RELEASE OF DOPAMINE AND CHEMORECEPTOR DISCHARGE INDUCED BY LOW PH AND HIGH PCO2 STIMULATION OF THE CAT CAROTID-BODY

1. Cat carotid bodies were incubated with the precursor [H-3] tyrosine to label the catecholamine deposits and then mounted in a superfusion chamber which allowed simultaneous collection of the released [H-3] dopamine (DA) and recording of action potentials from the carotid sinus nerve. 2. Low pH (7.2-6.6) superfusion of the carotid bodies for periods of 10 min produced a parallel increase in the release of [H-3] DA and chemoreceptor discharge. 3. Carotid sinus nerve denervation of the carotid body 12-15 days prior to the experiments did not modify the release of [H-3] DA elicited by low pH. 4. Superfusion of the carotid bodies with Ca2+ -free, high-Mg2+ (1.6 mM) media reduced basal release of [H-3] DA and chemoreceptor discharge by about 30 %. Release evoked by low pH was reduced by 82 %. Peak and average chemoreceptor discharge recorded in response to low pH were reduced by 28 %. 5. Solutions containing weak acids (sodium acetate, 10 mM), adjusted at pH 7.4, elicited release of [H-3] DA and increased chemoreceptor discharge. 6. With HCO3 -CO2-buffered superfusion media, a reduction of bicarbonate to 5.6 mM (pH 6.8), an increase in CO2 to 20 % (pH 6.8), or a simultaneous increase in CO2 to 20 % and bicarbonate to 90 mM (pH 7.4), resulted in all cases in a corresponding increase in [H-3] DA release and chemoreceptor discharge. The most effective stimulus was 20 % CO2 -pH 6.8 and the least effective 5 % CO2 -5.6 mM-HCO3 -pH 6.8. 7. Inhibition of carbonic anhydrase with acetazolamide while perfusing the carotid bodies with a 20 % CO2-equilibrated (pH 7.4) solution resulted in comparable reductions in the release of [H-3] DA and chemoreceptor discharge. 8. It is concluded that the effective acidic stimulus at the carotid body chemoreceptors is an increase in hydrogen ion concentrations in type I cells. It is also concluded that DA plays a critical role in the genesis of carotid sinus nerve discharges.