INHIBITORY SYMPATHETIC ACTION ON THE CAROTID-BODY RESPONSES TO SUSTAINED HYPOXIA

The mammalian carotid bodies receive sympathetic innervation from the superior cervical ganglion. The purposes of the present study were: (1 ) to investigate whether sympathetic innervation influences the caroti d body response to hypoxia, and, if so, (2) to determine the involveme nt of adrenoceptors in these influence. Chemo-sensory activity was rec orded from clearly identifiable action potentials from the carotid sin us nerve in 20 anaesthetized, paralyzed and artifically ventilated cat s. Chemoreceptor responses to sustained isocapnic hypoxia (30 min, dur ation) were compared before and after carotid body sympathectomy (n = 8 cats). In response to low P-o2, chemoreceptor discharge increased du ring the first 10 min, and plateaued for the rest of the hypoxic chall enge. After sympathectomy, chemoreceptor response in the initial 10 mi n was the same; whereas, the magnitude of the response in remaining 20 min was significantly greater than controls (P<0.01). Systemic admini stration of SKF-86466, an alpha(2)-adrenoceptor antagonist augmented t he hypoxic response by 80% (n = 6 cats). In presence of alpha(2)-antag onist, sympathectomy had no further effect on the hypoxic response. Ad ministration of alpha(2)-antagonist in sympathectomized carotid bodies potentiated the hypoxic response, but the magnitude of potentiation w as less than with intact sympathetic innervation (34% vs 80 %; P<0.01; n = 6 cats). From these results, it is concluded that (1) sympathetic innervation exerts an inhibitory influence on chemoreceptor response to sustained hypoxia, and (2) this inhibitory influence is mediated at least in part by alpha(2)-adrenoceptors. The inhibitory effects of sy mpathetic innervation could be of importance in the efferent regulatio n of the carotid body activity during sustained hypoxia.