Genetically defined risk of salt sensitivity in an intercross of Brown Norway and Dahl S rats

Identification of hypertension-related QTLs in African American sib pairs

To link hypertension-related phenotypes with chromosomal loci, genome scans were performed in 150 African American sib pairs concordant for essential hypertension. Phenotypes included blood pressure, anthropomorphic measurements, and estimates of body fluid compartments as determined by impedance plethysmography. These phenotypes were also measured in 335 normotensive African Americans. Phenotypes with LOD scores >3.3 were further evaluated for significance by use of permutation procedures. Significant linkage was detected for body mass index (BMI) on chromosomes 1 and 8 and for the ratio of extracellular water to total body water (ECF/TBW) on chromosomes 3, 5, 6, and 7. Both BMI and ECF/TBW were greater in hypertensive sibs than in normotensive subjects (P<0.001). In a subset of hypertensive sibs and normotensive subjects, average 24-hour blood pressures were correlated with ECF/TBW (P<0.01). A region linked to BMI in the hypertensive sibs corresponds to a region of conserved synteny containing blood pressure-related QTLs in an F2 cross of Brown NorwayxDahl salt-sensitive rats. Focusing on hypertension-related phenotypes is a promising approach for identifying the genetic determinants of hypertension.

Dissecting the genetic architecture of the cardiovascular and renal stress response

We review the evidence for a genetic basis of the cardiovascular and renal stress response. A bio-behavioral model of stress-induced hypertension is presented that explains how repeated exposure to stress in combination with genetic susceptibility might lead to the development of hypertension. In this model, we focus on three underlying physiological systems that mediate the stress response of the heart, vasculature and kidney: the sympathetic nervous system (SNS), the renin-angiotensin-aldosterone system (RAAS) and the endothelial system (ES). We then review the evidence for a genetic influence on cardiovascular reactivity to psychological stress and stress-induced sodium retention using data from twin and family studies and a limited number of candidate gene studies. Finally, by describing the underlying physiological systems of our model and their genetic underpinning we emphasize the importance of inclusion of genetic measurements in any future studies testing the reactivity hypothesis.

Ventilatory phenotypes among four strains of adult rats

Our purpose in this study was to identify different ventilatory phenotypes among four different strains of rats. We examined 114 rats from three in-house, inbred strains and one outbred strain: Brown Norway (BN; n = 26), Dahl salt-sensitive (n = 24), Fawn-hooded Hypertensive (FHH: n = 27), and outbred Sprague-Dawley rats (SD; n = 37). We measured eupneic (room air) breathing and the ventilatory responses to hypoxia (12% O(2)-88% N(2)), hypercapnia (7% CO(2)), and two levels of submaximal exercise. Primary strain differences were between BN and the other strains. BN rats had a relatively attenuated ventilatory response to CO(2) (P < 0.001), an accentuated ventilatory response to exercise (P < 0.05), and an accentuated ventilatory roll-off during hypoxia (P < 0.05). Ventilation during hypoxia was lower than other strains, but hyperventilation during hypoxia was equal to the other strains (P > 0.05), indicating that the metabolic rate during hypoxia decreased more in BN rats than in other strains. Another strain difference was in the frequency and timing components of augmented breaths, where FHH rats frequently differed from the other strains, and the BN rats had the longest expiratory time of the augmented breaths (probably secondary to the blunted CO(2) sensitivity). These strain differences not only provide insight into physiological mechanisms but also indicate traits (such as CO(2) sensitivity) that are genetically regulated. Finally, the data establish a foundation for physiological genomic studies aimed at elucidating the genetics of these ventilatory control mechanisms.

CYP4A mRNA, protein, and product in rat lungs: novel localization in vascular endothelium

The vasodilatory effect of 20-hydroxyeicosatetraenoic acid (20-HETE) on lung arteries is opposite to the constrictor effect seen in cerebral and renal vessels. These observations raise questions about the cellular localization of 20-HETE-forming isoforms in pulmonary arteries and other tissues. Using in situ hybridization, we demonstrate for the first time CYP4A (a family of cytochrome P-450 enzymes catalyzing formation of 20-HETE from the substrate arachidonic acid) mRNA in pulmonary arterial endothelial and smooth muscle cells, bronchial smooth muscle and bronchial epithelial cells, type I epithelial cells, and macrophages in adult male rat lungs. Moreover, we detect CYP4A protein in rat pulmonary arteries and bronchi as well as cultured endothelial cells. Finally, we identify endogenously formed 20-HETE by using fluorescent HPLC techniques, as well as the capacity to convert arachidonic acid into 20-HETE in pulmonary arteries, bronchi, and endothelium. These data show that 20-HETE is an endogenous product of several pulmonary cell types and is localized to tissues that optimally position it to modulate physiological functions such as smooth muscle tone or electrolyte flux.

Renal medullary nitric oxide deficit of Dahl S rats enhances hypertensive actions of angiotensin II

Studies were designed to examine the hypothesis that the renal medulla of Dahl salt-sensitive (Dahl S) rats has a reduced capacity to generate nitric oxide (NO), which diminishes the ability to buffer against the chronic hypertensive effects of small elevations of circulating ANG II. NO synthase (NOS) activity in the outer medulla of Dahl S rats (arginine-citrulline conversion assay) was significantly reduced. This decrease in NOS activity was associated with the downregulation of protein expression of NOS I, NOS II, and NOS III isoforms in this region as determined by Western blot analysis. In anesthetized Dahl S rats, we observed that a low subpressor intravenous infusion of ANG II (5 ng. kg(-1). min(-1)) did not increase the concentration of NO in the renal medulla as measured by a microdialysis with oxyhemoglobin trapping technique. In contrast, ANG II produced a 38% increase in the concentration of NO (87 +/- 8 to 117 +/- 8 nmol/l) in the outer medulla of Brown-Norway (BN) rats. The same intravenous dose of ANG II reduced renal medullary blood flow as determined by laser-Doppler flowmetry in Dahl S, but not in BN rats. A 7-day intravenous ANG II infusion at a dose of 3 ng. kg(-1). min(-1) did not change mean arterial pressure (MAP) in the BN rats but increased MAP in Dahl S rats from 120 +/- 2 to 138 +/- 2 mmHg (P < 0.05). ANG II failed to increase MAP after NO substrate was provided by infusion of L-arginine (300 microg. kg(-1). min(-1)) into the renal medulla of Dahl S rats. Intravenous infusion of L-arginine at the same dose had no effect on the ANG II-induced hypertension. These results indicate that an impaired NO counterregulatory system in the outer medulla of Dahl S rats makes them more susceptible to the hypertensive actions of small elevations of ANG II.

Mapping baroreceptor function to genome: a mathematical modeling approach

To gain information about the genetic basis of a complex disease such as hypertension, blood pressure averages are often obtained and used as phenotypes in genetic mapping studies. In contrast, direct measurements of physiological regulatory mechanisms are not often obtained, due in large part to the time and expense required. As a result, little information about the genetic basis of physiological controlling mechanisms is available. Such information is important for disease diagnosis and treatment. In this article, we use a mathematical model of blood pressure to derive phenotypes related to the baroreceptor reflex, a short-term controller of blood pressure. The phenotypes are then used in a quantitative trait loci (QTL) mapping study to identify a potential genetic basis of this controller.

Renal medullary genes in salt-sensitive hypertension: a chromosomal substitution and cDNA microarray study

Substitution of chromosome 13 from Brown Norway BN/SsNHsd/Mcw (BN/Mcw) rats into the Dahl salt-sensitive SS/JrHsd/Mcw (SS/Mcw) rats resulted in substantial reduction of blood pressure salt sensitivity in this consomic rat strain designated SSBN13. In the present study, we attempted to identify genes associated with salt-sensitive hypertension by utilizing a custom, known-gene cDNA microarray to compare the mRNA expression profiles in the renal medulla (a tissue playing a pivotal role in long-term blood pressure regulation) of SS/Mcw and SSBN13 rats on either low-salt (0.4% NaCl) or high-salt (4% NaCl, 2 wk) diets. To increase the reliability of microarray data, we designed a four-way comparison experiment incorporating several levels of replication and developed a conservative yet robust data analysis method. Using this approach, from the 1,751 genes examined (representing more than 80% of all currently known rat genes), we identified 80 as being differentially expressed in at least 1 of the 4 comparisons. Substantial agreements were found between the microarray results and the results predicted on the basis of the four-way comparison as well as the results of Northern blots of 20 randomly selected genes. Analysis of the four-way comparison further indicated that approximately 75% of the 80 differentially expressed genes were likely related to salt-sensitive hypertension. Many of these genes had not previously been recognized to be important in hypertension, whereas several genes/pathways known to be involved in hypertension were confirmed. These results should provide an informative source for designing future functional studies in salt-sensitive hypertension.

A genomic-systems biology map for cardiovascular function

With the draft sequence of the human genome available, there is a need to better define gene function in the context of systems biology. We studied 239 cardiovascular and renal phenotypes in 113 male rats derived from an F2 intercross and mapped 81 of these traits onto the genome. Aggregates of traits were identified on chromosomes 1, 2, 7, and 18. Systems biology was assessed by examining patterns of correlations ("physiological profiles") that can be used for gene hunting, mechanism-based physiological studies, and, with comparative genomics, translating these data to the human genome.

Microfocal X-ray CT imaging and pulmonary arterial distensibility in excised rat lungs

The objective of this study was to develop an X-ray computed tomographic method for measuring pulmonary arterial dimensions and locations within the intact rat lung. Lungs were removed from rats and their pulmonary arterial trees were filled with perfluorooctyl bromide to enhance X-ray absorbance. The lungs were rotated within the cone of the X-ray beam projected from a microfocal X-ray source onto an image intensifier, and 360 images were obtained at 1 degrees increments. The three-dimensional image volumes were reconstructed with isotropic resolution using a cone beam reconstruction algorithm. The vessel diameters were obtained by fitting a functional form to the image of the vessel circular cross section. The functional form was chosen to take into account the point spread function of the image acquisition and reconstruction system. The diameter measurements obtained over a range of vascular pressures were used to characterize the distensibility of the rat pulmonary arteries. The distensibility coefficient alpha [defined by D(P) = D(0)(1 + alphaP), where D(P) is the diameter at intravascular pressure (P)] was approximately 2.8% mmHg and independent of vessel diameter in the diameter range (about 100 to 2,000 mm) studied.

Evidence that reduced renal medullary nitric oxide synthase activity of dahl s rats enables small elevations of arginine vasopressin to produce sustained hypertension

On the basis of observations supporting the functional importance of nitric oxide (NO) in the regulation of renal medullary function, and a reduced nitric oxide synthase (NOS) enzyme activity in the outer medulla of the Dahl salt-sensitive (SS/Mcw) rats, we hypothesized that these inbred rats would have reduced capacity to synthesize renal medullary NO. This reduced capacity would sensitize them to the hypertensive effects of small elevations of circulating arginine vasopressin (AVP). SS/Mcw and Brown Norway (BN/Mcw) rats with implanted arterial and venous catheters were fed a 0.4% salt diet and infused intravenously for 14 days with a subpressor dose of AVP (2 ng/kg per min). Mean arterial pressure (MAP) was measured 2 hours daily in unanesthetized rats maintained in their home cages. MAP in SS/Mcw rats increased during day 1 of AVP infusion from a control level of 127+/-0.9 mm Hg to an average of 147+/-1.6 mm Hg after 14 days. MAP did not return to control values during the 3 days after the end of AVP infusion. BN/Mcw rats showed no changes of MAP during 14 days of AVP infusion (90.4+/-0.6 mm Hg and 92.3+/-0.4 mm Hg). Northern blot analysis of renal tissue from vehicle (saline) -infused rats demonstrated that NOS I and NOS III mRNA expression was significantly less in SS/Mcw rats in the renal outer medulla compared with BN/Mcw rats. We conclude that small, normally subpressor elevations of plasma AVP can produce chronic hypertension in SS/Mcw rats and that this phenomenon is related to the reduced medullary NOS enzyme activity, which in turn reduces the AVP-stimulated NO synthesis.

Brown Norway chromosome 13 confers protection from high salt to consomic Dahl S rat

Consomic rats (SS.BN13), in which chromosome 13 from normotensive inbred Brown Norway rats from a colony maintained at the Medical College of Wisconsin (BN/Mcw) was introgressed into the background of Dahl salt-sensitive (SS/Mcw) rats, also maintained in a colony at the Medical College of Wisconsin, were bred. The present studies determined the mean arterial pressure (MAP) responses to salt and renal and peripheral vascular responses to norepinephrine and angiotensin II; 24-hour protein excretion and histological analyses were used to assess renal pathology in rats that received a high salt (4% NaCl) diet for 4 weeks. MAP of rats measured daily during the fourth week averaged 170+/-3.3 mm Hg in SS/Mcw rats, 119+/-2.1 mm Hg in SS.BN13 rats, and 103+/-1.3 mm Hg in BN/Mcw rats. After salt depletion, MAP fell an average of 27+/-4.5 mm Hg in SS/Mcw rats, 9+/-2.6 mm Hg in SS.BN13 rats, and 11+/-3.0 mm Hg in BN/Mcw rats. Protein excretion of SS/Mcw rats on a high salt diet averaged 189+/-30 mg/24 h, 63+/-18 mg/24 h in SS.BN13 rats, and 40+/-6.4 mg/24 h in BN/Mcw rats. Compared with SS.BN13 and BN/Mcw rats, SS/Mcw rats exhibited significantly greater increases of renal vascular resistance in response to intravenous norepinephrine and angiotensin II. Severe medullary interstitial fibrosis and tubular necrosis after a high salt diet were found consistently in SS/Mcw rat kidneys but were largely absent in the SS.BN13 and BN/Mcw rat kidneys. A similar degree of glomerular sclerosis was found in both SS/Mcw and SS.BN13 rats. In rats fed a 0.4% salt diet, the glomerular filtration rate of SS/Mcw rats was significantly less than that of BN/Mcw and SS.BN13 rats. These results reveal a powerful gene, or set of genes, within chromosome 13 of BN/Mcw rats that confers protection from the detrimental effects of high salt to the SS/Mcw rats.