Circulatory and respiratory effects of capsaicin and resiniferatoxin on guinea pigs.

The cardiorespiratory effects of capsaicin and its novel analogue resiniferatoxin (RTX) have been investigated in urethan anaesthetized guinea pigs. Intravenously administered capsaicin (5-20 micrograms/kg) failed to elicit the full pulmonary chemoreflex, but after a latency of 2 seconds caused a short period of tachypnea (6-9 sec) and hypotension without bradycardia. An initial tachypnea was observed in response to 1 microgram/kg intravenously administered RTX, which was followed by a slowly developing shallow breathing, accompanied by an increase in blood pressure after a transient hypotensive effect. RTX inhibited the reflex response evoked by capsaicin for about 10 minutes. After bilateral vagotomy neither tachypnea nor hypotension was observed in response to capsaicin. These results show that in the guinea pig the vagally mediated pulmonary chemoreflex evoked by capsaicin and inhibited by RTX is qualitatively different from that described on other mammalian species (cat, dog, rat, etc.).