Sabra Hypertension-prone Rats Research Articles

Predisposition to cortical neurodegenerative changes in brains of hypertension prone rats

BackgroundSubstantial evidence suggests that hypertension is a significant risk factor for cognitive decline. However, it is unclear whether the genetic predisposition to hypertension is also associated with cellular dysfunction that promotes neurodegeneration.MethodsChanges in blood pressure were evaluated following dietary salt-loading or administration of a regular diet in Sabra Normotensive (SBN/y) and Sabra Hypertension-prone rats (SBH/y). We performed quantitative RT-PCR and immunofluorescence staining in brain cortical tissues before salt loading and 6 and 9 months after salt loading. To examine the expression of brain cortical proteins involved in the gene regulation (Histone Deacetylase-HDAC2; Histone Acetyltransferase 1-HAT1), stress response (Activating Transcription Factor 4-ATF4; Eukaryotic Initiation Factor 2- eIF2α), autophagy (Autophagy related 4A cysteine peptidase- Atg4a; light-chain 3-LC3A/B; mammalian target of rapamycin complex 1- mTORC1) and apoptosis (caspase-3).ResultsPrior to salt loading, SBH/y compared to SBN/y expressed a significantly higher level of cortical HAT1 (protein), Caspase-3 (mRNA/protein), LC3A, and ATF4 (mRNA), lower levels of ATG4A (mRNA/protein), LC3A/B, HDAC2 (protein), as well as a lower density of cortical neurons. Following dietary salt loading, SBH/y but not SBN/y developed high blood pressure. In hypertensive SBH/y, there was significant upregulation of cortical HAT1 (protein), Caspase-3 (protein), and eIF2α ~ P (protein) and downregulation of HDAC2 (protein) and mTORC1 (mRNA), and cortical neuronal loss.ConclusionsThe present findings suggest that genetic predisposition to hypertension is associated in the brain cortex with disruption in autophagy, gene regulation, an abnormal response to cellular stress, and a high level of cortical apoptosis, and could therefore exacerbate cellular dysfunction and thereby promote neurodegeneration.

Primed polymorphonuclear leukocytes, oxidative stress, and inflammation antecede hypertension in the Sabra rat.

Hypertension is accompanied by systemic oxidative stress, inflammation, and priming of peripheral polymorphonuclear leukocytes (PMNLs), yet the involvement of these factors in the pathophysiology of hypertension is incompletely understood. We investigated the relationship between oxidative stress, primed PMNLs, and inflammation and the development of hypertension in the Sabra rat model of salt-sensitive hypertension. Sabra hypertension-resistant rats (SBN/y) (salt-resistant) and Sabra hypertension-prone rats (SBH/y) (salt-sensitive) were studied under normal conditions or during salt loading. Systolic blood pressure (BP) was measured by the tail-cuff method. The extent of oxidative stress was evaluated by the rate of superoxide release from PMNLs, plasma-reduced glutathione (GSH) levels, malondialdehyde (MDA) levels (estimated by thiobarbituric acid-reacting substances), and plasma-carbonylated fibrinogen (Western blotting). Plasma fibrinogen levels and the peripheral PMNL count served as indices of inflammation. In SBH/y and SBN/y provided regular chow without salt loading, BP did not rise above baseline values, yet superoxide release, plasma MDA, carbonylated fibrinogen, and PMNL count were higher in SBH/y than in SBN/y, whereas GSH levels were lower in SBH/y. Four weeks of salt loading resulted in a gradual increase in systolic BP in SBH/y to 205+/-3 mm Hg, whereas BP remained in SBN/y at baseline normotensive levels. All the parameters reflecting oxidative stress and inflammation were further aggravated with the development of hypertension in salt-loaded SBH/y. We conclude that primed PMNLs, oxidative stress, and inflammation antecede the development of hypertension in this experimental model of hypertension.

Role of chromosome X in the Sabra rat model of salt-sensitive hypertension.

We carried out a total genome screen in the Sabra rat model of hypertension to detect salt-susceptibility genes. We previously reported in male animals the presence of 2 major quantitative trait loci (QTLs) on chromosome 1 that together accounted for most of the difference in the blood pressure (BP) response to salt loading between Sabra hypertension-prone rats (SBH/y) and Sabra hypertension-resistant rats (SBN/y). In females, we reported on 2 major QTLs on chromosomes 1 and 17 that together accounted for only two thirds of the difference in the BP response between the strains. On the basis of phenotypic patterns of inheritance in reciprocal F2 crosses, we proposed a role of the X chromosome. We therefore continued the search for the missing QTL in females that would account for the remaining difference in the BP response between the 2 strains using newly developed microsatellite markers and focusing on chromosome X. We screened an F2 cross, consisting of 371 females and 336 males, using 19 polymorphic chromosome X microsatellite markers. We analyzed the averages of BP by genotype using ANOVA and the individual data using MAPMAKER/QTL. In female F2 progeny, we identified a segment on chromosome X that spans over 33.4 cM and shows significant cosegregation (P<0.001) of 14 microsatellite markers (demarcated by DXRat4 and DXMgh10) with systolic BP after salt loading. This segment has 2 apparent peaks at DXRat4 and DXRat13, with a BP effect of 14 mm Hg for each. Multipoint linkage analysis with a free model detected 3 peaks (logarithm of the odds ratio [LOD] score >4.3) within the same chromosomal segment: One between DXMgh9 and DXMit4 (LOD 4.9; 6.1% of variance), a second between DXMgh12 and DXRat8 (LOD 5.2; 7.2% of variance), and a third between DXRat2 and DXRat4 (LOD 5.8; 7.5% of variance). On the basis of these findings and until congenic strains become available, our working assumption is that within chromosome X, 1 to 3 genetic loci contribute importantly to the BP response of female Sabra rats to salt. In male F2 progeny, we detected no significant cosegregation of any region on chromosome X with the BP response to salt loading. We conclude that in the female rat, salt susceptibility is mediated by 3 to 5 gene loci on chromosomes 1, 17, and X, whereas in the male rat, the X chromosome does not affect the BP response to salt.

Nitric oxide and the regulation of blood pressure in the hypertension-prone and hypertension-resistant Sabra rat.

We examined the role of nitric oxide (NO) in the inherited resistance or susceptibility to hypertension in the Sabra hypertension-prone (SBH) and hypertension-resistant (SBN) rat. Basal mean arterial blood pressure was significantly greater in SBH than in SBN rats. Phenylephrine elevated blood pressure to a similar extent in both substrains, whereas the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) had a greater pressor effect in SBN rats. The vasoconstrictor potency of phenylephrine was significantly higher in endothelium-intact aortic rings from the SBH rat, whereas the vasoconstrictor potency of L-NMMA was higher in those from the SBN substrain. Acetylcholine-induced endothelium-dependent relaxation was greater in aortic rings from SBN rats. The vasodilator potency of glyceryl trinitrate was significantly higher in aortic rings from SBH rats and was enhanced after endothelium removal. Both the activity of calcium-dependent NO synthase from aortic endothelial cells and the basal concentration of nitrite/nitrate in plasma were significantly greater in SBN than in SBH rats. In normotensive Wistar rats, basal mean arterial blood pressure, the pressor effect of L-NMMA, endothelial NO synthase activity, and plasma nitrite/ nitrate concentrations were all between the values in SBH and SBN rats. These results indicate that a decrease in NO generation plays a role in the susceptibility of SBH rats to hypertension. Furthermore, the resistance to hypertension in the SBN strain may be related to increased NO generation.

Erythrocyte membrane microviscosity and blood pressure in rats with salt-induced and spontaneous hypertension

To study membrane viscosity in various rat strains with genetic forms of experimental hypertension. The relationship between blood pressure and membrane dynamics was investigated in erythrocytes from three different rat strains with experimental hypertension, namely two models of salt-induced hypertension (Sabra and Dahl rats) and Lyon hypertensive rats with spontaneous hypertension. Membrane microviscosity was evaluated by diphenylhexatriene and trimethylamino-diphenylhexatriene fluorescence steady-state anisotropy. There were no significant differences among particular experimental groups in trimethylamino-diphenylhexatriene anisotropy that reflect microviscosity changes at the water-lipid interface of the external membrane leaflet. In contrast, the diphenylhexatriene anisotropy, which is related to the core membrane microviscosity, was significantly reduced in the Dahl salt-sensitive rats (irrespective of salt intake level) and in the Sabra hypertension-prone rats with developed salt hypertension. Erythrocyte membranes of Lyon hypertensive rats also had lower values of diphenylhexatriene anisotropy than the respective normotensive controls but this difference was not statistically significant. Systolic (and often also diastolic) blood pressure correlated negatively with the diphenylhexatriene anisotropy in each of the three strains studied, whereas the trimethylamino-diphenylhexatriene anisotropy of the erythrocyte membranes had no significant relationship to the blood pressure. Further experiments should clarify whether the observed relationship of the diphenylhexatriene anisotropy to blood pressure reflects true pathogenetic mechanisms or is a consequence of haemodynamic changes.

Nifedipine Corrects the Blunted Renal Response to Saline Loading in Hypertension-Prone SBH Rats

The elimination of an acute oral saline load is markedly blunted in adult Sabra hypertension-prone (SBH) rats compared with hypertension-resistant Sabra normotensive (SBN) rats. Within 2 h, urinary output and the excretion of sodium and potassium are significantly reduced, while urine osmolality is markedly elevated in SBH rats. The long-term administration of nifedipine, 20-30 mg/kg body weight enhanced the diuretic and natriuretic response to saline loading in members of both strains. The effect was significantly more pronounced in SBH, especially in adult animals where the diuretic and natriuretic response averaged 150 and 130% of control, while in SBN the enhanced response was 50 and 20%, respectively. As a result of the disparate effect of nifedipine in the two strains, the blunted response of SBH was abolished. The mechanism of the preferential response to nifedipine of SBH rats remains to be determined.

Sympathetic Component of Baroreflex Control of Heart Rate is Impaired in Hypertension-prone (SBH) Sabra Rats

Baroreflex control of heart rate in response to phenylephrine was studied in conscious Sabra hypertension-prone (SBH) rats, at a prehypertensive stage, and hypertension-resistant (SBN) rats. Baroreflex sensitivity as determined from the slope of the relationship of mean arterial blood pressure and heart period was significantly lower in SBH rats (0.58 +/- 0.06 versus 1.71 +/- 0.11 ms/mmHg in SBN rats, P less than 0.01) before the development of hypertension. Sympathetic nerve blockade with guanethidine (15 mg/kg) significantly reduced the slope of the mean arterial blood pressure-heart period relationship in SBN rats to 0.45 +/- 0.05 ms/mmHg (P less than 0.01) and increased the pressor response to phenylephrine, without having any effect on these parameters in SBH rats. Atropine methyl nitrate (1 mg/kg) abolished reflex vagal bradycardia in response to phenylephrine in both groups of rats. This suggests that SBH rats are unable to withdraw the sympathetic cardiac component of the baroreflex in response to a pressor stimulus and appear to rely only on increased vagal activity to effect bradycardia.

Stress-induced secretion of adrenocorticotropin, corticosterone, and prolactin in experimentally and genetically hypertensive rats.

Plasma levels of immunoreactive ACTH, corticosterone (CS), and PRL in two-kidney, one clip (2K1C) hypertensive SABRA, hypertension-prone (SBH), hypertension-resistant (SBN), and normotensive SABRA rats were compared under both quiescent conditions and after acute (2 min) cold water stress. Serum levels of CS were higher in 2K1C hypertensive compared with normotensive SABRA rats under both quiescent and stressful conditions. Circulating levels of ACTH and PRL were similar in both groups under quiescent conditions. Resting circulating levels of CS were higher in the SBH rats compared with SABRA or SBN rats. Serum PRL levels were similar in SBH and SABRA rats under both quiescent and stressful conditions. Resting PRL levels in the SBN rats were lower compared with the SABRA rats. Resting serum levels of ACTH and CS in the SBN rats were similar to those found in the SABRA rats. After stress exposure serum ACTH and CS levels were elevated in all groups. Serum PRL levels in SBN rats were not affected by stress, unlike the marked elevation seen in the other groups. Our study demonstrates increased secretion of CS in both 2K1C hypertensive and SBH rats under quiescent conditions. Both 2K1C hypertensive and SBH rats have normal hormonal capacity to respond to stress. SBN rats exhibited reduced PRL secretion under both quiescent and stressful conditions. It is suggested that abnormal activity of the hypothalamic-pituitary-adrenal system may play a role in the pathogenesis of 2K1C and genetic hypertension as well as in resistance to hypertension.