Effects of low oxygen on the release of dopamine from the rabbit carotid body in vitro

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Abstract
Rabbit carotid bodies were preloaded with [3H]dopamine (DA) synthesized from [3H]tyrosine and then mounted in a vertical drop-type superfusion chamber which permitted simultaneous collection of released [3H]DA and recording of chemoreceptor discharge from the carotid sinus nerve. The time course of the spontaneous release [3H]DA (superfusion with media equilibrated with 100% O2) in the presence of monoamine oxidase inhibitors exhibited 2 linear components, an initial steep phase followed after 3-4 h by a later slower phase of release. When a 5 min low O2 stimulus was delivered during the initial steep linear component of resting [3H]DA release, there was an abrupt increase in release, the magnitude of which was stimulus-dependent. The efflux of total radioactivity from the preparation declined exponentially with time; under resting conditions it was principally nonmetabolized [3H]tyrosine. During stimulation, the efflux increased, and 60-80% of the radioactivity could be attributed to [3H]DA. For a given low O2 stimulus, the ratio of [3H]DA release during the stimulus period over that in the preceding control period remained approximately the same throughout a single experiment. Ratios for different low O2 stimuli (50, 40, 30, 20, 10 and 0% O2 in N2) yielded a parabolic relationship when plotted against stimulus intensity. Transection of the carotid sinus nerve or removal of the superior cervical ganglion 12-15 days prior to the experiment did not affect the release of [3H]DA at moderate stimulus intensities (superfusion with media equilibrated with 30 or 10% O2 in N2) but both procedures significantly depressed release at the highest stimulus intensity (100% N2). Chemoreceptor discharge and [3H]DA release were simultaneously monitored in experiments using superfusion media free of monoamine oxidase inhibitors. In these experiments, the efflux of [3H]dihydroxyphenyl acetic acid (DOPAC) was also measured. The increase in peak chemosensory discharge was closely correlated with the increase in total release ([3H]DA + [3H]DOPAC) during stimulation with a series of low O2 stimuli. Release of [3H]DA was almost completely abolished during superfusion with Ca2+-free, high Mg2+ (2.1 mM) media, and the stimulus-related efflux of [3H]DOPAC was significantly reduced. Chemoreceptor discharge was diminished by only 55%. These data are discussed with respect to their implications for DA as a chemosensory transmitter in rabbit carotid body.