BACKGROUND Airway hyperresponsiveness (AHR) is one of the characteristic features of human asthma. The presence of AHR and the precise mechanisms immediately after establishment of sensitization in guinea pigs are unclear, although there are many reports showing allergen exposure that causes an increase in bronchial responsiveness associated with eosinophil influx into the airway in sensitized guinea pigs. OBJECTIVE We investigated the inhibitory effects on AHR to histamine of ONO-1078, a leukotriene antagonist; indomethacin, a cyclooxygenase inhibitor; S-145, a thromboxane A(2) (TXA(2)) antagonist, and Y-24180, a platelet-activating factor (PAF) antagonist, to assess the involvement of chemical mediators in AHR employing ovalbumin (OA) sensitized guinea pig models. METHODS Male Hartley guinea pigs were used. Each group comprised 4-7 animals. The animals were sensitized to OA, injecting intraperitoneally 30 mg of cyclophosphamide and 2,000 microg of OA together with 100 mg of aluminum hydroxide as the adjuvant. The guinea pigs were artificially ventilated via a cannula using a small-animal respirator after intraperitoneal anesthesia with pentobarbital sodium for tracheotomy. The pressure at the airway opening (PAO) was measured using a differential pressure transducer, and a differential pressure of peak PAO (peak DeltaPAO) at inspiratory phase as an overall index of bronchial response to bronchoactive agents was used. While being artificially ventilated, the animals were exposed to physiological saline solution containing various concentrations of histamine (4.9, 9.8, 20, 39, 78, and 156 microg/ml) by inhalation for 30 s at 3-min intervals. Determinations were made at 1 min after each inhalation. The chemical mediators were each (30 mg/kg of ONO-1078, 3 mg/kg of S-1452, and 1 mg/kg of Y-24180) administered orally to sensitized guinea pigs, and the airway response to histamine was assessed. Each group comprised 4-7 animals. RESULTS The airway response to histamine was significantly greater in the sensitized group than in the nonsensitized group at histamine concentrations of 36 (p < 0.05), 78, and 156 mg/ml (p < 0.01). Leukotrienes C(4) and D(4): 30 mg/kg of ONO-178 did not show any inhibitory effect on airway response to inhaled histamine. Cyclooxygenase: 5 mg/kg of indomethacin did not show any inhibitory effect on the airway response to inhaled histamine. TXA(2): the AHR to inhaled histamine at doses of 9.8, 39, 78, and 156 microg/ml was significantly inhibited by prior administration of 3 mg/kg of S-1452. PAF: the AHR to inhaled histamine at doses of 9.8, 39, and 78 microg/ml was significantly inhibited by prior administration of 1 mg/kg of Y-24180. CONCLUSIONS S-1452 (3 mg/kg) and Y-24180 (1 mg/kg) significantly inhibited AHR to histamine, while ONO-108 (30 mg/kg) and indomethacin (5 mg/kg) did not. The results suggest that TXA(2) and PAF are involved in AHR in OA-sensitized guinea pigs.