An update on hypertension

Abstract

Arterial hypertension is a serious medical condition that significantly increases the risks of heart, brain, kidney, and other diseases affecting 1.28 billion adults worldwide. Hypertension is a major cause of premature death worldwide. This pathological condition is also called a “silent killer.” Most people with hypertension are unaware of the problem because it may have no symptoms until the first complications occur. This is why it is so important that blood pressure is measured on a regular basis.¹

The aim of the following contribution is to highlight some of recent papers that appeared in Acta Physiologica with focus on articles that might be of importance to the field of arterial hypertension research and related topics. The scope that was covered in this field in Acta Physiologica ranged from basic research conducted in animal models to studies closely related to clinical questions.

Form a historic perspective, comparative physiology models have been a hallmark of studies on animal osmoregulation.² The basic idea is that animal experiments might be used to study fundamental mechanisms that are involved also in humans in the following particular example case for mechanisms relating to blood pressure regulation such as sodium and potassium transporters. In this specific case Clifford et al³ determined whether Na+ uptake in adult zebrafish (Danio rerio) exposed to acidic water adheres to traditional models reliant on Na+/H+ Exchangers (NHEs), Na+ channels and Na+/Cl− Cotransporters (NCCs) or if it might occur through a novel mechanism. In order to achieve this the zebrafish were exposed to control or acidic (pH 4.0) water for 0–12 h during which radioactive Na+ uptake, ammonia excretion, net acidic equivalent flux, and net K+ flux were measured. The involvement of the possible transporters was evaluated by exposure to Cl− -free or elevated [K+] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT-PCR. The authors found that the uptake of sodium was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led the authors to consider K+ as a counter substrate for Na+ uptake during acid exposure. The elevated environmental potassium inhibited sodium uptake during acid exposure in a concentration-dependent manner. Analysis of mRNA revealed that six NCKX isoforms were present in zebrafish gills. The main conclusion of this article is that during acid exposure, zebrafish engage a novel Na+ uptake mechanism that utilizes the outwardly directed K+ gradient as a counter-substrate for Na+ and is sensitive to tetraethylammonium. NKCXs are promising candidates to mediate this potassium-dependent sodium uptake.

How these findings relate to human physiology remains to be determined. One possible approach is to check whether the genes analyzed in the study are present in humans as well. To this end, it is interesting that the tissue distributions of the human NCKX2 (SLC24A2) (http://www.genome.ucsc.edu/cgi-bin/hgGene?hgg_gene=ENST00000341998.7), for example is focused on brain tissue whereas the human NCKX1, (SLC24A1) (http://www.genome.ucsc.edu/cgi-bin/hgGene?hgg_gene=ENST00000546330.1) shows a much broader tissue distribution involving kidney tissue as well.

One of the most powerful regulation systems of osmolarity involves the release of ADH and one of its target tissues – the kidney with the appropriate receptors and targeting of aquaporins. Regulation of the plasma membrane location of aquaporins is important for water reabsorption in the collection duct of the kidney. Once the aquaporins are targeted to the plasma membrane and then more present, the cell layer becomes more permeable for water and water can follow the concentration gradient leading to an increased reabsorption of water. The molecule cAMP is an important second messenger in transmitting the signal for water reabsorption. The dysregulation of AQP2 is associated with water balance disorders. In a study by Ernstsen et al.,⁴ the authors aimed to analyze AQP2 trafficking in response to acute pyelonephritis. From clinical observations it is known that children and adults with acute pyelonephritis have a urinary concentration defect and studies in children revealed increased AQP2 excretion in the urine. This study aimed to analyze AQP2 trafficking in response to acute pyelonephritis. To address this, the authors used immunofluorescence imaging to analyze the subcellular localization of AQP2 and AQP2-S256A (mimics non-phosphorylated AQP2 at serine 256) in cells stimulated with bacterial lysates and of AQP2 and pS256-AQP2 in a mouse model on day 5 of acute pyelonephritis. Further they employed western blotting to evaluate AQP2 levels and AQP2 phosphorylation on S256 upon incubation with bacterial lysates. Since cAMP is an important second messenger the authors used an imaging technique to study cAMP levels with time-lapse imaging after stimulation with bacterial lysates. Interestingly, the researchers found that lysates from both uropathogenic and nonpathogenic bacteria mediated AQP2 plasma membrane targeting and increased AQP2 phosphorylation at serine 256 (pS256) without increasing the cAMP levels in cell cultures. In animals, immunofluorescence analysis of renal sections from mice after 5 days of acute pyelonephritis revealed apical plasma membrane targeting of AQP2 and pS256-AQP2 in inner medullary collecting ducts. The study concludes that bacteria induce AQP2 plasma membrane targeting in vitro and in vivo. However, the cAMP levels were not elevated by the bacterial lysates and AQP2 plasma membrane targeting could occur without S256 phosphorylation. The findings may explain increased AQP2 excretion in the urine during acute pyelonephritis.

Aquaporin is also a research object of a study by Xu et al. In this study,⁵ the authors investigated whether enhanced histone acetylation, achieved by inhibiting histone deacetylases (HDACs), could prevent decreased aquaporin-2 (AQP2) expression during hypokalemia.

The authors fed male Wistar rats with a potassium-free diet with or without 4-phenylbutyric acid (4-PBA) or the selective HDAC3 inhibitor RGFP966 for 4 days. Primary renal inner medullary collecting duct (IMCD) cells and immortalized mouse cortical collecting duct (mpkCCD) cells were cultured in potassium-deprivation medium with or without HDAC inhibitors.⁵ The researchers found that 4-PBA increased the levels of AQP2 mRNA and protein in the kidney inner medullae in hypokalemic (HK) rats, which was associated with decreased urine output and increased urinary osmolality. The level of acetylated H3K27 protein was decreased in the inner medullae of HK rat kidneys; this decrease was mitigated by 4-PBA. To get more insights into the mechanisms the research group also performed experiments in collecting duct cell culture. To this end, the H3K27ac levels were decreased in cortical collecting duct cells cultured in potassium-deprivation medium. Decreased acetylated H3K27 in the Aqp2 promoter region was associated with reduced Aqp2 mRNA levels. HDAC3 protein expression was upregulated in the model cells in response to potassium deprivation, and the binding of HDAC3 to the Aqp2 promoter was also increased. The substance RGFP966 increased the levels of H3K27ac and AQP2 proteins and enhanced binding between H3K27ac and AQP2 in mpkCCD cells. In addition the substance RGFP966 reversed the hypokalaemia-induced downregulation of AQP2 and H3K27ac and alleviated polyuria in rats. RGFP966 increased interstitial osmolality in the kidney inner medulla of HK rats but did not affect urinary cAMP levels. The researchers have demonstrated that renal medullary HDAC3 plays an important role in the regulation of Aqp2 transcription and, potentially, urine concentration. Further, they conclude that the investigated HDAC inhibitors prevented the downregulation of AQP2 induced by potassium deprivation, probably by enhancing H3K27 acetylation.⁵

Potassium balance in mammals relies on regulated renal potassium excretion matching unregulated fluctuating potassium intake. A high potassium intake has to be followed by a rapid potassium excretion which possibly goes in line with an increased tubular flow which was addressed⁶ in a recent study in Acta Physiologica. The researchers challenged mice with potassium through diet or gavage. Afterward, the urinary and plasma concentrations of potassium, sodium, and osmolarity were determined. Further detailed analyses were performed in isolated thick ascending limb collecting ducts in potassium switching experiments. Immunoblotting was employed to quantify the abundance of transport proteins. Svendsen et al found that mice that switched from a 1% to 2% K+ diet showed increased diuresis within 12 h and reciprocally reduced diuresis when switched from 1% to 0.01% K+ diet. Diuresis was doubled after potassium gavage load of approximately 50% of daily potassium load. Interestingly, this occurred despite augmented plasma osmolarity and AVP synthesis. In contrast, this gavage did not change GFR. The experiments in isolated kidney sections revealed that the increase of potassium load from 3.6 to 6.5 mM in the isolated perfused thick ascending limbs, did not affect AVP-induced NaCl transport. Most interestingly, this was in sharp contrast to the findings in isolated perfused CDs. The same increase in potassium load markedly reduced CD AVP sensitivity, that is, inhibited water absorption.

Svendsen et al concluded that the dietary K+ loading induces a rapidly on-setting diuresis. It was further concluded that the rapid mechanism of potassium-induced diuresis involves the desensitization of the tubular distal convoluted segment to vasopressin. It has to be pointed out that this desensitization effect is of particular interest for further research since it might add dynamical considerations in potential dietary recommendations.

The following study refers to pulmonary hypertension and was performed in animals. Group 2 pulmonary hypertension (PH) is a condition for which there are currently no approved treatments.⁷ It is known that metabolic remodeling, specifically a biventricular increase in pyruvate kinase muscle (PKM) isozyme 2 to 1 ratio occurs in rats with group 2 pulmonary hypertension that was induced by supra-coronary aortic banding (SAB). Xiong et al hypothesize that increased ratio of PKM2/PKM1 is maladaptive and inhibiting PKM2 would possibly improve right ventricular (RV) function. To solve this, the researchers performed a pulmonary hypertension study in male, Sprague–Dawley SAB rats randomized to (a) treatment with a PKM2 inhibitor (intraperitoneal shikonin, 2 mg/kg/day) versus (b) 5% DMSO or small interfering RNA-targeting PKM2 (siPKM2) versus (c) siRNA controls by airway nebulization. The pulmonary hypertension was confirmed by echocardiography.

Xiong et al found that shikonin-treated SAB rats had milder PH and lower RV systolic pressure (RVSP) versus DMSO-SAB rats. siPKM2 nebulization reduced PKM2 expression in the RV, increased PAAT, lowered RVSP and reduced diastolic RVFW thickness. Both substances regressed pulmonary hypertension-induced medial hypertrophy of small pulmonary arteries. The researchers concluded that increases in PKM2/PKM1 in the RV contribute to right ventricular dysfunction in group 2 pulmonary hypertension. Chemical or molecular inhibition of PKM2 restores the normal PKM2/PKM1 ratio, reduces pulmonary hypertension, right ventricular systolic pressure, and regresses adverse remodeling. These results suggest that PKM2 may be a potential therapeutic target for group 2 PH and should be further investigated in the future.

Systemic arterial hypertension and heart failure are common cardiovascular diseases that are characterized by an imbalance in the autonomic nervous system, with an increase in sympathetic activity and a decrease in parasympathetic activity.⁸ Most therapeutic approaches seek to treat these diseases by medications that attenuate sympathetic activity. However, there is a growing number of studies demonstrating that the improvement of parasympathetic function, by means of pharmacological or electrical stimulation, can be an effective tool for the treatment of these cardiovascular diseases. In a systematic review by⁸ that appeared in Acta physiologica it is aimed by the researchers to describe the advances reported by experimental and clinical studies that addressed the potential of cholinergic stimulation to prevent autonomic and cardiovascular imbalance in hypertension and heart failure. Cavalcante et al conclude that pharmacological and electrical stimulation of the parasympathetic nervous system has the potential to be used as a therapeutic tool for the treatment of hypertension and heart failure, deserving to be more explored in the clinical setting.

Renin is a key enzyme in the regulation of long-term arterial blood pressure. The main locus of production of secreted renin is the afferent arteriole of the kidney. In recent years, it became apparent that other locations of production might be of importance too. To this end, the following study conducted by Xu et al⁹ might contribute to a deeper understanding. This study⁹ aimed to investigate the role of renin produced within the collecting duct (CD) of the kidney.

Xu et al addressed in a very recent study the involvement of intrarenal RAAS in K+ homeostasis with emphasis on locally generated renin within the collecting duct (CD).

The authors employed an animal model with wild-type (Floxed) and CD-specific deletion of renin (CD renin KO) mice. The animals were treated for one week with a high K+ (HK) diet to investigate the role of CD renin in kaliuresis regulation and further define the underlying mechanism with emphasis on analysis of intrarenal aldosterone biosynthesis. Xu et al found that in floxed mice, renin levels were elevated in the renal medulla and urine following a 1-week HK diet, indicating activation of intrarenal renin. CD renin KO mice had blunted HK-induced intrarenal renin response and developed impaired kaliuresis and elevated plasma potassium level. Among other findings that can be found in the study, the authors conclude that the results of the study support a kaliuretic action of collecting duct renin during HK intake.

Among other pharmacological substances that are used to fight arterial hypertension the class of the angiotensin-converting enzyme inhibitors (ACEi) are important drugs. To investigate potential side effects, the study conducted by Hillmeister et al¹⁰ is noteworthy to mention. In this study, the researchers demonstrate a potent stimulatory effect of ACEi on cerebral arteriogenesis in rats, presumably via bradykinin receptor 1.

There are many molecules involved in sodium handling relevant in the regulation of blood pressure. One is the so-called EnaC sodium channel that can be found for example in the collecting duct of the kidney. EnaC might be involved in the development of hypertension.¹¹ In a recent review by Anand et al¹² that appeared in Acta physiologica activating proteases are summarized in a systematic manner with focus on recent animal models. ENaC is also topic of another interesting review in Acta Physiologica that focuses on rodent models to study sodium retention by Xiao et al.¹³

In addition to the mentioned articles, there were a lot of articles about cardiorespiratory disorders with focus for example on the central nervous system¹⁴ that might be related to hypertension.

Taken together, we find many interesting aspects and topics in recent articles in Acta physiologica that shed new light on basic mechanisms such as the renin system⁹ up to studies closely related to clinical questions. Furthermore, we find articles about previously neglected fields such as the role of gut microbiome¹⁵ and on hypertension or sex differences in the field which was addressed by Ref. [16]. In addition, there are from time to time very interesting and surprising findings in the field such as the finding that the coagulation factor FXI has a protective role of in heart injury that is distinct from its role in coagulation¹⁷ or the discovery of the molecular basis for blood pressure sensing.¹⁸

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