Relative responses of aortic body and carotid body chemoreceptors to carboxyhemoglobinemia.

The effects of carbon monoxide inhalation and of consequent carboxyhemoglobinemia (HbCO) on the discharge rates of aortic body and carotid body chemoreceptor afferents were investigated in 18 anesthetized cats. In 10 experiments both aortic and carotid chemoreceptor activities were monitored simultaneously. Carbon monoxide inhalation during normoxia always stimulated aortic chemoreceptors before carotid chemoreceptors, and the steady-state response of aortic chemoreceptors to HbCO was greater than that of most carotid chemoreceptors. Only 2 of the 18 carotid chemoreceptor fibers tested showed a distinct increase in activity in response to moderate increases in HbCO%. Thus, oxyhemoglobin contributed substantially to maintain tissue PO2 of all aortic chemoreceptors and of a few carotid chemoreceptors. Hyperoxia diminished the response of both aortic and carotid chemoreceptors to HbCO, indicating a lowered tissue PO2 as the stimulus source. We hypothesize that the aortic bodies have a much lower perfusion relative to their O2 utilization compared to the carotid bodies. As a consequence, the aortic chemoreceptors are able to act as a sensitive monitor of O2 delivery and to generate a circulatory chemoreflex for O2 homeostasis. carotid chemoreceptors monitor O2 tension and initiate strong reflex effects on the level of ventilation.