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Hypoxic pulmonary hypertension (HPH) is characterized by progressive pulmonary vasoconstriction, vascular remodeling, and right ventricular hypertrophy, causing right heart failure. This study aimed to investigate the therapeutic effects of exosomes from Tibetan umbilical cord mesenchymal stem cells on HPH via the TGF-β1/Smad2/3 pathway, comparing them with exosomes from Han Chinese individuals. An HPH rat model was established in vivo, and a hypoxia-induced injury in the rat pulmonary artery smooth muscle cells (rPASMCs) was simulated in vitro. Exosomes from human umbilical cord mesenchymal stem cells were administered to HPH model rats or added to cultured rPASMCs. The therapeutic effects of Tibetan-mesenchymal stem cell-derived exosomes (Tibetan-MSC-exo) and Han-mesenchymal stem cell-derived exosomes (Han-MSC-exo) on HPH were investigated through immunohistochemistry, western blotting, EdU, and Transwell assays. The results showed that Tibetan-MSC-exo significantly attenuated pulmonary vascular remodeling and right ventricular hypertrophy in HPH rats compared with Han-MSC-exo. Tibetan-MSC-exo demonstrated better inhibition of hypoxia-induced rPASMCs proliferation and migration. Transcriptome sequencing revealed upregulated genes (Nbl1, Id2, Smad6, and Ltbp1) related to the TGFβ pathway. Nbl1 knockdown enhanced hypoxia-induced rPASMCs proliferation and migration, reversing Tibetan-MSC-exo-induced downregulation of TGFβ1 and p-Smad2/3. Furthermore, TGFβ1 overexpression hindered the therapeutic effects of Tibetan-MSC-exo and Han-MSC-exo on hypoxic injury. These findings suggest that Tibetan-MSC-exo favors HPH treatment better than Han-MSC-exo, possibly through the modulation of the TGFβ1/Smad2/3 pathway via Nbl1., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

During periods of severe hypoxia or anoxia, Carassius spp. are known for their ability to produce ethanol as their anaerobic end product, which diffuses into the environment thereby reducing the osmotic and acidotic load associated with "anaerobic" glycolysis. However, the relationship between alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALDH) activities, key ethanol metabolizing enzymes, and hypoxia tolerance among Carassius spp. and their closely related non-ethanol-producing cyprinids remains unclear. To address this, we quantified the activity levels of key anaerobic enzymes in liver and muscle in species of cyprinids over 48 h of severe hypoxia exposure (0.7 kPa). As predicted, muscle ADH activity was highest in the two most hypoxia-tolerant species (Carassius spp.), with very low levels present in the other species examined. However, liver ADH activities showed an inverse relationship with hypoxia tolerance, with the most hypoxia-tolerant fish having the lowest ADH activity. There was no correlation between hypoxia tolerance and ALDH and LDH activities in muscle or liver. All species produced lactate, reaching their highest levels after 8 h, but returning to near-baseline levels by 48 h of sustained exposure to hypoxia, suggesting lactate oxidation or depressed ATP demand. Liver glycogen content was not affected by 48 h hypoxia exposure in the most hypoxia-tolerant species, whereas the least tolerant species consumed the majority of the liver glycogen stores, which is probably due to the greater relative hypoxia exposure experienced by these species. Our findings that liver ADH activities were inversely related to hypoxia tolerance suggests that in all but Carassius spp., the ethanol metabolizing pathways in cyprinids is largely similar to that observed in other vertebrates and plays a role in the detoxification of ethanol. Furthermore, conservation of glycogen stores may be the result of metabolic-depressing pathways in the more tolerant species, regardless of the ability to produce ethanol, or adaptations that improve oxygen uptake to reduce metabolic demands due to hypoxia.

The gills of many fish, but in particular those of crucian carp (Carassius carassius) and goldfish (Carassius auratus), are capable of extensive remodeling in response to changes in oxygen (O2), temperature, and exercise. In this study, we investigated the interspecific variation in hypoxia-induced gill modeling and hypoxia tolerance in 10 closely related groups of cyprinids (nine species, with two strains of Cyprinus carpio). There was significant variation in hypoxia tolerance, measured as the O2 tension (P(O2)) at which fish lost equilibrium (LOEcrit), among the 10 groups of carp. In normoxia, there was a significant, phylogenetically independent relationship between mass-specific gill surface area and LOEcrit, with the more hypoxia-tolerant carp having smaller gills than their less hypoxia-tolerant relatives. All groups of carp, except the Chinese bream (Megalobrama pellegrini), increased mass-specific gill surface area in response to 48 h of exposure to hypoxia (0.7 kPa) through reductions in the interlamellar cell mass (ILCM) volume. The magnitude of the hypoxia-induced reduction in the ILCM was negatively correlated with LOEcrit (and thus positively correlated with hypoxia tolerance), independent of phylogeny. The hypoxia-induced changes in gill morphology resulted in reduced variation in mass-specific gill surface area among species and eliminated the relationship between LOEcrit and mass-specific gill surface area. While behavioral responses to hypoxia differed among the carp groups, there were no significant relationships between hypoxia tolerance and the Po2 at which aquatic surface respiration (ASR) was initiated or the total number of ASR events observed during progressive hypoxia. Our results are the first to show that the extent of gill remodeling in cyprinids is associated with hypoxia tolerance in a phylogenetically independent fashion.

Cardiovascular function and metabolic responses of the heart and other tissues during hypoxia exposure were compared between the hypoxia-tolerant epaulette shark (Hemiscyllium ocellatum) and the hypoxia-sensitive shovelnose ray (Aptychotrema rostrata). In both species, progressive hypoxia exposure caused increases in stroke volume and decreases in heart rate, cardiac output, cardiac power output (CPO, an assessment of cardiac energy demand) and dorsal aortic blood pressure, all of which occurred at or below each species' critical P(O2) for whole-animal O(2) consumption rate, M(O2) (P(crit)). In epaulette sharks, which have a lower P(crit) than shovelnose rays, routine levels of cardiovascular function were maintained to lower water P(O2) levels and the changes from routine levels during hypoxia exposure were smaller compared with those for the shovelnose ray. The maintenance rather than depression of cardiovascular function during hypoxia exposure may contribute to the superior hypoxia tolerance of the epaulette shark, presumably by improving O(2) delivery and waste removal. Compared with shovelnose rays, epaulette sharks were also better able to maintain a stable cardiac high-energy phosphate pool and to minimize metabolic acidosis and lactate accumulation in the heart (despite higher CPO) and other tissues during a 4 h exposure to 40% of their respective P(crit) (referred to as a relative hypoxia exposure), which results in similar hypoxaemia in the two species (∼16% Hb-O(2) saturation). These different metabolic responses to relative hypoxia exposure suggest that variation in hypoxia tolerance among species is not solely dictated by differences in O(2) uptake and transport but also by tissue-specific metabolic responses. In particular, lower tissue [lactate] accumulation in epaulette sharks than in shovelnose rays during relative hypoxia exposure suggests that enhanced extra-cardiac metabolic depression occurs in the former species. This could facilitate strategic utilization of available O(2) for vital organs such as the heart, potentially explaining the greater hypoxic cardiovascular function of epaulette sharks.

The critical O(2) tension of whole-animal O(2) consumption rate (M(O2)), or P(crit), is the water P(O2) (Pw(O(2))) at which an animal transitions from an oxyregulator to an oxyconformer. Although P(crit) is a popular measure of hypoxia tolerance in fishes because it reflects the capacity for O(2) uptake from the environment at low Pw(O(2)), little is known about the interrelationships between P(crit) and blood O(2) transport characteristics and increased use of anaerobic metabolism during hypoxia exposure in fishes, especially elasmobranchs. We addressed this knowledge gap using progressive hypoxia exposures of two elasmobranch species with differing hypoxia tolerance. The P(crit) of the hypoxia-tolerant epaulette shark (Hemiscyllium ocellatum, 5.10±0.37 kPa) was significantly lower than that of the comparatively hypoxia-sensitive shovelnose ray (Aptychotrema rostrata, 7.23±0.40 kPa). Plasma [lactate] was elevated above normoxic values at around P(crit) in epaulette sharks, but increased relative to normoxic values at Pw(O(2)) below P(crit) in shovelnose rays, providing equivocal support for the hypothesis that P(crit) is associated with increased anaerobic metabolism. The M(O2), arterial P(O2) and arterial blood O(2) content (Ca(O(2))) were similar between the two species under normoxia and decreased in both species with progressive hypoxia, but as Pw(O(2)) declined, epaulette sharks had a consistently higher M(O2) and Ca(O(2)) than shovelnose rays, probably due to their significantly greater in vivo haemoglobin (Hb)-O(2) binding affinity (in vivo Hb-O(2) P(50)=4.27±0.57 kPa for epaulette sharks vs 6.35±0.34 kPa for shovelnose rays). However, at Pw(O(2)) values representing the same percentage of each species' P(crit) (up to ∼175% of P(crit)), Hb-O(2) saturation and Ca(O(2)) were similar between species. These data support the hypothesis that Hb-O(2) P(50) is an important determinant of P(crit) and suggest that P(crit) can predict Hb-O(2) saturation and Ca(O(2)) during hypoxia exposure, with a lower P(crit) being associated with greater O(2) supply at a given Pw(O(2)) and consequently better hypoxia tolerance. Thus, P(crit) is a valuable predictor of environmental hypoxia tolerance and hypoxia exposures standardized at a given percentage of P(crit) will yield comparable levels of arterial hypoxaemia, facilitating cross-species comparisons of responses to hypoxia.

Hypoxia and warm ischemia are primary concerns in ischemic heart disease and transplant and trauma. Hypoxia impacts tissue ATP supply and can induce mitochondrial dysfunction that elevates reactive species release. The epaulette shark, Hemiscyllum ocellatum, is remarkably tolerant of severe hypoxia at temperatures up to 34 °C, and therefore provides a valuable model to study warm hypoxia tolerance. Mitochondrial function was tested in saponin permeabilised ventricle fibres using high-resolution respirometry coupled with purpose-built fluorospectrometers. Ventricular mitochondrial function, stability and reactive species production of the epaulette shark was compared with that of the hypoxia-sensitive shovelnose ray, Aptychotrema rostrata. Fibres were prepared from each species acclimated to normoxic water conditions, or following a 2 h, acute hypoxic exposure at levels representing 40% of each species' critical oxygen tension. Although mitochondrial respiratory fluxes for normoxia-acclimated animals were similar for both species, reactive species production in the epaulette shark was approximately half that of the shovelnose ray under normoxic conditions, even when normalised to tissue oxidative phosphorylation flux. The hypoxia-sensitive shovelnose ray halved oxidative phosphorylation flux and cytochrome c oxidase flux was depressed by 34% following hypoxic stress. In contrast, oxidative phosphorylation flux of the epaulette shark ventricular fibres isolated from acute hypoxia exposed the animals remained similar to those from normoxia-acclimated animals. However, uncoupling of respiration revealed depressed electron transport systems in both species following hypoxia exposure. Overall, the epaulette shark ventricular mitochondria showed greater oxidative phosphorylation stability and lower reactive species outputs with hypoxic exposure, and this may protect cardiac bioenergetic function in hypoxic tropical waters.

In this study, we explored how environmental oxygen levels affect the metabolic phenotype of sympatric sunfish known to differ in their hypoxia tolerance. We examined bluegill (Lepomis macrochirus) and pumpkinseed (Lepomis gibbosus), two species commonly found in the same water bodies, though pumpkinseed are considered more hypoxia tolerant, and survive in hypoxic lakes that exclude bluegill. Freshly caught Lake Opinicon pumpkinseed possessed significantly higher glycolytic enzyme activities (PGI, ALD, GAPDH, ENO, and LDH) than bluegill, but after holding the fish in an oxygenated environment for 7days, pumpkinseed glycolytic enzymes (PGI, ALD, and LDH) and mRNA (LDHA and HIF1α) declined to bluegill's levels. When glycolytic enzymes and mRNA were compared in pumpkinseed populations from seven lakes, only Penyck Lake pumpkinseed had significantly elevated glycolytic enzyme activity that did not diminish with normoxic holding. The levels of mRNA for LDHA and HIF1α did not differ between lakes and did not change in response to normoxic holding in the Penyck Lake fish. Collectively, these studies on sunfish show that hypoxia tolerance contributes to ecological niche specialization between species, and provides an example of a population that has adapted chronically elevated glycolytic enzyme activity independent of current dissolved oxygen in the water., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Monocarboxylate transporters (MCTs) have been shown to be important in regulating metabolism during hypoxia in mammals. However, the role of MCTs in hypoxic survival in lower vertebrates is currently unclear. The goal of this study was to investigate the coordinated responses of MCTs along with other metabolic genes during hypoxia. Therefore, we subjected zebrafish (Danio rerio) to 1.5mgL(-1) O(2) over 48 and 96h and measured tissue-specific transcriptional changes of MCTs (1, 2 and 4), lactate dehydrogenase A (LDHa), citrate synthase (CS), and other metabolic proteins using real-time RT-PCR. There were no changes in mRNA detected in muscle at 48 and 96h. When data from both time points were pooled in brain, a significant increase was found in MCT4 (+102%) and LDHa (+28%) mRNA suggesting a preference towards glycolysis. In gills, there were increases in LDHa at 48h (+101%) and MCT1 (+24%) mRNA from pooled data suggesting that both anaerobic and aerobic metabolisms are being utilized. The heart showed the greatest changes in transcriptional regulation compared to other tissues. At 48h, increases were found in MCT1 (+117%), MCT4 (+86%), LDHa (+197%) mRNA, and pooled data showed an increase in CS (+18%) mRNA. These results suggest that the influx and efflux of lactate are both employed as strategies in cardiac tissue during hypoxia. This study has shown that fish utilize tissue-specific transcriptional regulation of MCTs along with other metabolic genes during hypoxia.

Goldfish and crucian carp at low temperature exhibit plasticity in gill morphology during exposure to hypoxia to enhance gas exchange. Hypoxia-induced changes in gill morphology and cellular ultrastructure of the high altitude scaleless carp from Lake Qinghai, China, were investigated to determine whether this is a general characteristic of cold water carp species. Fish were exposed to acute hypoxia (0.3 mg O2 l(-1)) for 24 h followed by 12 h recovery in normoxic water (6 mg O2 l(-1) at 3200 m altitude), with no mortality. Dramatic alterations in gill structure were initiated within 8 h of hypoxia and almost complete by 24 h, and included a gradual reduction of filament epithelial thickness (>50%), elongation of respiratory lamellae, expansion of lamellar respiratory surface area (>60%) and reduction in epithelial water-blood diffusion distance (<50%). An increase in caspase 3 activity in gills occurred following 24 h exposure to hypoxia, indicating possible involvement of apoptosis in gill remodeling. Extensive gill mucous production during hypoxia may have been part of a general stress response or may have played a role in ion exchange and water balance. The large increase in lamellar surface area and reduction in diffusion distance presumably enhances gas transfer during hypoxia (especially in the presence of increased mucous production) but comes with an ionoregulatory cost, as indicated by a 10 and 15% reduction in plasma [Na+] and [Cl-], respectively, within 12-24 h of hypoxia. Within 12 h of hypoxia exposure, ;wavy-convex'-mitochondria rich cells (MRCs) with large apical crypts and numerous branched microvilli were transformed into small ;shallow-basin' cells with a flattened surface. As the apical membrane of MRCs is the site for active ion uptake from the water, a reduction in apical crypt surface area may have contributed to the progressive reduction in plasma [Na+] and [Cl-] observed during hypoxia. The changes in the macro- and ultra-structure of fish gills, and plasma [Na+] and [Cl-] during hypoxia were reversible, showing partial recovery by 12 h following return to normoxia. Although the large morphological changes in the gill observed in the scaleless carp support the hypothesis that gill remodeling during hypoxia is a general characteristic of cold water carp species, the reduced magnitude of the response in scaleless carp relative to goldfish and crucian carp may be a reflection of their more active lifestyle or because they reside in a moderately hypoxic environment at altitude.

Corticotropin-releasing factor (CRF) peptide release was activated by hypoxia in the rat hypothalamus. The mechanisms, however, of the hypoxia-induced CRF release remains unclear. In this study, we demonstrated that the norepinephrine (NE) and its receptors in the paraventricular nucleus (PVN) mediated the CRF release in a simulated altitude hypoxia. When rats were exposed to 5 or 7 km altitude of hypoxia for a short or long term: (1) NE levels in the PVN and the CeA, using the HPLC analysis, were intensity and time course dependently increased, but the increase in the PVN were potential than in the CeA. Restraint-induced NE increase was much higher in both the PVN and the CeA, compared with hypoxia-induced response. (2) Hypoxia and restraint significantly enhanced CRF release in the ME and the PVN but not in the CeA, through RIA assay, which result in stimulating corticosterone secretion. (3) Hypoxia-induced CRF release was reversed by an injection of prazosin (icv), an α-1 adrenoceptor antagonist, while administration of yohimbine (icv), an α-2 receptor antagonist, facilitated further CRF release. These data suggested that hypoxia induced NE activation centrally, via α-1 and -2 receptors, leading to improving hypothalamic CRF release, which in turn stimulated pituitary and adrenal cortex. Restraint presented much potential action on NE activation than hypoxia. [Copyright &y& Elsevier]