Analysis by microcomputer of the effect of capsaicin on pulmonary mechanics in the rat and guinea-pig.

Analysis of airways resistance and lung compliance in anaesthetized rats and guinea-pigs was accomplished on-line by microcomputer. In spontaneously respiring guinea-pigs capsaicin (0.5-2.5 microgram/kg i.v.) increased both resistance by 0.33 +/- 0.13 cmH20/ml/sec and the end-expiratory lung volume and it decreased compliance by 0.24 +/- 0.09 ml/cmH2O. These effects of capsaicin were resistant to vagotomy. In the rat, doses of capsaicin (40-80 micrograms/kg i.v. and i.a.) which produced respiratory apnoea had no effect on resistance or compliance in artificially ventilated animals but it did cause a decrease of end-expiratory volume, an effect which was abolished by vagotomy. The results confirm that reduction in tidal volume seen in spontaneously respiring rats following capsaicin (Mitchell et al., 1984) appears to be due to an action other than one on the bronchial calibre.