ELECTROCHEMICAL DETECTION OF CATECHOLAMINE RELEASE FROM RAT CAROTID-BODY INVITRO

Neurotransmitter secretion from carotid body glomus cells is hypothesized to be an essential element of chemotransduction. To address one aspect of this hypothesis, catecholamine release in response to hypoxic hypoxia and histotoxic hypoxia was examined using electrically treated carbon-fiber microelectrodes placed in rat carotid bodies in vitro. Carotid bodies of mature rats were removed, along with a portion of the sinus nerve, and suspended in oxygenated (95% O2-5% CO2) Ringer saline at 35-degrees-C. The microelectrode differential current after a 50-mV step was recorded over the potential range of -300 to +500 mV. In some preparations, a suction electrode applied to the sinus nerve recorded single-fiber chemoreceptor afferent activity. Stimulation by severe hypoxia (PO2 approximately 0-10 Torr for 3 min, n = 10) and cyanide (2 mM for 2 min) caused an increase in sinus nerve activity and an increase in the carbon-fiber electrode current at a potential corresponding to the oxidation potential of dopamine. As measured in the amperometric mode (constant voltage), tissue catecholamine was 0.35 +/- 0.05 muM (n = 6) and increased to 1.64 +/- 0.43 muM by 1 min of severe hypoxia or to 1.06 +/- 0.17 muM at 2 min of moderate hypoxia (PO2 approximately 50 Torr). Exposure to calcium-free Ringer saline before hypoxia ablated the increase in electrode current, and the response was restored after reperfusion with calcium-containing saline. Repeated exposures to hypoxia (3-min duration) every 15 min resulted in significantly smaller nerve and catecholamine responses. By the third hypoxia exposure, nerve and catecholamine responses were diminished by 30-50%. A single prolonged (20-min) exposure to hypoxia resulted in a peak catecholamine current at 3 min that diminished to a steady-state value of approximately 30% of peak. Nerve activity also peaked early and decreased to a steady-state level. These results show that an oxidizable chemical (likely dopamine) is released by the rat carotid body during chemostimulation, and this release is dependent on extracellular calcium. The results suggest that catecholamine release may be causal to hypoxia transduction, because the nerve and catecholamine responses change in parallel.