This study investigated the mechanisms underlying epithelium-derived hyperpolarizing factor (EpDHF)-type relaxation in rat bronchioles. Immunohistochemistry was performed, and rat bronchioles and pulmonary arteries were mounted in microvascular myographs for functional studies. An opener of small (SK(Ca)) and intermediate (IK(Ca))-conductance calcium-activated potassium channels, NS309 (6,7-dichloro-1H-indole-2,3-dione 3-oxime) was used to induce EpDHF-type relaxation. IK(Ca) and SK(Ca)3 positive immunoreactions were observed mainly in the epithelium and endothelium of bronchioles and arteries, respectively. In 5-hydroxytryptamine (1 microM)-contracted bronchioles (828 +/- 20 microm, n = 84) and U46619 (0.03 microM)-contracted arteries (720 +/- 24 microm, n = 68), NS309 (0.001-10 microM) induced concentration-dependent relaxations that were reduced by epithelium/endothelium removal and by blocking IK(Ca) channels with charybdotoxin and in bronchioles also by blocking SK(Ca) channels with apamin. Inhibition of cyclooxygenase, nitric oxide synthase, and cytochrome 2C isoenzymes, or blockade of large (BK(Ca))-conductance calcium-activated potassium channels with iberiotoxin, failed to reduce NS309 relaxation. In contrast to the pulmonary arteries, relaxations to a beta(2)-adrenoceptor agonist, salbutamol, were reduced in bronchioles by removing the epithelium or blocking IK(Ca) and/or SK(Ca) channels. Extracellular K(+) (2-20 mM) induced relaxation in both bronchioles and arteries. An inhibitor of Na(+)-K(+)-ATPase, ouabain, abolished relaxations to NS309, salbutamol, and K(+). These results suggest that IK(Ca) and SK(Ca)3 channels are located in the epithelium of bronchioles and endothelium of pulmonary arteries. Analog to the endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in pulmonary arteries, these channels may be involved in EpDHF-type relaxation of bronchioles caused by epithelial K(+) efflux followed by activation of Na(+)-K(+)-ATPase in the underlying smooth muscle layer.