Why is erythropoietin made in the kidney? The kidney functions as a 'critmeter' to regulate the hematocrit

Jian-Pi-Yi-Shen formula ameliorates renal fibrosis-induced anemia in rats with chronic kidney disease

ETHNOPHARMACOLOGICAL RELEVANCE

Jian-Pi-Yi-Shen (JPYS) formula is an effective herbal therapy against renal injury, and JPYS has been clinically applied to ameliorate chronic kidney disease (CKD) and CKD-associated anemia. Increasing evidence supports the link between renal fibrosis and anemia in CKD. JPYS possessed anti-fibrosis effects in experimental CKD. Nevertheless, research on the mechanisms of JPYS in ameliorating renal anemia (RA) through suppressing renal fibrosis remains to be clarified.

AIM OF THE STUDY

Our study here was carried out to investigate the mechanisms of JPYS in protecting against RA.

MATERIALS AND METHODS

An adenine-induced anemia model in rats with CKD at three different time points was established, and bio-samples taken from each group were analyzed. Biochemical analysis was employed to detect kidney function and hematological parameters. Masson staining was used to evaluate renal fibrosis of rats. Western blot and immunohistochemistry were utilized to evaluate the expressions of fibrotic markers, erythropoietin (EPO) and hypoxia inducible factor-2α (HIF-2α) in the kidneys of rats. Subsequently, transcriptomic analysis was conducted to disclose the possible mechanisms of JPYS in treating RA. Finally, the expression levels of key targets were analyzed and validated by using Western blot and enzyme-linked immunosorbent assay (ELISA).

RESULTS

JPYS treatment improved kidney function, suppressed renal fibrosis and enhanced hematological parameters in CKD rats. Moreover, JPYS treatment restored the increased expression levels of fibrotic markers and the declined EPO with time dependence. In parallel, data indicated JPYS treatment stimulated the translocation of HIF-2α into nucleus in the renal interstitium and thus promoted the expression of EPO. Transcriptomic profiling disclosed that activations of both nuclear factor kappa B (NF-κB) and transforming growth factor-β (TGF-β)/Smad pathways were closely associated with RA. Ultimately, experimental validation results presented that the increased expressions of target proteins from the above-mentioned two pathways in the kidneys were decreased significantly after JPYS treatment.

CONCLUSION

Our findings suggest that JPYS may improve RA by alleviating renal fibrosis, and the mechanisms of which involve in inhibiting the NF-κB and TGF-β/Smad pathways.

Using the daily rate of rise in hemoglobin S to manage RBC depletion/exchange treatment in sickle cell disease

BACKGROUND

Red blood cell exchange is often used prophylactically in patients with sickle cell disease, with the goal to maintain hemoglobin S (HbS) below a target threshold level. We reviewed whether the daily "rate of rise" (RoR) in HbS that occurs between procedures can be used for patient management. For some patients not achieving their HbS goals despite efficient exchanges, the post-procedure hematocrit (Hct) target is increased to potentially suppress HbS production. This case series explores the utility of this approach, other clinical uses of the daily RoR in HbS, and the factors that influence it.

STUDY DESIGN AND METHODS

A total of 660 procedures from 24 patients undergoing prophylactic RBC depletion/exchange procedures were included. Laboratory values and clinical parameters were collected and used to calculate the daily RoR in HbS. Factors such as Hct or medications that might influence the RoR in HbS were evaluated.

RESULTS

The RoR in HbS varied widely between patients but remained relatively stable within individuals. Surprisingly, this value was not significantly influenced by changes in post-procedure Hct or concurrent hydroxyurea use. A patient's average RoR in HbS effectively predicted the pre-procedure HbS at the following visit (R2 = 0.65).

DISCUSSION

The RoR in HbS is a relatively consistent parameter for individual patients that is unaffected by medication use or procedural Hct targets and may be useful in determining intervals between procedures.

The effect of chronic habitual exercise on oxygen carrying capacity and blood compartment volumes in older adults

Absolute total hemoglobin mass (tHbmass) and blood compartment volumes are often considered to be higher in endurance athletes compared with nonathletes, yet little data support a fitness effect in older age. Therefore, we measured tHbmass and blood compartment volumes (carbon monoxide rebreathing) in 77 healthy individuals (23% female; aged, 60-87 yr). Participants were recruited into groups based upon their lifelong (>25 yr) exercise "dose": 1) 15 sedentary individuals, <2 sessions/wk; 2) 25 casual exercisers, 2-3 sessions/wk; 3) 24 committed exercisers, 4-5 sessions/wk; and 4) 13 competitive Masters athletes, 6-7 sessions/wk, plus regular competitions. Absolute (L/min) and relative (mL/kg/min) V̇o2peak were higher with increasing exercise "dose" (P = 0.0005 and P < 0.0001, respectively). Hemoglobin concentration, hematocrit, and absolute tHbmass and blood compartment volumes were not significantly different between groups (all, P > 0.1328). When scaled to body mass, tHbmass (Sedentary, 9.2 ± 1.7 mL/kg; Casual, 9.2 ± 1.3; Committed, 10.2 ± 1.4; Competitive, 11.5 ± 1.4, ANOVA P < 0.0001) and blood volume were significantly different between groups [Sedentary, 63.4 (59.2-68.5) mL/kg; Casual, 67.3 (64.4-72.6); Committed, 73.5 (67.5-80.2); Competitive, 83.4 (78.9-88.6), ANOVA P < 0.0001], whereby all values were highest in Masters athletes. However, when scaled to fat-free mass (FFM), tHbmass and blood compartment volumes were greater in Competitive compared with Casual exercisers (all, P < 0.0340) and tHbmass and erythrocyte volume were also higher in Committed compared with Casual exercisers (both, P < 0.0134). In conclusion, absolute tHbmass and blood compartment volumes are not different between groups, with dose-dependent differences only among exercisers when scaled for FFM, with the highest tHbmass and blood compartment volumes in competitive Masters athletes.NEW & NOTEWORTHY We observed that absolute oxygen carrying capacity (total hemoglobin mass, tHbmass) and blood compartment volumes were not associated with lifelong exercise dose. However, hematological adaptations associated with lifelong habitual exercise are only present among exercisers, whereby competitive Masters athletes have a greater oxygen carrying capacity (tHbmass) and expanded blood compartment volumes when scaled to fat-free mass.

Anemia and Hypoxia Impact on Chronic Kidney Disease Onset and Progression: Review and Updates

Chronic kidney disease (CKD) is caused by hypoxia in the renal tissue, leading to inflammation and increased migration of pathogenic cells. Studies showed that leukocytes directly sense hypoxia and respond by initiating gene transcription, encoding the 2-integrin adhesion molecules. Moreover, other mechanisms participate in hypoxia, including anemia. CKD-associated anemia is common, which induces and worsens hypoxia, contributing to CKD progression. Anemia correction can slow CKD progression, but it should be cautiously approached. In this comprehensive review, the underlying pathophysiology mechanisms and the impact of renal tissue hypoxia and anemia in CKD onset and progression will be reviewed and discussed in detail. Searching for the latest updates in PubMed Central, Medline, PubMed database, Google Scholar, and Google search engines were conducted for original studies, including cross-sectional studies, cohort studies, clinical trials, and review articles using different keywords, phrases, and texts such as "CKD progression, anemia in CKD, CKD, anemia effect on CKD progression, anemia effect on CKD progression, and hypoxia and CKD progression". Kidney tissue hypoxia and anemia have an impact on CKD onset and progression. Hypoxia causes nephron cell death, enhancing fibrosis by increasing interstitium protein deposition, inflammatory cell activation, and apoptosis. Severe anemia correction improves life quality and may delay CKD progression. Detection and avoidance of the risk factors of hypoxia prevent recurrent acute kidney injury (AKI) and reduce the CKD rate. A better understanding of kidney hypoxia would prevent AKI and CKD and lead to new therapeutic strategies.

Exercise and Experiments of Nature

In this article, we highlight the contributions of passive experiments that address important exercise-related questions in integrative physiology and medicine. Passive experiments differ from active experiments in that passive experiments involve limited or no active intervention to generate observations and test hypotheses. Experiments of nature and natural experiments are two types of passive experiments. Experiments of nature include research participants with rare genetic or acquired conditions that facilitate exploration of specific physiological mechanisms. In this way, experiments of nature are parallel to classical "knockout" animal models among human research participants. Natural experiments are gleaned from data sets that allow population-based questions to be addressed. An advantage of both types of passive experiments is that more extreme and/or prolonged exposures to physiological and behavioral stimuli are possible in humans. In this article, we discuss a number of key passive experiments that have generated foundational medical knowledge or mechanistic physiological insights related to exercise. Both natural experiments and experiments of nature will be essential to generate and test hypotheses about the limits of human adaptability to stressors like exercise. © 2023 American Physiological Society. Compr Physiol 13:4879-4907, 2023.

Cysteine as a Multifaceted Player in Kidney, the Cysteine-Related Thiolome and Its Implications for Precision Medicine

In this review encouraged by original data, we first provided in vivo evidence that the kidney, comparative to the liver or brain, is an organ particularly rich in cysteine. In the kidney, the total availability of cysteine was higher in cortex tissue than in the medulla and distributed in free reduced, free oxidized and protein-bound fractions (in descending order). Next, we provided a comprehensive integrated review on the evidence that supports the reliance on cysteine of the kidney beyond cysteine antioxidant properties, highlighting the relevance of cysteine and its renal metabolism in the control of cysteine excess in the body as a pivotal source of metabolites to kidney biomass and bioenergetics and a promoter of adaptive responses to stressors. This view might translate into novel perspectives on the mechanisms of kidney function and blood pressure regulation and on clinical implications of the cysteine-related thiolome as a tool in precision medicine.

Metformin and the Risk of Anemia of Advanced Chronic Kidney Disease in Patients With Type 2 Diabetes Mellitus

This retrospective cohort study determines whether metformin monotherapy or combination therapies can decrease anemia risk in the progress of advanced chronic kidney disease for patients with type 2 diabetes mellitus. The data set was obtained from the National Health Insurance Research Database, containing 1 million randomly selected beneficiaries. After matching, 9303 pairs (1:1) of metformin users and nonusers were acquired. Every patient was individually recorded from 1997 to 2012 to identify anemia incidence (hemoglobin <9 gm/dL). Cox regression models were used to compute hazard ratios and 95% confidence intervals (CIs). There were 305 (0.7%) and 76 (0.8%) erythropoietin-stimulating agent cases in the metformin and non-metformin cohorts over a mean follow-up period of 6.8 and 5.6 years. After matching, the use of metformin decreased the risk of usage of erythropoietin-stimulating agents with an adjusted hazard ratio of 0.76 (95%CI, 0.45-1.29) for dosage of <357 g to 0.30 (95%CI, 0.17-0.56) for >1368 g. The combination of metformin and dipeptidyl peptidase-4 inhibitors decreased with a hazard ratio of 0.42 (95%CI, 0.18-0.99), compared to metformin alone. Metformin combined with dipeptidyl peptidase-4 inhibitors is superior to metformin monotherapy or non-metformin antidiabetic therapies for reducing the risk of anemia in the progress of advanced chronic kidney disease among patients with type 2 diabetes.

Global REACH 2018: the adaptive phenotype to life with chronic mountain sickness and polycythaemia

KEY POINTS

Humans suffering from polycythaemia undergo multiple circulatory adaptations including changes in blood rheology and structural and functional vascular adaptations to maintain normal blood pressure and vascular shear stresses, despite high blood viscosity. During exercise, several circulatory adaptations are observed, especially involving adrenergic and non-adrenergic mechanisms within non-active and active skeletal muscle to maintain exercise capacity, which is not observed in animal models. Despite profound circulatory stress, i.e. polycythaemia, several adaptations can occur to maintain exercise capacity, therefore making early identification of the disease difficult without overt symptomology. Pharmacological treatment of the background heightened sympathetic activity may impair the adaptive sympathetic response needed to match local oxygen delivery to active skeletal muscle oxygen demand and therefore inadvertently impair exercise capacity.

ABSTRACT

Excessive haematocrit and blood viscosity can increase blood pressure, cardiac work and reduce aerobic capacity. However, past clinical investigations have demonstrated that certain human high-altitude populations suffering from excessive erythrocytosis, Andeans with chronic mountain sickness, appear to have phenotypically adapted to life with polycythaemia, as their exercise capacity is comparable to healthy Andeans and even with sea-level inhabitants residing at high altitude. By studying this unique population, which has adapted through natural selection, this study aimed to describe how humans can adapt to life with polycythaemia. Experimental studies included Andeans with (n = 19) and without (n = 17) chronic mountain sickness, documenting exercise capacity and characterizing the transport of oxygen through blood rheology, including haemoglobin mass, blood and plasma volume and blood viscosity, cardiac output, blood pressure and changes in total and local vascular resistances through pharmacological dissection of α-adrenergic signalling pathways within non-active and active skeletal muscle. At rest, Andeans with chronic mountain sickness had a substantial plasma volume contraction, which alongside a higher red blood cell volume, caused an increase in blood viscosity yet similar total blood volume. Moreover, both morphological and functional alterations in the periphery normalized vascular shear stress and blood pressure despite high sympathetic nerve activity. During exercise, blood pressure, cardiac work and global oxygen delivery increased similar to healthy Andeans but were sustained by modifications in both non-active and active skeletal muscle vascular function. These findings highlight widespread physiological adaptations that can occur in response to polycythaemia, which allow the maintenance of exercise capacity.

High-Altitude Adaptation: Mechanistic Insights from Integrated Genomics and Physiology

Population genomic analyses of high-altitude humans and other vertebrates have identified numerous candidate genes for hypoxia adaptation, and the physiological pathways implicated by such analyses suggest testable hypotheses about underlying mechanisms. Studies of highland natives that integrate genomic data with experimental measures of physiological performance capacities and subordinate traits are revealing associations between genotypes (e.g., hypoxia-inducible factor gene variants) and hypoxia-responsive phenotypes. The subsequent search for causal mechanisms is complicated by the fact that observed genotypic associations with hypoxia-induced phenotypes may reflect second-order consequences of selection-mediated changes in other (unmeasured) traits that are coupled with the focal trait via feedback regulation. Manipulative experiments to decipher circuits of feedback control and patterns of phenotypic integration can help identify causal relationships that underlie observed genotype–phenotype associations. Such experiments are critical for correct inferences about phenotypic targets of selection and mechanisms of adaptation.

Physiological Genomics of Adaptation to High-Altitude Hypoxia

Population genomic studies of humans and other animals at high altitude have generated many hypotheses about the genes and pathways that may have contributed to hypoxia adaptation. Future advances require experimental tests of such hypotheses to identify causal mechanisms. Studies to date illustrate the challenge of moving from lists of candidate genes to the identification of phenotypic targets of selection, as it can be difficult to determine whether observed genotype-phenotype associations reflect causal effects or secondary consequences of changes in other traits that are linked via homeostatic regulation. Recent work on high-altitude models such as deer mice has revealed both plastic and evolved changes in respiratory, cardiovascular, and metabolic traits that contribute to aerobic performance capacity in hypoxia, and analyses of tissue-specific transcriptomes have identified changes in regulatory networks that mediate adaptive changes in physiological phenotype. Here we synthesize recent results and discuss lessons learned from studies of high-altitude adaptation that lie at the intersection of genomics and physiology.

Angiotensin-converting enzyme 2 and COVID-19 in cardiorenal diseases

The rapid spread of the novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has brought into focus the key role of angiotensin-converting enzyme 2 (ACE2), which serves as a cell surface receptor required for the virus to enter cells. SARS-CoV-2 can decrease cell surface ACE2 directly by internalization of ACE2 bound to the virus and indirectly by increased ADAM17 (a disintegrin and metalloproteinase 17)-mediated shedding of ACE2. ACE2 is widely expressed in the heart, lungs, vasculature, kidney and the gastrointestinal (GI) tract, where it counteracts the deleterious effects of angiotensin II (AngII) by catalyzing the conversion of AngII into the vasodilator peptide angiotensin-(1-7) (Ang-(1-7)). The down-regulation of ACE2 by SARS-CoV-2 can be detrimental to the cardiovascular system and kidneys. Further, decreased ACE2 can cause gut dysbiosis, inflammation and potentially worsen the systemic inflammatory response and coagulopathy associated with SARS-CoV-2. This review aims to elucidate the crucial role of ACE2 both as a regulator of the renin-angiotensin system and a receptor for SARS-CoV-2 as well as the implications for Coronavirus disease 19 and its associated cardiovascular and renal complications.

The overlooked significance of plasma volume for successful adaptation to high altitude in Sherpa and Andean natives

In contrast to Andean natives, high-altitude Tibetans present with a lower hemoglobin concentration that correlates with reproductive success and exercise capacity. Decades of physiological and genomic research have assumed that the lower hemoglobin concentration in Himalayan natives results from a blunted erythropoietic response to hypoxia (i.e., no increase in total hemoglobin mass). In contrast, herein we test the hypothesis that the lower hemoglobin concentration is the result of greater plasma volume, rather than an absence of increased hemoglobin production. We assessed hemoglobin mass, plasma volume and blood volume in lowlanders at sea level, lowlanders acclimatized to high altitude, Himalayan Sherpa, and Andean Quechua, and explored the functional relevance of volumetric hematological measures to exercise capacity. Hemoglobin mass was highest in Andeans, but also was elevated in Sherpa compared with lowlanders. Sherpa demonstrated a larger plasma volume than Andeans, resulting in a comparable total blood volume at a lower hemoglobin concentration. Hemoglobin mass was positively related to exercise capacity in lowlanders at sea level and in Sherpa at high altitude, but not in Andean natives. Collectively, our findings demonstrate a unique adaptation in Sherpa that reorientates attention away from hemoglobin concentration and toward a paradigm where hemoglobin mass and plasma volume may represent phenotypes with adaptive significance at high altitude.

Erythropoietin inhibits SGK1-dependent TH17 induction and TH17-dependent kidney disease

IL-17-producing CD4+ cells (TH17) are pathogenically linked to autoimmunity including to autoimmune kidney disease. Erythropoietin's (EPO) newly recognized immunoregulatory functions and its predominant intra-renal source suggested that EPO physiologically regulates TH17 differentiation, thereby serving as a barrier to the development of autoimmune kidney disease. Using in vitro studies of human and murine cells and in vivo models, we show that EPO ligation of its receptor (EPO-R) on CD4+ T cells directly inhibits TH17 generation and promotes trans-differentiation of TH17 into IL-17-FOXP3+CD4+ T cells. Mechanistically, EPO/EPO-R ligation abrogates upregulation of SGK1 gene expression and blocks p38 activity to prevent SGK1 phosphorylation, thereby inhibiting RORC-mediated transcription of IL-17 and IL-23 receptor genes. In a murine model of TH17-dependent aristolochic acid (ArA)-induced, interstitial kidney disease associated with reduced renal EPO production, we demonstrate that transgenic EPO overexpression or recombinant EPO (rEPO) administration limits TH17 formation and clinical/histological disease expression. EPO/EPO-R ligations on CD4+ T cells abrogate, while absence of T cell-expressed EPO-R augments, TH17 induction and clinical/histological expression of pristane-induced glomerulonephritis (associated with decreased intrarenal EPO). rEPO prevents spontaneous glomerulonephritis and TH17 generation in MRL-lpr mice. Together, our findings indicate that EPO physiologically and therapeutically modulate TH17 cells to limit expression of TH17-associated autoimmune kidney disease.

Association of Estimated Glomerular Filtration Rate with Hemoglobin Level in Korean Adults: The 2010-2012 Korea National Health and Nutrition Examination Survey

PURPOSE

Little is known about anemia in patients with early renal dysfunction. We aimed to investigate the association of hemoglobin level and anemia prevalence with estimated glomerular filtration rate (eGFR) decline using a nation-wide representative sample of the adult Korean population.

METHODS

In total, 17,373 participants (7,296 men; weighted n = 18,330,187; mean age, 44.2±0.3 years; 9,886 women, weighted n = 18,317,454; mean age, 46.9±0.3 years) were included. eGFR was divided into 5 groups: Group 1, ≥105; Group 2, 90-104; 75-89; Group 4, 60-74; and Group 5, <60 mL/min/1.73m2.

RESULTS

The weighted anemia prevalence rates were 2.6% in men and 12.8% in women. In men, the weighted hemoglobin level increased with a decrease in eGFR; this value peaked at an eGFR of 60-89 mL/min/1.73m2 and decreased thereafter at an eGFR of <60 mL/min/1.73m2 (15.19±0.03, 15.35±0.03, 15.53±0.03, 15.52±0.06, and 14.90±0.12 g/dL from Groups 1 to 5) after adjustment for age, college graduation, cancer history, current smoking, waist circumference, serum cholesterol level, serum triglyceride level, and diastolic blood pressure. In women, the weighted hemoglobin level increased with a decrease in eGFR; this value peaked with an eGFR of 75-89 mL/min/1.73m2 and decreased thereafter (12.90±0.03, 13.08±0.02, 13.20±0.04, 13.14±0.05, and 12.47±0.11 g/dL from Groups 1 to 5) after adjustment for menstruation, pregnancy, estrogen replacement, and the above-mentioned variables. In both sexes, the weighted prevalence of anemia with an eGFR of 60-104 mL/min/1.73m2 was significantly lower than that with an eGFR of ≥105 mL/min/1.73m2 (men, 3.2±0.4%, 1.9±0.3%, 1.8±0.3%, 2.0±0.9%, and 18.1±3.1%; women, 14.0±0.8%, 11.2±0.7%, 10.5±1.0%, 13.2±1.6%, and 32.3±3.2% from Groups 1 to 5).

CONCLUSIONS

We noted a compensatory increase in the hemoglobin level with a minor decline in kidney function (in the range of eGFR ≥60 mL/min/1.73m2) prior to a marked decrease in hemoglobin level with severe renal dysfunction.

Tubulovascular cross-talk by vascular endothelial growth factor a maintains peritubular microvasculature in kidney

Vascular endothelial growth factor A (VEGFA) production by podocytes is critical for glomerular endothelial health. VEGFA is also expressed in tubular epithelial cells in kidney; however, its physiologic role in the tubule has not been established. Using targeted transgenic mouse models, we found that Vegfa is expressed by specific epithelial cells along the nephron, whereas expression of its receptor (Kdr/Vegfr2) is largely restricted to adjacent peritubular capillaries. Embryonic deletion of tubular Vegfa did not affect systemic Vegfa levels, whereas renal Vegfa abundance was markedly decreased. Excision of Vegfa from renal tubules resulted in the formation of a smaller kidney, with a striking reduction in the density of peritubular capillaries. Consequently, elimination of tubular Vegfa caused pronounced polycythemia because of increased renal erythropoietin (Epo) production. Reducing hematocrit to normal levels in tubular Vegfa-deficient mice resulted in a markedly augmented renal Epo production, comparable with that observed in anemic wild-type mice. Here, we show that tubulovascular cross-talk by Vegfa is essential for maintenance of peritubular capillary networks in kidney. Disruption of this communication leads to increased renal Epo production and resulting polycythemia, presumably to counterbalance microvascular losses.

Stability of tissue PO2 in the face of altered perfusion: a phenomenon specific to the renal cortex and independent of resting renal oxygen consumption

1. Oxygen tension (PO(2)) in renal cortical tissue can remain relatively constant when renal blood flow changes in the physiological range, even when changes in renal oxygen delivery (DO(2)) and oxygen consumption (VO(2)) are mismatched. In the current study, we examined whether this also occurs in the renal medulla and skeletal muscle, or if it is an unusual property of the renal cortex. We also examined the potential for dysfunction of the mechanisms underlying this phenomenon to contribute to kidney hypoxia in disease states associated with increased renal VO(2) . 2. In both the kidney and hindlimb of pentobarbitone anaesthetized rabbits, whole organ blood flow was reduced by intra-arterial infusion of angiotensin-II and increased by acetylcholine infusion. In the kidney, this was carried out before and during renal arterial infusion of the mitochondrial uncoupler, 2,4-dinitrophenol (DNP), or its vehicle. 3. Angiotensin-II reduced renal (-34%) and hindlimb (-25%) DO(2) , whereas acetylcholine increased renal (+38%) and hindlimb (+66%) DO(2) . However, neither renal nor hindlimb VO(2) were altered. Tissue PO(2) varied with local perfusion in the renal medulla and biceps femoris, but not the renal cortex. DNP increased renal VO(2) (+38%) and reduced cortical tissue PO(2) (-44%), but both still remained stable during subsequent infusion of angiotensin-II and acetylcholine. 4. We conclude that maintenance of tissue PO(2) in the face of mismatched changes in local perfusion and VO(2) is an unusual property of the renal cortex. The underlying mechanisms remain unknown, but our current findings suggest they are not compromised when resting renal VO(2) is increased.

Inhibition of prolyl hydroxylases increases erythropoietin production in ESRD

The reasons for inadequate production of erythropoietin (EPO) in patients with ESRD are poorly understood. A better understanding of EPO regulation, namely oxygen-dependent hydroxylation of the hypoxia-inducible transcription factor (HIF), may enable targeted pharmacological intervention. Here, we tested the ability of fibrotic kidneys and extrarenal tissues to produce EPO. In this phase 1 study, we used an orally active prolyl-hydroxylase inhibitor, FG-2216, to stabilize HIF independent of oxygen availability in 12 hemodialysis (HD) patients, six of whom were anephric, and in six healthy volunteers. FG-2216 increased plasma EPO levels 30.8-fold in HD patients with kidneys, 14.5-fold in anephric HD patients, and 12.7-fold in healthy volunteers. These data demonstrate that pharmacologic manipulation of the HIF system can stimulate endogenous EPO production. Furthermore, the data indicate that deranged oxygen sensing--not a loss of EPO production capacity--causes renal anemia.

Effects of recombinant human erythropoietin in normal humans

This review describes some of the physiological effects of recombinant human erythropoietin (EPO) in healthy humans. At the blood level EPO increases the arterial O(2) content not only by increasing red blood cell volume, but also by an equally important decrease in plasma volume. Well before that, EPO causes a prompt decrease in plasma levels of renin and aldosterone. Renal clearance studies suggest that EPO decreases renal proximal tubular reabsorption rate leading to activation of the tubuloglomerular feedback mechanism and a fall in glomerular filtration rate. Thus, treatment with EPO may result in suppression of endogenous EPO production through a decrease in intrarenal oxygen consumption. EPO elevates the arterial blood pressure even in healthy subjects. The receptor for EPO is present in many tissues. However, the functional effects of EPO in the skeletal muscle seem limited, and although it has been speculated that non-erythropoietic effects of EPO (angiogenesis, shift in muscle fibre types, cognitive effects) may be responsible for the increase in exercise performance, this has not been confirmed. EPO-induced haemodynamic effects call for careful monitoring during the administration period. The metabolic, hormonal and renal effects of EPO do not seem to range beyond physiologically acceptable limits and are reversible. Taken together, EPO seems safe to use for experimental purposes in healthy volunteers.

Erythropoietin down-regulates proximal renal tubular reabsorption and causes a fall in glomerular filtration rate in humans

Recombinant human erythropoietin (rHuEPO) elevates haemoglobin concentration both by increasing red blood cell volume and by a decrease in plasma volume. This study delineates the association of rHuEPO-induced changes in blood volumes with changes in the renin–aldosterone system and renal function. Sixteen healthy males were given rHuEPO for 28 days in doses raising the haematocrit to 48.3±4.1%.Renal clearance studieswith urine collections (N = 8) were done at baseline and at days 4, 11, 29 and 42. Glomerular filtration rate (GFR) was measured by 51Cr-EDTA.Renal clearance of lithium (CLi)was used as an index of proximal tubular outflow and to assess segmental renal tubular handling of sodium and water. rHuEPO-induced increases in haematocrit occurred from day 10 onwards and was caused by both an increase in red cell volume and a fall in plasma volume. Well before that (from day 2 and throughout the treatment time), rHuEPO decreased plasma levels of renin and aldosterone (N = 8) by 21–33% (P < 0.05) and 15–36% (P < 0.05), respectively. After cessation of rHuEPO, values returned to baseline. On days 11 and 29, CLi increased (P < 0.02) indicating a significant 10–16% decrease in absolute proximal reabsorption of sodium and water (APR = GFR − CLi, P < 0.05). GFR decreased slightly, albeit significantly, on day 4 (P < 0.05). In conclusion, rHuEPO promptly, and before any changes in blood volumes and haematocrit can be detected, causes a down-regulation of the renin–aldosterone system. The results are compatible with a rHuEPO-induced reduction in proximal reabsorption rate leading to activation of the tubuloglomerular feedback mechanism and a fall in GFR. Therefore, treatment with rHuEPO may result in suppression of endogenous EPO synthesis secondary to a decrease in intrarenal oxygen consumption.

Intrarenal oxygenation: unique challenges and the biophysical basis of homeostasis

The kidney is faced with unique challenges for oxygen regulation, both because its function requires that perfusion greatly exceeds that required to meet metabolic demand and because vascular control in the kidney is dominated by mechanisms that regulate glomerular filtration and tubular reabsorption. Because tubular sodium reabsorption accounts for most oxygen consumption (Vo2) in the kidney, renal Vo2 varies with glomerular filtration rate. This provides an intrinsic mechanism to match changes in oxygen delivery due to changes in renal blood flow (RBF) with changes in oxygen demand. Renal Vo2 is low relative to supply of oxygen, but diffusional arterial-to-venous (AV) oxygen shunting provides a mechanism by which oxygen superfluous to metabolic demand can bypass the renal microcirculation. This mechanism prevents development of tissue hyperoxia and subsequent tissue oxidation that would otherwise result from the mismatch between renal Vo2 and RBF. Recent evidence suggests that RBF-dependent changes in AV oxygen shunting may also help maintain stable tissue oxygen tension when RBF changes within the physiological range. However, AV oxygen shunting also renders the kidney susceptible to hypoxia. Given that tissue hypoxia is a hallmark of both acute renal injury and chronic renal disease, understanding the causes of tissue hypoxia is of great clinical importance. The simplistic paradigm of oxygenation depending only on the balance between local perfusion and Vo2 is inadequate to achieve this goal. To fully understand the control of renal oxygenation, we must consider a triad of factors that regulate intrarenal oxygenation: local perfusion, local Vo2, and AV oxygen shunting.

`Blood-doping' effects on hematocrit regulation and oxygen consumption in late-stage chicken embryos (Gallus gallus)

The extent to which hematocrit (Hct) is regulated and the impact of altered Hct on blood oxygen transport in avian embryos are largely unknown. Consequently, we investigated how acute blood removal or Ringer solution injection modified Hct in day 15 embryos, and how `blood doping' with erythrocyte-enriched whole blood influenced O2 consumption in day 15–17 chicken embryos. Mean Hct (±s.e.m.) at day 15, 16 and 17 was 26.7±0.6%, 28.0±0.4% and 30.7±0.5%, respectively. Blood withdrawal (19 increments of 125 μl each, separated by 30 min) caused a progressive fall in Hct to ∼12% at day 15. Hct decline was strictly proportional to the extent of blood withdrawal. Incremental Ringer solution injection over an 8 h period, transiently increasing blood volume up to 85%over initial values, did not decrease Hct, indicating that injected Ringer solution rapidly left the circulating blood compartment. Blood doping with erythrocyte-enriched whole blood artificially elevated Hct from 27% to 38%,but caused no significant change in routine O2 consumption(0.35–0.39 ml O2 min–1egg–1) at any point over the subsequent 6 h period in day 15–17 embryos. We conclude that Hct is not protected acutely in day 15 chicken embryos, with no evidence of erythrocyte sequestration or release. Additionally, at day 15–17, Hct increases of ∼10% do not enhance embryonic oxygen consumption, suggesting that blood oxygen carrying capacity per se is not limiting to oxygen consumption.

Development of an AT2-deficient proximal tubule cell line for transport studies

Angiotensin II is a major regulatory peptide for proximal tubule Na(+) reabsorption acting through two distinct receptor subtypes: AT(1) and AT(2). Physiological or pathological roles of AT(2) have been difficult to unravel because angiotensin II can affect Na(+) transport either directly via AT(2) on luminal or peritubular plasma membranes of proximal tubule cells or indirectly via the renal vasculature. Furthermore, separate systemic and intratubular renin-angiotensin systems impart considerable complexity to angiotensin's regulation. A transport-competent, proximal tubule cell model that lacks AT(2) is a potentially useful tool to assess cellular angiotensin II regulation. To this end, AT(2)-receptor-deficient mice were bred with an Immortomouse, which harbors the thermolabile immortalization gene SV40 large-T antigen (Tag), and AT(2)-receptor-deficient [AT(2) (-/-)], Tag heterozygous [Tag (+/-)] F(2) offspring were selected for cell line generation. S1 proximal tubule segments were microdissected, and epithelial cell outgrowth was expanded in culture. Cells that formed confluent, electrically resistive monolayers were selected for cryopreservation, and one isolate was extensively characterized for conductance (2 mS/cm(2)), short-circuit current (Isc; 0.2 microA/cm(2)), and proximal tubule-specific Na3(+) - succinate (DeltaIsc = 0.8 microA/cm(2) at 2 mM succinate) and Na3(+) - phosphate cotransport (DeltaIsc = 3 microA/cm(2) at 1 mM phosphate). Light microscopy showed a uniform, cobblestone-shaped monolayer with prominent cilia and brush borders. AT(2) receptor functionality, as demonstrated by angiotensin II inhibition of ANF-stimulated cGMP synthesis, was absent in AT(2)-deficient cells but prominent in wild-type cells. This transport competent cell line in conjunction with corresponding wild type and AT(1)-deficient lines should help explain angiotensin II signaling relevant to Na(+) transport.

The Role of the Kidney in Blood Volume Regulation: The Kidney as a Regulator of the Hematocrit

The kidney plays a pivotal role in the regulation of blood volume by controlling the plasma volume and red blood cell (RBC) mass. Further, it is proposed that the kidney coordinates the relative volumes of these 2 blood components and in so doing regulates the hematocrit. This novel function as proposed is a functional concept whereby the kidney does not simply produce erythropoietin, but that the kidney regulates the hematocrit is termed the critmeter function. The kidney is unique in that it can indirectly report on blood volume as a tissue oxygen signal. It is proposed that the kidneys detect small changes in tissue oxygen tension for erythropoietin production at the critmeter, a functional unit of marginal oxygen tension within the kidneys. As the production of erythropoietin is modulated by angiotensin II, the renin-angiotensin system entrains the production of erythropoietin as part of the effector signals of the feedback loop of blood volume regulation. Collectively, the consideration of these points generates a paradigm shift in our understanding of blood volume regulation in that the role of the kidney may be expanded from simply "producing" erythropoietin to regulating the hematocrit. Further, this concept broadens the scope of the traditionally identified effector mechanisms of plasma volume regulation to include the modulation of erythropoietin production and hence RBC mass. The inclusion of both plasma volume and RBC mass as factors targeted by the effector signals recapitulates that whole blood volume is sensed and reported in the afferent signals. In summary, distinct sensing and effector mechanisms for regulating the volume of the two components of whole blood (plasma and red cell mass) are recognized. The coupling of the regulation of these 2 components of blood volume is highlighted.

Blood Volume Perturbations in the Postural Tachycardia Syndrome

The postural tachycardia syndrome (POTS) is characterized by excessive orthostatic tachycardia with chronic symptoms that are associated with upright posture. These chronic symptoms (of at least 6 months' duration) include tachycardia, exercise intolerance, lightheadedness, extreme fatigue, headache, and mental clouding. Patients with POTS demonstrate an increase in heart rate of at least 30 beats/min within 5 to 30 minutes of assuming an upright posture, in the absence of orthostatic hypotension (a fall in blood pressure >20/10 mm Hg) and in the absence of other medical disorders that might cause tachycardia. POTS can be associated with a high degree of functional disability. The blood volume has been found to be low in many patients with POTS. This article will review some of the data regarding blood volume perturbations in POTS, blood volume regulation in POTS, and potential treatment approaches.

Anemia in diabetes: marker or mediator of microvascular disease?

Anemia is a common finding in patients with diabetes due to the high burden of chronic kidney disease in this population. Anemia is more prevalent and is found earlier in patients with diabetes than in those with kidney disease from other causes. The increased risk of anemia in diabetes probably reflects changes in the renal tubulointerstitium associated with diabetic kidney disease, which disrupt the delicate interaction between interstitial fibroblasts, capillaries and tubular cells required for normal hemopoietic function. In particular, the uncoupling of the hemoglobin concentration from renal erythropoietin synthesis seems to be the key factor underlying the development of anemia. Systemic inflammation, functional hematinic deficiencies, erythropoietin resistance and reduced red cell survival also drive anemia in the setting of impaired renal compensation. Although anemia can be considered a marker of kidney damage, reduced hemoglobin levels independently identify diabetic patients with an increased risk of microvascular complications, cardiovascular disease and mortality. Nevertheless, a direct role in the development or progression of diabetic complications remains to be clearly established and the clinical utility of correcting anemia in diabetic patients has yet to be demonstrated in randomized controlled trials. Correction of anemia certainly improves performance and quality of life in diabetic patients. In the absence of additional data, treatment should be considered palliative, and any functional benefits must be matched against costs to the patient and the health system.

QTL influencing baseline hematocrit in the C57BL/6J and DBA/2J lineage: age-related effects

Baseline serum hematocrit varies substantially in the population. While additive genetic factors account for a large part of this variability, little is known about the genetic architecture underlying the trait. Because hematocrit levels vary with age, it is plausible that quantitative trait loci (QTL) that influence the phenotype also show an age-specific profile. To investigate this possibility, hematocrit was measured in three different age cohorts of mice (150, 450, and 750 days) of the C57BL/6J (B6) and the DBA2/J (D2) lineage. QTL were searched in the B6D2F(2) intercross and the BXD recombinant inbred (RI) strains. The effects of these QTL were explored across the different age groups. On the phenotypic level, baseline serum hematocrit declines with age in a sex-specific manner. In the B6D2F(2) intercross, suggestive QTL that influence the phenotype were located on Chromosomes (Chr) 1, 2, 7, 11, 13, and 16. With the exception of the QTL on Chr 2, all of these QTL exerted their largest effect at 750 days. The QTL on Chr 1, 2, 7, 11 and 16 were confirmed in the BXD RIs in a sex- and age-specific manner. Linkage analysis in the BXD RIs revealed an additional significant QTL on Chr 19. Baseline serum hematocrit is influenced by several QTL that appear to vary with the age and sex of the animal. These QTL primarily overlap with QTL that have been shown to regulate hematopoietic stem cell phenotypes.

Mouse model for noninvasive imaging of HIF prolyl hydroxylase activity: assessment of an oral agent that stimulates erythropoietin production

Many human diseases are characterized by the development of tissue hypoxia. Inadequate oxygenation can cause cellular dysfunction and death. Tissues use many strategies, including induction of angiogenesis and alterations in metabolism, to survive under hypoxic conditions. The heterodimeric transcription factor hypoxia-inducible factor (HIF) is a master regulator of genes that promote adaptation to hypoxia. HIF activity is linked to oxygen availability because members of the EGLN family hydroxylate HIFalpha subunits on specific prolyl residues when oxygen is present, which marks them for ubiquitination and proteasomal degradation. We created a mouse that ubiquitously expresses a bioluminescent reporter consisting of firefly luciferase fused to a region of HIF that is sufficient for oxygen-dependent degradation. Our validation studies suggest that this mouse will be useful for monitoring hypoxic tissues and evaluating therapeutic agents that stabilize HIF. One such agent, the HIF prolyl hydroxylase inhibitor FG-4383, was active in the liver and kidney after systemic administration as determined by bioluminescence imaging, transcription profiling, and production of erythropoietin, indicating that the HIF transcriptional program can be manipulated in vivo with orally active organic small molecules.

Serum erythropoietin levels in healthy humans after a short period of normobaric and hyperbaric oxygen breathing: the "normobaric oxygen paradox"

Renal (peritubular) tissue hypoxia is a well-known physiological trigger for erythropoietin (EPO) production. We investigated the effect of rebound relative hypoxia after hyperoxia obtained under normo- and hyperbaric oxygen breathing conditions. A group of 16 healthy volunteers were investigated before and after a period of breathing 100% normobaric oxygen for 2 h and a period of breathing 100% oxygen at 2.5 ATA for 90 min (hyperbaric oxygen). Serum EPO concentration was measured using a radioimmunoassay at various time points during 24-36 h. A 60% increase (P < 0.001) in serum EPO was observed 36 h after normobaric oxygen. In contrast, a 53% decrease in serum EPO was observed at 24 h after hyperbaric oxygen. Those changes were not related to the circadian rhythm of serum EPO of the subjects. These results indicate that a sudden and sustained decrease in tissue oxygen tension, even above hypoxia thresholds (e.g., after a period of normobaric oxygen breathing), may act as a trigger for EPO serum level. This EPO trigger, the "normobaric oxygen paradox," does not appear to be present after hyperbaric oxygen breathing.

Microcytic anemia, erythropoietic protoporphyria, and neurodegeneration in mice with targeted deletion of iron-regulatory protein 2

Iron-regulatory proteins (IRPs) 1 and 2 posttranscriptionally regulate expression of transferrin receptor (TfR), ferritin, and other iron metabolism proteins. Mice with targeted deletion of IRP2 overexpress ferritin and express abnormally low TfR levels in multiple tissues. Despite this misregulation, there are no apparent pathologic consequences in tissues such as the liver and kidney. However, in the central nervous system, evidence of abnormal iron metabolism in IRP2-/- mice precedes the development of adult-onset progressive neurodegeneration, characterized by widespread axonal degeneration and neuronal loss. Here, we report that ablation of IRP2 results in iron-limited erythropoiesis. TfR expression in erythroid precursors of IRP2-/- mice is reduced, and bone marrow iron stores are absent, even though transferrin saturation levels are normal. Marked overexpression of 5-aminolevulinic acid synthase 2 (Alas2) results from loss of IRP-dependent translational repression, and markedly increased levels of free protoporphyrin IX and zinc protoporphyrin are generated in IRP2-/- erythroid cells. IRP2-/- mice represent a new paradigm of genetic microcytic anemia. We postulate that IRP2 mutations or deletions may be a cause of refractory microcytic anemia and bone marrow iron depletion in patients with normal transferrin saturations, elevated serum ferritins, elevated red cell protoporphyrin IX levels, and adult-onset neurodegeneration.

Renin-Aldosterone Paradox and Perturbed Blood Volume Regulation Underlying Postural Tachycardia Syndrome

BACKGROUND

Patients with postural tachycardia syndrome (POTS) experience considerable disability, but in most, the pathophysiology remains obscure. Plasma volume disturbances have been implicated in some patients. We prospectively tested the hypothesis that patients with POTS are hypovolemic compared with healthy controls and explored the role of plasma renin activity and aldosterone in the regulation of plasma volume.

METHODS AND RESULTS

Patients with POTS (n=15) and healthy controls (n=14) underwent investigation. Heart rate (HR), blood pressure (BP), plasma renin activity, and aldosterone were measured with patients both supine and upright. Blood volumes were measured with 131I-labeled albumin and hematocrit. Patients with POTS had a higher orthostatic increase in HR than controls (51+/-18 versus 16+/-10 bpm, P<0.001). Patients with POTS had a greater deficit in plasma volume (334+/-187 versus 10+/-250 mL, P<0.001), red blood cell volume (356+/-128 versus 218+/-140 mL, P=0.010), and total blood volume (689+/-270 versus 228+/-353 mL, P<0.001) than controls. Despite the lower plasma volume in patients with POTS, there was not a compensatory increase in plasma renin activity (0.79+/-0.58 versus 0.79+/-0.74 ng x mL(-1) x h(-1), P=0.996). There was a paradoxically low level of aldosterone in the patients with POTS (190+/-140 pmol/L versus 380+/-230 pmol/L; P=0.017).

CONCLUSIONS

Patients with POTS have paradoxically unchanged plasma renin activity and low aldosterone given their marked reduction in plasma volume. These patients also have a significant red blood cell volume deficit, which is regulated by the renal hormone erythropoietin. These abnormalities suggest that the kidney may play a key role in the pathophysiology of POTS.