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

Abstract

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 implications for DA as a chemosensory transmitter in rabbit carotid body.