LOCALIZATION AND ACTIONS OF NITRIC-OXIDE IN THE CAT CAROTID-BODY

An extensive plexus of nerve fibers capable of synthesizing nitric oxi de was demonstrated in the cat carotid body by immunocytochemical and biochemical studies of nitric oxide synthase. Denervation experiments indicated that the axons originate from: (i) microganglial neurons loc ated within the carotid body and along the glossopharyngeal and caroti d sinus nerves, whose ramifications primarily innervate carotid body b lood vessels; and (ii), sensory neurons in the petrosal ganglion, whos e terminals end in association with lobules of type I cells. In the in vitro superfused cat carotid body, the nitric oxide synthase substrat e, L-arginine, induced a dose-dependent inhibition of carotid sinus ne rve discharge evoked by hypoxia. In contrast, the nitric oxide synthas e inhibitor, L-N-G-nitroarginine methylester, augmented the chemorecep tor response to hypoxia, and this effect was markedly enhanced when th e preparation was both perfused and superfused in vitro. The nitric ox ide donor, nitroglycerine, inhibited carotid sinus nerve discharge, an d immunocytochemistry revealed that this drug stimulated the formation of cyclic 3', 5'-guanosine monophosphate in both type I cells and blo od vessels. Our data indicate that nitric oxide is an inhibitory neuro nal messenger in the carotid body, which affects the process of chemor eceptor transduction/transmission via actions on both the receptor ele ments and their associated blood vessels.