Carbon monoxide, formed as a product of heme oxygenase activity, has been postulated to act as an intra- and intercellular messenger molecule. We addressed the hypothesis that heme oxygenase is involved in the relaxation of the guinea pig trachealis elicited by vasoactive intestinal peptide (VIP) or by electrical field stimulation. Immunohistochemical studies revealed the presence of heme oxygenase-II in airway smooth muscle and epithelium. Zinc protoporphyrin-IX (ZnPPn), an inhibitor of heme oxygenase, effectively inhibited VIP-induced relaxations of tracheal smooth muscle. Surprisingly, the potency of ZnPPn was increased if the drug was preincubated with the VIP solution before addition to the tissue bath. The relaxant responses to 3-morpholinosydnonimine were unaffected by ZnPPn. Zinc deuteroporphyrin-IX 2,4 bisglycol, a more potent inhibitor of heme oxygenase than ZnPPn, did not affect the VIP responses. ZnPPn (300 microM) had no effect on nonadrenergic, noncholinergic relaxations of the guinea pig trachea. These data indicate that although ZnPPn is an efficacous inhibitor of VIP-induced relaxations of the guinea pig trachealis, it is unlikely that heme oxygenase plays an important role in this response. Rather, the data are consistent with the hypothesis that ZnPPn inhibits the VIP response via an interaction with the VIP molecules themselves. Although the results demonstrate the existence of heme oxygenase-II in the guinea pig trachealis, they do not support the hypothesis that it plays a role in electrical field stimulation-induced nonadrenergic, noncholinergic relaxations.