Angiotensin (ANG) IV stimulation of pulmonary artery (PA) endothelial cells (PAECs) but not of PA smooth muscle cells (PASMCs) resulted in significant increased production of cGMP in PASMCs. ANG IV receptors are not present in PASMCs, and PASMC nitric oxide synthase activity was not altered by ANG IV. ANG IV caused a dose-dependent vasodilation of U-46619-precontracted endothelium-intact but not endothelium-denuded PAs, and this response was blocked by the ANG IV receptor antagonist divalinal ANG IV but not by ANG II type 1 and 2 receptor blockers. ANG IV receptor-mediated increased intracellular Ca(2+) concentration ([Ca(2+)](i)) release from intracellular stores in PAECs was blocked by divalinal ANG IV as well as by the G protein, phospholipase C, and phosphoinositide (PI) 3-kinase inhibitors guanosine 5'-O-(2-thiodiphosphate), U-73122, and LY-294002, respectively, and was regulated by both PI 3-kinase- and ryanodine-sensitive Ca(2+) stores. Basal and ANG IV-mediated vasorelaxation of endothelium-denuded PAs was restored by exogenous PAECs but not by exogenous PAECs pretreated with the intracellular Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM. These results demonstrate that ANG IV-mediated vasodilation of PAs is endothelium dependent and regulated by [Ca(2+)](i) release through receptor-coupled G protein-phospholipase C-PI 3-kinase signaling mechanisms.