The alpha 1 Na(+)-K+ pump of the Dahl salt-sensitive rat exhibits altered Na+ modulation of K+ transport in red blood cells

A functional 12T-insertion polymorphism in the ATP1A1 promoter confers decreased susceptibility to hypertension in a male Sardinian population

Identification of susceptibility genes for essential hypertension in humans has been a challenge due to its multifactorial pathogenesis complicated by gene-gene and gene-environment interactions, developmental programing and sex specific differences. These concurrent features make identification of causal hypertension susceptibility genes with a single approach difficult, thus requiring multiple lines of evidence involving genetic, biochemical and biological experimentation to establish causal functional mutations. Here we report experimental evidence encompassing genetic, biochemical and in vivo modeling that altogether support ATP1A1 as a hypertension susceptibility gene in males in Sardinia, Italy. ATP1A1 encodes the α1Na,K-ATPase isoform, the sole sodium pump in vascular endothelial and renal tubular epithelial cells. DNA-sequencing detected a 12-nucleotide long thymidine (12T) insertion(ins)/deletion(del) polymorphism within a poly-T sequence (38T vs 26T) in the ATP1A1 5’-regulatory region associated with hypertension in a male Sardinian population. The 12T-insertion allele confers decreased susceptibility to hypertension (P = 0.035; OR = 0.50 [0.28–0.93]) accounting for 12.1 mmHg decrease in systolic BP (P = 0.02) and 6.6 mmHg in diastolic BP (P = 0.046). The ATP1A1 promoter containing the 12T-insertion exhibited decreased transcriptional activity in in vitro reporter-assay systems, indicating decreased α1Na,K-ATPase expression with the 12T-insertion, compared with the 12T-deletion ATP1A1 promoter. To test the effects of decreased α1Na,K-ATPase expression on blood pressure, we measured blood pressure by radiotelemetry in three month-old, highly inbred heterozygous knockout ATP1A1+/− male mice with resultant 58% reduction in ATP1A1 protein levels. Male ATP1A1+/− mice showed significantly lower blood pressure (P < 0.03) than age-matched male wild-type littermate controls. Concordantly, lower ATP1A1 expression is expected to lower Na-reabsorption in the kidney thereby decreasing sodium-associated risk for hypertension and sodium-induced endothelial stiffness and dysfunction. Altogether, data support ATP1A1 as a hypertension susceptibility gene in a male Sardinian population, and mandate further investigation of its involvement in hypertension in the general population.

Effect of complete protein 4.1R deficiency on ion transport properties of murine erythrocytes

Moderate hemolytic anemia, abnormal erythrocyte morphology (spherocytosis), and decreased membrane stability are observed in mice with complete deficiency of all erythroid protein 4.1 protein isoforms (4.1(-/-); Shi TS et al. J Clin Invest 103: 331, 1999). We have examined the effects of erythroid protein 4.1 (4.1R) deficiency on erythrocyte cation transport and volume regulation. 4.1(-/-) mice exhibited erythrocyte dehydration that was associated with reduced cellular K and increased Na content. Increased Na permeability was observed in these mice, mostly mediated by Na/H exchange with normal Na-K pump and Na-K-2Cl cotransport activities. The Na/H exchange of 4.1(-/-) erythrocytes was markedly activated by exposure to hypertonic conditions (18.2 +/- 3.2 in 4.1(-/-) vs. 9.8 +/- 1.3 mmol/10(13) cell x h in control mice), with an abnormal dependence on osmolality (EC(50) = 417 +/- 42 in 4.1(-/-) vs. 460 +/- 35 mosmol/kgH(2)O in control mice), suggestive of an upregulated functional state. While the affinity for internal protons was not altered (K(0.5) = 489.7 +/- 0.7 vs. 537.0 +/- 0.56 nM in control mice), the V(max) of the H-induced Na/H exchange activity was markedly elevated in 4.1(-/-) erythrocytes (V(max) 91.47 +/- 7.2 compared with 46.52 +/- 5.4 mmol/10(13) cell x h in control mice). Na/H exchange activation by okadaic acid was absent in 4.1(-/-) erythrocytes. Altogether, these results suggest that erythroid protein 4.1 plays a major role in volume regulation and physiologically downregulates Na/H exchange in mouse erythrocytes. Upregulation of the Na/H exchange is an important contributor to the elevated cell Na content of 4.1(-/-) erythrocytes.

Sex-specific QTLs and interacting loci underlie salt-sensitive hypertension and target organ complications in Dahl S/jrHS hypertensive rats

Sex-specific differences in polygenic (essential) hypertension are commonly attributed to the role of sex steroid hormone-receptor systems attenuating sex-common disease mechanisms in premenopausal women. However, emerging observations indicate sex-specific genetic susceptibility in various traits, thus requiring systematic study. Here we report a comparative analysis of independent total genome scans for salt-sensitive hypertension susceptibility quantitative trait loci (QTLs) in male and female F2 [Dahl R/jrHS x S/jrHS] intercross rats exposed to high-salt (8% NaCl) rat diets. Hypertension was phenotyped with three quantitative traits: blood pressure (BP) elevation associated with increased hypertensive renal disease [glomerular injury score (GIS)] and increased cardiac mass [relative heart weight (RHW)] obtained 8-12 wk after high-salt challenge; 24-h nonstress, telemetric BP measurements were used. Although sex-common QTLs were detected for BP [chromosome (chr) 1-144.3 Mbp; chr 1-208.8 Mbp], GIS (chr 1-208.8 Mbp), and cardiac mass (chr 5-150.3 Mbp), most QTLs across the three phenotypes studied are gender specific as follows: female QTLs for BP (chr 2-106.7 Mbp, chr 2-181.7 Mbp, chr 5-113.9 Mbp, chr 5-146.7 Mbp, chr 12-12.8 Mbp), GIS (chr 15-59.6 Mbp), and RHW (chr 2-31.5 Mbp, chr 5-154.7 Mbp, chr 5-110.9 Mbp); male QTLs for BP (chr 2-196.7 Mbp, chr 11-48.0 Mbp, chr 20-35.7 Mbp), GIS (chr 6-3.3 Mbp, chr 20-40.7 Mbp), and RHW (chr 6-3.3 Mbp, chr 20-40.7 Mbp). Furthermore, interacting loci with significant linkage were detected only in female F2 intercross rats for BP and hypertensive renal disease. Comparative analyses revealed concordance of BP QTL peaks with previously reported rat model and human hypertension susceptibility genes and with BP QTLs in previous Dahl S-derived F2 intercross studies and also suggest strain-specific genetic modifiers of sex-specific determinants. Altogether, the data provide key experimental bases for sex-specific investigation of mechanisms and intervention and prevention strategies for polygenic hypertension in humans.

Relationships between membrane lipids and ion transport in red blood cells of Dahl rats

Distinct changes of membrane lipid content could contribute to the abnormalities of ion transport that take part in the development of salt hypertension in Dahl rats. The relationships between lipid content and particular ion transport systems were studied in red blood cells (RBC) of Dahl rats kept on low- and high-salt diets for 5 weeks since weaning. Dahl salt-sensitive (SS/Jr) rats on high-salt diet had increased blood pressure, levels of plasma triacylglycerols and total plasma cholesterol compared to salt-resistant (SR/Jr) rats. Furthermore, RBC of SS/Jr rats differed from SR/Jr ones by increased content of total membrane phospholipids, but membrane cholesterol was not changed significantly. SS/Jr rats had higher RBC intracellular Na+ (Na(i)+) content and enhanced bumetanide-sensitive Rb+ uptake. RBC membrane content of cholesterol and phospholipids correlated positively with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and also with Rb+ leak. The content of phosphatidylserines plus phosphatidylinositols was positively associated with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and with Rb+ leak. The content of sphingomyelins was positively related to Na+-K+-2Cl- cotransport activity and negatively to ouabain-sensitive Rb+-K+ exchange. We can conclude that observed relationships between ion transport and the membrane content of cholesterol and/or sphingomyelins, which are known to regulate membrane fluidity, might participate in the pathogenesis of salt hypertension in Dahl rats.

Corroboration of Dahl S Q276L alpha1Na,K-ATPase protein sequence: impact on affinities for ligands and on E1 conformation

OBJECTIVE

Multifactorial analyses support the hypothesis that alpha1Na,K-ATPase is a hypertension susceptibility gene in Dahl S rats. However, two studies report non-detection of the A1079T transversion underlying the Q276L substitution in Dahl S alpha1Na,K-ATPase questioning the validity of ATP1A1 as a hypertension susceptibility gene. To resolve this discordance, we investigated the issue at the protein level.

DESIGN AND METHODS

We employed protein blot analysis using Q276L- and Q276-specific; antipeptide-specific antibodies; tested differential chymotrypsin cleavage efficiency, measured differential Na and K affinities of alpha1Na,K-ATPases in Dahl S and Dahl R renal membranes and determined amino acid sequences of purified Dahl S alpha1Na,K-ATPase chymotryptic-digest peptides.

RESULTS

We detected Q276L variant protein in Dahl S rats; and Q276 wild-type variant in Dahl R, spontaneously hypertensive (SHR), Lewis and Wistar-Kyoto (WKY) rat kidney membranes. Q276L variant exhibits less chymotrypsin cleavage efficiency than the Q276 wild-type variant, consistent with the substitution of hydrophobic L for hydrophilic Q. Kinetic studies of kidney membranes detect increased Na affinity and decreased K affinity in renal Dahl S alpha1Na,K-ATPase compared with Dahl R. Protein sequencing of high pressure liquid chromatography (HPLC)-purified chymotrypsin digested 77 kDa peptide confirms Q276L substitution in the Dahl S alpha1Na,K-ATPase.

CONCLUSIONS

Data demonstrate the existence and functional significance of the Q276L variant in Dahl S rats.

Erythrocyte ion transport and membrane lipid composition in young and adult rats with NO-deficient hypertension

The aim of our study was to search for abnormalities of sodium and potassium transport in erythrocytes of male Wistar rats subjected to chronic L-NAME treatment (40 mg/kg/day) for 4 weeks either from weaning (4-week-old) or in adulthood (12-week-old). Sodium content, Na(+),K(+)-pump and Na(+),K(+)-cotransport activity, cation leaks as well as membrane cholesterol and phospholipid contents were determined in fresh erythrocytes. Chronic inhibition of NO synthase elicited similar blood pressure rise in both age groups which did not differ in the degree of NO synthase inhibition. No significant ion transport abnormalities were disclosed in erythrocytes of young NO-deficient rats, whereas erythrocyte Na(+) content, outward Na(+),K(+)-cotransport and inward Na(+) leak were significantly reduced in adult hypertensive animals compared to age-matched controls. It should be noted that the erythrocytes of adult control rats were characterized by higher activity of Na(+),K(+)-pump and Na(+),K(+)-cotransport, increased Na(+) and Rb(+) leaks and elevated membrane cholesterol content compared to those of young normotensive controls. Increased Na(+) leak and elevated membrane cholesterol content but reduced membrane phospholipid content were revealed in erythrocytes of adult hypertensive rats when compared to young hypertensive rats. It can be concluded that young and adult Wistar rats did not differ in the extent of NO synthase inhibition and blood pressure rise elicited by chronic L-NAME treatment. Our results exclude the important participation of classical sodium transport abnormalities in the pathogenesis of this NO-deficient form of experimental hypertension.

Endogenous cardiac glycosides, a new class of steroid hormones

The search for endogenous digitalis has led to the isolation of ouabain as well as several additional cardiotonic steroids of the cardenolide and bufadienolide type from blood, adrenals, and hypothalamus. The concentration of endogenous ouabain is elevated in blood upon increased Na(+) uptake, hypoxia, and physical exercise. Changes in blood levels of ouabain upon physical exercise occur rapidly. Adrenal cortical cells in tissue culture release ouabain upon addition of angiotensin II and epinephrine, and it is thought that ouabain is released from adrenal cortex in vivo. Ouabain levels in blood are elevated in 50% of Caucasians with low-renin hypertension. Infusion over several weeks of low concentrations of ouabain, but not of digoxin, induces hypertension in rats. A digoxin-like compound, which has been isolated from human urine and adrenals, as well various other endogenous cardiac glycosides may counterbalance their actions within a regulatory framework of water and salt metabolism. Marinobufagenin, for instance, whose concentration is increased after cardiac infarction, may show natriuretic properties because it inhibits the alpha1 isoform of Na(+)/K(+)-ATPase, the main sodium pump isoform of the kidney, much better than other sodium pump isoforms. In analogy to other steroid hormones, cardiotonic steroid hormones in blood are bound to a specific cardiac glycoside binding globulin. The discovery of ouabain as a new adrenal hormone affecting Na(+) metabolism and the development of the new ouabain antagonist PST 2238 allows for new possibilities for the therapy of hypertension and congestive heart failure. This will lead in turn to a better understanding of the disease on a physiological and endocrinological level and of the action of ouabain on the cellular level as a signal that is transduced to the plasma membrane as well as to the cell nucleus.

Replacement of (alpha)1-Na-K-ATPase of Dahl rats by Milan rats lowers blood pressure but does not affect its activity

Both linkage and use of congenic strains have shown that a chromosome region near the gene for the Na-K-ATPase alpha(1)-subunit (Atp1a1) contained a quantitative trait locus (QTL) for blood pressure (BP). Currently, two congenic strains, designated S.M5 and S.M6, were made by replacing a segment of the Dahl salt-sensitive SS/Jr (S) rat by the homologous region of the Milan normotensive rat (MNS). In S.M5, the gene for Atp1a1 is from the MNS strain; whereas in S.M6, Atp1a1 is from the S strain. The baseline activity of the alpha(1)-Na-K-ATPase and its stoichiometry were evaluated by an assay of ouabain-sensitive inwardly and outwardly directed (86)Rb and (22)Na fluxes in erythrocytes. The two congenic strains showed a similar BP, but both had a BP lower than that of S rats (P < 0.0001). Neither the alpha(1)-Na-K-ATPase activity nor its stoichiometry was affected by the substitution of the Atp1a1 alleles of S by those of MNS. Thus the BP-lowering effects observed in S.M5 and S.M6 could not be attributed to the alpha(1)-Na-K-ATPase activity or its stoichiometry. Atp1a1 is not supported as a candidate to be a BP QTL.

Altered Na+-K+ pump activity and plasma lipids in salt-hypertensive Dahl rats: relationship to Atp1a1 gene

A genetic variant of the gene for the alpha(1)-isoform of Na(+)-K(+)-ATPase (Atp1a1) was suggested to be involved in the pathogenesis of salt hypertension in Dahl rats through altered Na(+):K(+) coupling ratio. We studied Na(+)-K(+) pump activity in erythrocytes of Dahl salt-sensitive (SS/Jr) rats in relation to plasma lipids and blood pressure (BP) and the linkage of polymorphic microsatellite marker D2Arb18 (located within intron 1 and exon 2 of Atp1a1 gene) with various phenotypes in 130 SS/Jr x SR/Jr F(2) rats. Salt-hypertensive SS/Jr rats had higher erythrocyte Na(+) content, enhanced ouabain-sensitive (OS) Na(+) and Rb(+) transport, and higher Na(+):Rb(+) coupling ratio of the Na(+)-K(+) pump. BP of F(2) hybrids correlated with erythrocyte Na(+) content, OS Na(+) extrusion, and OS Na(+):Rb(+) coupling ratio, but not with OS Rb(+) uptake. In F(2) hybrids there was a significant association indicating suggestive linkage (P < 0.005, LOD score 2.5) of an intragenic marker D2Arb18 with pulse pressure but not with mean arterial pressure or any parameter of Na(+)-K(+) pump activity (including its Na(+):Rb(+) coupling ratio). In contrast, plasma cholesterol, which was elevated in salt-hypertensive Dahl rats and which correlated with BP in F(2) hybrids, was also positively associated with OS Na(+) extrusion. The abnormal Na(+):K(+) stoichiometry of the Na(+)-K(+) pump is a consequence of elevated erythrocyte Na(+) content and suppressed OS Rb(+):K(+) exchange. In conclusion, abnormal cholesterol metabolism but not the Atp1a1 gene locus might represent an important factor for both high BP and altered Na(+)-K(+) pump function in salt-hypertensive Dahl rats.

Renal proximal tubule sodium transport and genetic mechanisms of essential hypertension

Renal sodium re-absorption is a closely regulated process serving to maintain both extracellular fluid volume and arterial blood pressure. Proteins participating in sodium re-absorption and its regulation are therefore important candidate proteins whose genes may contain sequence variation contributing to the inherited tendency for increased arterial blood pressure (essential hypertension). Important insight has come from rare forms of single-gene hypertension in human subjects and from polygenic animal models of genetic hypertension. Both indicate the primacy of altered renal function in the genesis of hypertension, and suggest that genes contributing to the disease are members of the subset of genes expressed in the kidney. This review examines evidence for abnormalities in renal sodium re-absorption in hypertension and focuses on the proximal tubule as a site of relevant dysfunction. Identification of the proteins participating in renal sodium re-absorption and its regulation, particularly those involved in the renal pressure-natriuresis mechanism, will allow gene cloning and sequencing which in turn may lead to the identification of novel gene sequence variation participating in hypertension.

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.

The alpha1 Na,K-ATPase gene is a susceptibility hypertension gene in the Dahl salt-sensitiveHSD rat

Despite the prevalence of essential hypertension, its underlying genetic basis has not been elucidated due to the complexities of its determinants. To identify a hypertension susceptibility gene, we used an approach that integrates molecular, transgenic, and genetic analysis using Dahl salt-sensitive (S) and Dahl salt-resistant (R) rats ascertained for genotype and phenotype. To determine the role of the Dahl S Q276L alpha1 Na,K-ATPase gene variant, we developed transgenic Dahl S rats bearing the Dahl R wild-type (wt) alpha1 Na, K-ATPase cDNA directed by the cognate wt promoter region, Tg[wtalpha1]. Transgenic Dahl S rats exhibited less salt-sensitive hypertension, less hypertensive renal disease, and longer life span when compared with non-transgenic Dahl S controls. Total chromosome 2 linkage analysis of F2(SxR) male rats detects cosegregation of the alpha1 Na,K-ATPase locus with salt-sensitive hypertension. These data support the alpha1 Na,K-ATPase gene as a susceptibility gene for salt-sensitive hypertension in the Dahl S rat model, and provide the basis for the study of the alpha1 Na,K-ATPase locus in human hypertension.

Immortalization and characterization of proximal tubule cells derived from kidneys of spontaneously hypertensive and normotensive rats

Epithelial cell lines from the proximal tubule of SHR and WKY rats were generated by microdissection, cell growth on 3T3 cell feeder layers, and transduction of the SV40 large T-antigen gene. The cell lines that formed confluent, electrically-resistive monolayers (basal conductance 1 to 20 mS/cm2) were selected for further study. Of these, cell lines generated from one rat did not show evidence of T-antigen expression or integration, and apparently immortalized spontaneously. Cell lines from three other rats expressed high levels of T-antigen, and showed evidence of integration of one or more copies of T-antigen. All cell lines formed polarized monolayers with apical microvilli, tight junctional complexes, and convolutions of the basolateral plasma membrane. Most cell lines grew in the absence of extracellular glucose indicating a capacity for gluconeogenesis. Sodium succinate cotransport and P2-purinergic receptor mediated signaling were demonstrated in all lines tested. The cell lines also showed that Na/H exchanger activity is regulated by angiotensin II. The results indicate that these cell lines express a proximal tubular phenotype, and are morphologically and functionally similar to primary cultures. These rat cell lines represent a new, potentially useful cell model for elucidating the cellular and molecular mechanisms of genetic differences in proximal tubule Na+ reabsorption.