Stimulus-specific mobilization of dopamine and norepinephrine stores in cat carotid body.

The catecholamines (CAs), dopamine (DA) and norepinephrine (NE), are synthesized and stored in carotid body chemosensory type I cells. Previous studies in our laboratory demonstrated that low concentrations of nicotine preferentially evoke the release of NE from rabbit type I cells, whereas hypoxia mobilizes DA and NE in proportion to their stores in the tissue. The primary objective of the present study was to examine whether hypoxia, nicotine and elevated concentrations (30 mM) of K+ evoke the preferential release of DA vs. NE from cat carotid bodies superfused in vitro. In this species, where tissue stores of DA and NE are nearly equal, hypoxia evoked the preferential release of DA from normal carotid bodies. This pattern of release evoked by low O2 was also present following chronic removal of the superior cervical ganglion, which eliminated NE contained in the sympathetic innervation to the carotid body. In contrast, nicotine and high-K+ preferentially mobilized NE in these sympathectomized animals. Sympathectomy also reduced the percent of DA (but not NE) content released from type I cells in response to any of the three stimuli. Our findings suggest that chemosensory type I cells possess stimulus-specific mechanisms for CA mobilization and that the sympathetic innervation modulates the metabolism and release of CAs in the cat carotid body.