Effects of CO2-HCO3- on catecholamine efflux from cat carotid body

Using a chronoamperometric technique with carbon-fiber microelectrodes and neural recordings, we simultaneously measured the effects of the following procedures on catecholamine efflux (Delta CA) and frequency of chemosensory discharges (f(x)) from superfused cat carotid body: 1) the addition of CO2-HCO3- to Tyrode solution previously buffered with N-2-hydroxyethylpiperazine-N'-2-ethane-sulfonic acid, maintaining pH at 7.40; 2) hypercapnia (10% CO2, pH 7.10); 3) hypoxia (Po-2 h approximate to 40 Torr) with and without CO2-HCO3-; and 4) the impact of several boluses of dopamine (DA; 10-100 mu g) on hypoxic and hypercapnic challenges. With CO2-HCO3-, hypoxia increased f(x) which preceded Delta CA increases, whereas hypercapnia raised f(x) but did not consistently increase Delta CA. Repeated stimuli induced similar f(x) increases, but attenuated Delta CA. After DA, hypoxia produced larger Delta CA, which preceded chemosensory responses. Without CO2-HCO3-, hypoxia produced a similar pattern of Delta CA and f(x) responses. Switching to Tyrode solution with CO2-HCO3- at pH 7.40 raised f(x) but did not increase Delta CA. With CO2-HCO3- and after DA, hypoxic-induced Delta CAs were larger than in its absence. Results suggest that DA release is not essential for chemosensory excitation.