Intermittent Hypoxia and Diet-Induced Obesity on the Intestinal Wall Morphology in a Murine Model of Sleep Apnea.

This work analyzes the impact of two conditions, intermittent hypoxia exposure and high-fat diet in rats as models of sleep apnea. We studied the autonomic activity and histological structure of the rat jejunum and whether the overlapping of both conditions, as often observed in patients, induces more deleterious effects on the intestinal barrier. We found alterations in jejunum wall histology, predominantly in HF rats, based on increased crypt depth and submucosal thickness, as well as decreased muscularis propria thickness. These alterations were maintained with the IH and HF overlap. An increase in the number and size of goblet cells in the villi and crypts and the infiltration of eosinophils and lymphocytes in the lamina propria suggest an inflammatory status, confirmed by the increase in plasma CRP levels in all experimental groups. Regarding the CAs analysis, IH, alone or combined with HF, causes a preferential accumulation of NE in the catecholaminergic nerve fibers of the jejunum. In contrast, serotonin increases in all three experimental conditions, with the highest level in the HF group. It remains to be elucidated whether the alterations found in the present work could affect the permeability of the intestinal barrier, promoting sleep apnea-induced morbidities.

Effects of Gestational Intermittent Hypoxia on Placental Morphology and Fetal Development in a Murine Model of Sleep Apnea.

Obstructive sleep apnea (OSA) during pregnancy is characterized by episodes of intermittent hypoxia (IH) during sleep, resulting in adverse health outcomes for mother and offspring. Despite a prevalence of 8-20% in pregnant women, this disorder is often underdiagnosed.We have developed a murine model of gestational OSA to study IH effects on pregnant mothers, placentas, fetuses, and offspring. One group of pregnant rats was exposed to IH during the last 2 weeks of gestation (GIH). One day before the delivery date, a cesarean section was performed. Other group of pregnant rats was allowed to give birth at term to study offspring's evolution.Preliminary results showed no significant weight differences in mothers and fetuses. However, the weight of GIH male offspring was significantly lower than the controls at 14 days (p < 0.01). The morphological study of the placentas showed an increase in fetal capillary branching, expansion of maternal blood spaces, and number of cells of the external trophectoderm in the tissues from GIH-exposed mothers. Additionally, the placentas from the experimental males were enlarged (p < 0.05). Further studies are needed to follow the long-term evolution of these changes to relate the histological findings of the placentas with functional development of the offspring in adulthood.

Peripheral Dopamine 2-Receptor Antagonist Reverses Hypertension in a Chronic Intermittent Hypoxia Rat Model.

The sleep apnea-hypopnea syndrome (SAHS) involves periods of intermittent hypoxia, experimentally reproduced by exposing animal models to oscillatory PO2 patterns. In both situations, chronic intermittent hypoxia (CIH) exposure produces carotid body (CB) hyperactivation generating an increased input to the brainstem which originates sympathetic hyperactivity, followed by hypertension that is abolished by CB denervation. CB has dopamine (DA) receptors in chemoreceptor cells acting as DA-2 autoreceptors. The aim was to check if blocking DA-2 receptors could decrease the CB hypersensitivity produced by CIH, minimizing CIH-related effects. Domperidone (DOM), a selective peripheral DA-2 receptor antagonist that does not cross the blood-brain barrier, was used to examine its effect on CIH (30 days) exposed rats. Arterial pressure, CB secretory activity and whole-body plethysmography were measured. DOM, acute or chronically administered during the last 15 days of CIH, reversed the hypertension produced by CIH, an analogous effect to that obtained with CB denervation. DOM marginally decreased blood pressure in control animals and did not affect hypoxic ventilatory response in control or CIH animals. No adverse effects were observed. DOM, used as gastrokinetic and antiemetic drug, could be a therapeutic opportunity for hypertension in SAHS patients' resistant to standard treatments.

Exploring the Mediators that Promote Carotid Body Dysfunction in Type 2 Diabetes and Obesity Related Syndromes.

Carotid bodies (CBs) are peripheral chemoreceptors that sense changes in blood O2, CO2, and pH levels. Apart from ventilatory control, these organs are deeply involved in the homeostatic regulation of carbohydrates and lipid metabolism and inflammation. It has been described that CB dysfunction is involved in the genesis of metabolic diseases and that CB overactivation is present in animal models of metabolic disease and in prediabetes patients. Additionally, resection of the CB-sensitive nerve, the carotid sinus nerve (CSN), or CB ablation in animals prevents and reverses diet-induced insulin resistance and glucose intolerance as well as sympathoadrenal overactivity, meaning that the beneficial effects of decreasing CB activity on glucose homeostasis are modulated by target-related efferent sympathetic nerves, through a reflex initiated in the CBs. In agreement with our pre-clinical data, hyperbaric oxygen therapy, which reduces CB activity, improves glucose homeostasis in type 2 diabetes patients. Insulin, leptin, and pro-inflammatory cytokines activate the CB. In this manuscript, we review in a concise manner the putative pathways linking CB chemoreceptor deregulation with the pathogenesis of metabolic diseases and discuss and present new data that highlight the roles of hyperinsulinemia, hyperleptinemia, and chronic inflammation as major factors contributing to CB dysfunction in metabolic disorders.

High fat diet blunts the effects of leptin on ventilation and on carotid body activity.

KEY POINTS: Leptin plays a role in the control of breathing, acting mainly on central nervous system; however, leptin receptors have been recently shown to be expressed in the carotid body (CB), and this finding suggests a physiological role for leptin in the regulation of CB function. Leptin increases minute ventilation in both basal and hypoxic conditions in rats. It increases the frequency of carotid sinus nerve discharge in basal conditions, as well as the release of adenosine from the CB. However, in a metabolic syndrome animal model, the effects of leptin in ventilatory control, carotid sinus nerve activity and adenosine release by the CB are blunted. Although leptin may be involved in triggering CB overactivation in initial stages of obesity and dysmetabolism, resistance to leptin signalling and blunting of responses develops in metabolic syndrome animal models. ABSTRACT: Leptin plays a role in the control of breathing, acting mainly on central nervous system structures. Leptin receptors are expressed in the carotid body (CB) and this finding has been associated with a putative physiological role of leptin in the regulation of CB function. Since, the CBs are implicated in energy metabolism, here we tested the effects of different concentrations of leptin administration on ventilatory parameters and on carotid sinus nerve (CSN) activity in control and high-fat (HF) diet fed rats, in order to clarify the role of leptin in ventilation control in metabolic disease states. We also investigated the expression of leptin receptors and the neurotransmitters involved in leptin signalling in the CBs. We found that in non-disease conditions, leptin increases minute ventilation in both basal and hypoxic conditions. However, in the HF model, the effect of leptin in ventilatory control is blunted. We also observed that HF rats display an increased frequency of CSN discharge in basal conditions that is not altered by leptin, in contrast to what is observed in control animals. Leptin did not modify intracellular Ca2+ in CB chemoreceptor cells, but it produced an increase in the release of adenosine from the whole CB. We conclude that CBs represent an important target for leptin signalling, not only to coordinate peripheral ventilatory chemoreflexive drive, but probably also to modulate metabolic variables. We also concluded that leptin signalling is mediated by adenosine release and that HF diets blunt leptin responses in the CB, compromising ventilatory adaptation.

Adrenal Medulla Chemo Sensitivity Does Not Compensate the Lack of Hypoxia Driven Carotid Body Chemo Reflex in Guinea Pigs.

Guinea pigs (GP), originally from the Andes, have absence of hypoxia-driven carotid body (CB) reflex. Neonatal mammals have an immature CB chemo reflex and respond to hypoxia with metabolic changes arising from direct effects of hypoxia on adrenal medulla (AM). Our working hypothesis is that adult GP would mimic neonatal mammals. Plasma epinephrine (E) has an AM origin, while norepinephrine (NE) is mainly originated in sympathetic endings, implying that specific GP changes in plasma E/NE ratio, and in blood glucose and lactate levels during hypoxia would be observed. Experiments were performed on young adult GP and rats. Hypoxic ventilation (10% O2) increased E and NE plasma levels similarly in both species but PaO2 was lower in GP than in rats. Plasma E/NE ratio in GP was higher (≈1.0) than in rats (≈0.5). The hypoxia-evoked increases in blood glucose and lactate were smaller in GP than in the rat. The AM of both species contain comparable E content, but NE was four times lower in GP than in rats. GP superior cervical ganglion also had lower NE content than rats and an unusual high level of dopamine, a negative modulator of sympathetic transmission. Isolated AM from GP released half of E and one tenth of NE than the rat AM, and hypoxia did not alter the time course of CA outflow. These data indicate the absence of direct effects of hypoxia on AM in the GP, and a lower noradrenergic tone in this species. Pathways for hypoxic sympatho-adrenal system activation in GP are discussed.

Functional abolition of carotid body activity restores insulin action and glucose homeostasis in rats: key roles for visceral adipose tissue and the liver.

AIMS/HYPOTHESIS: We recently described that carotid body (CB) over-activation is involved in the aetiology of insulin resistance and arterial hypertension in animal models of the metabolic syndrome. Additionally, we have demonstrated that CB activity is increased in animal models of insulin resistance, and that carotid sinus nerve (CSN) resection prevents the development of insulin resistance and arterial hypertension induced by high-energy diets. Here, we tested whether the functional abolition of CB by CSN transection would reverse pre-established insulin resistance, dyslipidaemia, obesity, autonomic dysfunction and hypertension in animal models of the metabolic syndrome. The effect of CSN resection on insulin signalling pathways and tissue-specific glucose uptake was evaluated in skeletal muscle, adipose tissue and liver. METHODS: Experiments were performed in male Wistar rats submitted to two high-energy diets: a high-fat diet, representing a model of insulin resistance, hypertension and obesity, and a high-sucrose diet, representing a lean model of insulin resistance and hypertension. Half of each group was submitted to chronic bilateral resection of the CSN. Age-matched control rats were also used. RESULTS: CSN resection normalised systemic sympathetic nervous system activity and reversed weight gain induced by high-energy diets. It also normalised plasma glucose and insulin levels, insulin sensitivity lipid profile, arterial pressure and endothelial function by improving glucose uptake by the liver and perienteric adipose tissue. CONCLUSIONS/INTERPRETATION: We concluded that functional abolition of CB activity restores insulin sensitivity and glucose homeostasis by positively affecting insulin signalling pathways in visceral adipose tissue and liver.

Hypoxic pulmonary vasoconstriction, carotid body function and erythropoietin production in adult rats perinatally exposed to hyperoxia.

KEY POINTS: Adult animals that have been perinatally exposed to oxygen-rich atmospheres (hyperoxia), recalling those used for oxygen therapy in infants, exhibit a loss of hypoxic pulmonary vasoconstriction, whereas vasoconstriction elicited by depolarizing agents is maintained. Loss of pulmonary hypoxic vasoconstriction is not linked to alterations in oxygen-sensitive K(+) currents in pulmonary artery smooth muscle cells. Loss of hypoxic vasoconstriction is associated with early postnatal oxidative damage and corrected by an antioxidant diet. Perinatal hyperoxia damages carotid body chemoreceptor cell function and the antioxidant diet does not reverse it. The hypoxia-elicited increase in erythropoietin plasma levels is not affected by perinatal hyperoxia. The potential clinical significance of the findings in clinical situations such as pneumonia, chronic obstructive pulmonary disease or general anaesthesia is considered. ABSTRACT: Adult mammalians possess three cell systems that are activated by acute bodily hypoxia: pulmonary artery smooth muscle cells (PASMC), carotid body chemoreceptor cells (CBCC) and erythropoietin (EPO)-producing cells. In rats, chronic perinatal hyperoxia causes permanent carotid body (CB) atrophy and functional alterations of surviving CBCC. There are no studies on PASMC or EPO-producing cells. Our aim is to define possible long-lasting functional changes in PASMC or EPO-producing cells (measured as EPO plasma levels) and, further, to analyse CBCC functional alterations. We used 3- to 4-month-old rats born and reared in a normal atmosphere or exposed to perinatal hyperoxia (55-60% O2 for the last 5-6 days of pregnancy and 4 weeks after birth). Perinatal hyperoxia causes an almost complete loss of hypoxic pulmonary vasoconstriction (HPV), which was correlated with lung oxidative status in early postnatal life and prevented by antioxidant supplementation in the diet. O2 -sensitivity of K(+) currents in the PASMC of hyperoxic animals is normal, indicating that their inhibition is not sufficient to trigger HPV. Perinatal hyperoxia also abrogated responses elicited by hypoxia on catecholamine and cAMP metabolism in the CB. An increase in EPO plasma levels elicited by hypoxia was identical in hyperoxic and control animals, implying a normal functioning of EPO-producing cells. The loss of HPV observed in adult rats and caused by perinatal hyperoxia, comparable to oxygen therapy in premature infants, might represent a previously unrecognized complication of such a medical intervention capable of aggravating medical conditions such as regional pneumonias, atelectases or general anaesthesia in adult life.

Effects of low glucose on carotid body chemoreceptor cell activity studied in cultures of intact organs and in dissociated cells.

The participation of the carotid body (CB) in glucose homeostasis and evidence obtained in simplified cultured CB slices or dissociated cells have led to the proposal that CB chemoreceptor cells are glucoreceptors. However, data generated in intact, freshly excised organs deny CB chemoreceptor cells' glucosensing properties. The physiological significance of the contention has prompted the present study, performed in a newly developed preparation of the intact CB organ in culture that maintains chemoreceptor cells' microenvironment. Chemoreceptor cells of intact CBs in culture retained their capacity to store, synthesize, and secrete catecholamine in response to hypoxia for at least 6 days. Aglycemia did not elicit neurosecretion in dissociated chemoreceptor cells or in intact CB in culture, but potentiated hypoxia-elicited neurosecretion, exclusively, in 1-day-old intact CB cultures and dissociated chemoreceptor cells cultured for 24 h. In fura 2-loaded cells, aglycemia (but not 1 mM) caused a slow Ca(2+)-dependent and nifedipine-insensitive increase in fluorescence at 340- to 380-nm wavelength emission ratio and augmented the fluorescent signal elicited by hypoxia. Association of nifedipine and KBR7943 (a Na(+)/Ca(2+) exchanger inhibitor) completely abolished the aglycemic Ca(2+) response. We conclude that chemoreceptor cells are not sensitive to hypoglycemia. We hypothesize that cultured chemoreceptor cells become transiently more dependent on glycolysis. Consequently, aglycemia would partially inhibit the Na(+)/K(+) pump, causing an increase in intracellular Na(+) concentration, and a reversal of Na(+)/Ca(2+) exchanger. This would slowly increase intracellular Ca(2+) concentration and cause the potentiation of the hypoxic responses. We discuss the nature of the signals detected by chemoreceptor cells for the CB to achieve its glycemic homeostatic role.

Chronic caffeine intake in adult rat inhibits carotid body sensitization produced by chronic sustained hypoxia but maintains intact chemoreflex output.

Sustained hypoxia produces a carotid body (CB) sensitization, known as acclimatization, which leads to an increase in carotid sinus nerve (CSN) activity and ensuing hyperventilation greater than expected from the prevailing partial pressure of oxygen. Whether sustained hypoxia is physiological (high altitude) or pathological (lung disease), acclimatization has a homeostatic implication because it tends to minimize hypoxia. Caffeine, the most commonly ingested psychoactive drug and a nonselective adenosine receptor antagonist, alters CB function and ventilatory responses when administered acutely. Our aim was to investigate the effect of chronic caffeine intake on CB function and acclimatization using four groups of rats: normoxic, caffeine-treated normoxic, chronically hypoxic (12% O2, 15 days), and caffeine-treated chronically hypoxic rats. Caffeine was administered in drinking water (1 mg/ml). Caffeine ameliorated ventilatory responses to acute hypoxia in normoxic animals without altering the output of the CB (CSN neural activity). Caffeine-treated chronically hypoxic rats exhibited a decrease in the CSN response to acute hypoxia tests but maintained ventilation compared with chronically hypoxic animals. The findings related to CSN neural activity combined with the ventilatory responses indicate that caffeine alters central integration of the CB input to increase the gain of the chemoreflex and that caffeine abolishes CB acclimatization. The putative mechanisms involved in sensitization and its loss were investigated: expression of adenosine receptors in CB (A(2B)) was down-regulated and that in petrosal ganglion (A(2A)) was up-regulated in caffeine-treated chronically hypoxic rats; both adenosine and dopamine release from CB chemoreceptor cells was increased in chronic hypoxia and in caffeine-treated chronic hypoxia groups.

Interactions between postnatal sustained hypoxia and intermittent hypoxia in the adulthood to alter brainstem structures and respiratory function.

Neural plasticity is defined as a persistent change in the morphology and/or function based on prior experiences. Plasticity is well evident when the triggering experience occurs early in life, but in the case of respiratory control plasticity, it also can be triggered in adult life. We have combined a 10 days postnatal hypoxic (PH) (0-10 days of age;11% O(2)) and a 15 days intermittent hypoxia (IH) exposures in the adulthood (90-105 days of age; 5% O(2), 40 s/20% O(2), 80 s; 8 h/day) to test if early PH interacts with IH of the adulthood to generate detrimental plastic changes. After recording of ventilatory parameters, the brains were studied immunocytochemically for localization of the organization pattern of non-phosphorylated subunit of neurofilament H (NFH) and tyrosine hydroxylase (TH) expression in the nucleus tractus solitarius (Sol) and caudal (CVL) and rostral ventrolateral reticular (RVL) nuclei, areas related to central cardio-respiratory regulation. In comparison to control, PH male rats (but not females) at 1 month of age hyperventilated at rest, in response to moderate hypoxia (12% O(2)) and 5% CO(2), the effect being due to increased tidal volume. At 3.5 months sex differences in ventilation disappeared and it was indistinguishable between control and PH. IH tended to decrease ventilation in both control (C) and PH animals. PH augmented PENH values in air and in hypoxic conditions when compared with C group. IH in both groups, tended to decrease the PENH value, being statistically different in PH+IH. Results also show an increment of disorganization of NFH-positive labeled structures at the level of Sol and CVL/RVL nuclei in PH, IH and HP+HI groups. PH rats showed differences in the number of TH-positive neurons at the level of CVL/RVL nuclei, which was increased in the PH and PH+IH groups with respect to C one. In conclusion, PH alters the central morpho-physiological organization and the catecholaminergic components of cardio-respiratory nuclei, whose effects were enhanced after a period of IH in the adulthood.

Effect of chronic caffeine intake on carotid body catecholamine dynamics in control and chronically hypoxic rats.

Caffeine is the most commonly psychoactive drug, an habitual drink in high altitude sporting, and when acutely taken, it causes profound alterations in carotid body (CB) function and ventilation via adenosine receptors antagonism. In the present work we have investigated the effects of chronic caffeine ingestion in catecholamine (CA) dynamics in the carotid body of control and chronic hypoxic rats. Four groups of animals were used: normoxic (N), caffeine-treated normoxic (1 mg/mL in drinking water 15 days; CafN), chronic hypoxic (CH, 12%O(2), 15 days) and chronically hypoxic-caffeine-treated (CafH).. Caffeine intake in controls rats did not modify CA content, synthesizing, and releasing responses, and the expression of tyrosine hydroxylase. CH increased dopamine content, synthesis, and basal and acute hypoxia-induced release; chronic caffeine ingestion augmented CH effects. Findings indicate that chronic caffeine ingestion in normoxic rats did not modify dopamine dynamics at the CB, but increases dopaminergic system during chronic hypoxia.

Serotonin dynamics and actions in the rat carotid body: preliminary findings.

Serotonin or 5-HT is a biogenic amine present in the carotid body (CB) of several species as evidenced in many immunocytochemical studies and in a few biochemical measurements. Early literature on 5-HT actions in the CB in all studied species has lead to the conclusion that it does not participate in the setting of conducted action potentials in the sensory nerve of the CB. However, during the last 10 years very important roles in the cellular physiology of the CB have been proposed for this biogenic amine. These roles include a primary role in setting the excitability of chemoreceptor cells via an autocrine or paracrine action, and thereby, the conducted activity in the carotid sinus nerve, and a critical role in the genesis of long term sensory facilitation observed in CBs of animals exposed to intermittent hypoxia. These facts, along with important discrepancies in the endogenous levels of 5-HT in the CB prompted present study conducted in rat CBs. We measured CB endogenous 5-HT content by HPLC with electrochemical detection and found levels of 5-HT in the range of 15-22 pmole/mg tissue in control and chronically hypoxic animals either sustained or intermittent, with no significant differences among them. 5-HT and the 5-HT2A antagonist ketanserin dose-dependently activated chemoreceptor cells as assessed by their capacity to release catecholamines from freshly isolated CB. In preliminary experiments we have observed that intense hypoxia and high extracellular K(+) promote a small release of 5-HT from CB which is not dependent on the presence of extracellular Ca(2+). Further studies are needed to firmly establish the dynamics of 5-HT in the CB of the rat.

Some reflections on intermittent hypoxia. Does it constitute the translational niche for carotid body chemoreceptor researchers?

The views presented in this article are the fruit of reflections and discussion with my colleagues at Valladolid and with the members of the Sleep Apnea Hypopnea Syndrome Group of the CIBERES (Spain). We have assembled the article in three sections. In the first one we provide a mechanistic description of obstructive sleep apnea (OSA) and all of its components, including the repetitive episodes of upper airways (UA) obstruction and accompanying hypoxic hypoxia, the respiratory efforts to fight and overcome the obstruction, and the sleep fragmentation due to the hypoxia-triggered arousal reactions, all events occurring during sleep hours with frequencies that might reach up >40-50 episodes/sleep hour. When OSA is accompanied by some of the elements of a big cohort of associated pathologies (vascular, metabolic, and neuropsychiatric) it conforms the obstructive sleep apnea syndrome (OSAS). The high frequency of OSAS in adults (>35 years old) and the costs in every regard of the treatment makes the syndrome a primary importance socio-sanitary problem. In the second section, we describe the experimental models of OSAS, basically the episodic repetitive hypoxic model described by Fletcher and coworkers in 1992, today named in short intermittent hypoxia (IH). From these lines, we want to call for some kind of consensus among researchers to lessen the dispersion of IH protocols. Finally, in the last section we intend to share our optimism with all ISAC members. The optimism is based on the recognition that carotid body (CB) chemoreceptors are critical elements of one of the main pathophysiologic loops in the genesis of OSAS. Therefore, we believe that all of us, as ISAC members, are well qualified to contribute in multidisciplinary research teams with well defined translational interests.

Effects of cigarette smoke and chronic hypoxia on ventilation in guinea pigs. Clinical significance.

Ventilatory effects of chronic cigarette smoke (CS) alone or associated to chronic hypoxia (CH), as frequently occurs in chronic obstructive pulmonary disease (COPD), remain unknown. We have addressed this problem using whole-body plethysmography in guinea-pigs, common models to study harmful effects of CS on the respiratory system. Breathing frequencies (Bf) in control (2-5 months old) guinea pigs is 90-100 breaths/min, their tidal volume (TV) increased with age but lagged behind body weight gain and, as consequence, their minute volume (MV)/Kg decreased with age. MV did not change by acutely breathing 10% O(2) but doubled while breathing 5% CO(2) in air. Exposure to chronic sustained hypoxia (15 days, 12% O(2), CH) did not elicit ventilatory acclimatization nor adaptation. These findings confirm the unresponsiveness of the guinea pig CB to hypoxia. Exposure to CS (3 months) increased Bf and MV but association with CH blunted CS effects. We conclude that CS and CH association accelerates CS-induced respiratory system damage leading to a hypoventilation that can worsen the ongoing COPD process.

Tetrodotoxin as a tool to elucidate sensory transduction mechanisms: the case for the arterial chemoreceptors of the carotid body.

Carotid bodies (CBs) are secondary sensory receptors in which the sensing elements, chemoreceptor cells, are activated by decreases in arterial PO(2) (hypoxic hypoxia). Upon activation, chemoreceptor cells (also known as Type I and glomus cells) increase their rate of release of neurotransmitters that drive the sensory activity in the carotid sinus nerve (CSN) which ends in the brain stem where reflex responses are coordinated. When challenged with hypoxic hypoxia, the physiopathologically most relevant stimulus to the CBs, they are activated and initiate ventilatory and cardiocirculatory reflexes. Reflex increase in minute volume ventilation promotes CO(2) removal from alveoli and a decrease in alveolar PCO(2) ensues. Reduced alveolar PCO(2) makes possible alveolar and arterial PO(2) to increase minimizing the intensity of hypoxia. The ventilatory effect, in conjunction the cardiocirculatory components of the CB chemoreflex, tend to maintain an adequate supply of oxygen to the tissues. The CB has been the focus of attention since the discovery of its nature as a sensory organ by de Castro (1928) and the discovery of its function as the origin of ventilatory reflexes by Heymans' group (1930). A great deal of effort has been focused on the study of the mechanisms involved in O(2) detection. This review is devoted to this topic, mechanisms of oxygen sensing. Starting from a summary of the main theories evolving through the years, we will emphasize the nature and significance of the findings obtained with veratridine and tetrodotoxin (TTX) in the genesis of current models of O(2)-sensing.

Maladaptive Pulmonary Vascular Responses to Chronic Sustained and Chronic Intermittent Hypoxia in Rat.

Chronic sustained hypoxia (CSH), as found in individuals living at a high altitude or in patients suffering respiratory disorders, initiates physiological adaptations such as carotid body stimulation to maintain oxygen levels, but has deleterious effects such as pulmonary hypertension (PH). Obstructive sleep apnea (OSA), a respiratory disorder of increasing prevalence, is characterized by a situation of chronic intermittent hypoxia (CIH). OSA is associated with the development of systemic hypertension and cardiovascular pathologies, due to carotid body and sympathetic overactivation. There is growing evidence that CIH can also compromise the pulmonary circulation, causing pulmonary hypertension in OSA patients and animal models. The aim of this work was to compare hemodynamics, vascular contractility, and L-arginine-NO metabolism in two models of PH in rats, associated with CSH and CIH exposure. We demonstrate that whereas CSH and CIH cause several common effects such as an increased hematocrit, weight loss, and an increase in pulmonary artery pressure (PAP), compared to CIH, CSH seems to have more of an effect on the pulmonary circulation, whereas the effects of CIH are apparently more targeted on the systemic circulation. The results suggest that the endothelial dysfunction evident in pulmonary arteries with both hypoxia protocols are not due to an increase in methylated arginines in these arteries, although an increase in plasma SDMA could contribute to the apparent loss of basal NO-dependent vasodilation and, therefore, the increase in PAP that results from CIH.

Chronic Intermittent Hypoxia Induces Early-Stage Metabolic Dysfunction Independently of Adipose Tissue Deregulation.

Several studies demonstrated a link between obstructive sleep apnea (OSA) and the development of insulin resistance. However, the main event triggering insulin resistance in OSA remains to be clarified. Herein, we investigated the effect of mild and severe chronic intermittent hypoxia (CIH) on whole-body metabolic deregulation and visceral adipose tissue dysfunction. Moreover, we studied the contribution of obesity to CIH-induced dysmetabolic states. Experiments were performed in male Wistar rats submitted to a control and high-fat (HF) diet. Two CIH protocols were tested: A mild CIH paradigm (5/6 hypoxic (5% O2) cycles/h, 10.5 h/day) during 35 days and a severe CIH paradigm (30 hypoxic (5% O2) cycles, 8 h/day) during 15 days. Fasting glycemia, insulinemia, insulin sensitivity, weight, and fat mass were assessed. Adipose tissue hypoxia, inflammation, angiogenesis, oxidative stress, and metabolism were investigated. Mild and severe CIH increased insulin levels and induced whole-body insulin resistance in control animals, effects not associated with weight gain. In control animals, CIH did not modify adipocytes perimeter as well as adipose tissue hypoxia, angiogenesis, inflammation or oxidative stress. In HF animals, severe CIH attenuated the increase in adipocytes perimeter, adipose tissue hypoxia, angiogenesis, and dysmetabolism. In conclusion, adipose tissue dysfunction is not the main trigger for initial dysmetabolism in CIH. CIH in an early stage might have a protective role against the deleterious effects of HF diet on adipose tissue metabolism.

MaxiK potassium channels in the function of chemoreceptor cells of the rat carotid body.

Hypoxia activates chemoreceptor cells of the carotid body (CB) promoting an increase in their normoxic release of neurotransmitters. Catecholamine (CA) release rate parallels the intensity of hypoxia. Coupling of hypoxia to CA release requires cell depolarization, produced by inhibition of O(2)-regulated K(+) channels, and Ca(2+) entering the cells via voltage-operated channels. In rat chemoreceptor cells hypoxia inhibits large-conductance, calcium-sensitive K channels (maxiK) and a two-pore domain weakly inward rectifying K(+) channel (TWIK)-like acid-sensitive K(+) channel (TASK)-like channel, but the significance of maxiK is controversial. A proposal envisions maxiK contributing to set the membrane potential (E(m)) and the hypoxic response, but the proposal is denied by authors finding that maxiK inhibition does not depolarize chemoreceptor cells or alters intracellular Ca(2+) concentration or CA release in normoxia or hypoxia. We found that maxiK channel blockers (tetraethylammonium and iberiotoxin) did not modify CA release in rat chemoreceptor cells, in either normoxia or hypoxia, and iberiotoxin did not alter the Ca(2+) transients elicited by hypoxia. On the contrary, both maxiK blockers increased the responses elicited by dinitrophenol, a stimulus we demonstrate does not affect maxiK channels in isolated patches of rat chemoreceptor cells. We conclude that in rat chemoreceptor cells maxiK channels do not contribute to the genesis of the E(m), and that their full inhibition by hypoxia, preclude further inhibition by maxiK channel blockers. We suggest that full inhibition of this channel is required to generate the spiking behavior of the cells in acute hypoxia.