INTRACELLULAR PH AND OXYGEN CHEMORECEPTION IN THE CAT CAROTID-BODY INVITRO

To test the hypothesis that O2 chemoreception in the carotid body (CB) is mediated by cellular acidosis, we simultaneously measured responses of the chemosensory and intracellular pH (pH(i)) to agents that are known to change pH(i) and studied the effects of hypoxia and ischemia on these variables in the cat CB. The CB was perfused and superfused in vitro with a modified Tyrode's solution at 36.0 +/- 0.5-degrees-C with or without CO2-HCO3- (pH 7.40) and equilibrated at a given PO2. Chemosensory discharges were recorded from the,whole carotid sinus nerve. To measure pH(i) changes, the CB was loaded with the pH-sensitive indicator 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, and the fluorescence (excitation 420-490 nm, emission >515 nm) was detected by an intensified charged coupled device camera with an epifluorescence macroscope. Boluses of Tyrode's solution (0.5 ml, free of CO2-HCO3-) containing sodium acetate or NH4Cl prolonged perfusion of acid Tyrode's solution (pH 7.20-6.50), and boluses of Tryrode's solution with CO2-HCO3- were used. A decrease of florescence indicated pH(i) turning acid, and an increase of fluorescence indicated a change in alkaline pH(i). Chemosensory activity varied inversely with the fluorescence change after application of these agents. Interruption of perfusate flow or application of hypoxic perfusate resulted in large increases in chemosensory discharge without any change in the fluorescence. The results indicated that chemosensory responses to brief ischemia and hypoxia were not mediated by a fall of pH(i) of CB cells, whereas those to CO2 and extracellular acidity were associated with decreases in pH(i).