Hypoxia-Induced amine secretion from rodent carotid body and adrenal chromaffin cells: Evidence against NADPH oxidase as an 02 sensor

Abstract

An adequate supply of oxygen (02) is essential to the survival of all higher organisms. The mammalian carotid body, located at the common carotid artery senses blood levels of 0 2, carbon dioxide (C02) and acidity. Glomus cells, or type I cells in the carotid body are the main 0 2-sensors which regulate blood p02 via reflex control of ventilation. The carotid body secretes multiple neurotransmitters including dopamine (DA), which is potentiated during low p02 levels and is thought to modulate sensory signaling by apposing afferent nerve fibers. Catecholamine (CA) release is also critical for the animal's ability to survive hypoxic stress associated with the birthing process and the transition to extrauterine life. However, the source for this CA release (primarily epinephrine; EPI) is from adrenal chromaffin cells. The primary 02-sensor in both adrenal chromaffin cells and carotid body type I cells is unknown. One potential candidate is the cytochrome b55s/NADPH oxidase complex that generates the respiratory burst in phagocytes. To test this hypothesis, cultured adrenal medulla chromaffin cells and intact carotid bodies from wild type (WT) and oxidase deficient (OD) mice (knockout gp91 phox, the glycoprotein subunits in the NADPH oxidase complex) were investigated. High performance liquid chromatography and immunocytochemistry were used to quantify amine release in these two chemoreceptors following exposure to hypoxia. Both WT and OD chromaffin cells and carotid bodies responded to the hypoxic challenge with increased monoamine secretion. Norepinephrine and epinephrine were the principal amines released from chromaffin cells, compared to dopamine and serotonin from carotid bodies. These findings suggest that NADPH oxidase is not the primary 02- sensor in either chemosensory system. Quantification of monoamine secretion in intact carotid body from mouse and rat was also compared under basal conditions and after exposure to hypoxia and acid/hypercapnia (pH 7.10). Significantly larger amounts of basal serotonin was secreted from mouse carotid body as compared to the rat. Interestingly, serotonin release was potentiated by hypoxia in mouse carotid body, but this was not observed in the rat. Additionally, ratio of basal level serotonin-to-dopamine secretion was significantly higher in mouse than rat CB. Surprisingly, acid/hypercapnic (pH 7.1 0) had no detectable effect on amine secretion from either mouse or rat carotid body.