Role of vagal lung C-fibres in the cardiorespiratory effects of capsaicin in monkeys

Abstract

Apnoea, bradycardia and hypotension were elicited by right atrial injections of capsaicin in anaesthetized monkeys. At the threshold dose (2.5 ± 0.3 μg/kg), tachypnoea was elicited (latency 1.6 ± 0.2 s) which got replaced by apnoea with higher doses of capsaicin. These responses persisted (1) after cooling the cervical vagi to 6–8°C, and (2) after instilling xylocaine into the pericardial sac. Tachypnoea and apnoea were elicited after bilateral cervical vagotomy also, but only with higher doses and after a longer latency (5.0 ± 0.3 s). Right atrial injection of capsaicin and insufflation of halothane stimulated vagal pulmonary C-fibre receptors with a latency of 1.7 ± 0.7 s and 0.2 ± 0.1 s, respectively. Tachypnoea/apnoea, bradycardia and hypotension were elicited by left atrial injection of capsaicin also (threshold dose: 5.0 ± 1.2 μg/kg). The respiratory responses persisted (1) after instilling xylocaine into the pericardial sac, and (2) after bilateral cervical vagotomy suggesting that they were due to stimulation of non-cardiac receptors with sympathetic afferents. It is concluded that the initial respiratory responses elicited by right atrial injection of capsaicin were due to stimulation of pulmonary C-fibre receptors with vagal afferents.