The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Specialized O 2 -sensing cells exhibit a particularly low threshold to regulation by O 2 supply and function to maintain arterial pO 2 within physiological limits. For example, hypoxic pulmonary vasoconstriction optimizes ventilation-perfusion matching in the lung, whereas carotid body excitation elicits corrective cardio-respiratory reflexes. It is generally accepted that relatively mild hypoxia inhibits mitochondrial oxidative phosphorylation in O 2 -sensing cells, thereby mediating, in part, cell activation. However, the mechanism by which this process couples to Ca 2؉ signaling mechanisms remains elusive, and investigation of previous hypotheses has generated contrary data and failed to unite the field. We propose that a rise in the cellular AMP/ATP ratio activates AMP-activated protein kinase and thereby evokes Ca 2؉ signals in O 2 -sensing cells. Co-immunoprecipitation identified three possible AMP-activated protein kinase subunit isoform combinations in pulmonary arterial myocytes, with ␣1␤2␥1 predominant. Furthermore, their tissue-specific distribution suggested that the AMP-activated protein kinase-␣1 catalytic isoform may contribute, via amplification of the metabolic signal, to the pulmonary selectivity required for hypoxic pulmonary vasoconstriction. Immunocytochemistry showed AMPactivated protein kinase-␣1 to be located throughout the cytoplasm of pulmonary arterial myocytes. In contrast, it was targeted to the plasma membrane in carotid body glomus cells. Consistent with these observations and the effects of hypoxia, stimulation of AMPactivated protein kinase by phenformin or 5-aminoimidazole-4-carboxamide-riboside elicited discrete Ca 2؉ signaling mechanisms in each cell type, namely cyclic ADP-ribose-dependent Ca 2؉ mobilization from the sarcoplasmic reticulum via ryanodine receptors in pulmonary arterial myocytes and transmembrane Ca 2؉ influx into carotid body glomus cells. Thus, metabolic sensing by AMP-activated protein kinase may mediate chemotransduction by hypoxia.Specialized O 2 -sensing cells within the body have evolved as vital homeostatic mechanisms that monitor O 2 supply and alter respiratory and circulatory function, as well as the capacity of the blood to transport O 2 . By these means, arterial pO 2 is maintained within physiological limits. Two key systems involved are the pulmonary arteries and the carotid body. Constriction of pulmonary arteries by hypoxia optimizes ventilation-perfusion matching in the lung (1), whereas carotid body excitation by hypoxia initiates corrective changes in breathing patterns via increased sensory afferent discharge to the brain stem (2). Although O 2 -sensitive mechanisms independent of mitochondria may also play a role (3-5), it is generally accepted that relatively mild hypoxia inhibits mitochondrial oxidative phosphorylation and that this underpins, at least in part, cell activation (2, 6 -10). Despite this consensus, the mechanism by which inhibition of mitochondrial oxidative phosphorylation couples to discrete cell-specific Ca 2ϩ signaling ...

Rationale: Modulation of breathing by hypoxia accommodates variations in oxygen demand and supply during, for example, sleep and ascent to altitude, but the precise molecular mechanisms of this phenomenon remain controversial. Among the genes influenced by natural selection in high-altitude populations is one for the adenosine monophosphate-activated protein kinase (AMPK) a1-catalytic subunit, which governs cell-autonomous adaptations during metabolic stress.Objectives: We investigated whether AMPK-a1 and/or AMPK-a2 are required for the hypoxic ventilatory response and the mechanism of ventilatory dysfunctions arising from AMPK deficiency.Methods: We used plethysmography, electrophysiology, functional magnetic resonance imaging, and immediate early gene (c-fos) expression to assess the hypoxic ventilatory response of mice with conditional deletion of the AMPK-a1 and/or AMPK-a2 genes in catecholaminergic cells, which compose the hypoxia-responsive respiratory network from carotid body to brainstem.Measurements and Main Results: AMPK-a1 and AMPK-a2 deletion virtually abolished the hypoxic ventilatory response, and ventilatory depression during hypoxia was exacerbated under anesthesia. Rather than hyperventilating, mice lacking AMPK-a1 and AMPK-a2 exhibited hypoventilation and apnea during hypoxia, with the primary precipitant being loss of AMPK-a1 expression. However, the carotid bodies of AMPK-knockout mice remained exquisitely sensitive to hypoxia, contrary to the view that the hypoxic ventilatory response is determined solely by increased carotid body afferent input to the brainstem. Regardless, functional magnetic resonance imaging and c-fos expression revealed reduced activation by hypoxia of well-defined dorsal and ventral brainstem nuclei.Conclusions: AMPK is required to coordinate the activation by hypoxia of brainstem respiratory networks, and deficiencies in AMPK expression precipitate hypoventilation and apnea, even when carotid body afferent input is normal.

The most physiologically important sensors for systemic glucoregulation are located in extracranial sites. Recent evidence suggests that the carotid body may be one such site. We assessed rat carotid body afferent neural output in response to lowered glucose, indirectly by measurement of ventilation, and directly by recording single or few-fibre chemoafferent discharge, in vitro. Insulin (0.4 U kg −1 min −1 )-induced hypoglycaemia (blood glucose reduced by ca 50% to 3.4 ± 0.1 mmol l −1 ) significantly increased spontaneous ventilation (V E ) in sham-operated animals but not in bilateral carotid sinus nerve sectioned (CSNX) animals. In both groups, metabolic rate (measured asV O 2 ) was almost doubled during hypoglycaemia. The ventilatory equivalent (V E /V O 2 ) was unchanged in the sham group leading to a maintained control level of P a,CO 2 , buṫ V E /V O 2 was significantly reduced in the CSNX group, giving rise to an elevation of 6.0 ± 1.3 mmHg in P a,CO 2 . When pulmonary ventilation in sham animals was controlled and maintained, phrenic neural activity increased during hypoglycaemia and was associated with a significant increase in P a,CO 2 of 5.1 ± 0.5 mmHg. Baseline chemoreceptor discharge frequency, recorded in vitro, was not affected, and did not increase when the superfusate [glucose] was lowered from 10 mM to 2 mM by substitution with sucrose: 0.40 ± 0.20 Hz to 0.27 ± 0.15 Hz, respectively (P > 0.20). We suggest therefore that any potential role of the carotid bodies in glucose homeostasis in vivo is mediated through its transduction of some other metabolically derived blood-borne factor rather than glucose per se and that this may also provide the link between exercise, metabolic rate and ventilation.

Early detection of an O 2 deficit in the bloodstream is essential to initiate corrective changes in the breathing pattern of mammals. Carotid bodies serve an essential role in this respect; their type I cells depolarize when O 2 levels fall, causing voltage-gated Ca 2؉ entry. Subsequent neurosecretion elicits increased afferent chemosensory fiber discharge to induce appropriate changes in respiratory function (1). Although depolarization of type I cells by hypoxia is known to arise from K ؉ channel inhibition, the identity of the signaling pathway has been contested, and the coupling mechanism is unknown (2). We tested the hypothesis that AMP-activated protein kinase (AMPK) is the effector of hypoxic chemotransduction. AMPK is co-localized at the plasma membrane of type I cells with O 2 -sensitive K ؉ channels. In isolated type I cells, activation of AMPK using 5-aminoimidazole-4-carboxamide riboside (AICAR) inhibited O 2 -sensitive K ؉ currents (carried by large conductance Ca 2؉ -activated (BK Ca ) channels and TASK (tandem pore, acidsensing potassium channel)-like channels, leading to plasma membrane depolarization, Ca 2؉ influx, and increased chemosensory fiber discharge. Conversely, the AMPK antagonist compound C reversed the effects of hypoxia and AICAR on type I cell and carotid body activation. These results suggest that AMPK activation is both sufficient and necessary for the effects of hypoxia. Furthermore, AMPK activation inhibited currents carried by recombinant BK Ca channels, whereas purified AMPK phosphorylated the ␣ subunit of the channel in immunoprecipitates, an effect that was stimulated by AMP and inhibited by compound C. Our findings demonstrate a central role for AMPK in stimulus-response coupling by hypoxia and identify for the first time a link between metabolic stress and ion channel regulation in an O 2 -sensing system.Chronic and intermittent deficits in O 2 supply to the body precipitate a variety of pathologies including dementia (3) and pulmonary hypertension (4). To develop effective therapies, it is necessary to understand the homeostatic mechanisms that monitor O 2 supply to the body and elicit corrective changes in respiratory and circulatory function to maintain O 2 levels. O 2 -sensitive ion channels, which were first identified in the carotid body type I cell, play a pivotal role in this respect and have now been reported in a diverse range of highly specialized O 2 -sensing tissues (5). Within the carotid body, clusters of type I cells lie in presynaptic contact with afferent sensory fibers, whose discharge increases in proportion to the degree of systemic arterial O 2 deficit, providing information concerning blood O 2 levels to the central respiratory centers (1, 2). This occurs subsequent to hypoxic inhibition of type I cell K ϩ channels, membrane depolarization, voltage-gated Ca 2ϩ influx (6), and consequent neurotransmitter release. For many years, there has existed compelling evidence that mitochondria serve an important role in O 2 sensing by type I cells (2, 7). Inde...

1. The effect of PCO2 upon the discharge response to changes in P02 (between ca 450 and 30 mmHg) was observed in adult (> 5 weeks old) and neonatal (5-7 days old) rat carotid body chemoreceptors using an in vitro, superfused preparation.2. In both adult and neonatal rats, regression analysis revealed that increasing Pco2 was without effect upon the shape of the P02-response curves (P > 0 30), but did cause an upward shift in the position of the curves, as indicated by a significant increase in the baseline chemoreceptor discharge during hyperoxia (0 22 + 0 02% maximum discharge per mmHg PC02, P< 0001, and 0-25 + 007% maximum discharge per mmHg PCo2' P < 0 005, respectively). However, whilst increasing Pco2 caused a significant rightward shift of the response curves in adults (0 75 + 0-23 mmHg P02 per mmHg PC02; P < 0 005), it was without effect in neonates (Ofi21 + 0 fi22 mmHg Po2 per mmHg Pco2; P > 0 200). Thus increasing levels of hypoxia increased CO2 chemosensitivity in adult but not in neonatal rats as shown by multiple regression analysis of the C02-response curves which revealed a significant interaction between Pco2 and P02 for adult (P < 0 010) but not for neonatal (P> 0 150) rats. 3. We suggest that the previously reported maturation of peripheral chemoreceptor hypoxic sensitivity (resetting) may be due to the postnatal emergence of a significant degree of interaction between Pco, and Po0 at the level afferent innervation.

Key pointsr Hypoglycaemia is counteracted by release of hormones and an increase in ventilation and CO 2 sensitivity to restore blood glucose levels and prevent a fall in blood pH.r The full counter-regulatory response and an appropriate increase in ventilation is dependent on carotid body stimulation.r We show that the hypoglycaemia-induced increase in ventilation and CO 2 sensitivity is abolished by preventing adrenaline release or blocking its receptors.r Physiological levels of adrenaline mimicked the effect of hypoglycaemia on ventilation and CO 2 sensitivity.r These results suggest that adrenaline, rather than low glucose, is an adequate stimulus for the carotid body-mediated changes in ventilation and CO 2 sensitivity during hypoglycaemia to prevent a serious acidosis in poorly controlled diabetes.Abstract Hypoglycaemia in vivo induces a counter-regulatory response that involves the release of hormones to restore blood glucose levels. Concomitantly, hypoglycaemia evokes a carotid body-mediated hyperpnoea that maintains arterial CO 2 levels and prevents respiratory acidosis in the face of increased metabolism. It is unclear whether the carotid body is directly stimulated by low glucose or by a counter-regulatory hormone such as adrenaline. Minute ventilation was recorded during infusion of insulin-induced hypoglycaemia (8-17 mIU kg −1 min −1 ) in Alfaxan-anaesthetised male Wistar rats. Hypoglycaemia significantly augmented minute ventilation (123 ± 4 to 143 ± 7 ml min −1 ) and CO 2 sensitivity (3.3 ± 0.3 to 4.4 ± 0.4 ml min −1 mmHg −1 ). These effects were abolished by either β-adrenoreceptor blockade with propranolol or adrenalectomy. In this hypermetabolic, hypoglycaemic state, propranolol stimulated a rise in P aCO 2 , suggestive of a ventilation-metabolism mismatch. Infusion of adrenaline (1 μg kg −1 min −1 ) increased minute ventilation (145 ± 4 to 173 ± 5 ml min −1 ) without altering P aCO 2 or pH and enhanced ventilatory CO 2 sensitivity (3.4 ± 0.4 to 5.1 ± 0.8 ml min −1 mmHg −1 ). These effects were attenuated by either resection of the carotid sinus nerve or propranolol. Physiological concentrations of adrenaline increased the CO 2 sensitivity of freshly dissociated carotid body type I cells in vitro. These findings suggest that adrenaline release can account for the ventilatory hyperpnoea observed during hypoglycaemia by an augmented carotid body and whole body ventilatory CO 2 sensitivity.

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