Effect of hypoxia and hypercapnia on catecholamine content in cat carotid body

The purpose of this study was to determine the content of catecholamines (CA) in the cat carotid body before and after 0.5 h exposures to normoxic normocapnia [arterial O₂ partial pressure (PaO₂) 126 ± 28 Torr, arterial CO₂ partial pressure (PaCO₂) 36.4 ± 1.5 Torr], hypoxic normocapnia (PaO₂ 25 ± 3 Torr, PaCO₂ 36.7 ± 3.3 Torr), and normoxic hypercapnia (PaO₂ 132 ± 13 Torr, PaCO₂ = 98.2 ± 7.6 Torr). CA synthesis was blocked using α-methylparatyrosine methyl ester (AMPT) prior to alterations in the inspired air. There was a significant decrease in carotid body content of dopamine (DA), norepinephrine (NE), and epinephrine (E) 1 h after AMPT administration. Analysis of variance and Duncan new multiple range procedures revealed that during the subsequent 0.5-h exposures to normoxia, hypoxia, or hypercapnia, only the decrease in DA during hypoxia was significantly greater than that during normoxia; the loss during hypercapnia was not. The decreases in NE during the three exposures were indistinguishable among themselves as were the decreases in E. The decrease in CA content is probably attributable to increased release. The data reveal that the release of CAs during the chemoreception of hypoxia is different from that during the chemoreception of hypercapnia and support the concept of different mechanisms for the chemoreception of hypoxia and hypercapnia.