Female Spontaneously Hypertensive Rats Research Articles

Role of oxidative stress in the sex differences in blood pressure in spontaneously hypertensive rats

The hypothesis was tested that differences in oxidative stress play a role in the sex differences in the development and maintenance of hypertension in spontaneously hypertensive rats (SHR). Male and female SHR [and Wistar-Kyoto (WKY) rats in the long-term study] (n = 6-12 per group) received tempol (30 mg/kg per day) or tap water for 6 weeks from 9 to 15 weeks of age or from birth until 15 weeks of age. Blood pressure [mean arterial pressure (MAP)] and kidney tissue F2-isoprostane (IsoP) were measured at 15 weeks of age. In SHR given tempol for 6 weeks, blood pressure and IsoP were reduced in males, but not in females. In SHR given tempol from birth, MAP was higher in SHR than WKY rats (SHR males, 181 +/- 2 mmHg; SHR females, 172 +/- 3 mmHg; WKY males, 100 +/- 2 mmHg; WKY females, 101 +/- 2 mmHg, P < 0.01), and tempol reduced MAP by 14% (156 +/- 3) and 26% (127 +/- 4) in male and female SHR, respectively, but had no effect on WKY rats. IsoP was higher in SHR than WKY rats and higher in male SHR than female SHR (SHR males, 5.18 +/- 0.23 ng/mg; SHR females, 3.71 +/- 0.19 ng/mg, P < 0.01; WKY males, 1.72 +/- 0.45 ng/mg; WKY females, 2.21 +/- 0.08 ng/mg, P < 0.05, compared with SHR). Tempol reduced IsoP in SHR to levels found in WKY rats, but had no effect on IsoP in WKY rats. Development of hypertension in SHR is mediated in part by oxidative stress independent of sex. Also, tempol is effective in reducing blood pressure in females only when given prior to the onset of hypertension.

Sexual Dimorphism in Renal Production of Prostanoids in Spontaneously Hypertensive Rats

Male spontaneously hypertensive rats (SHR) have higher blood pressure, blunted pressure-natriuresis relationship, and accelerated progression of renal injury compared with female SHR. Renal medullary prostanoids mediate vascular tone, salt and water balance, and renin release and, as a result, are involved in the maintenance of renal blood flow and the pathogenesis of hypertension. The aim of this study was to determine whether a gender difference exists in prostanoid production in SHR and whether sex steroids influence prostaglandin (PG) production. Thirteen-week-old intact and gonadectomized male and female SHR rats were placed in metabolic cages for 24-hour urine collection. Prostanoid excretion was determined using enzyme immunoassay. Kidneys were isolated and separated into outer and inner medulla for Western blot analysis. Female SHR had enhanced urinary excretion of PG E2 (PGE2) metabolites and thromboxane B2, an indicator of renal thromboxane production, compared with male SHR. There were no gender differences in excretion of systemic thromboxane or prostacyclin. Correspondingly, female SHR had enhanced microsomal PGE2 synthase protein expression in the renal inner medulla and greater cyclooxygenase-2 (COX-2) expression in the outer medulla. Orchidectomy was associated with increased PGE2 metabolite excretion and microsomal PGE synthase protein expression. Thromboxane B2 excretion was not affected by gonadectomy in either male or female SHR. Protein expressions of COX and cytoplasmic PGE2 synthase in the renal medulla were unchanged by gonadectomy in both sexes. These results demonstrate a sexual dimorphism in renal production of prostanoids in SHR and that PGE production is testosterone sensitive and estrogen insensitive.

Gender differences in blood pressure and heart rate in spontaneously hypertensive and Wistar‐Kyoto rats

1. In general, premenopausal women are known to have lower blood pressure than men and animal models have shown a similar sexual dimorphism. However, many studies in animals have been performed using anaesthetized or restrained models. Current experiments were conducted to investigate the relationships among resting heart rate, blood pressure and gender in conscious, unrestrained normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). 2. Biotelemetry transmitters were implanted in 6-month-old animals. Values for heart rate, diastolic blood pressure, systolic blood pressure and pulse pressure were recorded continuously at 10 min intervals after all animals recovered completely from surgery. 3. Normal circadian rhythms in heart rate were found in all rats, with no significant differences among the four groups; the circadian variation in blood pressure was evident in all groups, although much smaller. Heart rate was found to be higher in WKY female rats than in the other three groups. Male WKY rats, male SHR and female SHR had similar heart rates. Male SHR had significantly higher systolic and diastolic blood pressures than female SHR. Male and female WKY rats had similar diastolic blood pressure, but males had slightly higher systolic pressure than females. No significant difference in pulse pressure was found in WKY male and female rats. Male SHR showed significantly higher pulse pressure than female SHR at most times during the day. 4. In conclusion, these results indicate that hypertension is exacerbated in male SHR compared with females under conscious resting conditions and demonstrate that the higher heart rate observed in WKY female rats is not present in the SHR model.

Gender difference in response to thromboxane A2/prostaglandin H2 receptor antagonism in spontaneously hypertensive rats

Blood pressure (BP) is typically higher in men than in women. As in humans, BP is higher in male spontaneously hypertensive rats (SHRs) than in female SHRs. The mechanism(s) responsible for the higher BP in men and male rats has not been elucidated. The present study tested the hypothesis that thromboxane and/or the thromboxane A(2)/prostaglandin H(2) receptor (TxR) plays a role in the gender difference in BP in SHRs. Male and female SHRs were treated with the TxR antagonist SQ29548 (2 microg/kg/min into the jugular vein) via minipump for 2 weeks; mean arterial pressure (MAP) and renal hemodynamic function were measured. To determine whether thromboxane played a role in the higher MAP in SHRs, male and female rats were treated with the thromboxane synthase inhibitor (TxI) furegrelate, 30 mug/kg/min, via minipump, for 2 weeks. MAP was reduced with TxR antagonism in male but not in female SHRs. In addition, TxR antagonism in males caused renal vasodilation with increases in glomerular filtration rate and renal plasma flow, and reductions in renal vascular resistance. In contrast, MAP was not affected by the TxI in either males or females. These data support a role for TxR, but not thromboxane, in mediating the gender difference in BP in SHRs.

Gender difference in functional properties of Na,K-ATPase in the heart of spontaneously hypertensive rats

The aim of present study was the investigation of functional properties of the cardiac Na,K-ATPase in 16 weeks old male and female spontaneously hypertensive rats (SHR). The Na,K-ATPase activity in the presence of increasing concentrations of ATP, as well as Na + was lower in SHR of both genders, as compared to respective normotensive controls. Evaluation of kinetic parameters revealed a significant decrease of the maximum velocity (V max) in males (30% for ATP-activation, 40% for Na +-activation), as well as in females (24% for ATP, 29% for Na +), indicating a hypertension-induced diminution of the number of active enzyme molecules in cardiac sarcolemma. Insignificant changes were observed in the value of Michaelis-Menten constant (K m) in both cases. The concentration of sodium that gives half-maximal reaction velocity (K Na), increased by 38% in male and by 70% in female SHR. This impairment in the affinity of the Na +-binding site together with decreased number of active Na,K-ATPase molecules are probably responsible for the deteriorated enzyme-function in hearts of SHR. Direct comparison of SHR of both genders showed, that the enzyme from female hearts seems to be adapted better to hypertension as documented by its increased activity as a consequence of improved ability to bind and utilize ATP, as suggested by 32% decrease of K m value in females. In addition, the enzyme from female hearts is able to increase its activity (by 41%) in the presence of increasing sodium concentration even in the range where the enzyme from male hearts is already saturated.

Strain and sex differences in the expression of nociceptive behavior and stress-induced analgesia in rats

Evidence indicates that genetic, gender, and emotional/attentional aspects modulate the pain sensation. The present study examined the effect of swim-stress on nociceptive responses in Lewis (LEW) and spontaneously hypertensive (SHR) inbred rats (contrasting for anxiety-related behaviors), as well as in Wistar (WIS) rats of both sexes. Furthermore, we explored possible neurochemical mechanisms involved. In addition, we investigated whether habituation in the hot-plate apparatus could modify the hypoalgesic phenotype of SHR. Male and female LEW, SHR, and WIS rats were tested immediately before and 2 min after a 3-min swim in 15 °C water. The swim-stress induced analgesia in LEW and WIS, but not in SHR male rats. The same stressor induced analgesia in females of all three strains. In WIS female rats, the stress-induced analgesia (SIA) seems to involve, at least partially, a nonopioid N-methyl- d-aspartate (NMDA) analgesic system. Moreover, five brief exposures (90 s; 10-min intertrial interval) to the unheated hot-plate apparatus completely abolished the differences in basal hot-plate latencies observed in SHR compared with LEW and WIS strains. The present results demonstrate genetic and gender differences in nociceptive sensitivity and in the activation of endogenous analgesic systems in rats and highlight the influence of emotional reactivity. The SHR's hypoalgesic phenotype seems to involve central cognitive processes. Therefore, the LEW and SHR inbred strains may provide an important tool for study of the molecular bases underlying nociception and its modulation and the relationship with emotional/attentional processes.

Role of endothelin in mediating postmenopausal hypertension in a rat model.

Cardiovascular disease is the leading cause of death in women after menopause. Hypertension, a major cardiovascular risk factor, becomes more prevalent after menopause. The mechanisms responsible for the increase in blood pressure (BP) in postmenopausal women are unknown. We have recently characterized the aged, postestrous-cycling (PMR) spontaneously hypertensive rats (SHR) as a model of postmenopausal hypertension. The purpose of the present study was to determine whether endothelin plays a role in the increased BP in PMR. Premenopausal female SHR, aged 4-5 mo (YF), and PMR, aged 16 mo, were studied. Expression of preproendothelin-1 mRNA was not different in either renal cortex or medulla between PMR and YF (n = 7-8/group). In contrast, ET-1 peptide expression was significantly higher in renal cortex of PMR than in renal cortex of YF, but there was no difference in medullary ET-1. Expression of endothelin ET(A) receptor (ET(A)R) mRNA was lower in renal cortex and medulla of PMR than of YF. Additional groups of rats (n = 6-7/group) were treated for 3 wk with the ET(A)R antagonist ABT-627 (5 mg.kg(-1).day(-1)). BP was significantly higher in PMR than in YF. ET(A)R antagonist reduced BP in PMR by 20% to the level found in control YF. ET(A)R antagonist had no effect on BP in YF. These data support the hypothesis that the increase in BP in PMR is mediated in part by endothelin and the ET(A)R.

Novel mechanisms responsible for postmenopausal hypertension.

Blood pressure increases in many women after menopause. Hypertension is one of the major risk factors for cardiovascular disease. However, the mechanisms responsible for the postmenopausal increase in blood pressure are yet to be elucidated. Various humoral systems have been proposed to play a role in postmenopausal hypertension, such as changes in estrogen/androgen ratios, increases in endothelin and oxidative stress, and activation of the renin-angiotensin system (RAS). In addition, obesity, type II diabetes, and activation of the sympathetic nervous system are common in postmenopausal women and may also play important roles. However, progress in elucidating the mechanisms responsible for postmenopausal hypertension has been hampered by the lack of a suitable animal model. The aging female spontaneously hypertensive rat (SHR) exhibits many of the characteristics found in postmenopausal women. In this review, some of the possible mechanisms that could play a role in postmenopausal hypertension are discussed, as well as the characteristics of the aged female SHR as a model to study.

Temporary losartan or captopril in young SHR induces malignant hypertension despite initial normotension

Exposure of normotensive rats to angiotensin-converting enzyme (ACE) inhibitors in early life causes hypertrophy of intrarenal arteries. Similar defects have been found in knockout mice lacking angiotensinogen, ACE, or angiotensin II type 1 (AT1) receptors. On the other hand, transient inhibition of the renin-angiotensin system from 2 weeks of age in spontaneously hypertensive rats (SHR), either with ACE inhibitors or with AT1 receptor antagonists partially prevents the increase in blood pressure. However, permanent treatment of SHR from conception onwards with ACE inhibitors completely prevents hypertension. Although these studies demonstrated protection from hypertension-induced changes in the heart and large arteries, renal arteries were not studied and follow-up did not extend beyond 6 months of age. We postulated that while brief exposure to ACE inhibitors or AT1 receptor antagonists in young SHR would temporarily decrease blood pressure, it would also be associated with development of intrarenal arterial malformation, and ultimately have deleterious effects. Direct effects on intrarenal arterial morphology of an ACE inhibitor (captopril, 100 mg/kg/day) and an AT1 receptor antagonist (losartan, 50 mg/kg/day), administered from the last week of gestation until 8 weeks of age were examined in SHR. After stopping treatment at 8 weeks, we continued to monitor blood pressure until spontaneous death. Systolic blood pressure at 8 weeks was normalized by captopril and losartan (SHR control 187 +/- 8 mm Hg; captopril 118 +/- 5 mm Hg; and losartan 120 +/- 9 mm Hg). However, by 30 weeks, blood pressure had increased to control SHR levels. At 4 weeks, the media of renal arteries and arterioles was hypertrophied. Marked smooth muscle cell hyperplasia of cortical arteries resulted in significantly increased wall thickness by 8 weeks, despite similar external diameter. Arterial wall structure was disrupted, with fragmentation of elastic fibers and irregular distribution of collagen type I fibers. After stopping treatment, the rats gradually began to show poor health and all had died by 1 year of age, while all 1-year-old control SHR females were in good health. The cause of morbidity and mortality in the rats treated in early life was clearly malignant hypertension. Severe hypertrophy of renal arterioles was found, as well as cerebral hemorrhage. Despite initial normalization of blood pressure interference with the renin-angiotensin system during a crucial stage of development in SHR can initiate marked smooth muscle cell hyperplasia and disruption of the wall structure of the intrarenal arteries. Subsequent progression of this intrarenal process after cessation of treatment suggests an independent process that eventually results in malignant hypertension and early death.

Spatial learning/memory and social and nonsocial behaviors in the Spontaneously Hypertensive, Wistar–Kyoto and Sprague–Dawley rat strains

The Spontaneously Hypertensive rat (SHR) is often described as less behaviorally reactive than its normotensive strain, the Wistar–Kyoto (WKY), although results are somewhat inconsistent across studies. In part, this may be due to the lack of a definitive characterization of “reactivity.” Still, results from identical behavioral tests of SHR and WKY across studies are sometimes conflicting. Further, few comparisons with other rodent strains are available and these might provide guidance in outlining the meaning of reactivity. Here, social and nonsocial behaviors and spatial learning and memory were measured in male and female SHR, WKY, and Sprague–Dawley (SD) rats. Systolic blood pressure measurements at adulthood confirmed hypertension in the SHR. Juvenile play behavior indicated that SHRs were more sensitive to the strain of their play partner than were the WKY or SD, playing less with different strain partners than with same strain partners. However, adult dominance behavior (restricted access in a water competition test) indicated no strain differences. The SHR appeared to exhibit attenuated acoustic startle relative to the WKY and SD and their prepulse inhibition was substantially less at higher prepulse decibel intensities; however, this decreased prepulse inhibition was not the result of decreased startle during the test. Anxiety-related behavior in the elevated plus maze was most prominent in the SD strain, possibly as a result of poorer motor coordination as measured by rotarod performance. Elevated plus maze behavior as well as motor coordination did not differ between the SHR and WKY strains. Performance in the NCTR complex maze and the Morris water maze was significantly better in the SHR. These results do not support hypotheses of decreased behavioral reactivity in the SHR strain. Rather, they suggest complex interactions between social and nonsocial environments and the behavioral capabilities and requirements of the rat strain.

Age-Related Reduction in Estrogen Receptor–Mediated Mechanisms of Vascular Relaxation in Female Spontaneously Hypertensive Rats

Hypertension increases with aging, and changes in vascular estrogen receptors (ERs) may play a role in age-related hypertension in women. We tested whether age-related increases in blood pressure in female spontaneously hypertensive rats (SHRs) are associated with reduction in amount and/or vascular relaxation effects of estrogen and ER. Arterial pressure and plasma estradiol were measured in adult (12 weeks) and aging (16 months) female SHRs, and thoracic aorta was isolated for measurement of active stress, 45Ca2+ influx, and ERs. Arterial pressure was greater and plasma estradiol was less in aging females than in adult females. In aorta of adult females, Western blots revealed alpha- and beta-ERs that were slightly reduced in aging rats. In endothelium-intact vascular strips, phenylephrine (Phe; 10(-5) mol/L) caused greater active stress in aging rats (9.3+/-0.2) than in adult rats (6.2+/-0.3x10(4) N/m2). 17beta-estradiol (E2) caused relaxation of Phe contraction and stimulation of vascular nitrite/nitrate production, which was reduced in aging rats. In endothelium-denuded strips, E2 still caused relaxation of Phe contraction, which was smaller in aging rats than adult rats. KCl (51 mmol/L), which stimulates Ca2+ influx, produced greater active stress in aging rats (9.1+/-0.3) than in adult rats (5.9+/-0.2x10(4) N/m2). E2 caused relaxation of KCl contraction and inhibition of Phe- and KCl-induced 45Ca2+ influx, which were reduced in aging rats. Thus, aging in female SHR is associated with reduction in ER-mediated NO production from endothelial cells and decrease in inhibitory effects of estrogen on Ca2+ entry mechanisms of smooth muscle contraction. The age-related decrease in ER-mediated vascular relaxation may explain the increased vascular contraction and arterial pressure associated with aging in females.

Improvement of endothelial dysfunction by selective estrogen receptor-alpha stimulation in ovariectomized SHR.

Both known estrogen receptors, ERalpha and ERbeta, are expressed in blood vessels. To gain further insight into the role of ERalpha in a functional setting, we investigated the effect of the novel highly selective ERalpha agonist Cpd1471 on vascular reactivity in ovariectomized spontaneously hypertensive rats (SHR). After ovariectomy or sham operation, 12-week-old female SHR received either 17beta-estradiol (E2, 2 microg/kg body wt per day), the selective ERalpha agonist Cpd1471 (30 microg/kg body wt per day), or placebo. Acetylcholine-induced endothelium-dependent vasorelaxation was significantly blunted in aortas from ovariectomized rats (Rmax, 53%+/-3% versus sham, 79%+/-2%; P<0.001). Treatment with E2 or Cpd1471 significantly augmented acetylcholine-induced relaxation in ovariectomized rats (Rmax, 70%+/-2%; resp, 73%+/-2%). Endothelium-independent relaxation induced by sodium nitroprusside was not different among the four groups. The contractile response induced by the nitric oxide (NO) synthase inhibitor Nomega-nitro-l-arginine, an index of basal NO formation, was significantly lower in ovariectomized rats compared with sham-operated animals (53+/-2% versus 77%+/-5%; P<0.01) and was normalized by both E2 (70%+/-2%) and Cpd1471 (70%+/-3%). Aortic endothelial NO synthase (eNOS) expression and phosphorylation of the vasodilator-stimulated phosphoprotein, an index of NO/cGMP-signaling, was reduced in ovariectomized SHR and normalized by E2 and Cpd1471. In SHR after ovariectomy, endothelium-dependent NO-mediated vasorelaxation and eNOS expression are attenuated. The novel selective ERalpha agonist Cpd1471 prevented these pathophysiological changes to a similar extent as E2. Thus, the pharmacological principle of selective ERalpha activation mediates positive vascular effects.

Oestrogen modulates vascular adrenergic reactivity of the spontaneously hypertensive rat.

Male spontaneously hypertensive rats (SHRs) show an increased vascular neurogenic response compared with normotensive Wistar-Kyoto (WKY) control rats. To study the vascular adrenergic response in hypertensive and normotensive female rats, with a focus on the influence of oestrogen. Female SHRs and WKY rats were allocated randomly to a control group or to groups to undergo ovariectomy or ovariectomy combined with oestrogen supplementation (17beta-oestradiol 150 microg/kg per day) for either 1 day (group 1E2) or 10 days (group 10E2). Mean arterial pressure (MAP) was recorded and small mesenteric arteries were mounted in a Multi Myograph 610M. Vascular reactivities to transmural nerve stimulation (TNS), exogenous noradrenaline and acetylcholine were analysed. MAP was significantly greater in SHRs than in WKY rats in all groups studied. Sensitivity to cumulative TNS (0.12-32 Hz) did not differ between vessels from control SHRs and WKY rats, expressed as the frequency giving 50% of maximal neurogenic response (Ef(50): 4.1 +/- 1.1 and 4.0 +/- 1.6 Hz, respectively). However, there was a greater reactivity to TNS in ovariectomized SHRs than in ovariectomized WKY rats (Ef(50) 1.8 +/- 0.7 and 6.8 +/- 2.2 Hz, respectively; P < 0.05). Oestradiol treatment significantly decreased the sensitivity to TNS in ovariectomized SHRs (P < 0.05), and after 10 days the frequency-response curves were almost identical (Ef(50) 6.3 +/- 1.9 Hz for group 10E2 SHRs and 5.6 +/- 0.8 Hz for group 10E2 WKY rats). The increased adrenergic reactivity in ovariectomized SHRs was inhibited by prazosin, an alpha(1)-adrenergic antagonist, and could not be explained by differences in endothelial function or sensitivity to applied noradrenaline. Increased adrenergic reactivity is not present in small arteries from female SHRs. The findings of this study suggest that oestrogen acts on prejunctional mechanisms, reducing full expression of hypertension and peripheral vascular pathology.

Enalapril alters the formation of the collagen matrix in spontaneously hypertensive rats.

To assess the effect of the inhibition of the angiotensin-converting enzyme on the collagen matrix (CM) of the heart of newborn spontaneously hypertensive rats (SHR) during embryonic development. The study comprised the 2 following groups of SHR (n=5 each): treated group - rats conceived from SHR females treated with enalapril maleate (15 mg. kg-1.day-1) during gestation; and nontreated group - offspring of nontreated females. The newborns were euthanized within the first 24 hours after birth and their hearts were removed and processed for histological study. Three fields per animal were considered for computer-assisted digital analysis and determination of the volume densities (Vv) of the nuclei and CM. The images were segmented with the aid of Image Pro Plus 4.5.029 software (Media Cybernetics). No difference was observed between the treated and nontreated groups in regard to body mass, cardiac mass, and the relation between cardiac and body mass. A significant reduction in the Vv[matrix] and a concomitant increase in the Vv[nuclei] were observed in the treated group as compared with those in the nontreated group. The treatment with enalapril of hypertensive rats during pregnancy alters the collagen content and structure of the myocardium of newborns.

Blood pressure produces deleterious effects in genital structures of female spontanously hypertensive rat

Emerging knowledge from clinical and experimental studies provides a new insight for understanding the pathophysiology of female sexual dysfunction in patients with arterial hypertension. The aim of the present study was to evaluate possible morphological alteration in clitoris and vagina from spontaneous hypertensive rats (SHR) and compared with normotensive Wistar-Kyoto (WKY) rats. Twelve-week-old female SHR and WKY rats, were studied during six months. G1 Control WKY (n = 10) and G2 SHR (n = 10). Systolic blood pressure (SBP) was recorded monthly. At end of the experiment, clitoris and vagina were dissected out and processed by Masson's trichrome, monoclonal anti-a-smooth-muscle actin, anti- collagen type I and type III, and anti TGFb1. Histomorphometric evaluation revealed that SHR presented higher amount of clitoral cavernous smooth muscle (20 ± 6.9 % versus 7 ± 4.4 %; p< 0.01), as well as vascular smooth muscle (16.1 ± 3.6 % versus 9 ± 2 %; p< 0.01); higher TGFb1 in clitoral vessel wall (2.7 ± 1.4 % versus 1.4 ± 0.5 %; p< 0.01); higher wall/lumen ratio (p< 0.01) in both vaginal and clitoral vessels; and remarkable interstitial fibrosis (60.7 ± 4.9 % versus 38.6 ± 2.7 %; p< 0.01), expressed by a higher amount in interstitial collagen type III and I (p< 0.01) in both clitoris and vagina, when compared to WKY rats. In addition, nerve fibers from clitoral and vaginal tissue in SHR showed a notable fibrosis at perineurium and endoneurium. SHR showed positive correlation between SBP and clitoral cavernous smooth muscle (r= 0.87 p< 0.01); SBP and fibrosis in clitoris (r= 0.81 p< 0.01); and SBP and collagen type I and III (r= 0.92 p< 0.01; r= 0.84 p< 0.01) in clitoris respectively: Similar findings were observed between SBP and collagen type I and III in vagina (r= 0.68 p< 0.05; r= 0.75 p< 0.05). According to these results we conclude that SHR present morphologic changes in clitoral vessels as well as in clitoral cavernous space, which have a high positive correlation with the high blood pressure level. Moreover, the increase in extracellular matrix expansion affects not only the clitoral and vaginal interstitium but also the nerve structures from both clitoris and vagina.

Estrogen depletion increases blood pressure and hypothalamic norepinephrine in middle-aged spontaneously hypertensive rats.

In male spontaneously hypertensive rats (SHR) a high NaCl diet increases arterial pressure via a reduction in anterior hypothalamic nucleus norepinephrine release. Young female SHR are relatively well protected from this NaCl-sensitive hypertension, but depletion of both endogenous and dietary estrogens greatly exacerbates NaCl-sensitive hypertension. This study tests the hypothesis that estrogen also protects late middle-aged female SHR from NaCl-sensitive hypertension and that this effect is mediated by an estrogen-related effect on hypothalamic norepinephrine release. Ten-month-old female SHR were ovariectomized and placed on a phytoestrogen-free diet containing either basal or high NaCl. Each rat was implanted with a silastic tube containing 17beta estradiol or vehicle. Three months later, arterial pressure and hypothalamic norepinephrine metabolite levels (MOPEG) were measured. On the basal NaCl diet, estrogen-depleted rats displayed increased arterial pressure (12 mm Hg) and decreased anterior hypothalamic nucleus MOPEG (20%). Both effects were reversed by estrogen treatment. In all groups, the high NaCl diet increased arterial pressure by over 35 mm Hg and reduced anterior hypothalamic nucleus MOPEG by >60%. Across all groups, there was a significant inverse correlation between arterial pressure and anterior hypothalamic nucleus MOPEG. These data suggest that both dietary NaCl excess and estrogen depletion raise arterial pressure in middle-aged female SHR by a decreasing hypothalamic norepinephrine.

Effects of vitamin C on high blood pressure induced by salt in spontaneously hypertensive rats.

By breeding and feeding salt to spontaneously hypertensive rats (SHR) continuously over a long period (until 60 wk old), rats with systolic blood pressures (SBP) of over 270 mmHg were prepared. It was studied whether or not supplying large amounts of vitamin C (200 mg/rat/d) over this period might bring any beneficial effect to blood pressure. Moreover, physico-chemical studies were performed to measure the components and enzymes in the blood and urine at 53 and 60 wk-old, and biochemical studies on vitamin C were also carried out in this experiment. Male (14 rats: 7 wk-old, 100-105 g) and female (15 rats: 7 wk-old, 95-100 g) SHR were divided into three groups and bred continuously for 53 wk. The A group rats were given salt (2.5 g/100 g of diet), the B group rats were given salt and vitamin C (500 mg/100 mL of drinking water), and the C group rats were controls. The results showed almost the same tendencies between male and female rats. The body weights of the SHR in groups A and B were slightly lower than group C. The amount of food intake in groups A and B was almost the same as group C. The amount of water intake was, in the order from highest to lowest, group A, B and C. The SBP of group A rats exhibited the highest value among the three groups. The SBP of group B rats given vitamin C simultaneously with the salt resulted in a low blood pressure level close to that of the controls (group C). Furthermore, the DBP (diastolic blood pressure) also reflected the antihypertensive effect of vitamin C as well. The heartbeat of the rats was highest in group A, and was comparable to the value in the rats receiving vitamin C simultaneously with salt. For the tests on occult blood and protein in the urine, group A rats showed strong positive reactions, whereas the group B and C rats had decreased results for both tests. The organ weights of the liver, stomach, spleen, adrenal gland and kidneys per 100 g rat body weight were not different among the three groups. The values for the bilirubin content, and the enzyme activities of ALT and AST in the blood showed to be the highest in the male rats of group A. The values from the group B rats decreased near to the normal value like the control group. Vitamin C was found to decrease the blood pressure in SHR, and also to work effectively to protect liver and kidney functions even under the condition of very high blood pressure, as high as 250 mmHg.

Renal ischemia-reperfusion injury in the rat is prevented by a novel immune modulation therapy.

Vasogen Inc.'s (Mississauga, Ontario, Canada) immune modulation therapy (IMT) is a therapy in which cells from the patient's own blood are modified by ex vivo exposure to specific physicochemical stressors, including oxidation, ultraviolet (UV) light, and an elevated temperature. The therapy has been shown to have a beneficial effect in models of inflammation and vascular diseases. This study tested the hypothesis that IMT can prevent renal ischemia-reperfusion (I/R) injury in rats. Whole blood was collected from syngeneic age-matched donors by cardiac puncture. It was treated with a combination of controlled physiochemical stressors consisting of elevated temperature, a gas mixture of medical oxygen containing ozone, and UV light. The treated blood (150 microL) was injected in the gluteal muscle. Control animals received the same volume of untreated blood or physiological saline. Transient (45 or 60 minutes) left-renal ischemia was produced with simultaneous contralateral nephrectomy in treated and control spontaneously hypertensive rats (SHR). Young and old male and female rats were studied. Plasma creatinine, diuresis, and the survival rates of each group were compared. Renal apoptosis-necrosis was estimated by DNA laddering, histology, and in situ terminal deoxynucleotidyl transferase assay. mRNA levels of several regulators of apoptosis-regeneration were determined in control and postischemic kidneys by Northern blotting. IMT pretreatment of SHR significantly reduced renal I/R injury compared with equivalent placebo treatments consisting of untreated blood- or saline-injected SHR, as evidenced by a significant increase of the survival rate curves in young and old male SHR, which correlated with 24-hour postischemic diuresis. The increases in plasma creatinine following renal I/R were significantly lower in IMT-treated young male and old female SHR compared with saline or untreated blood-injected controls. Dilution analysis showed that the protective effect of treated blood was lost by dilution. Loss of epithelial cells was reduced in IMT-treated rats, with a significant decline in the peak of apoptosis 12 hours after acute ischemic renal injury. IMT did not modify the pattern of mRNA levels of several genes involved in the inflammation and regeneration processes. Our data demonstrate that IMT prevents the destruction of kidney tissue and the resulting animal death caused by renal I/R injury.

Effects of ovariectomy on indices of insulin resistance, hypertension, and cardiac energy metabolism in middle-aged spontaneously hypertensive rats (SHR).

Insulin resistance is a risk factor for coronary heart disease. The protection of young women from coronary events is sharply reduced with menopause. To assess the impact of menopause on glucose tolerance, insulin resistance, body weight gain, heart size, and cardiac energy metabolism, we studied 28-week-old female SHR and Wistar-Kyoto (WKY) rats, who were either ovariectomized (SHR(OVX) and WKY(OVX)) or sham-operated (SHR(SHAM) and WKY(SHAM)). Animals underwent blood-pressure measurement and an oral glucose tolerance test (OGTT). Hearts were weighed and assayed for metabolic enzyme activities. Female SHR were 33 % lighter and hypertensive (+ 36 mmHg), with 33 % larger hearts (when corrected for body weight differences) compared to WKY. Although ovariectomized animals of both strains were heavier overall than their sham-operated counterparts, when heart weights were corrected for body weight, both OVX strains had lighter hearts than both SHAM strains. Glucose and insulin responses during OGTT were similar between the four groups; however, free fatty acid (FFA) responses were approximately 50 % greater in SHR than WKY, although less in SHR(OVX) than SHR(SHAM). WKY(OVX) demonstrated 8 % lower ventricular hexokinase activity than WKY(SHAM), which may reflect reduced cardiac glucose utilization. We also noted 16 % higher citrate synthase activity in WKY hearts. In conclusion, the insulin resistance characteristic of younger SHR is blunted in middle-aged female rats, although FFA responses remain elevated. Ovariectomy did not alter in vivo glucose tolerance in this group; however, sex hormones may be important in maintaining normal heart size and the potential for cardiac glucose metabolism.

Estradiol increases salt intake in female normotensive and hypertensive rats.

The objective of this study was to examine whether or not estradiol (E2) alters sodium intake in hypertensive and normotensive female rats. It was hypothesized that higher doses of E2 would increase sodium consumption and that this response would be greater in spontaneously hypertensive rats (SHR) compared with Wistar Kyoto (WKY) rats. The study involved female SHR and WKY (n = 12/group). All animals were ovariectomized. Six of twelve rats from each strain received three progressively larger doses of beta-estradiol propionate (each dose lasting 2 wk), whereas the other six rats from each strain received sham implants. Blood E2 levels were measured by radioimmunoassay after each 2-wk period, allowing a 10-day washout period before the next E2 dose. Rats had access to 0.0, 0.5, 1.0, and 1.5% NaCl solutions to drink throughout the experiment. There was a significant positive correlation between sodium intake and plasma E2 (r = 0.8, P < 0.001). Both strains avoided the 1.5% NaCl, and the increased sodium intake was achieved by an increase in consumption of the 0.5% NaCl. SHR females consumed more sodium than WKY females, which is similar to what has been observed in males of these strains. In conclusion, E2 was positively correlated with sodium intake in both strains of rat, with the hypertensive rats consuming more sodium than the normotensive rats.