Abstract
Carotid bodies are sensory organs for monitoring arterial oxygen and CO2. Previous studies have shown that chemoreceptor tissue contains substance P (SP) and exogenously administered SP augments chemosensory discharge. In the present study, we examined the physiological importance of SP in carotid body chemoreception by using a selective nonpeptide SP [neurokinin (NK) 1] receptor antagonist CP-96,345. In experiments performed on anesthetized cats, sensory discharge was recorded from the carotid body in situ. To control for alterations in blood flow, additional studies were conducted on the carotid body in vitro. In in vivo studies, close carotid body (intraarterial) administration of CP-96,345 attenuated the sensory response to hypoxia in a dose-dependent manner with 73% of the response abolished at doses of 0.3-0.6 mg/kg. Comparable doses of the (2R,3R)-enantiomer had no effect on hypoxia-induced excitation, indicating that the effect of CP-96,345 was not due to nonspecific action. In contrast, the carotid body response to high CO2 was not affected by CP-96,345, implying that only the hypoxic response is mediated by NK-1 receptor and confirming that the effect of the SP antagonist was not due to nonspecific actions. Marked attenuation of the sensory response to hypoxia was also obtained in the carotid body in vitro, suggesting that the effects of the NK-1 antagonist were not secondary to cardiovascular changes. These results demonstrate that CP-96,345 attenuates or abolishes the chemosensory response to hypoxia but not to CO2 and suggest that SP mediates the hypoxia-induced sensory excitation in the cat carotid body via NK-1 receptor activation.