Gap junction-dependent increases in smooth muscle cAMP underpin the EDHF phenomenon in rabbit arteries

We have investigated the role of cAMP in nitric oxide (NO)- and prostanoid- independent vascular relaxations evoked by acetylcholine (ACh) in isolated arteries and perfused ear preparations from the rabbit. These EDHF-type res ponses are shown to be associated with elevated cAMP levels specifically in smooth muscle and are attenuated by blocking adenylyl cyclase or protein k inase A (PKA). Relaxations are amplified by 3-isobutyl-1-methylxanthine, wh ich prevents cAMP hydrolysis, while remaining susceptible to inhibition by the combination of two K-Ca channel blockers, apamin and charybdotoxin. Ana logous endothelium- and cAMP-dependent relaxations were evoked by cyclopiaz onic acid (CPA) which stimulates Ca2+ influx via channels linked to the dep letion of Ca2+ stores. Responses to ACh and CPA were both inhibited by inte rrupting cell-to-cell coupling via gap junctions with 18 alpha -glycyrrheti nic acid and a connexin-specific Gap 27 peptide. The findings suggest that EDHF-type responses are initiated by capacitative Ca2+ influx into the endo thelium and propagated by direct intercellular communication to effect rela xation via cAMP/PKA-dependent phosphorylation events in smooth muscle. (C) 2001 Academic Press.