Genetic mapping of modifier loci affecting malignant hypertension in TGRmRen2 rats

Loci associated with variation in gene expression and growth in juvenile salmon are influenced by the presence of a growth hormone transgene

BACKGROUND

Growth regulation is a complex process influenced by genetic and environmental factors. We examined differences between growth hormone (GH) transgenic (T) and non-transgenic (NT) coho salmon to elucidate whether the same loci were involved in controlling body size and gene expression phenotypes, and to assess whether physiological transformations occurring from GH transgenesis were under the influence of alternative pathways. The following genomic techniques were used to explore differences between size classes within and between transgenotypes (T vs. NT): RNA-Seq/Differentially Expressed Gene (DEG) analysis, quantitative PCR (qPCR) and OpenArray analysis, Genotyping-by-Sequencing, and Genome-Wide Association Study (GWAS).

RESULTS

DEGs identified in comparisons between the large and small tails of the size distributions of T and NT salmon (NT_Large, NT_Small, T_Large and T_Small) spanned a broad range of biological processes, indicating wide-spread influence of the transgene on gene expression. Overexpression of growth hormone led to differences in regulatory loci between transgenotypes and size classes. Expression levels were significantly greater in T fish at 16 of 31 loci and in NT fish for 10 loci. Eleven genes exhibited different mRNA levels when the interaction of size and transgenotype was considered (IGF1, IGFBP1, GH, C3-4, FAS, FAD6, GLUT1, G6PASE1, GOGAT, MID1IP1). In the GWAS, 649 unique SNPs were significantly associated with at least one study trait, with most SNPs associated with one of the following traits: C3_4, ELA1, GLK, IGF1, IGFBP1, IGFII, or LEPTIN. Only 1 phenotype-associated SNP was found in common between T and NT fish, and there were no SNPs in common between transgenotypes when size was considered.

CONCLUSIONS

Multiple regulatory loci affecting gene expression were shared between fast-growing and slow-growing fish within T or NT groups, but no such regulatory loci were found to be shared between NT and T groups. These data reveal how GH overexpression affects the regulatory responses of the genome resulting in differences in growth, physiological pathways, and gene expression in T fish compared with the wild type. Understanding the complexity of regulatory gene interactions to generate phenotypes has importance in multiple fields ranging from applications in selective breeding to quantifying influences on evolutionary processes.

Modeling Human Diabetic Kidney Disease by Combining Hyperglycemia and Hypertension in a Transgenic Rodent Model

Traditional animal models mimic only the earliest stages of human diabetic nephropathy (DN), which limits their utility to dissect the pathogenesis of progressive disease or test novel therapeutics. In this chapter we describe in detail the experimental procedures required to conduct the Cyp1a1mRen2 rodent model, in which hyperglycemia and renin-dependent hypertension synergize to promote moderate proteinuria, renal fibrosis, and induction of many of the transcriptomic changes observed in the kidney of patients with progressive DN.

Renal disease pathophysiology and treatment: contributions from the rat

The rat has classically been the species of choice for pharmacological studies and disease modeling, providing a source of high-quality physiological data on cardiovascular and renal pathophysiology over many decades. Recent developments in genome engineering now allow us to capitalize on the wealth of knowledge acquired over the last century. Here, we review rat models of hypertension, diabetic nephropathy, and acute and chronic kidney disease. These models have made important contributions to our understanding of renal diseases and have revealed key genes, such as Ace and P2rx7, involved in renal pathogenic processes. By targeting these genes of interest, researchers are gaining a better understanding of the etiology of renal pathologies, with the promised potential of slowing disease progression or even reversing the damage caused. Some, but not all, of these target genes have proved to be of clinical relevance. However, it is now possible to generate more sophisticated and appropriate disease models in the rat, which can recapitulate key aspects of human renal pathology. These advances will ultimately be used to identify new treatments and therapeutic targets of much greater clinical relevance.

Summary: This Review highlights the key role that the rat continues to play in improving our understanding of the etiologies of renal pathologies, and how these insights have opened up new therapeutic avenues.

Towards Precision Medicine for Hypertension: A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans

Compelling evidence for the inherited nature of essential hypertension has led to extensive research in rats and humans. Rats have served as the primary model for research on the genetics of hypertension resulting in identification of genomic regions that are causally associated with hypertension. In more recent times, genome-wide studies in humans have also begun to improve our understanding of the inheritance of polygenic forms of hypertension. Based on the chronological progression of research into the genetics of hypertension as the "structural backbone," this review catalogs and discusses the rat and human genetic elements mapped and implicated in blood pressure regulation. Furthermore, the knowledge gained from these genetic studies that provide evidence to suggest that much of the genetic influence on hypertension residing within noncoding elements of our DNA and operating through pervasive epistasis or gene-gene interactions is highlighted. Lastly, perspectives on current thinking that the more complex "triad" of the genome, epigenome, and the microbiome operating to influence the inheritance of hypertension, is documented. Overall, the collective knowledge gained from rats and humans is disappointing in the sense that major hypertension-causing genes as targets for clinical management of essential hypertension may not be a clinical reality. On the other hand, the realization that the polygenic nature of hypertension prevents any single locus from being a relevant clinical target for all humans directs future studies on the genetics of hypertension towards an individualized genomic approach.

Impaired Neovascularization and Reduced Capillary Supply in the Malignant vs. Non-malignant Course of Experimental Renovascular Hypertension

Malignant hypertension develops in some cases of hypertension but not in others. We hypothesized that an impaired neovascularization and a reduced capillary supply characterizes the malignant course of experimental hypertension. Two-kidney, one-clip renovascular hypertension was induced in rats; controls (sham) were sham operated. To distinguish malignant hypertension from non-malignant hypertension, we considered two factors: weight loss, and the number of typical vascular lesions (onion skin lesions and fibrinoid necroses) per kidney section of the nonclipped kidney. Animals in the upper half for both criteria were defined as malignant hypertensives. After 5 weeks, mean arterial blood pressure was elevated to the same degree in malignant hypertension and non-malignant hypertension whereas plasma renin and aldosterone were significantly higher in malignant hypertensives. The expression of plasminogen activator inhibitor-1 was elevated (up to 14-fold) in non-malignant but significantly more increased (up to 36-fold) in malignant hypertensive rats, compared to sham. As a bioassay for neovascularization, the area of granulation tissue ingrowth in polyvinyl discs (implanted subcutaneously) was reduced in malignant hypertension compared to non-malignant hypertension and sham, while there was no difference between non-malignant hypertension and sham. The number of renal and left ventricular capillaries was significantly lower in malignant hypertension compared to non-malignant hypertension, as was the number of proliferating endothelial cells. We conclude that an impaired neovascularization and capillarization occurs in malignant renovascular hypertension but not in the non-malignant course of the disease despite comparable blood pressure levels. This might contribute to the unique vascular lesions and progressive target organ damage observed in malignant hypertension.

Fischer-344 Tp53-knockout rats exhibit a high rate of bone and brain neoplasia with frequent metastasis

Somatic mutations in the Tp53 tumor suppressor gene are the most commonly seen genetic alterations in cancer, and germline mutations in Tp53 predispose individuals to a variety of early-onset cancers. Development of appropriate translational animal models that carry mutations in Tp53 and recapitulate human disease are important for drug discovery, biomarker development and disease modeling. Current Tp53 mouse and rat models have significant phenotypic and genetic limitations, and often do not recapitulate certain aspects of human disease. We used a marker-assisted speed congenic approach to transfer a well-characterized Tp53-mutant allele from an outbred rat to the genetically inbred Fischer-344 (F344) rat to create the F344-Tp53^{tm1(EGFP-Pac)Qly}/Rrrc (F344-Tp53) strain. On the F344 genetic background, the tumor spectrum shifted, with the primary tumor types being osteosarcomas and meningeal sarcomas, compared to the hepatic hemangiosarcoma and lymphoma identified in the original outbred stock model. The Fischer model is more consistent with the early onset of bone and central nervous system sarcomas found in humans with germline Tp53 mutations. The frequency of osteosarcomas in F344-Tp53 homozygous and heterozygous animals was 57% and 36%, respectively. Tumors were highly representative of human disease radiographically and histologically, with tumors found primarily on long bones with frequent pulmonary metastases. Importantly, the rapid onset of osteosarcomas in this promising new model fills a current void in animal models that recapitulate human pediatric osteosarcomas and could facilitate studies to identify therapeutic targets.

Editors' choice: Transferring a Tp53-knockout allele from an outbred rat stock to the F344 inbred rat genetic background alters the spectrum of tumors, providing a model of early-onset brain and bone sarcomas.

Inhibition of the purinergic P2X7 receptor improves renal perfusion in angiotensin-II-infused rats

Chronic activation of the renin-angiotensin system promotes hypertension, renal microvascular dysfunction, tissue hypoxia, and inflammation. Despite similar hypertension, an injurious response to excess angiotensin II is greater in F344 than in Lewis rats; the latter displaying renoprotection. Here we studied whether p2rx7, encoding the P2X7 receptor (P2X7R), is a candidate gene for the differential susceptibility to vascular dysfunction under high angiotensin II tone. A 14-day infusion of angiotensin II into F344 rats increased blood pressure by about 15 mm Hg without inducing fibrosis or albuminuria. In vivo pressure natriuresis was suppressed, medullary perfusion reduced by half, and the corticomedullary oxygenation gradient disrupted. Selective P2X7R antagonism restored pressure natriuresis, promoting a significant leftward shift in the intercept and increasing the slope. Sodium excretion was increased sixfold and blood pressure normalized. The specific P2X7R antagonist AZ11657312 increased renal medullary perfusion, but only in angiotensin II-treated rats. Tissue oxygenation was improved by P2X7R blockade, particularly in poorly oxygenated regions of the kidney. Thus, activation of P2X7R induces microvascular dysfunction and regional hypoxia when angiotensin II is elevated and these effects may contribute to progression of renal injury induced by chronic angiotensin II.

Effect of P2X4 and P2X7 receptor antagonism on the pressure diuresis relationship in rats

Reduced glomerular filtration, hypertension and renal microvascular injury are hallmarks of chronic kidney disease, which has a global prevalence of ~10%. We have shown previously that the Fischer (F344) rat has lower GFR than the Lewis rat, and is more susceptible to renal injury induced by hypertension. In the early stages this injury is limited to the pre-glomerular vasculature. We hypothesized that poor renal hemodynamic function and vulnerability to vascular injury are causally linked and genetically determined. In the present study, normotensive F344 rats had a blunted pressure diuresis relationship, compared with Lewis rats. A kidney microarray was then interrogated using the Endeavour enrichment tool to rank candidate genes for impaired blood pressure control. Two novel candidate genes, P2rx7 and P2rx4, were identified, having a 7− and 3− fold increased expression in F344 rats. Immunohistochemistry localized P2X₄ and P2X₇ receptor expression to the endothelium of the pre-glomerular vasculature. Expression of both receptors was also found in the renal tubule; however there was no difference in expression profile between strains. Brilliant Blue G (BBG), a relatively selective P2X₇ antagonist suitable for use in vivo, was administered to both rat strains. In Lewis rats, BBG had no effect on blood pressure, but increased renal vascular resistance, consistent with inhibition of some basal vasodilatory tone. In F344 rats BBG caused a significant reduction in blood pressure and a decrease in renal vascular resistance, suggesting that P2X₇ receptor activation may enhance vasoconstrictor tone in this rat strain. BBG also reduced the pressure diuresis threshold in F344 rats, but did not alter its slope. These preliminary findings suggest a physiological and potential pathophysiological role for P2X₇ in controlling renal and/or systemic vascular function, which could in turn affect susceptibility to hypertension-related kidney damage.

Cardiac remodeling during and after renin-angiotensin system stimulation in Cyp1a1-Ren2 transgenic rats

This study investigated renin-angiotensin system (RAS)-induced cardiac remodeling and its reversibility in the presence and absence of high blood pressure (BP) in Cyp1a1-Ren2 transgenic inducible hypertensive rats (IHR). In IHR (pro)renin levels and BP can be dose-dependently titrated by oral administration of indole-3-carbinol (I3C). Young (four-weeks old) and adult (30-weeks old) IHR were fed I3C for four weeks (leading to systolic BP >200 mmHg). RAS-stimulation was stopped and animals were followed-up for a consecutive period. Cardiac function and geometry was determined echocardiographically and the hearts were excised for molecular and immunohistochemical analyses. Echocardiographic studies revealed that four weeks of RAS-stimulation incited a cardiac remodeling process characterized by increased left ventricular (LV) wall thickness, decreased LV volumes, and shortening of the left ventricle. Hypertrophic genes were highly upregulated, whereas in substantial activation a fibrotic response was absent. Four weeks after withdrawal of I3C, (pro)renin levels were normalized in all IHR. While in adult IHR BP returned to normal, hypertension was sustained in young IHR. Despite the latter, myocardial hypertrophy was fully regressed in both young and adult IHR. We conclude that (pro)renin-induced severe hypertension in IHR causes an age-independent fully reversible myocardial concentric hypertrophic remodeling, despite a continued elevated BP in young IHR.

Imatinib ameliorates renal morphological changes in Cyp1a1-Ren2 transgenic rats with inducible ANG II-dependent malignant hypertension

The present study was performed to assess the effects of the platelet-derived growth factor (PDGF) receptor kinase inhibitor imatinib mesylate on the renal morphological changes occurring during the development of malignant hypertension in transgenic rats with inducible expression of the Ren2 gene [TGR(Cyp1a1Ren2)]. Arterial blood pressure was measured by radiotelemetry in male Cyp1a1-Ren2 rats during control conditions and during dietary administration of indole-3-carbinol (I3C; 0.3%) for 14 days to induce malignant hypertension. Rats induced with I3C (n = 5) had higher mean arterial pressures (178 ± 4 vs. 109 ± 2 mmHg, P < 0.001) and increased urinary albumin excretion (Ualb; 13 ± 5 vs. 0.6 ± 0.2 mg/day) compared with noninduced rats (n = 5). Chronic administration of imatinib (60 mg·kg(-1)·day(-1) in drinking water, n = 5) did not alter the magnitude of the hypertension (176 ± 8 mmHg) but prevented the increase in Ualb (1.6 ± 0.3 mg/day). Quantitative analysis of proliferating cell nuclear antigen using immunohistochemistry demonstrated increased proliferating cell number in cortical tubules (38 ± 5 vs. 18 ± 1 cells/mm(2)) and cortical interstitium (40 ± 7 vs. 13 ± 6 cells/mm(2)) of hypertensive rat kidneys. Renal cortical fibrosis evaluated by picrosirius red staining showed increased collagen deposition in kidneys of the hypertensive rats (1.6 ± 0.1 vs. 0.4 ± 0.1% of cortical area). Imatinib attenuated the increase in proliferating cell number in cortical tubules and interstitium (22 ± 5 vs. 38 ± 5 and 22 ± 6 vs. 40 ± 7 cells/mm(2), respectively) and reduced the degree of collagen deposition (0.8 ± 0.2 vs. 1.6 ± 0.1%) in the kidneys of hypertensive rats. These findings demonstrate that the renal pathological changes that occur during the development of malignant hypertension in Cyp1a1-Ren2 rats involve activation of PDGF receptor kinase.

Enhanced urinary angiotensinogen excretion in Cyp1a1-Ren2 transgenic rats with inducible ANG II-dependent malignant hypertension

INTRODUCTION

Previous studies have demonstrated that the urinary excretion of angiotensinogen is significantly increased in ANG II-infused hypertensive rats, which is associated with an augmentation of intrarenal ANG II levels. These findings suggest that urinary angiotensinogen excretion rates provide an index of intrarenal ANG II levels in ANG II-dependent hypertensive states. However, little information is available regarding the urinary excretion of angiotensinogen in ANG II-dependent malignant hypertension.

METHODS

This study was performed to determine if urinary angiotensinogen excretion is increased in Cyp1a1-Ren2 transgenic rats [strain name: TGR(Cyp1aRen2)] with inducible ANG II-dependent malignant hypertension. Adult male Cyp1a1-Ren2 rats (n = 6) were fed a normal diet containing 0.3% indole-3-carbinol (I3C) for 10 days to induce ANG II-dependent malignant hypertension.

RESULTS

Rats induced with I3C exhibited pronounced increases in systolic blood pressure (208 ± 7 versus 127 ± 3 mm Hg; P < 0.001), marked proteinuria (29.4 ± 3.6 versus 5.9 ± 0.3 mg/d; P < 0.001) and augmented urinary angiotensinogen excretion (996 ± 186 versus 241 ± 31 ng/d; P < 0.01). Chronic administration of the AT₁ receptor antagonist, candesartan (25 mg/L in drinking water, n = 6), prevented the I3C-induced increases in systolic blood pressure (125 ± 5 mm Hg; P < 0.001), proteinuria (7.3 ± 1.0 mg/d; P < 0.001) and urinary angiotensinogen excretion (488 ± 51 ng/d, P < 0.01).

CONCLUSIONS

These data demonstrate that the urinary excretion of angiotensinogen is markedly augmented in ANG II-dependent malignant hypertension. Such increased urinary angiotensinogen excretion may contribute to augmented intrarenal ANG II levels and, thereby, to the increased blood pressure in Cyp1a1-Ren2 transgenic rats with inducible ANG II-dependent malignant hypertension.

Standing genetic variation and compensatory evolution in transgenic organisms: a growth-enhanced salmon simulation

Genetically modified strains usually are generated within defined genetic backgrounds to minimize variation for the engineered characteristic in order to facilitate basic research investigations or for commercial application. However, interactions between transgenes and genetic background have been documented in both model and commercial agricultural species, indicating that allelic variation at transgene-modifying loci are not uncommon in genomes. Engineered organisms that have the potential to allow entry of transgenes into natural populations may cause changes to ecosystems via the interaction of their specific phenotypes with ecosystem components and services. A transgene introgressing through natural populations is likely to encounter a range of natural genetic variation (among individuals or sub-populations) that could result in changes in phenotype, concomitant with effects on fitness and ecosystem consequences that differ from that seen in the progenitor transgenic strain. In the present study, using a growth hormone transgenic salmon example, we have modeled selection of modifier loci (single and multiple) in the presence of a transgene and have found that accounting for genetic background can significantly affect the persistence of transgenes in populations, potentially reducing or reversing a "Trojan gene" effect. Influences from altered life history characteristics (e.g., developmental timing, age of maturation) and compensatory demographic/ecosystem controls (e.g., density dependence) also were found to have a strong influence on transgene effects. Further, with the presence of a transgene in a population, genetic backgrounds were found to shift in non-transgenic individuals as well, an effect expected to direct phenotypes away from naturally selected optima. The present model has revealed the importance of understanding effects of selection for background genetics on the evolution of phenotypes in populations harbouring transgenes.

Differential contribution of diabetes and the Ren2 gene to glomerular pathology in diabetic (mREN-2)27 rats

The effect of diabetes mellitus vs the effect of the Ren2 gene on the glomerular pathology of (mREN-2)27 heterozygous male rats is controversial. As discrete diabetes-induced glomerular lesions may have been overlooked, we performed a detailed morphometric analysis of glomeruli in diabetic and non-diabetic heterozygous male (mREN-2)27 rats and their normotensive (non-diabetic and diabetic Sprague-Dawley) controls. Glomeruli were scored by light microscopy for nine discrete histological parameters, some of which were graded for extent and/or severity. Mesangiolysis, segmental hypocellularity, and severe tuft-to-capsule adhesions were specific to diabetes; severe mesangial matrix expansion, glomerulosclerosis, thickening of Bowman's capsule, and dilatation of the urinary space were specific to the Ren2 gene. Hyalinosis and hypercellularity were associated with both diabetes and the Ren2 gene: the effect was additive for hyalinosis and synergistic for hypercellularity. The histological parameters were then combined with two physiological indices (systolic blood pressure and proteinuria) and principle components analysis (PCA) was used to detect correlations between the variables. Four discrete patterns of pathology were identified; three were statistically associated with diabetes and/or the Ren2 gene. These findings suggest that both diabetes and the Ren2 gene make significant, albeit different, contributions to the glomerular pathology of diabetic heterozygous male (mREN-2)27 rats. Despite defining the contribution of diabetes, our work does not support the (mREN-2)27 rat as a model of diabetic nephropathy (DN). Rather, it suggests that these animals remain useful for investigating a particular and limited constellation of DN features.

Transient induction of ANG II-dependent malignant hypertension causes sustained elevation of blood pressure and augmentation of the pressor response to ANG II in CYP1A1-REN2 transgenic rats

INTRODUCTION

Transgenic rats with inducible expression of the mouse Ren2 renin gene [strain name: TGR(Cyp1a1Ren2)] allow induction of various degrees of ANG II-dependent hypertension. Dietary administration of the aryl hydrocarbon indole-3-carbinol (I3C) at a dose of 0.15% induces a slowly developing form of ANG II-dependent hypertension, whereas dietary administration of a higher dose (0.3%) of I3C results in the development of ANG II-dependent malignant hypertension. Cessation of administration of 0.15% I3C results in the normalization of blood pressure, indicating the reversibility of hypertension induced by this dose of I3C. The present study was performed to determine if ANG II-dependent malignant hypertension is similarly reversible following cessation of dietary administration of 0.3% I3C.

METHODS

Cyp1a1-Ren2 rats (n = 6) were fed a normal diet containing 0.3% I3C for 11 days to induce malignant hypertension.

RESULTS

Cyp1a1-Ren2 rats induced with I3C exhibited pronounced increases in systolic blood pressure (SBP) (132 +/- 3-229 +/- 11 mm Hg, P < 0.001) and marked decreases in body weight (303 +/- 4-222 +/- 2 g, P < 0.001). When I3C administration was terminated, SBP decreased to 167 +/- 4 mm Hg (P < 0.01) and body weight increased to normal levels (309 +/- 2 g, P < 0.01) within 12 days. However, SBP remained significantly elevated (172 +/- 1 mm Hg, P < 0.01) for up to 3 weeks after termination of dietary administration of 0.3% I3C. In addition, the magnitude of the blood pressure response to intravenous bolus administration of 50 ng of ANG II (50 microL in volume) 3 weeks after cessation of dietary I3C administration was substantially higher than that observed in normotensive control rats (134 +/- 1 mm Hg, n = 6) not previously induced with 0.3% I3C (53 +/- 2 versus 38 +/- 3 mm Hg, P < 0.05).

CONCLUSIONS

The present findings demonstrate that transient induction of ANG II-dependent malignant hypertension results in prolonged elevations of arterial blood pressure and marked augmentation of the magnitude of the pressor response to ANG II in Cyp1a1-Ren2 transgenic rats.

Syndactyly lethal: new mutation with multiple malformations occurring in Sprague Dawley rats

We previously found newborns exhibiting syndactyly of both fore- and hindlimbs in a litter from a pair of Sprague Dawley rats. Continuous breeding of the parental animals yielded pups with the same anomaly in following litters, suggesting that the syndactyly was genetic in origin. In the present study, as all the syndactylous pups died on postnatal day 0, we conducted genetic analyses using 30 phenotypically normal female progeny and the sire. The females were subjected to caesarean section on day 20 of gestation and the fetuses were examined for the phenotypes. The results of the mating experiments suggest that the mutant phenotype is caused by a single autosomal recessive gene at a homozygous condition. As homozygous mutants are lethal at the neonatal stage, the mutant gene was named syndactyly lethal, gene symbol syl. The mutant rats have multiple abnormalities, such as syndactyly, micrognathia, fused/absent/small lung lobes, absent kidney and ureter, small spleen, small uterus, fused phalanges, sternoschisis, absent/detached rib, and splitting/fused/absent/small thoracic vertebra, some of which must be the cause of death on postnatal day 0. This mutant is considered to be useful for investigating the mechanisms and/or pathogenesis of syndactyly, as well as the accompanying malformations.

Angiotensin-converting enzyme is a modifier of hypertensive end organ damage

Severe forms of hypertension are characterized by high blood pressure combined with end organ damage. Through the development and refinement of a transgenic rat model of malignant hypertension incorporating the mouse renin gene, we previously identified a quantitative trait locus on chromosome 10, which affects malignant hypertension severity and morbidity. We next generated an inducible malignant hypertensive model where the timing, severity, and duration of hypertension was placed under the control of the researcher, allowing development of and recovery from end organ damage to be investigated. We have now generated novel consomic Lewis and Fischer rat strains with inducible hypertension and additional strains that are reciprocally congenic for the refined chromosome 10 quantitative trait locus. We have captured a modifier of end organ damage within the congenic region and, using a range of bioinformatic, biochemical and molecular biological techniques, have identified angiotensin-converting enzyme as the modifier of hypertension-induced tissue microvascular injury. Reciprocal differences between angiotensin-converting enzyme and the anti-inflammatory tetrapeptide, N-acetyl-Ser-Asp-Lys-Pro in the kidney, a tissue susceptible to end organ damage, suggest a mechanism for the amelioration of hypertension-dependent damage.

Renoprotective effects of neuronal NOS-derived nitric oxide and cyclooxygenase-2 metabolites in transgenic rats with inducible malignant hypertension

The present study was performed to determine the effects of neuronal nitric oxide synthase (nNOS) and cyclooxygenase-2 (COX-2) inhibition on blood pressure and renal hemodynamics in transgenic rats with inducible ANG II-dependent malignant hypertension [strain name: TGR(Cyp1a1Ren2)]. Male Cyp1a1-Ren2 rats (n = 7) were fed a normal diet containing indole-3-carbinol (I3C; 0.3%) for 6-9 days to induce malignant hypertension. Mean arterial pressure (MAP) and renal hemodynamics were assessed in pentobarbital sodium-anesthetized Cyp1a1-Ren2 rats before and during intravenous infusion of the nNOS inhibitor S-methyl-l-thiocitrulline (l-SMTC; 1 mg/h). In hypertensive Cyp1a1-Ren2 rats, l-SMTC increased MAP from 169 +/- 3 to 188 +/- 4 mmHg (P < 0.01), which was a smaller increase than in noninduced rats (124 +/- 9 to 149 +/- 9 mmHg, P < 0.01, n = 5). Additionally, l-SMTC decreased renal plasma flow (RPF) to a similar extent (-34 +/- 13 vs. -35 +/- 12%) in the hypertensive and normotensive rats (4.1 +/- 0.2 to 2.7 +/- 0.5 and 3.1 +/- 0.3 to 2.0 +/- 0.3 ml x min(-1) x g(-1), respectively, P < 0.01) but did not alter glomerular filtration rate (GFR) in either group. In additional experiments, administration of the COX-2 inhibitor, nimesulide (3 mg/kg i.v.), during simultaneous infusion of l-SMTC decreased MAP in both hypertensive and noninduced rats (182 +/- 2 to 170 +/- 3 mmHg and 153 +/- 3 to 140 +/- 3 mmHg, respectively, P < 0.01). Nimesulide also decreased RPF (1.9 +/- 0.2 to 0.8 +/- 0.1 ml x min(-1) x g(-1), P < 0.01) and GFR (0.9 +/- 0.1 to 0.4 +/- 0.1 ml x min(-1) x g(-1), P < 0.01) in hypertensive rats but did not alter RPF or GFR in noninduced rats. The present findings demonstrate that both nNOS-derived NO and COX-2 metabolites exert pronounced renal vasodilator influences in hypertensive Cyp1a1-Ren2 rats. The data also indicate that the renal vasodilator effects of COX-2-derived prostanoids in hypertensive Cyp1a1-Ren2 rats are not dependent on nNOS activity.

Genetic clamping of renin gene expression induces hypertension and elevation of intrarenal Ang II levels of graded severity in Cyp1a1-Ren2 transgenic rats

INTRODUCTION

Transgenic rats with inducible angiotensin II (Ang II)-dependent hypertension (strain name: TGR[Cyp1a1-Ren2]) were generated by inserting the mouse Ren2 renin gene, fused to the cytochrome P450 1a1 (Cyp1a1) promoter, into the genome of the rat. The present study was performed to characterise the changes in plasma and kidney tissue Ang II levels and in renal haemodynamic function in Cyp1a1-Ren2 rats following induction of either slowly developing or malignant hypertension in these transgenic rats.

MATERIALS AND METHODS

Arterial blood pressure (BP) and renal haemodynamics and excretory function were measured in pentobarbital sodium-anaesthetised Cyp1a1- Ren2 rats fed a normal diet containing either a low dose (0.15%, w/w for 1415 days) or high dose (0.3%, w/w for 1112 days) of the aryl hydrocarbon indole-3-carbinol (I3C) to induce slowly developing and malignant hypertension, respectively. In parallel experiments, arterial blood samples and kidneys were harvested for measurement of Ang II levels by radioimmunoassay.

RESULTS

Dietary I3C increased plasma renin activity (PRA), plasma Ang II levels, and arterial BP in a dose-dependent manner. Induction of different fixed levels of renin gene expression and PRA produced hypertensive phenotypes of varying severity with rats developing either mild or malignant forms of hypertensive disease. Administration of I3C, at a dose of 0.15% (w/w), induced a slowly developing form of hypertension whereas administration of a higher dose (0.3%) induced a more rapidly developing hypertension and the clinical manifestations of malignant hypertension including severe weight loss. Both hypertensive phenotypes were characterised by reduced renal plasma flow, increased filtration fraction, elevated PRA, and increased plasma and intrarenal Ang II levels. These I3C-induced changes in renal haemodynamics, PRA and kidney Ang II levels were more pronounced in Cyp1a1-Ren2 rats with malignant hypertension. Chronic administration of the AT1-receptor antagonist, hypertension, the associated changes in renal haemodynamics, and the augmentation of intrarenal Ang II levels.

CONCLUSIONS

Activation of AT1-receptors by Ang II generated as a consequence of induction of the Cyp1a1-Ren2 transgene mediates the increased arterial pressure and the associated reduction of renal haemodynamics and enhancement of intrarenal Ang II levels in hypertensive Cyp1a1-Ren2 transgenic rats.

Cyclooxygenase-2 inhibition normalizes arterial blood pressure in CYP1A1-REN2 transgenic rats with inducible ANG II-dependent malignant hypertension

The present study was performed to determine the effects of cyclooxygenase (COX)-1 and COX-2 inhibition on blood pressure and renal hemodynamics in transgenic rats with inducible malignant hypertension [strain name: TGR(Cyp1a1Ren2)]. Male Cyp1a1-Ren2 rats ( n = 7) were fed a normal diet containing the aryl hydrocarbon, indole-3-carbinol (I3C; 0.3%), for 6–9 days to induce malignant hypertension. Mean arterial pressure (MAP) and renal hemodynamics were measured in pentobarbital sodium-anesthetized Cyp1a1-Ren2 rats during control conditions, following administration of the COX-2 inhibitor nimesulide (3 mg/kg iv), and following administration of the nonspecific COX inhibitor meclofenamate (5 mg/kg iv). Rats induced with I3C had higher MAP than noninduced rats ( n = 7; 188 ± 6 vs. 136 ± 4 mmHg, P < 0.01). There was no difference in renal plasma flow (RPF) or glomerular filtration rate (GFR) between induced and noninduced rats. Nimesulide elicited a larger decrease in MAP in hypertensive rats (188 ± 6 to 140 ± 8 mmHg, P < 0.01) than in normotensive rats (136 ± 4 to 113 ± 8 mmHg, P < 0.01). Additionally, nimesulide decreased GFR (0.9 ± 0.13 to 0.44 ± 0.05 ml·min−1·g−1, P < 0.05) and RPF (2.79 ± 0.27 to 1.35 ± 0.14 ml·min−1·g−1, P < 0.05) in hypertensive rats but did not alter GFR or RPF in normotensive rats. Meclofenamate further decreased MAP in hypertensive rats (to 115 ± 10 mmHg, P < 0.05) but did not decrease MAP in normotensive rats. Meclofenamate did not alter GFR or RPF in either group. These findings demonstrate that COX-1- and COX-2-derived prostanoids contribute importantly to the development of malignant hypertension in Cyp1a1-Ren2 transgenic rats. The data also indicate that COX-2-derived vasodilatory metabolites play an important role in the maintenance of RPF and GFR following induction of malignant hypertension in Cyp1a1-Ren2 transgenic rats.

Hypertension, kidney, and transgenics: a fresh perspective

In this review, we outline the application and contribution of transgenic technology to establishing the genetic basis of blood pressure regulation and its dysfunction. Apart from a small number of examples where high blood pressure is the result of single gene mutation, essential hypertension is the sum of interactions between multiple environmental and genetic factors. Candidate genes can be identified by a variety of means including linkage analysis, quantitative trait locus analysis, association studies, and genome-wide scans. To test the validity of candidate genes, it is valuable to model hypertension in laboratory animals. Animal models generated through selective breeding strategies are often complex, and the underlying mechanism of hypertension is not clear. A complementary strategy has been the use of transgenic technology. Here one gene can be selectively, tissue specifically, or developmentally overexpressed, knocked down, or knocked out. Although resulting phenotypes may still be complicated, the underlying genetic perturbation is a starting point for identifying interactions that lead to hypertension. We recognize that the development and maintenance of hypertension may involve many systems including the vascular, cardiac, and central nervous systems. However, given the central role of the kidney in normal and abnormal blood pressure regulation, we intend to limit our review to models with a broadly renal perspective.

Germ Line Polymorphism in Metastatic Progression: Figure 1

Somatic genetic analysis of tumors and metastases has yielded a plethora of information regarding genes associated with cancer progression. However, somatic alterations in tumor cells are only one source of variability. Genetic polymorphism may also play a significant role in person-to-person variability in metastasis frequency, raising the intriguing possibility that some individuals could be predisposed to secondary tumor development. The identification and characterization of these polymorphisms may have significant implications for the development of tailored treatment or prevention of recurrent disease.

Interactive effects of superoxide anion and nitric oxide on blood pressure and renal hemodynamics in transgenic rats with inducible malignant hypertension

Superoxide anion contributes to the pathogenesis of various forms of hypertension, but its role in the development of malignant hypertension remains unclear. The present study was performed to determine the influence of superoxide anion on blood pressure and renal hemodynamics in transgenic rats with inducible malignant hypertension [strain name: TGR(Cyp1a1Ren2)]. Malignant hypertension was induced in male Cyp1a1-Ren2 rats ( n = 6) through dietary administration of the aryl hydrocarbon, indole-3-carbinol (0.3%) for 7–9 days. Mean arterial pressure (MAP) and renal hemodynamics were measured in pentobarbital sodium-anesthetized Cyp1a1-Ren2 rats before and during intravenous infusion of the superoxide dismutase mimetic tempol (100 μmol/h). Basal MAP and renal vascular resistance (RVR) were elevated in rats induced with indole-3-carbinol compared with noninduced rats ( n = 5) (184 ± 4 vs. 127 ± 3 mmHg, P < 0.01, and 29 ± 2 vs. 21 ± 1 mmHg·ml−1·min·g, P < 0.01, respectively). Hypertensive rats had elevated excretion of urinary 8-isoprostane compared with normotensive rats (41 ± 4 vs. 13 ± 6 pg·min−1·g−1, P < 0.01). There were no differences in renal plasma flow and glomerular filtration rate between groups. Systemic administration of tempol decreased MAP (184 ± 4 to 151 ± 4 mmHg, P < 0.01) and RVR (29 ± 2 to 25 ± 2 mmHg·ml−1·min·g, P < 0.05) in hypertensive but not in normotensive Cyp1a1-Ren2 rats. In addition, tempol administration decreased urinary excretion of 8-isoprostane (41 ± 4 to 25 ± 4 pg·min−1·g−1, P < 0.05). Renal plasma flow and glomerular filtration rate remained unaltered during tempol administration in both groups. The administration of the nitric oxide synthase inhibitor nitro-l-arginine attenuated the decrease in MAP and RVR in response to tempol. These findings indicate that superoxide anion contributes to the elevated RVR and increased arterial blood pressure, by a mechanism that is at least in part nitric oxide dependent, in Cyp1a1-Ren2 rats with malignant hypertension.

Enhanced tubuloglomerular feedback in Cyp1a1-Ren2 transgenic rats with inducible ANG II-dependent malignant hypertension

The present study was performed to evaluate tubuloglomerular feedback responses in transgenic rats [TGR(Cypa1a1Ren2)] with inducible malignant hypertension and to determine the degree to which feedback responsiveness is modulated by ANG II in these rats. Male Cyp1a1-Ren2 rats were fed a normal diet containing the aryl hydrocarbon indole-3-carbinol (I3C; 0.3%), for 5-6 days to stimulate expression of the Cyp1a1-Ren2 transgene and, thereby, to induce malignant hypertension. Stop-flow pressure (SFP) feedback responses to a late proximal perfusion rate of 40 nl/min were assessed in pentobarbital sodium-anesthetized rats during control conditions and after administration of the AT(1) receptor antagonist candesartan (0.1 mg/kg iv). Rats induced with I3C (n = 6) exhibited elevated mean arterial pressure and increased maximal SFP feedback responses compared with noninduced rats (n = 4; 163 +/- 4 vs. 130 +/- 2 mmHg, P < 0.01 and 16.3 +/- 1.4 vs. 11.7 +/- 0.5 mmHg, P < 0.05, respectively). Systemic candesartan decreased arterial pressure (to 98 +/- 7 and to 101 +/- 5 mmHg, respectively, P < 0.001) and attenuated SFP feedback responses (to 2.0 +/- 0.4 and to 3.3 +/- 0.9 mmHg, respectively, P < 0.01) in both hypertensive and normotensive rats. In additional experiments, peritubular capillary infusion of 10(-3) M candesartan did not alter arterial pressure but attenuated feedback responses in both hypertensive (19.3 +/- 1.4 to 8.8 +/- 0.9 mmHg, P < 0.01, n = 9) and normotensive Cyp1a1-Ren2 rats (9.0 +/- 0.8 to 4.7 +/- 0.6 mmHg, P < 0.01, n = 7). The present findings indicate that Cyp1a1-Ren2 rats with ANG II-dependent malignant hypertension exhibit augmented tubuloglomerular feedback responses. The data also show that AT(1) receptor activation by ANG II contributes to the enhanced feedback responsiveness in Cyp1a1-Ren2 rats with malignant hypertension.

A novel transmembrane MSP-containing protein that plays a role in right ventricle development

We have identified and characterized a gene, Mospd3 on mouse chromosome 5 using gene trapping in ES cells. MOSPD3 is part of a family of proteins, including MOSPD1, which is defined by the presence of a major sperm protein (MSP) domain and two transmembrane domains. Interestingly Mospd3 is mammalian specific and highly conserved between mouse and man. Insertion of the gene trap vector at the Mospd3 locus is mutagenic and breeding to homozygosity results in a characteristic right ventricle defect and neonatal lethality in 50% of mice. The phenotypic defect is dependent on the genetic background, indicating the presence of genetic modifier loci. We speculate that the further characterization of Mospd3 will shed light on the complex genetic interactions involved in cardiac development and disease.

Angiotensin-converting enzyme i/d polymorphism in patients with malignant hypertension

The angiotensin-converting enzyme (ACE) gene has been implicated in the manifestation of the phenotype of malignant hypertension (MH). In 1990 the ACE gene polymorphism characterized by the insertion or deletion of a 287-base pair fragment in the 17q23 chromosome was identified. The DD genotype is associated with increased tissue and circulating ACE levels and elevated angiotensin II. ACE polymorphism was studied in 48 patients with MH, 25 patients with non-MH, and a control group of 78 normotensive individuals by real-time polymerase chain reaction using the LightCycler system (Roche Diagnostics Corporation, Indianapolis, IN). The DD genotype was found statistically more frequently in MH patients than controls (p=0.028; odds ratio, 2.5; confidence interval, 1.1-5.5). Presence of the DD genotype of the ACE gene is more frequent in MH patients than in controls, indicating that this genotype could be a significant risk factor and a predictor for the development of MH.

Blockade of Endothelin Receptors Attenuates End-Organ Damage in Homozygous Hypertensive Ren-2 Transgenic Rats

BACKGROUND/AIMS

A growing body of evidence suggests that the interplay between the endothelin (ET) and the renin-angiotensin systems (RAS) plays an important role in the development of the malignant phase of hypertension. The present study was performed to evaluate the role of an interaction between ET and RAS in the development of hypertension and hypertension-associated end-organ damage in homozygous male transgenic rats harboring the mouse Ren-2 renin gene (TGRs) under conditions of normal-salt (NS, 0.45% NaCl) and high-salt (HS, 2% NaCl) intake.

METHODS

Twenty-eight-day-old homozygous male TGRs and age-matched transgene-negative male normotensive Hannover Sprague-Dawley (HanSD) rats were randomly assigned to groups with NS or HS intake. Nonselective ET(A/B) receptor blockade was achieved with bosentan (100 mg/kg/day). Systolic blood pressure (BP) was measured in conscious animals by tail plethysmography. Rats were placed into metabolic cages to determine proteinuria and clearance of endogenous creatinine. At the end of the experiment the final arterial BP was measured directly in anesthetized rats. Kidneys were taken for morphological examination.

RESULTS

All male HanSD fed either the NS or HS diet exhibited a 100% survival rate until 180 days of age (end of experiment). The survival rate in untreated homozygous male TGRs fed the NS diet was 41%, which was markedly improved by treatment with bosentan to 88%. The HS diet reduced the survival rate in homozygous male TGRs to 10%. The survival rate in homozygous male TGRs on the HS diet was significantly improved by bosentan to 69%. Treatment with bosentan did not influence either the course of hypertension or the final levels of BP in any of the experimental groups of HanSD rats or TGRs. Although the ET-1 content in the renal cortex did not differ between HanSD rats and TGRs, ET-1 in the left heart ventricle of TGRs fed the HS diet was significantly higher compared with all other groups. Administration of bosentan to homozygous male TGRs fed either the NS or HS diet markedly reduced proteinuria, glomerulosclerosis and attenuated the development of cardiac hypertrophy compared with untreated TGR.

CONCLUSIONS

Our data show that nonselective ET(A/B) receptor blockade markedly improves the survival rate and ameliorates end-organ damage in homozygous male TGRs without significantly lowering BP.

Vascular injury in systemic sclerosis: Angiotensin-converting enzyme insertion/deletion polymorphism

The microvascular involvement in systemic sclerosis (SSc) is characterized by endothelial damage and smooth muscle cell migration in the intima. The vascular pathologic modifications in SSc are strikingly similar to those of atherosclerosis. SSc also is characterized by an accelerated macrovascular disease. The gene encoding for angiotensin-converting enzyme (ACE) is a 21-kb, 26-exon gene, localized on chromosome 17 (17q23). Polymorphic sites are an insertion/deletion (I/D) that consists of three genotypes: DD and II homozygotes, and ID heterozygote. ACE gene polymorphisms have been linked to vascular disorders (coronary artery disease, hypertension, cerebrovascular disease, hypertrophic cardiomyopathy, and diabetic or nondiabetic nephropathy). In particular, the possession of ACE D allele was associated with an increased risk of developing malignant vascular injury. ACE D allele frequency of the I/D polymorphism was associated with an increased risk of SSc, suggesting a genetic contribution to the disease. The discrepancy between the high prevalence of D allele and reduced ACE plasma levels in SSc demonstrate the lack of knowledge on the regulation and function of renin-angiotensin system in SSc.

Genetically modified laboratory animals--what welfare problems do they face?

In this article, we respond to public concern expressed about the welfare of genetically modified (GM) nonhuman animals. As a contribution to the debate on this subject, we attempt in this article to determine in what situations the practice of genetic modification in rodents may generate significant welfare problems. After a brief discussion of the principles of animal welfare, we focus on the problem of animal suffering and review some types of gene modifications likely to cause predictable welfare problems. In this article, we also consider suffering that may be involved in the process of generating GM animals. Finally, we discuss the role of GM animals in attempts to reduce, replace, and refine the use of animals in research.

The role of the renin–angiotensin system in malignant vascular injury affecting the systemic and cerebral circulations

Malignant hypertension is a rare but serious syndrome complicating 1% of essential hypertension and causing neurological, renal and cardiac complications. Despite improved anti-hypertensive medication, the incidence of this condition fails to decline. In the first part of this review, we discuss transgenic rat models of malignant hypertension, generated by over-expressing renin, to illustrate the role of the renin-angiotensin system in the development of systemic hypertensive vascular remodelling and hypertension. In the second part, we focus on the cerebrovascular response to hypertension and discuss new data using a conditional, transgenic model of malignant hypertension, the inducible hypertensive rat (IHR). Cerebral infarction associates strongly with hypertension in man and the mechanisms by which hypertension predisposes to different types of stroke remains poorly understood. Rats have similar cerebrovascular anatomy and structure to humans and as such provide a good experimental tool. To date, such models lack controllability and blood-pressure matched controls. Using the IHR, we have manipulated dietary salt and water intake to generate a novel, controllable stroke phenotype. Hypertensive small-vessel stroke develops over a predictable time period, permitting the study of developing cerebrovascular lesions. Systemic end-organ injury and hypertension are not affected. Dissociation of the systemic and central vascular responses in this way, will allow for comparative study of animals with equivalent hypertension, genetic background and systemic features of hypertension with or without stroke.

The use of animal models in the study of complex disease: all else is never equal or why do so many human studies fail to replicate animal findings?

The study of the genetics of complex human disease has met with limited success. Many findings with candidate genes fail to replicate despite seemingly overwhelming physiological data implicating the genes. In contrast, animal model studies of the same genes and disease models usually have more consistent results. We propose that one important reason for this is the ability to control genetic background in animal studies. The fact that controlling genetic background can produce more consistent results suggests that the failure to replicate human findings in the same diseases is due to variation in interacting genes. Hence, the contrasting nature of the findings from the different study designs indicates the importance of non-additive genetic effects on human disease. We discuss these issues and some methodological approaches that can detect multilocus effects, using hypertension as a model disease. This article contains supplementary material, which may be viewed at the BioEssays website at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html.

Hypertensive cerebral small vessel disease and stroke

Lacunar infarcts and "hypertensive" primary intracerebral hemorrhages, collectively often referred to as hypertensive small vessel strokes, constitute about one third of all strokes. However, despite their public health importance, their etiopathogenesis remains ill-understood. Like all strokes, they are a heterogeneous entity, but the autopsy pathology evidence suggests that the majority are caused by a limited number of cerebral small vessel lesions. Small vessel atherosclerosis is causally implicated in a proportion of lacunar infarcts, although modern concepts of atherosclerotic plaque biology and natural history have yet to be applied to small cerebral vessels. A lesion characterized in its acute form by fibrinoid necrosis appears to be important in causing both lacunar infarcts and primary intracerebral bleeds. Advances in molecular genetics may prove instrumental in understanding the cause of this lesion and therefore in designing its targeted prevention.

Association of the D allele of the angiotensin I converting enzyme polymorphism with malignant vascular injury

AIMS

To determine whether there is an association between the insertion/deletion (I/D) polymorphism of the human angiotensin I converting enzyme (ACE) gene and malignant vascular injury (MVI).

METHODS

The polymerase chain reaction was used to genotype DNA extracted from archival, paraffin wax embedded renal biopsy material from 48 patients with MVI, made up from cases of malignant hypertension (n = 23), scleroderma (n = 10), and haemolytic uraemic syndrome (n = 15), and from whole blood samples from 191 healthy controls.

RESULTS

The D allele was found more frequently in cases of MVI than in healthy controls, (65% v 52%). Both the DD and I/D genotypes occurred significantly more frequently in patients with MVI than did the II genotype (chi(2) = 7.26, p = 0.007; and chi(2) = 4.06, p = 0.04, respectively).

CONCLUSIONS

Possession of at least one copy of the D allele is associated with an increased risk of developing MVI. Our data support a dominant mode of effect for the D allele. Use of the I/D polymorphism as a genetic marker for MVI may be of value clinically in identifying at risk individuals before the development of target end organ damage. Furthermore, those at risk may benefit from early ACE inhibition.

Controlled hypertension, a transgenic toggle switch reveals differential mechanisms underlying vascular disease

A novel inbred rat model with inducible hypertension has been generated using a renin transgene under the transcriptional control of the cytochrome P450, Cyp1a1 promoter. The degree and duration of hypertension are regulated tightly by administration of the natural xenobiotic indole-3 carbinol and can be readily reversed. Induction experiments reveal distinct temporal and mechanistic responses to hypertensive injury in different vascular beds, which is indicative of differential susceptibility of organs to a hypertensive stimulus. The mesentery and heart exhibited the greatest sensitivity to damage, and the kidney showed an adaptive response prior to the development of malignant hypertensive injury. Quantitative analysis of morphological changes induced in mesenteric resistance arteries suggest eutrophic remodeling of the vessels. Kinetic evidence suggests that locally activated plasma prorenin may play a critical role in mediating vascular injury. This model will facilitate studies of the cellular and genetic mechanisms underlying vascular injury and repair and provide a basis for the identification of novel therapeutic targets for vascular disease.

Malignant hypertension - the role of the paracrine renin-angiotensin system

Malignant hypertension remains one of the life-threatening complications of blood pressure elevation. It is a clinico-pathological syndrome of severe blood pressure elevation combined with malignant vascular injury. This is a characteristic form of vascular damage, with two elements: fibrinoid necrosis and endarteritis proliferans. Although the morphology of these has been well described, the molecular events are not fully understood. This review summarizes the evidence from transgenic animals for a role for the activation of a local paracrine renin-angiotensin system in the pathogenesis of malignant vascular injury. These animal models provide pathological, pharmacological, and genetic evidence supporting the hypothesis that intra-renal generation of angiotensin 2 and exposure of the microcirculation to elevated blood pressure co-operate in causing tissue damage in malignant hypertension.