Effect of chronic continuous or intermittent hypoxia and reoxygenation on cerebral capillary density and myelination

Chronic hypoxia, whether continuous (CCH) or intermittent (CIH), occurs in many neonatal pathological conditions, such as bronchopulmonary dysplasia and obstructive sleep apnea. In this study, we explored the effect of CCH and CIH on cerebral capillary density and myelination. We subjected CD-1 mice starting at postnatal day 2 to either CCH 11% oxygen (O(2)), or CIH 11% O(2) (4-min cycles), for periods of 2 and 4 wk followed by reoxygenation for 4 wk. Mice were deeply anesthetized and perfused. Brains were removed to fixative for 24 h, then paraffin-embedded. Coronal brain sections were taken for analysis. Immunocytochemistry for glucose transporter 1 was used to assess angiogenesis, and Luxol fast blue and fluoromyelin stains were used to assess myelination. Capillary density increased after 2-wk exposure to CIH and CCH. By 4 wk, capillary density increased in both CIH and CCH by 25% and 47%, respectively, in cortex and by 29% and 44%, respectively, in hippocampus (P < 0.05). There was a decrease in myelination in the corpus callosum of mice exposed to CIH (75% of control) and CCH (50% of control) (P < 0.05). Reoxygenation reversed the increased capillary density seen in CCH to normoxic values. However, dysmyelination that occurred in CCH-exposed mice did not show any improvement upon reoxygenation. We conclude that neonatal chronic hypoxia 1) induces brain angiogenesis, which is reversible with reoxygenation, and 2) irreversibly reduces the extent of myelination in the corpus callosum. This potential irreversible effect on myelination in early life can, therefore, have long-term and devastating effects.     

Chronic intermittent hypoxia increases the CO2 reserve in sleeping dogs.

We hypothesized that chronic intermittent hypoxia (CIH) would induce a predisposition to apnea in response to induced hypocapnia. To test this, we used pressure support ventilation to quantify the difference in end-tidal partial pressure of CO(2) (Pet(CO(2))) between eupnea and the apneic threshold ("CO(2) reserve") as an index of the propensity for apnea and unstable breathing during sleep, both before and following up to 3-wk exposure to chronic intermittent hypoxia in dogs. CIH consisted of 25 s of Pet(O(2)) = 35-40 Torr followed by 35 s of normoxia, and this pattern was repeated 60 times/h, 7-8 h/day for 3 wk. The CO(2) reserve was determined during non-rapid eye movement sleep in normoxia 14-16 h after the most recent hypoxic exposure. Contrary to our hypothesis, the slope of the ventilatory response to CO(2) below eupnea progressively decreased during CIH (control, 1.36 +/- 0.18; week 2, 0.94 +/- 0.12; week 3, 0.73 +/- 0.05 l.min(-1).Torr(-1), P < 0.05). This resulted in a significant increase in the CO(2) reserve relative to control (P < 0.05) following both 2 and 3 wk of CIH (control, 2.6 +/- 0.6; week 2, 3.7 +/- 0.8; week 3, 4.5 +/- 0.9 Torr). CIH also 1) caused no change in eupneic, air breathing Pa(CO(2)); 2) increased the slope of the ventilatory response to hypercapnia after 2 wk but not after 3 wk compared with control; and 3) had no effect on the ventilatory response to hypoxia. We conclude that 3-wk CIH reduced the sensitivity of the ventilatory response to transient hypocapnia and thereby increased the CO(2) reserve, i.e., the propensity for apnea was reduced.     

Effect of two paradigms of chronic intermittent hypoxia on carotid body sensory activity.

Reflexes arising from the carotid bodies may play an important role in cardiorespiratory changes evoked by chronic intermittent hypoxia (CIH). In the present study, we examined whether CIH affects the hypoxic sensing ability of the carotid bodies and, if so, by what mechanisms. Experiments were performed on adult male rats (Sprague-Dawley, 250-300 g) exposed to two paradigms of CIH for 10 days: 1) multiple exposures to short durations of intermittent hypoxia per day (SDIH; 15 s of 5% O(2) + 5 min of 21% O(2), 9 episodes/h, 8 h/day) and 2) single exposure to longer durations of intermittent hypoxia per day [LDIH; 4 h of hypobaric hypoxia (0.4 atm/day) + 20 h of normoxia]. Carotid body sensory response to graded isocapnic hypoxia was examined in both groups of animals under anesthetized conditions. Hypoxic sensory response was significantly enhanced in SDIH but not in LDIH animals. Similar enhancement in hypoxic sensory response was also elicited in ex vivo carotid bodies from SDIH animals, suggesting that the effects were not secondary to cardiovascular changes. SDIH, however, had no significant effect on the hypercapnic sensory response. The effects of SDIH on the hypoxic sensory response completely reversed after SDIH animals were placed in a normoxic environment for an additional 10 days. Previous treatment with systemic administration of O(2)(-)* radical scavenger prevented SDIH-induced augmentation of the hypoxic sensory response. These results demonstrate that SDIH but not LDIH results in selective augmentation of the hypoxic response of the carotid body and O(2)(-)* radicals play an important role in SDIH-induced sensitization of the carotid body.     

Comparative analysis of neonatal and adult rat carotid body responses to chronic intermittent hypoxia.

Previous studies suggest that carotid body responses to long-term changes in environmental oxygen differ between neonates and adults. In the present study we tested the hypothesis that the effects of chronic intermittent hypoxia (CIH) on the carotid body differ between neonates and adult rats. Experiments were performed on neonatal (1-10 days) and adult (6-8 wk) males exposed either to CIH (9 episodes/h; 8 h/day) or to normoxia. Sensory activity was recorded from ex vivo carotid bodies. CIH augmented the hypoxic sensory response (HSR) in both groups. The magnitude of CIH-evoked hypoxic sensitization was significantly greater in neonates than in adults. Seventy-two episodes of CIH were sufficient to evoke hypoxic sensitization in neonates, whereas as many as 720 CIH episodes were required in adults, suggesting that neonatal carotid bodies are more sensitive to CIH than adult carotid bodies. CIH-induced hypoxic sensitization was reversed in adult rats after reexposure to 10 days of normoxia, whereas the effects of neonatal CIH persisted into adult life (2 mo). Acute intermittent hypoxia (IH) evoked sensory long-term facilitation of the carotid body activity (sensory LTF, i.e., increased baseline neural activity following acute IH) in CIH-exposed adults but not in neonates. The effects of CIH were associated with hyperplasia of glomus cells in neonatal but not in adult carotid bodies. These observations demonstrate that responses to CIH differ between neonates and adults with regard to the magnitude of sensitization of HSR, susceptibility to CIH, induction of sensory LTF, reversibility of the responses, and morphological remodeling of the chemoreceptor tissue.     

Modulation of the contractile responses of guinea pig isolated tracheal rings after chronic intermittent hypobaric hypoxia with and without cold exposure.

Previous studies have documented that repetitive exposure to intermittent hypoxia, such as that encountered in preparation to high-altitude ascent, influences breathing. However, the impact of intermittent hypoxia on airway smooth muscle has not been explored. Ascents to high altitude, in addition to hypoxia, expose individuals to cold air. The objective of the present study is to examine the effect of chronic intermittent hypobaric hypoxia (CIH) and CIH combined with cold exposure (CIHC) on tracheal smooth muscle responses to various contractile and relaxant agonists. Experiments were performed on tracheal rings harvested from adult guinea pigs exposed either to CIH or CIHC [14 days (6 h/day) at barometric pressure of 350 mmHg with and without cold exposure of 5 degrees C] or to room air (normoxia). CIH and CIHC attenuated maximum contractile responses to ACh compared with normoxia. The maximum contractile response to histamine decreased with CIH, whereas CIHC restored the response back to normoxia. Both CIH and CIHC attenuated maximum contractile responses to 5-HT. Altered contractile responses after CIH and CIHC were independent of epithelium. Isoproterenol-induced relaxation was not altered by CIH, whereas it was enhanced after CIHC, and these responses were independent of the epithelium. The data demonstrate that intermittent exposure to hypoxia profoundly influences contractile response of tracheal smooth muscle, and cold exposure can further modulate the response, implying the importance of cold at high altitude.     

Gene expression in mouse brain following chronic hypoxia: role of sarcospan in glial cell death.

Hypoxia is a hallmark of respiratory, neurological, or hematological diseases as well as life at high altitude. For example, chronic constant hypoxia (CCH) occurs in chronic lung diseases or at high altitude, whereas chronic intermittent hypoxia (CIH) occurs in diseases such as sleep apnea or sickle cell disease. Despite the fact that such conditions are frequent, the cellular and molecular mechanisms underlying the effect of hypoxia, whether constant or intermittent, are not well understood. In this study, we first determined the effect of CCH and CIH on global gene expression in different regions of mouse brain using microarrays and then investigated the biological role of genes of interest. We found that: 1) in the cortical region, the expression level of 80 genes was significantly altered by CIH (16 up- and 64 downregulated), and this number increased to 137 genes following CCH (34 up- and 103 downregulated); 2) a similar number of gene alterations was identified in the hippocampal area, and the majority of the changes in this region were upregulations; 3) two genes (Sspn and Ttc27) were downregulated in both brain regions and following both treatments; and 4) RNA interference-mediated knockdown of Sspn increased cell death in hypoxia in a cell culture system. We conclude that CIH or CCH induced significant and distinguishable alterations in gene expression in cortex and hippocampus and that Sspn seems to play a critical role in inducing cell death under hypoxic conditions.     

Intermittent hypoxia increases ventilation and Sa(O2) during hypoxic exercise and hypoxic chemosensitivity.

The purpose of this study was 1) to test the hypothesis that ventilation and arterial oxygen saturation (Sa(O2)) during acute hypoxia may increase during intermittent hypoxia and remain elevated for a week without hypoxic exposure and 2) to clarify whether the changes in ventilation and Sa(O2) during hypoxic exercise are correlated with the change in hypoxic chemosensitivity. Six subjects were exposed to a simulated altitude of 4,500 m altitude for 7 days (1 h/day). Oxygen uptake (VO2), expired minute ventilation (VE), and Sa(O2) were measured during maximal and submaximal exercise at 432 Torr before (Pre), after intermittent hypoxia (Post), and again after a week at sea level (De). Hypoxic ventilatory response (HVR) was also determined. At both Post and De, significant increases from Pre were found in HVR at rest and in ventilatory equivalent for O2 (VE/VO2) and Sa(O2) during submaximal exercise. There were significant correlations among the changes in HVR at rest and in VE/VO2 and Sa(O2) during hypoxic exercise during intermittent hypoxia. We conclude that 1 wk of daily exposure to 1 h of hypoxia significantly improved oxygenation in exercise during subsequent acute hypoxic exposures up to 1 wk after the conditioning, presumably caused by the enhanced hypoxic ventilatory chemosensitivity.     

N-acetylcysteine treatment prevents the up-regulation of MnSOD in chronically hypoxic rat hearts

Chronic intermittent hypoxia (CIH) is associated with increased production of reactive oxygen species that contributes to the adaptive mechanism underlying the improved myocardial ischemic tolerance. The aim was to find out whether the antioxidative enzyme manganese superoxide dismutase (MnSOD) can play a role in CIH-induced cardioprotection. Adult male Wistar rats were exposed to intermittent hypobaric hypoxia (7000 m, 8 h/day, 25 exposures) (n=14) or kept at normoxia (n=14). Half of the animals from each group received N-acetylcysteine (NAC, 100 mg/kg) daily before the hypoxic exposure. The activity and expression of MnSOD were increased by 66 % and 23 %, respectively, in the mitochondrial fraction of CIH hearts as compared with the normoxic group; these effects were suppressed by NAC treatment. The negative correlation between MnSOD activity and myocardial infarct size suggests that MnSOD can contribute to the improved ischemic tolerance of CIH hearts.     

Role of gp91phox-containing NADPH oxidase in left ventricular remodeling induced by intermittent hypoxic stress

Intermittent hypoxia due to sleep apnea syndrome is associated with cardiovascular diseases. However, the precise mechanisms by which intermittent hypoxic stress accelerates cardiovascular diseases are largely unclear. The aim of this study was to investigate the role of gp91(phox)-containing NADPH oxidase in the development of left ventricular (LV) remodeling induced by intermittent hypoxic stress in mice. Male gp91(phox)-deficient (gp91(-/-)) mice (n = 26) and wild-type (n = 39) mice at 7-12 wk of age were exposed to intermittent hypoxia (30 s of 4.5-5.5% O(2) followed by 30 s of 21% O(2) for 8 h/day during daytime) or normoxia for 10 days. Mean blood pressure and LV systolic and diastolic function were not changed by intermittent hypoxia in wild-type or gp91(-/-) mice, although right ventricular systolic pressure tended to be increased. In wild-type mice, intermittent hypoxic stress significantly increased the diameter of cardiomyocytes and interstitial fibrosis in LV myocardium. Furthermore, intermittent hypoxic stress increased superoxide production, 4-hydroxy-2-nonenal protein, TNF-alpha and transforming growth factor-beta mRNA, and NF-kappaB binding activity in wild-type, but not gp91(-/-), mice. These results suggest that gp91(phox)-containing NADPH oxidase plays a crucial role in the pathophysiology of intermittent hypoxia-induced LV remodeling through an increase of oxidative stress.     

Chronic intermittent hypoxia reduces ventilatory long-term facilitation and enhances apnea frequency in newborn rats

Ventilatory long-term facilitation (LTF; defined as gradual increase of minute ventilation following repeated hypoxic exposures) is well described in adult mammals and is hypothesized to be a protective mechanism against apnea. In newborns, LTF is absent during the first postnatal days, but its precise developmental pattern is unknown. Accordingly, this study describes this pattern of postnatal development. Additionally, we tested the hypothesis that chronic intermittent hypoxia (CIH) from birth alters this development. LTF was estimated in vivo using whole body plethysmography by exposing rat pups at postnatal days 1, 4, and 10 (P1, P4, and P10) to 10 brief hypoxic cycles (nadir 5% O2) and respiratory recordings during the following 2 h (recovery, 21% O2). Under these conditions, ventilatory LTF (gradual increase of minute ventilation during recovery) was clearly expressed in P10 rats but not in P1 and P4. In a second series of experiments, rat pups were exposed to CIH during the first 10 postnatal days (6 brief cyclic exposures at 5% O2 every 6 min followed by 1 h under normoxia, 24 h a day). Compared with P10 control rats, CIH enhanced hypoxic ventilatory response (estimated during the hypoxic cycles) specifically in male rat pups. Ventilatory LTF was drastically reduced in P10 rats exposed to CIH, which was associated with higher apnea frequency during recovery. We conclude that CIH from birth enhances hypoxic chemoreflex and disrupts LTF development, thus likely contributing to increase apnea frequency.     

消痰化瘀利窍方对慢性间歇性低氧大鼠肠系膜动脉损伤的改善作用

目的:观察消痰化瘀利窍方对慢性间歇性低氧(CIH)大鼠肠系膜动脉功能损伤的作用,并探讨其可能机制。方法:48只雄性SD大鼠随机分为4组(n=12),常氧对照组(Normoxia)、慢性间歇性低氧组(CIH)、慢性间歇性低氧中药干预组(Formula+CIH)、中药对照组(Formula)。CIH与Formula+CIH组置于间歇性低氧装置,通过充入氮气、氧气使O2含量在9%至21%间循环,每循环3min;Normoxia和Formula组则充入空气。其中,Formula+CIH与Formula组于每日造模前中药水煎液灌胃(24g/kg),而CIH组与Normoxia组给予等量生理盐水。造模结束后,应用HE染色观察各组大鼠肠系膜动脉的组织病理学改变,通过微血管环技术观察ACh、L-Arg诱导的肠系膜动脉舒张反应,通过ELISA技术检测大鼠造模前及造模21d血清一氧化氮(NO)的含量并应用Westernblot技术测定肠系膜动脉eNOS和p-eNOS的蛋白水平。结果:与Normoxia组相比,CIH组大鼠肠系膜动脉内皮明显损伤、中膜增厚,ACh、L-Arg诱导的肠系膜动脉舒张反应明显减弱,血清中NO水平及肠系膜动脉p-eNOS/eNOS比值显著降低。消痰化瘀利窍方干预能够减轻大鼠肠系膜动脉的内膜与中膜病理损伤,改善肠系膜动脉舒张功能,提高血清NO含量及肠系膜动脉eNOS磷酸化水平。而单纯给予消痰化瘀利窍方大鼠与Normoxia组相比各指标均未发现显著变化。结论:消痰化瘀利窍方可以减轻慢性间歇性低氧引起的大鼠肠系膜动脉功能损伤,其机制与提高NO的生物利用度有关。     

Hyperlipidemia and lipid peroxidation are dependent on the severity of chronic intermittent hypoxia.

Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia (CIH) and associated with dysregulation of lipid metabolisms and atherosclerosis. Causal relationships between OSA and metabolic abnormalities have not been established because of confounding effects of underlying obesity. The goal of the study was to determine if CIH causes lipid peroxidation and dyslipidemia in the absence of obesity and whether the degrees of dyslipidemia and lipid peroxidation depend on the severity of hypoxia. Lean C57BL/6J mice were exposed to CIH for 4 wk with a fractional inspired O2 (FI(O2)) nadir of either 10% (moderate CIH) or 5% (severe CIH). Mice exposed to severe CIH exhibited significant increases in fasting serum levels of total cholesterol (129 +/- 2.9 vs. 113 +/- 2.8 mg/dl in control mice, P < 0.05) and low-density lipoprotein cholesterol (85.7 +/- 8.9 vs. 56.4 +/- 9.7 mg/dl, P < 0.05) in conjunction with a 1.5- to 2-fold increase in lipoprotein secretion, and upregulation of hepatic stearoyl coenzyme A desaturase 1 (SCD-1). Severe CIH also markedly increased lipid peroxidation in the liver (malondialdehyde levels of 94.4 +/- 5.4 vs. 57.4 +/- 5.2 nmol/mg in control mice, P < 0.001). In contrast, moderate CIH did not induce hyperlipidemia or change in hepatic SCD-1 levels but did cause lipid peroxidation in the liver at a reduced level relative to severe CIH. In conclusion, CIH leads to hypercholesterolemia and lipid peroxidation in the absence of obesity, and the degree of metabolic dysregulation is dependent on the severity of the hypoxic stimulus.     

Chronic intermittent hypoxia alters NMDA and AMPA-evoked currents in NTS neurons receiving carotid body chemoreceptor inputs

Chronic exposure to intermittent hypoxia (CIH) has been used in animals to mimic the arterial hypoxemia that accompanies sleep apnea. Humans with sleep apnea and animals exposed to CIH have elevated blood pressures and augmented sympathetic nervous system responses to acute exposures to hypoxia. To test the hypothesis that exposure to CIH alters neurons within the nucleus of the solitary tract (NTS) that integrate arterial chemoreceptor afferent inputs, we measured whole cell currents induced by activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in enzymatically dispersed NTS neurons from normoxic (NORM) and CIH-exposed rats (alternating cycles of 3 min at 10% O2 followed by 3 min at 21% O2 between 8 AM and 4 PM for 7 days). To identify NTS neurons receiving carotid body afferent inputs the anterograde tracer 4- (4-(dihexadecylamino)styryl-N-methylpyridinum iodide (DiA) was placed onto the carotid body 1 wk before exposure to CIH. AMPA dose-response curves had similar EC50 but maximal responses increased in neurons isolated from DiA-labeled CIH (20.1 +/- 0.8 microM, n = 9) compared with NORM (6.0 +/- 0.3 microM, n = 8) rats. NMDA dose-response curves also had similar EC50 but maximal responses decreased in CIH (8.4 +/- 0.4 microM, n = 8) compared with NORM (19.4 +/- 0.6 microM, n = 9) rats. These results suggest reciprocal changes in the number and/or conductance characteristics of AMPA and NMDA receptors. Enhanced responses to AMPA receptor activation could contribute to enhanced chemoreflex responses observed in animals exposed to CIH and humans with sleep apnea.     

消痰化瘀利窍方对慢性间歇性低氧小鼠认知障碍的改善作用*

目的: 探讨消痰化瘀利窍方对慢性间歇性低氧小鼠认知障碍的改善作用。方法: 48只雄性C57小鼠随机分为4组(n=12),常氧对照组(Normoxia),慢性间歇性低氧组(CIH)、慢性间歇性低氧中药干预组(Formula+CIH)、中药对照组(Formula)。Normoxia和Formula组暴露于常氧环境,CIH与Formula+CIH组暴露于间歇性低氧环境(低氧舱中前1.5 min充入氮气使舱内氧浓度降至9%,后1.5 min充入氧气使氧浓度恢复至21%,3 min/循环,每天上舱8 h,共35 d)。其中,Formula +CIH与Formula组于每日灌胃中药水煎液灌胃(26.8 g/kg),同时CIH组与Normoxia组灌胃给予同体积生理盐水。实验在26～35 d连续应用水迷宫观测各组小鼠的学习和记忆能力,35 d造模结束后,首先进行Y-迷宫实验,麻醉后断头取脑,分离海马组织。应用尼氏染色和电镜观察海马神经元的形态学改变,通过Western blot检测海马神经元synapsin和PSD-95的表达水平。结果: 与Normoxia组相比,CIH组小鼠水迷宫和Y-迷宫的成绩显著下降(P＜0.01,P＜0.01),海马神经元尼氏小体的数量和突触后致密物质的厚度均减少,PSD-95蛋白表达下调(P＜0.01),而synapsin表达无明显改变。与CIH组小鼠比较,消痰化瘀利窍方干预可显著提高小鼠水迷宫和Y-迷宫的成绩(P＜0.01),增加海马神经元尼氏小体的数量和突触后致密物质的厚度,上调PSD-95蛋白表达水平(P＜0.01)。结论: 消痰化瘀利窍方可改善由CIH诱导的突触后致密区的结构和功能受损,进而对认知功能障碍起到保护作用。     

Selected contribution: chronic intermittent hypoxia enhances respiratory long-term facilitation in geriatric female rats.

Age and the estrus cycle affect time-dependent respiratory responses to episodic hypoxia in female rats. Respiratory long-term facilitation (LTF) is enhanced in middle-aged vs. young female rats (72). We tested the hypothesis that phrenic and hypoglossal (XII) LTF are diminished in acyclic geriatric rats when fluctuating sex hormone levels no longer establish conditions that enhance LTF. Chronic intermittent hypoxia (CIH) enhances LTF (41); thus we further predicted that CIH would restore LTF in geriatric female rats. LTF was measured in young (3-4 mo) and geriatric (20-22 mo) female Sasco Sprague-Dawley rats and in a group of geriatric rats exposed to 1 wk of nocturnal CIH (11 vs. 21% O2 at 5-min intervals, 12 h/night). In anesthetized, paralyzed, vagotomized, and ventilated rats, time-dependent hypoxic phrenic and XII responses were assessed. The short-term hypoxic response was measured during the first of three 5-min episodes of isocapnic hypoxia (arterial Po2 35-45 Torr). LTF was assessed 15, 30, and 60 min postepisodic hypoxia. Phrenic and XII short-term hypoxic response was not different among groups, regardless of CIH treatment (P > 0.05). LTF in geriatric female rats was smaller than previously reported for middle-aged rats but comparable to that in young female rats. CIH augmented phrenic and XII LTF to levels similar to those of middle-aged female rats without CIH (P < 0.05). The magnitude of phrenic and XII LTF in all groups was inversely related to the ratio of progesterone to estradiol serum levels (P < 0.05). Thus CIH and sex hormones influence the magnitude of LTF in geriatric female rats.     

Chronic sustained and intermittent hypoxia reduce function of ATP-sensitive potassium channels in nucleus of the solitary tract

Activation of neuronal ATP-sensitive potassium (K(ATP)) channels is an important mechanism that protects neurons and conserves neural function during hypoxia. We investigated hypoxia (bath gassed with 95% N(2)-5% CO(2) vs. 95% O(2)-5% CO(2) in control)-induced changes in K(ATP) current in second-order neurons of peripheral chemoreceptors in the nucleus of the solitary tract (NTS). Hypoxia-induced K(ATP) currents were compared between normoxic (Norm) rats and rats exposed to 1 wk of either chronic sustained hypoxia (CSH) or chronic intermittent hypoxia (CIH). Whole cell recordings of NTS second-order neurons identified after 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) labeling of the carotid bodies were obtained in a brain stem slice. In Norm cells (n = 9), hypoxia (3 min) induced an outward current of 12.7 +/- 1.1 pA with a reversal potential of -73 +/- 2 mV. This current was completely blocked by the K(ATP) channel blocker tolbutamide (100 muM). Bath application of the K(ATP) channel opener diazoxide (200 muM, 3 min) evoked an outward current of 21.8 +/- 5.8 pA (n = 6). Hypoxia elicited a significantly smaller outward current in both CSH (5.9 +/- 1.4 pA, n = 11; P < 0.01) and CIH (6.8 +/- 1.7 pA, n = 6; P < 0.05) neurons. Diazoxide elicited a significantly smaller outward current in CSH (3.9 +/- 1.0 pA, n = 5; P < 0.05) and CIH (2.9 +/- 0.9 pA, n = 3; P < 0.05) neurons. Western blot analysis showed reduced levels of K(ATP) potassium channel subunits Kir6.1 and Kir6.2 in the NTS from CSH and CIH rats. These results suggest that hypoxia activates K(ATP) channels in NTS neurons receiving monosynaptic chemoreceptor afferent inputs. Chronic exposure to either sustained or intermittent hypoxia reduces K(ATP) channel function in NTS neurons. This may represent a neuronal adaptation that preserves neuronal excitability in crucial relay neurons in peripheral chemoreflex pathways.     

The Na+/Ca2+ exchanger-1 mediates left ventricular dysfunction in mice with chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) and cardiovascular dysfunction occur in patients with obstructive sleep apnea. We hypothesized that the Na(+)/Ca(2+) exchanger-1 (NCX1) mediates, at least partially, left ventricular (LV) dysfunction in CIH. Four groups of mice (N = 15-17 per group), either cardiac-specific NCX1 knockouts (KO) or wild types (WT), were exposed to either CIH or normoxia [i.e., handled controls (HC)] 10 h/day for 8 wk. As expected, myocardial expression of NCX1 was greater in WT than in KO animals, both in HC and CIH-exposed groups. In both CIH groups (WT or KO), but not the HC groups, blood pressure increased by 10% at week 1 over their baseline and remained elevated for all 8 wk, with no differences between WT and KO. LV dilation (increased diastolic and systolic dimension) and hypertrophy (increased left heart weight), along with LV dysfunction (greater end-diastolic pressure and lower ejection fraction), were observed in the WT animals compared with the KO following CIH exposure. Compared with HC, CIH exposure was associated with apoptosis (terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling and caspase-3) in WT, but not KO, mice. We conclude that myocardial NCX1 does not mediate changes in blood pressure, but is one of the mediators for LV global dysfunction and cardiomyocyte injury in CIH.     

Effects of Cyclic Intermittent Hypoxia on ET-1 Responsiveness and Endothelial Dysfunction of Pulmonary Arteries in Rats

Obstructive sleep apnoea (OSA) is a risk factor for cardiovascular disorders and in some cases is complication of pulmonary hypertension. We simulated OSA by exposing rats to cyclic intermittent hypoxia (CIH) to investigate its effect on pulmonary vascular endothelial dysfunction. Sprague-Dawley Rats were exposed to CIH (FiO₂ 9% for 1 min, repeated every 2 min for 8 h/day, 7 days/wk for 3 wk), and the pulmonary arteries of normoxia and CIH treated rats were analyzed for expression of endothelin-1 (ET-1) and ET receptors by histological, immunohistochemical, RT-PCR and Western Blot analyses, as well as for contractility in response to ET-1. In the pulmonary arteries, ET-1 expression was increased, and ET-1 more potently elicited constriction of the pulmonary artery in CIH rats than in normoxic rats. Exposure to CIH induced marked endothelial cell damage associated with a functional decrease of endothelium-dependent vasodilatation in the pulmonary artery. Compared with normoxic rats, ET_A receptor expression was increased in smooth muscle cells of the CIH rats, while the expression of ET_B receptors was decreased in endothelial cells. These results demonstrated endothelium-dependent vasodilation was impaired and the vasoconstrictor responsiveness increased by CIH. The increased responsiveness to ET-1 induced by intermittent hypoxia in pulmonary arteries of rats was due to increased expression of ET_A receptors predominantly, meanwhile, decreased expression of ET_B receptors in the endothelium may also participate in it.