Influence of sulphur dioxide breathing on defensive reflexes of the airways.

The influence of sulphur dioxide (SO2) on cough and expiratory reflexes was studied in 22 anaesthetized (pentobarbital, 30 mg/kg i.v.) rabbits. The cough reflex (CR) was elicited by tracheobronchial mucous membrane stimulation, using a soft venous catheter. In order to induce the expiratory reflex (ER) irritation of the larynx was performed by a silon fibre loop. The strength of both reflexes was assessed from the interpleural pressure fluctuation before SO2 breathing, immediately after, and 30-120 minutes after SO2 breathing had been stopped. The animals breathed SO2 in 200-300 ppm concentration through a tracheal cannula. The influence of SO2 on the direct dependence of lung inflation pressure magnitude (LIPM) on strength of the expiratory reflex (ERS) was tested. It was found that elicitability and strength of the cough reflex decreased immediately after SO2 breathing had been stopped, and did not reach the preexposure level at the end of the experiment, i.e. 2 hours after discontinuation of SO2 breathing. The direct dependence of LIPM on ERS in unaffected rabbits [10] was abolished immediately after SO2 breathing had been stopped but was present again 30-60 minutes later, and did not differ significantly from the control values. The results give evidence of the strong depressive influence of SO2 on the defensive reflexes of the airways in rabbits.