Genetic analysis of blood pressure in spontaneously hypertensive rats

Integrative genomic analysis of blood pressure and related phenotypes in rats

Despite remarkable progress made in human genome-wide association studies, there remains a substantial gap between statistical evidence for genetic associations and functional comprehension of the underlying mechanisms governing these associations. As a means of bridging this gap, we performed genomic analysis of blood pressure (BP) and related phenotypes in spontaneously hypertensive rats (SHR) and their substrain, stroke-prone SHR (SHRSP), both of which are unique genetic models of severe hypertension and cardiovascular complications. By integrating whole-genome sequencing, transcriptome profiling, genome-wide linkage scans (maximum n=1415), fine congenic mapping (maximum n=8704), pharmacological intervention and comparative analysis with transcriptome-wide association study (TWAS) datasets, we searched causal genes and causal pathways for the tested traits. The overall results validated the polygenic architecture of elevated BP compared with a non-hypertensive control strain, Wistar Kyoto rats (WKY); e.g. inter-strain BP differences between SHRSP and WKY could be largely explained by an aggregate of BP changes in seven SHRSP-derived consomic strains. We identified 26 potential target genes, including rat homologs of human TWAS loci, for the tested traits. In this study, we re-discovered 18 genes that had previously been determined to contribute to hypertension or cardiovascular phenotypes. Notably, five of these genes belong to the kallikrein-kinin/renin-angiotensin systems (KKS/RAS), in which the most prominent differential expression between hypertensive and non-hypertensive alleles could be detected in rat Klk1 paralogs. In combination with a pharmacological intervention, we provide in vivo experimental evidence supporting the presence of key disease pathways, such as KKS/RAS, in a rat polygenic hypertension model.

Genetics of hypertension: an assessment of progress in the spontaneously hypertensive rat

The application of gene mapping methods to uncover the genetic basis of hypertension in the inbred spontaneously hypertensive rat (SHR) began over 25 yr ago. This animal provides a useful model of genetic high blood pressure, and some of its features are described. In particular, it appears to be a polygenic model of disease, and polygenes participate in human hypertension genetic risk. The SHR hypertension alleles were fixed rapidly by selective breeding in just a few generations and so are presumably common genetic variants present in the outbred Wistar strain from which SHR was created. This review provides a background to the origins and genesis of this rat line. It considers its usefulness as a model organism for a common cardiovascular disease. The progress and obstacles facing mapping are considered in depth, as are the emergence and application of other genome-wide genetic discovery approaches that have been applied to investigate this model. Candidate genes, their identification, and the evidence to support their potential role in blood pressure elevation are considered. The review assesses the progress that has arisen from this work has been limited. Consideration is given to some of the factors that have impeded progress, and prospects for advancing understanding of the genetic basis of hypertension in this model are discussed.

The stroke-prone spontaneously hypertensive rat: still a useful model for post-GWAS genetic studies?

The stroke-prone spontaneously hypertensive rat (SHRSP) is a unique genetic model of severe hypertension and cerebral stroke. SHRSP, as well as the spontaneously hypertensive rat, the parental strain of SHRSP, has made a tremendous contribution to cardiovascular research. However, the genetic mechanisms underlying hypertension and stroke in these rats have not yet been clarified. Recent studies using whole-genome sequencing and comprehensive gene expression analyses combined with classical quantitative trait loci analyses provided several candidate genes, such as Ephx2, Gstm1 and Slc34a1, which still need further evidence to define their pathological roles. Currently, genome-wide association studies can directly identify candidate genes for hypertension in the human genome. Thus, genetic studies in SHRSP and other rat models must be focused on the pathogenetic roles of 'networks of interacting genes' in hypertension, instead of searching for individual candidate genes.

Baroreflex gain in normotensive and GH hypertensive rats before and after early gonadectomy

The authors have assessed arterial baroreflex gain in urethane-anesthetized normotensive and New Zealand genetically hypertensive (GH) rats and investigated the effect of gonadectomy in adult animals at 3 weeks of age postnatally. No gender differences in resting blood pressures existed for either normotensive or GH strains. In normotensive animals, bradycardic gain was greater than tachycardic gain and was lower in females than in males. Tachycardic gain was similar in GH and normotensive rats of either sex, but bradycardic gain was lower in GH. Gonadectomy had no effect on baroreflex gain in male or female animals of either strain.

Relative contribution of the prenatal versus postnatal period on development of hypertension and growth rate of the spontaneously hypertensive rat

1. To determine the relative roles of the prenatal and postnatal (preweaning) environment on the development of blood pressure and growth rate in the spontaneously hypertensive rat (SHR) of the Okamoto strain, we used combined embryo transfer and cross-fostering techniques between SHR and normotensive Wistar-Kyoto (WKY) rats to produce offspring whose development was examined during the first 20 weeks of life. 2. We measured litter sizes, bodyweights and tail-cuff blood pressures in offspring at 4, 8, 12 and 20 weeks of age. We also recorded heart, kidney and adrenal weights at 20 weeks of age, when the study concluded. 3. We found that both the in utero and postnatal environments provided by the SHR mother could significantly affect WKY rat offspring growth rates, but blood pressure was unaffected in this strain. In SHR offspring, the SHR maternal in utero and suckling period both contributed to the rate of blood pressure development in the SHR, but not the final blood pressure of offspring at 20 weeks of age. This effect was greater for male than female offspring. Organ weights were largely unaffected by the perinatal environment in either strain. 4. We conclude that although the SHR maternal in utero and immediate postnatal environment both contribute to the rate of blood pressure development in the SHR, they do not appear to contribute to the final blood pressure of offspring at maturity. The SHR maternal environment also alters growth rate that may, in turn, underlie these effects on SHR blood pressure development, particularly in males.

Genome-wide searches for blood pressure quantitative trait loci in the stroke-prone spontaneously hypertensive rat of a Japanese colony

OBJECTIVES

Although several quantitative trait loci for blood pressure have been reported in stroke-prone spontaneously hypertensive rats (SHRSP), the results are not always concordant among different crosses. To evaluate potential confounding factors in linkage analysis, we performed genome-wide screens in F2 populations derived from SHRSP and Wistar-Kyoto rats of a Japanese colony.

METHODS

Two F cohorts were independently produced: F2-1 (110 male and 110 female rats), and F2-2 (174 male and 184 female rats). Blood pressure was measured longitudinally (from 2 to 5 months of age and 1 month after salt-loading) in F2-1, while it was measured at 13 weeks of age in F2-2. Subsequent to an initial screen with 251 markers in F2-1 male progeny, 170 markers were selected and characterized in the remaining populations.

RESULTS

When 578 rats were analyzed together, markers from five chromosomal regions showed significant linkage to blood pressure at 13 weeks of age. The strongest and the most consistent linkage was found on rat chromosome 1 (a maximal log of the odds score reached 8.3). In the other regions, the degree of linkage was more prominent in either of sexes. Some evidence of age-specific and sex-specific linkage was detected in five additional regions in the F2-1 cohort. In the Japanese colony, however, there was no significant linkage to several chromosomal regions previously reported in other SHRSP colonies.

CONCLUSIONS

Our data provide solid evidence of a chromosome-1 linkage and demonstrate the importance of aging, sex, and dietary manipulation in linkage analysis. Also, the combination of parental rat strains seems to be critical when searching for blood pressure quantitative trait loci.

Rat genetics: attaching physiology and pharmacology to the genome

During the past five years, the Rat Genome Project has been rapidly gaining momentum, especially since the announcement in August 2000 of plans to sequence the rat genome. Combined with the wealth of physiological and pharmacological data for the rat, the genome sequence should facilitate the discovery of mammalian genes that underlie the physiological pathways that are involved in disease. Most importantly, this combined physiological and genomic information should also lead to the development of better pre-clinical models of human disease, which will aid in the development of new therapeutics.

The genetics of essential hypertension

Essential hypertension is an escalating problem for industrialized populations. It is currently seen as a 'complex' genetic trait caused by multiple susceptibility genes the effects of which are modulated by gene-environment and gene-gene interactions. Nevertheless, the success to date in identifying these susceptibility genes has been very limited. A number of candidates has been proposed, but demonstrating consistently the linkage or association with hypertension has been problematic. The data for angiotensinogen is undoubtedly the most extensive and meta-analysis has confirmed a significant association overall, although the risk contributed by this gene appears to be modest (odds ratio of 1.2). Identifying further genes - probably conferring even smaller attributable risks - represents a major challenge for future developments in this area. This contrasts markedly with the success that has been achieved in the past 5 years in solving the molecular genetics of a number of rare familial hypertension syndromes. The true incidences of some of these disorders may be higher than first appreciated, but it is still unclear if the genes for these syndromes also play a part in essential hypertension. A more complete understanding of the genetic basis of essential hypertension should be possible in the coming years using new strategies that take advantage of the information provided by the human genome project. This knowledge will irrevocably change the way we approach this disease in terms of its diagnosis, risk assessment for end-points such as stroke and heart disease, and the customised treatment that might be offered in the future.

Selective genotyping with epistasis can be utilized for a major quantitative trait locus mapping in hypertension in rats

Epistasis used to be considered an obstacle in mapping quantitative trait loci (QTL) despite its significance. Numerous epistases have proved to be involved in quantitative genetics. We established a backcross model that demonstrates a major QTL for hypertension (Ht). Seventy-eight backcrossed rats (BC), derived from spontaneously hypertensive rats (SHR) and normotensive Fischer 344 rats, showed bimodal distribution of systolic blood pressure (BP) values and a phenotypic segregation ratio consistent with 1:1. In this backcross analysis, sarco(endo)plasmic reticulum Ca(2+)-dependent ATPase (Serca) II heterozygotes showed widespread bimodality in frequency distribution of BP values and obviously demonstrated Ht. First, in genome-wide screening, Mapmaker/QTL analysis mapped Ht at a locus between D1Mgh8 and D1Mit4 near Sa in all 78 BC. The peak logarithm of the odds (LOD) score reached 5.3. Second, Serca II heterozygous and homozygous BC were analyzed separately using Mapmaker/QTL. In the 35 Serca II heterozygous BC, the peak LOD score was 3.8 at the same locus whereas it did not reach statistical significance in the 43 Serca II homozygotes. Third, to map Ht efficiently, we selected 18 Serca II heterozygous BC with 9 highest and 9 lowest BP values. In these 18 BC, the peak LOD score reached 8.1. In 17 of the 18, D1Mgh8 genotypes (homo or hetero) qualitatively cosegregated with BP phenotypes (high or low) (P < 0.0001, by chi-square analysis). In conclusion, selective genotyping with epistasis can be utilized for a major QTL mapping near Sa on chromosome 1 in SHR.

Cosegregation of spontaneously hypertensive rat renin gene with elevated blood pressure in an F2 generation

OBJECTIVE

To investigate the role of the renin gene in the hypertension of the spontaneously hypertensive rat (SHR) of the Okamoto strain.

METHODS

We determined whether the SHR renin allele was cosegregated with high blood pressure in 137 F2 rats derived from inbred SHR and Wistar-Kyoto rats. Systolic blood pressure in conscious rats was measured by the tail-cuff method, whereas mean arterial pressure was determined from an indwelling catheter in the left carotid artery. Renin genotypes of F2 rats were determined using a SHR-specific Bg/II restriction fragment length polymorphism that we have previously described.

RESULTS

The SHR renin allele was cosegregated significantly with higher systolic blood pressure in male F2 rats aged 8-24 weeks and in females aged 12-24 weeks. The greatest differences in blood pressure between SHR renin allele homozygotes and Wistar-Kyoto rat renin allele homozygotes were 35 mmHg for males and 17 mmHg for females aged 24 weeks. The SHR renin allele was also associated with a higher mean arterial pressure in rats aged 24 weeks and cosegregated with higher body weight of male F2 rats aged 12-24 weeks but not with that of the females. In contrast to the relationship with blood pressure, the SHR renin allele was segregated with lower plasma renin concentrations in rats aged 24 weeks.

CONCLUSION

These results are consistent with the SHR renin gene being one of the loci determining high blood pressure in rats of this strain, possibly through action at some extra-renal site subserving control of blood pressure.

Sex differences in the abundance of endothelial nitric oxide in a model of genetic hypertension

A deficiency of nitric oxide may be responsible for the increased vascular resistance associated with human essential hypertension and that seen in animal models of hypertension. Premenopausal females are relatively protected from hypertension and cardiovascular complications. Levels of superoxide can influence the availability of nitric oxide. We hypothesize that there are differences in nitric oxide availability between stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto rats (WKY) and that superoxide may be responsible for at least some of these differences. We studied vascular reactivity in endothelium-intact aortic rings from WKY and SHRSP. We measured nitric oxide synthase activity in endothelial cells removed from aortas and also measured circulating nitrite/nitrate levels. We found the response to N(G)-nitro-L-arginine methyl ester to be significantly greater in WKY compared with SHRSP (95% CI: 20 to 174; P=.015) and in females compared with males in WKY (95% CI: 143 to 333; P=.00004) and SHRSP (95% CI: 70 to 224; P=.0006). Endothelial nitric oxide synthase activity was significantly greater in SHRSP compared with WKY (95% CI: 2.3 to 17.6; P=.016). The EC50 for relaxation to carbachol was significantly greater in male rats compared with female rats (95% CI: -1.1 to -0.2; P=.003) within the SHRSP strain. The maximum relaxation to carbachol was significantly attenuated in stroke prone spontaneously hypertensive compared with Wistar-Kyoto rats (95% CI: 1.7 to 14.4; P=.015). Diethyldithiocarbamate had a significantly greater effect on the stroke prone spontaneously hypertensive rats' carbachol response than that of Wistar-Kyoto rats (95% CI: 14.3 to 47.0; P=.0008). We conclude that superoxide may be responsible for strain differences in vascular reactivity, whereas nitric oxide availability may be responsible for sex differences independently of endothelial nitric oxide synthase activity and superoxide.

Diabetes and hypertension in rodent models

As shown by ourselves and others, animals models closely resembling human complex diseases like IDDM in BB/OK and hypertension in SHR/Mol rats can be used to dissect a complex disease into discrete genetic factors as has been done for hypertension in (BB/OK x SHR/Mol) cross hybrids. Discrete genetic factors, so-called QTLs, were detected on chromosomes 1, 10, 18, 20, and X. To gain additional information about the physiologic effect of the mapped blood pressure QTLs, genetically defined regions of the SHR rat were transferred onto the genetic background of diabetes-prone BB/OK rats. Four new congenic BB.SHR rats named BB.Sa, BB.Bp2, BB.1K, and BB.Xs were generated and characterized telemetrically for blood pressure, heart rate, and motor activity. The data demonstrate clearly that each single blood pressure QTL of the SHR rat causes a significant increase of the systolic blood pressure and has a different influence on diastolic blood pressure, heart rate, and motor activity. The effects were modified differently by the diabetic state in BB.Sa, BB.Bp2, and BB.Xs rats carrying all diabetogenic genes of the BB/OK rats. The results demonstrate that these newly established congenic strains are a unique tool to study the physiological control of blood pressure by a single blood pressure QTL on the one hand and their interaction with hyperglycemia on the other. It is well within the bounds of possibility that diabetic congenics reflect the diabetic hypertension seen in diabetic patients. Because of the synteny conservation in gene order between different mammals, genes of the appropriate human region could therefore be candidate genes for hypertension in diabetics. Furthermore, these congenic strains can also be used to study interactions between a blood pressure QTL and various selected environmental conditions. In this way, one could learn which QTL can be influenced by environmental factors and to what extent. Another point is the study of gene interactions. Because congenics are genetically identical except for the defined transferred region, congenics can be crossed to investigate the interaction between two or three blood pressure QTLs selected by the investigator and not by nature. These QTL combinations can be studied in the nondiabetic as well as diabetic state. Although the advantage of congenic strains has been shown, the transferred chromosomal regions are too large to pinpoint the gene responsible for the phenotypic change. Therefore, regions on each chromosome must be systematically whittled down, which can be done by crossing the congenics with BB/OK rats and intercrossing their progeny to generate recombinants. These can then be used for the creation of new congenic lines carrying a much smaller region of the SHR/Mol rat. This has been started for the region on chromosome 1 spanning a 16-cM region from the Sa to the Igf2 gene. BB.Sa rats were therefore backcrossed onto BB/OK rats and the resulting progeny were intercrossed. The aim will be to create at least three new congenic BB.Sa rat strains homozygous for the SHR alleles of Sa, Lsn, or Igf2 genes. However, new problems will emerge with these new congenics. To genetically define small regions requires more dense polymorphic markers than are currently available. Dense polymorphic markers will also be necessary to split the other regions on chromosomes 10, 18, 20, and X. We expect that in the near future it will be possible using this approach to define small regions of < 0.5 cM. The recent progress in gene mapping in the rat gives hope that the use of such congenic lines will allow the identification and recovery of the blood pressure genes in the near future.

Angiotensin I converting enzyme gene cosegregates with blood pressure and heart weight in F2 progeny derived from spontaneously hypertensive and normotensive Wistar-Kyoto rats

The present investigation examines the association of angiotesin I converting enzyme (ACE) genotypes with blood pressure and heart weight in an F2 population of rats derived from a cross between spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. ACE genotype of rats in the F2 population was determined using a microsatellite polymorphism. Our investigation revealed that cardiac mass was not correlated with blood pressure at 12 weeks of age within the SHR, WKY, F1 or F2 groups of rats. In male rats, ACE genotype accounted for approximately 20% of the difference in mean blood pressure between SHR and WKY rats. There was no effect in females. It was also responsible for 21%-29% of the difference in heart weight both in female and male animals. The allele derived from the SHR parent appeared recessive to the allele from WKY parent for both heart weight and blood pressure. These results suggest that a gene in the region of the ACE locus is one of the genetic factor influencing blood pressure and heart weight in SHR.

Different cholesterol deposition in aorta of Dahl salt-sensitive rats and spontaneously hypertensive rats fed a high-cholesterol diet

1. We compared the serum and aortic lipid levels in spontaneously hypertensive rats (SHR) and Dahl salt-sensitive rats (DSR) fed a high-cholesterol (HC) diet. 2. In SHR fed the HC diet, the serum cholesterol level significantly increased, but no aortic cholesterol deposition was observed. 3. The serum cholesterol level in DSR fed the HC diet markedly increased compared to that in DSR fed the basal diet, and this change was greater with the diet containing 8% NaCl than 0.4% NaCl. A significant increase in the content of aortic cholesterol, notably cholesteryl ester, was observed in only DSR fed the HC diet containing 8% NaCl. 4. These results suggest that the combination of hypercholesterolaemia with salt-induced hypertension acts as a greater risk factor for atherosclerosis than that with genetic hypertension.

Gender difference in endothelial dysfunction in the aorta of spontaneously hypertensive rats

We investigated endothelium-dependent responses of thoracic aorta isolated from age-matched male and female spontaneously hypertensive rats (SHR) to explore gender differences in endothelial dysfunction that may contribute to the sexual dimorphism observed in the development of hypertension in this strain. Endothelium-dependent relaxation in response to acetylcholine (10(-9) to 10(-4) mol/L) was significantly greater in female rats than in male rats, although impaired responses were seen in both sexes compared with normotensive controls. Inhibition of cyclooxygenase by indomethacin (10(-5) mol/L) improved endothelium-dependent relaxation, but it did not abolish the gender difference. Relaxations in response to sodium nitroprusside were identical in denuded aortic rings from male and female SHR. Acetylcholine at higher concentrations (10(-6) to 10(-4) mol/L) induced endothelium-dependent contraction in intact, quiescent aortic rings from male SHR but not in those from female SHR. After incubation with NG-nitro-L-arginine (10(-4) mol/L), contraction in response to acetylcholine became apparent in rings from female SHR, but it was still significantly less pronounced than in similarly treated rings from male SHR. Endothelium-dependent contraction was prevented by indomethacin in both sexes, suggesting that a cyclooxygenase product such as endoperoxide may be mediating this effect. Because responses evoked by the thromboxane/endoperoxide receptor agonist U46619 (10(-10) to 10(-6) mol/L) were not greater in rings from male SHR than those from female SHR, increased smooth muscle responsiveness or higher thromboxane/endoperoxide receptor density in the males could not account for the differences in endothelium-dependent contraction. These results suggest that sex steroid hormones may control endothelium-dependent vascular reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic and environmental influences on blood pressure in elderly twins

We used 289 pairs of Swedish twins reared apart or together to evaluate the importance of genetic and environmental influences on blood pressure. Unlike other twin and family studies, the adoption/twin design allows a distinction between estimates of the importance of shared rearing environments and genetic effects. Genetic factors were observed to play an important role for individual differences in blood pressure. Model-fitting analyses suggested upper limits of heritability for systolic and diastolic blood pressures in the entire sample of 0.44 and 0.34, respectively. More interestingly, substantial influences of shared family effects accounting for up to 27% of the variation were also revealed. Effects of correlated environment, which might reflect, for example, the intrauterine environment, existed to some extent later in life. The influence of genetic factors tended to decrease across age groups for systolic blood pressure (0.62 in individuals less than 65 years old; 0.12 in those 65 years and older) but not for diastolic blood pressure (0.22 for the middle-aged group; 0.26 for the older group). However, this declining trend for systolic blood pressure did not reach significance (chi 2 = 8.07, df = 4, P = .09).

Cosegregation of the renin gene with an increase in mean arterial blood pressure in the F2 rats of SHR-WKY cross

Using the restriction endonuclease, BgI I, Samani et al. found a restriction fragment length polymorphism (RFLP) for the renin gene in spontaneously hypertensive rats (SHR) and its normotensive control Wistar-Kyoto (WKY) rats. This RFLP was confirmed in our laboratory in SHR and WKY rats using a rat renin cDNA probe. The correlation of blood pressure and the renin RFLP was examined in 106 F2 rats produced from F1 rats, the offspring of a cross between SHR males and WKY females. Systolic blood pressure was measured by the tail cuff method at 12 weeks of age. Mean arterial blood pressure of anesthetized rats was measured by cannulation of the femoral artery prior to sacrifice. The frequency of renin genotype showed a typical 1:2:1 Mendelian ratio in F2 rats of SHR and WKY cross. The mean arterial blood pressure of F2 rats homozygous with the SHR allele was significantly higher than F2 rats that were heterozygous or homozygous for the WKY allele. No significant difference in systolic blood pressure was observed in these F2 rats. Thus, the renin gene RFLP cosegregates with an increase in mean arterial blood pressure in the F2 rats of SHR and WKY cross.

The borderline hypertensive rat (BHR) as a model for environmentally-induced hypertension: a review and update

In recent years, the burgeoning disciplines of health psychology and behavioral medicine have renewed interest in the important role that environmental factors can play in the disease process. Nowhere is this concern more well-founded than in the area of cardiovascular disorders, particularly hypertension. Epidemiologists and clinicians have long suspected that stressful life events can be a sufficient trigger for the expression of hypertension in some individuals. To understand better the ways in which these variables interact in the disease process, researchers have tried, with limited success, to produce experimental hypertension in animals by exposing them to stressful environmental paradigms. Additionally, recent investigations using the borderline hypertensive rat (BHR) have demonstrated the important role genetic factors can play in mediating both the behavioral and cardiovascular responses to environmental stressors. The current paper will review these attempts and discuss recent data from experiments using a relatively new animal model that appears to be especially appropriate for the study of environmental-genetic factors in the elaboration of essential hypertension. We will also discuss potential mechanisms by which environmental stress influences arterial pressure and suggest avenues for further inquiry into the stress-disease relationship.

Polymerase chain reaction-based DNA fingerprinting in the spontaneously hypertensive rat: potential source of cDNA library probes

1. Using the Polymerase Chain Reaction with a primer homologous to a repetitive unit found at irregular distances in the rat genome, we were able to synthesize DNA fragments spanning such units and visualize them as a ladder of discrete bands following electrophoresis. 2. Differences between fingerprint patterns generated using SHR, WKY and SD DNA, in the form of band deletions or duplications, were readily detected. 3. Unlike traditional fingerprinting, PCR fingerprints allow the ready extraction and characterization of polymorphic bands. Such bands could then be used as probes with which to screen a cDNA library that might, in turn, identify expressed genes involved in the development of hypertension.

Molecular genetics of hypertension

During the last decades the evidence that a genetic component contributes to the development of primary hypertension has been accumulating. The identification of the genes involved in blood pressure regulation, however, is only starting to emerge. The recent advances in recombinant DNA technology provide new molecular genetic strategies in cardiovascular research. In this review we will discuss the testing of candidate genes in vivo by transgenic techniques. Furthermore, we will describe the possibilities to identify the genes implicated in primary hypertension by genetic linkage analysis using polymorphic DNA markers.

Blood pressure cosegregates with a microsatellite of angiotensin I converting enzyme (ACE) in F2 generation from a cross between original normotensive Wistar-Kyoto rat (WKY) and stroke-prone spontaneously hypertensive rat (SHRSP)

We investigated the linkage between high blood pressure and the ACE gene in the F2 generation between SHRSP/Izm and WKY/Izm. The male F2 rats were categorized into 3 genotypes according to a microsatellite polymorphism in the ACE gene. Significantly high blood pressure was observed in the SHRSP homozygotes when it was compared to the blood pressure of the heterozygotes. Further, after 2 or 3 months salt-loading, the blood pressure was significantly higher in the SHRSP homozygotes than in the heterozygotes and the WKY homozygotes. The heterozygotes had a blood pressure similar to that in the WKY homozygotes, indicating that the effect of the ACE gene genotype was recessive. Salt appetite was neither correlated with the salt-sensitivity nor cosegregated with the ACE genotype. The results indicate that the locus of ACE gene associates with the development of hypertension, especially salt-sensitive hypertension.

Chromosomal mapping of two genetic loci associated with blood-pressure regulation in hereditary hypertensive rats

The spontaneously hypertensive rat and the stroke-prone spontaneously hypertensive rat are useful models for human hypertension. In these strains hypertension is a polygenic trait, in which both autosomal and sex-linked genes can influence blood pressure. Linkage studies in crosses between the stroke-prone spontaneously hypertensive rat and the normotensive control strain Wistar-Kyoto have led to the localization of two genes, BP/SP-1 and BP/SP-2, that contribute significantly to blood pressure variation in the F2 population. BP/SP-1 and BP/SP-2 were assigned to rat chromosomes 10 and X, respectively. Comparison of the human and rat genetic maps indicates that BP/SP-1 could reside on human chromosome 17q in a region that also contains the angiotensin I-converting enzyme gene (ACE). This encodes a key enzyme of the renin-angiotensin system, and is therefore a candidate gene in primary hypertension. A rat microsatellite marker of ACE was mapped to rat chromosome 10 within the region containing BP/SP-1.

Genetic mapping of a gene causing hypertension in the stroke-prone spontaneously hypertensive rat

The stroke-prone spontaneously hypertensive rat (SHRSP) is a well-characterized model for primary hypertension in humans. High blood pressure in SHRSP shows polygenic inheritance, but none of the loci responsible have previously been identified. To locate genes controlling this quantitative trait, we mapped a large collection of DNA polymorphisms in a cross between SHRSP and the normotensive WKY strain. Here we report strong genetic evidence that a gene, Bp1, having a major effect on blood pressure maps to rat chromosome 10 with a LOD score of 5.10 and is closely linked to the rat gene encoding angiotensin-converting enzyme (ACE), an enzyme that plays a major role in blood pressure homeostasis and is an important target of anti-hypertensive drugs. We also find significant, albeit weaker, linkage to a locus, Bp2, on chromosome 18. We discuss the implications of genetic dissection of quantitative disease-related phenotypes in mammals.

Genetic factors regulate the rise in blood pressure in F2 generation crossed between stroke-prone spontaneously hypertensive rats and Wistar-Kyoto rats

1. There was no significant difference between the systolic blood pressure (SBP) of offspring derived from SHRSP mother and WKY father and the SBP of offspring derived from WKY mother and SHRSP father at the developing stage (5-13 weeks of age). 2. The degree of genetic determinations of SBP in stroke-prone spontaneously hypertensive rat (SHRSP) at 5, 7, 10 and 13 weeks of age, determined by genetic crosses between SHRSP and WKY, was 73.9, 70.8, 50.2 and 55.3% respectively. 3. Significant correlations between SBP at 5 and 7 weeks, 7 and 10 weeks, 10 and 13 weeks, also at 5 and 13 weeks of age in F2 generation crossed between SHRSP and WKY were observed. SBP falling at or above the 80th percentile group in F2 generation at 5 weeks of age were constantly higher than SBP falling at or below the 20th percentile group from 7 weeks of age onwards. 4. These results indicate that there exists 'tracking phenomenon of SBP in SHRSP' and that genetic factors regulate the rise in SBP. Tracking of SBP in F2 generation gives us new methodological insight into hypertensive mechanism in SHR.

Two new Wistar-Kyoto rat strains in which hypertension and hyperactivity are expressed separately

Two inbred strains have been developed from a cross between SHR and WKY. WK-HTs are hypertensive but not hyperactive, and WK-HAs are hyperactive but normotensive. Together with SHR (that express both traits) and WKY (expressing neither trait) we used four strains to follow correlations of biological changes with the expression of hyperactivity or hypertension. We show that the well known sympathetic hyperreactivity of SHRs to acute stress is associated with the hyperactivity trait and not the hypertension among the four strains. Similarly, the well known ventricular hypertrophy in SHRs is more prominent among the hyperactive strains than the hypertensives. Examination of regional brain amine levels revealed an imbalance in forebrain serotonin transmission in the hyperactive strains, and no significant correlations with hypertension. On the other hand, neuropeptides in brainstem and spinal cord revealed a decrease, in hypertension, in neuropeptide Y and PNMT content of terminals of C1 fibers that innervate the spinal cord sympathetic outflow. Also, the two hypertensive strains showed increased TRH-and proctolin-like immunoreactivity in fibers that innervate the C1 cells in the rostral ventrolateral medulla. These findings illustrate the unique advantage provided by WK-HA and WK-HT strains as additional controls for SHRs in studying hypertension and hyperactivity.

The role of dominance and epistasis in the genetic control of blood pressure in rodent models of hypertension

Genetic analyses of crosses between hypertensive rodent models and their normotensive controls were performed on 43 sets of data published between 1970-1989. In each case, the cross involved F1, F2, and both backcross generations for a "complete genetic cross." Biometrical analysis estimated genetic parameters and their standard errors associated with dominance and epistasis (interaction of alleles that are not at the same locus). The statistical significance of these parameters was determined by comparing the parameter to its standard error. A purely additive inheritance pattern was seldom found. Additive/dominance inheritance was apparent in only two models. The prevailing pattern of inheritance was one with partial dominance for alleles for normal blood pressures and epistatic interactions. Finding epistasis in so many models will have implications for the application of cosegregation and linkage analyses in hypertension research.

Hypertension in the spontaneously hypertensive rat is linked to the Y chromosome

The objective of our study was to determine the genetic influence on blood pressure in spontaneously hypertensive rats (SHR), and normotensive Wistar-Kyoto (WKY) rats using genetic crosses. Blood pressure was measured by tail sphygmomanometry from 8 to 20 weeks of age. Blood pressure was significantly higher from 12 to 20 weeks in the male offspring derived from WKY mothers x SHR fathers as compared with male offspring derived from SHR mothers X WKY fathers (180 +/- 4 versus 160 +/- 5 mm Hg, p less than 0.01). There was no significant difference between the blood pressure of the F1 females, further supporting Y chromosome linkage and not parental imprinting. The blood pressure data from F2 males derived from reciprocal crosses of parental strains were consistent with the presence of a Y-linked locus, but not with an X-linked locus controlling blood pressure. The data strongly suggest that hypertension in the SHR has two primary components of equal magnitude, one consisting of a small number of autosomal loci with a second Y-linked component.

Cosegregation of the renin allele of the spontaneously hypertensive rat with an increase in blood pressure

The spontaneously hypertensive rat (SHR) exhibits alterations in the renin-angiotensin-aldosterone system which are similar to those that characterize patients with "nonmodulating" hypertension, a common and highly heritable form of essential hypertension. Accordingly, we determined whether the inheritance of a DNA restriction fragment length polymorphism (RFLP) marking the renin gene of the SHR was associated with greater blood pressure than inheritance of a RFLP marking the renin gene of a normotensive control rat. In an F2 population derived from inbred SHR and inbred normotensive Lewis rats, we found the blood pressure in rats that inherited a single SHR renin allele to be significantly greater than that in rats that inherited only the Lewis renin allele. To the extent that the SHR provides a suitable model of "nonmodulating" hypertension, these findings raise the possibility that a structural alteration in the renin gene, or a closely linked gene, may be a pathogenetic determinant of increased blood pressure in one of the most common forms of essential hypertension in humans.

Dam strain affects cardiovascular reactivity to acute stress in BHR

The effect of maternal strain on reactivity to acute stress was studied in F1 reciprocals produced by crossing the spontaneously hypertensive rat (SHR) with its normotensive progenitor, the Wistar-Kyoto (WKY). This F1 generation, known as the borderline hypertensive rat (BHR), is genetically predisposed to develop hypertension in response to chronic stress or high dietary sodium. Reciprocals, considered to be genetically equivalent aside from sex-linked traits, differ in strain of dam during intrauterine and preweanling development. At 17 weeks of age, reciprocal F1 males did not differ in open-field behavior (squares crossed, rearings, and defecation measured over 3 days in 15-min sessions) or in home-cage measurements of mean arterial pressure (MAP) and heart rate (HR). However, different patterns of cardiovascular reactivity were displayed to transfer and footshock. While WKY-mothered rats reacted with graded pressor responses, SHR-mothered rats responded maximally to transfer, showed no additional increase to footshock, and maintained peak responding after footshock was terminated. Such reactivity differences may mediate the impact of environmental variables on the genetic disposition to hypertension.

Spontaneously hypertensive Wistar-derived male rats are more aggressive than those of their normotensive progenitor strain

We compared a group of spontaneously hypertensive rats (SHRs) to a group of Wistar-Kyoto (WKY) rats on each of the three most commonly studied forms of aggressive behavior in rats: muricide, intraspecific aggression, and shock-induced fighting (SIF). A significantly higher proportion of SHRs were muricidal; they also fought more at the lowest shock level. A trend for a higher incidence of intraspecific offense behaviors by SHRs was not significant. SHR flinch and jump thresholds were lower than the respective WKY thresholds. Although there were no significant correlations between shock thresholds and any aspects of SIF, the possibility that strain differences in shock sensitivity may contribute to differences in SIF cannot be ruled out. Within strains, there were no correlations among the different forms of aggression. Several different inherited characteristics may be associated with the accentuation of different forms of aggression in SHRs.

Enhanced growth rate of cultured smooth muscle cells from spontaneously hypertensive rats

Smooth muscle cells were isolated and cultured from the aortic media of age-matched, stroke-prone and stroke-resistant spontaneously hypertensive rats (SHRSP, SHRSR:SHRs) and Wistar-Kyoto rats (WKY), and the growth rate of cells from the three strains was compared. Under electron microscopical observation the cells were identified as modified (or synthetic-type) smooth muscle cells. Cells occurring at low densities showed the same morphology for both SHRs and WKY, but cells occurring at higher densities were observed to be smaller in SHRs than in WKY. An analysis of the growth curves of cells showed a significantly enhanced replication rate in cells from SHRs compared with those from WKY, especially in the early passages. In later passages (repeated until the 9th passage), however, this distinction was not clear. These growth characteristics were also confirmed in cells from both 12- and 24-week-old, age-matched SHRs and WKY. We could not find any difference between the growth characteristics of cells from SHRSP and SHRSR. It is possible to hypothesize from these findings that the abnormality relating to hypertension (hypertension being a common characteristic of SHRSR and SHRSP) is found in the smooth muscle cells, and is reflected as an enhanced growth rate when they are exposed to mitogenic stimuli, such as in atherosclerosis.

Blood pressure, plasma and pituitary prolactin levels and their responses to bromocriptine in the segregating F2 generation of the SHR/WKY hybrid

Our previous study in the spontaneously hypertensive rat (SHR) and its normotensive control, the Wistar-Kyoto (WKY), had shown differences in plasma and pituitary prolactin (PRL) levels as well as in their responses to bromocriptine (BRC), a centrally acting dopaminergic agonist. The results provided evidence for a central dopaminergic deficiency in the SHR and suggested a possible genetic linkage between blood pressure (BP) and PRL and their responses to BRC in the SHR. To investigate this possibility further, we have repeated the experiment, using the same protocol, in the segregating F2 generation of the SHR/WKY hybrid which was classified according to BP into low, intermediate and high BP groups. The results from this study show that the differences between BP and PRL and their responses to BRC are similarly present in the three groups of the F2 generation. They provide further evidence that suggests a genetic linkage between BP and PRL in the SHR.

Genetic analysis of blood pressure and sodium balance in spontaneously hypertensive rats

Blood pressure and parameters of sodium balance were measured during the first 16 weeks of life in male Okamoto spontaneously hypertensive rats (SHR, n = 22), Wistar-Kyoto rats (WKY, n = 25), and the F1 (n = 27) and F2 (n = 81) hybrids of the SHR and WKY. Genetic analysis revealed that blood pressure in SHR was controlled by approximately four independent genetic loci and the degree of genetic determination was 64.5%. No difference in blood pressure was discernible before 12 weeks of age between those F2 rats that at 16 weeks had blood pressures either higher or lower than one standard deviation from the mean. Exchangeable sodium was measured sequentially in individual rats of all populations by determining their whole-body radioactivity while receiving 37.5 mM 22Na/23NaCl drinking fluid of constant specific activity as their sole source of sodium. The SHR had consistently higher exchangeable sodium levels than WKY and showed evidence of relative sodium retention during the early developmental phase of hypertension. Sodium intake was higher in SHR than WKY from 4 to 16 weeks of age, although saline preference was the same in both strains. None of these parameters of sodium balance were found to correlate with blood pressure in the F2 population. It is concluded that the heritable abnormalities of sodium balance in SHR appear to represent coincidental inbred characteristics controlled by genetic loci that are unrelated to those loci responsible for the expression of hypertension in this model.

Inbreeding of Wistar-Kyoto rat strain with hyperactivity but without hypertension

A genetic inbreeding program using Wistar-Kyoto rat strains as progenitors was used to combine the hyperactivity trait of the spontaneously hypertensive rat (SHR) with the normotensive trait of the WKY genetic control strain. From an SHR X WKY cross we produced a gene-assorting F2 population from which selected brother-sister matings were carried out through seven successive inbred populations. This program produced a new strain of hyperactive rats with normotensive mean systolic blood pressure levels, and we have designated the new strain as the Wistar-Kyoto hyperactive (WK/HA) rat. Another behavioral characteristic of the SHR rat, poor habituation in a nonreinforcing novel environment, did not appear as a characteristic trait of the new strain of WK/HA rats, suggesting a separate underlying genetic basis for the two traits that had been apparently fortuitously fixed in the SHR genotype as a result of intensive inbreeding of that strain. The new WK/HA strain, together with the WKY control strain, is considered as more suitable for subjects in studying hyperactivity in rats than the original SHR strain with its concomitant hypertension and poor habituation traits.

Vasopressin-induced calcium increases in smooth muscle cells from spontaneously hypertensive rats

Cytosolic free Ca2+ concentrations [( Ca2+]i) were measured in smooth muscle cells (SMC) from spontaneously hypertensive rats (SHR) and age and sex matched Wistar-Kyoto rats (WKY). Resting levels of [Ca2+]i were 114 +/- 6 nM and 116 +/- 5 nM in SMC from WKY and SHR, respectively. Angiotensin II (AII) induced a dose-dependent large increases in [Ca2+]i in SMC. There were no significant differences in resting or AII-stimulated levels of [Ca2+]i when SMC from WKY and SHR were compared. Arg-vasopressin (AVP) caused a similar but smaller [Ca2+]i increase than AII in SMC. AVP caused larger [Ca2+]i increases in SMC from SHR than in SMC from WKY. Although concentrations of AVP higher than those ordinarily detected in plasma were necessary to obtain different responses between SHR and WKY, these differences may be related to the pathogenesis of hypertension.

Factors influencing the expression of stress fibers in vascular endothelial cells in situ

The organization of actin and myosin in vascular endothelial cells in situ was studied by immunofluorescence microscopy. Examination of perfusion-fixed, whole mounts of normal mouse and rat descending thoracic aorta revealed the presence of axially oriented stress fibers containing both actin and myosin within the endothelial cells. In both species, the proportion of cells containing stress fibers varied from region to region within the same vessel. Some endothelial cells in mouse mesenteric vein and in rat inferior vena cava also contained stress fibers. Quantitative studies of the proportion of endothelial cells containing stress fibers in the descending thoracic aorta of age-matched normotensive and spontaneously hypertensive rats revealed significant differences. When animals of the same sex of the two strains were compared, the proportion was approximately two times greater in the spontaneously hypertensive rats. The proportion of endothelial cells containing stress fibers was about two times greater in males than in females of both strains. These observations suggest that multiple factors, including anatomical, sex, and hemodynamic differences, influence the organization of the endothelial cell cytoskeleton in situ.

A paradigm for identification of primary genetic causes of hypertension in rats

A paradigm is developed for identifying the genes (and the biochemical-physiological traits for which the genes code) that cause differences in blood pressure in inbred strains of rats. A biochemical-physiological trait which meets the following four criteria is one which can reasonably be accepted as causing genetic differences in blood pressure: 1) a difference in a biochemical or physiological trait between two strains must be demonstrated; 2) the trait must be shown to follow Mendelian inheritance; 3) the genes identified in criterion 2 must co-segregate with an increment in blood pressure which is significantly different from zero; and 4) there must be some logical biochemical and/or physiological link between the trait and blood pressure. Traits which do not show discrete phenotypes following Mendelian inheritance may correlate with blood pressure in segregating populations. In this case no rigorous cause and effect genetic argument is possible because such correlations could arise from complex primary genetic causes or as secondary effects of blood pressure on the biochemical-physiological trait.

Dissociation of genetic hyperactivity and hypertension in SHR

The Wistar Kyoto strain of spontaneously hypertensive rat (SHR) has been characterized as behaviorally hyperactive as well as hypertensive. The relationship between these two inbred traits remains uncertain, and their coexistence in the SHR has complicated studies of central nervous system mechanisms underlying the hypertensive process. A breeding program was initiated to examine the possible genetic linkage of these two traits which, if separable, would allow us to develop substrains of SHR that are hypertensive without being hyperactive, or hyperactive without being hypertensive. We crossed SHR males with Wistar Kyoto, normotensive (WKY) female rats and produced F1 hybrids which were then randomly inbred to produce an F2 population. When tested at 12 weeks of age, F2 rats exhibited the expected wide range of mean systolic blood pressures (BP), from 111 to 174 mm Hg, as determined using indirect tail plethysmography. The BP in the parental rats at the time of breeding (16 weeks) was 187 +/- 4.5 mm Hg (SHR males, n = 7) and 111 +/- 2.4 (WKY females, n = 7). Locomotor activity was determined in an automated activity cage in F1 and F2 rats at 12 weeks of age. These strains exhibited a wide range of phenotypic distribution of locomotor activity scores, and the mean scores were intermediary between those of SHR rats and WKY rats of the same age. Among individual rats of both the F1 and F2 hybrid strains, there was no correlation between the activity score and the level of the BP at 12 weeks of age. These findings indicated that the genes responsible for the hypertensive trait and those responsible for the hyperactivity trait were not tightly linked in the hybrid populations, suggesting that different genetic factors were involved in the transmission of each of these traits. Accordingly, it should be possible to separate the two traits by further selective, recombinant inbreeding procedures.

A genetic locus (Hyp-2) controlling vascular smooth muscle response in spontaneously hypertensive rats

Aortic vascular smooth muscle from spontaneously hypertensive rats (SHR) is known to respond in vitro to nonphysiologic cations with greater contraction than vascular smooth muscle from certain normotensive control stocks. The pattern of inheritance of the response of aortic rings to cobalt (Co2+) in vitro was determined. The test characteristic utilized was the cobalt response ratio (CRR) defined as the contractile response to 0.6 microM Co2+ divided by the response given by maximal stimulation with 10 microM Co2+. The SHR were crossed with Dahl salt-resistant rats that had been inbred for 21 generations (R/JR strain) to produce F1, F2, and backcross populations. The CRR was 0.90 +/- 0.011 in SHR, 0.74 +/- 0.016 in F1, and 0.38 +/- 0.031 in R/JR. The F1 value was significantly higher than the midparental value indicating partial SHR dominance. The F1 males from reciprocal crosses had similar CRR indicating no sex-linked effect. In the backcross to the R/JR, the CRR showed a bimodal distribution segregating one intermediate type: one R/JR type. In the backcross to the SHR the CRR showed a unimodal distribution. In F2 rats there was a bimodal distribution segregating three intermediate type: one R/JR type. In F2 rats there was a significant (p less than 0.005) blood pressure difference of 9.7 mm Hg between phenotypes. It is concluded that there is an autosomal locus (named "Hyp-2" for hypertension locus number 2) controlling vascular smooth muscle response to Co2+, which exhibits partially dominant inheritance. Alleles at Hyp-2 segregate with an increment in blood pressure.

Abnormal membrane characteristics of erythrocytes in rat models and men with predisposition to stroke

Erythrocyte membrane characteristics were examined in normotensive Wistar-Kyoto rats (WK), stroke-resistant spontaneously hypertensive rats (SHRSR) and stroke-prone SHR (SHRSP). The membrane of SHRSP was more labile against osmotic shock than that of SHRSR or WK at the age of 2 months. This membrane fragility was not dependent on their blood pressure. The membrane permeability of lipophilic ion, tetraphenylphosphonium, was increased in 1-month old SHRSP. These membrane characteristics in SHRSP were partially dependent on cholesterol amount which was significantly decreased in SHRSP. The method determining the membrane fragility was applied to human erythrocytes. Two groups of people were selected from about 2,000 inhabitants under our community health control program. The erythrocytes from the group with stroke in their family history were more labile than erythrocytes from the corresponding group without stroke in their family history. This result suggests that the examination of these membrane characteristics may give us a new method to detect a genetic risk to hypertension or stroke-proneness also in human.

Regional changes in the activities of aminergic biosynthetic enzymes in the brains of hypertensive rats

The activities of monoamine biosynthetic enzymes were measured in brain regions of several hypertensive rat models at various ages. The types of hypertensive rats were the spontaneously hypertensive rat (SHR) and a stroke-prone substrain of the SHR as well as DOCA-salt and renal hypertensive rats. The genetically hypertensive rats had significantly elevated blood pressures as compared to the Wistar-Kyoto control rat after 5 weeks of age. During the early development of hypertension in the SHR, the activities of tyrosine hydroxylase in the hypothalamus and corpus striatum and of dopamine-beta-hydroxylase in the hypothalamus and pons-medulla were significantly higher than in the control rats. Tryptophan-hydroxylase was also elevated in the hypothalamus in SHR. From 3 to 8 weeks of age there appeared to be a significant correlation between hypothalamic dopamine-beta-hydroxylase activity and blood pressure in the hypertensive rats. In contrast, the activities of tyrosine hydroxylase and dopamine-beta-hydroxylase were slightly decreased in the DOCA-salt and renal hypertensive rats. It is suggested that noradrenergic or adrenergic neurons in the hypothalamus may participate in the initiation of elevated blood pressure in the genetic, but not in the DOCA-salt or renal hypertensive rats.