog mesenteric venular microvessels to 8-Br-cGMP (2 mM) and ATP (10 mu M). Sim ilarly, in rat mesenteric venular microvessels, application of 8-BrcAMP (2 mM) abolished the increases in L-p due to exposure to 8-BrcGMP alone (2 mM) or with the combination of bradykinin (1 nM). In addition, application of erythro-9-(2-hydroxy-3-nonyl)adenine, an inhibitor of cGMP-stimulated phosp hodiesterase, significantly attenuated both 8-BrcGMP- and bradykinin-induce d increases in L-p. These results demonstrate that basal levels of cAMP are critical to maintaining normal permeability under resting conditions, and that increased levels of cAMP are capable of overcoming the activation of c GMP-dependent pathways, therefore preventing increases in microvessel perme ability. The balance between endothelial concentrations of these two opposi ng cyclic nucleotides controls microvessel permeability, and cAMP levels pl ay a dominant role.