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

1. Rabbit carotid bodies were pre-loaded with [ 3 H]dopamine (DA) synthesized from [ 3 H]tyrosine and then mounted in a vertical drop-type superfusion chamber which permitted simultaneous collection of released [ 3 H]DA and recording of chemoreceptor discharge from the carotid sinus nerve. 2. The time course of the spontaneous release of [ 3 H]DA (superfusion with media equilibrated with 100% O 2 ) in the presence of monoamine oxidase inhibitors exhibited two linear components, an initial steep phase followed after 3-4 hr by a later slower phase of release. 3. When a 5 min low O 2 stimulus was delivered during the initial steep linear component of resting [ 3 H]DA release, there was an abrupt increase in release, the magnitude of which was stimulus-dependent. 4. The efflux of total radioactivity from the preparation declined exponentially with time; under resting conditions it was principally non-metabolized [ 3 H]tyrosine. During stimulation, however the efflux increased, and 60-80% of the radioactivity could be attributed to [ 3 H]DA. 5. For a given low O 2 stimulus, the ratio of [ 3 H]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 O 2 stimuli (50, 40, 30, 20, 10 and 0% O 2 in N 2 ) yielded a parabolic relationship when plotted against stimulus intensity. 6. 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 [ 3 H]DA at moderate stimulus intensities (superfusion with media equilibrated with 30% or 10% O 2 in N 2 ) but both procedures significantly depressed release at the highest stimulus intensity (100% N 2 ). 7. Chemoreceptor discharge and [ 3 H]DA release were simultaneously monitored in experiments using superfusion media free of monoamine oxidase inhibitors. In these experiments, the efflux of [ 3 H]dihydroxyphenyl acetic acid (DOPAC) was also measured. The increase in peak chemosensory discharge was closely correlated with the increase in total release ([ 3 H]DA + [ 3 H]DOPAC) during stimulation with a series of low O 2 stimuli. 8. Release of [ 3 H]DA was almost completely abolished during superfusion with Ca 2+ -free, high Mg 2+ (2·1 mM) media, and the stimulus-related efflux of [ 3 H]DOPAC was significantly reduced. However, chemoreceptor discharge was diminished by only 55%. These data are discussed with respect to their implica