The Endocrine Heart: Natriuretic Peptides and Oxygen Metabolism in Cardiac Diseases.

Circulating natriuretic peptides are widely used as tools in the diagnosis and follow-up of cardiac diseases, and their use has been increasing throughout other medical branches. After 40 years and more than 40,000 publications, their function in healthy human adults of reproductive age appears to remain confusing-with every physiology and pharmacology textbook telling a different story. In cardiology, mechanical load upon the heart is generally regarded as the condition that regulates the synthesis and release of natriuretic peptides. The key issue in cardiology remains how mechanical activity and oxygen consumption are related, and yet no published paper has shown that mechanical load does not increase oxygen consumption, as wall tension is a major determinant of myocardial oxygen consumption. However, this relationship has been largely neglected in studies on natriuretic peptides. Based on published papers, an outline is presented of how oxygen metabolism, related to mechanical stress, could play an important role in the pathophysiology of natriuretic peptides. The natriuretic peptide system might enhance oxygen transport by causing diuresis, natriuresis, and water transfer from the intra- to extravascular space, resulting in volume contraction and hemoconcentration, thus indirectly promoting the transfer of oxygen into tissues and organs. Mechanical stress and oxygen consumption are 2 sides of the same coin. The relationship between mechanical stress and oxygen metabolism, in the particular case of natriuretic peptides, represents a new avenue for clinical studies and will better explain the results of studies that have been published previously.

Les formes circulantes de peptides natriurétiques sont des outils très utilisés dans le diagnostic et le suivi des maladies cardiaques, et leur utilisation ne cesse de croître dans toutes les autres branches de la médecine. Après 40 ans et plus de 40 000 publications, leur fonctionnement chez les humains adultes en bonne santé et en âge de reproduire semble encore confus (devant tous ces manuels de physiologie et de pharmacologie qui brossent des tableaux différents). En cardiologie, la charge mécanique sur le cœur est généralement considérée comme la condition qui régule la synthèse et la libération des peptides natriurétiques. L'enjeu principal en cardiologie reste à connaître le lien entre l'activité mécanique et la consommation d'oxygène, mais jusqu'à ce jour, aucun des articles publiés n'a montré que la charge mécanique n'augmentait pas la consommation d'oxygène, alors que la tension sur la paroi est un déterminant majeur de la consommation en oxygène du myocarde. Toutefois, dans les études sur les peptides natriurétiques, on a grandement négligé cette relation. Selon les articles publiés, une ébauche sur la façon dont le métabolisme de l'oxygène, en relation avec le stress mécanique, jouerait un rôle important dans la physiopathologie des peptides natriurétiques est présentée. Le système des peptides natriurétiques pourrait accroître le transport en oxygène en provoquant la diurèse, la natriurèse et le transfert d'eau de l'espace intravasculaire à l'espace extravasculaire, qui entraînerait une contraction du volume et une hémoconcentration et, par conséquent, favoriserait indirectement le transfert d'oxygène dans les tissus et les organes. Le stress mécanique et la consommation en oxygène sont les deux côtés de la médaille. La relation entre le stress mécanique et le métabolisme de l'oxygène, particulièrement des peptides natriurétiques, représente la nouvelle avenue des études cliniques et permettra de mieux expliquer les résultats des études publiées antérieurement.

Systemic Hypoxia Increases Circulating Concentration of Apelin in Humans.

Heinonen, Ilkka, Olli Vuolteenaho, Juha Koskenvuo, Olli Arjamaa, and Mikko Nikinmaa. Systemic hypoxia increases circulating concentration of apelin in humans. High Alt Med Biol. 18:292-295, 2017. BACKGROUND: Apelin is a hormone that regulates cardiovascular function, and its concentration is increased by hypoxia based on cell culture and animal studies. As it remains unknown as to whether hypoxia could affect apelin levels in humans, we investigated whether breathing normobaric hypoxic gas mixture increases the circulating apelin concentration in healthy male subjects. METHODS: Ten healthy young men (age 29 ± 5 years, body mass index 24.7 ± 2.8 kg/m2) breathed normobaric hypoxic gas mixture (11% O2/89% N2) for 1 hour. Venous blood samples were obtained immediately before, and 2 and 24 hours after the start of the hypoxic exposure and analyzed for circulating apelin concentrations. RESULTS: Arterial oxygen saturation decreased steadily from a baseline value of 99% ± 1% after the initiation hypoxia challenge and reached a steady-state level of 73% ± 6% within 20-30 minutes. Baseline apelin concentration was 3.3 ± 1.3 pmol/L and remained comparable (3.3 ± 1.4 pmol/L) to the baseline concentration at a 2-hour time point. However, apelin concentration at the 24-hour time point (5.5 ± 2.8 pmol/L) was significantly (by ∼67%) higher as compared with at both baseline and 2-hour time points (p < 0.05). CONCLUSION: In conclusion, in line with cell culture and animal studies, acute systemic hypoxia increases circulating apelin concentration also in humans.

Hypoxia and inflammation in the release of VEGF and interleukins from human retinal pigment epithelial cells.

PURPOSE: Retinal diseases are closely associated with both decreased oxygenation and increased inflammation. It is not known if hypoxia-induced vascular endothelial growth factor (VEGF) expression in the retina itself evokes inflammation, or whether inflammation is a prerequisite for the development of neovascularization. METHODS: Human ARPE-19 cell line and primary human retinal pigment epithelium (RPE) cells were used. ARPE-19 cells were kept either under normoxic (24 h or 48 h) or hypoxic conditions (1% O2, 24 h). Part of the cells were re-oxygenated (24 h). Some ARPE-19 cells were additionally pre-treated with bacterial lipopolysaccharide (LPS). The levels of IL-6, IL-8, IL-1ß, and IL-18 were determined from medium samples by an enzyme-linked immunosorbent assay (ELISA) method. Primary human RPE cells were exposed to hypoxia for 24 h, and the subsequent release of IL-6 and IL-8 was measured with ELISA. VEGF secretion from ARPE-19 cells was determined up to 24 h. RESULTS: Hypoxia induced significant (P < 0.01) increases in the levels of both IL-6 and IL-8 in ARPE-19 cells, and LPS pre-treatment further enhanced these responses. Hypoxia exposure did not affect the IL-1ß or IL-18 release irrespective of LPS pre-treatment. If primary RPE cells were incubated for 4 h in hypoxic conditions, IL-6 and IL-8 concentrations were increased by 7 and 8-fold respectively. Hypoxia increased the VEGF secretion from ARPE-19 cells in a similar manner with or without pre-treatment with LPS. CONCLUSIONS: Hypoxia causes an inflammatory reaction in RPE cells that is potentiated by pre-treatment with the Toll-like receptor-activating agent, LPS. The secretion of VEGF from these cells is regulated directly by hypoxia and is not mediated by inflammation.

Projected changes in age-related macular degeneration and driving license holders in Finland.

PURPOSE: The aim of the study was to approximate the occurrence of all forms of age-related macular degeneration (AMD) of the retina among the driving license holders aged 80 or more, and to project the changes to 2030 in Finland. AMD, destroying the visual cells in the central part of the retina, is a common disease of older age: one out of three individuals aged 70 or older shows early signs of AMD progressing later to relentless loss of vision. This eye disease can be detected only by an ophthalmologist. In general, little is known about the prevalence of AMD among driving license holders aged 80 or older. METHODS: At first the prevalence of individuals with either drusen or AMD in Finland among those 80 or older was approximated. Then the number of license holders in this age group was extracted from the statistics of the Finnish Transport Safety Agency and Eurostat provided us with the demographical data. The changes were projected to 2030. RESULTS: In Finland, with a population of 5.35 million, the number of those aged 80 or over will increase by 175,000 by 2030. The total number of individuals with either drusen or AMD will increase from 118,000 to 193,000 by the year 2030 and an increasing proportion of them will have a driving license. The proportion of women in 2012 having a driving license in the groups 60 or younger is about 45%, while in those aged 80 or older it is only 20%. CONCLUSION: The number of people aged 80 years or older will increase in Finland by 2030. The number of those in this age group having a driving license will increase more rapidly as the population ages because the proportion of women with a driving license will increase in this age group. As the prevalence of drusen and AMD among women aged 80 or over is higher than among men at comparable age, this means that AMD will increase even more rapidly among drivers in this age group.

Circulating N-terminal brain natriuretic peptide and cardiac function in response to acute systemic hypoxia in healthy humans.

BACKGROUND: As it remains unclear whether hypoxia of cardiomyocytes could trigger the release of brain natriuretic peptide (BNP) in humans, we investigated whether breathing normobaric hypoxic gas mixture increases the circulating NT-proBNP in healthy male subjects. METHODS: Ten healthy young men (age 29 ± 5 yrs, BMI 24.7 ± 2.8 kg/m2) breathed normobaric hypoxic gas mixture (11% O2/89% N2) for one hour. Venous blood samples were obtained immediately before, during, and 2 and 24 hours after hypoxic exposure. Cardiac function and flow velocity profile in the middle left anterior descending coronary artery (LAD) were measured by Doppler echocardiography. RESULTS: Arterial oxygen saturation decreased steadily from baseline value of 99 ± 1% after the initiation hypoxia challenge and reached steady-state level of 73 ± 6% within 20-30 minutes. Cardiac output increased from 6.0 ± 1.2 to 8.1 ± 1.6 L/min and ejection fraction from 67 ± 4% to 75 ± 6% (both p < 0.001). Peak diastolic flow velocity in the LAD increased from 0.16 ± 0.04 to 0.28 ± 0.07 m/s, while its diameter remained unchanged. In the whole study group, NT-proBNP was similar to baseline (60 ± 32 pmol/ml) at all time points. However, at 24 h, concentration of NT-proBNP was higher (34 ± 18%) in five subjects and lower (17 ± 17%), p = 0.002 between the groups) in five subjects than at baseline. CONCLUSION: In conclusion, there is no consistent increase in circulating NT-proBNP in response to breathing severely hypoxic normobaric gas mixture in healthy humans, a possible reason being that the oxygen flux to cardiac myocytes does not decrease because of increased coronary blood flow. However, the divergent individual responses as well as responses in different cardiac diseases warrant further investigations.

Hypoxic conditions stimulate the release of B-type natriuretic peptide from human retinal pigment epithelium cell culture.

PURPOSE: A-type peptide, a natriuretic peptide belonging to the natriuretic peptide family, has been shown to be increased in the vitreous of patients suffering from diabetic retinopathy and that human retina has a well-developed natriuretic peptide system. The stimulus to which the synthesis of natriuretic peptides responded in these patients has, however, remained unknown. As the natriuretic peptides have recently been shown to respond to hypoxic conditions, the genes of both A-type and B-type have a hypoxia-response element (HRE) in their promoter sequence, we therefore hypothesized that hypoxia in the human retinal pigment epithelium will increase the secretion of NT-proBNP, the most common natriuretic peptide monitored in clinical medicine. METHODS: We used cultured human retinal pigment epithelium cell line (ARPE-19) which was exposed either to normoxia or to hypoxia for 2 hr, 4 hr, 6 hr and 24 hr. NT-proBNP was measured with enzyme immunoassay, VEGF with ELISA and HIF-1α with Western blotting. RESULTS: Hypoxia induced VEGF 165 release in culture medium and HIF-1α expression in cultured ARPE-19 cells. Time-dependent NT-proBNP release was detected when the ARPE-19 cells were cultured under normoxia. When hypoxia was induced, a statistically significant increase in NT-proBNP release was demonstrated in the culture medium. CONCLUSIONS: Hypoxic conditions increase the release of a natriuretic peptide from retinal pigment epithelium (RPE) cells. The secretion of VEGF was also enhanced. The responses were associated with the up-regulation of the HIF-1α transcription factor. These results explain the previous findings from patients with diabetes, which also suggest that hypoxia is a ubiquitous stimulus for the secretion of natriuretic peptides in human body.

Physiology of natriuretic peptides: The volume overload hypothesis revisited.

The discovery of the natriuretic peptide system in the early 1980s aroused great interest among clinical cardiologists. The heart was not a mechanical pump alone, but also an endocrine organ that had powerful effects on blood circulation. Natriuretic peptides caused both natriuresis and diuresis, and they responded to a volume overload which caused either stretch or pressure on the heart. As a result, the findings led to the conclusion that the human body had a hormone with effects similar to those of a drug which treats high blood pressure. Later, it became evident that the volume contraction was fortified by extrarenal plasma shift. Here, a hypothesis is presented in which the role of natriuretic peptides is to regulate oxygen transport as the volume contraction leads to hemoconcentration with an increased oxygen-carrying capacity. Wall stress, either chemical or mechanical, changes the oxygen gradient of the myocardium and affects the diffusion of oxygen within a myocyte. In support of this hypothesis, hypoxia-response elements have been found in both the atrial natriuretic peptide and the brain natriuretic peptide genes.

Hypoxia increases the release of salmon cardiac peptide (sCP) from the heart of rainbow trout (Oncorhynchus mykiss) under constant mechanical load in vitro.

Our aim was to study the effects of hypoxia on the release of salmon cardiac peptide (sCP) from an isolated heart ventricle of trout during a constant mechanical load. Trout heart ventricles were studied in vitro. The ventricle was placed in an organ bath at 12 °C in which a constant mechanical load could be imposed on the ventricle while buffer solution was circulating. Ventricles were field-stimulated with a supramaximal voltage pulse at a rate of about 0.3 s⁻¹. Samples of 1 ml were collected at an interval of 10 min for 200 min from the organ bath and assessed with a radioimmunoassay for sCP. After a control period of 20 min, ventricles were exposed to hypoxia produced with N2 gassing (n = 9) or to hypoxia with 20 mM BDM, a nonselective myosin ATPase inhibitor locking cross-bridges in a pre-power-stroke state inhibiting force production with normal electrical activity (n = 10). In this model and setup, hypoxia stimulated the release of sCP, but the interindividual variation in the response was large. At the end of hypoxia exposure, the concentration of sCP in the organ bath was about sixfold higher than at the start of the exposure (P < 0.05, one-way ANOVA for repeated measurements, followed by Dunnett's multiple comparison test). When BDM was introduced into the bath, the ventricle still secreted sCP but the hypoxic response was smaller than in the experiments without BDM. In the trout heart ventricle, there is a hypoxia-sensitive component in the release mechanism of sCP which is independent of contraction.

Corneal reconstruction by stem cells and bioengineering.

Almost 300 million people are visually impaired worldwide due to various eye diseases such as cataracts, glaucoma, age-related macular degeneration, diabetic retinopathy, and corneal diseases. Notably, ten million people are blind because of severe ocular surface diseases and the majority of cases occur in developing countries. Blinding ocular surface diseases have, however, become treatable by grafting of surface layers, or by full-thickness transplantation of the cornea. As the demand for human corneal tissue for surface reconstruction and transplantation far exceeds the supply, methods are being developed to supplement tissue donation. Xenotransplantation of the cornea or cells from genetically modified pigs may become one of the solutions. Transplantation of limbal stem cells within tissue biopsies, to restore the transparency of the cornea is another remarkable method, which has shown its potential in several clinical studies. The combination of stem cell technology and engineering of biocompatible tissue equivalent, still at preclinical stage, has shown us how synthetic corneal tissue is able to guide cultured corneal stromal stem cells of human origin, to become native-like stroma, the most important layer of the cornea. These findings give hope for a large-quantity production of biomaterial for corneal reconstruction. As such, clinical ophthalmologists should become more familiar with the methods of laboratory science.

High lactose tolerance in North Europeans: a result of migration, not in situ milk consumption.

The main carbohydrate in milk is lactose, which must be hydrolyzed to glucose and galactose before the sugars can be digested. While 65% or more of the total human population are lactose intolerant, in some human populations lactase activity commonly persists into adulthood. Lactose tolerance is exceptionally widespread in Northern European countries such as Sweden and Finland, with tolerance levels of 74% and 82%, respectively. Theoretically, this may result either from a strong local selection pressure for lactose tolerance, or from immigration of lactose tolerant people to Northern Europe. We provide several lines of archaeological and historical evidence suggesting that the high lactose tolerance in North Europeans cannot be explained by selection from in situ milk consumption. First, fresh cow milk has not belonged to the traditional diet of Swedes or Finns until recent times. Second, not enough milk has been available for adult consumption. Cattle herding has been neither widespread nor productive enough in Northern Europe to have provided constant access to fresh milk. We suggest that the high prevalence of lactose tolerance in Finland in particular may be explained by immigration of people representing so-called Corded Ware Culture, an early culture representing agricultural development in Europe.

Hypoxia regulates the natriuretic peptide system.

Numerous clinical studies have addressed the role of the natriuretic peptide system either as a diagnostic tool or as a guide to treatment in many cardiac diseases. The concept behind these studies has been that intravascular overload produces cardiac wall stress that alone stimulates the synthesis and release of natriuretic peptides the result of which is diuresis, natriuresis, and vasodilatation. However, almost thirty years after the discovery of the natriuretic peptides the measurement of these peptides, especially the BNP, has not met all the expectations of a simple and useful diagnostic tool in clinical cardiology, possibly due to confounding factors confusing the interpretation of the wall stress effect. In the same way as in pressure studies, it has been shown that hypoxia is a direct and sufficient stimulus for the synthesis and release of ANP and BNP. Additionally, hypoxia-response elements have been characterized from the promoter sequence of both the ANP and the BNP genes. Furthermore, a physiological rhythm (eupnea-apnea), causing changes in blood oxygen tension, regulates the plasma levels of ANP in sleeping seal pups which are spontaneously able to hold back their breathing. We suggest, on the basis of the extensive published literature, that the stimulus for the synthesis and release of natriuretic peptides is the oxygen gradient which always occurs in all human tissues in physiological conditions. The plasma volume contraction caused by natriuretic peptides (natriuresis, diuresis, and plasma shift) leads to hemoconcentration and ultimately to the increased oxygen-carrying capacity of unit volume of blood.

Increased IL-6 levels are not related to NF-κB or HIF-1α transcription factors activity in the vitreous of proliferative diabetic retinopathy.

PURPOSE: The purpose was to assess the activity of nuclear factor (NF)-κB and hypoxia inducible factor (HIF)-1α transcription factors and the expression levels of inflammation markers [interleukin (IL)-6 and IL-8] in the vitreous of patients suffering from proliferative diabetic retinopathy (PDR) scheduled for elective vitreous surgery in a single academic-based retina practice in a prospective clinical study. METHODS: Twenty-seven patients with PDR were enrolled in the study. The severity of retinopathy was classified (0, 1, 2, 3, 4) and the activity of neovascularization was graded (0, 1, 2, 3, 4) by the surgeon intraoperatively. Samples of the vitreous were collected during surgery, and the activity of NF-κB and HIF-1α transcription factors and the expression levels of IL-6 and IL-8 were measured. RESULTS: The majority of samples fell into the retinopathy class 3 (n = 12) or 4 (n = 13). The level of IL-6 increased from 68.9 ± 46.8 pg/ml to 102.7 ± 94.1 pg/ml, and IL-8 increased from 165.1 ± 136.0 pg/ml to 521.0 ± 870.9 pg/ml (mean ± S.D., nonsignificant change: normality test followed with Mann-Whitney Rank Sum Test). According to the neovascularization activity, the samples fell into grade 1 (n = 7), 2 (n = 12) or 3 (n = 7). In IL-6, there was a statistically significant increase (P < .05) from grade 2 to 3: 58.6 ± 40.3 pg/ml and 158.4 ± 102.5 pg/ml, respectively (Kruskal-Wallis One-Way Analysis of Variance on Ranks followed with Dunn's Method). The level of IL-8 was as follows: in grade 1: 118.0 ± 62.4 pg/ml, in grade 2: 192.3 ± 127.1 pg/ml and in grade 3: 884.3 ± 1161.0 pg/ml (statistically nonsignificant change). There was a statistically significant linear regression between IL-6 and IL-8 (P < .001): IL-6 = 51.88 pg/ml + (0.092*IL-8), r = 0.772. Increased activity of the NF-κB and HIF-1α transcription factors was not observed. CONCLUSION: Interleukin-6 is a candidate to indicate activity of neovascularization process in PDR. It might be a new molecular therapeutic target to regulate innate immunity response in vitreous.

Regulatory role of HIF-1alpha in the pathogenesis of age-related macular degeneration (AMD).

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness in the elderly throughout the world. AMD is attributed to a complex interaction of genetic and environmental factors. It is characterized by degeneration involving the retinal photoreceptors, retinal pigment epithelium (RPE), and Bruch's membrane, as well as alterations in choroidal capillaries. Aging and age-associated degenerative diseases, such as AMD, are intimately associated with decreased levels of tissue oxygenation and hypoxia that may induce accumulation of detrimental RPE-associated deposits, inflammation and neovascularization processes in retina. Hypoxia-inducible factor (HIF) is the master regulator for hypoxia-induced cellular adaptation that is involved in NF-kappaB signaling and the autophagic protein clearance system. In this review, we discuss role of HIF in AMD pathology and as a possible therapeutic target.

Natriuretic peptides in hormonal regulation of hypoxia responses.

The possibility that natriuretic peptides' effects are important in hypoxia responses of vertebrates is reviewed. Both the transcription and release of natriuretic peptides are affected by oxygen tension. Furthermore, many of the effects observed in hypoxia, such as diuresis and a reduction of plasma volume, are also caused by treatment of the animal with natriuretic peptides. Also, several clinical observations about changes in natriuretic peptide levels in, e.g., sleep apnea and cyanotic congenital heart disease, are consistent with the idea that hypoxia is involved in the etiology of conditions, in which natriuretic peptide levels increase. Virtually all published information on the relationship between oxygen and natriuretic peptides is based on human studies. Because hypoxic conditions are more common in aquatic than terrestrial environments, future studies about the possible role of natriuretic peptides in hypoxia, as well as the role of hypoxia in the evolution of natriuretic peptides, including the different subtypes, should increasingly involve also aquatic organisms.