Hyperoxia Improves Hemodynamic Status During Head-up Tilt Testing in Healthy Volunteers: A Randomized Study.

Head-up tilt test is useful for exploring neurally mediated syncope. Adenosine is an ATP derivative implicated in cardiovascular disturbances that occur during head-up tilt test. The aim of the present study was to investigate the impact of hyperoxia on adenosine plasma level and on hemodynamic changes induced by head-up tilt testing.Seventeen healthy male volunteers (mean age 35 ±â€Š11 years) were included in the study. The experiment consisted of 2 head-up tilt tests, 1 session with subjects breathing, through a mask, medical air (FiO2 = 21%) and 1 session with administration of pure oxygen (FiO2 = 100%) in double-blind manner. Investigations included continuous monitoring of hemodynamic data and measurement of plasma adenosine levels.No presyncope or syncope was found in 15 of the 17 volunteers. In these subjects, a slight decrease in systolic blood pressure was recorded during orthostatic stress performed under medical air exposure. In contrast, hyperoxia led to increased systolic blood pressure during orthostatic stress when compared with medical air. Furthermore, mean adenosine plasma levels decreased during hyperoxic exposure before (0.31 ±â€Š0.08 µM) and during head-up tilt test (0.33 ±â€Š0.09 µM) when compared with baseline (0.6 ±â€Š0.1 µM). Adenosine plasma level was unchanged during medical air exposure at rest (0.6 ±â€Š0.1 µM), and slightly decreased during orthostatic stress. In 2 volunteers, the head-up tilt test induced a loss of consciousness when breathing air. In these subjects, adenosine plasma level increased during orthostatic stress. In contrast, during hyperoxic exposure, the head-up tilt test did not induce presyncope or syncope. In these 2 volunteers, biological study demonstrated a decrease in adenosine plasma level at both baseline and during orthostatic stress for hyperoxic exposure compared with medical air.These results suggest that hyperoxia was able to increase blood pressure during head-up tilt test via a decrease in plasma adenosine concentration. Our results also suggest that adenosine receptor antagonists are worth trying in neurocardiogenic syncope.

Effect of hyperoxic and hyperbaric conditions on the adenosinergic pathway and CD26 expression in rat.

The nucleoside adenosine acts on the nervous and cardiovascular systems via the A2A receptor (A2AR). In response to oxygen level in tissues, adenosine plasma concentration is regulated in particular via its synthesis by CD73 and via its degradation by adenosine deaminase (ADA). The cell-surface endopeptidase CD26 controls the concentration of vasoactive and antioxidant peptides and hence regulates the oxygen supply to tissues and oxidative stress response. Although overexpression of adenosine, CD73, ADA, A2AR, and CD26 in response to hypoxia is well documented, the effects of hyperoxic and hyperbaric conditions on these elements deserve further consideration. Rats and a murine Chem-3 cell line that expresses A2AR were exposed to 0.21 bar O2, 0.79 bar N2 (terrestrial conditions; normoxia); 1 bar O2 (hyperoxia); 2 bar O2 (hyperbaric hyperoxia); 0.21 bar O2, 1.79 bar N2 (hyperbaria). Adenosine plasma concentration, CD73, ADA, A2AR expression, and CD26 activity were addressed in vivo, and cAMP production was addressed in cellulo. For in vivo conditions, 1) hyperoxia decreased adenosine plasma level and T cell surface CD26 activity, whereas it increased CD73 expression and ADA level; 2) hyperbaric hyperoxia tended to amplify the trend; and 3) hyperbaria alone lacked significant influence on these parameters. In the brain and in cellulo, 1) hyperoxia decreased A2AR expression; 2) hyperbaric hyperoxia amplified the trend; and 3) hyperbaria alone exhibited the strongest effect. We found a similar pattern regarding both A2AR mRNA synthesis in the brain and cAMP production in Chem-3 cells. Thus a high oxygen level tended to downregulate the adenosinergic pathway and CD26 activity. Hyperbaria alone affected only A2AR expression and cAMP production. We discuss how such mechanisms triggered by hyperoxygenation can limit, through vasoconstriction, the oxygen supply to tissues and the production of reactive oxygen species.

Improved cryosections and specific immunohistochemical methods for detecting hypoxia in mouse and rat cochleae.

The present study was undertaken to develop an improved cryoembedding method for analysis of mice and rat cochleae, which permits high-quality cryosections and preserves overall structure and cellular resolution as shown by hematoxylin/eosin staining. The preservation of morphology and antigenicity is mandatory to achieve optimal results. A total of 20 male cd/1 mice and 14 male Sprague-Dawley rats were used in experiments for optimization of preservation, fixative, decalcification, embedding and cryosectioning of cochleae from adult and aged rodents. In addition, a novel immunohistochemical procedure (using Hydroxyprobe-1 kit) was developed for detecting regions of hypoxia in mice and rat cochlea. This method employs a primary fluorescent-conjugated monoclonal antibody directed against pimonidazole protein adducts that are created in hypoxic tissues. Subsequent studies of hypoxia inducible factor-1alpha (HIF-1alpha) by immunofluorescence in the cochlea of these animals were performed in order to confirm that immunochemical detection of pimonidazole protein is representative of a hypoxic environment. We conclude that the present method results in high-quality cryosections of cochlear tissues presenting good anatomical and histological preservation. Furthermore, our optimized procedures provide novel tools for the investigation of neuro-sensory-epithelium in physio-pathological situations associated with hypoxia and/or ischemia, such as inner ear development, plasticity, regeneration and senescence.

Enhanced cochlear acoustic sensitivity and susceptibility to endotoxin are induced by adrenalectomy and reversed by corticosterone supplementation in rat.

Glucocorticoid receptors are widely distributed in the cochlea but their role remains poorly known. Previous studies provided contradictory reports on a possible cochlear acoustic hypersensitivity induced by adrenal insufficiency, while several experiments agree on a significant action of glucocorticoid receptors in adverse conditions such as acoustic trauma and restraint stress. The present experiments confirmed a cochlear acoustic hypersensitivity induced by adrenalectomy and reversed by corticosterone supplementation. These observations point to a significant role of corticosteroids in basal cochlear functioning. The glucocorticoids are known to be essential for limiting and resolving inflammatory processes. The endotoxin Escherichia coli lipopolysaccharide is widely used to induce inflammatory reactions. However, in various organs several toxic processes of this endotoxin are not influenced by glucocorticoids. From previous experiments on the cochlea there is no evidence that glucocorticoids are an essential factor against endotoxin cochlear toxicity. In the present experiments it was found that adrenalectomy greatly increased the cochlear susceptibility to endotoxin; the effect was reversed by providing corticosterone supplementation. This shows the essential role of corticosterone in this cochlear inflammation model. In previous studies local administration (at the cochlear base) of endotoxin was used and losses of cochlear acoustic sensitivity were found predominantly at high frequencies; in contrast, the systemic injection used in this study produced a cochlear loss of acoustic sensitivity at all frequencies, indicating a uniform cochlear sensitivity to the toxic effects of endotoxin.