Expanding role of ATP as a versatile messenger at carotid and aortic body chemoreceptors.

In mammals, peripheral arterial chemoreceptors monitor blood chemicals (e.g. O(2), CO(2), H(+), glucose) and maintain homeostasis via initiation of respiratory and cardiovascular reflexes. Whereas chemoreceptors in the carotid bodies (CBs), located bilaterally at the carotid bifurcation, control primarily respiratory functions, those in the more diffusely distributed aortic bodies (ABs) are thought to regulate mainly cardiovascular functions. Functionally, CBs sense partial pressure of O(2) ( ), whereas ABs are considered sensors of O(2) content. How these organs, with essentially a similar complement of chemoreceptor cells, differentially process these two different types of signals remains enigmatic. Here, we review evidence that implicates ATP as a central mediator during information processing in the CB. Recent data allow an integrative view concerning its interactions at purinergic P2X and P2Y receptors within the chemosensory complex that contains elements of a 'quadripartite synapse'. We also discuss recent studies on the cellular physiology of ABs located near the aortic arch, as well as immunohistochemical evidence suggesting the presence of pathways for P2X receptor signalling. Finally, we present a hypothetical 'quadripartite model' to explain how ATP, released from red blood cells during hypoxia, could contribute to the ability of ABs to sense O(2) content.