Impaired diastolic function and coronary reserve in genetic hypertension. Role of interstitial fibrosis and medial thickening of intramyocardial coronary arteries.

Left ventricular hypertrophy (LVH) in rats with genetic hypertension is accompanied by abnormal myocardial diastolic stiffness and impaired coronary reserve. Whether these functional defects are related to a structural remodeling of the myocardium that includes an interstitial and perivascular fibrosis, myocyte hypertrophy, and medial thickening of intramyocardial coronary arteries is uncertain. To address these issues, 14-week-old male spontaneously hypertensive rats with established hypertension and LVH were treated with low-dose (SLO group: 2.5 mg/kg/day, n = 11) or high-dose (SHI group: 20 mg/kg/day, n = 9) oral lisinopril for 12 weeks to sustain hypertension and LVH or to normalize arterial pressure and myocardial mass, respectively. When SHI and SLO groups were compared with age- and sex-matched 26-week-old untreated spontaneously hypertensive rats (n = 11) and normotensive Wistar-Kyoto rats (n = 9), we found 1) normalization of blood pressure (p less than 0.005) and complete regression of LVH (p less than 0.005) in the SHI group and no significant blood pressure or LVH reduction in the SLO group, 2) complete regression of morphometrically determined myocardial interstitial and perivascular fibrosis in SHI and SLO groups (p less than 0.025) associated with normalization of diastolic stiffness, measured in the isolated heart (p less than 0.025), and 3) regression of medial wall thickening of intramyocardial coronary arteries only in the SHI group (P less than 0.005), accompanied by a normalization of coronary vasodilator reserve to adenosine (p less than 0.005). Thus, interstitial fibrosis and not LVH is responsible for abnormal myocardial diastolic stiffness, whereas medical wall thickening of intramyocardial resistance vessels, influenced by arterial pressure, is associated with impaired coronary reserve.     

Arterial hypertension, myocardial hypertrophy and disorders of cardiac rhythm induced by ligation of the left coronary artery in the rat

Cardiac hypertrophy and hypertension are major elements in sudden cardiac death in patients with coronary artery disease. To investigate in animals the hypothesis that left ventricular hypertrophy (LVH) and/or hypertension increase the incidence of severe ventricular arrhythmias, we have undertaken a 30 min period of coronary artery ligation in anaesthetized spontaneously hypertensive rats (SHR), normotensive (NT) Wistar Kyoto (WKY) and Wistar (W) rats. Mean systolic blood pressure (SBP) was 190 +/- 4 mmHg in SHR vs 123 +/- 5 mmHg in WKY and 116 +/- 4 mmHg in W (p less than 0.001). LVH index was 2.81 +/- 0.04 in SHR vs 196 +/- 0.03 in WKY and 1.65 +/- 0.05 in W (p less than 0.01). Incidence (IVF) and duration (DVF) of ventricular fibrillation were significantly more elevated in SHR than in NT rats. IVF was 100 p. 100 in SHR vs 36 p. 100 in WKY and 27 p. 100 in W (p less than 0.001); DVF was 61 +/- 17 s in SHR vs 6 +/- 6 s in WKY and W (p less than 0.001). In addition the calcium channel blocker nicardipine (N) has been administered orally to SHR either chronically during eight weeks (20 mg/kg-1 per os twice daily) or acutely as a single dose of 20 mg/kg. After long term treatment (LT) with N the LVH index and SBP were significantly reduced when compared to vehicle treated (VT) SHR; whereas a single administration of N (AT) only decreased SBP without affecting LVH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myocardial collagen and mechanics after preventing hypertrophy in hypertensive rats

To determine if a remodeling of the collagen matrix would occur in the absence of hypertrophy and cell necrosis and if such a remodeling could alter active and passive stiffness of the intact myocardium, five rats with genetic hypertension (SHR) were treated (SHRT) with hydralazine for 32 weeks, beginning at four weeks of age, and compared to six age- and sex-matched SHR and seven Wistar-Kyoto genetic control rats (WKY). Left ventricular (LV) weight of SHRT was 17% lower (P less than .001) than that of SHR and 19% higher (P less than .01) than that of WKY. Collagen volume fraction of SHR (13.7 +/- 3.2%) and SHRT (9.9 +/- 1.8%) were greater (P less than .01) than WKY (5.0 +/- 1.9%). Diastolic and systolic stress-strain relations were determined in the isolated heart. A comparison of these relations revealed: 1) a 24% increase in passive stiffness for SHR and SHRT; and 2) a reduced zero-strain intercept (41% to 54%) and slope (36% to 48%) of the developed stress-strain relation for the SHRT. Thus, in SHR, collagen remodeling occurred in the absence of hypertrophy which suggests that the muscular and collagenous compartments of the myocardium are under separate controls. The excess accumulation of collagen in SHR and SHRT leads to abnormal passive stiffness, and the prevention of hypertrophy with hydralazine reduces active stiffness.     

Altered angiotensin II-induced small artery contraction during the development of hypertension in spontaneously hypertensive rats.

This study assessed whether the angiotensin-II (Ang II)-induced contractile responsiveness of resistance arteries is altered during the development of hypertension in spontaneously hypertensive rats (SHR). Structural parameters and Ang II-stimulated contraction were determined in small mesenteric arteries from 6-week-old (phase of developing hypertension) and 21-week-old SHR (phase of established hypertension), compared with age-matched Wistar-Kyoto rats (WKY). To ascertain whether effects were specific for Ang II, contractile responses to another vasoactive agonist, vasopressin (AVP), were also determined. Systolic blood pressure was measured in conscious rats by the tail-cuff method. Segments of third-order mesenteric arteries (approximately 200 microm in diameter and 2 mm in length) were mounted in a pressurized system with the intraluminal pressure maintained at 45 mm Hg. Blood pressure was significantly increased in SHR (P < .001) and was higher in adult than in young SHR (P < .001). Ang II dose-dependently increased contraction, with responses significantly greater (P < .05) in SHR than in age-matched WKY. SHR, in the early phase of hypertension, exhibited significantly augmented contractile responses (Emax = 70 +/- 5%), compared with SHR with established hypertension (Emax = 33 +/- 5%). These effects were not generalized, as responses to AVP were not significantly different between young and adult SHR. Functional Ang II-elicited alterations were associated with structural modifications: 6-week-old SHR had smaller media to lumen ratio compared with 21-week-old SHR (8.1% +/- 0.17% v 10.6% +/- 0.20%, P < .01). In young SHR vessels the media cross-sectional area was unchanged relative to age-matched WKY rats, suggesting eutrophic remodeling (remodeling index 101.4% v 93.3% young v adult), whereas the cross-sectional area of adult vessels was increased in comparison to WKY rats, suggesting mild hypertrophic remodeling (growth index -1.0% v 15.2%, young v adult). In conclusion, the present study demonstrates that in SHR with early hypertension and slight medial thickening, Ang II-mediated vascular contractile responsiveness is significantly augmented compared with SHR with established hypertension and more severe vascular structural changes. These findings indicate attenuation, as hypertension progresses, of the initially enhanced vascular reactivity to Ang II that is present during the development of hypertension in SHR.     

Prolonged QT interval is associated with blood pressure rather than left ventricular mass in spontaneously hypertensive rats

QT interval is prolonged in hypertensive individuals, although the factors responsible for this increase are not completely understood. We questioned whether enhanced left ventricular mass (LVM) or increased systemic blood pressure represents the principal factor determining QT prolongation in the period of development of hypertension and left ventricular hypertrophy (LVH) in spontaneously hypertensive rats (SHR). In 12-and 20-week-old SHR (SHR12 and SHR20) and age-matched normotensive Wistar-Kyoto rats (WKY12 and WKY20), arterial systolic blood pressure (sBP) was measured using tail-cuff technique. Orthogonal Frank ECG was registered in anaesthetized animals in vivo, and bipolar ECG was measured in spontaneously beating isolated hearts in vitro. Progressive increase of sBP and LVM resulted in significant QT prolongation in SHR20 as compared to WKY12, WKY20, and also to SHR12 in vivo (WKY12: 82 +/- 9 ms, WKY20: 81 +/- 9 ms, SHR12: 88 +/- 15 and SHR20: 100 +/- 10, respectively; p < 0.05) but not in isolated hearts (WKY20: 196 +/- 39 ms and SHR20: 220 +/- 55, respectively; NS). In whole animals, QT duration was positively related to sBP (r = 0.6842; p < 0.001) but not to LVM (r = 0.1632, NS) in SHR20. The results suggest that QT prolongation in SHR developing hypertension and LVH depends on blood pressure rather than increase in LVM. In this period, myocardial hypertrophy is probably the predisposition for QT prolongation, but the significant change manifests only in the presence of elevated systemic factors.     

Vascular remodeling and improvement of coronary reserve after hydralazine treatment in spontaneously hypertensive rats

The purpose of these studies was to evaluate cardiovascular structural and functional changes in a model of hypertension-induced myocardial hypertrophy in which vasodilator therapy decreased blood pressure to normal levels. Thus, we determined the separate contributions of hypertension and hypertrophy on myocardial and coronary vascular function and structure. Twelve-month-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) with and without 12 weeks of vasodilator antihypertensive treatment (hydralazine) were studied using an isolated perfused rat heart model. Hydralazine treatment normalized blood pressure in SHR but did not cause regression of cardiac hypertrophy (heart weight to body weight ratio of SHR + hydralazine 4.33 +/- 0.098 vs. SHR 4.66 +/- 0.091; WKY 3.21 +/- 0.092 and WKY + hydralazine 3.38 +/- 0.152; mean +/- SEM). Coronary flow reserve, elicited by adenosine vasodilation in the perfused heart, was decreased in SHR (29%) compared with WKY (105%) and WKY + hydralazine (100%) and was significantly improved in SHR + hydralazine (75%). Morphometric evaluation of perfusion-fixed coronary arteries and arterioles (30-400 microns diameter) demonstrated a significant increase in the slope of the regression line comparing the square root of medial area versus outer diameter in SHR (0.444) compared with WKY (0.335) and WKY + hydralazine (0.336, p less than 0.05). Blood vessels from SHR + hydralazine were not different from control (0.338). Cardiac oxygen consumption was decreased in SHR (10.9 +/- 0.74 mumols oxygen/min/g/60 mm Hg left ventricular pressure) compared with WKY (22.4 +/- 1.47) and WKY + hydralazine (23.4 +/- 1.90; p less than 0.01), while SHR + hydralazine was intermediate (16.0 +/- 1.60). These studies suggest that significant alterations in myocardial and coronary vascular structure and function occur in hypertension-induced cardiac hypertrophy. The coronary vasculature is responsive to blood pressure, independent of cardiac hypertrophy, although moderate coronary deficits do remain after chronic antihypertensive therapy.     

Vascular oxidative stress precedes high blood pressure in spontaneously hypertensive rats

This study examines whether longitudinal antioxidant treatment initiated in prehypertensive spontaneously hypertensive rats (SHR) can attenuate vascular oxidant stress and prevent blood pressure elevation during development. Male SHR and age-matched Wistar-Kyoto rats (WKY) were treated from 6 to 11 weeks of age with Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl) (1 mmol/l in drinking water), a membrane-permeable superoxide dismutase mimetic. Mean systolic blood pressures (SBPs) were measured by tail-cuff Agonist-induced and basal O2- production was measured in thoracic aortas of 6- and 11-week-old SHR and WKY by lucigenin-derived chemiluminescence and oxidative fluorescent microscopy, respectively. SBP of 6-week-old SHR (131 +/- 5 mmHg) and WKY (130 +/- 4 mmHg) were not different; however, 11-week-old SHR SBP (171 +/- 4 mmHg) was significantly greater (p = .0001) than 11-week-old WKY SBP (143 +/- 5 mmHg). Tempol treatment completely, but reversibly, prevented this age-related rise in SHR SBP (SHR + Tempol: 137 +/- 4 mmHg; p < .0001 versus untreated SHR). Agonist-induced vascular O2- was increased in 6- (p = .03) and 11-week-old SHR (p < .0001) and 11-week-old WKY (p = .03) but not in 6-week-old WKY. Long-term Tempol treatment significantly lowered O2- production in both strains. Basal O2- measurements in both 6- and 11-week-old SHR were qualitatively increased compared with age-matched WKY; this increase in SHR was inhibited with in vitro Tempol treatment. These data show that antioxidant treatment to reduce oxidative stress prevents the age-related development of high blood pressure in an animal model of genetic hypertension.     

Hypertensive myocardial hypertrophy and rhythm disorders: 2 possible origins

It has been suggested that complex ventricular arrhythmias commonly occur in hypertensive patients with left ventricular hypertrophy. We have previously demonstrated that coronary artery ligation in anesthetized spontaneously hypertensive rats (SHR) and their normotensive controls (WKY) resulted in a significantly increased incidence and duration of ventricular fibrillation in SHR compared with WKY. The object of the present study was to characterize the structural and electrophysiological abnormalities in hypertrophied hearts, associated with the occurrence of arrhythmias. We used a double tissue bath in which a ventricular strip was exposed simultaneously to normal and to altered conditions (low pH, hypoxia and high potassium). Electrical activity recorded using standard micro-electrode techniques showed the occurrence of arrhythmias in all preparations and the development of major alterations in conduction (a conduction block appeared at 11 +/- 1 mn in SHR vs 16 +/- 1 mn in WKY, p less than 0.05), and maximal upstroke velocity (Vmax values before and 3 mn after the beginning of ischemia were 229 +/- 12 to 46 +/- 7 v/s for the SHR and 227 +/- 10 to 106 +/- 12 v/s for the WKY; p less than 0.001). These changes were associated in hypertrophied ventricles with a marked sub-endocardial collagen fibrosis as estimated by the use of automated image analysis (subendocardial collagen density = 4.39 +/- 0.34 p. 100 in SHR vs 1.66 +/- 0.15 p. 100 in WKY; p less than 0.001). Action potential duration measured using conventional glass micro-electrodes in a single chamber tissue bath revealed a highly significant difference (p less than 0.001) in APD 90 p. 100 of papillary muscles between SHR (114.7 +/- 2.8 ms) and WKY (76.9 +/- 1.7 ms). The addition of tetra-ethylammonium to block potassium channels induced triggered activity arising from early afterdepolarizations only in muscles hypertrophied SHR hearts.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endothelium-dependent mechanical properties of the carotid artery in WKY and SHR. Role of angiotensin converting enzyme inhibition

An experimental model of in situ isolated carotid arteries has been used to evaluate the static mechanical properties of the arterial wall in 12-week-old Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The effects of endothelium removal and of local incubation with the converting enzyme inhibitor lisinopril (ICI Pharma 209000) on the carotid compliance (CC) were compared with the effects of total abolition of the vascular smooth muscle tone by potassium cyanide. CC measured for pressures ranging from 50 to 175 mm Hg had maximal values (0.22 +/- 0.07 microliter/mm Hg and 0.13 +/- 0.03 microliter/mm Hg, respectively, for WKY and SHR, p less than 0.001) for pressure values close to the operating pressures in both groups. Maximal values of CC were increased by 35% and 45% in WKY and SHR, respectively, after potassium cyanide poisoning (p less than 0.01). The endothelium removal induced a significant increase in CC compared with their control values (+37%, p less than 0.01, and +25%, p less than 0.01, respectively, in WKY and SHR). CC measured after endothelium removal did not significantly differ from its values measured after potassium cyanide poisoning in normotensive animals. In contrast, in hypertensive animals, CC was significantly lower after endothelium, removal than after potassium cyanide poisoning (p less than 0.01). In the presence of intact endothelium, local incubation with converting enzyme inhibitor increased CC by 23% (p less than 0.05) in WKY rats and by 14% (p less than 0.01) in SHR. In contrast, after endothelium removal, converting enzyme inhibitors did not significantly increase further CC in either strain.(ABSTRACT TRUNCATED AT 250 WORDS)     

High angiotensin converting enzyme (kininase II) activity in the cerebrospinal fluid of spontaneously hypertensive adult rats

Angiotensin converting enzyme (ACE, Kininase II, E.C. 3.4.15.1) activity was measured in the cerebrospinal fluid of 4- and 16-week-old spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) normotensive controls. Adult SHR showed higher cerebrospinal fluid enzyme activity than normotensive age-matched WKY (19.6 +/- 1 and 32.3 +/- 5 nmol/h per ml in WKY and SHR, respectively, P less than 0.025). Conversely, there were no significant differences in enzyme activity in the cerebrospinal fluid of young animals. Our results support the hypothesis of enhanced activity of the central angiotensin system during the established phase of spontaneous hypertension in rats.     

Attenuation of cardiac failure, dilatation, damage, and detrimental interstitial remodeling without regression of hypertrophy in hypertensive rats

Whether left ventricular (LV) hypertrophy is important in the development of LV failure associated with advanced myocardial damage and detrimental chamber and interstitial remodeling in hypertension has not been established. We examined the effect of an antihypertensive agent without the ability to regress LV hypertrophy on the development of LV changes in spontaneously hypertensive rats (SHR). Hydralazine given to SHR from 5.2 to 26 months of age returned systolic blood pressure to Wistar Kyoto (WKY) control values but failed to prevent the increase in LV mass noted in SHR (at 26 months of age: WKY, 0.99+/-0.02 g; untreated SHR, 1.40+/-0.02 g; treated SHR, 1.36+/-0.02 g; P<0.001 in SHR versus WKY). In comparison to both 16-month-old SHR and age-matched WKY, 26-month-old untreated SHR developed signs consistent with heart failure, LV dilatation (an increased LV internal radius), an eccentric LV geometry, advanced myocyte necrosis, an increase in myocardial collagen solubility (an index of decreases in myocardial collagen cross-linking), and marked increases in myocardial total, type III, and non-cross-linked myocardial collagen concentrations. Despite the inability of hydralazine to regress LV hypertrophy, treated SHR did not develop signs of heart failure, myocyte necrosis, decreases in myocardial collagen cross-linking, or increases in myocardial total, type III, and non-cross-linked collagen at 26 months of age. Moreover, treatment attenuated the development of LV dilatation and an eccentric LV geometry. In conclusion, antihypertensive therapy that does not attenuate LV hypertrophy but achieves normal blood pressure in SHR, is able to hinder the development of heart failure associated with advanced myocardial damage and detrimental chamber and interstitial remodeling.     

Renal kallikrein activity in spontaneously hypertensive rats

To assess possible roles of the renal kallikrein-kinin system in the development of spontaneous hypertension, we determined daily excretion of urinary total and active kallikrein in 6-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats for up to 2 weeks. We also evaluated the effect of aprotinin, a reversible inhibitor of kallikrein and other serine proteases, on the development of hypertension in the 6-week-old SHR on ordinary intakes of sodium or on sodium loading with 1% NaCl for up to 2 weeks. Active kallikrein was determined by its kininogenase activity, and the generated kinins were radio-immunologically measured. Total kallikrein was also determined by measuring kininogenase activity after inactive kallikrein had been activated with trypsin (200 micrograms/ml). Urinary active kallikrein excretion was significantly reduced in 7-week-old SHR (1.5 +/- 0.2 microgram/day compared to 2.8 +/- 0.3 micrograms/day in WKY, P less than 0.05) and in 8-week-old SHR (1.6 +/- 0.2 microgram/day compared to 3.2 +/- 0.4 micrograms/day in WKY, P less than 0.01). Urinary total kallikrein excretion was also reduced in the 7- and 8-week-old SHR whereas the ratio of active to total kallikrein did not change. In addition, renal contents of total and active kallikrein were significantly lower in the 8-week-old SHR than in the controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structural vs. contractile protein remodeling and myocardial stiffness in hypertrophied rat left ventricle

Left ventricular pressure overload will result in the hypertrophic growth of the myocardium and in the rat may include a remodeling of both the structural and contractile proteins. As a result, an adaptive rise in active stiffness, or the force generating capacity of the myocardium, may occur. The relative importance of structural vs. contractile protein remodeling to the hypertrophic response and active stiffness is unclear. Accordingly, we monitored the ratio of V1/V3 isomyosin and the fibrillar nature and volume fraction of myocardial collagen, together with the developed systolic stress-strain relation of the intact myocardium in the adult male Wistar rat after 8 weeks of abdominal aorta banding. In comparison to controls and for the 20% increase in left ventricular mass obtained with banding we found: (a) collagen volume fraction had increased significantly (6.2 +/- 2.0 vs. 3.6 +/- 1.0%) while the V1/V3 ratio did not change; (b) interstitial compartment remodeling included a perivascular accumulation of collagen around small intramyocardial coronary arteries and the appearance of more extensive fibrillar collagens; and (c) active stiffness increased significantly. Thus, the increase in active stiffness of the hypertrophied adult rat myocardium, seen with abdominal aorta banding, appears to be related to interstitial fibrosis and not a conversion of myosin isoforms.     

Reduced calcium sensitivity of dihydropyridine binding to calcium channels in spontaneously hypertensive rats.

To explore the role of calcium channels in hypertension, dihydropyridine ([3H]PN200-110) binding to heart, brain, and skeletal muscle microsomes of 4-, 8- and 15-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats was measured. At a constant Ca2+ ion concentration (pCa 3.0), maximal binding (Bmax) of dihydropyridine binding to heart and brain microsomes was significantly enhanced in 8- and 15-week-old SHR compared with WKY rats (p less than 0.01), whereas this phenomenon was not observed in 4-week-old SHR and WKY rats. Bmax and dissociation constant (Kd) values for skeletal muscle microsomes from SHR showed no difference compared with WKY rats irrespective of age. Dihydropyridine binding to heart microsomes, brain microsomes, and solubilized skeletal muscle microsomes exhibited strong calcium dependence. The Ca2(+)-dependent dihydropyridine binding curves for heart showed a Hill slope, and pK 0.5 values for 15-week-old SHR and WKY rats were 0.70 +/- 0.12 and 4.66 +/- 0.12 versus 0.72 +/- 0.12 and 5.66 +/- 0.08 (n = 4, mean +/- SD), respectively, indicating that 15-week-old SHR require 10-fold higher calcium concentration than WKY rats to promote dihydropyridine binding. The pK 0.5 values of calcium for brain and solubilized skeletal muscle calcium channels in 15-week-old SHR were also significantly lower than in WKY rats. This difference first became apparent in SHR and WKY rats as early as 4 and 8 weeks after birth. These results suggest that enhancement of calcium channel density might occur in the heart and brain of SHR in response to elevated blood pressure and that reduced calcium sensitivity of dihydropyridine binding to calcium channels might be a primary characteristic of this rat strain.     

Urinary kallikrein activity in conscious spontaneously-hypertensive rats: development as a function of age

Urinary excretion of active kallikrein was determined every day (amidolytic assay) in 6 male Okamoto-Aoki spontaneously hypertensive rats (SHR) and in 6 male normotensive Wistar-Kyoto rats (WKY) from ages 3 to 6 weeks and from 12 to 16 weeks. The rats were placed into individual metabolic cages and allowed free access to food having normal sodium content and to tap water. Urinary kallikrein excretion (UKall V, nKat/24 h) was lower in 3-week-old SHR (7.8 +/- 1.4 nKat/24 h) than in WKY (15.5 +/- 2.3 nKat/24 h, p less than 0.01) at a moment when systolic blood pressure (BP) in SHR (85.5 +/- 4.0 mmHg) was already higher than in WKY (76.3 +/- 4.6 mmHg, p less than 0.01). The slope of the increase in kallikrein excretion from week 3 to week 6 was not different for SHR and WKY (y = 6.39 x - 12.09, r = 0.95 vs y = 7.49 x - 9.40, r = 0.93). In contrast, from week 12 to week 15, this slope was slightly negative for SHR (y = 1.08 x + 59.38, r = 0.66) and became significantly different (p less than 0.05) from the slope in WKY which remained positive (y = 5.09 x + 7.05, r = 0.48). The relation between kallikrein excretion and systolic BP was an exponential curve for both SHR and WKY. But the curve of SHR (y = 1.22.e0.03x, r = 0.91) was significantly different (p less than 0.01) from the curve of WKY (y = 1.08.e0.03x, r = 0.95). For each identical systolic BP, UKall V was always lower in SHR than in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for a difference in vitamin D metabolism between spontaneously hypertensive and Wistar-Kyoto rats

It has been contended that the metabolism of vitamin D in spontaneously hypertensive rats (SHR) is different from that in Wistar-Kyoto rats (WKY). To investigate this possibility, the plasma concentration of 1,25-dihydroxycholecalciferol (1,25[OH]2D) and several known determinants of its production rate were measured in SHR and WKY given normal and restricted amounts of dietary phosphorus. In 12-week-old male SHR given a normal amount of dietary phosphorus, the mean plasma concentration of 1,25(OH)2D (72 +/- 5 pg/ml) was significantly lower than that in age-matched WKY (129 +/- 6 pg/ml; p less than 0.001). The lower plasma concentration of 1,25(OH)2D in the SHR could not be attributed to higher circulating levels of inorganic phosphorus or ionized calcium, lower plasma concentrations of 25-hydroxycholecalciferol, or acidosis. However, in the SHR, urinary excretion of cyclic adenosine 3',5'-monophosphate (12.5 +/- 0.4 nmol/mg creatinine) was significantly lower than that in WKY (15.2 +/- 0.3 nmol/mg creatinine; p less than 0.001). In both SHR and WKY, restriction of dietary phosphorus for 1 week induced an increase in the plasma concentration of 1,25(OH)2D without affecting blood pressure. The current findings indicate that in 12-week-old male SHR, 1,25(OH)2D metabolism is different from that in age-matched WKY. The activity of 25-hydroxyvitamin D-1 alpha-hydroxylase, however, appears to be at least partially responsive to short-term restriction of dietary phosphorus. In SHR, the activity of 25-hydroxyvitamin D-1 alpha-hydroxylase may be lower than that in WKY, perhaps due in part to some impairment in the renal metabolism of, or responsiveness to, cyclic adenosine 3',5'-monophosphate.     

Impaired nitric oxide production and enhanced autoregulation of coronary circulation in young spontaneously hypertensive rats at prehypertensive stage.

In the current study, we investigated the NO-generation pathway in response to mechanical stimuli in SHR at the prehypertensive stage. To examine the role of NO in coronary autoregulation, we evaluated the effects of L-NAME on the coronary flow in SHR at both the prehypertensive and hypertensive stages. Isolated perfused hearts from 5- and 15-week-old SHR and from age-matched Wistar-Kyoto rats (WKY) were used. After stabilization at 60 mmHg, perfusion pressure was immediately raised to 90 mmHg to record the change in coronary flow for 10 min without (control) or with NO synthesis blockade by Nomega-nitro-L-arginine methyl ester (L-NAME). NOx- (nitrite/nitrate) was measured in coronary effluent. At 5 weeks of age, SHR did not have hypertension, while the coronary autoregulation was enhanced. L-NAME did not affect this enhanced autoregulation in 5-week-old SHR. At perfusion pressures of both 60 and 90 mmHg, 5-week-old SHR showed less coronary NOx- production than age-matched WKY. At 15 weeks, SHR showed a higher blood pressure than WKY. The coronary autoregulation in SHR remained higher than that in WKY, but was below that in 5-week-old SHR. NOx- production in 15-week-old SHR recovered to the level of age-matched WKY. These results indicate that NOx- production induced by mechanical stimulation was markedly reduced in 5-week-old SHR at the prehypertensive stage, which may have enhanced coronary autoregulation. An impaired nitric oxide production may precede the onset of hypertension in SHR.     

Increased expression of vascular endothelial growth factor and capillary density in hearts of spontaneously hypertensive rats

OBJECTIVE: The role of VEGF in vascular remodeling of target organs exposed to chronic hypertension is poorly understood. The authors compared capillary density (CD), capillary-to-fiber ratio (C/F), and VEGF mRNA expression in the hearts (left ventricle [LV]), and skeletal muscles (soleus and anterior tibialis [AT]) of 18-week-old male spontaneously hypertensive rats (SHR) and age-matched male Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats. METHODS: CD or C/F in LV, soleus, and AT of SHR, WKY, and SD rats was determined by analysis of randomly acquired digital images of cryosections stained with FITC-conjugated GS-I lectin. VEGF mRNA expressions in the tissues were determined by Northern blot. RESULTS: VEGF mRNA expressions in LV of SHR were 3.84- and 5.05-fold higher, compared to SD and WKY rats, respectively (n = 6; p < .01). There were no significant differences in VEGF mRNA expression in soleus or AT among SHR, WKY, and SD rats (p > .05). CD in LV of SHR (4975 +/- 167) was significantly higher than WKY or SD rats, 4151 +/- 169 and 3807 +/- 187 mm(-2), respectively (p < .05). In LV of SHR, C/F increased (35%) more significantly than CD (increased 20%), compared to WKY rats. CD, or C/F in soleus or AT of SHR was similar to that observed in WKY or 8D rats. CONCLUSIONS: VEGF expression, CD, and C/F in the heart (LV) of SHR are significantly increased, compared to WKY and SD rats. The data are consistent with the possibility that VEGF may contribute to capillary growth as a compensatory response to hypertension.     

Adrenocorticotropin responses to corticotropin releasing factor and vasopressin in spontaneously hypertensive rats

The effects of exogenous corticotropin releasing factor and arginine vasopressin were evaluated in 6- and 11-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Basal adrenocorticotropic hormone (ACTH) and vasopressin levels did not differ between SHR and WKY, but basal corticosterone level was higher in 6-week-old SHR (p less than 0.01). To block endogenous corticotropin releasing factor secretion and nonspecific systemic responses, both groups were pretreated with chlorpromazine, morphine, and sodium pentobarbital anesthesia before measurement of ACTH responses to corticotropin releasing factor and vasopressin infusion. Basal ACTH level was lower in anesthetized 6-week-old SHR than in age-matched WKY (p less than 0.01), but no difference was seen between 11-week-old WKY and SHR. The ACTH response to corticotropin releasing factor in 6-week-old WKY was significantly greater than that in age-matched SHR (p less than 0.01), whereas in 11-week-old SHR and WKY the response was similar. Compared with responses in WKY, SHR showed an increased ACTH response to high doses of vasopressin (0.25 micrograms/100 g body weight) at both ages (p less than 0.05). These results indicate that the ACTH response to corticotropin releasing factor is blunted in the early stages of hypertension in SHR but later recovers. These abnormal responses to corticotropin releasing factor and vasopressin may be related to the development of spontaneous hypertension.     

自发性高血压大鼠左心室重构中的细胞凋亡

为探讨细胞凋亡在高血压大鼠左心室重构中的变化及其作用 ,以 16周龄雄性自发性高血压大鼠和正常血压对照大鼠为研究对象 ,左心室中轴切片 ,应用TdT介导的dUTP缺口末端标记法检测心肌组织细胞凋亡 ,透射电镜观察凋亡细胞的超微结构和细胞类型 ,LSAB免疫组织化学法检测细胞膜死亡受体分子Fas蛋白的细胞定位。结果发现 ,与同龄正常血压对照大鼠相比较 ,16周龄自发性高血压大鼠左心室肥厚指数显著增高 ;光镜观察发现 ,心肌细胞凋亡指数显著增加 [以 10 4 个核为计算单位 (下同 ) ,自发性高血压组为 19.5 4± 7.2 7,正常血压对照组为 6 .92± 4.35 ,P <0 .0 5 ],并可见小血管内皮细胞和浸润心肌的白细胞凋亡 ,但纤维细胞凋亡指数显著下降 (自发性高血压组为 5 .88± 2 .15 ,正常血压对照组为 14.34± 9.5 6 ,P <0 .0 5 )。电镜观察发现 ,自发性高血压大鼠可见凋亡的内皮细胞和心肌细胞 ,内皮细胞凋亡多发生于萎缩的毛细血管 ,血管周围有纤维组织增生 ;正常血压对照大鼠可见凋亡的纤维细胞。Fas蛋白在自发性高血压大鼠主要定位于心肌细胞 ,在正常血压对照大鼠主要定位于纤维细胞。实验提示高血压左心室重构中细胞凋亡类型发生了改变 ,Fas受体途径可能与此改变有关。寻找靶细胞特异的细胞凋亡调节剂可能具有重