Analysis of the cardiovascular changes induced in the rat by graded levels of systemic hypoxia

The sigh and related behaviors

Breathing is a critical, complex, and highly integrated behavior. Normal rhythmic breathing, also referred to as eupnea, is interspersed with different breathing related behaviors. Sighing is one of such behaviors, essential for maintaining effective gas exchange by preventing the gradual collapse of alveoli in the lungs, known as atelectasis. Critical for the generation of both sighing and eupneic breathing is a region of the medulla known as the preBötzinger Complex (preBötC). Efforts are underway to identify the cellular pathways that link sighing as well as sneezing, yawning, and hiccupping with other brain regions to better understand how they are integrated and regulated in the context of other behaviors including chemosensation, olfaction, and cognition. Unraveling these interactions may provide important insights into the diverse roles of these behaviors in the initiation of arousal, stimulation of vigilance, and the relay of certain behavioral states. This chapter focuses primarily on the function of the sigh, how it is locally generated within the preBötC, and what the functional implications are for a potential link between sighing and cognitive regulation. Furthermore, we discuss recent insights gained into the pathways and mechanisms that control yawning, sneezing, and hiccupping.

Left ventricular depression and pulmonary edema in rats after short-term normobaric hypoxia: effects of adrenergic blockade and reduced fluid load

Acute normobaric hypoxia may induce pulmonary injury with edema (PE) and inflammation. Hypoxia is accompanied by sympathetic activation. As both acute hypoxia and high plasma catecholamine levels may elicit PE, we had originally expected that adrenergic blockade may attenuate the severity of hypoxic pulmonary injury. In particular, we investigated whether administration of drugs with reduced fluid load would be beneficial with respect to both cardiocirculatory and pulmonary functions in acute hypoxia. Rats were exposed to normobaric hypoxia (10% O₂) over 1.5 or 6 h and received 0.9% NaCl or adrenergic blockers either as infusion (1 ml/h, increased fluid load) or injection (0.5 ml, reduced fluid load). Control animals were kept in normoxia and received infusions or injections of 0.9% NaCl. After 6 h of hypoxia, LV inotropic function was maintained with NaCl injection but decreased significantly with NaCl infusion. Adrenergic blockade induced a similar LV depression when fluid load was low, but did not further deteriorate LV depression after 6 h of infusion. Reduced fluid load also attenuated pulmonary injury after 6 h of hypoxia. This might be due to an effective fluid drainage into the pleural space. Adrenergic blockade could not prevent PE. In general, increased fluid load and impaired LV inotropic function promote the development of PE in acute hypoxia. The main physiologic conclusion from this study is that fluid reduction under hypoxic conditions has a protective effect on cardiopulmonary function. Consequently, appropriate fluid management has particular importance to subjects in hypoxic conditions.

Neuronal Networks in Hypertension: Recent Advances

Neurogenic hypertension is associated with excessive sympathetic nerve activity to the kidneys and portions of the cardiovascular system. Here we examine the brain regions that cause heightened sympathetic nerve activity in animal models of neurogenic hypertension, and we discuss the triggers responsible for the changes in neuronal activity within these regions. We highlight the limitations of the evidence and, whenever possible, we briefly address the pertinence of the findings to human hypertension. The arterial baroreflex reduces arterial blood pressure variability and contributes to the arterial blood pressure set point. This set point can also be elevated by a newly described cerebral blood flow-dependent and astrocyte-mediated sympathetic reflex. Both reflexes converge on the presympathetic neurons of the rostral medulla oblongata, and both are plausible causes of neurogenic hypertension. Sensory afferent dysfunction (reduced baroreceptor activity, increased renal, or carotid body afferent) contributes to many forms of neurogenic hypertension. Neurogenic hypertension can also result from activation of brain nuclei or sensory afferents by excess circulating hormones (leptin, insulin, Ang II [angiotensin II]) or sodium. Leptin raises blood vessel sympathetic nerve activity by activating the carotid bodies and subsets of arcuate neurons. Ang II works in the lamina terminalis and probably throughout the brain stem and hypothalamus. Sodium is sensed primarily in the lamina terminalis. Regardless of its cause, the excess sympathetic nerve activity is mediated to some extent by activation of presympathetic neurons located in the rostral ventrolateral medulla or the paraventricular nucleus of the hypothalamus. Increased activity of the orexinergic neurons also contributes to hypertension in selected models.

Effects of Adrenergic Agonists and Antagonists on Cardiopulmonary Function During Normobaric Hypoxia in Rat

Pulmonary edema (PE) is an issue widely noted in acute exposure to hypoxia as seen in high altitude climbers, yet the etiology of this is not defined. Previous studies in rats showed that both hypoxia and strong sympathetic activation may induce PE. As acute exposure to hypoxia is accompanied by sympathetic activation, we assume that this may impair pulmonary circulation and contribute to the development of hypoxic PE. The aim of the present study was to investigate the effects of adrenergic agonists and antagonists as models for overstimulation and suppression, respectively, of sympathetic activity on cardiovascular function and formation of PE in hypoxic rats. Norepinephrine or adrenergic blockers were infused to rats exposed to normobaric hypoxia with 10% O₂ over time intervals up to 24 h. Normoxic and hypoxic controls received 0.9% NaCl infusion. We evaluated hemodynamic function and lung histology. A significant decrease of left ventricular systolic function was observed after 6 h of hypoxia. This effect was less pronounced with α-adrenergic blockade but was more severe with combined α-plus β-adrenergic blockade. Norepinephrine delayed the onset of hypoxic left ventricular depression but did not reduce its degree. Significant PE developed after 16 h of hypoxia. It regressed under α- but not with β-adrenergic blockade, and was aggravated by combining hypoxia with norepinephrine. Almost half of the animals exposed to hypoxia over 16–24 h suffered cardiorespiratory arrest during the experiment and presented with signs of acute right ventricular failure. They had significantly elevated serum catecholamine concentrations and significantly stronger PE than the others. Notably, most of them had received norepinephrine or combined adrenergic blockade. Mild changes in serum catecholamine concentrations indicated that hypoxic sympathoadrenergic activation was only weak. Hence, it was not sufficient to prevent left ventricular depression. However, the results show that α-adrenergic mechanisms contribute to the formation of hypoxic PE. Adrenergic blockade but also sympathetic overactivity may induce pulmonary congestion, PE and acute right ventricular failure indicating that a fine balance of sympathetic activation under hypoxic conditions is crucial. This has important implications for climbers to high altitude as well as for patients suffering from hypoxia.

Impaired neonatal cardiorespiratory responses to hypoxia in mice lacking PAC1 or VPAC2 receptors

The stress peptide pituitary adenylate cyclase activating polypeptide (PACAP) and its specific receptor PACAP type 1 receptor (PAC1) have been implicated in sudden infant death syndrome (SIDS). PACAP is also critical to the neonatal cardiorespiratory response to homeostatic stressors identified in SIDS, including hypoxia. However, which of PACAP's three receptors, PAC1, vasoactive intestinal peptide receptor type 1 (VPAC1), and/or vasoactive intestinal peptide receptor type 2 (VPAC2), are involved is unknown. In this study, we hypothesized that PAC1, but not VPAC2, is involved in mediating the cardiorespiratory response to hypoxia during neonatal development. To test this hypothesis, head-out plethysmography and surface ECG electrodes were used to assess the cardiorespiratory variables of unanesthetized postnatal day 4 PAC1 and VPAC2-knockout (KO) and wild-type (WT) mice in response to a 10% hypoxic challenge. Our results demonstrate that compared with WT pups, the early and late hypoxic rate of expired CO₂ (V̇co₂), V̇co₂ and ventilatory responses were blunted in PAC1-KO neonates, and during the posthypoxic period, minute ventilation (V̇e), V̇co₂ and heart rate were increased, while the increase in apneas normally associated with the posthypoxic period was reduced. Consistent with impaired cardiorespiratory control in these animals, the V̇e/V̇co₂ slope was reduced in PAC1-KO pups, suggesting that breathing was inappropriately matched to metabolism. In contrast, VPAC2-KO pups exhibited elevated heart rate variability during hypoxia compared with WT littermates, but the effects of the VPAC2-KO genotype on breathing were minimal. These findings suggest that PAC1 plays the principal role in mediating the cardiorespiratory effects of PACAP in response to hypoxic stress during neonatal development and that defective PACAP signaling via PAC1 may contribute to the pathogenesis of SIDS.

Variable role of carotid bodies in cardiovascular responses to exercise, hypoxia and hypercapnia in spontaneously hypertensive rats

KEY POINTS

Carotid bodies play a critical role in maintaining arterial pressure during hypoxia and this has important implications when considering resection therapy of the carotid body in disease states such as hypertension. Curbing hypertension in patients whether resting or under stress remains a major global health challenge. We demonstrated previously the benefits of removing carotid body afferent input into the brain for both alleviating sympathetic overdrive and reducing blood pressure in neurogenic hypertension. We describe a new approach in rats for selective ablation of the carotid bodies that spares the functional integrity of the carotid sinus baroreceptors, and demonstrate the importance of the carotid bodies in the haemodynamic response to forced exercise, hypoxia and hypercapnia in conditions of hypertension. Selective ablation reduced blood pressure in hypertensive rats and re-set baroreceptor reflex function accordingly; the increases in blood pressure seen during exercise, hypoxia and hypercapnia were unaffected, abolished and augmented, respectively, after selective carotid body removal. The data suggest that carotid body ablation may trigger potential cardiovascular risks particularly during hypoxia and hypercapnia and that suppression rather than obliteration of their activity may be a more effective and safer route to pursue.

ABSTRACT

The carotid body has recently emerged as a promising therapeutic target for treating cardiovascular disease, but the potential impact of carotid body removal on the dynamic cardiovascular responses to acute stressors such as exercise, hypoxia and hypercapnia in hypertension is an important safety consideration that has not been studied. We first validated a novel surgical approach to selectively resect the carotid bodies bilaterally (CBR) sparing the carotid sinus baroreflex. Second, we evaluated the impact of CBR on the cardiovascular responses to exercise, hypoxia and hypercapnia in conscious, chronically instrumented spontaneously hypertensive (SH) rats. The results confirm that our CBR technique successfully and selectively abolished the chemoreflex, whilst preserving carotid baroreflex function. CBR produced a sustained fall in arterial pressure in the SH rat of ∼20 mmHg that persisted across both dark and light phases (P < 0.001), with baroreflex function curves resetting around lower arterial pressure levels. The cardiovascular and respiratory responses to moderate forced exercise were similar between CBR and Sham rats. In contrast, CBR abolished the pressor response to hypoxia seen in Sham animals, although the increases in heart rate and respiration were similar between Sham and CBR groups. Both the pressor and the respiratory responses to 7% hypercapnia were augmented after CBR (P < 0.05) compared to sham. Our finding that the carotid bodies play a critical role in maintaining arterial pressure during hypoxia has important implications when considering resection therapy of the carotid body in disease states such as hypertension as well as heart failure with sleep apnoea.

Organs Blood Flow during Elevation of Body Temperature in Sevoflurane Anesthetized Rats

The aim of this study is to investigate how elevation of body temperature changes organs blood flow during sevoflurane anesthesia. We conducted in vivo research on 14 male Wistar rats to monitor pulse rate and arterial blood pressure and measure hepatic, small intestinal, renal, and descending aortic blood flow using a laser Doppler blood flowmeter. We assessed the changes in organ blood flow, pulse rate, and arterial blood pressure during elevation of the rats' body temperatures up to 41.5°C under anesthesia with 2.0% or 3.0% sevoflurane. We concluded that elevation of body temperature up to 39.5°C does not change hepatic, small intestinal, and renal blood flow during 2.0 and 3.0% sevoflurane anesthesia.

Search for an animal model to investigate selective pulmonary vasodilation

Pulmonary arterial hypertension is a life-threatening disease with a poor prognosis. Oral treatment with vasodilators is often limited by systemic hypotension. Inhalation of vasodilators offers the opportunity for selective pulmonary vasodilation. Testing selective pulmonary vasodilation by inhaled nitric oxide or alternative substances in animal models requires an increased pulmonary vascular tone. The aim of this study was to identify animal models that are suitable for investigating selective pulmonary vasodilation. To do so, a haemodynamic stable pulmonary hypertension was initiated, with a 30 min duration deemed to be a sufficient time interval before and after a possible intervention. In anaesthetized and mechanically-ventilated Sprague-Dawley rats pulmonary hypertension was induced either by acute hypoxia due to reduction of the inspired oxygen fraction from 0.21 to 0.1 ( n = 6), a fixed infusion rate of the thromboxane analogue U46619 (240 ng/min; n = 6) or a monocrotaline injection (MCT; 60 mg/kg applied 23 days before the investigation; n = 7). The animals were instrumented to measure right ventricular and systemic arterial pressures. Acute hypoxia caused a short, and only transient, increase of pulmonary artery pressure as well as profound systemic hypotension which suggested haemodynamic instability. U46619 infusion induced variable changes in the pulmonary and systemic vascular tone without sufficient stabilization within 30 min. MCT provoked sustained pulmonary hypertension with normal systemic pressure values and inhalation of nitric oxide caused selective pulmonary vasodilation. In conclusion, out of the three examined rat animal models only MCT-induced pulmonary hypertension is a solid and reliable model for investigating selective pulmonary vasodilation.

Role of muscular eNOS in skeletal arteries: Endothelium-independent hypoxic vasoconstriction of the femoral artery is impaired in eNOS-deficient mice

We previously reported that hypoxia augments α-adrenergic contraction (hypoxic vasoconstriction, HVC) of skeletal arteries in rats. The underlying mechanism may involve hypoxic inhibition of endothelial nitric oxide synthase (eNOS) expressed in skeletal arterial myocytes (16). To further explore the novel role of muscular eNOS in the skeletal artery, we compared HVC in femoral arteries (FAs) from eNOS knockout (KO) mice with that from wild-type (WT) and heterozygous (HZ) mice. Immunohistochemical assays revealed that, in addition to endothelia, eNOS is also expressed in the medial layer of FAs, albeit at a much lower level. However, the medial eNOS signal was not evident in HZ FAs, despite strong expression in the endothelium; similar observations were made in WT carotid arteries (CAs). The amplitude of contraction induced by 1 μM phenylephrine (PhE) was greater in HZ than in WT FAs. Hypoxia (3% Po2) significantly augmented PhE-induced contraction in WT FAs but not in HZ or KO FAs. No HVC was observed in PhE-pretreated WT CAs. The NOS inhibitor nitro-l-arginine methyl ester (0.1 mM) also augmented PhE contraction in endothelium-denuded WT FAs but not in WT CAs. Inhibitors specific to neuronal NOS and inducible NOS did not augment PhE-induced contraction of WT FAs. NADPH oxidase 4 (NOX4) inhibitor (GKT137831, 5 μM), but not NOX2 inhibitor (apocynin, 100 μM), suppressed HVC. Consistent with the role of reactive oxygen species (ROS), HVC was also inhibited by pretreatment with tiron or polyethylene glycol-catalase. Taken together, these data suggest that the eNOS expressed in smooth muscle cells in FAs attenuates α-adrenergic vasoconstriction; this suppression is alleviated under hypoxia, which potentiates vasoconstriction in a NOX4/ROS-dependent mechanism.

Catecholaminergic neurons projecting to the paraventricular nucleus of the hypothalamus are essential for cardiorespiratory adjustments to hypoxia

Brainstem catecholamine neurons modulate sensory information and participate in control of cardiorespiratory function. These neurons have multiple projections, including to the paraventricular nucleus (PVN), which contributes to cardiorespiratory and neuroendocrine responses to hypoxia. We have shown that PVN-projecting catecholaminergic neurons are activated by hypoxia, but the function of these neurons is not known. To test the hypothesis that PVN-projecting catecholamine neurons participate in responses to respiratory challenges, we injected IgG saporin (control; n = 6) or anti-dopamine β-hydroxylase saporin (DSAP; n = 6) into the PVN to retrogradely lesion catecholamine neurons projecting to the PVN. After 2 wk, respiratory measurements (plethysmography) were made in awake rats during normoxia, increasing intensities of hypoxia (12, 10, and 8% O2) and hypercapnia (5% CO2-95% O2). DSAP decreased the number of tyrosine hydroxylase-immunoreactive terminals in PVN and cells counted in ventrolateral medulla (VLM; -37%) and nucleus tractus solitarii (nTS; -36%). DSAP produced a small but significant decrease in respiratory rate at baseline (during normoxia) and at all intensities of hypoxia. Tidal volume and minute ventilation (VE) index also were impaired at higher hypoxic intensities (10-8% O2; e.g., VE at 8% O2: IgG = 181 ± 22, DSAP = 91 ± 4 arbitrary units). Depressed ventilation in DSAP rats was associated with significantly lower arterial O2 saturation at all hypoxic intensities. PVN DSAP also reduced ventilatory responses to 5% CO2 (VE: IgG = 176 ± 21 and DSAP = 84 ± 5 arbitrary units). Data indicate that catecholamine neurons projecting to the PVN are important for peripheral and central chemoreflex respiratory responses and for maintenance of arterial oxygen levels during hypoxic stimuli.

Diastolic dysfunction precedes hypoxia-induced mortality in dystrophic mice

Duchenne muscular dystrophy (DMD) is a progressive striated muscle disease that is characterized by skeletal muscle weakness with progressive respiratory and cardiac failure. Together respiratory and cardiac disease account for the majority of mortality in the DMD patient population. However, little is known regarding the effects of respiratory dysfunction on the dystrophic heart. The studies described here examine the effects of acute hypoxia on cardiac function. These studies demonstrate, for the first time, that a mouse model of DMD displays significant mortality following acute exposure to hypoxia. This mortality is characterized by a steady decline in systolic function. Retrospective analysis reveals that significant decreases in diastolic dysfunction, especially in the right ventricle, precede the decline in systolic pressure. The initial hemodynamic response to acute hypoxia in the mouse is similar to that observed in larger species, with significant increases in right ventricular afterload and decreases in left ventricular preload being observed. Significant increases in heart rate and contractility suggest hypoxia-induced activation of the sympathetic nervous system. These studies provide evidence that while hypoxia presents significant hemodynamic challenges to the dystrophic right ventricle, global cardiac dysfunction precedes hypoxia-induced mortality in the dystrophic heart. These findings are clinically relevant as the respiratory insufficiency evident in patients with DMD results in significant bouts of hypoxia. The results of these studies indicate that hypoxia may contribute to the acceleration of the heart disease in DMD patients. Importantly, hypoxia can be avoided through the use of ventilatory support.

Microvascular dilation evoked by chemical stimulation of C-fibers in rats

Activation of pulmonary C-fibers can reflexively decrease heart rate, blood pressure, and peripheral vascular resistance. However, the effects of these afferents on microvascular tone remain incompletely understood. In this study, we examined the effects of these afferents on microvascular tone in a striated muscle vascular bed. The right cremaster muscle in pentobarbital-anesthetized rats with intact circulation and innervation was suspended in a tissue bath, and diameters of small arterioles were measured by intravital video microscopy. Stimulation of pulmonary C-fibers by injecting capsaicin (5 μg/kg) or phenylbiguanide (20 μg/kg) into the right atrium dilated small arterioles and decreased blood pressure and heart rate. The effects persisted when the cervical vagus nerves were cooled to 5 to 7°C (blocking myelinated fibers), but were prevented by cooling to 0°C (blocking C-fibers and myelinated fibers), by cutting the genital femoral nerve (GFN) supplying the cremaster to block the nerve supply to the muscle, or by adding 6-hydroxydopamine to the bathing medium to selectively block sympathetic effects by depleting norepinephrine from adrenergic nerve terminals. Our results show that stimulation of pulmonary C-fibers reflexively dilates small arterioles in striated muscle by a mechanism that could involve withdrawal of sympathetic adrenergic tone. In conclusion, pulmonary C-fibers can exert an inhibitory influence on neural tone of the microcirculation at an important site where microvascular resistance and tissue blood flow are regulated.

Dyspnea, tachycardia, and new onset seizure as a presentation of wilms tumor: a case report

Wilms tumor is found in 1 in 10,000 children and most commonly presents in asymptomatic toddlers whose care givers notice a nontender abdominal mass in the right upper quadrant. This case of Wilms tumor presented as a critically ill eleven-year old with significant tachypnea, dyspnea, vague abdominal pain, intermittent emesis, new onset seizure, metabolic acidosis, and hypoxemia. This is the first case in the literature of Wilms Tumor with cavoatrial involvement and seizure and pulmonary embolism resulting in aggressive resuscitation and treatment. Treatment included anticoagulation, chemotherapy, nephrectomy, and surgical resection of thrombi, followed by adjunctive chemotherapy with pulmonary radiation.

Consequences of peripheral chemoreflex inhibition with low-dose dopamine in humans

Low-dose dopamine inhibits peripheral chemoreceptors and attenuates the hypoxic ventilatory response (HVR) in humans. However, it is unknown: (1) whether it also modulates the haemodynamic reactions to acute hypoxia, (2) whether it also modulates cardiac baroreflex sensitivity (BRS) and (3) if there is any effect of dopamine withdrawal. We performed a double-blind, placebo-controlled study on 11 healthy male volunteers. At sea level over 2 days every subject was administered low-dose dopamine (2 μg kg(-1) min(-1)) or saline infusion, during which we assessed both ventilatory and haemodynamic responses to acute hypoxia. Separately, we evaluated effects of initiation and withdrawal of each infusion and BRS. The initiation of dopamine infusion did not affect minute ventilation (MV) or mean blood pressure (MAP), but increased both heart rate (HR) and cardiac output. Concomitantly, it decreased systemic vascular resistance. Dopamine blunted the ventilatory, MAP and HR reactions (hypertension, tachycardia) to acute hypoxia. Dopamine attenuated cardiac BRS to falling blood pressure. Dopamine withdrawal evoked an increase in MV. The magnitude of the increment in MV due to dopamine withdrawal correlated with the size of the HVR and depended on the duration of dopamine administration. The ventilatory reaction to dopamine withdrawal constitutes a novel index of peripheral chemoreceptor function.

Dissociation between blood pressure and heart rate response to hypoxia after bilateral carotid body removal in men with systolic heart failure

While the ventilatory response to hypoxia is known to be mediated by the carotid bodies, the origin of the haemodynamic alterations evoked by hypoxia is less certain. Bilateral carotid body removal (CBR) performed to treat congestive heart failure may serve as a model to improve our understanding of haemodynamic responses to hypoxia in humans. We studied six congestive heart failure patients before and 1 month after CBR [median (interquartile range): age, 58.5 (56-61) years old; and ejection fraction, 32 (25-34)%]. Peripheral chemosensitivity (hypoxic ventilatory response) was equated to the slope relating lowest oxygen saturation to highest minute ventilation following exposures to hypoxia. Likewise, systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR) slopes were calculated as slopes relating the lowest oxygen saturations to the highest SBP, DBP and HR responses. We found that CBR reduces the hypoxic ventilatory response (91%, P < 0.05), SBP (71%, P < 0.05) and DBP slopes (59%, P = 0.07). In contrast, the HR slope remained unchanged. The dissociation between the blood pressure and HR responses after CBR shows involvement of a different chemoreceptive site(s) maintaining the response to acute hypoxia. We conclude that carotid bodies are responsible for ventilatory and blood pressure responses, while the HR response might be mediated by the aortic bodies. The significant reduction of the blood pressure response to hypoxia after CBR suggests a decrease in sympathetic tone, which is of particular clinical relevance in congestive heart failure.

The impact of acute and chronic catecholamines on respiratory responses to hypoxic stress in the rat

Chronic catecholamine production is associated with desensitisation and down-regulation of adrenergic receptors and occurs in conditions, such as heart failure and myocardial infarction. The effects of further acute adrenergic stimulation, which may occur during exercise, and their subsequent effects on chemosensitivity and ventilation are unclear. Chronic isoprenaline (ISO) increased ventilation by 50 % (P < 0.05) yet the sensitivity to graded hypoxia was preserved. Acute noradrenaline (NA) in control animals led to a doubling of ventilation in hyperoxia (P < 0.001), and this difference was preserved in graded hypoxia (P < 0.001). Yet, combination of NA + ISO did not increase ventilation beyond ISO at baseline or in hypoxia. ISO, NA, and NA + ISO all induced a metabolic acidosis (P < 0.05) with enhanced ventilation in partial compensation. Carotid sinus nerve (CSN) section led to a partial loss of catecholamine-induced augmentation in ventilation (P < 0.05), yet direct recording from CSN in vitro suggests catecholamine is inhibitory for CSN discharge. These observations suggest that chronic catecholamine exposure may result in decreased exercise performance as a direct consequence of the hyperpnea to compensate for an increased metabolic rate coupled with acidosis and leading to increased central chemosensitivity. A limited contribution from peripheral chemoreceptors was noted but was not a consequence of catecholamine stimulation of the carotid body.

Hemodynamic and ventilatory response to different levels of hypoxia and hypercapnia in carotid body-denervated rats

OBJECTIVE

Chemoreceptors play an important role in the autonomic modulation of circulatory and ventilatory responses to changes in arterial O(2) and/or CO(2). However, studies evaluating hemodynamic responses to hypoxia and hypercapnia in rats have shown inconsistent results. Our aim was to evaluate hemodynamic and respiratory responses to different levels of hypoxia and hypercapnia in conscious intact or carotid body-denervated rats.

METHODS

Male Wistar rats were submitted to bilateral ligature of carotid body arteries (or sham-operation) and received catheters into the left femoral artery and vein. After two days, each animal was placed into a plethysmographic chamber and, after baseline measurements of respiratory parameters and arterial pressure, each animal was subjected to three levels of hypoxia (15, 10 and 6% O(2)) and hypercapnia (10% CO(2)).

RESULTS

The results indicated that 15% O(2) decreased the mean arterial pressure and increased the heart rate (HR) in both intact (n = 8) and carotid body-denervated (n = 7) rats. In contrast, 10% O(2) did not change the mean arterial pressure but still increased the HR in intact rats, and it decreased the mean arterial pressure and increased the heart rate in carotid body-denervated rats. Furthermore, 6% O(2) increased the mean arterial pressure and decreased the HR in intact rats, but it decreased the mean arterial pressure and did not change the HR in carotid body-denervated rats. The 3 levels of hypoxia increased pulmonary ventilation in both groups, with attenuated responses in carotid body-denervated rats. Hypercapnia with 10% CO(2) increased the mean arterial pressure and decreased HR similarly in both groups. Hypercapnia also increased pulmonary ventilation in both groups to the same extent.

CONCLUSION

This study demonstrates that the hemodynamic and ventilatory responses varied according to the level of hypoxia. Nevertheless, the hemodynamic and ventilatory responses to hypercapnia did not depend on the activation of the peripheral carotid chemoreceptors.

Cross talk between S-nitrosylation and S-glutathionylation in control of the Na,K-ATPase regulation in hypoxic heart

Oxygen-induced regulation of Na,K-ATPase was studied in rat myocardium. In rat heart, Na,K-ATPase responded to hypoxia with a dose-dependent inhibition in hydrolytic activity. Inhibition of Na,K-ATPase in hypoxic rat heart was associated with decrease in nitric oxide (NO) production and progressive oxidative stress. Accumulation of oxidized glutathione (GSSG) and decrease in NO availability in hypoxic rat heart were followed by a decrease in S-nitrosylation and upregulation of S-glutathionylation of the catalytic α-subunit of the Na,K-ATPase. Induction of S-glutathionylation of the α-subunit by treatment of tissue homogenate with GSSG resulted in complete inhibition of the enzyme in rat a myocardial tissue homogenate. Inhibitory effect of GSSG in rat sarcolemma could be significantly decreased upon activation of NO synthases. We have further tested whether oxidative stress and suppression of the Na,K-ATPase activity are observed in hypoxic heart of two subterranean hypoxia-tolerant blind mole species (Spalax galili and Spalax judaei). In both hypoxia-tolerant Spalax species activity of the enzyme and tissue redox state were maintained under hypoxic conditions. However, localization of cysteines within the catalytic subunit of the Na,K-ATPase was preserved and induction of S-glutathionylation by GSSG in tissue homogenate inhibited the Spalax ATPase as efficiently as in rat heart. The obtained data indicate that oxygen-induced regulation of the Na,K-ATPase in the heart is mediated by a switch between S-glutathionylation and S-nitrosylation of the regulatory thiol groups localized at the catalytic subunit of the enzyme.

Hypoxic preconditioning in an autohypoxic animal model

Hypoxic preconditioning refers to the exposure of organisms, systems, organs, tissues or cells to moderate hypoxia/ischemia that results in increased resistance to a subsequent episode of severe hypoxia/ischemia. In this article, we review recent research based on a mouse model of repeated exposure to autohypoxia. Pre-exposure markedly increases the tolerance to or protection against hypoxic insult, and preserves the cellular structure of the brain. Furthermore, the hippocampal activity amplitude and frequency of electroencephalogram, latency of cortical somatosensory-evoked potential and spinal somatosensory-evoked potential progressively decrease, while spatial learning and memory improve. In the brain, detrimental neurochemicals such as free radicals are down-regulated, while beneficial ones such as adenosine are up-regulated. Also, antihypoxia factor(s) and gene(s) are activated. We propose that the tolerance and protective effects depend on energy conservation and plasticity triggered by exposure to hypoxia via oxygen-sensing transduction pathways and hypoxia-inducible factor-initiated cascades. A potential path for further research is the development of devices and pharmaceuticals acting on antihypoxia factor(s) and gene(s) for the prevention and treatment of hypoxia and related syndromes.

Hypoxia activates nucleus tractus solitarii neurons projecting to the paraventricular nucleus of the hypothalamus

Peripheral chemoreceptor afferent information is sent to the nucleus tractus solitarii (nTS), integrated, and relayed to other brain regions to alter cardiorespiratory function. The nTS projects to the hypothalamic paraventricular nucleus (PVN), but activation and phenotype of these projections during chemoreflex stimulation is unknown. We hypothesized that activation of PVN-projecting nTS neurons occurs primarily at high intensities of hypoxia. We assessed ventilation and cardiovascular parameters in response to increasing severities of hypoxia. Retrograde tracers were used to label nTS PVN-projecting neurons and, in some rats, rostral ventrolateral medulla (RVLM)-projecting neurons. Immunohistochemistry was performed to identify nTS cells that were activated (Fos-immunoreactive, Fos-IR), catecholaminergic, and GABAergic following hypoxia. Conscious rats underwent 3 h normoxia (n = 4, 21% O(2)) or acute hypoxia (12, 10, or 8% O(2); n = 5 each). Hypoxia increased ventilation and the number of Fos-IR nTS cells (21%, 13 ± 2; 12%, 58 ± 4; 10%, 166 ± 22; 8%, 186 ± 6). Fos expression after 10% O(2) was similar whether arterial pressure was allowed to decrease (-13 ± 1 mmHg) or was held constant. The percentage of PVN-projecting cells activated was intensity dependent, but contrary to our hypothesis, PVN-projecting nTS cells exhibiting Fos-IR were found at all hypoxic intensities. Notably, at all intensities of hypoxia, ∼75% of the activated PVN-projecting nTS neurons were catecholaminergic. Compared with RVLM-projecting cells, a greater percentage of PVN-projecting nTS cells was activated by 10% O(2). Data suggest that increasing hypoxic intensity activates nTS PVN-projecting cells, especially catecholaminergic, PVN-projecting neurons. The nTS to PVN catecholaminergic pathway may be critical even at lower levels of chemoreflex activation and more important to cardiorespiratory responses than previously considered.

The effect of altitude-induced hypoxia on heart disease: do acute, intermittent, and chronic exposures provide cardioprotection?

With the global prevalence of heart disease continuing to increase and large populations living at altitude around the world, we review the concept of altitude and cardioprotection. Current epidemiologic data, as well as the basic science and molecular mechanisms involved in acute, intermittent, and chronic exposure to altitude, are discussed. Intermittent and chronic exposures have been demonstrated to increase coronary vasculature, decrease infarction size, and provide more efficient metabolism and better cardiac functional recovery postischemia. Mechanisms demonstrated in these situations include those mediated by the hypoxia inducible factor, as well as reactive oxygen species, certain ion channels, and protein kinases. Although current epidemiologic studies are difficult to interpret owing to many confounders, many studies point to the possibility that living at altitude provides cardiovascular protection. Further research is needed to determine if the bench studies showing mechanisms consistent with cardioprotection translate to the population living at altitude.

Changes in muscle sympathetic nerve activity and vascular responses evoked in the spinotrapezius muscle of the rat by systemic hypoxia

Responses evoked in muscle sympathetic nerve activity (MSNA) by systemic hypoxia have received relatively little attention. Moreover, MSNA is generally identified from firing characteristics in fibres supplying whole limbs: their actual destination is not determined. We aimed to address these limitations by using a novel preparation of spinotrapezius muscle in anaesthetised rats. By using focal recording electrodes, multi-unit and discriminated single unit activity were recorded from the surface of arterial vessels.This had cardiac- and respiratory-related activities expected of MSNA, and was increased by baroreceptor unloading, decreased by baroreceptor stimulation and abolished by autonomic ganglion blockade. Progressive, graded hypoxia (breathing sequentially 12, 10, 8% O2 for 2min each) evoked graded increases in MSNA.In single units, mean firing frequency increased from 0.2±0.04 in 21% O2 to 0.62 ± 0.14 Hz in8% O2, while instantaneous frequencies ranged from 0.04–6Hz in 21% O2 to 0.09–20 Hz in 8%O2. Concomitantly, arterial pressure (ABP), fell and heart rate (HR) and respiratory frequency(RF) increased progressively, while spinotrapezius vascular resistance (SVR) decreased (Spinotrapezius blood flow/ABP), indicating muscle vasodilatation. During 8% O2 for 10 min, the falls in ABP and SVR were maintained, but RF, HR and MSNA waned towards baselines from the second to the tenth minute. Thus, we directly show that MSNA increases during systemic hypoxia to an extent that is mainly determined by the increases in peripheral chemoreceptor stimulation and respiratory drive, but its vasoconstrictor effects on muscle vasculature are largely blunted by local dilator influences, despite high instantaneous frequencies in single fibres.

Effects of carvedilol on oxygen uptake and heart rate kinetics in patients with chronic heart failure at simulated altitude

Background: The response to moderate exercise at altitude in heart failure (HF) is unknown.

Methods and results: We evaluated 30 HF patients, (NYHA I-III, 25 M/5 F; 59 ± 10 years; LVEF = 39.6 ± 7.1%), in stable clinical conditions, treated with carvedilol at the maximal tolerated dose. We performed a maximal cardiopulmonary exercise test (CPET) with ramp protocol at sea level to evaluate patients’ performance and two moderate intensity constant workload CPETs (50% of peak workload) at sea level (normoxia) and simulated altitude (hypoxia). Oxygen uptake ([Formula: see text]) and heart rate (HR) on-kinetics at constant workload were assessed calculating the time constant (τ) with a monoexponential equation. [Formula: see text] and HR were higher in hypoxia (0.944 ± 0.233 vs 1.031 ± 0.264 l/min; 100 ± 23 vs 108 ± 22 bpm; p < 0.001). On-kinetics showed a different behavior of τ being [Formula: see text] faster in hypoxia (67.1 ± 23.0 vs. 56.3 ± 19.7 s; p = 0.026) and HR faster in normoxia (49.3 ± 19.4 vs. 62.2 ± 22.5 s; p = 0.018). Ten patients, who lowered oxygen kinetics in hypoxia, had greater HR increase during maximal CPET suggesting lower functional betablockade. The higher τ of [Formula: see text] in hypoxia is likely to be due to a peripheral effect of carvedilol mediated either by β- or α-receptor.

Conclusion: HF patients performing moderate exercise at 2000 m simulated altitude have 20% [Formula: see text] increase without trouble at the beginning of exercise when treated with carvedilol.

Influence of acute hypoxia combined with nitrous oxide on cardiovascular variability in conscious hypertensive rats

Anesthetics have been reported to depress autonomic nervous system (ANS) responses to hypoxia. The mechanisms by which cardiovascular variability responds to acute progressive hypoxia (APH) under nitrous oxide (N(2)O) inhalation, however, remain unclear. Additionally, the effect of hypertension on ANS responses in such cases has not been fully clarified. The present study examined the influence of APH (10% O(2)) under 60% N(2)O inhalation on cardiovascular variability in conscious, spontaneously hypertensive rats (SHR). Twenty-seven male SHR were randomly assigned to 3 treatment groups receiving N(2)O inhalation alone, APH stress alone or APH stress under N(2)O inhalation, using Wistar Kyoto rats (WKY) or non-N(2)O inhalation rats as controls. Systolic blood pressure (SBP) and heart rate (HR) variability were evaluated time-dependently using the wavelet method. While inhalation of N(2)O alone induced more powerful sympathomimetic actions in SHR than in WKY, circulatory and parasympathetic reactions were weaker. APH stress alone evoked significant inhibition of cardiac parasympathetic activity from immediately after exposure to hypoxic stress in SHR in contrast to WKY, facilitating tachycardia. This inhibition of parasympathetic activity in SHR continued without coupled changes in sympathetic activity. In SHR, APH under N(2)O inhalation decreased SBP and sympathetic activity more prominently and earlier than APH alone, and earlier than APH under N(2)O inhalation in WKY. Additionally, APH under N(2)O inhalation inhibited cardiac parasympathetic activity in SHR as compared to APH stress alone. In conclusion, APH under N(2)O inhalation in SHR potentially results in exacerbation of circulatory suppression from the earlier hypoxic phase, compared with non-N(2)O inhalation.

Permanent occlusion of feeding arteries and draining veins in solid mouse tumors by vascular targeted photodynamic therapy (VTP) with Tookad

Background

Antiangiogenic and anti-vascular therapies present intriguing alternatives to cancer therapy. However, despite promising preclinical results and significant delays in tumor progression, none have demonstrated long-term curative features to date. Here, we show that a single treatment session of Tookad-based vascular targeted photodynamic therapy (VTP) promotes permanent arrest of tumor blood supply by rapid occlusion of the tumor feeding arteries (FA) and draining veins (DV), leading to tumor necrosis and eradication within 24–48 h.

Methodology/Principal Findings

A mouse earlobe MADB106 tumor model was subjected to Tookad-VTP and monitored by three complementary, non-invasive online imaging techniques: Fluorescent intravital microscopy, Dynamic Light Scattering Imaging and photosensitized MRI. Tookad-VTP led to prompt tumor FA vasodilatation (a mean volume increase of 70%) with a transient increase (60%) in blood-flow rate. Rapid vasoconstriction, simultaneous blood clotting, vessel permeabilization and a sharp decline in the flow rates then followed, culminating in FA occlusion at 63.2 sec±1.5SEM. This blockage was deemed irreversible after 10 minutes of VTP treatment. A decrease in DV blood flow was demonstrated, with a slight lag from FA response, accompanied by frequent changes in flow direction before reaching a complete standstill. In contrast, neighboring, healthy tissue vessels of similar sizes remained intact and functional after Tookad-VTP.

Conclusion/Significance

Tookad-VTP selectively targets the tumor feeding and draining vessels. To the best of our knowledge, this is the first mono-therapeutic modality that primarily aims at the larger tumor vessels and leads to high cure rates, both in the preclinical and clinical arenas.

Sympathetic restraint of muscle blood flow during hypoxic exercise

Control of exercising muscle blood flow is a balance between local vasodilatory factors and the increase in global sympathetic vasoconstrictor outflow. Hypoxia has been shown to potentiate the muscle sympathetic nerve response to exercise, potentially limiting the increase in muscle blood flow. Accordingly, we investigated sympathetic restraint to exercising muscle during whole body exercise in hypoxia. Six dogs chronically instrumented with ascending aortic and hindlimb flow probes and a terminal aortic catheter were studied at rest and mild [2.5 miles/h (mph), 5% grade] and moderate (4.0 mph, 10% grade) exercise while breathing room air or hypoxia (Pa(O(2)) approximately 45 mmHg) in the intact control condition and following systemic alpha-adrenergic blockade (phentolamine). Hypoxia caused an increase in cardiac output (CO), hindlimb flow (Flow(L)), and blood pressure (BP), while total (Cond(T)) and hindlimb conductance (Cond(L)) were unchanged at rest and mild exercise but increased with moderate exercise. During both mild and moderate exercise, alpha-blockade in normoxia resulted in significant vasodilation as evidenced by increases in CO (10%), Flow(L) (17%), Cond(T) (33%), Cond(L) (43%), and a decrease in BP (-18%), with the increase in Cond(L) greater than the increase in Cond(T) during mild exercise. Compared with the normoxic response, alpha-blockade in hypoxia during exercise resulted in a significantly greater increase in Cond(T) (59%) and Cond(L) (74%) and a correspondingly greater decrease in BP (-34%) from baseline. These findings indicate that there is considerable hypoxia-induced sympathetic restraint of muscle blood flow during both mild and moderate exercise, which helps to maintain arterial blood pressure in hypoxia.

Time-dependent adaptation in the hemodynamic response to hypoxia

In rats, acute exposure to hypoxia causes a decrease in mean arterial pressure (MAP) caused by a predominance of hypoxic vasodilation over chemoreflex-induced vasoconstriction. We previously demonstrated that exposure to chronic intermittent hypoxia (CIH) impairs hypoxic vasodilation in isolated resistance arteries; therefore, we hypothesized that the acute systemic hemodynamic responses to hypoxia would be altered by exposure to CIH. To test this hypothesis, rats were exposed to CIH for 14 days. Heart rate (HR) and MAP were monitored by telemetry. On the first day of CIH exposure, acute episodes of hypoxia caused a decrease in MAP (-9+/-5 mmHg) and an increase in HR (+45+/-4 beats/min). On the 14th day of CIH exposure the depressor response was attenuated (-4+/-1mmHg; 44% of the day 1 response) and the tachycardia was enhanced (+68+/-2 beats/min; 151% of the day 1 response). The observed time-dependent modulation of the acute hemodynamic responses to hypoxia may reflect important changes in neurocirculatory regulation that contribute to CIH-induced hypertension.

Influence of acute progressive hypoxia on cardiovascular variability in conscious spontaneously hypertensive rats

The purpose of this study is to examine the influence of acute progressive hypoxia on cardiovascular variability and striatal dopamine (DA) levels in conscious, spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). After preparation for measurement, the inspired oxygen concentration of rats was decreased to 10% within 5 min (descent stage), maintained at 10% for 10 min (fixed stage), and then elevated back to 20% over 5 min (recovery stage). The systolic blood pressure (SBP) and heart rate (HR) variability at each stage was calculated to evaluate the autonomic nervous system response using the wavelet method. Striatal DA during each stage was measured using in vivo microdialysis. We found that SHR showed a more profound hemodynamic response to progressive hypoxia as compared to WKY. Cardiac parasympathetic activity in SHR was significantly inhibited by acute progressive hypoxia during all stages, as shown by the decrease in the high frequency band of HR variability (HR-HF), along with transient increase in sympathetic activity during the early hypoxic phase. This decrease in the HR-HF continued even when SBP was elevated. Striatal DA levels showed the transient similar elevation in both groups. These findings suggest that acute progressive hypoxic stress in SHR inhibits cardiac parasympathetic activity through reduction of baroreceptor reflex sensitivity, with potentially severe deleterious effects on circulation, in particular on HR and circulatory control. Furthermore, it is thought that the influence of acute progressive hypoxia on striatal DA levels is similar in SHR and WKY.

Time course of hypoxia-induced lung injury in rats

We investigated the effects of normobaric hypoxia on rat lungs and hypothesized that the hypoxic exposure would induce lung injury with pulmonary edema and inflammation ensued by development of fibrosis. Rats were exposed to 10% O(2) in nitrogen over 6-168h. We analyzed cardiovascular function and pulmonary changes, lung histology and mRNA expression of extracellular matrix (ECM) molecules in the lung. Significant hemodynamic changes occurred after 168h of hypoxic exposure. Moderate pulmonary edema appeared after 8h and peaked after 16h of hypoxia. It was accompanied by inflammation, fibrosis and vascular hypertrophy. mRNA expression of transforming growth factor-beta2 and -beta3 was up-regulated in lung tissue after 8h of hypoxia. After 8-16h, mRNA expression of collagen types I and III and of other ECM molecules was significantly elevated and increased further with longer exposure to hypoxia. The time course of hypoxia-induced pulmonary injury resembled that previously observed after continuous norepinephrine infusion in rats.

Contribution of alpha2-adrenoceptors and Y1 neuropeptide Y receptors to the blunting of sympathetic vasoconstriction induced by systemic hypoxia in the rat

There is evidence that sympathetically evoked vasoconstriction in skeletal muscle is blunted in systemic hypoxia, but the mechanisms underlying this phenomenon are not clear. In Saffan-anaesthetized Wistar rats, we have studied the role of α₂-adrenoceptors and neuropeptide Y (NPY) Y₁ receptors in mediating vasoconstriction evoked by direct stimulation of the lumbar sympathetic chain by different patterns of impulses in normoxia (N) and systemic hypoxia (H: breathing 8% O₂). Patterns comprised 120 impulses delivered in bursts over a 1 min period at 40 or 20 Hz, or continuously at 2 Hz. Hypoxia attenuated the evoked increases in femoral vascular resistance (FVR) by all patterns, the response to 2 Hz being most affected (40 Hz bursts: N = 3.25 ± 0.75 arbitrary resistance units (RU); H = 1.14 ± 0.29 RU). Yohimbine (Yoh, α₂-adrenoceptor antagonist) or BIBP 3226 (Y₁-receptor antagonist) did not affect baseline FVR. In normoxia, Yoh attenuated the responses evoked by high frequency bursts and 2 Hz, whereas BIBP 3226 only attenuated the response to 40 Hz (40 Hz bursts: N + Yoh = 2.1 ± 0.59 RU; N + BIBP 3226 = 1.9 ± 0.4 RU). In hypoxia, Yoh did not further attenuate the evoked responses, but BIBP 3226 further attenuated the response to 40 Hz bursts. These results indicate that neither α₂-adrenoceptors nor Y₁ receptors contribute to basal vascular tone in skeletal muscle, but both contribute to constrictor responses evoked by high frequency bursts of sympathetic activity. We propose that in systemic hypoxia, the α₂-mediated component represents about 50% of the sympathetically evoked constriction that is blunted, whereas the contribution made by Y₁ receptors is resistant. Thus we suggest the importance of NPY in the regulation of FVR and blood pressure increases during challenges such as systemic hypoxia.

α2-Adrenoreceptor mediated sympathoinhibition of heart rate during acute hypoxia is diminished in conscious prostacyclin synthase deficient mice

Acute hypoxia increases ventilatory drive in conscious animals, resulting in tachycardia. Sustained hypoxia changes the initial chemoreflex ventilatory increase to secondary ventilatory depression, which then evokes a gradual secondary heart rate (HR) reduction. Prostacyclin (PGI(2)) release is known to potentiate alpha(2)-adrenoreceptor (alpha(2)-AR) mediated inhibition of sympathoactivation during ischaemia and hypoxia. We examined whether alpha(2)-AR mediated sympathoinhibition was responsible for limiting hypoxic heart rate increases during initial sympathoactivation, and subsequent secondary HR depression, and if PGI(2) is required for sympathoinhibition of HR. The responses of unrestrained PGI(2) synthase deficient (PGID) and wild type (WT) mice to acute hypoxia (10% O(2) for 30 min) were investigated by simultaneous telemetry, whole body plethysmography and open-flow respirometry. PGID mice exhibited potentiated .V(E) (p < 0.007) after intraperitoneal vehicle injection (n = 8), but not so HR responses compared to WT mice during sustained hypoxia. Idazoxan (alpha(2)-AR antagonist, i.p. bolus 3 mg/kg) pretreatment did not change hypoxic ventilatory response in either group, but significantly elevated hypoxic HR in WT mice only (p < 0.013). Sodium meclofenamate (cyclooxygenase inhibition, i.p. bolus 25 mg/kg) pretreatment eliminated the potentiated .V(E) of PGID and caused significant basal hypotension that led to a transient hypertensive response to hypoxia. From these results, we suggest that alpha(2)-AR activation is required for coupling HR to central inspiratory drive during acute hypoxia, and that PGI(2) is required to enhance the inhibition of sympathoactivation.

Decreased supply-dependent oxygen consumption in the skeletal muscle of the spontaneously hypertensive rat during acute hypoxia

The purpose of this study was to correlate microvascular oxygen delivery (DO2) and consumption (VO2) in the skeletal muscle of spontaneously hypertensive rats (SHRs) and Wistar Kyoto rats (WKY) with hemodynamics during acute hypoxia. We expected greater abnormalities in central and microvascular hemodynamics during hypoxic induced shock in the SHR compared with the WKY due to microvascular rarefaction. The inspired oxygen fraction (FiO2) was lowered from 0.21 to 0.15, 0.1, 0.08, and 0.05 in anesthetized, mechanically ventilated rats. Lactate and base deficit values were similar for both groups at 0.21 and 0.15 FiO2, but higher in SHR at lower FiO2. Baseline aortic blood flow (SHR, 56.2+/-4.0 mL min; WKY, 61.8+/-5.3 mL min) and systemic DO2 (SHR, 9.02+/-0.82 mL min; WKY, 9.32+/-0.54 mL min) increased similarly when FiO2 was lowered to 0.15. Further reductions in FiO2 caused lower aortic flow and systemic DO2 in the SHR than WKY at 0.08 and 0.05 FiO2. Spinotrapezius blood flow increased from baseline (SHR, 24.8+/-1.8 nL s; WKY, 22.7+/-2.1 nL s) in both groups when FiO2 was reduced to 0.15; further reductions in FiO2 decreased blood flow in both groups, with lower values in the SHR group at 0.1 and 0.08 FiO2. The SHR group demonstrated higher venous oxygen saturation at low values of FiO2 compared with WKY. This reduced oxygen extraction in SHR resulted in a lower supply-dependent VO2 at low values of spinotrapezius DO2, perhaps attributed to arteriolar thickening and rarefaction seen in chronic hypertension.

Hypoxic preconditioning: a novel intrinsic cytoprotective strategy

A concept of tissue-cell adaptation to hypoxia (hypoxic preconditioning) is raised and its corresponding animal model is introduced. A significantly strengthened tolerance to hypoxia and a protective effect of the brain extracts from the preconditioned animals are presented. Changes in animals' behavior, neuromorphology, neurophysiology, neurochemistry and molecular neurobiology during preconditioning are described. Energy saving, hypometabolism, and cerebral protection in particular are thought to be involved in the development of hypoxic tolerance and tissue-cell protection. The essence and significance of the hypoxic tissue-cell adaptation or preconditioning are discussed in terms of biological evolution and practical implication.

Respiratory and body temperature modulation by adenosine A1 receptors in the anteroventral preoptic region during normoxia and hypoxia

The present study was undertaken to investigate adenosine as a simultaneous mediator of hypoxia-induced hyperventilation and regulated hypothermia in the anteroventral preoptic region (AVPO), the thermointegrative region of the central nervous system (CNS). Accordingly, we predicted that injection of aminophylline and DPCPX, non-selective and A(1) receptor antagonists, respectively, into the AVPO would exacerbate the ventilatory response and lessen the drop in body temperature (T(b)) caused by hypoxia. We measured ventilation (V ) and T(b) of conscious Wistar rats before and after AVPO injection of aminophylline (1 and 10 microg/100 nL) or DPCPX (17.5 and 175 ng/100 nL), or their respective vehicles, followed by 30 min of hypoxia (7% O(2)). Vehicles and the lower doses of both antagonists had no effect on V and T(b) during normoxia or hypoxia. The higher doses of aminophylline and DPCPX increased (P<0.05) the hypoxia-induced hyperventilation, whereas the drop in T(b) elicited by hypoxia was attenuated (P<00.05) by DPCPX only. This higher DPCPX dose also increased T(b) during normoxia. The present data is consistent with the notion that adenosine plays an inhibitory role in respiratory and metabolic regulation, in a way that A(1) receptors stimulation in the AVPO inhibits ventilatory drive during hypoxia and tonically modulates basal T(b).

Cardiovascular and autonomic nervous functions during acclimatization to hypoxia in conscious rats

The time courses of changes in cardiovascular and autonomic nervous functions during acclimatization to hypoxia were studied in conscious Sprague-Dawley rats. The animals were kept under a 12:12-h light-dark cycle and exposed to hypoxia (1 atm, 10% O2). Implanted telemetry transmitters were used to record blood pressure (BP). Changes in heart rate (HR) and BP were monitored over a 21-day period, and variations before and during hypoxia were analyzed using the wavelet transform method. The HR, high-frequency power of HR variability (HR-HF) and low-frequency power of BP variability (BP-LF) were all significantly increased after 1 h of hypoxia, whereas the LF/HF ratio of HR variability did not change. After this initial increase, both HR and the BP-LF were found to decrease. On the first day of hypoxia, HR and BP-LF values were significantly lower than those of the control rats, whereas the HR-HF was higher. Subsequently, these values altered so that they were similar to the control after 14 days of hypoxia. In addition, the amplitude of diurnal variation in HR was reduced during hypoxia. These results suggest that a sequence of dynamic interactions between sympathetic and parasympathetic nervous activities might have important roles in the regulation of cardiovascular function during acclimatization to hypoxia.

Effect of estradiol sulfamate (ES-J995) on affinity of hemoglobin for oxygen, cardiovascular function and acid-base balance in ovariectomized rats

Oral administration of estradiol sulfamate (ES, prodrug of estradiol) leads to increased systemic and reduced hepatic effects than estradiol because ES is accumulated in erythrocytes. However, possible alterations of erythrocytic oxygen transport by intraerythrocytic ES accumulation has not been studied. Therefore, ovariectomized adult female rats (n = 58; body wt.) were randomly treated orally either with single doses (day 1) or multiple dose (days 1-4) with vehicle, with estradiol sulfamate (ES-J995, 1 mg x kg(-1) b.w.) or with estradiol (30 mg x kg(-1) b.w.). Under general anesthesia arterial blood pressure, heart rate, blood gases, and acid-base balance were measured. Hypoxia was performed by lowering the inspired fraction of oxygen from 0.35 to 0.12. In addition, individual oxygen dissociation curves and ES-J995 distribution in blood and plasma were estimated. ES-J995 was accumulated in erythrocytes by approximately 98% (P < 0.01), but oxygen transport capacity was not altered (P50: 35.6 +/- 1.0 mm Hg to 37.1 +/- 1.1 mm Hg). Blood gases and acid-base balance parameters were not altered after ES-J995 treatment under normoxic and hypoxic conditions. In conclusion, ES-J995 accumulation in erythrocytes does not alter the affinity of hemoglobin for oxygen nor any function which would indicate an impaired oxygen delivery to the body.

Roles of adenosine in skeletal muscle during systemic hypoxia

1. The present review is concerned with the effects of acute systemic hypoxia on the gross vascular conductance of skeletal muscle (MVC) and on the behaviour of muscle microcirculation. 2. On the basis of experiments performed in the rat, it is argued that adenosine released from the vascular endothelium plays a major role in dilating muscle vasculature by acting on adenosine A1 receptors. 3. The dilatation of the proximal arterioles is primarily important in increasing MVC and in limiting the fall in O(2) delivery to muscle. It is suggested that the action of adenosine on proximal arterioles is dependent on nitric oxide (NO) rather than mediated by NO, such that adenosine dilates the proximal arterioles via other mechanisms when synthesis of NO is blocked. 4. In contrast, dilatation of terminal arterioles, particularly in regions within muscle where the hypoxia is most severe, helps to improve the distribution of available O(2), allowing muscle O(2) consumption to be maintained by increased O(2) extraction. It is concluded that the action of adenosine on terminal arterioles is mainly mediated by NO arising from stimulation of endothelial A1 receptors. 5. Therefore, adenosine plays a major role in coordinating the behaviour of muscle vasculature such that the relationship between O(2) supply and O(2) demand can be optimized even when the O(2) content of the arterial blood is greatly reduced.

Roles of adenosine and nitric oxide in skeletal muscle in acute and chronic hypoxia

In experiments on anaesthetised rats, the roles played by adenosine and nitric oxide (NO) were determined in resting skeletal muscle in acute systemic hypoxia and during acclimation to chronic systemic hypoxia. It is concluded that adenosine acting on A1 receptors, at least in part in an NO-dependent manner, plays essential roles in causing the dilation of proximal and terminal arterioles that helps to maintain muscle O2 consumption when O2 delivery is reduced by acute systemic hypoxia. It is proposed that adenosine and NO are similarly responsible for causing the tonic vasodilation that gradually wanes in the first 7 days of chronic hypoxia and that concomitantly, adenosine and hypoxia stimulate VEGF expression, so increasing venular permeability and triggering angiogenesis. By 7 days of chronic hypoxia, arteriolar remodelling is well established and within 18-21 days, substantial capillary angiogenesis alleviates tissue hypoxia. At this time, vasoconstrictor responses to the sympathetic transmitter norepinephrine are reduced, but dilator responses to adenosine released by acute hypoxia are enhanced, as may be explained by increased sensitivity to NO. Thus, preservation of tissue oxygenation is apparently associated with impaired ability to regulate arterial pressure and vulnerability to further hypoxia.

Oxygen delivery and oxygen consumption in rat hindlimb during systemic hypoxia: role of adenosine

1. In anaesthetised rats, the increase in femoral vascular conductance (FVC) evoked by moderate systemic hypoxia is mediated by adenosine acting on A(1) receptors. It is also nitric oxide (NO) dependent: it is attenuated by NO synthase (NOS) inhibition, but restored when baseline FVC is restored by sodium nitroprusside (SNP), a NO donor. However, under these conditions there was in increase in the critical O(2) delivery (D(O2,crit)) at which hindlimb O(2) consumption (V(O2)) becomes directly dependent upon O(2) delivery (D(O2)), indicating that V(O2) is regulated by newly synthesised NO. 2. In the present study, after NOS inhibition, when baseline FVC was restored with SNP infusion, the increases in FVC evoked by breathing 12 and 8 % O(2) were reduced by the A(1) receptor antagonist DPCPX, by 60 and 40 %, respectively (n = 8). The A(2A) receptor antagonist ZM241385 reduced the FVC increase evoked by 12 % O(2) (by 45 %, n = 8), but did not alter that evoked by 8 % O(2). 3. DPCPX also reduced the increases in FVC evoked by graded systemic hypoxia, breathing 14-6 % O(2) and increased D(O2,crit), from 0.64 +/- 0.06 to 0.95 +/- 0.07 ml O(2) min(-1) kg(-1) (control vs. DPCPX). However, ZM241385 (n = 8) had no effect on the FVC increases or on D(O2,crit) (0.70 +/- 0.02 ml O(2) min(-1) kg(-1), n = 8). 4. Thus, the increases in FVC evoked by mild to severe systemic hypoxia are mediated by A(1) receptors. These responses, which are attributable to proximal arteriolar dilatation, help maintain D(O2). Even after NOS inhibition, adenosine still increases FVC via A(2A) (moderate hypoxia only) and A(1) receptors, providing baseline levels of NO are present. Furthermore, adenosine, acting via A(1) receptors, is important in determining D(O2,crit) and therefore in maintaining V(O2). We propose that this is achieved by A(1)-evoked dilatation of terminal arterioles and is mediated by increased synthesis of NO.

Downregulation of nitric oxide in the brain of mice during their hypoxic preconditioning

An animal model of hypoxic preconditioning was produced in mice by repeated exposure to autohypoxic condition. The animals' tolerance times to hypoxia were 1.7, 1.8, 2.1, and 2.3 times longer in runs 2, 3, 4, and 5, respectively, than that in run 1, and their oxygen consumption and heart and respiration rates were progressively and significantly slowed down during the repetitive exposure to hypoxia. L-Arginine concentration, nitric oxide (NO) synthase-positive cells, NO synthase activity, and NO content in the whole brain and the subregions telencephalon, diencephalon, and brain stem were significantly increased during the first exposure and were, instead of continuing to increase, significantly decreased in run 4 after the second and third exposure. Tolerance times under the hypoxic condition were significantly shortened and prolonged when preadministration of L-arginine and its analog, respectively, was made. These results indicate that NO in the brain is downregulated under condition of hypoxic preconditioning and negatively involved in increased tolerance to hypoxia.

The changes in leg blood flow during and after mild or severe acute hypoxaemia in healthy humans

The present study examines the leg blood flow changes in resting healthy humans during and after a 10-min period of mild (PaO2=5.60 kPa) or severe hypoxaemia (PaO2=4.53 kPa) induced by breathing hypoxic gas mixtures. A Colour Duplex Scan system allowed to measure the cross-sectional area (CSA) and mean blood flow (Q) in a femoral artery (FA) and a femoral vein (FV) and also in an artery supplying leg muscles (medial gastrocnemius artery, MGA). During the mild as well as the severe hypoxaemia and their recovery periods, no significant variations of Q and CSA occurred in FA and FV. During the mild hypoxaemia and the first 10 min of the recovery period, Q and CSA of MGA increased (maximal changes: +84 and +20%, respectively). By contrast, a marked Q decrease and a reduced CSA were measured in MGA during the severe hypoxaemia (-67 and -60%, respectively). This reduced muscle blood flow was followed by a vasodilatation (CSA increase = +30%), which began 10 min after the hypoxaemia ended and persisted for a further 10-min period. This study shows that the time course of muscle blood flow changes in response to acute hypoxaemia depends on the PaO2 level. Reverse effects were measured during the mild or the severe hypoxaemia, whereas a post-hypoxaemic vasodilatation occurred in all circumstances.

Vasodilatation, oxygen delivery and oxygen consumption in rat hindlimb during systemic hypoxia: roles of nitric oxide

We have investigated the relationship between O2 delivery (DO2) and O2 consumption (VO2) in hindlimb muscle of anaesthetised rats during progressive systemic hypoxia. Since muscle vasodilatation that occurs during hypoxia is nitric oxide (NO) dependent, we examined the effects of the NO synthase (NOS) inhibitor nitro-L-arginine methyl ester (L-NAME). In control rats (n = 8), femoral vascular conductance (FVC) increased at each level of hypoxia. Hindlimb DO2 decreased with the severity of hypoxia, but muscle VO2 was maintained until the critical DO2 value (DO2,crit) was reached at 0.64 +/- 0.06 ml O2 min-1 kg-1; below this VO2 declined linearly with DO2. This is a novel finding for the rat but is comparable to the biphasic relationship seen in the dog. In another group of rats (n = 6), L-NAME caused hindlimb vasoconstriction and attenuated the hypoxia-evoked increases in FVC DO2 was so low after L-NAME administration that VO2 was dependent on DO2 at all levels of hypoxia. In a further group (n = 8), femoral blood flow and DO2 were restored after L-NAME by infusion of the NO donor sodium nitroprusside (20 g x kg(-1) x min(-1). Thereafter, hypoxia-evoked increases in FVC were fully restored. Nevertheless, DO2,crit was increased relative to control (0.96 +/- 0.07 ml O2 min(-1) x kg(-1), P < 0.01). As NOS inhibition limited the ability of muscle to maintain VO2 during hypoxia, we propose that hypoxia-induced dilatation of terminal arterioles, which improves tissue O2 distribution, is mediated by NO. However, since the hypoxia-evoked increase in FVC was blocked by L-NAME but restored by the NO donor, we propose that the dilatation of proximal arterioles is dependent on tonic levels of NO, rather than mediated by NO.

Adenosine and muscle vasodilatation in acute systemic hypoxia

Adenosine is released by skeletal and cardiac muscles when their metabolism increases: it serves to couple O2 supply with O2 demand by causing vasodilatation. This review argues that adenosine plays a similar role in skeletal muscle in systemic hypoxia. It accounts for approximately 50% of the increase in muscle vascular conductance and, within muscle, it causes dilatation of individual arterioles, thus maximizing the distribution of O2 and allowing O2 consumption to remain constant when O2 delivery is reduced. In vivo and in vitro studies have indicated that adenosine can induce dilatation in several different ways. This review argues that during systemic hypoxia, adenosine is predominantly released from the endothelium and acts on endothelial A1 receptors to produce dilatation in a nitric oxide (NO)-dependent manner. A1 receptor stimulation increases the synthesis of NO by a process initiated by opening of ATP-sensitive K+ (KATP) channels. Moreover, recent findings suggest that prostaglandins also make a major contribution to the hypoxia-induced dilatation, but that the dilator pathways for adenosine, NO and prostaglandins are interdependent. In addition, adenosine released from the skeletal muscle fibres contributes indirectly to the dilatation by stimulating A1 and A2 receptors on the muscle fibres, opening KATP channels and allowing efflux of K+, which is a vasodilator. Finally, by acting on endothelial A1 receptors, adenosine attenuates the vasoconstrictor effects of constant or bursting patterns of sympathetic activity. This limits the extent to which the sympathetic nervous system can reduce O2 delivery to muscle when it is already compromised by systemic hypoxia.

Hypoxic pressor response, cardiac size, and natriuretic peptides are modified by long-term intermittent hypoxia

We investigated whether the effect of long-term intermittent hypoxia (LTIH) on cardiovascular function may be modified by preexisting genetic traits. To induce LTIH experimentally, cycles of 90-s hypoxia (nadir 6%) followed by 90-s normoxia were applied to six Wistar-Kyoto and six spontaneously hypertensive rats during 8 h daily. Comparison with the same number of control animals after 70 days revealed no alteration of intra-arterial blood pressure or heart rate. Blood pressure responsiveness to a brief hypoxic stimulus was enhanced in the LTIH animals, regardless of strain, whereas the hypoxia-induced increase in heart rate was abolished. In the spontaneously hypertensive but not the Wistar-Kyoto rats, LTIH increased left ventricular weight-to-body weight ratio and content of atrial natriuretic peptide mRNA. Expression of B-type natriuretic peptide was unchanged (Northern blot). Slightly increased right ventricular weight-to-body weight ratios in the LTIH animals were associated with higher right ventricular atrial natriuretic peptide and B-type natriuretic peptide mRNA amounts. Consequently, the effects of LTIH on different components of cardiovascular function appear incompletely related to each other and differentially influenced by constitutional traits.