The aim of the present study was to provide a mechanistic insight into how 14,15-epoxyeicosatrienoic acid (EET) relaxes organ-cultured human bronchi. Tension measurements, performed on either fresh or 3-d-cultured bronchi, revealed that the contractile responses to 1 microM methacholine and 10 microM arachidonic acid were largely relaxed by the eicosanoid regioisomer in a concentration-dependent manner (0.01-10 microM). Pretreatments with 14,15-epoxyeicosa-5(Z)-enoic acid, a specific 14,15-EET antagonist, prevented the relaxing effect, whereas iberitoxin pretreatments (10 nM) partially abolished EET-induced relaxations. In contrast, pretreatments with 1 microM indomethacin amplified relaxations in explants and membrane hyperpolarizations triggered by 14,15-EET on airway smooth muscle cells. The relaxing responses induced by 14,15-EET were likely related to reduced Ca2+ sensitivity of the myofilaments, because free Ca2+ concentration-response curves performed on beta-escin-permeabilized cultured explants were shifted toward higher [Ca2+] (lower pCa2+ values). 14,15-EET also abolished the tonic responses induced by phorbol-ester-dybutyrate (PDBu) (a protein kinase C [PKC]-sensitizing agent), on both fresh (intact) and beta-escin-permeabilized explants. Western blot analyses, using two specific primary antibodies against CPI-17 and its PKC-dependent phosphorylated isoform (p-CPI-17), confirmed that the eicosanoid interferes with this intracellular process. These data indicate that 14,15-EET hyperpolarizes airway smooth muscle cells and relaxes precontracted human bronchi while reducing Ca2+ sensitivity of fresh and cultured explants. The intracellular effects are related to a PKC-dependent process involving a lower phosphorylation level of CPI-17.