Vasopressin and oxytocin in genetically hypertensive rats of the Lyon strain

To investigate the role of arginine vasopressin (AVP) in genetic hypertension, we measured 24-h urinary volume, osmolality, and 24-h AVP excretion, as well as pituitary and pineal AVP and oxytocin levels, in genetically hypertensive (LH), normotensive (LN), and low blood pressure (LL) 5-and 45-week-old female rats of the Lyon strains. We also determined vascular sensitivity to AVP, norepinephrine, and angiotensin II in 6- and 21-week-old rats. AVP secretion was increased in both LH and LL rats compared with LN controls. Previous reports of increased AVP secretion in spontaneously hypertensive rats have suggested that AVP might play a role in high blood pressure. The existence of a similar increase in LL rats indicates that genetic hypertension of LH rats is not related directly to their increased AVP secretion. Furthermore, the vascular sensitivity to AVP was not specifically enhanced in 21-week-old LH rats compared with LN and LL controls. This study provides evidence against a major role of vasopressin in the genesis and maintenance of high blood pressure in this model of experimental hypertension, and emphasizes that the choice of controls in such investigations is of crucial importance.     

Lyon genetically hypertensive rats: an animal model of "low renin hypertension"

This paper reviews the data which demonstrate that Lyon genetically hypertensive (LH) rats exhibit a low renin form of hypertension. Since when compared to normotensive controls, LH rats exhibit a low renin release and are salt-sensitive. Despite this, the blockade of the renin-angiotensin system normalizes the blood pressure level and the regional blood flows in LH rats. Such a discrepancy between the compulsory presence of angiotensin II for the hypertension to develop and the low level of renin release seen in LH rats led to two hypotheses: 1) the existence of an early, short lasting increase of renin release which would be sufficient for the occurrence of a stable hypertension; 2) an hypersensitivity to the effects of angiotensin II. The study of the long-term effects of early short lasting blockades of the renin-angiotensin system allowed to exclude the first hypothesis. Concerning a hypersensitivity to the effects of angiotensin II, it was found to exist in the preglomerular vessels of LH rats. This increased response of renal vessels to angiotensin II may well explain the low renin form of hypertension of our model and therefore represents an important field for further research.     

Visfatin is associated with lipid metabolic abnormalities in Lyon hypertensive rats

1. Visfatin (also known as nicotinamide phosphoribosyltransferase or Nampt) is a novel adipokine associated with metabolic abnormalities in obesity and diabetes. The aim of the present study was to determine whether visfatin levels in the circulation and visfatin expression in fat tissues are altered in Lyon hypertensive (LH) rats, a hypertensive strain with numerous metabolic abnormalities, and, if so, to explore the possible correlations between blood visfatin levels and the metabolic abnormalities in LH rats. 2. Male 18–20‐week‐old LH rats and their control, Lyon normotensive (LN) rats, were used. Blood pressure was recorded in conscious, unrestrained rats. Fat tissue weight was determined. Serum visfatin concentrations were determined by enzyme immunoassay, whereas visfatin expression in fat tissues was determined by western blotting. Relationships between two parameters were evaluated by linear regression analysis. 3. The LH rats had significantly higher bodyweight and fat tissue weight compared with LN rats. Serum lipid levels of total cholesterol (TC), high‐density lipoprotein–cholesterol (HDL‐C), low‐density lipoprotein–cholesterol (LDL‐C) and triglyceride (TG) were all significantly higher in LH rats than in LN rats. Moreover, serum visfatin levels were higher in LH rats than in LN rats and were positively correlated with bodyweight, fat tissue weight and serum lipid levels (i.e. TC, HDL‐C, LDL‐C, TG and the TG:HDL‐C ratio). There were no significant differences in serum glucose and insulin concentrations, or in the whole‐body insulin resistance index, between LN and LH rats, and serum visfatin levels were not correlated with any of these parameters. Visfatin expression in visceral fat tissue, but not in subcutaneous fat tissue, was markedly upregulated in LH compared with LN rats. 4. In conclusion, the results of the present study demonstrate that serum visfatin levels in LH rats are elevated and are associated with lipid metabolic abnormalities.     

20-Hydroxyeicosatetraenoic acid and renal function in Lyon hypertensive rats.

To evaluate the contribution of cytochrome P450 (CYP450) metabolites of arachidonic acid in the increased renal vascular resistance and blunted pressure-natriuresis response exhibited by Lyon hypertensive (LH) rats, the effects of an intrarenal infusion of 17-octadecynoic acid (3 microM), an inhibitor of the formation of epoxyeicosatrienoic and 20-hydroxyeicosatetraenoic acids, were compared in 8-week-old LH and low blood pressure (LL) control rats. 17-Octadecynoic acid failed to affect renal function in LL rats. In contrast, it reduced renal vascular resistance and shifted the pressure-natriuresis relationship to lower pressures in LH rats. Blockade of thromboxane-endoperoxide (TP) receptors with GR 32191B prevented the renal vasodilator response to 17-octadecynoic acid but not its natriuretic action. Miconazole (1 microM), an inhibitor of epoxygenase activity, had no effect on renal function in LH rats. These results indicate that CYP450 metabolites of arachidonic acid, likely 20-hydroxyeicosatetraenoic acid, contribute to the resetting of the pressure-natriuresis relation in LH rats and that the renal vasoconstrictor effects of 20-hydroxyeicosatetraenoic acid in LH rats may be related to activation of TP receptors.     

Effects of fetal and neonatal renin-angiotensin system blockade in Lyon hypertensive rats

It has been shown that a brief period of angiotensin-converting enzyme (ACE) inhibition in growing spontaneously hypertensive rats (SHRs) induces long-term decrease of the blood pressure (BP) level. This study assessed whether persistent effects of ACE inhibition could be disclosed in Lyon genetically hypertensive (LH) rats treated from conception to age 3 weeks. ACE inhibition was obtained with captopril (100 mg/kg/24 h in the drinking water of the breeders) because this compound crosses the placental barrier. For each of the six treated pairs, the first litter was discarded, the second served as control, whereas the third and the fourth were obtained during captopril treatment. Six other pairs remained untreated. Aortic BP was beat-to-beat recorded in freely moving 14-week-old rats. It was observed that captopril reduced the number of newborns (42 in the second vs. 17 rats in the third litter of six LH pairs). BP and left ventricle weight did not differ between control and treated animals. It is concluded that, unlike SHRs, in LH rats, ACE inhibition is devoid of persistent effects on BP after cessation of the treatment.     

The susceptibility of ventricular arrhythmia to aconitine in conscious Lyon hypertensive rats

AIM: The present work was designed to investigate the relationship between hemodynamic parameters and the susceptibility of ventricular arrhythmia to aconitine in conscious Lyon hypertensive rats (LH). METHODS: Male LH and Lyon low blood pressure rats (LL) were used. After the determination of baroreflex sensitivity (BRS), ventricular arrhythmia was induced by aconitine infusion in conscious rats. Blood pressure (BP) was recorded during the period of infusion. RESULTS: Compared with the LL rats, the LH rats possessed significantly higher BP, blood pressure variability and lower BRS. The threshold of aconitine required for ventricular fibrillation and cardiac arrest in the LH rats were significantly lower than those in the LL rats. It was found that all the hemodynamic parameters studied were not correlated with the threshold of aconitine required for arrhythmia, with the exception of BRS, which was positively related to the threshold of aconitine required for ventricular premature beat. CONCLUSION: The LH rats possessed greater susceptibility to aconitine-induced ventricular arrhythmias when compared to the LL rats. This greater susceptibility could not be attributed to any one of the hemodynamic parameters alone studied in the LH rats. It is proposed that various hypertension-associated abnormalities, including the abnormal hemodynamics, may co-contribute to this vulnerability to ventricular arrhythmias.     

Durable improvement of renal function after perindopril withdrawal in Lyon hypertensive rats

To determine whether the persistent antihypertensive effect observed after withdrawal of angiotensin-converting enzyme inhibition was specific and whether it was associated with durable improvement of renal function. Lyon hypertensive (LH) rats were treated from 3 and 12 weeks of age with perindopril at the dose of 0.4 (P0.4) or 1.5 (P1.5) mg·kg(-1)·d(-1), with double (hydralazine 75 mg·kg(-1)·d(-1) and hydrochlorothiazide 15 mg·kg(-1)·d(-1)), or triple antihypertensive therapy (reserpine 0.75 mg·kg(-1)·d(-1) was added). Blood pressure (BP) was recorded by telemetry, and renal functions were evaluated in freely moving rats. Despite similar BP reduction after perindopril withdrawal in P0.4 and P1.5 groups, greater decrease in proteinuria was observed in P1.5 group. Double or triple therapy prevented the hypertension of LH rats, but without any persistent antihypertensive or antiproteinuric effect after its withdrawal. Salt load elicited an increase in BP that was similar in untreated LH and both perindopril-pretreated groups (+30 mm Hg), but more pronounced in double therapy–pretreated or triple therapy–pretreated groups (+50 mm Hg). The pressure–natriuresis curve shifted to lower BP levels in P0.4 and P1.5 groups, whereas it was blunted in double and triple therapy groups. Angiotensin-converting enzyme inhibition has a durable renoprotection after treatment withdrawal that is independent of BP lowing.     

Decreased cAMP content of the hypothalamus of genetically hypertensive rats

Summary In genetically hypertensive rats an altered catecholamine content of hypothalamic structures has been reported. In the present study cAMP was estimated in the hypothalamus and cortex of genetically hypertensive rats and compared with normotensive controls of the same strain. It is shown, that the cAMP content of the hypothalamus of the hypertensive animals was decreased to about 60% of control values, whereas there was no difference of the cAMP content in the cortical regions of the same animals. These results indicate an alteration of the adenyl cyclase-cAMP-phosphodiesterase system in hypothalamic structures of genetically hypertensive rats.Supported by the Deutsche Forschungsgemeinschaft     

Cardiovascular effects of prazosin in normotensive and genetically hypertensive rats

1. The cardiovascular effects of prazosin, a new antihypertensive drug, were studied in normotensive and genetically hypertensive rats. 2. Prazosin, infused intra-arterially, lowered vascular resistance in the blood-perfused rat hind limb. This effect was dependent on the presence of intact sympathetic innervation to the limb; no direct vasodilatation was demonstrated. In this preparation prazosin infusion reduced vasoconstrictor responses to noradrenaline. 3. In the saline-perfused rat mesenteric artery preparation prazosin reduced responses to noradrenaline and sympathetic nerve stimulation but not those to serotonin and vasopressin. Prazosin was more potent than phentolamine, on a molar basis, in reducing the vasoconstrictor effects of noradrenaline. 4. A comparison of the effects of prazosin injected intravenously and into a lateral cerebral ventricle failed to show any central action of the drug on blood pressure. Experiments using the donor blood-perfused, vascularly isolated rat hind limb preparation confirmed that the sympatholytic effect of prazosin occurred within the limb itself.     

Function of renal angiotensin AT2 receptors is not enhanced in Lyon hypertensive rats

1. Because we previously observed that angiotensin AT2 receptor stimulation decreased pressure-natriuresis, in the present study we examined the possible involvement of these receptors in altered sodium excretion shown by Lyon hypertensive (LH) rats. 2. In 9-week-old male anaesthetized LH rats and normotensive (LL) controls pretreated with an angiotensin-converting enzyme inhibitor (quinapril; 10 mg/kg) and an AT1 receptor antagonist (losartan; 10 mg/kg), angiotensin (Ang) II was infused (30 ng/kg per min) to stimulate AT2 receptors. In other groups of rats, an AT2 receptor antagonist (PD123319; 50 micro g/kg per min) was added before AngII infusion. 3. During AT2 receptor stimulation, LH differed from LL rats by significantly reduced renal blood flow (RBF), glomerular filtration rate and pressure diuresis and natriuresis. The addition of PD123319 did not change total RBF, whereas it did increase pressure diuresis and natriuresis in both strains. However, the effects of PD123319 were less marked in LH rats than in LL rats. 4. These findings confirm that, under the present experimental conditions, AT2 receptors are antinatriuretic and are not of greater functional importance in hypertensive animals of the Lyon strain.     

Cardiovascular effects of chronic nitric oxide synthase inhibition in genetically hypertensive rats

1. The possible role of an endothelial defect in the hypertension of the New Zealand genetically hypertensive (GH) rat strain was assessed by examining cardiovascular responses to the nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) and the endothelium-dependent depressor agent acetylcholine (ACh). The vascular sensitivity of the hindquarter to nitric oxide (NO) was examined using the NO donor sodium nitroprusside (SNP). 2. NG-Nitro-L-arginine methyl ester (10 mg/kg per day in drinking water) was given to GH and normotensive (N) rats from age 7-9 weeks, with GH and N untreated control groups. Systolic blood pressure (tail-cuff) was monitored weekly from age 5-9 weeks. At age 9 weeks, pressure responses to various vasoactive agents were measured in vivo and in the rat isolated hindquarter. Left ventricular (LV) mass was measured at the time of death. 3. NG-Nitro-L-arginine methyl ester induced a greater hypertensive effect in GH (P < 0.001) compared with N (P < 0.05) rats and caused a significant increase in hindquarter perfusion pressure in GH rats only (P < 0.01). 4. Genetically hypertensive rats had LV hypertrophy that was exacerbated by L-NAME (P < 0.01). Left ventricular hypertrophy was not induced by L-NAME in N rats. 5. The normalized response to ACh did not differ between GH and N control rats and was unaffected by L-NAME treatment in vivo and in vitro except at the highest ACh dose (3 micrograms/kg) in GH hindquarters (P < 0.01). The response to SNP was similar in GH and N hindquarters and enhanced by L-NAME in GH (0.1 microgram; P < 0.05) and N rats (0.01 microgram, P < 0.01; 0.01 microgram, P < 0.001). 6. These results suggest that the L-arginine/NO system is not deficient in GH rats and that endothelial function in the GH hindquarter is preserved. They confirm that NO is involved in mediating blood pressure in GH and N rats and raise the possibility that a non-NO-mediated mechanism may underlie ACh-induced vasodilation in GH and N.     

Remodelling of mesenteric arteries in genetically hypertensive rats cross-fostered from birth to normotensive Wistar rats

1. The effects of cross-fostering genetically hypertensive (GH) with normotensive (N) Wistar rats on the structure of mesenteric resistance arteries (MRA) in young (6 week old) and adult (18 week old) rats were investigated to see whether the abnormal remodelling known to exist in GH rats could be prevented by changing the maternal environment. 2. Genetically hypertensive and normotensive rat pups were reared either by their natural mothers or a foster mother of the opposite strain (NX and GHX) with fostering done within 24 h of birth. 3. Blood pressure (BP) was measured from age 6-18 weeks; at 6 and 18 weeks MRA structure was assessed. 4. At the time of death, intra-arterial mean BP was measured (via the femoral artery), after which MRA were fixed by perfusion (at the systolic BP of the rat) via the abdominal aorta, first with 75% Tyrode's solution containing heparin and papaverine, followed by 2% glutaraldehyde in 75% Tyrode's solution. Arteries were dissected out, processed and embedded in Technovit (Heraeus Kulzer, Werheim, Germany) and serial sections were cut and stained with Giemsa. 5. Stereological techniques were used to determine media width, lumen diameter and medial cross-sectional area (CSA); in addition, the ratio of media width to lumen diameter was calculated. Smooth muscle cell density was also calculated. 6. In MRA from 6-week-old rats, GH rats compared with N rats had increased media width and CSA and an increased ratio of media width to lumen diameter. 7. There were no significant changes in structure in the GHX group compared with GH rats. The NX group compared with N rats had increased media width and CSA and lumen diameter, but no change in the ratio of media width to lumen diameter. Smooth muscle cell density, reduced in GH compared with N rats, was increased (P < 0.001) in the NX group, but not changed in the GHX group compared with GH rats. 8. In 18-week-old GH rats compared with N rats, the MRA had a decreased media width and medial CSA and smaller lumen diameter, but there was no change in the ratio of media width to lumen diameter. 9. In the GHX group compared with GH rats, media width and CSA were reduced; in the NX group compared with N rats, media width was increased, lumen decreased and the ratio of media width to lumen diameter increased. Smooth muscle cell density was increased (P < 0.001) in the GHX group, but not in the NX group. 10. Changing the maternal environment significantly affected BP in GHX and NX groups up to 9-10 weeks of age but, in adult rats, the BP differences were no longer present. Thus, structural changes were seen at 6 weeks of age in MRA from NX rats and also at 18 weeks in GHX and NX rats (even though the BP differences were no longer significant); structural remodelling occurred independently of BP.     

Increased NADPH oxidase activity mediates spontaneous aortic tone in genetically hypertensive rats

NADPH oxidase is critically involved in increased blood pressure, vascular hypertrophy, inflammation and endothelial dysfunction in experimental and clinical hypertension. We hypothesized that NADPH oxidase might also play a role in the development of spontaneous aortic tone in spontaneously hypertensive rats (SHR). Wistar Kyoto rats (WKY) were used as normotensive controls. Tone was recorded under isometric conditions. NADPH oxidase activity was measured by both lucigenin luminescence and dihydroethidium fluorescence. p47phox protein was localized by immunohistochemistry. SHR (but not WKY rat) aortae showed spontaneous tone in the absence of exogenous vasoconstrictors as evidenced by a stronger relaxant effect of Ca2+-free sodium nitroprusside solution. This tone was enhanced in endothelium-denuded arteries and was inhibited by superoxide dismutase, apocynin, diphenylene iodonium and quercetin. Aortic NADPH oxidase activity, measured by both lucigenin luminescence and dihydroethidium fluorescence, was increased in SHR compared with WKY rats. Immunohistochemical analysis revealed a strong increase in p47phox expression in the medial layer in SHR. Taken together, the present results indicate that enhanced NADPH oxidase activity and, hence, NADPH driven O2- production, is involved in the spontaneous aortic tone in SHR. This was associated with an increased expression of p47phox in the medial layer of the aorta.     

Decreased vascular permeability response to substance P in airways of genetically hypertensive rats

1. The inbred genetically hypertensive strain (GH) of the Otago Wistar rat possesses more sensory neurons containing the neuropeptide substance P (SP) than does its genetically related control normotensive strain. 2. As SP contributes to airway inflammation by increasing microvascular permeability, we assessed the extravasation of Evans Blue dye in trachea and main bronchus of anaesthetized GH and control rats, in the presence of endogenous (capsaicin-liberated) or exogenous SP. 3. Following intravenous administration of either capsaicin (75 microg kg(-1)) or SP (3.3 nmol kg(-1)), extravasation of Evans Blue in airways from GH rats was only about 60% of that in airways of control rats. This difference was not gender-specific and responses to capsaicin were abolished by pretreatment with a selective NK1 receptor antagonist SR 140333 (360 nmol kg(-1)). 4. By contrast, the extravasation of dye caused by intravenous 5-hydroxytryptamine (0.5 micromol kg(-1)) was similar in magnitude in both GH and control strains. 5. Falls in systemic arterial blood pressure in response to exogenous SP (0.1-3 nmol kg(-1)) or acetylcholine (0.2-2 nmol kg(-1)) were also very similar between strains, but those in response to capsaicin (75 microg kg(-1)) in the GH rats were about double those in control rats. The hypotensive response to SP was abolished by SR 140333, but that to capsaicin was unaffected. 6. Our results indicate that the increased peripheral innervation density by SP-nerves in GH rats is accompanied by reduced inflammatory responses to SP. This does not involve decreased vasodilator potency of SP and is therefore probably related to altered endothelial responsiveness.     

Catecholamine uptake sites in the rat heart after 6-hydroxydopamine treatment and in a genetically hypertensive strain

Summary Uptake of ³H-NA was measured in hearts from genetically hypertensive and chemically sympathectomised rats. The endogenous cardiac NA content and NA uptake₁ were depressed in genetically hypertensive animals, although NA uptake₂ per heart was normal. After treatment of normal rats with 6-hydroxydopamine (chemical sympathectomy) endogenous cardiac NA, uptake₁ and uptake₂ were markedly lowered. Uptake₂ inhibition by corticosterone varied in the two conditions tested, whereas no change in the potency of clonidine was detected in the genetically hypertensive rats.     

-Adrenoceptors of aortae from genetically hypertensive rats: reaction with 2-halogenoethylamines

The responses of aortic rings from adult spontaneously hypertensive and Carworth normotensive Wistar rats to noradrenaline were compared. The former developed slightly more tension at low, and markedly less tension at high concentrations (> 10^?8m ) of noradrenaline. The α-adrenoceptors of these tissues were probed using 2-halogenoethylamines to compare quantitatively α-adrenoceptors in normotensive and hypertensive tissue. The kinetics of the recovery of response to noradrenaline from the short-lived antagonism produced by NN-dimethyl-2-bromo-2-phenylethylamine were identical in aortae from both strains of rat. The rate of recovery of response was the same as the rate of loss of tritium from the tissues, suggesting the absence of spare receptors in rat aorta. An estimation of the number of α-adrenoceptors in aortic tissue did not show a difference between normotensive and hypertensive tissues. Evidence is presented that 2-halogenoethylamines react at two kinetically distinguishable sites in rat aorta. The rates of return of response to noradrenaline after receptor blockade with N-(2-bromoethyl)-N-ethyl-N-1-naphthylmethylamine (SY-28) in tissues pretreated with NN-dimethyl-2-bromo-2-phenylethylamine were the same in aortic smooth muscle from both strains of rat. No significant difference was found in the reactions of 2-halogenoethylamine α-adrenoceptor antagonists in aortic tissue from either strain of rat.