Endothelins and nitric oxide: Vasoactive modulators of carotid body chemoreception

The carotid body (CB) is the main arterial chemoreceptor that senses arterial PO2, PCO2 and pH. The structural unit of the CB is the glomoid, which is formed by clusters of chemoreceptor (glomus) cells located around the capillaries. The glomus cells are synaptically connected to nerve terminals of petrosal ganglion (PG) neurons and surrounded by sustentacular cells. The most accepted model of CB chemoreception states that glomus cells are the primary sensors. In response to hypoxia, hypercapnia and acidosis, glomus cells release one or more transmitters, which acting on the nerve terminals of sensory PG neurons, increase the chemosensory discharge. The CB has a high blood flow and an elevated metabolism that correlate to its oxygen-sensing function. Thus, vasoactive molecules produced within the CB may modulate the chemosensory process by controlling the CB blood flow and tissue PO2. In this review, we examine recent evidence supporting the idea that endothelins (ETs) and nitric oxide (NO) modulate the CB function acting upon chemoreceptor cells and chemosensory neurons or by regulating the blood flow through the CB parenchyma.