Renovascular adaptive changes in chronic hypoxic polycythemia

Effects of high altitude on renal physiology and kidney diseases

The hypobaric and hypoxic conditions of high-altitude areas exert adverse effects on the respiratory, circulatory and nervous systems. The kidneys have an abundant blood supply (20%–25% of cardiac output) and high blood flow; thus, they are susceptible to the effects of hypoxia. However, the effects of acute and chronic exposure to high altitudes on renal physiology and pathology are not fully understood. Moreover, few studies have investigated the impact of high-altitude exposure on patients with chronic kidney disease or acute kidney injury. In this review, we summarized changes in renal physiology and renal pathology due to high-altitude exposure as well as the impact of high-altitude exposure on existing kidney diseases, with the aim of informing the prevention and treatment of kidney diseases at high altitudes.

Where do you live and what do you do? Two questions that might impact your kidney health

In many cases the social determinants of health need to be assessed through their interaction with environmental factors. This review looks at the impact of physical location and occupation of individuals on their kidney health. It examines the effect of living at high altitude on kidney function and the relationship between extreme cold or hot temperatures and the incidence of kidney injury. It reviews as well the many occupations that have been linked to kidney disease in high-income and low-and-middle-income countries. As a conclusion, this overview proposes preventive recommendations that could be individualized based on weather, altitude, socio-economic level of the country and occupation of the individual.

Pharmacological HIF-PHD inhibition reduces renovascular resistance and increases glomerular filtration by stimulating nitric oxide generation

AIM

Hypoxia-inducible factors (HIFs) are O₂ -sensitive transcription factors that regulate multiple biological processes which are essential for cellular adaptation to hypoxia. Small molecule inhibitors of HIF-prolyl hydroxylase domain (PHD) dioxygenases (HIF-PHIs) activate HIF-dependent transcriptional programs and have broad clinical potential. HIF-PHIs are currently in global late-stage clinical development for the treatment of anaemia associated with chronic kidney disease. Although the effects of hypoxia on renal haemodynamics and function have been studied in animal models and in humans living at high altitude, the effects of pharmacological HIF activation on renal haemodynamics, O₂ metabolism and metabolic efficiency are not well understood.

METHODS

Using a cross-sectional study design, we investigated renal haemodynamics, O₂ metabolism, gene expression and NO production in healthy rats treated with different doses of HIF-PHIs roxadustat or molidustat compared to vehicle control.

RESULTS

Systemic administration of roxadustat or molidustat resulted in a dose-dependent reduction in renovascular resistance (RVR). This was associated with increased glomerular filtration rate (GFR), urine flow and tubular sodium transport rate (T_Na ). Although both total O₂ delivery and T_Na were increased, more O₂ was extracted per transported sodium in rats treated with high-doses of HIF-PHIs, suggesting a reduction in metabolic efficiency. Changes in RVR and GFR were associated with increased nitric oxide (NO) generation and substantially suppressed by pharmacological inhibition of NO synthesis.

CONCLUSIONS

Our data provide mechanistic insights into dose-dependent effects of short-term pharmacological HIF activation on renal haemodynamics, glomerular filtration and O₂ metabolism and identify NO as a major mediator of these effects.

Global REACH 2018: Volume regulation in high-altitude Andeans with and without chronic mountain sickness

The high-altitude maladaptation syndrome known as chronic mountain sickness (CMS) is characterized by polycythemia and is associated with proteinuria despite unaltered glomerular filtration rate. However, it remains unclear if indigenous highlanders with CMS have altered volume regulatory hormones. We assessed N-terminal pro-B-type natriuretic peptide (NT pro-BNP), plasma aldosterone concentration, plasma renin activity, kidney function (urinary microalbumin, glomerular filtration rate), blood volume, and estimated pulmonary artery systolic pressure (ePASP), in Andean males without (n=14; age=39±11) and with (n=10; age=40±12) CMS at 4330 meters (Cerro de Pasco, Peru). Plasma renin activity (non-CMS: 15.8±7.9 vs. CMS: 8.7±5.4 ng/ml; p=0.025) and plasma aldosterone concentration (non-CMS: 77.5±35.5 vs. CMS: 54.2±28.9 pg/ml; p=0.018) were lower in highlanders with CMS compared to non-CMS, while NT pro-BNP was not different between groups (non-CMS: 1394.9±214.3 vs. CMS: 1451.1±327.8 pg/ml; p=0.15). Highlanders had similar total blood volume (non-CMS: 90±15 vs. CMS: 103±18 ml • kg^-1; p=0.071), but Andeans with CMS had greater total red blood cell volume (non-CMS: 46±10 vs. CMS 66±14 ml • kg^-1; p<0.01) and smaller plasma volume (non-CMS 43±7 vs. CMS 35±5 ml • kg^-1; p=0.03) compared to non-CMS. There were no differences in ePASP between groups (non-CMS 32±9 vs. CMS 31±8 mmHg; p=0.6). A negative correlation was found between plasma renin activity and glomerular filtration rate in both groups (group: r=-0.66; p<0.01; non-CMS: r=-0.60; p=0.022; CMS: r=-0.63; p=0.049). A smaller plasma volume in Andeans with CMS may indicate an additional CMS maladaptation to high-altitude, causing potentially greater polycythemia and clinical symptoms.

Global REACH 2018: renal oxygen delivery is maintained during early acclimatization to 4,330 m

Early acclimatization to high altitude is characterized by various respiratory, hematological, and cardiovascular adaptations that serve to restore oxygen delivery to tissue. However, less is understood about renal function and the role of renal oxygen delivery (RDO₂) during high altitude acclimatization. We hypothesized that 1) RDO₂ would be reduced after 12 h of high altitude exposure (high altitude day 1) but restored to sea level values after 1 wk (high altitude day 7) and 2) RDO₂ would be associated with renal reactivity, an index of acid-base compensation at high altitude. Twenty-four healthy lowlander participants were tested at sea level (344 m, Kelowna, BC, Canada) and on day 1 and day 7 at high altitude (4,330 m, Cerro de Pasco, Peru). Cardiac output, renal blood flow, and arterial and venous blood sampling for renin-angiotensin-aldosterone system hormones and NH₂-terminal pro-B-type natriuretic peptides were collected at each time point. Renal reactivity was calculated as follows: (Δarterial bicarbonate)/(Δarterial Pco₂) between sea level and high altitude day 1 and sea level and high altitude day 7. The main findings were that 1) RDO₂ was initially decreased at high altitude compared with sea level (ΔRDO₂: -22 ± 17%, P < 0.001) but was restored to sea level values on high altitude day 7 (ΔRDO₂: -6 ± 14%, P = 0.36). The observed improvements in RDO₂ resulted from both changes in renal blood flow (Δ from high altitude day 1: +12 ± 11%, P = 0.008) and arterial oxygen content (Δ from high altitude day 1: +44.8 ± 17.7%, P = 0.006) and 2) renal reactivity was positively correlated with RDO₂ on high altitude day 7 (r = 0.70, P < 0.001) but not high altitude day 1 (r = 0.26, P = 0.29). These findings characterize the temporal responses of renal function during early high altitude acclimatization and the influence of RDO₂ in the regulation of acid-base balance.

Renal Physiological Adaptation to High Altitude: A Systematic Review

Background: Under normal physiological conditions, renal tissue oxygen is tightly regulated. At high altitude, a physiological challenge is imposed by the decrease in atmospheric oxygen. At the level of the kidney, the physiological adaptation to high altitude is poorly understood, which might relate to different integrated responses to hypoxia over different time domains of exposure. Thus, this systematic review sought to examine the renal physiological adaptation to high altitude in the context of the magnitude and duration of exposure to high altitude in the healthy kidney model.

Methods: To conduct the review, three electronic databases were examined: OVID, PubMed, and Scopus. Search terms included: Altitude, renal, and kidney. The broad, but comprehensive search, retrieved 1,057 articles published between 1997 and April 2020. Fourteen studies were included in the review.

Results: The inconsistent effect of high altitude on renal hemodynamic parameters (glomerular filtration rate, renal blood flow, and renal plasma flow), electrolyte balance, and renal tissue oxygen is difficult to interpret; however, the data suggest that the nature and extent of renal physiological adaptation at high altitude appears to be related to the magnitude and duration of the exposure.

Conclusion: It is clear that renal physiological adaptation to high altitude is a complex process that is not yet fully understood. Further research is needed to better understand the renal physiological adaptation to hypoxia and how renal oxygen homeostasis and metabolism is defended during exposure to high altitude and affected as a long-term consequence of renal adaptation at high altitude.

Impaired renal endothelial nitric oxide synthase and reticulocyte production as modulators of hypertension induced by rHuEPO in the rat

Our aim was to study the effect of a broad range of recombinant human erythropoietin (rHuEPO) doses on hematological and biochemical parameters, blood pressure (BP), renal function and damage in the rat, focusing on endothelial nitric oxide synthase (eNOS) and hypoxia-inducible factors (HIFs). Male Wistar rats were divided in 5 groups receiving different doses of rHuEPO (100, 200, 400 and 600IU/kg body weight (BW)/week) and saline solution (control), during 3weeks. Blood and 24h urine were collected to perform hematological and biochemical analysis. BP was measured by the tail-cuff method. Kidney tissue was collected to mRNA and protein expression assays and to characterize renal lesions. A dose-dependent increase in red blood cells count, hematocrit and hemoglobin levels was found with rHuEPO therapy, in rHuEPO200, rHuEPO400 and rHuEPO600 groups. Increased reticulocyte count was found in rHuEPO400 and rHuEPO600 groups. BP raised in all groups receiving rHuEPO. The rHuEPO200 and rHuEPO600 groups presented increased kidney protein levels of HIF2α, a reduction in kidney protein levels of eNOS, and the highest grade of vascular and tubular renal lesions. Our study showed that rHuEPO-induced hypertension is present before significant hematological changes occur and, therefore, might involve direct (renal) and indirect (hematological) effects, which varies according to the dose used. The presence of renal hypoxia reduces eNOS activity. Excessive erythrocytosis increases blood hyperviscosity, which can be modulated by an increase in reticulocytes. Hypertension leads to early renal damage without alterations in traditional markers of renal function, thus underestimating the serious adverse effects and risks.

Strain variation in the adaptation of C57Bl6 and BALBc mice to chronic hypobaric hypoxia

The interplay of environmental and genetic factors may lead to a spectrum of physiological and behavioral outcomes. How environmental stress factors interact with the diverse mouse genomes is still poorly understood and elucidating the underlying interactions requires specific stress models that can target integrated physiological systems. Here, we employ behavioral tests and whole-body plethysmography to examine the effects of 12 weeks of simulated high altitude (HA) exposure on two inbred mouse strains, BALBc and C57Bl6. We find that HA induced- weight loss recovers at significantly different rates in these two strains. Even at 12 weeks, however, both strains fail to reach body weight levels of controls. Performance on two motor tasks, rotarod and treadmill, improve with HA exposure but more prominently in BALBc mice. Whole-body plethysmography outcomes indicate that compensation to chronic HA includes increased respiratory frequencies and tidal volumes in both strains. However, the effects on tidal volume are significantly greater in BALBc mice and showed a biphasic course. Whole- body metabolic rates are also increased in both strains with prolonged HA exposure, but were more pronounced in BALBc mice suggestive of less successful adaptation in this strain. These adaptations occur in the absence of gross pathological changes in all major organs. Together these results indicate that chronic HA exposure results in environmental stressors that impact the specific physiological responses of BALBc more than C57Bl6 mice. Thus, these strains provide a promising platform for investigating how genetic backgrounds can differentially reinforce the effects of long-lasting environmental stressors and their potential to interact with psychological stressors.

The exceptional and far-flung manifestations of heart failure in Eisenmenger syndrome

Dramatic advances in the diagnosis and treatment of congenital heart disease (CHD), the most common inborn defect, has resulted in a growing population of adults with CHD. Eisenmenger syndrome (ES) represents the extreme form of pulmonary arterial hypertension associated with CHD, characterized by markedly increased pulmonary vascular resistance with consequently reversed or bidirectional shunting. While ES is a direct consequence of a heart defect, it is a fundamentally multisystem syndrome with wide-ranging clinical manifestations. The introduction of targeted pulmonary hypertension therapies aimed has subtly shifted clinical focus from preventing iatrogenic and other adverse events toward cautious therapeutic activism.

Effects of exposure to a simulated altitude of 5500 m on energy metabolic pathways in rats

We examined the effect of exposure to 5500 m on three closely related metabolic pathways: anaerobic glycolysis, the pentose phosphate shunt (PPS), and fatty acid metabolism. Rats were exposed to simulated altitude of 5500 m for up to 3 months. The maximal rate of lactate production in tissue homogenates, tissue lactic acid dehydrogenase and blood lactate levels were measured to evaluate the capacity for anaerobic glycolysis. The uptake of 14C-1-palmitate, oxidation of 14C-1-palmitate to 14CO2, incorporation of 14C-1-palmitate into tissue lipids, plasma and tissue free fatty acids (FFA) levels and total lipid contents were measured to assess the magnitude of lipid metabolism. Activities of glucose-6-phosphate dehydrogenase (G-6-PD) and 6-phophogluconate dehydrogenase (6-PGD) in the PPS pathway were measured to assess the capacity to generate reducing power. Acute and chronic hypoxia did not affect most of the measurements of anaerobic glycolysis, but depressed lactate production in liver and kidney. Chronic hypoxia enhanced all aspects of lipid metabolism in liver and enhanced the uptake and oxidation to CO2 of palmitate in skeletal muscle. Chronic hypoxia did not alter the activity of the G-6-PD in any tissue studied, but the activity of 6-PGD was depressed in heart, kidney, thymus and adrenal gland. The lack of major changes in the capacities of anaerobic glycolytic pathways and the activities of the PPS dehydrogenases is consistent with the maintenance of normal aerobic metabolism in rats at 5500 m. We found no evidence that anaerobic metabolic processes were upregulated to sustain energy consumption during chronic hypoxia. On the other hand, enhanced fatty acid metabolism may spare carbohydrate for metabolic fuel under conditions of extreme hypoxic limitation.

Vascular Remodeling Marks Tumors That Recur During Chronic Suppression of Angiogenesis

The potential for avoiding acquired resistance to therapy has been proposed as one compelling theoretical advantage of antiangiogenic therapy based on the normal genetic status of the target vasculature. However, previous work has demonstrated that tumors may resume growth after initial inhibition if antiangiogenic blockade is continued for an extended period. The mechanisms of this recurrent growth are unclear. In these studies, we characterized molecular changes in vasculature during apparent resumption of xenograft growth after initial inhibition by vascular endothelial growth factor blockade, “metronome” topotecan chemotherapy, and combined agents in a xenograft murine model of human Wilms' tumor. Tumors that grew during antiangiogenic blockade developed as viable clusters surrounding strikingly remodeled vessels. These vessels displayed significant increases in diameter and active proliferation of vascular mural cells and expressed platelet-derived growth factor-B, a factor that functions to enhance vascular integrity via stromal cell recruitment. In addition, remodeled vessels were marked by expression of ephrinB2, required for proper assembly of stromal cells into vasculature. Thus, enhanced vascular stability appears to characterize tumor vessel response to chronic antiangiogenesis, features that potentially support increased perfusion and recurrent tumor growth.

Microvascular and tubulointerstitial injury associated with chronic hypoxia-induced hypertension

BACKGROUND

Rats submitted to chronic hypoxia develop hypertension that persists despite cessation of the hypoxia or correction of the hematocrit. We examined whether chronic hypoxia might induce subtle renal injury since studies in other animal models of hypertension suggest this may cause persistent hypertension.

METHODS

Chronic hypoxia was induced in rats by placement in a hypobaric chamber for up to 24 days. Blood pressure and kidney biopsies were performed at baseline, 6 hours, 24 hours, and 24 days of hypoxia.

RESULTS

Chronic hypoxia induced hypertension and erythrocytosis at 24 days. Acute hypoxia was associated with endothelial cell swelling in arterioles (6 and 24 hours), followed by thickening of the arterioles at 24 days. Subtle tubulointerstitial injury and inflammation occurred and was progressive. The influx of macrophages increased steadily over the 24 days and was associated with a progressive increase in interstitial collagen III deposition. Hypoxia was associated with increased tubular expression of osteopontin as early as 6 hours, the same period when an increase of proximal tubular cell proliferation occurred. Interstitial cell proliferation peaked twice, at 6 hours and at 24 days. Glomerular hypertrophy was manifest at 24 days.

CONCLUSION

Both afferent arteriolar disease and subtle tubulointerstitial inflammation and injury occur early in hypoxic rats. These changes may predispose these animals to persistent hypertension.

Renal vasodilatory influence of endogenous carbon monoxide in chronically hypoxic rats

Chronic hypoxia (CH) attenuates systemic vasoconstriction to a variety of agonists in conscious rats. Recent evidence suggests that similarly diminished responses to vasoconstrictors in aortic rings from CH rats may be due to increased endothelial heme oxygenase (HO) activity and enhanced production of the vasodilator carbon monoxide (CO). Thus we hypothesized that a hypoxia-induced increase in HO activity is responsible for decreased vasoconstrictor responsiveness observed in conscious CH rats. CH (4 wk at 0.5 atm) and control rats were renal denervated and instrumented for the measurement of renal blood flow (RBF) and blood pressure. First, renal vasoconstrictor responses to graded intravenous infusion of phenylephrine (PE) were assessed in conscious rats. CH rats demonstrated significantly diminished renal vasoconstrictor responses to PE compared with control responses that persisted even with acute restoration of normoxia. In additional experiments, CH rats exhibited increased renal vascular resistance and decreased RBF in response to the HO inhibitor zinc protoporphyrin IX (11 micromol/kg iv), whereas renal hemodynamics were unaffected by the inhibitor in control animals. Furthermore, we demonstrated greater HO enzyme activity in renal tissue from CH rats compared with controls. These data suggest that enhanced HO activity contributes a tonic vasodilatory influence in the renal vasculature of CH rats that may be responsible for the diminished sensitivity to vasoconstrictor agonists observed under these conditions.

Pinniped diving response mechanism and evolution: a window on the paradigm of comparative biochemistry and physiology

Starting even before the end of World War II, the discipline of comparative physiology and biochemistry experienced a period of unprecedented growth and development that pioneers in this field thought would never end. However, by the mid-1970s many of the major mechanistic problems in the field were pretty well understood in principle, and by the mid-1980s workers in the field widely recognized that the discipline was at the point of diminishing returns. One response to this was disillusionment, which turned out to be premature because the field was already absorbing molecular biology tools which has now caused a kind of renaissance in mechanistic physiology studies. The second major response to the sense of disillusionment led to a search for new approaches, and out of this endeavor the newly rejuvenated field of evolutionary physiology arose, and this research area too is now in a growth phase. These general patterns of growth and development in our discipline as a whole are particularly clearly evident in the field of aquatic mammals and birds. Between the 1930s and the 1970s, studies of diving physiology and biochemistry made great progress in mechanistically explaining the basic diving response of aquatic mammals and birds. Key components of the diving response (apnea, bradycardia, peripheral vasoconstriction, redistribution of cardiac output) were found in essentially all species analyzed and were generally taken to be biological adaptations. By the mid-1970s, this approach to unraveling the diving response had run 'out of steam' and was in conceptual stasis. The breakthrough which gave renewal to the field at this time was the development of microprocessor based monitoring of diving animals in their natural environments, which led to a flurry of studies mostly confirming the essential outlines of the diving response based upon laboratory studies and firmly placing it into a proper biological context, underlining its plasticity and species specificities. Now as we begin a new millenium, despite ever more detailed field monitoring of physiology, behavior and ecology, studies aimed at improving understanding of physiological mechanisms in diving are again approaching a point of diminishing returns. To avoid another conceptual stasis, what seems required are new initiatives which may arise from two differing approaches. The first is purely experimental, relying on magnetic resonance imaging (MRI) and spectroscopy (MRS) to expand the framework of the original 'diving response' concept. The second, evolutionary study of the diving response, is synthetic, linked to both field and laboratory studies. To date the evolution of the diving response has only been analyzed in pinnipeds and from these studies two kinds of patterns have emerged. (1) Some physiological and biochemical characters, required and used in diving animals, are highly conserved not only in pinnipeds but in all vertebrates; these traits are necessarily similar in all pinnipeds and include diving apnea, bradycardia, tissue specific hypoperfusion, and hypometabolism of hypoperfused tissues. (2) Another group of functionally linked characters are more malleable and include (i) spleen mass, (ii) blood volume, and (iii) hemoglobin (Hb) pool size. Increases in any of these traits (or in a morphological character, body size) improve diving capacity. Assuming that conserved physiological function means conserved sequences in specific genes and their products (and that evolving function requires changes in such sequences), it is possible to rationalize both the above trait categories in pinniped phylogeny. However, it is more difficult for molecular evolution theory to explain how complex regulatory systems like those involved in bradycardia and peripheral vasoconstriction remain the same through phylogenetic time than it is to explain physiological change driven by directional natural selection.