Pulmonary oedema produced by scorpion venom augments a phenyldiguanide-induced reflex response in anaesthetized rats.

1. The involvement of pulmonary oedema produced by scorpion venom in augmenting a phenyldiguanide (PDG)-induced reflex response was evaluated in urethane-anaesthetized rats. 2. PDG-induced bradycardiac, hypotensive and apnoeic responses, expressed as time-response area, exhibited similarities before or after venom treatment. Hence, the time-response area of bradycardia was taken as a reflex parameter. Pulmonary oedema was determined by physical evaporation and histological methods. 3. Exposure to Indian red scorpion (Buthus tamulus, BT; i.v.) venom for 30 min increased the pulmonary water content (P < 0.05; Student's t test) and augmented the PDG-induced bradycardiac reflex response by more than 2 times (P < 0.001). The increase of pulmonary water content was maximal with 100 microg kg-1 of venom and the augmentation was maximal with 10 microg kg-1. In a separate series of experiments, the venom (100 microg kg-1)-induced pulmonary oedema was confirmed by histological and physical methods. In this group also, the venom augmented the reflex to the same magnitude. 4. Pulmonary oedema (physical and histological) and augmentation of the bradycardiac reflex response after BT venom (100 microg kg-1; i.v.) were absent in animals pretreated with aprotinin, a kallikrein-kinin inhibitor (6000 KIU; i. v.). 5. Ondansetron (10 microg kg-1; i.v.), a 5-HT3 receptor antagonist, failed to block the venom-induced pulmonary oedema (physical and histological) but blocked the venom-induced augmentation of the reflex. 6. The results of this study indicate that the venom-induced augmentation of the PDG reflex is associated with pulmonary oedema involving kinins utilizing 5-HT3 receptors.