Dopamine increases in cat carotid body during excitation by carbon monoxide: Implications for a chromophore theory of chemoreception

Studies of dopamine (DA) release were conducted with 10 perfused/superfused cat carotid bodies using shallow recessed Nafion polymer-coated microsensors (tips approximately 5 microns). Simultaneous measurements of tissue DA and neuronal discharge (ND) from the sinus nerve were made after switching from normoxic, normocapnic control perfusate (20% O2, 5% CO2, balance N2) to a normoxic, normocapnic perfusate equilibrated with a high tension (> 550 Torr) of carbon monoxide (CO). When high PCO perfusate was delivered in the dark, ND increased from a baseline of 89 +/- 24 (SE) impulses/s, to a peak excitation of 374 +/- 44 impulses/s within 15-30 s. Excitation then diminished to a plateau of 281 +/- 36 impulses/s within 1-2 min. Both peak and plateau ND were significantly above baseline (P < 0.05). Average tissue DA values increased above basal levels by +7.2 +/- 1.0 and +5.6 +/- 0.6 microns, respectively, during the peak and plateau ND phases (P < 0.05). Bright light restored the chemosensory activity to baseline, but had no effect on DA. Both chemosensory excitation and tissue DA responses to high CO in the dark were diminished in 3 carotid bodies perfused with Ca(2+)-free solutions. Responses were reduced even further with Ca2+ chelator (EGTA) in the perfusate. The results suggest that the effect of high PCO on DA release and chemosensory excitation are dependent on Ca2+ in the media, but the two events are not coupled.