Na,K-ATPase activity in renal tubule cells from Milan hypertensive rats

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Personalized Therapy of Hypertension: the Past and the Future

During the past 20 years, the studies on genetics or pharmacogenomics of primary hypertension provided interesting results supporting the role of genetics, but no actionable finding ready to be translated into personalized medicine. Two types of approaches have been applied: a "hypothesis-driven" approach on the candidate genes, coding for proteins involved in the biochemical machinery underlying the regulation of BP, and an "unbiased hypothesis-free" approach with GWAS, based on the randomness principles of frequentist statistics. During the past 10-15 years, the application of the latter has overtaken the application of the former leading to an enlargement of the number of previously unknown candidate loci or genes but without any actionable result for the therapy of hypertension. In the present review, we summarize the results of our hypothesis-driven approach based on studies carried out in rats with genetic hypertension and in humans with essential hypertension at the pre-hypertensive and early hypertensive stages. These studies led to the identification of mutant adducin and endogenous ouabain as candidate genetic-molecular mechanisms in both species. Rostafuroxin has been developed for its ability to selectively correct Na(+) pump abnormalities sustained by the two abovementioned mechanisms and to selectively reduce BP in rats and in humans carrying the gene variants underlying the mutant adducin and endogenous ouabain (EO) effects. A clinical trial is ongoing to substantiate these findings. Future studies should apply both the candidate gene and GWAS approaches to fully exploit the potential of genetics in optimizing the personalized therapy.

Possible role of brain salt-inducible kinase 1 in responses to central sodium in Dahl rats

In Dahl salt-sensitive (S) rats, Na+ entry into the cerebrospinal fluid (CSF) and sympathoexcitatory and pressor responses to CSF Na+ are enhanced. Salt-inducible kinase 1 (SIK1) increases Na+/K+-ATPase activity in kidney cells. We tested the possible role of SIK1 in regulation of CSF [Na+] and responses to Na+ in the brain. SIK1 protein and activity were lower in hypothalamic tissue of Dahl S (SS/Mcw) compared with salt-resistant SS.BN13 rats. Intracerebroventricular infusion of the protein kinase inhibitor staurosporine at 25 ng/day, to inhibit SIK1 further increased mean arterial pressure (MAP) and HR but did not affect the increase in CSF [Na+] or hypothalamic aldosterone in Dahl S on a high-salt diet. Intracerebroventricular infusion of Na+-rich artificial CSF caused significantly larger increases in renal sympathetic nerve activity, MAP, and HR in Dahl S vs. SS.BN13 or Wistar rats on a normal-salt diet. Intracerebroventricular injection of 5 ng staurosporine enhanced these responses, but the enhancement in Dahl S rats was only one-third that in SS.BN13 and Wistar rats. Staurosporine had no effect on MAP and HR responses to intracerebroventricular ANG II or carbachol, whereas the specific protein kinase C inhibitor GF109203X inhibited pressor responses to intracerebroventricular Na+-rich artificial CSF or ANG II. These results suggest that the SIK1-Na+/K+-ATPase network in neurons acts to attenuate sympathoexcitatory and pressor responses to increases in brain [Na+]. The lower hypothalamic SIK1 activity and smaller effect of staurosporine in Dahl S rats suggest that impaired activation of neuronal SIK1 by Na+ may contribute to their enhanced central responses to sodium.

The role of the kidney in salt-sensitive hypertension

Primary hypertension is one of the leading risk factors for cardiovascular disease. Although the pathogenesis is not completely understood, an imbalance of sodium and chloride homeostasis seems to be relevant both in the induction and in the maintenance of salt-sensitive hypertension. Besides individual renal phenotypes, salt intake is one of the most important environmental determinants of this condition. The Milan hypertensive strain (MHS) of rats is an interesting model to investigate the molecular mechanisms underling the development of salt-sensitive hypertension. In young MHS rats, hypertension is anticipated by a phase of increased salt reabsorption localized along the medullary thick ascending limb associated with the up-regulation of the apical sodium-potassium-chloride cotransporter (NKCC2). Later, the frank hypertensive status of adult MHS rats is accompanied by the activation of the luminal and basal lateral transporters of sodium chloride (NaCl) in the distal convoluted tubule (DCT). Several lines of evidence have proven the key role of DCT in the maintenance of hypertension in MHS rats; more importantly, hypertensive patients carrying a mutation of α-adducin (resembling the MHS model) have a high sensitivity to thiazides, suggesting that the Na(+)-Cl(-) cotransporter also plays a pivotal role in humans.

Blocking the salt-inducible kinase 1 network prevents the increases in cell sodium transport caused by a hypertension-linked mutation in human alpha-adducin

OBJECTIVES

Because a newly described salt-inducible kinase 1 (SIK1) network is responsible for increases in active cell sodium transport in response to elevated intracellular sodium, we hypothesized that this network could mediate the effects of the mutant (hypertensive) form of alpha-adducin on Na,K-ATPase activity.

METHODS

Studies were performed in normotensive and hypertensive Milan rats and in a cell line of proximal tubule origin expressing transiently variants of alpha-adducin (human G460W/S586C; rat F316Y) that are associated with elevated blood pressure and result in increased Na,K-ATPase activity. Na,K-ATPase activity was determined as ouabain-sensitive rubidium transport.

RESULTS

SIK1 activity (T182 phosphorylation) was significantly elevated in renal proximal tubule cells from Milan hypertensive rats (carrying a alpha-adducin mutation) when compared with normotensive controls. Similarly, SIK1 activity (T182 phosphorylation) was elevated in a normal renal proximal tubule cell line when transfected with the alpha-adducin variant carrying the human hypertensive mutation. Blocking the SIK1 network using negative mutants as well as different stages of its activation pathway prevented the effects induced by the hypertensive alpha-adducin.

CONCLUSION

The SIK1 network may constitute an alternative target by which agents can modulate active sodium transport in renal epithelia and avoid the increases in systemic blood pressure that are associated with genetic mutations in the human alpha-adducin molecule.

Effect of maturation on renal Na+/K+-atpase and its susceptibility to nitric oxide-deficient hypertension in rats

1. The present study deals with the effect of maturation on the kinetic properties of renal Na(+)/K(+)-ATPase and its susceptibility to nitric oxide (NO)-deficient hypertension induced by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). 2. Immature (4-week-old) and adult (12-week-old) male Wistar rats were administered L-NAME (40 mg/kg per day) in their drinking water for 4 weeks. 3. The properties of the ATP- and Na(+)-binding sites of Na(+)/K(+)-ATPase were investigated by activation of the enzyme with increasing concentrations of the energy substrate ATP and/or cofactor Na(+). Unchanged values of K(m) suggest that energy utilization by the enzyme in the kidney of control rats remains unaffected during maturation. Conversely, the decrease in K(Na) values (the concentration of Na(+) necessary to achieve half-maximal reaction velocity) indicates improved affinity for Na(+) in the older group of control rats. 4. Application of L-NAME to all young animals had no significant effect on the functional properties of Na(+)/K(+)-ATPase. 5. In adult animals, the V(max) values remained unchanged after treatment with L-NAME, but the affinities of the ATP- and Na(+)-binding sites were decreased, as indicated by significant increase in K(m) and K(Na) values. 6. Maturation of control rats was accompanied by an increase in the Na(+) affinity of renal Na(+)/K(+)-ATPase without affecting ATP utilization. However, maturation increased the susceptibility of renal Na(+)/K(+)-ATPase to the harmful effects of L-NAME.

INTRARENAL OXYGEN AND HYPERTENSION

The kidney has a unique environment that results in relatively low tissue oxygen tension (Po2). However, recent studies have shown that renal hypoxia is more severe during hypertension and may reflect changes in the way O2 is used. The present review summarizes studies that explore the relationship between renal oxygen tension (Po2), oxygen consumption and hypertension. More recent studies suggest that oxidative stress accompanying hypertension, rather than the elevated blood pressure per se reduces Po2. The Po2 in various sections of the kidney often reflects the level of oxygen consumption, which varies depending on the sites of Na+ reabsorption, a process that consumes nearly 90% of total renal oxygen. The efficient use of oxygen for the transport of Na+ in the kidney is reduced during hypertension, which may contribute to the resulting hypoxia. Conversely, the defect in renal oxygen usage due to oxidative stress may exacerbate hypertension in animal models. The goal of many of these studies is to determine the impact of renal hypoxia in the generation of hypertension.

Genetic variations of tubular sodium reabsorption leading to “primary” hypertension: from gene polymorphism to clinical symptoms

The definition of the most appropriate strategy to demonstrate causation of a given genetic-molecular mechanism in a complex multifactorial polygenic disease like hypertension is hampered by the underestimation of the complexity arising from the genetic and environmental interactions. To disentangle this complexity, we developed a strategy based on six steps: 1) isolation of a rodent model of hypertension (Milan hypertensive strain and Milan normotensive strain) that shares some pathophysiological abnormalities with human primary hypertension; 2) definition in the model of the sequence of events linking these abnormalities to a genetic molecular mechanism; 3) determination of the polymorphism of the three adducin genes discovered in the model both in rats and in humans; 4) comparison at biochemical and physiological levels between the rodent models and the hypertensive carriers of the “mutated” gene variants; 5) evaluation of the impact of the adducin genes in hypertension and its organ complications with association and linkage studies in humans, also considering the genetic and environmental interactions; and 6) development of a pharmacogenomic approach aimed at establishing the therapeutic benefit of a drug interfering with the sequence of events triggered by adducin and their effect's size. The bulk of data obtained demonstrates the importance of a multidisciplinary approach considering a variety of genetic and environmental interactions. Adducin functions within the cells as a heterodimer composed of a combination of three subunits. Each of these subunits is coded by genes mapping to different chromosomes. Therefore, the interaction among these genes, taken together with the interactions with other modulatory genes or with the environment, is indispensable to establish the adducin clinical impact. The hypothesis that adducin polymorphism favors the development of hypertension via an increased tubular sodium reabsorption is well supported by a series of consistent experimental and clinical data. Many mechanistic aspects, underlying the link between these genes and clinical symptoms, need to be clarified. The clinical effect size of adducin must be established also with the contribution of pharmacogenomics with a drug that selectively interferes with the sequence of events triggered by the mutated adducin.

Inhibition of Na,K-ATPase by Dopamine in Proximal Tubule Epithelial Cells

In the current report we review the results that lay grounds for the model of intracellular sodium-mediated dopamine-induced endocytosis of Na,K-ATPase. Under conditions of a high salt diet, dopamine activates PKCzeta, which phosphorylates NKA alpha1 Ser-18. The phosphorylation produces a conformational change of alpha1 NH2-terminus, which through interaction with other domains of alpha1 exposes PI3K- and AP-2-binding domains. PI3K bound to the NKA alpha1 induces the recruitment and activation of other proteins involved in endocytosis, and PI3K-generated 3-phosphoinositides affect the local cytoskeleton and modify the biophysical conditions of the membrane for development of clathrin-coated pits. Plasma membrane phosphorylated NKA is internalized to specialized intracellular compartments where the NKA will be dephosphorylated. The NKA internalization results in a reduced Na+ transport by proximal tubule epithelial cells.

Hypertension-linked mutation in the adducin alpha-subunit leads to higher AP2-mu2 phosphorylation and impaired Na+,K+-ATPase trafficking in response to GPCR signals and intracellular sodium

Alpha-adducin polymorphism in humans is associated with abnormal renal sodium handling and high blood pressure. The mechanisms by which mutations in adducin affect the renal set point for sodium excretion are not known. Decreases in Na+,K+-ATPase activity attributable to endocytosis of active units in renal tubule cells by dopamine regulates sodium excretion during high-salt diet. Milan rats carrying the hypertensive adducin phenotype have a higher renal tubule Na+,K+-ATPase activity, and their Na+,K+-ATPase molecules do not undergo endocytosis in response to dopamine as do those of the normotensive strain. Dopamine fails to promote the interaction between adaptins and the Na+,K+-ATPase because of adaptin-mu2 subunit hyperphosphorylation. Expression of the hypertensive rat or human variant of adducin into normal renal epithelial cells recreates the hypertensive phenotype with higher Na+,K+-ATPase activity, mu2-subunit hyperphosphorylation, and impaired Na+,K+-ATPase endocytosis. Thus, increased renal Na+,K+-ATPase activity and altered sodium reabsorption in certain forms of hypertension could be attributed to a mutant form of adducin that impairs the dynamic regulation of renal Na+,K+-ATPase endocytosis in response to natriuretic signals.

Renal oxidative stress in medullary thick ascending limbs produced by elevated NaCl and glucose

The effects of NaCl, glucose, and thyroid hormone on the production of superoxide (O2*-) within the renal medulla of Sprague-Dawley rats were examined. Responses of intracellular superoxide [O2*-]i in isolated medullary thick ascending limbs (mTALs) were studied using real-time fluorescent microscopy with measurement of the dehydroethidium (DHE) to ethidium (Eth) conversion ratio (Eth/DHE ratio unit). The results demonstrated that elevations of extracellular NaCl (from 152 to 252 mmol/L), D-glucose (from 5 to 25 mmol/L), and triiodo-thyronine (T3; 10 micromol/L) significantly increased [O2*-]i levels. Preincubation with superoxide scavenger 4,5-dihydroxy-1,3-benzene-disulfonic acid (1 mmol/L) significantly inhibited these responses. Stimulation with equamolar amounts of choline chloride or L-glucose failed to increase [O2*]i, indicating that these O2*- responses were not determined by changes in osmolality. The responses to NaCl, D-glucose, and T3 were abolished by pretreatment with the Na+/K+-ATPase pump inhibitor ouabain (4 mmol/L) and with Na+/H+ -exchanger inhibitor dimethylamiloride (100 micromol/L). We conclude that elevations of extracellular NaCl, D-glucose, or T3 levels can activate both the Na+/K+-ATPase pump and Na+/H+ exchanger in mTAL, which, in turn, is associated with increased intracellular concentrations of superoxide.

Pharmacogenomics of primary hypertension--the lessons from the past to look toward the future

A number of recent reviews have addressed the issue of the pharmacogenomics of primary hypertension and related complications by considering the data on the genotype-drug response relationship. Here we mainly discuss the methodological aspects of this issue, trying to integrate 'traditional' clinical and experimental pathophysiology and therapy-pharmacology with the 'new' genetics. Such integration is indispensable to: a). define the appropriate 'context' (genetic background, environment, age, gender, phase of hypertension, previous therapy etc.) in which a given genotype-drug response relationship should be tested (it is indeed likely that many discrepancies among published data originate from context's interference); b). assign the correct clinical meaning to the results obtained by statistics and functional genetics methodologies; c). define a novel clinical entity caused by a disease favoring allele, alone or in combination with other alleles, with a consistent clinical picture, prognosis and responsiveness to the appropriate drug; d). estimate the size of the population target amenable to benefit from a therapeutic intervention developed according to the pharmacogenomics' principles; e). develop a novel drug that selectively interferes with the sequence of events triggered by the genetic mechanism(s) underlying the clinical entity. Peculiar to this strategy is to look for consistency among findings gathered from different 'contexts' after having properly accounted for the context's dependency of the results.

Differential regulation of Na(+),K(+)-ATPase and the Na(+)-coupled glucose transporter in hypertensive rat kidney

Several Na(+) transporters are functionally abnormal in the hypertensive rat. Here, we examined the effects of a high-salt load on renal Na(+),K(+)-ATPase and the sodium-coupled glucose transporter (SGLT1) in Dahl salt-resistant (DR) and salt-sensitive (DS) rats. The protein levels of Na(+),K(+)-ATPase and SGLT1 in the DS rat were the same as those in the DR rat, and were not affected by the high-salt load. In the DS rat, a high-salt load decreased Na(+),K(+)-ATPase activity, and this decrease coincided with a decrease in the apparent Mechaelis constant (K(m)) for ATP, but not with a change of maximum velocity (V(max)). On the contrary, a high-salt load increased SGLT1 activity in the DS rat, which coincided with an increase in the V(max) for alpha-methyl glucopyranoside. The protein level of phosphorylated tyrosine residues in Na(+),K(+)-ATPase was decreased by the high-salt load in the DS rat. The amount of phosphorylated serine was not affected by the high-salt load in DR rats, and could not be detected in DS rats. On the other hand, the amount of phosphorylated serine residues in SGLT1 was increased by the high-salt load. However, the phosphorylated tyrosine was the same for all samples. Therefore, we concluded that the high-salt load changes the protein kinase levels in DS rats, and that the regulation of Na(+),K(+)-ATPase and SGLT1 activity occurs via protein phosphorylation.

Genetic determination of human essential hypertension

Recent advances in genetic determination of human essential hypertension (EHT) are discussed by reviewing the candidate genes. Candidate genes have been selected based on genetic information from classical linkage analysis (affected sib-pair analysis) or mendelian hypertension (autosomal dominant inheritance of hypertension). Most of these genes are, directly or indirectly, coupled to salt handling of the kidney, being included in the renin-angiotensin system (RAS), steroid-hormone metabolism, and renal sodium transporters. Angiotensinogen (AGT) gene in RAS was first described as a strong candidate associated with the onset of hypertension, since sib-pair linkage analysis has demonstrated the trait loci for hypertension which includes the coding region for AGT. M235T polymorphism of AGT has been studied extensively in many populations including Japanese, and the results suggest a weak, but significant linkage with hypertension. The presence (insertion [I]) or absence (deletion [D]) of 287bp in intron 16 of angiotensin converting enzyme gene has also been examined in RAS, and the results suggest D polymorphism as a risk factor for hypertension in men. Other components in RAS, such as renin, angiotensinogen II type I receptor, or kallikrein have also been studied, but the available information is still incomplete. Genetic investigations of mendelian hypertension has identified the genetic mechanisms for glucocorticoid remediable aldosteronism, apparent mineral corticoid excess, and Liddle's syndrome as chimeric gene duplications of CYP11B1 (aldosterone synthase gene) and CYP11B2 (11beta-hydroxylase gene), mutations in the gene of 11beta-hydroxysteroid dehydrogenase type 2 that catalyzes the conversion of cortisol to cortisone, and mutations in beta or gamma subunit of epithelial sodium channel (ENaC), respectively. Subsequently, genetic variants of CYP11B2 and beta or gamma subunit of ENaC have been found, suggesting the -344C polymorphism of CYP11B2, 594S variant of betaENaC, and two rare variants of gammaENaC as risk factors for EHT. In spite of the extensive research, haplotypes in individual populations remain to be elucidcated in most candidate genes. Even casual conclusions of possible linkage with EHT need to be further examined with better determinations of phenotypes, such as ambulatory and home blood pressure monitoring or identification of onset of hypertension in cohort studies.

Genetic and biochemical determinants of abnormal monovalent ion transport in primary hypertension

Data obtained during the last two decades show that spontaneously hypertensive rats, an acceptable experimental model of primary human hypertension, possess increased activity of both ubiquitous and renal cell-specific isoforms of the Na+/H+ exchanger (NHE) and Na+-K+-2Cl- cotransporter. Abnormalities of these ion transporters have been found in patients suffering from essential hypertension. Recent genetic studies demonstrate that genes encoding the beta- and gamma-subunits of ENaC, a renal cell-specific isoform of the Na+-K+-2Cl- cotransporter, and alpha3-, alpha1-, and beta2-subunits of the Na+-K+ pump are localized within quantitative trait loci (QTL) for elevated blood pressure as well as for enhanced heart-to-body weight ratio, proteinuria, phosphate excretion, and stroke latency. On the basis of the homology of genome maps, several other genes encoding these transporters, as well as the Na+/H+ exchanger and Na+-K+-2Cl- cotransporter, can be predicted in QTL related to the pathogenesis of hypertension. However, despite their location within QTL, analysis of cDNA structure did not reveal any mutation in the coding region of the above-listed transporters in primary hypertension, with the exception of G276L substitution in the alpha1-Na+-K+ pump from Dahl salt-sensitive rats and a higher occurrence of T594M mutation of beta-ENaC in the black population with essential hypertension. These results suggest that, in contrast to Mendelian forms of hypertension, the altered activity of monovalent ion transporters in primary hypertension is caused by abnormalities of systems involved in the regulation of their expression and/or function. Further analysis of QTL in F2 hybrids of normotensive and hypertensive rats and in affected sibling pairs will allow mapping of genes causing abnormalities of these regulatory pathways.

Role of the ouabain-like factor and Na-K pump in rat and human genetic hypertension

Endogenous ouabain-like factor (OLF) is present in mammal tissues and after standardized extraction procedure can be similarly quantified by two independent assays: RIA and Na-KATPase inhibition. OLF was quantified both from plasma and tissues obtained from MHS hypertensive and MNS normotensive rats, maintained under the same environmental and dietary conditions, and from plasma of healthy volunteers and essential hypertensive patients. OLF biochemical characterization shows that it behaves like ouabain except for a 1000-fold higher affinity for the ouabain low-affinity Na-KATPase isoforms than ouabain. Tissue and plasma levels of OLF are higher in MHS than in MNS rats and are not influenced by exogenous OLF sources. Plasma OLF is also increased in a subgroup of hypertensive patients. Both in rats and humans a primary cell membrane alteration affecting ion transports seems to be linked to the increased levels of OLF. An antihypertensive compound which selectively antagonizes the pressor effect of OLF and corrects the ion transport defect is under development and can represent a new pharmacological approach to the treatment of hypertension.

Human alpha-adducin gene, blood pressure, and sodium metabolism

The adducin genes contribute significantly to population variation in rat blood pressure and cell membrane sodium transport. The 460Trp mutation of the human alpha-adducin gene has been associated with hypertension, in particular hypertension sensitive to sodium restriction. We studied the relationship between the 460Trp mutation and population variation in blood pressure and sodium metabolism. From 603 Scottish families, we selected 151 offspring and 224 parents with blood pressures in either the upper (high) or bottom (low) 30% of the population distribution and measured the 460Trp mutation using allele-specific hybridization. In offspring, we also measured exchangeable sodium, plasma volume, and total body water. Plasma levels of components of the renin-angiotensin system, atrial natriuretic peptide, and cellular sodium and transmembrane sodium efflux were also estimated. The overall frequency of the 460Trp mutation was 27.1%. In offspring and parent groups, we found no difference in the genotype or allele frequencies of the 460Trp mutation between subjects with high or low blood pressure. There was no overall association between the alpha-adducin genotypes and blood pressure variation. In offspring, the 460Trp mutation was not associated with any significant differences in body fluid volumes or exchangeable sodium; levels of plasma renin, angiotensin II, aldosterone, or atrial natriuretic peptide; intracellular sodium; or ouabain-sensitive transmembrane sodium efflux. These findings suggest that in our Scottish population, the alpha-adducin 460Trp polymorphism is not related to blood pressure and does not affect whole body or cellular sodium metabolism.

Renal Na,K-ATPase in genetic hypertension

Milan hypertensive rats (MHS) develop hypertension because of a primary renal alteration. Both apical and basolateral sodium transport are faster in membrane vesicles derived from renal tubules of MHS than in those of Milan normotensive control rats (MNS). These findings suggest that the increased renal sodium retention and concomitant development of hypertension in MHS may be linked to an altered transepithelial sodium transport. Since this transport is mainly under the control of the Na-K pump, we investigated whether an alteration of the enzymatic activity and/or protein expression of the renal Na,K-ATPase is detectable in prehypertensive MHS. We measured the Na,K-ATPase activity, Rb+ occlusion, turnover number, alpha 1- and beta 1-subunit protein abundance, and alpha 1 and beta 1 mRNA levels in microsomes from renal outer medulla of young (prehypertensive) and adult (hypertensive) MHS and in age-matched MNS. In both young and adult MHS, the Na,K-ATPase activity was significantly higher because of an enhanced number of active pump sites, as determined by Rb+ occlusion maximal binding. The higher number of pump sites was associated with a significant pretranslational increase of alpha 1 and beta 1 mRNA levels that preceded the development of hypertension in MHS. Since a molecular alteration of the cytoskeletal protein adducin is genetically associated with hypertension in MHS and is able to affect the actin-cytoskeleton and Na-K pump activity in transfected renal cells, we propose that the in vivo upregulation of Na-K pump in MHS is primary and linked to a genetic alteration of adducin.

Hypertension-associated point mutations in the adducin alpha and beta subunits affect actin cytoskeleton and ion transport

The adducin heterodimer is a protein affecting the assembly of the actin-based cytoskeleton. Point mutations in rat adducin alpha (F316Y) and beta (Q529R) subunits are involved in a form of rat primary hypertension (MHS) associated with faster kidney tubular ion transport. A role for adducin in human primary hypertension has also been suggested. By studying the interaction of actin with purified normal and mutated adducin in a cell-free system and the actin assembly in rat kidney epithelial cells (NRK-52E) transfected with mutated rat adducin cDNA, we show that the adducin isoforms differentially modulate: (a) actin assembly both in a cell-free system and within transfected cells; (b) topography of alpha V integrin together with focal contact proteins; and (c) Na-K pump activity at V(max) (faster with the mutated isoforms, 1281 +/- 90 vs 841 +/- 30 nmol K/h.mg pt., P < 0.0001). This co-modulation suggests a role for adducin in the constitutive capacity of the epithelia both to transport ions and to expose adhesion molecules. These findings may also lead to the understanding of the relation between adducin polymorphism and blood pressure and to the development of new approaches to the study of hypertension-associated organ damage.

Renal mechanisms of genetic hypertension: from the molecular level to the intact organism

Human primary hypertension is a polygenic disease; its phenotypic expression is modulated by the environment. Though the kidney can play a major role in the initiation and maintainance of hypertension, many questions remain open. Kidney cross-transplantation demonstrated that hypertension can be transplanted with the kidney in all strains of genetically hypertensive rats where such experiments have been carried out. Data consistent with those in rats were also obtained in humans. Many abnormalities in kidney function and ion transport were described in hypertensive rats and humans, but the logical sequence from genetic-molecular to cellular abnormality that causes hypertension via modification of kidney function is difficult to prove. We established this sequence in Milan hypertensive rats using a variety of experimental techniques (isolated kidney and renal cell function, cell membrane ion transport, cross-immunization with membrane proteins, molecular biology, genetic crosses and manipulation). Such studies led to the identification of a polymorphism in the cytoskeletal protein adducin. This polymorphism seems involved in blood pressure regulation both in rats and humans. Preliminary results suggest that adducin polymorphism affects kidney function by modulating the overall capacity of tubular epithelial cells to transport ions modifying the assembly of actin cytoskeleton.

Association of the alpha-adducin locus with essential hypertension

Previous studies on genetic rat hypertension have shown that polymorphism within the alpha-adducin gene may regulate blood pressure. Adducin is a cytoskeletal protein that may be involved in cellular signal transduction and interacts with other membrane-skeleton proteins that affect ion transport across the cell membrane. There is a high homology between rat and human adducin and pathophysiological similarities between the Milan hypertensive rat strain and a subgroup of patients with essential hypertension. Thus, we designed a case-control study to test the possible association between the alpha-adducin locus and hypertension. One hundred ninety primary hypertensive patients were compared with 126 control subjects. All subjects were white and unrelated. Four multiallelic markers surrounding the alpha-adducin locus located in 4p16.3 were selected: D4S125 and D4S95 mapping at 680 and 20 kb centromeric, and D4S43 and D4S228/E24 mapping at 660 and 2500 kb telomeric. Alleles for each marker were pooled into groups. Comparisons between control subjects and hypertensive patients were carried out by testing the allele-disease association relative to the marker genotype. The maximal association occurred for D4S95 (chi 2(1) 13.33), which maps closest to alpha-adducin. These data suggest that a polymorphism within the alpha-adducin gene may affect blood pressure in humans.

Recognition of markers of response to potassium-canrenoate in essential hypertension

Potassium canrenoate (K-Can) prevents hypertension in Milan hypertensive strain (MHS) but not in spontaneously hypertensive rats (SHR). Essential hypertensive patients (HT) may have differential sensitivity to diuretics, since a subgroup of HT insensitive to hydrochlorothiazide (HCTZ) but sensitive to K-Can has previously been found. The aims of this study were: 1) to seek markers of response in essential hypertensive patients selectively sensitive to K-Can: and 2) to test whether selective sensitivity to furosemide may also be demonstrated. After 2 weeks of placebo (P) 50 uncomplicated, mild to moderate HT (46 +/- 9 yrs, mean +/- SD) received K-Can (50 mg/day) for 4 weeks. After 2 more weeks of P, patients received HCTZ (25 mg) and furosemide (25 mg) for 4 weeks each in a single blind crossover design, with 2 weeks P between each treatment. Dosages were doubled after 2 weeks if diastolic blood pressure (DBP) was > 90 mmHg. Responders (R) were those HT whose DBP was < or = 90 mmHg and/or at least 10 mmHg lower than before treatment. Systolic blood pressure (SBP)/DBP was measured every 2 weeks with plasma renin activity (PRA), red blood cell Na(+)-K(+)-Cl- cotransport (COT) and Na(+)-K+ ATPase pump activity measured at the end of the first P period, and serum electrolytes at the end of each period. Four HT dropped out because of low compliance, 6 because of reversible side effects, and 1 because blood pressure was not back to pre-treatment value after the second placebo period.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic models of arterial hypertension--role of tubular ion transport

The description of pathogenetic mechanisms underlying different genetic models of essential hypertension is a useful way of illustrating the logical sequence needed to dissect a complex phenotypic condition such as hypertension. The abnormalities in renal function observed in spontaneously hypertensive rats of the Okamoto strain and Milan strain will be emphasized. The description may proceed "downward" from alterations that affect the whole body function to cellular and subcellular levels. However, the identification in the Milan strain rats of a point mutation in the gene coding for adducin, a skeletal protein able to modulate transepithelial sodium transport, provides the opportunity to reconstruct, in an "upward" direction, the sequence of events leading from the single point mutation to the final complex phenotype of essential hypertension.

Relationships among alterations in renal membrane sodium transport, renin and aminopeptidase M activities in genetic hypertension

Rats of the Milan Hypertensive Strain (MHS) may be considered a useful model for understanding the genetic molecular mechanism underlying a primary form of hypertension in at least a subgroup of patients. Many differences between MHS and its normotensive control strain (MNS) were found at the organ, cellular and biochemical level. In the present investigation renal cell membrane proteins (BBMV) were analysed by two-dimensional electrophoresis and a difference between MHS and MNS was shown in a polypeptide of 32 kDa, subsequently identified as the C-terminal fragment of aminopeptidase M (APM). The activity of the enzyme was higher in MHS. Genetic relationships between this enzyme and the other biochemical cellular abnormalities of MHS, namely sodium transport in BBMV and renin activity in kidney cortex were investigated in MHS, MNS and in two inbred recombinant strains. This analysis showed that faster sodium transport, low kidney levels of renin and hypertension, but not differences in two-dimensional electrophoretic pattern and in aminopeptidase M activity, cosegregated in recombinant strains. These results are consistent with the hypothesis that the faster sodium transport can be considered a primary cellular abnormality responsible for hypertension in MHS and that the aminopeptidase difference is not involved in the cellular abnormalities.

Increased renal tubular Na-K-ATPase activity in Milan hypertensive rats in the prehypertensive period

Milan hypertensive (MSH) rats develop hypertension around the 3rd-4th week of life and exhibit increased Na-pump activity in adulthood. The present study was performed to evaluate whether or not hypertension is preceded by an increase in Na-K-ATPase activity. Total and ouabain-sensitive ATPase activities were studied in single microdissected medullary thick ascending limb of Henle (mTAL) tubules from MHS, Milan normotensive (MNS) and Sprague-Dawley (SD) rats at 22-24, 26-28 and 45-60 days of age. Data are given as mean +/- SEM. Total and Na-K-ATPase activity exhibited a developmental pattern in MHS, MNS and SD rats. At 22-24 days no difference was seen between MHS and MNS animals. At 26-28 days MHS had a higher total and Na-K-ATPase activity than MNS (3031 + 171 vs 2471 + 178 pmol phosphate/mm tubule per hour, P less than 0.05; 2289 + 205 vs 1653 + 151, n = 10, P less than 0.05). At this age there was still no difference in mean arterial blood pressure (88 + 4 vs 86 + 3 mm Hg, n = 15). Adult MHS rats had higher blood pressure (140 + 9 vs 112 + 8 mm Hg, P less than 0.001) and higher total (3544 + 136 vs 2718 + 215 pmol phosphate/mm tubule per hour, n = 10, P less than 0.01) and Na-K-ATPase activity (2670 + 99 vs 1942 + 217 pmol phosphate/mm tubule per hour, n = 10, P less than 0.05) than adult MNS rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased Na pump activity in the kidney cortex of the Milan hypertensive rat strain

The (Na+,K+)-ATPase activity from the kidney cortex of the Milan hypertensive rat strain (MHS) and the corresponding normotensive control (MNS) was measured both in active solubilized enzyme preparations and in isolated basolateral membrane vesicles. Kinetic analysis of the purified enzyme showed that the Vmax value was significantly higher in MHS rats. The difference between MHS and MNS was not linked to a different number of sodium pumps, but was related to the molecular activity of the enzyme. Using basolateral membrane vesicles, an increased ATP-dependent ouabain-sensitive sodium transport was also demonstrated in MHS rats. These results support the hypothesis that a higher tubular sodium reabsorption may be involved in the pathogenesis of hypertension in this rat strain.