Micro-Physiological-Systems enable investigation of hypoxia induced pathological processes in human aortic valve cells and tissues

Aortic valve (AV) stenosis is characterized by tissue fibrosis and calcification. Fibrous thickening can result in reduced tissue oxygen supply leading to pathological valvular interstitial cell (VIC) differentiation and calcification. Static 2D VIC cultures and animal models are limited in the ability to reflect human AV calcification. Culturing of VICs in micro-physiological-systems (MPS) in a pulsatile flow and the establishment of a modular AV tissue incubation chamber (TIC) are new approaches to evaluate pathophysiological processes of AV disease. Therefore, a MPS able to adjust hypoxic conditions was applied for VIC culture. A significant increase of mRNA-expression of EGLN1 and HIF1α- regulated LDHA and HIF1α nuclear localisation were proven under hypoxia. AV tissue culture was established within a TIC and viability was monitored by Resazurin-reduction in the incubation medium and visualized by LDH-activity in tissue cryosections. Viability was compared between fluid and static incubated tissues revealing an advantageous effect of the fluidic assay condition. Consecutively, the application of MPS in AV research allows i) the investigation of VIC cultures with efficient oxygen regulation and ii) the culture of porcine or human AV tissues preserving viability and specifically reflecting in vivo parameters. These methods open up new possibilities beyond static 2D culture and facilitate a reduction of animal experiments in AV research.

The oxygen cascade in hemodialysis patients and native high altitude dwellers- Lessons from extreme physiology to benefit patients with end-stage renal disease

Hemodialysis patients repeatedly undergo intradialytic low arterial oxygen saturation, as well as low central venous oxygen saturation, reflecting an imbalance between upper body systemic oxygen supply and demand, which are associated with increased mortality. Abnormalities along the entire oxygen cascade, with impaired diffusive and convective oxygen transport, contribute to the reduced tissue oxygen supply. Dialysis treatment impairs pulmonary gas exchange and reduces ventilatory drive, whereas ultrafiltration can reduce tissue perfusion due to a decline in cardiac output. In addition to these factors, capillary rarefaction and reduced mitochondrial efficacy can further affect the balance between cellular oxygen supply and demand. Whereas it has been convincingly demonstrated that a reduced perfusion of heart and brain during dialysis contributes to organ damage, the significance of systemic hypoxia remains uncertain, although it may contribute to oxidative stress, systemic inflammation and accelerated senescence. These abnormalities along the oxygen cascade of dialysis patients appear to be diametrically opposite to the situation in Tibetan highlanders and Sherpa, whose physiology adapted to the inescapable hypobaric hypoxia of their living environment over many generations. Their adaptation includes pulmonary, vascular and metabolic alterations with enhanced capillary density, nitric oxide production and mitochondrial efficacy without oxidative stress. Improving the tissue oxygen supply in dialysis patients depends primarily on preventing hemodynamic instability by increasing dialysis time/frequency or prescribing cool dialysis. Whether dietary or pharmacological interventions, such as the administration of L-arginine, fermented food, nitrate, Nrf2 agonists or prolyl hydroxylase 2 inhibitors improve clinical outcome in dialysis patients warrants future research.

Possible contribution of erythrocytes to the purinergic regulation of tissue oxygen delivery

ATP release occurs in virtually all cell types and tissues. It is considered to be a key component of a ubiquitous, evolutionary ancient cell-to-cell communication system. The regulated release of ATP is also believed to be a part of a mechanism which facilitates matching tissue oxygen supply with demand. In this paper, ATP signaling is reviewed, regulation of tissue oxygen supply is outlined, and a concept attributing a role in this process to erythrocytes releasing ATP is discussed. Oxygen saturation of hemoglobin in erythrocytes traveling through a tissue reflects the level of oxygen utilization of that tissue. Therefore, erythrocytes can serve as local oxygen sensors. In the proposed mechanism, upon hemoglobin deoxygenation, Gi protein in the erythrocyte plasma membrane is activated. The activation of Gi protein initiates a cAMP-dependent pathway, leading to ATP efflux through pannexin channels. Extracellular ATP triggers vasodilatation via P2Y receptors on the surface of vascular endothelial cells, increasing blood flow in tissue regions of elevated oxygen consumption. Despite the abundance of compelling evidence in support of this concept, some details remain elusive, in particular, the process of Gi protein activation in response to hemoglobin desaturation. Furthermore, the involvement of cAMP, as well as the final conduit of ATP release from erythrocytes, remains controversial. Finally, the actual physiological relevance of the proposed regulatory mechanism will require further in vivo research.

MODELLING HYDROGEN CLEARANCE FROM THE RETINA

The human retina is supplied by two vascular systems: the highly vascular choroidal, situated behind the retina; and the retinal, which is dependent on the restriction that the light path must be minimally disrupted. Between these two circulations, the avascular retinal layers depend on diffusion of metabolites through the tissue. Oxygen supply to these layers may be threatened by diseases affecting microvasculature, for example diabetes and hypertension, which may ultimately cause loss of sight.An accurate model of retinal blood flow will therefore facilitate the study of retinal oxygen supply and, hence, the complications caused by systemic vascular disease. Here, two simple models of the blood flow and exchange of hydrogen with the retina are presented and compared qualitatively with data obtained from experimental measurements. The models capture some interesting features of the exchange and highlight effects that will need to be considered in a more sophisticated model and in the interpretation of experimental results.