Vascular wall renin in spontaneously hypertensive rats. Potential relevance to hypertension maintenance and antihypertensive effect of captopril

Relationships among systolic blood pressure (SBP), plasma renin activity (PRA), arterial renin concentrations (ARC), and venous renin concentrations (VRC) were examined in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats before and after treatment with captopril. The ARC was elevated in SHR relative to WKY whereas VRC was not. Similarly, ARC was related to SBP (r = 0.69, p less than 0.01) whereas PRA was not (r = 0.04). Captopril (100 mg/kg daily by mouth for 8 days) decreased blood pressure significantly in both SHR and WKY. PRA as well as ARC and VRC were all increased by captopril. Bilateral nephrectomy virtually eliminated PRA but ARC was not significantly reduced over a 24-hour period. Bilateral nephrectomy also markedly attenuated the acute antihypertensive effects of captopril in SHR; however, a modest effect was still apparent. It is suggested that ARC in SHR, being higher than in WKY, may play a role in the genesis or maintenance of hypertension in this model. Furthermore, the effects of captopril in both intact and nephrectomized SHR may be related to the ability of captopril to inhibit the vascular formation of angiotensin II. Finally, vascular renin is probably not renal in origin and responds to typical feedback inhibition as unmasked by captopril administration.     

The Effect of Captopril Treatment and Its Withdrawal on the Gene Expression of the Renin-Angiotensin System

The mRN A expression for renin, angiotensinogen and angiotensin converting enzyme (ACE) was determined in the kidneys and livers from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) during chronic treatment with captopril and following its withdrawal. Chronic captopril treatment was associated with a dramatic rise in renin mRNA in the kidney and an elevation in mRNA for ACE in the liver. The release from captopril treatment was associated with a reversal of the increase in kidney renin mRNA but no reversal of the sustained elevation of ACE m RNA in the liver. In situ hybridisation revealed a localisation of renin to the area of the juxtaglomerular apparatus in the kidneys from untreated animals, but recruitment of vascular sites of renin expression in kidneys from captopril-treated animals. In kidneys from released animals, renin mRNA expression was once again confined to the juxtaglomerular apparatus. ACE mRNA was expressed in hepatocytes throughout the livers from animals in all treatment groups. The results highlight a differential effect of captopril withdrawal upon the gene expression of the components of the renin-angiotensin system in kidney and liver.     

Blood pressure responses of conscious normotensive and spontaneously hypertensive rats to intracerebroventricular and peripheral administration of captopril

In conscious spontaneously hypertensive rats, intraxerebroventricular injection of captopril (2 mg/kg) resulted in a rapid hypotensive response that lasted several hours. The same dose given by intracerebroventricular injection had no significant effect on blood pressure (BP) of normotensive Wistar-Kyoto (WK) rats over 7 hours. There was no significant change in BP of conscious spontaneously hpertensive rats (SHR) in response to intracerebroventricular injection of vehicle and only a transitory fall in BP in response to intravenous injection of captopril (2 mg/kg). There was no significant differences between plasma renin activity (PRA) of conscious normotensive WKY rats and the PRA of SHR. These results suggest biochemical differences between the brains of SHR and normotensive WKY control rats. These differences could involve the brain renin-angiotensin system or other neuropeptides.     

Dietary sodium restriction and the renin-angiotensin system in young spontaneously hypertensive rats

Severe dietary sodium restriction initiated early in life is required to prevent development of hypertension in spontaneously hypertensive rats (SHR). Moderate sodium restriction does not affect hypertension development. This relative insensitivity to sodium restriction may be related to compensatory increases in other pressor mechanisms, specifically the renin-angiotensin system. We evaluated this possibility by measuring plasma renin activity, the blood pressure response to the angiotensin converting enzyme inhibitor captopril as well as blood pressure responsiveness to exogenous angiotensin II in SHR and Wistar-Kyoto rats (WKY) raised from birth until 6 or 16 weeks on control (101 mumol Na+/g food), moderate (26 mumol/g) or two severe (17 or 9 mumol/g) sodium-restricted diets. Moderate sodium restriction did not affect development of hypertension, but also did not cause significant increases in PRA or the blood pressure response to captopril in SHR or WKY. In contrast, severe sodium restriction blunted or prevented the development of hypertension in SHR and was associated with (1) marked increases in plasma renin activity (2) increased maintenance of blood pressure by the renin-angiotensin system (as assessed by captopril), and (3) a marked decrease in the blood pressure response to angiotensin II. We conclude that the relative insensitivity of hypertension development in SHR to dietary sodium restriction does not relate to a compensatory increase in the activity of the renin-angiotensin system. The moderate sodium restriction employed (26 mumol/g) may rather represent the lower end of the normal range.     

Brain Renin Angiotensin System Contributes to the Salt-Induced Enhancement of Hypertension in Shr

The study was performed to determine whether the brain renin angiotensin system may contribute to the acceleration in hypertension following long-term salt loading in spontaneously hypertensive rats(SHR). Five weeks old SHR and normotensive Wistar-Kyoto(WKY) were given 1% NaCl solution or plain tap water as drinking for 7 weeks. The salt treatment exaggerated the development of hypertension in SHR, but did not change the blood pressure (BP) in WKY. The hypotensive actions of intracerebroventricular(ICV) captopril was greater in SHR treated with salt than in those without treatment, whereas ICV All increased BP to a similar degree between salt and control SHR. In WKY, the effects of ICV captopril and All were not altered by the salt loading. The increases in BP induced by ICV hypertonic saline were not different between the rats with and without saline drinking in either SHR or WKY. The intravenous (IV) hexamethonium led to a greater fall in BP in SHR treated with saline than in those without salt, while it tended to produce a smaller decrease in BP in WKY with salt overload than in those without loading. Both duration and magnitude of the depressor effects of IV captopril were reduced by the chronic saline treatment in SHR. The plasma renin concentration (PRC) in both SHR and WKY was significantly suppressed by the salt load. The present results suggest that long-term salt overload may result in the enhanced activity of brain renin angiotensin system, which could be responsible for the exaggerated development of hypertension in SHR. Our observations also provide further evidence that the central renin angiotensin system is independent of the peripheral system.     

Corticosteroid Modulation of the Renin System and Blood Pressure in the Spontaneously Hypertensive Rat

This study examines the role of gluco- and mineralcorticoids in the regulation of the renin-angiotensin system and blood pressure in the spontaneously hypertensive rat (SHR). Effects of adrenalectomy and selective treatment with either aldosterone (30 μg/kg/day) or dexamethasone (60 μg/kg/day) on plasma renin substrate, active renin (PRA), total renin and blood pressure were studied in 10 week old SHR and control WKY rats. Systolic blood pressure was moderately lower in adrenalectomized rats (129±2 mm Hg vs 137±4 mm Hg in control WKY and 145±4 mm Hg vs 160±3 mm Hg in control SHR) but could be restored to the control range by aldosterone. Dexamethasone repletion induced substantial increments of systolic blood pressure to comparable levels in both species (202±8 mm Hg in WKY and 192±6 mm Hg in SHR). Renin substrate was markedly lower in adrenalectomized, saline repleted rats. This could be reversed by dexamethasone in both species and by aldosterone in WKY rats only. Both PRA and total renin were higher (p<0.01) in the adrenalectomized, saline repleted state. This increase was not observed in aldosterone repleted rats. However, dexamethasone inhibited the adrenalectomy associated increase of PRA and total renin in SHR but not in WKY rats. Differences in blood pressure between SHR and WKY persist even in adrenalectomized state despite comparable stimulation of the renin system. Conversely, while blood pressure of both species responds similarly to selective corticosteroids therapy, the response of the renin-angiotensin system in SHR and WKY rats is distinct. Therefore factors other than the adrenal gland and the renin system must be involved in the determination of the high blood pressure in SHR.     

Increased PI3-kinase in presympathetic brain areas of the spontaneously hypertensive rat

Existing evidence led us to hypothesize that increases in p85alpha, a regulatory subunit of PI3-kinase, in presympathetic brain areas contribute to hypertension. PI3-kinase p85alpha, p110alpha, and p110delta mRNA was 1.5- to 2-fold higher in the paraventricular nucleus (PVN) of spontaneously hypertensive rats (SHR) compared with their controls, Wistar Kyoto rats (WKY). The increase in p85alpha/p110delta was attenuated in SHR treated with captopril, an angiotensin (Ang)-converting enzyme inhibitor, from in utero to 6 months of age. In the rostral ventrolateral medulla (RVLM), p110delta mRNA was approximately 2-fold higher in SHR than in WKY. Moreover, the increases in mRNA were associated with higher PI3-kinase activity in both nuclei. The functional relevance was studied in neuronal cultures because SHR neurons reflect the augmented p85alpha mRNA and PI3-kinase activity. Expression of a p85 dominant-negative mutant decreased norepinephrine (NE) transporter mRNA and [3H]NE uptake by approximately 60% selectively in SHR neurons. In summary, increased p85alpha/p110delta expression in the PVN and RVLM is associated with increased PI3-kinase activity in the SHR. Furthermore, normalized PI3-kinase p85alpha/p110delta expression within the PVN might contribute to the overall effect of captopril, perhaps attributable to a consequent decrease in NE availability.     

The Mechanism of Distinct Diurnal Variations of Renin-Angiotensin System in Aorta and Heart of Spontaneously Hypertensive Rats

Diurnal variations in plasminogen activator inhibitor-1 mRNA expression are different between the spontaneously hypertensive rats (SHRs) and the Wistar-Kyoto (WKY) rats, and between the aorta and the heart. To elucidate the mechanisms, we examined diurnal changes in the circulating renin-angiotensin system in the SHR and WKY rats. Diurnal variations in plasma renin activity (PRA), plasma angiotensin I, and aldosterone concentrations were similar between the SHR and WKY rats. On the other hand, plasma angiotensin II (Ang II) concentration in the SHR was lower than that in the WKY rats at most time points, but increased to the level of the WKY rats in the late light phase. Treatment with AT1 receptor antagonist candesartan increased plasma Ang II concentration except at ZT 8 and lessened its diurnal variation in the SHR. At the peak in plasma Ang II in the SHR, Ang II regulated genes such as transforming growth factor-β1 and p22phox were upregulated in the aorta. On the other hand, these genes were upregulated throughout the day in the heart of SHR. Candesartan treatment increased AT1a receptor mRNA expression in the heart but not in the aorta of SHR. These findings suggest that an AT1 receptor-mediated mechanism might cause a surge in plasma Ang II concentration at the late light phase in the SHR. Homologous down-regulation of AT1a receptor by Ang II may dampen the effect of a surge in plasma Ang II concentration in the heart of SHR.     

Isolation of preferentially expressed genes in the kidneys of hypertensive rats

By differential hybridization, three complementary DNAs designated as S3, S2, and SA were isolated, and the corresponding messenger RNAs (mRNAs) were differentially expressed between the kidneys of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. S3 is identical to cytochrome P450 IV A2. SA encoded a protein of 546 amino acid residues, and its carboxyl terminal region had a slight homology to luciferase. No homologous sequence has been reported in S2 sequences. S3 mRNA was about four times more abundantly expressed in the kidneys of 28-day-old SHR than in those of age-matched WKY rats, but there was no difference at age 16 weeks. A low NaCl diet positively modulated the expression of the S3 gene. S2 mRNA was almost undetectable in the kidneys of 28-day-old WKY rats but was clearly detected in those of age-matched SHR. The expression level of S2 mRNA in the livers of 16-week-old SHR was about five times higher than that of age-matched WKY rats. The expression of S2 mRNA in the livers was modulated by dietary NaCl and captopril. SA mRNA was more than 10 times more abundantly expressed in the kidneys of SHR than in those of WKY rats from age 4 weeks. With the administration of captopril, the expressions of SA mRNA in the livers of SHR were positively modulated. Because these three genes are not only differentially expressed between SHR and WKY rats but also related to sodium metabolism or blood pressure control, the identification of these genes may provide important probes to examine the mechanisms of hypertension.     

Effect of stress on the control of renin release in spontaneously hypertensive rats

Recent reports suggest that centrally induced increases in sympathetic outflow to the kidney have the potential to enhance the sensitivity of pressure-dependent renin release. In the present study, the possibility was investigated that spontaneously hypertensive rats (SHR), which are thought to have increased tonic sympathetic outflow to the kidney, exhibit enhanced renin release in response to reduced renal perfusion pressure. The increase in plasma renin activity in response to a graded suprarenal aortic constriction was determined in conscious young (6-9 weeks of age) and adult (14-16 weeks of age) SHR and age-matched Wistar-Kyoto (WKY) control rats. Under conditions of relatively little stress, the renin response to reduced renal perfusion pressure was not enhanced in young or adult SHR when compared with age-matched WKY rats. That is, this regulatory mechanism was not "reset" in the hypertensive animals. When challenged with an acute stress (air to the face) both age groups of SHR exhibited a significantly enhanced response. Neither age group of WKY rats was affected by the acute air stress. These data suggest that, under unstressed conditions, pressure-dependent renin release probably does not contribute to the elevation of arterial pressure in the SHR. However, under stressful conditions, the contribution of this system may be significant. Intermittent increases in sympathetic outflow to the kidney that can occur in the SHR in response to daily stresses have the potential to render it more sensitive to spontaneous reductions in perfusion pressure. Occasional exaggerated release of renin could then contribute to the hypertensive process.     

Interaction between lifetime captopril treatment and NaCI-sensitive hypertension in spontaneously hypertensive rats and Wistar-Kyoto rats.

DESIGN: Previous studies that were based on daytime arterial pressure recordings indicate that lifetime treatment with captopril exacerbates the hypertensive response to a high NaCl diet in spontaneously hypertensive rats (SHR) but has no such effect in normotensive Wistar-Kyoto (WKY) rats. The present study used 24-h recording methods to examine the hypothesis that during the normal waking hours of rats (night-time) the hypertensive response to a high NaCl diet is exacerbated in SHR and induced in WKY rats treated with lifetime captopril. METHODS: SHR and WKY rats were (1) untreated, (2), lifetime captopril treated or (3) lifetime captopril treated but removed from the treatment 2 weeks prior to exposure to a high (8%) NaCl diet RESULTS: Compared to untreated SHR, in SHR that were continuously treated with captopril, the high NaCl diet caused a more rapid and greater rise in arterial pressure. Discontinuation of the captopril treatment did not significantly diminish this NaCl-sensitivity. In untreated WKY rats, the high NaCl diet did not alter mean arterial pressure, but in the lifetime captopril-treated WKY rats the high NaCl diet induced a rapid rise in arterial pressure. In WKY rats, discontinuation of the lifetime captopril treatment did not diminish this NaCl-induced rise in arterial pressure, even though baseline mean arterial pressure in this group is similar to that in untreated WKY rats. CONCLUSIONS: Lifetime captopril treatment accelerates the hypertensive response to a high NaCl diet in SHR, and it induces a similar response in WKY rats. In both strains, the lifetime captopril treatment causes a change in the response that is not dependent on concurrent administration of the drug. This finding further suggests that lifetime captopril treatment causes a long-term resetting of cardiovascular response mechanisms.     

Expression of the Na+/H+ exchanger regulatory protein family in genetically hypertensive rats

OBJECTIVE: To examine a possible involvement of a regulatory protein of Na+/H+ exchanger (NHE) in the increased renal NHE activity in spontaneously hypertensive rats (SHR), we investigated mRNA expression of inhibitory members of the NHE regulatory protein family, NHERF1 and NHERF2, in the kidney. DESIGN: Prehypertensive 4-week-old and hypertensive 11-week-old SHR and age-matched Wistar-Kyoto (WKY) rats were used to determine the changes in NHE activity and NHERF family expression in the kidney. Dahl salt sensitive (DS) and resistant rats were also used to examine whether these changes are specific for SHR. METHODS: mRNA expression in the kidney was quantified by RNase protection assay. The NHE activity in primary cultured proximal tubular cells was measured as Na-dependent pHi recovery rate by the NH4Cl prepulse technique with 2'7'-bis-(2-carboxyethyl)-5.6-carboxyfluorescein (BCECF). RESULTS: NHERF1 mRNA expression was significantly decreased in both prehypertensive and hypertensive SHR in comparison with age-matched WKY rats, whereas NHERF2 mRNA expression was significantly increased in SHR only in the hypertensive period. Antihypertensive treatment did not abolish these changes seen in control SHR. On the other hand, hypertensive DS rats fed a high-salt diet showed significant decreases in NHE activity and NHE3 mRNA expression compared with normotensive DS rats fed a low-salt diet, without significant changes in NHERF1 and NHERF2 mRNA expression. CONCLUSION: These results suggest that decreased expression of NHERF1 may be related to the enhanced NHE activity in SHR and that these changes are likely to be genetically determined, whereas the increased NHERF2 expression may be induced as a compensatory mechanism.     

Cardiac hypertrophy and antihypertensive therapy.

Biochemical (myocardial DNA, RNA, and hydroxyproline) and humoral (plasma [PRA] and kidney [KRA] renin activity) factors were determined in spontaneously hypertensive rats (SHR) and normotensive Wistar controls (NR) before and following treatment with minoxidil or propranolol. Minoxidil (150 mg.litre-1 drinking water) effectively controlled blood pressure (17.3 kPa vs 24.9 kPa [130 mmHg vs 187 mmHg], P less than 0.001) despite marked and sustained increases in both PRA and KRA ventricular weight which were not reduced and myocardial DNA, RNA, and hyperdroxyproline which were increased by minoxidil (P less than 0.01). In contrast propranolol did not reduce blood pressure in SHR but ventricular weight was reduced somewhat (3.1 +/- 0.4 mg.g-1 vs 3.4 +/- 0.09 mg.g-1, P less than 0.05); in both SHR and NR, KRA, and PRA were lowered by pranolol. Methyldopa which controlled blood pressure and lowered PRA led to a reversal of hypertrophy. Thus, although blood pressure control is obviously important for reversing cardiac hypertrophy, it may not be the sole factor for the development and reversal of cardiac hypertrophy.     

Plasma renin activity determined by two different methods in spontaneously hypertensive rats

As a discrepancy exists between two reports from our laboratory on plasma renin activity (PRA) in spontaneously hypertensive rats (SHR), we undertook to examine the reasons for this discrepancy. PRA in SHR at 15 weeks of age was determined by two different methods, utilizing the procedures of Boucher et al or of Haber et al. For normotensive controls, Donryu (DON) and Wistar-Kyoto (WKY) rats were used. By Boucher's method, PRA in SHR was significantly lower than in DON, but did not differ from WKY. By Haber's method, PRA in SHR did not significantly differ from DON or WKY. However, the value in DON was significantly higher than in WKY. The present study basically confirmed our previous results, concluding that PRA in SHR at 15 weeks of age is within the normal range. A previous suggestion that PRA in SHR is suppressed as a result of blood pressure elevation must be revised, because it was derived from results considering only the DON strain as the control.     

Significance of brain renin for pathogenesis of hypertension: effect of antihypertensive drugs on active and inactive renins in the brain of spontaneously hypertensive rat

Relationship between blood pressure and brain renin was studied in four groups of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY); as controls (n = 5), administered captopril (n = 5), trichlormethiazide (n = 5) and atenolol (n = 5). 1) Inactive renin in the hypothalamus of captopril-administered SHR was significantly lower than that of control SHR and captopril-administered WKY. On the other hand, active renin in the hypothalamus, thalamus and striatum of captopril-administered SHR was significantly lower than that of control SHR and captopril-administered WKY. 2) Inactive renin in the hypothalamus of trichlormethiazide administered SHR was significantly lower than that of control SHR and trichlormethiazide-administered WKY. On the other hand, active renin in the hypothalamus, thalamus and midbrain of trichlormethiazide-administered SHR was significantly lower than that of control SHR and trichlormethiazide-administered WKY. 3) Inactive renin in the hypothalamus of atenolol-administered SHR was significantly lower than that of control SHR and atenolol-administered WKY. On the other hand, active renin in the hypothalamus, thalamus and midbrain of atenolol-administered SHR was significantly lower than that of control SHR and atenolol-administered WKY. These results suggest that the production and/or activation of renin in the hypothalamus, thalamus, midbrain and striatum play an important role in the initiation and/or development of hypertension of SHR by the local generation of angiotensin II.     

Central and Peripheral Contributions of the Renin-Angiotensin System in Two Models Experimental Hypertension in Rats

To examine the relationships between the central and pepirheral renin angiotensin system in normotensive Wistar Kyoto (WKY) rats, two-kidney, one-clip Goldblatt renovascular hypertension (RVH), spontaneously hypertensive rats (SHR), SQ 14225 (captopril) was administered intraventricularly (IVT and intravenously (IV) in the alternative manner and there combination. Also, the effects of IVT captopril on the peripheral sympathetic nervous system were evaluated using an intravenous injection of prazosin.

IVT captopril induced a significant reduction of blood pressure in both types hypertensive rats but not in normotensive rats. Greater depressor effects of IV captopril not IV prazosin following IVT captopril were observed in RVH. compared to those in SHR.

These results indicate that the pressor action of the brain renin angiotensin system is closely related with the sympathetic nervous system in hypertensive conditions and that these functions are independent from the peripheral renin angiotensin system. Furthermore. their roles were different in different types of experimental hypertension in rats.     

Studies on the Mechanism of the Enhancement of the Antihypertensive Activity of Captopril by A Diuretic in Spontaneously Hypertensive Rats

Captopril (30 mg/kg/day orally for two days) in spontaneously hypertensive rats (SHR) inhibited serum angiotensin converting enzyme (ACE) activity 92.3%; increased plasma renin activity (PRA) 18-fold and reduced mean arterial blood pressure (MABP) 19 mm Hg. Hydrochlorothiazide (HCTZ) (100 mg/kg-day 1; 10 mg/kg-day 2, orally) increased PRA 3-fold but did not affect serum ACE or MABP. HCTZ plus captopril inhibited serum ACE 95.2%; increased PRA 38-fold and reduced MABP 47.5 mm Hg. Captopril or HCTZ plus captopril did not alter the responses of isolated aortic strips to norepinephrine (NE), serotonin, angiotensin II (AII) or isoproterenol. Pressor responses of conscious SHR to AII and NE were unaltered by captopril or HCTZ plus captopril although the bradykinin-induced depressor responses were significantly but equally potentiated. These results suggest that the potentiating effect of HCTZ is due to some mechanism that shifts the animal's blood pressure maintenance system to a renin-dependent state and is not due to changes in vascular reactivity.