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Conde SV, Monteiro EC, Rigual R, Obeso A, Gonzalez C. Hypoxic intensity: a determinant for the contribution of ATP and adenosine to the genesis of carotid body chemosensory activity. J Appl Physiol 112: 2002-2010, 2012. First published April 12, 2012 doi:10.1152/japplphysiol.01617.2011.-Excitatory effects of adenosine and ATP on carotid body (CB) chemoreception have been previously described. Our hypothesis is that both ATP and adenosine are the key neurotransmitters responsible for the hypoxic chemotransmission in the CB sensory synapse, their relative contribution depending on the intensity of hypoxic challenge. To test this hypothesis we measured carotid sinus nerve (CSN) activity in response to moderate and intense hypoxic stimuli (7 and 0% O 2) in the absence and in the presence of adenosine and ATP receptor antagonists. Additionally, we quantified the release of adenosine and ATP in normoxia (21% O 2) and in response to hypoxias of different intensities (10, 5, and 2% O 2) to study the release pathways. We found that ZM241385, an A 2 antagonist, decreased the CSN discharges evoked by 0 and 7% O 2 by 30.8 and 72.5%, respectively. Suramin, a P 2X antagonist, decreased the CSN discharges evoked by 0 and 7% O 2 by 64.3 and 17.1%, respectively. Simultaneous application of both antagonists strongly inhibited CSN discharges elicited by both hypoxic intensities. ATP release by CB increased in parallel to hypoxia intensity while adenosine release increased preferably in response to mild hypoxia. We have also found that the lower the O 2 levels are, the higher is the percentage of adenosine produced from extracellular catabolism of ATP. Our results demonstrate that ATP and adenosine are key neurotransmitters involved in hypoxic CB chemotransduction, with a more relevant contribution of adenosine during mild hypoxia, while vesicular ATP release constitutes the preferential origin of extracellular adenosine in high-intensity hypoxia.adenosine; hypoxia; carotid body CAROTID BODIES (CB) are major peripheral chemoreceptor organs that release neurotransmitters in response to hypoxia, generating action potentials at the carotid sinus nerve (CSN) that are integrated in the brain stem to induce a hyperventilatory compensatory response (17). Among the neurotransmitters released in the CB are adenosine and ATP (1-3). One of the first studies on the effects of ATP on CSN activity was carried out by McQueen and Ribeiro (24) where they showed in vivo in the cat CB that ATP increased CSN activity in a dosedependent manner. Additional experiments led them to conclude that the excitatory action of ATP was due to the adenosine formed by its extracellular degradation. More recently, Zhang and co-workers (47) showed that the coapplication of blockers of nicotinic and P2X receptors, hexamethonium and suramin, respectively, in cocultures of glomus cells and "juxtaposed" petrosal ganglions completely abolished the hypoxiaevoked excitatory postsynaptic responses. They concluded that ATP and ACh, acting as cotransmitters, supported the s...

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Caffeine, an unspecific antagonist of adenosine receptors, is commonly used to treat the apnea of prematurity. We have defined the effects of caffeine on the carotid body (CB) chemoreceptors, the main peripheral controllers of breathing, and identified the adenosine receptors involved.

The aim of this work was to determine contents and turnover rates for dopamine (DA) and norepinephrine (NE) and to identify the catecholamine (CA) released during stimulation of the rat carotid body (CB). Turnover rates and the release of CA were measured in an in vitro preparation using a combination of HPLC and radioisotopic methods. Mean rat CB levels of DA and NE were 209 and 45 pmol/mg tissue, respectively. With [(3)H]tyrosine as precursor, rat CB synthesized [(3)H]CA in a time- and concentration-dependent manner; calculated turnover times for DA and NE were 5.77 and 11.4 h, respectively. Hypoxia and dibutyryl adenosine 3',5'-cyclic monophosphate significantly increased [(3)H]CA synthesis. In normoxia, rat CB released [(3)H]DA and [(3)H]NE in a ratio of 5:1, comparable to that of the endogenous tissue CA. Hypoxia and high K(+) preferentially released [(3)H]DA, nicotine preferentially released [(3)H]NE, and acidic stimuli released both amines in proportion to tissue content. Release of [(3)H]CA induced by hypoxia and high K(+) was nearly fully dependent on extracellular Ca(2+), whereas basal normoxic release was not altered by removal of Ca(2+) from the incubating solution. We conclude that the rat CB is an organ with higher levels of DA than NE that preferentially releases DA or NE in a stimulus-specific manner.

Hypoxia increases the release of neurotransmitters from chemoreceptor cells of the carotid body (CB) and the activity in the carotid sinus nerve (CSN) sensory fibers, elevating ventilatory drive. According to previous reports, perinatal hyperoxia causes CSN hypotrophy and varied diminishment of CB function and the hypoxic ventilatory response. The present study aimed to characterize the presumptive hyperoxic damage. Hyperoxic rats were born and reared for 28 days in 55%-60% O 2 ; subsequent growth (to 3.5-4.5 months) was in a normal atmosphere. The parenchyma of the carotid body (CB) is formed by chemoreceptor and sustentacular cells organized in clusters surrounded by a dense network of capillaries. Sensory nerve terminals of the carotid sinus nerve (CSN) penetrate the clusters to synapse with chemoreceptor cells (Verna, 1997). Functionally, chemoreceptor cells are activated by hypoxia and hypercapnia, and they respond with an increased release of neurotransmitters that activate the sensory nerve endings of the CSN and produce an increase in the action potential frequency in the CSN. Central projections of the CSN produce a ventilatory response, which facilitates homeostasis of blood O 2 and CO 2 levels. Catecholamines (CA) are the most abundant neurotransmitters present in chemoreceptor cells. Many groups have demonstrated that CA metabolism, including rate of synthesis and release, parallel the level of CB stimulation and action potential frequency in the CSN (Gonzalez et al. , 1994 and references therein). In adult mammals the CB is responsible for the entire hyperventilation evoked by hypoxia and for about 30-50% of the hyperventilation triggered by hypercapnia and acidosis (Cherniack & Altose 1997;Gonzalez et al. 2002a). In the intact animal a decrease in arterial P O 2 from 100 to about 75 mmHg produces only minor changes in the basal level of CSN activity or ventilation. At P O 2 below the apparent threshold of 75 mmHg there is an almost exponential increase in either response. CSN activity and ventilation double at about 50-55 mmHg, and increase by a factor of four near 40 mmHg. In the case of CO 2 the activity in the CSN and the ventilation mediated by the CB increase linearly with the P CO 2 , doubling every 15-20 mmHg (Gonzalez et al. 1994). In neonatal animals, the apparent threshold for the hypoxic response is set at a much lower P O 2 , in the range of 20-25 mmHg, i.e. at P O 2 comparable to that found in utero. At lower P O 2 , the hypoxic response increases with a lower slope than in adults. In response to elevated CO 2 there is a comparable hyposensitivity in newborn animals (Hanson & Kumar 1994;Donnelly, 1997). Functional maturation of the CB during postnatal life occurs in the first few weeks after birth, with some differences among species. In the rat, at four weeks of age the responses are fully developed (Eden & Hanson 1987a;Donnelly & Doyle 1994;Rigual et al. 2000;Donnelly, 1997). Maturation of CB function is greatly affected by the ambient P O 2 in the perinatal period. Thus, if animals ...

Some age-related deficits in the ventilatory responses have been attributed to a decline in the functionality of the carotid body (CB) arterial chemoreceptors, but a systematic study of the CB function in ageing is lacking. In rats aged 3-24 months, we have performed quantitative morphometry on specific chemoreceptor tissue, assessed the function of chemoreceptor cells by measuring the content, synthesis and release of catecholamines (a chemoreceptor cell neurotransmitter) in normoxia and hypoxia, and determined the functional activity of the intact organ by measuring chemosensory activity in the carotid sinus nerve (CSN) in normoxia, hypoxia and hypercapnic acidosis. We found that with age CBs enlarge, but at the same time there is a concomitant decrease in the percentage of chemoreceptor tissue. CB content and turnover time for their catecholamines increase with age. Hypoxic stimulation of chemoreceptor cells elicits a smaller release of catecholamines in rats after 12 months of age, but a non-specific depolarizing stimulus elicits a comparable release at all ages. In parallel, there was a marked decrease in the responsiveness to hypoxia, but not to an acidic-hypercapnic stimulus, assessed as chemosensory activity in the CSN. We conclude that in aged mammals chemoreceptor cells become hypofunctional, leading to a decreased peripheral drive of ventilation.

Hypoxia elicits catecholamine (CA) secretion from the adrenal medulla (AM) in perinatal animals by acting directly on chromaffin cells. However, whether innervation of the AM, which in the rat occurs in the second postnatal week, suppresses this direct hypoxic response is the subject of debate. Opioid peptides have been proposed as mediators of this suppression. To resolve these controversies, we have compared CA-secretory responses with high external concentrations of K+ ([K+]e) and hypoxia in the AM of neonatal (1- to 2-day-old) and juvenile (14- or 15- and 30-day-old) rats subjected to superfusion in vitro. In addition, we studied the effect of hypercapnic acidosis on the CA-secretory responses in the AM during postnatal development and the possible interaction between acidic and hypoxic stimuli. Responses to high [K+]e were comparable at all ages, but responses to hypoxia and hypercapnic acidosis were maximal in neonatal animals. Suppression of the hypoxic response in the rat AM was not mediated by opioids, because their agonists did not affect the hypoxic CA response. The association of hypercapnic acidosis and hypoxia, mimicking the episodes of asphyxia occurring during delivery, generates a more than additive secretory response in the neonatal rat AM. Our data confirm the loss of the direct sensitivity to hypoxia of the AM in the initial weeks of life and demonstrate a direct response of neonatal AM to hypercapnic acidosis. The synergistic effect of hypoxia and acidosis would explain the CA outburst crucial for adaptation to extrauterine life observed in naturally delivered mammals.