Does capsaicin cause reflex bronchospasm in guinea-pigs?

Capsaicin given intravenously (i.v.) into the vena cava or intra-arterially (i.a.) into the aortic arch of anesthetized guinea-pigs induced dose-dependent increases in pulmonary-flow resistance (R) and dynamic thoracic elastance (E). Threshold doses were 0.5-1.0 micrograms/kg body weight; greater than 8.0 micrograms/kg induced tachyphylaxis. These responses to capsaicin, i.v. or i.a., were similar after decentralization, bilateral vagotomy, or glossopharyngealotomy, and after autonomic blockers (mecamylamine, atropine, mepyramine, and bethanidine). Morphine significantly reduced responses to capsaicin but had no effect on responses to substance P (SP). Naloxone did not reverse the inhibitory effect of morphine, and neither reduced nor enhanced capsaicin-induced increases. Of the two antagonists to SP examined (given i.v.), [D-Pro2,D-Trp7,9]SP had no effect on SP-induced increases but abolished capsaicin-induced increases in R, though without affecting increases in E, and [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP reduced increases in R induced by both SP and capsaicin but had no effect on increases in E. Capsaicin (2.0-32.0 micrograms/kg i.v. or i.a.) had no bronchospastic effect in guinea-pigs given this drug (50.0 mg/kg s.c.) 7 days earlier. We conclude that in guinea-pigs--unlike other species--capsaicin causes bronchospasm without stimulating any afferent receptors in a centrally mediated bronchospastic reflex arc.