Expression of HIF-1alpha, VEGF and VEGF receptors in the carotid body of chronically hypoxic rat

Short-term sustained hypoxia distinctly affects subpopulations of carotid body glomus cells from rats

The main O2 arterial chemoreceptors are the carotid bodies (CBs), which mediate hyperventilation in response to short-term sustained hypoxia (SH). CBs contain glomus cells expressing K+ channels, which are inhibited by hypoxia, leading to neurotransmitter release. ATP released by CBs and type II cells has been considered essential for chemosensory processing under physiological and pathophysiological conditions. Although the systemic effects of chronic activation of CBs by SH are well known, the early (first 24 h) cellular and molecular mechanisms in CBs as well as the effects of short-term SH on populations of glomus cells are still poorly understood. Here, we show that SH (10% O2 for 24 h) depolarizes the membrane potential of one population of glomus cells, mediated by increases in inward current, but does not affect the ATP release by CBs. In addition, SH promotes a reduction in their maximum outward current, mediated by voltage-gated K+ channels. SH also affected sensitivity to acute hypoxia in one glomus cell subpopulation. As for the content of mitochondrial proteins, we observed increases in the citrate synthase, Tom-20, and succinate dehydrogenase (mitochondrial complex II) per cell of CBs after SH. Our results demonstrate important cellular and molecular mechanisms of plasticity in CBs from rats after only 24 h of SH, which may contribute to the generation of cardiovascular and ventilatory adjustments observed in this experimental model.NEW & NOTEWORTHY Our study revealed two subpopulations of glomus cells of carotid bodies (CBs) with specific electrophysiological properties, which were differentially affected by short-term sustained hypoxia (SH; 10% O2 for 24 h). Our experiments showed that SH also affected the sensitivity to acute hypoxia of these glomus cell subpopulations differently. Our molecular analyses allowed us to identify important adaptations in the content of CB mitochondrial proteins that participate in the Krebs cycle and form the electron transport chain.

Altered Expression Levels of Angiogenic Peptides in the Carotid Body of Spontaneously Hypertensive Rats

The carotid body (CB), the main peripheral arterial chemoreceptor, exhibits considerable structural and neurochemical plasticity in response to pathological conditions such as high blood pressure. Previous studies have shown that morphological alterations in the hypertensive CB are characterized by enlarged parenchyma due to cellular hypertrophy and hyperplasia, and vasodilation. To test whether hypertension can also induce neoangiogenesis and modulate its chemosensory function, we examined the immunohistochemical expression of two angiogenic factors, vascular endothelial growth factor (VEGF) and endothelin-1 (ET), and their corresponding receptors in the CB of adult spontaneously hypertensive rats (SHRs), and compared their expression patterns to that of age-matched normotensive Wistar rats (NWR). We found an increased VEGF-A and B, and VEGFR-2 expression in glomus and endothelial cells in the enlarged CB glomeruli of SHRs compared with that in NWR. Conversely, weaker immunoreactivity to VEGFR-1 was detected in cell clusters of the hypertensive CB. The expression of endothelin-converting enzyme 1 and its receptor ETA was higher in a subset of glomus cells in the normotensive CB, while the immunoreactivity to the ETB receptor was enhanced in endothelial cells of CB blood vessels in SHRs. The elevated endothelial expression of VEGF and ET-1 suggests their role as local vascular remodeling factors in the adaptation to hypertension, though their involvement in the cellular rearrangement and modulation of chemosensory function could also be implied.

Altered Hypoxia-Induced and Heat Shock Protein Immunostaining in Secondary Hair Follicles Associated with Changes in Altitude and Temperature in Tibetan Cashmere Goats

This experiment compared secondary hair follicles (SFs) in Tibetan cashmere goats from two different steppes that were at different altitudes and had different temperatures. Twenty-four 2-year-old goats were studied. Twelve goats were from Rikaze in Tibet which is at an altitude of above 5000 m with an average temperature of 0 °C. The other 12 studied goats were from Huan County of Gansu Province which is around 2000 m above sea level with an average temperature of 9.2 °C. The structural features of SFs were assessed using light microscopy and transmission electron microscopy. The presence of HIF-1a, HIF-2a, HIF-3a, HSP27, and HOXC13 proteins was studied using immunohistochemistry and immunofluorescence. Light and electron microscopy revealed that the SFs of the Tibetan cashmere goats that lived in the Rikaze Steppe were in the proanagen stage in May. However, the SFs of the goats from the lower warmer Huan County were in the anagen stage at the same time. Immunohistochemistry revealed intense immunostaining for HIF-1a protein in the inner root sheath (IRS) and hair shaft (HS); immunostaining against HIF-2a in the outer root sheath (ORS) and IRS; HIF-3a protein immunostaining in the ORS; HSP27 immunostaining in the ORS, IRS, and HS; and HOXC13 immunostaining in the ORS and HS. HIF-1a protein expression in the IRS and HS was higher than the expression in the ORS (p < 0.05) while the expression of HIF-2a protein was higher in the ORS and IRS than the HS (p < 0.05). The expression of HIF-3a protein was higher in the ORS than in the IRS (p < 0.05). Expression of HOXC13 protein was higher in the ORS than in the IRS and HS (p < 0.05). Immunostaining of HIF-1a, HIF-2a, and HSP27 protein was significantly higher in SFs from cashmere goats from Rikaze than in goats from Huan (p < 0.05). In contrast, HOX13 protein immunostaining was significantly higher in cashmere goats from Huan than from Rikaze (p < 0.05). Significant differences were observed in the SFs of cashmere goats from two locations that differ in altitude and temperature. This suggests the differences in the secondary hair follicles could be due to the hypoxia and lower temperatures experienced by the goats in Rikaze. These results are useful in understanding how altitude and temperature influence SF development. Hair produced by the SFs are used for down fiber. Therefore, understanding of the factors that influence SF development will allow the production and harvest of these valuable fibers to be maximized.

Comparative morphological and molecular studies on the oxygen-chemoreceptive cells in the carotid body and fish gills

Oxygen-chemoreceptive cells play critical roles for the respiration control. This review summarizes the chemoreceptive cells in the carotid body and fish gills from a morphological and molecular perspective. The cells synthesize and secrete biogenic amines, neuropeptides, and neuroproteins and also express many signaling molecules and transcription factors. In mammals, birds, reptiles, and amphibians, the carotid body primordium is consistently formed in the wall of the third arch artery which gives rise to the common carotid artery and the basal portion of the internal carotid artery. Consequently, the carotid body is located in the carotid bifurcation region, except birds in which the organ is situated at the lateral side of the common carotid artery. The carotid body receives branches of the cranial nerves IX and/or X dependent on the location of the organ. The glomus cell progenitors in mammals and birds are derived from the neighboring ganglion, i.e., the superior cervical sympathetic ganglion and the nodose ganglion, respectively, and immigrate into the carotid body primordium, constituting a solid cell cluster. In other animal species, the glomus cells are dispersed singly or forming small cell groups in intervascular stroma of the carotid body. In fishes, the neuroepithelial cells, corresponding to the glomus cells, are distributed in the gill filaments and lamellae. All oxygen-chemoreceptive cells sensitively respond to acute or chronic hypoxia, exhibiting degranulation, hypertrophy, hyperplasia, and upregulated expression of many genes.

Growth Factors in the Carotid Body-An Update

The carotid body may undergo plasticity changes during development/ageing and in response to environmental (hypoxia and hyperoxia), metabolic, and inflammatory stimuli. The different cell types of the carotid body express a wide series of growth factors and corresponding receptors, which play a role in the modulation of carotid body function and plasticity. In particular, type I cells express nerve growth factor, brain-derived neurotrophic factor, neurotrophin 3, glial cell line-derived neurotrophic factor, ciliary neurotrophic factor, insulin-like-growth factor-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-α and -β, interleukin-1β and -6, tumor necrosis factor-α, vascular endothelial growth factor, and endothelin-1. Many specific growth factor receptors have been identified in type I cells, indicating autocrine/paracrine effects. Type II cells may also produce growth factors and express corresponding receptors. Future research will have to consider growth factors in further experimental models of cardiovascular, metabolic, and inflammatory diseases and in human (normal and pathologic) samples. From a methodological point of view, microarray and/or proteomic approaches would permit contemporary analyses of large groups of growth factors. The eventual identification of physical interactions between receptors of different growth factors and/or neuromodulators could also add insights regarding functional interactions between different trophic mechanisms.

Association of Age with the Expression of Hypoxia-Inducible Factors HIF-1α, HIF-2α, HIF-3α and VEGF in Lung and Heart of Tibetan Sheep

Simple Summary

The heart and lung play an essential role in physiological homeostasis, especially in a hypoxic environment. The effect of aging on HIF-1α, HIF-2α, HIF-3α and VEGF expression in the heart and lung of Tibetan sheep that were adapted to hypoxia was evaluated in this study. We conclude that HIF-3a and VEGF are important in how the heart responds to hypoxia and that HIF-1a and HIF-2a may help mediate the adaptation by the sheep to hypoxia. The results suggested that the altered expression of these proteins due to hypoxia is regulated at the protein as well as gene levels. The expression of these proteins in alveolar macrophages suggests these cells play an important role in adaption to hypoxia. The research could provide insight into the role of inflammation in response to reduced alveolar PO2, and is useful in understanding how age influences the hypoxia adaption mechanisms of the heart and lung. This may allow a better understanding of chronic mountain sickness that is commonly observed in Tibetan people living at high altitude on the Qinghai-Tibetan plateau.

Abstract

Hypoxia-inducible factors (HIFs) play an important role in mediating the physiological response to low oxygen environments. However, whether the expression of HIFs changes with age is unknown. In the present study, the effect of aging on HIF-1α, HIF-2α, HIF-3α and VEGF expression in the heart and lung of 30 Tibetan sheep that were adapted to hypoxia was evaluated. The 30 sheep were subdivided into groups of 10 animals that were 1, 2 or 6 years of age. Immunohistochemistry for HIF-1α, HIF-2α, HIF-3α and VEGF revealed that the immunostaining intensity of VEGF protein in the heart and lung was significantly higher than the intensity of immunostaining against the HIFs (p < 0.05). HIF-1α and HIF-2α protein translocated into the nucleus of cardiac muscle cells. However, immunostaining for HIF-3α was restricted to the cytoplasm of the myocardial cells. Immunostaining for HIF-1α, HIF-2α, HIF-3α and VEGF was detected within alveolar macrophages. The concentration of HIF-1α and HIF-2α was higher in the lung of 1-year-old than 6-year-old sheep (p < 0.05). In contrast, HIF-3α and VEGF immunostaining was most prominent in the hearts of the oldest sheep. However, when RT-PCR was used to evaluate RNA within the tissues, the expression of all four studied genes was higher in the lung than in the heart in the 1-year-old sheep (p < 0.05). Furthermore, VEGF and HIF-3α gene expression was higher in the heart from 1-year old than 6-year old sheep (p < 0.05). However, in the lung, HIF-1α and HIF-2α gene expression was lower in 1-year old than 6-year old sheep (p < 0.05). We conclude that HIF-3α and VEGF may play be important in how the heart responds to hypoxia. Additionally, HIF-1α and HIF-2α may have a role in the adaptation of the lung to hypoxia. The expression of these proteins in alveolar macrophages suggests a potential role of these cells in the physiological response to hypoxia. These results are useful in understanding how age influences the hypoxia adaption mechanisms of the heart and lung and may help to better understand chronic mountain sickness that is commonly observed in Tibetan people living on the Qinghai-Tibetan plateau.

Role of the HIF-1 signaling pathway in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is the most common cause of chronic morbidity and mortality. However, the molecular mechanisms underlying COPD remain largely unknown. The purpose of the present study was to investigate the expression patterns of hypoxia-inducible factor 1α (HIF-1α), vascular endothelial growth factor (VEGF), and VEGF receptor 2 (R2) in regard to the HIF-1 signaling pathway in COPD. The expressions of HIF-1α, VEGF and VEGFR2 were examined and quantified in the human lung tissues of 102 subjects with a defined smoking status, with or without COPD. The expressions of HIF-1α, VEGF and VEGFR2 were observed to be increased in the lung tissues collected from smoking COPD subjects when compared with those tissues from smoking subjects without COPD and non-smoking subjects without COPD. The expression of HIF-1α was shown to be positively associated with the expression of VEGF and VEGFR2. In addition, increased expression of HIF-1α, VEGF and VEGFR2 reflected the disease severity of COPD. The key findings obtained from the present study indicated that high expression of HIF-1α, VEGF and VEGFR2 may be associated with decreased lung function and reduced quality of life, contributing to disease progression in COPD.

Chicoric Acid Improves Heart and Blood Responses to Hypobaric Hypoxia in Tibetan Yaks

Yak is a wild bovine species living on the Qinghai Tibet Plateau that demonstrates good adaptability to the hypoxic environment. Chicoric acid, a natural phenolic compound, is known as having anti-oxidant, antiviral, anti-inflammatory and analgesic properties. However, its effect on hypoxia adaptability of yak is still unclear. In this study 40 yaks were selected that were of similar age, parity and weight, and divided into the control group and experimental groups 1, 2, 3, randomly. Results showed that chicoric acid significantly improved RBC, HGB, and WBC. There are significantly beneficial effects to increasing total protein contents ([Formula: see text]): all treatments increased HDL-C contents, and supplementations 100[Formula: see text]mg/h significantly decreased the content of TG on the 60th day ([Formula: see text]). Contents of the serum related enzymes like ALP, GOP and GPT showed varying degrees of change, but no significant differences and the indexes of anti-oxidant capacity (T-AOC and GSH-Px) were significantly improved ([Formula: see text]), but MDA was decreased ([Formula: see text]) under the action of the chicoric acid. Hypoxia-inducible factor in serum such as HIF-2[Formula: see text], EPO, ROS, Fe[Formula: see text] and Tf are all significantly decreased ([Formula: see text]). The myocardial mitochondrial parameters mtDNA, UCP2, PGC1-[Formula: see text], NRF1 and mitochondrial complexes were altered remarkably. Some indicators of glucose metabolism presented variation trends. Taken together, chicoric acid has shown a positive effect on the adaptive ability of yak in high altitude, hypoxic environment in plateau areas. Our findings reported a new potential means to enhance immunity and inflammatory response and improve the anti-oxidant capacity.

Guinea Pig Oxygen-Sensing and Carotid Body Functional Properties

Mammals have developed different mechanisms to maintain oxygen supply to cells in response to hypoxia. One of those mechanisms, the carotid body (CB) chemoreceptors, is able to detect physiological hypoxia and generate homeostatic reflex responses, mainly ventilatory and cardiovascular. It has been reported that guinea pigs, originally from the Andes, have a reduced ventilatory response to hypoxia compared to other mammals, implying that CB are not completely functional, which has been related to genetically/epigenetically determined poor hypoxia-driven CB reflex. This study was performed to check the guinea pig CB response to hypoxia compared to the well-known rat hypoxic response. These experiments have explored ventilatory parameters breathing different gases mixtures, cardiovascular responses to acute hypoxia, in vitro CB response to hypoxia and other stimuli and isolated guinea pig chemoreceptor cells properties. Our findings show that guinea pigs are hypotensive and have lower arterial pO₂ than rats, probably related to a low sympathetic tone and high hemoglobin affinity. Those characteristics could represent a higher tolerance to hypoxic environment than other rodents. We also find that although CB are hypo-functional not showing chronic hypoxia sensitization, a small percentage of isolated carotid body chemoreceptor cells contain tyrosine hydroxylase enzyme and voltage-dependent K⁺ currents and therefore can be depolarized. However hypoxia does not modify intracellular Ca²⁺ levels or catecholamine secretion. Guinea pigs are able to hyperventilate only in response to intense acute hypoxic stimulus, but hypercapnic response is similar to rats. Whether other brain areas are also activated by hypoxia in guinea pigs remains to be studied.

Changes in the Anatomic and Microscopic Structure and the Expression of HIF-1α and VEGF of the Yak Heart with Aging and Hypoxia

The study aimed to identify the changes of anatomic and microscopic structure and the expression and localization of hypoxia-inducible factor (HIF)-1α and vascular endothelial growth factor (VEGF) in the myocardium and coronary artery of the yak heart adapted to chronic hypoxia with aging. Thirty-two yaks (1 day, 6 months, 1 year, 2 years, and 5 year old) were included, and immunoelectronmicroscopy, immunohistochemistry, and enzyme-linked immunosorbent assay (ELISA) were used. Right ventricular hypertrophy was not present in yaks with aging. There was no intima thickening phenomenon in the coronary artery. The ultrastructure of myofibrils, mitochondria, and collagen fibers and the diameter and quantity of collagen changed significantly with aging. The enzymatic activity of complexes I, II, and V increased with age. Immunogold labeling showed the localization of HIF-1α protein in the cytoplasm and nuclei of endothelial cells and cytoplasm of cardiac muscle cells, and VEGF protein in the nuclei and perinuclei areas of smooth muscle cells of coronary artery, and in the cytoplasm and nuclei of endothelial cells. ELISA results showed that HIF-1α secretion significantly increased in the myocardium and coronary artery from an age of 1 day to 2 years of yaks and decreased in old yaks. However, VEGF protein always increased with aging. The findings of this study suggest that 6 months is a key age of yak before which there are some adaptive changes to deal with low-oxygen environment, and there is a maturation of the yak heart from the age of 6 months to 2 years.

Hypoxic Adaptation in the Nervous System: Promise for Novel Therapeutics for Acute and Chronic Neurodegeneration

Homeostasis is the process by which cells adapt to stress and prevent or repair injury. Unique programs have evolved to sense and activate these homeostatic mechanisms and as such, homeostatic sensors may be potent therapeutic targets. The hypoxic response mediated by hypoxia inducible factor (HIF) downstream of oxygen sensing by HIF prolyl 4-hydroxylases (PHDs) has been well-studied, revealing cell-type specific regulation of HIF stability, activity, and transcriptional targets. HIF's paradoxical roles in nervous system development, physiology, and pathology arise from its complex roles in hypoxic adaptation and normoxic biology. Understanding how to engage the hypoxic response so as to recapitulate the protective mechanism of ischemic preconditioning is a high priority. Indeed, small molecules that activate the hypoxic response provide broad neuroprotection in several clinically relevant injury models. Screens for PHD inhibitors have identified novel therapeutics for neuroprotection that are ready to proceed to clinical trials for ischemic stroke. Better understanding the mechanisms of how to engage hypoxic adaption without altering development or physiology may identify additional novel therapeutic targets for diverse acute and chronic neuropathologies.

Upregulation of a local renin-angiotensin system in the rat carotid body during chronic intermittent hypoxia

The carotid body (CB) plays an important role in the alteration of cardiorespiratory activity in chronic intermittent hypoxia (IH) associated with sleep-disordered breathing, which may be mediated by local expression of the renin-angiotensin system (RAS). We hypothesized a pathogenic role for IH-induced RAS expression in the CB. The CB expression of RAS components was examined in rats exposed to IH resembling a severe sleep-apnoeic condition for 7 days. In situ hybridization showed an elevated expression of angiotensinogen in the CB glomus cells in the hypoxic group when compared with the normoxic control group. Immunohistochemical studies and Western blot analysis revealed increases in the protein level of both angiotensinogen and angiotensin II type 1 (AT1) receptors in the hypoxic group, which were localized to the glomic clusters containing tyrosine hydroxylase. RT-PCR studies confirmed that levels of the mRNA expression of angiotensinogen, angiotensin-converting enzyme, AT1a and AT2 receptors were significantly increased in the CBs of the hypoxic rats. Functionally, the [Ca(2+)]i response to exogenous angiotensin II was enhanced in fura-2-loaded glomus cells dissociated from hypoxic rats when compared with those of the normoxic control animals. Pretreatment with losartan, but not PD123319, abolished the angiotensin II-induced [Ca(2+)]i response, suggesting an involvement of AT1 receptors. Moreover, daily treatment of the IH group of rats with losartan attenuated the levels of oxidative stress, gp91(phox) expression and macrophage infiltration in the CB. Collectively, the upregulated local RAS expression could play a pathogenic role in the augmented CB activity and local inflammation via AT1 receptor activation during IH conditions in patients with sleep-disordered breathing.

Upregulation of pituitary adenylate cyclase activating polypeptide and its receptor expression in the rat carotid body in chronic and intermittent hypoxia

The carotid body (CB) plays important roles in cardiorespiratory changes in chronic and intermittent hypoxia. Pituitary adenylate cyclase activating polypeptide (PACAP) is involved in the regulation of respiratory chemoresponse. We hypothesized an upregulation of the expressions of PACAP and its receptor (PAC1) in the rat CB in chronic and intermittent hypoxia. The CB expressions of PACAP and PAC1 were examined in rats breathing 10% O(2) (in isobaric chamber for chronic hypoxia, 24 h/day) or in intermittent hypoxia (cyclic between air and 5% O(2) per minute, 8 h/day) for 7 days. Immunohistochemical studies showed that the PACAP and PAC1 proteins were localized in CB glomic clusters containing tyrosine hydroxylase. The proportional amount of cells with positive staining of PACAP and PAC1 was significantly increased in both hypoxic groups when compared with the normoxic control. In addition, the mRNA level of PAC1 expression was markedly elevated in the hypoxic groups, despite no changes in the PACAP expression. These results suggest an upregulation of PACAP and its receptor expression in the rat CB under chronic and intermittent hypoxic conditions. The PACAP binding to its receptor could activate the PKA signaling pathway leading to an increased CB excitability under hypoxic conditions.

Contribution of increased VEGF receptors to hypoxic changes in fetal ovine carotid artery contractile proteins

Recent studies suggest that vascular endothelial growth factor (VEGF) can modulate smooth muscle phenotype and, consequently, the composition and function of arteries upstream from the microcirculation, where angiogenesis occurs. Given that hypoxia potently induces VEGF, the present study explores the hypothesis that, in fetal arteries, VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins. Pregnant ewes were acclimatized at sea level or at altitude (3,820 m) for the final 110 days of gestation. Endothelium-denuded carotid arteries from full-term fetuses were used fresh or after 24 h of organ culture in a physiological concentration (3 ng/ml) of VEGF. After 110 days, hypoxia had no effect on VEGF abundance but markedly increased abundance of the Flk-1 (171%) and Flt-1 (786%) VEGF receptors. Hypoxia had no effect on smooth muscle α-actin (SMαA), decreased myosin light chain (MLC) kinase (MLCK), and increased 20-kDa regulatory MLC (MLC(20)) abundances. Hypoxia also increased MLCK-SMαA, MLC(20)-SMαA, and MLCK-MLC(20) colocalization. Compared with hypoxia, organ culture with VEGF produced the same pattern of changes in contractile protein abundance and colocalization. Effects of VEGF on colocalization were blocked by the VEGF receptor antagonists vatalanib (240 nM) and dasatinib (6.3 nM). Thus, through increases in VEGF receptor density, hypoxia can recruit VEGF to help mediate remodeling of fetal arteries upstream from the microcirculation. The results support the hypothesis that VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins.

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.

Short-term hypoxia transiently increases dopamine β-hydroxylase immunoreactivity in glomus cells of the rat carotid body

Under long-term hypoxia, noradrenaline (NA) content in the carotid body (CB) increases, suggesting that NA plays an important role in CB chemotransduction. However, it is unknown whether short-term hypoxia upregulates NA biosynthesis in CB. Therefore, we examined dopamine β-hydroxylase (DBH) expression in the CB of rats exposed to hypoxia (10% O(2)) for 0 to 24 hr with immunoblotting and immunohistochemistry. Using immunoblotting, the signal intensity for DBH appeared to be the most intense in rats exposed to hypoxia for 12 hr. Using immunohistochemistry, DBH immunoreactivity was observed in the cytoplasm of some glomus cells and varicosities in controls and rats exposed to hypoxia for 6 hr. In rats exposed to hypoxia for 12 hr, DBH immunoreactive intensities in DBH-positive glomus cells were significantly higher compared with controls (p<0.05). In the CB of rats exposed to hypoxia for 18 and 24 hr, DBH immunoreactive intensities in DBH-positive glomus cells were significantly lower than that of rats exposed to hypoxia for 12 hr (p<0.05). These results demonstrate that DBH immunoreactivity is transiently increased in glomus cells by short-term hypoxia, suggesting that NA biosynthesis is transiently facilitated in glomus cells at an early stage of hypoxia.

Chronic hypoxia and VEGF differentially modulate abundance and organization of myosin heavy chain isoforms in fetal and adult ovine arteries

Chronic hypoxia increases vascular endothelial growth factor (VEGF) and thereby promotes angiogenesis. The present study explores the hypothesis that hypoxic increases in VEGF also remodel artery wall structure and contractility through phenotypic transformation of smooth muscle. Pregnant and nonpregnant ewes were maintained at sea level (normoxia) or 3,820 m (hypoxia) for the final 110 days of gestation. Common carotid arteries harvested from term fetal lambs and nonpregnant adults were denuded of endothelium and studied in vitro. Stretch-dependent contractile stresses were 32 and 77% of normoxic values in hypoxic fetal and adult arteries. Hypoxic hypocontractility was coupled with increased abundance of nonmuscle myosin heavy chain (NM-MHC) in fetal (+37%) and adult (+119%) arteries. Conversely, hypoxia decreased smooth muscle MHC (SM-MHC) abundance by 40% in fetal arteries but increased it 123% in adult arteries. Hypoxia decreased colocalization of NM-MHC with smooth muscle α-actin (SM-αA) in fetal arteries and decreased colocalization of SM-MHC with SM-αA in adult arteries. Organ culture with physiological concentrations (3 ng/ml) of VEGF-A(165) similarly depressed stretch-dependent stresses to 37 and 49% of control fetal and adult values. The VEGF receptor antagonist vatalanib ablated VEGF's effects in adult but not fetal arteries, suggesting age-dependent VEGF receptor signaling. VEGF replicated hypoxic decreases in colocalization of NM-MHC with SM-αA in fetal arteries and decreases in colocalization of SM-MHC with SM-αA in adult arteries. These results suggest that hypoxic increases in VEGF not only promote angiogenesis but may also help mediate hypoxic arterial remodeling through age-dependent changes in smooth muscle phenotype and contractility.

Peripheral chemoreceptors: function and plasticity of the carotid body

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.

Chronic intermittent hypoxia induces local inflammation of the rat carotid body via functional upregulation of proinflammatory cytokine pathways

Maladaptive changes in the carotid body (CB) induced by chronic intermittent hypoxia (IH) account for the pathogenesis of cardiovascular morbidity in patients with sleep-disordered breathing. We postulated that the proinflammatory cytokines, namely interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α, and cytokine receptors (IL-1r1, gp130 and TNFr1) locally expressed in the rat CB play a pathophysiological role in IH-induced CB inflammation. Results showed increased levels of oxidative stress (serum 8-isoprostane and nitrotyrosine in the CB) in rats with 7-day IH treatment resembling recurrent apneic conditions when compared with the normoxic control. Local inflammation shown by the amount of ED1-containing cells (macrophage infiltration) and the gene transcripts of NADPH oxidase subunits (gp91(phox) and p22(phox)) and chemokines (MCP-1, CCR2, MIP-1α, MIP-1β and ICAM-1) in the CB were significantly more in the hypoxic group than in the control. In addition, the cytokines and receptors were expressed in the lobules of chemosensitive glomus cells containing tyrosine hydroxylase and the levels of expressions were significantly increased in the hypoxic group. Exogenous cytokines elevated the intracellular calcium ([Ca(2+)](i)) response to acute hypoxia in the dissociated glomus cells. The effect of cytokines on the [Ca(2+)](i) response was significantly greater in the hypoxic than in the normoxic group. Moreover, daily treatment of IH rats with anti-inflammatory drugs (dexamethasone or ibuprofen) attenuated the levels of oxidative stress, gp91(phox) expression and macrophage infiltration in the CB. Collectively, these results suggest that the upregulated expression of proinflammatory cytokine pathways could mediate the local inflammation and functional alteration of the CB under chronic IH conditions.

Carotid body remodelling in l-NAME-induced hypertension in the rat

The carotid body (CB) is a chemoreceptor organ located at the bifurcation of the common carotid artery. It is made up of the carotid glomus, a structure containing type 1 cells surrounded by type 2 cells. The aim of this study was to evaluate the morphological changes of the CB and carotid glomus in the rat model of l-NAME-induced hypertension. Male Wistar rats were divided in two groups: control untreated rats (C) and rats receiving l-NAME 40 mg/kg/day (LN) for 6 weeks. At the end of the experiment, the systolic blood pressure was 63% higher in the LN group compared with the C group. Morphometric analysis showed that the area of the CB was 29% greater in the LN group compared with the C group. The density of nuclei in the CB was similar between groups, but it was 31% less in the carotid glomus of the LN group. Cells in the CB of the LN group displayed cytoplasmic vacuolation and expressed several biogenic amines. There were more elastic fibres, proteoglycans and collagen fibres in the LN group compared with the C group. Immunohistochemistry showed increased expression of nuclear factor kB, substance P, vascular endothelial growth factor and neuronal nitric oxide synthase in the LN group, while expression of the protein gene product 9.5 was decreased. l-NAME alters cell morphology and the expression of extracellular matrix molecules in the CB and carotid glomus in rats with l-NAME-induced hypertension.

Chronic hypoxia-induced acid-sensitive ion channel expression in chemoafferent neurons contributes to chemoreceptor hypersensitivity

Previously we demonstrated that chronic hypoxia (CH) induces an inflammatory condition characterized by immune cell invasion and increased expression of inflammatory cytokines in rat carotid body. It is well established that chronic inflammatory pain induces the expression of acid-sensitive ion channels (ASIC) in primary sensory neurons, where they contribute to hyperalgesia and allodynia. The present study examines the effect of CH on ASIC expression in petrosal ganglion (PG), which contains chemoafferent neurons that innervate oxygen-sensitive type I cells in the carotid body. Five isoforms of ASIC transcript were increased ∼1.5-2.5-fold in PG following exposure of rats to 1, 3, or 7 days of hypobaric hypoxia (380 Torr). ASIC transcript was not increased in the sympathetic superior cervical ganglion (SCG). In the PG, CH also increased the expression of channel-interacting PDZ domain protein, a scaffolding protein known to enhance the surface expression and the low pH-induced current density mediated by ASIC3. Western immunoblot analysis showed that CH elevated ASIC3 protein in PG, but not in SCG or the (sensory) nodose ganglion. ASIC3 transcript was likewise elevated in PG neurons cultured in the presence of inflammatory cytokines. Increased ASIC expression was blocked in CH rats concurrently treated with the nonsteroidal anti-inflammatory drug ibuprofen (4 mg·kg(-1)·day(-1)). Electrophysiological recording of carotid sinus nerve (CSN) activity in vitro showed that the specific ASIC antagonist A-317567 (100 μM) did not significantly alter hypoxia-evoked activity in normal preparations but blocked ∼50% of the hypoxic response following CH. Likewise, a high concentration of ibuprofen, which is known to block ASIC1a, reduced hypoxia-evoked CSN activity by ∼50% in CH preparations. Our findings indicate that CH induces inflammation-dependent phenotypic adjustments in chemoafferent neurons. Following CH, ASIC are important participants in chemotransmission between type I cells and chemoafferent nerve terminals, and these proton-gated channels appear to enhance chemoreceptor sensitivity.

Chronic intermittent hypoxia-induced vascular enlargement and VEGF upregulation in the rat carotid body is not prevented by antioxidant treatment

Chronic intermittent hypoxia (CIH), a characteristic of sleep obstructive apnea, enhances carotid body (CB) chemosensory responses to hypoxia, but its consequences on CB vascular area and VEGF expression are unknown. Accordingly, we studied the effect of CIH on CB volume, glomus cell numbers, blood vessel diameter and number, and VEGF immunoreactivity (VEGF-ir) in male Sprague-Dawley rats exposed to 5% O(2), 12 times/h for 8 h or sham condition for 21 days. We found that CIH did not modify the CB volume or the number of glomus cells but increased VEGF-ir and enlarged the vascular area by increasing the size of the blood vessels, whereas the number of the vessels was unchanged. Because oxidative stress plays an essential role in the CIH-induced carotid chemosensory potentiation, we tested whether antioxidant treatment with ascorbic acid may impede the vascular enlargement and the VEGF upregulation. Ascorbic acid, which prevents the CB chemosensory potentiation, failed to impede the vascular enlargement and the increased VEGF-ir. Thus present results suggest that the CB vascular enlargement induced by CIH is a direct effect of intermittent hypoxia and not secondary to the oxidative stress. Accordingly, the subsequent capillary changes may be secondary to the mechanisms involved in the neural chemosensory plasticity induced by intermittent hypoxia.

Expression of hypoxia-inducible factor-1α in human periodontal tissue

BACKGROUND

Hypoxia-inducible factor (HIF)-1 is a key transcription factor responding to hypoxia. It is composed of an oxygen-sensitive α subunit (HIF-1α) and a constitutively expressed β subunit. Increasing evidence indicates an essential role for HIF-1α in infection and immunity. Because inflamed periodontium is thought to be hypoxic, we hypothesize that HIF-1α is expressed and related to its upstream regulator tumor necrosis factor (TNF)-α and downstream effecter vascular endothelial growth factor (VEGF).

METHODS

Human gingival biopsies were collected from advanced periodontitis sites and clinically healthy sites, and immunohistochemically examined for HIF-1α and VEGF peptides. The messenger ribonucleic acid (mRNA) and protein levels of HIF-1α, VEGF, and TNF-α in the biopsies were then assessed by reverse transcription polymerase chain reaction and Western blotting.

RESULTS

HIF-1α-positive immunoreactivity was detected in the nuclei of epithelial and endothelial cells. In periodontal pockets, there was a marked increase in the proportion of fibroblast-like cells and leukocyte-like cells expressing HIF-1α. Protein levels of HIF-1α, VEGF, and TNF-α were significantly higher in periodontal pockets than in control gingival samples. The mRNA expression of VEGF and TNF-α was also increased in periodontal pockets.

CONCLUSION

HIF-1α is expressed in healthy and diseased periodontium and may be related to TNF-α and VEGF function during periodontitis.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Iron chelation and the ventilatory response to hypoxia

Chelation of iron in in vitro carotid body emulates the effects of hypoxia. The role iron plays in in vivo ventilatory responses is unclear. In the current study we addressed this issue by examining the effects of chronic iron chelation on the hypoxic ventilatory response in 9 conscious Wistar rats. Acute responses to 14 and 9% O(2)in N(2) were recorded in the same rat before and then after 7 and 14 days of continuous iron chelation. Iron chelation was carried out with ciclopirox olamine (CPX) in a dose of 20 mg/kg daily, i.p. Ventilation was recorded with whole body plethysmography. We found that the peak hypoxic ventilation (V(E) achieved during 14 and 9% hypoxia was lower by 239.6+/-55.4(SE) and 269.6.2+/-69.2 ml min(-1)kg(-1), respectively, in the rats treated with CPX for 7 days. The decreases were not intensified by a longer duration of iron chelation. CPX failed to alter hypoxic sensitivity, assessed from the gain of peak V(E) with increasing strength of the hypoxic stimulus. In conclusion, we believe we have shown that iron is operational in shaping the hypoxic ventilatory response, but is not liable to be the underlying determinant of the hypoxic chemoreflex.

Upregulation of erythropoietin and its receptor expression in the rat carotid body during chronic and intermittent hypoxia

The carotid body (CB) plays important roles in cardiorespiratory changes in intermittent hypoxia (IH). Erythropoietin (EPO), a hypoxia-inducible factor (HIF)-1 target gene, is present in the chemoreceptive type-I cells in the CB but its expression and role in IH resembling sleep apnoeic conditions are not known. We hypothesized that IH upregulates the expression of EPO and its receptor (EPOr) in the rat CB. The CB expressions of EPO and EPOr were examined in rats breathing 10% O(2) (in isobaric chamber for CH, 24 hour/day) or in IH (cyclic between air and 5% O(2) per minute, 8 hour/day) for 3-28 days. Immunohistochemical studies revealed that the EPO and EPOr proteins were localized in CB glomic clusters. The proportional amount of cells with positive staining of EPO immunoreactivities was significantly increased in both IH and CH groups when compared with the normoxic control. The EPO expression was more markedly increased in the CH than that of the IH groups throughout the time course, reaching a peak level at day 14. The positive EPOr immunostaining was increased significantly in the 3-day CH group. By day 14, the EPOr expression elevated considerably at peak levels in both IH and CH rats, whereas the elevation was greater in the CH rats. These results suggest an upregulation of EPO and its receptor expression in the rat CB under IH and CH conditions, presumably mediated by the activation of HIF-1 pathway. The increased EPO binding to its receptor might play a role in the enhancement of CB excitability during the early pathogenesis in patients with sleep-disordered breathing.

Chronic hypoxia upregulates the expression and function of proinflammatory cytokines in the rat carotid body

The structure and function of the carotid body are greatly altered during chronic hypoxia. Recent studies showed the expression of interleukin (IL)-1 receptor and IL-6 receptor in the carotid body, suggesting a role of proinflammatory cytokines in the chemoreceptor function. The present study aimed to examine the hypothesis that the expression of pro-inflammatory cytokines, namely IL-1beta, IL-6 and tumor necrosis factor (TNF)alpha, plays a role in the rat carotid body in chronic hypoxia. Levels of the mRNA expression of the cytokines and their receptors IL-1r1, gp130 and TNFr1, were significantly increased in the carotid body of hypoxic rats when compared with the normoxic control. Immunohistochemistry showed that the expressions of cytokines and receptors were localized in the lobules of chemosensitive glomus cells containing tyrosine hydroxylase. There were significantly more positive-staining cells in the hypoxic groups with treatment for 3, 7 and 28 days than those of the normoxic controls. Application of exogenous cytokines (0.1 nM) elevated intracellular calcium ([Ca(2+)](i)) responses to acute hypoxia in the dissociated fura-2-loaded glomus cells. The increased [Ca(2+)](i) response in the hypoxic group was significantly greater than that of the normoxic group. Moreover, the gene transcripts of inflammatory mediator inducible nitric oxide synthase and chemokines (MCP-1, CCR2, MIP-1alpha, and ICAM-1) were increased in the carotid body of hypoxic rats. Collectively, results suggest that the increased expressions of proinflammatory cytokines play a functional role in the carotid body with local inflammation during chronic hypoxia.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

Effect of the endothelin receptor antagonist bosentan on chronic hypoxia-induced morphological and physiological changes in rat carotid body

Previous experiments have repeatedly demonstrated that exposure to chronic hypoxia (CH) elicits remarkable structural changes and chemosensory hypersensitivity in the mammalian carotid body. Moreover, recent studies have shown that CH upregulates the neuroactive peptide, endothelin (ET), in oxygen-sensitive type I cells. The present study examines the possible involvement of ET in adaptation by concurrently exposing rats to hypobaric CH (B(P) = 380 Torr) and bosentan, a potent nonpeptide antagonist that blocks ET(A) and ET(B) receptors. Carotid body weight indicated that 14 days of CH induced organ enlargement, a response that was blunted in bosentan-treated rats (CH: 2.54 +/- 0.19-fold increase; CH plus bosentan: 1.92 +/- 0.14-fold increase; P < 0.05). Morphometric studies revealed that bosentan substantially eliminated CH-induced hyperplasia of chemosensory cell lobules as well as expansion of the connective tissue matrix. Vascular dilation associated with CH was not altered by the drug. In untreated animals exposed to 3 days of CH, expression of proliferating cell nuclear antigen (PCNA), a marker of mitosis, was increased in lobules of oxygen-sensitive type I cells and in extralobular vascular and connective tissue cells. The incidence of PCNA expression was significantly (P < 0.05) reduced in bosentan-treated animals. In vitro assessments of carotid sinus nerve (CSN) activity showed that enhancement of basal and hypoxia-evoked chemosensory activity following 9 days of CH was significantly (P < 0.001) blunted by concurrent treatment with bosentan. Collectively, our data are consistent with the hypothesis that CH-induced adaptation in the carotid body is at least partially mediated by signaling pathways involving ET receptors.

Increased endogenous nitric oxide release by iron chelation and purinergic activation in the rat carotid body

We examined the hypothesis that hypoxic chemotransduction with stabilization of HIF-1 and activation of purinoceptors stimulate the endogenous NO production in the rat carotid body. The effects of blockade of purinoceptors with suramin, or blockade of HIF-1α hydroxylation by suppressing prolyl hydroxylase (PAH) activity on the endogenous NO release measured electrochemically by microsensor inserted into the isolated carotid body superfused with bicarbonate-buffer were examined. Suramin did not change the resting NO level under normoxic conditions but it significantly decreased the hypoxia-induced NO elevation in a dose-dependent manner. Suramin (100μM) blocked the NO response to acute hypoxia by 53%. Intracellular iron chelator, ciclopirox olamine (CPX) significantly increased the resting NO release close to the hypoxic level, which was reversed by FeSO₄ or blocked by L-NMMA. Also, PAH inhibition with dimethy-loxalylglycine (DMOG) moderately increased the resting NO release. In the presence of CPX and DMOG the resting NO release was increased to the hypoxic level. Collectively, results suggest that iron chelation and purinoceptor stimulation play a role in the hypoxic chemotransduction for an increase in the endogenous NO production in the rat carotid body.

Age-related changes in adrenomedullin expression and hypoxia-inducible factor-1 activity in the rat lung and their responses to hypoxia

Male rats aged 3 months, 12 months and 20 months were subjected to breathing 8% oxygen for 6 hours. Lung preproadrenomedullin (AM) messenger RNA (mRNA) levels were measured by solution hybridization-RNase protection assay while AM was measured by radioimmunoassay. The binding of hypoxia-inducible factor-1alpha (HIF-1alpha) to DNA was determined by electrophoretic mobility shift. There was an age-related increase in basal levels of preproAM mRNA and AM and of the binding of hypoxia-inducible factor (HIF) to DNA. Upon hypoxic stimulation, HIF binding to DNA increased in the young and middle-aged rats, but not in the old rats. AM gene expression increased in response to hypoxia in rats of all ages, but the increase was much less in the old rats. AM peptide levels in the lung decreased with age in hypoxia. In a separate experiment, male rats aged 3 months and 20 months were subjected to hypoxia as described above. PreproAM, calcitonin receptor-like receptor (CRLR), receptor activity modifying protein (RAMP) mRNA, HIF-1 and peptidyl-glycine-amidating monooxygenase (PAM) mRNA levels were measured by reverse transcription-polymerase chain reaction. All except PAM showed a decrease in basal levels and a diminished response to hypoxia in the old rats. Polysome profiling demonstrated decreases in the percentages of translatable preproAM mRNA in response to hypoxia, with a greater decrease in the old than the young rats. It is concluded that an age-dependent decrease in the hypoxic response of the AM system in the lung was associated with high basal levels of HIF activity and AM expression in the old rats, and a lower proportion of translatable preproAM mRNA in the old rats in response to hypoxia. Thus, the HIF-AM pathway may be impaired in the aged lung, and other mechanisms may be present to maintain an AM response to hypoxia.

Developmental plasticity of the hypoxic ventilatory response after perinatal hyperoxia and hypoxia

Both genetic and environmental factors influence the normal development of the respiratory control system. This review examines the role perinatal O2 plays in the development of normoxic breathing and the hypoxic ventilatory response in mammals. Hyperoxia and hypoxia elicit plasticity in respiratory control that is unique to development and may persist weeks to years after return to normoxia. Specifically, both hyperoxia and hypoxia during early postnatal development attenuate the adult hypoxic ventilatory response, but the underlying mechanisms for this plasticity differ. Hyperoxia attenuates the hypoxic ventilatory response through potentially life-long changes in carotid body function. Neonatal hypoxia appears to have short-term effects on carotid body function, but persistent changes in the hypoxic ventilatory response may instead reflect changes in respiratory mechanics or related neural pathways. Overall, it appears that a relatively narrow range of environmental O2 is consistent with "normal" postnatal respiratory control development, predisposing animals to potentially maladaptive plasticity in the face of disease or atypical environmental conditions.

Developmental changes in HIF transcription factor in carotid body: relevance for O2 sensing by chemoreceptors

Before birth, the peripheral chemoreceptors located in the carotid bodies (CB) are adapted to the low fetal Po(2) and are relatively insensitive to hypoxia. After birth, the sensitivity of the CB to hypoxia is reset in response to the rise in Po(2). The mechanism underlying this resetting, which requires several days to complete, remains unknown. We have investigated the possibility that the hypoxia-inducible factors HIF-1alpha and HIF-2alpha, which are activated by oxygen deprivation, are involved in this resetting process. Accordingly, we used immunostaining and densitometry to quantitate the levels of the HIF-1alpha and HIF-2alpha proteins in the rat CB during early perinatal life and after exposure to in vivo hypoxia during adolescence. Tyrosine hydroxylase (TH) was used as a marker for catecholaminergic neurons and oxygen-sensitive cells in the CB. Double-immunostaining revealed constitutive expression of HIF-1alpha in both glomus cells (TH+) and sustentacular cells (TH-) of the CB of adolescent rats. However, immunoreactivity toward HIF-2alpha was restricted to glomus cells. After exposure to hypoxia (8% O(2), 6 h), the expression of HIF-1alpha was selectively up-regulated in glomus cells and apparent translocation of both HIF-1alpha and HIF-2alpha to the nucleus was observed. Both of these proteins were expressed constitutively in the CB during the perinatal transition period. During the first postnatal week, the intensity of immunostaining for HIF-1alpha in glomus cells decreased markedly, whereas the level of HIF-2alpha remained constant. We suggest that this selective down-regulation of HIF-1alpha may be involved in the postnatal maturation of CB responsiveness to hypoxia.

Comparative gene expression profile of mouse carotid body and adrenal medulla under physiological hypoxia

The carotid body (CB) is an arterial chemoreceptor, bearing specialized type I cells that respond to hypoxia by closing specific K+ channels and releasing neurotransmitters to activate sensory axons. Despite having detailed information on the electrical and neurochemical changes triggered by hypoxia in CB, the knowledge of the molecular components involved in the signalling cascade of the hypoxic response is fragmentary. This study analyses the mouse CB transcriptional changes in response to low PO2 by hybridization to oligonucleotide microarrays. The transcripts were obtained from whole CBs after mice were exposed to either normoxia (21% O2), or physiological hypoxia (10% O2) for 24 h. The CB transcriptional profiles obtained under these environmental conditions were subtracted from the profile of control non-chemoreceptor adrenal medulla extracted from the same animals. Given the common developmental origin of these two organs, they share many properties but differ specifically in their response to O2. Our analysis revealed 751 probe sets regulated specifically in CB under hypoxia (388 up-regulated and 363 down-regulated). These results were corroborated by assessing the transcriptional changes of selected genes under physiological hypoxia with quantitative RT-PCR. Our microarray experiments revealed a number of CB-expressed genes (e.g. TH, ferritin and triosephosphate isomerase) that were known to change their expression under hypoxia. However, we also found novel genes that consistently changed their expression under physiological hypoxia. Among them, a group of ion channels show specific regulation in CB: the potassium channels Kir6.1 and Kcnn4 are up-regulated, while the modulatory subunit Kcnab1 is down-regulated by low PO2 levels.

Activation of HIF-1α mRNA by hypoxia and iron chelator in isolated rat carotid body

The hypoxia inducible factor-1alpha (HIF-1alpha) protein level is increased by hypoxia and iron chelator (ciclopirox olamine) in isolated rat carotid body (CB) and glomus cells. Reverse transcription and polymerase chain reaction (RT-PCR) are performed to test whether this increase is caused, at least in part, by increased HIF-1alpha gene transcription. HIF-1alpha mRNA levels dose-dependently increased and decreased in the rat CBs incubated for 1 h in a medium saturated with O(2) levels that were varied around nominally normoxic level of 21% in the 0-95% range. The iron chelator, ciclopirox olamine (5 microM), stimulated HIF-1alpha mRNA production under normoxic condition. Thus, in the CB, the main systemic O(2)-sensing organ, HIF-1alpha transcription is regulated by O(2) supply around the normoxic level; this may contribute to cellular and organismal adaptations to chronic changes in ambient O(2).