Persistent effects on blood pressure and renal haemodynamics following chronic angiotensin converting enzyme inhibition with perindopril

Spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats aged 4 and 16 weeks were given an acute oral dose of either Perindopril (3 mg/kg) or vehicle. Direct blood pressure (BP), glomerular filtration rate (GFR) and renal blood flow (RBF) were measured, and renal vascular resistance (RVR) calculated. GFR and RBF were lower in vehicle-treated SHR than WKY at 4 weeks of age, but were not different at 16 weeks. Acute Perindopril increased GFR and RBF and reduced RVR in both strains at both 4 and 16 weeks. Total body sodium, sodium intake and blood pressure were measured in SHR and WKY from 1 to 28 weeks of age. Rats of both strains were treated daily between 4 and 16 weeks of age with either Perindopril (3 mg/kg per day) or vehicle. Chronic Perindopril treatment prevented the development of hypertension in the SHR. From 16 to 28 weeks of age, after stopping Perindopril, BP rose slowly in SHR, but remained lower than vehicle-treated SHR. No changes in total body sodium occurred during Perindopril treatment. GFR and RBF were measured in SHR and WKY chronically treated with either Perindopril or vehicle, 3 days or 12 weeks after stopping treatment. In WKY, GFR and RBF were not different between Perindopril-treated and untreated rats at either measurement. In SHR, GFR and RBF remained significantly higher in rats previously treated with Perindopril at both ages. These findings suggest that renal haemodynamic abnormalities may be important in the initiation of hypertension in the SHR. These renal circulatory abnormalities and the hypertension of the SHR depend, at least in part, on intact converting enzyme activity, yet appear to be independent of abnormalities of total body sodium. At a later age, hypertension seems to develop independently of renal vascular abnormalities.     

Nitric oxide inhibition accelerates hypertension and induces perivascular inflammation in rats

1. Inflammatory changes in peripheral arteries have been reported in animal models of hypertension. Whether they occur in cerebral arteries (CA) with hypertension induced by deprivation of endogenous nitric oxide (NO) remains unknown. 2. In the present study, we compared the arteriolar injury score (AIS) and perivascular inflammation in CA between hypertensive and normotensive rats following NO deprivation with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Five-week-old male spontaneously hypertensive rats (SHR) and Wistar -Kyoto (WKY) rats were fed with L-NAME (1 mg/mL) for 4 weeks. 3. Nitric oxide deprivation resulted in time-dependent elevations in tail-cuff pressure (representing systolic blood pressure (SBP)) in both SHR and WKY rats. The magnitude of increase in SBP was larger in SHR (+81.0 +/- 3.2 vs+25.0 +/- 2.2 mmHg; P < 0.01). Arteriolar hyalinosis and AIS in various segments of the CA were assessed with periodic acid-Schiff staining and inflammatory cells were immunostained with the antibody against macrophage/monocyte marker (ED1). The ED1+ cells appeared in the middle CA of L-NAME-treated SHR as early as 2 weeks after treatment. These cells were not observed in L-NAME-treated WKY rats and untreated SHR. More ED1+ cells were found in L-NAME-treated SHR than L-NAME-treated WKY rats after 4 weeks treatment. 4. The AIS and number of ED1+ cells around the perivascular area of the internal carotid artery were significantly higher in L-NAME-treated compared with untreated rats (AIS: 137 +/- 28 vs 46 +/- 10 for WKY rats, respectively; 169 +/- 18 vs 53 +/- 6 for SHR, respectively (P < 0.01); ED1+ cells: 7.9 +/- 0.6 vs 1.3 +/- 0.9 for WKY rats, respectively; 13.6 +/- 2.7 vs 2.1 +/- 0.9 for SHR, respectively (P < 0.01)), although SBP was higher in untreated SHR than in L-NAME-treated WKY rats (170 +/- 4 vs 137 +/- 4 mmHg, respectively; P < 0.05). 5. These findings suggest that ED1+ cells appeared in the middle CA of L-NAME-SHR as early as 2 weeks after treatment. Chronic inhibition of NO accelerates hypertension and induces perivascular inflammation.     

The bradykinin B1 receptor and the central regulation of blood pressure in spontaneously hypertensive rats.

1. We evaluated if the brain bradykinin (BK) B1 receptor is involved in the regulation of blood pressure (BP) in conscious rats. 2. Basal mean BP and HR were 115 +/- 2 and 165 +/- 3 mmHg and 345 +/- 10 and 410 +/- 14 beats min in Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR), respectively. Intracerebroventricular (i.c.v.) injection of 1 nmol B1 receptor agonist Lys-desArg9-BK significantly increased the BP of WKY and SHR by 7+/-1 and 19+/-2 mmHg, respectively. One nmol Sar[D-Phe8]-desArg9-BK, a kininase-resistant B1 agonist, increased the BP of WKY and SHR by 19+/-2 and 17+/-2 mmHg, respectively and reduced HR in both strains. 3. I.c.v. injection of 0.01 nmol B1 antagonists, LysLeu8-desArg9-BK or AcLys[D-betaNal7,Ile8]-desArg9-BK (R715), significantly decreased mean BP in SHR (by 9+/-2 mmHg the former and 14+/-3 mmHg the latter compound), but not in WKY. In SHR, the BP response to R715 was associated to tachycardia. 4. I.c.v. Captopril, a kininase inhibitor, increased the BP of SHR, this response being partially prevented by i.c.v. R715 and reversed into a vasodepressor effect by R715 in combination with the B2 antagonist Icatibant. 5. I.c.v. antisense oligodeoxynucleotides (ODNs) targeted to the B1 receptor mRNA decreased BP in SHR, but not in WKY. HR was not altered in either strain. Distribution of fluorescein-conjugated ODNs was detected in brain areas surrounding cerebral ventricles. 6. Our results indicate that the brain B1 receptor participates in the regulation of BP. Activation of the B1 receptor by kinin metabolites could participate in the pathogenesis of hypertension in SHR.     

PERTUSSIS TOXIN-SENSITIVE G-PROTEINS AND REGULATION OF BLOOD PRESSURE IN THE SPONTANEOUSLY HYPERTENSIVE RAT

1. Increased Gi-protein-mediated receptor-effector coupling in the vasculature of the spontaneously hypertensive rat (SHR) has been proposed as a contributing factor in the maintenance of elevated blood pressure. If increased Gi-protein-mediated activity plays an important role in hypertension in SHR, then inhibition of Gi-proteins by pertussis toxin would be expected to decrease blood pressure in this genetic hypertensive model. To address this hypothesis, studies were undertaken comparing the cardiovascular effects of pertussis toxin in SHR and normotensive Wistar-Kyoto (WKY) rats. 2. Spontaneously hypertensive and WKY rats were instrumented with radiotelemetry devices and blood pressure measurements were recorded in conscious rats. Following a single injection of pertussis toxin (10 micrograms/kg, i.v.), mean arterial blood pressure fell from 161 +/- 3 to 146 +/- 1 mmHg in the SHR and the effect was sustained for more than 2 weeks. In contrast, 10 micrograms/kg, i.v., pertussis toxin produced no significant effect on blood pressure in WKY rats (103 +/- 4 vs 101 +/- 5 mmHg). 3. In a separate study, SHR and WKY rats were administered 30 micrograms/kg, i.v., pertussis toxin or 150 microL/kg, i.v., saline and, 3-5 days later, rats were anaesthetized and instrumented to permit measurement of blood pressure and renal function. At this higher dose, pertussis toxin reduced blood pressure in both strains of rat, although the effect was markedly greater in SHR (approximately 40 mmHg decrease) compared with WKY rats (approximately 15 mmHg decrease). In SHR, pertussis toxin increased renal blood flow (from 5.7 +/- 0.3 to 7.5 +/- 0.8 mL/min per g kidney) and decreased renal vascular resistance (from 31 +/- 2 to 19 +/- 2 mmHg/mL per min per g kidney). In WKY rats, pertussis toxin had no significant effect on renal parameters. 4. Results from these studies indicate that a pertussis toxin-sensitive Gi-protein-mediated pathway contributes to the maintenance of hypertension and elevated renal vascular tone in the SHR.     

Development of hypertension in WKY rats after transplantation of parathyroid glands from SHR/SP.

We have investigated the role of the parathyroid gland (PTG) in the long-term development of blood pressure (BP) in stroke prone spontaneously hypertensive rats (SHR/SP) and Wistar Kyoto (WKY) rats. After ablation of their own PTGs, SHR/SP animals received PTGs from WKY rats and vice versa. Transplantation resulted in a normal calcium and parathyroid hormone status without signs of hypoparathyroidism. All animals received a high salt diet (8% NaCl) for 4 weeks after transplantation of PTGs. In SHR/SP, which received PTGs from WKY, development of high BP was clearly attenuated when compared to sham-operated SHR/SP rats. WKY rats with PTGs from SHR/SP rats became hypertensive, while WKY sham-operated animals remained normotensive. PTGs from SHR/SP rats are able to induce hypertension in normotensive WKY rats.     

Genetic influences on cardiovascular responses to an acoustic startle stimulus in rats

1. The aim of the present study was to assess the cardiovascular differences among five inbred rat strains (n=16 per strain), including spontaneously hypertensive rats (SHR), Wistar Kyoto (WKY) rats, Wistar Furth (WF) rats, Fischer (F344) rats and Lewis (Lew) rats and the usual outbred Wistar (W) rat strain (n=25). 2. These strains were compared under resting conditions for blood pressure (BP) and heart rate (HR) levels and for their baroreceptor-HR reflex sensitivity. In addition, their responses to an acoustic startle stimulus were measured. 3. A consistent rise in BP was observed among the groups as a result of the noise stimulus. This rise in systolic BP (SBP) averaged (+/-SEM) 37 +/- 2 mmHg in the SHR and 34 +/- 4 mmHg in F344 rats, while the response was only 23 +/- 3 mmHg in WKY rats. Pulse pressure (PP) was increased following noise in all groups. The delay for the BP response for all groups combined was 1.6 +/- 0.1 s. 4. Most animals had minimal HR variations, except F344 rats, responding with a 42 +/- 13 b.p.m. decrease 3.0 s after the stimulus (i.e. 1.3 s after the maximal 34 +/- 4 mmHg SBP rise). 5. The highest SBP (160 +/- 3 mmHg) and diastolic BP (104 +/- 3 mmHg) were observed in inbred SHR. Other groups were normotensive. Resting PP was elevated for SHR (56 +/- 2 mmHg) compared with the other groups (40 +/- 2 mmHg). The highest HR was found in F344 and WF rats, with 389 +/- 11 and 372 +/- 7 b.p.m., respectively. The lowest HR was observed in SHR and Lewis rats, with 335 +/- 7 and 323 +/- 7 b.p.m., respectively. The least sensitive baroreflex function was observed in SHR (0.8 +/- 0.1 b.p.m./mmHg) compared with the other strains (1.4 +/- 0.2 b.p.m./mmHg). 6. The present study confirms the importance of genetic factors on the cardiovascular responses of rats to a noise startle stimulus. Two inbred normotensive rat strains, namely F344 and WKY rats, which exhibit a substantial difference in pressor response to noise, may be used to unravel the mechanisms of sympathetic activation.     

EFFECT OF DELAPRIL ON THE VASCULAR ANGIOTENSIN II RELEASE IN ISOLATED HIND LEGS OF THE SPONTANEOUSLY HYPERTENSIVE RAT: EVIDENCE FOR POTENTIAL RELEVANCE OF VASCULAR ANGIOTENSIN II TO THE MAINTENANCE OF HYPERTENSION

1. In view of a recent interesting hypothesis that the vascular renin-angiotensin system (RAS) plays an important role in the maintenance of hypertension, we examined the effect of delapril (DP), a newly developed angiotensin converting enzyme inhibitor (ACEI), on angiotensin II (Ang II) release from isolated perfused hind legs of spontaneously hypertensive rats (SHR) in comparison with normotensive rats of Wistar-Kyoto strain (WKY). 2. Male SHR and WKY were given DP orally (10 mg/kg per day) for 2 weeks. Isolated hind legs of these rats were perfused with angiotensinogen-free Krebs-Ringer solution, and Ang II released into the perfusate was determined directly by extraction with Sep-Pak C18 cartridges connected to the perfusion system. 3. Delapril produced a sustained antihypertensive action in SHR but not in WKY. The spontaneous release of Ang II in SHR was 112.9 +/- 17.6 pg during the first 30 min of perfusion, which was somewhat greater than that in WKY (96.5 +/- 9.8 pg). An active metabolite of DP, delapril diacid (DPD), when added to the perfusion medium, suppressed the Ang II release in a dose-dependent manner in the two strains. Oral pretreatment of DP for 2 weeks suppressed the Ang II release by 60% in WKY and more pronouncedly by 73% in SHR. 4. These results suggest the presence of a functional RAS in vascular tissues which contributes to the maintenance of vascular tone of SHR, and that ACEI including DP exerts their antihypertensive effect through inhibition of vascular Ang II release in this animal model of human hypertension.     

Comparison of angiotensin II-induced blood pressure and structural changes in Fischer 344 and Wistar Kyoto rats

1. The purpose of the present study was to evaluate the blood pressure (BP) response, the BP and heart rate (HR) components of the startle reaction and the structure of the carotid artery and the aorta during chronic infusion of angiotensin (Ang) II in Fischer 344 (F344) compared with Wistar Kyoto (WKY) rats, two in-bred normotensive contrasted strains. 2. Osmotic mini-pumps filled with saline vehicle or AngII (120 ng/kg per min) were implanted subcutaneously in 8-week-old normotensive rats and infused for 4 weeks in F344 rats (saline, n = 10; AngII, n = 10) and WKY rats (saline, n = 10; AngII, n = 9). Basal BP, HR and the responses to an acoustic startle stimulus (duration 0.7 s, 115 dB) were recorded in conscious rats. The structure of the carotid artery and aorta was determined in 4% formaldehyde-fixed arteries. 3. Compared with WKY rats, vehicle-treated F344 rats had lower bodyweight (BW; 266 +/- 7 vs 299 +/- 9 g; P < 0.05) and heart weight (0.80 +/- 0.02 vs 0.98 +/- 0.04 g; P < 0.05) and higher aortic systolic BP (SBP; 131 +/- 1 vs 123 +/- 5 mmHg; P < 0.001) and diastolic BP (98 +/- 3 vs 89 +/- 2 mmHg; P < 0.001). In F344 rats, compared with the WKY rats, the wall thickness/BW ratio was increased in the carotid artery (156 +/- 9 vs 131 +/- 6 nm/g; P < 0.05) and abdominal aorta (264 +/- 13 vs 217 +/- 12 nm/g; P < 0.05) and decreased in the thoracic aorta (246 +/- 13 vs 275 +/- 8 nm/g; P < 0.05). There was no difference in elastin and collagen density. Angiotensin II differentially enhanced BP in both strains: (SBP: 163 +/- 5 and 132 +/- 4 mmHg in F344 and WKY rats, respectively; P(strain x treatment) < 0.05). Circumferential wall stress was increased in the aorta of F344 rats compared with WKY rats (1176 +/- 39 vs 956 +/- 12 kPa (P < 0.001) and 1107 +/- 42 vs 813 +/- 12 kPa (P < 0.001) in thoracic and abdominal aortas, respectively). The startle response was amplified in F344 rats, with enhanced increases in SBP and pulse pressure (PP) and bradycardia compared with responses of WKY rats (+44 +/- 9 mmHg, +10 +/- 2 mmHg and -40 +/- 17 b.p.m., respectively, in F344 rats vs+28 +/- 4 mmHg, + 4 +/- 2 mmHg and -19 +/- 10 b.p.m. in WKY rats, respectively; P(strain) < 0.05 for BP and PP). The startle response was not affected by AngII. 4. These results indicate a higher BP producing an increase in wall thickness in F344 rats compared with WKY rats. We propose that an increase in sympathetic nervous activity causes these haemodynamic differences, as suggested by the excessive increase in BP during an acoustic startle stimulus. Angiotensin II increased BP in F344 rats, but did not exaggerate the increase in BP during the startle reaction.     

Antihypertensive effects of the flavonoid quercetin in spontaneously hypertensive rats

1. The effects of an oral daily dose (10 mg kg(-1)) of the flavonoid quercetin for 5 weeks in spontaneously hypertensive (SHR) and normotensive Wistar Kyoto rats (WKY) were analysed. 2. Quercetin induced a significant reduction in systolic (-18%), diastolic (-23%) and mean (-21%) arterial blood pressure and heart rate (-12%) in SHR but not in WKY rats. 3. The left ventricular weight index and the kidney weight index in vehicle-treated SHR were significantly greater than in control WKY and these parameters were significantly reduced in quercetin-treated SHR in parallel with the reduction in systolic blood pressure. 4. Quercetin had no effect on the vasodilator responses to sodium nitroprusside or to the vasoconstrictor responses to noradrenaline or KCl but enhanced the endothelium-dependent relaxation to acetylcholine (E(max)=58+/-5% vs 78+/-5%, P<0.01) in isolated aortae. 5. The 24 h urinary isoprostane F(2 alpha) excretion and the plasma malonyldialdehyde (MDA) levels in SHR rats were increased as compared to WKY rats. However, in quercetin-treated SHR rats both parameters were similar to those of vehicle-treated WKY. 6. These data demonstrate that quercetin reduces the elevated blood pressure, the cardiac and renal hypertrophy and the functional vascular changes in SHR rats without effect on WKY. These effects were associated with a reduced oxidant status due to the antioxidant properties of the drug.     

Withdrawal reveals lack of effect of prolonged antihypertensive treatment on intrinsic aortic wall stiffness in senescent spontaneously hypertensive rats

1. Chronic antihypertensive treatment lowers cardiovascular morbidity and mortality. The beneficial effect on the blood vessel wall may be due to the lowering of blood pressure (BP) and, hence, wall stress (WS), or to a treatment-induced change in wall structure. 2. We have previously shown that, when evaluated at the same level of BP and WS, the stiffness of the aortic wall of old spontaneously hypertensive rats (SHR) is higher than that of young and adult SHR and that of age-matched Wistar-Kyoto (WKY) rats. In the present study, we tested the hypothesis that the intrinsic changes in wall composition and mechanics in old SHR can be modulated by long-term treatment with an angiotensin I-converting enzyme inhibitor (captopril; 40 mg/kg per day) combined with a diuretic (hydrochlorothiazide; 20 mg/kg per day) and that treatment withdrawal would reveal whether such changes are maintained when BP and WS return to pretreatment levels. 3. We evaluated aortic structure and mechanics in SHR following 1 week withdrawal of oral antihypertensive treatment from 3 to 15 months of age (n = 8). Results were compared with age-matched SHR that were maintained on treatment (n = 12) or were not treated (n = 13) and with WKY rats (no treatment n = 11; maintained n = 11; withdrawn n = 10). 4. Isobaric aortic wall stiffness was estimated from the ratio of baseline aortic pulse wave velocity (PWV) to BP and the slope relating aortic PWV to BP following sodium nitroprusside-induced hypotension. Relative wall stiffening was estimated as the ratio of elastic modulus (EM) to WS. We argued that if treatment produced a change in wall elastin or collagen content, with a subsequent decrease in isobaric wall stiffness, then this would be maintained when BP increased following withdrawal of treatment. 5. In old SHR, treatment lowered isobaric wall stiffness (baseline PWV/BP 4.6 +/- 0.3 cm/s per mmHg; slope relating PWV to BP 6.7 +/- 0.4 x 10-3 cm/s per mmHg and EM/WS 4.1 +/- 0.4 vs 6.1 +/- 0.4 cm/s per mmHg, 9.7 +/- 0.9 x 10-3 cm/s per mmHg and 8.9 +/- 1.1, respectively, without treatment; all P < 0.05). After 1 weeks treatment withdrawal, the indices (5.7 +/- 0.2 cm/s per mmHg, 9.1 +/- 0.2 x 10-3 cm/s per mmHg and 7.2 +/- 0.6) increased in parallel with the increase in WS to levels similar to those observed in untreated SHR. There were no significant differences among the WKY rat groups. 6. Treatment increased the elastin and collagen contents of the aortic wall in both SHR (196 +/- 13 and 128 +/- 5 vs 111 +/- 9 and 86 +/- 4 mg/g wet weight, respectively, in untreated; P < 0.05) and WKY rats (190 +/- 19 and 135 +/- 4 vs 115 +/- 7 and 114 +/- 5 mg/g wet weight, respectively, in untreated; P < 0.05). This increase remained following withdrawal (213 +/- 26 and 118 +/- 4 vs 161 +/- 14 and 127 +/- 4 mg/g wet weight in SHR and WKY rats, respectively). 7. In summary, 1 year of treatment with captopril plus hydrochlorothiazide increases wall elastin content and reduces WS and stiffness in old SHR. Following withdrawal, elastin content remains high, but wall stiffness parallels WS in a manner similar to that in untreated SHR.     

Delay of the initiation of hypertension in spontaneously hypertensive rats by CV-4151, a specific thromboxane A2 synthetase inhibitor

When CV-4151, a specific thromboxane (TX) A2 synthetase inhibitor, was given orally to 4 week old (4w) spontaneously hypertensive rats (SHR) daily for 3 weeks, the initiation of hypertension was delayed by about one week. The agent increased urinary excretion of water, sodium and creatinine, reduced that of TXA2 (as TXB2), increased that of PGI2 (as 6-keto-PGF1 alpha) and enhanced urinary PGI2/TXA2. In 4w Wistar-Kyoto rats (WKY) and 18w SHR was established hypertension, the agent had little effect on blood pressure and renal function. In isolated, perfused kidneys of 6w SHR, CV-4151 markedly inhibited both arachidonic acid-induced pressor action and production of TXA2. TXA2 synthetase activity in renal cortical microsomes of 5w SHR was approximately 1.5 times higher than that in age-matched WKY. CV-4151 inhibited TXA2 synthetase activity of medullary and cortical microsomes more effectively in 5w SHR than in age-matched WKY. Thus, in young SHR, the TXA2 synthetase inhibitor seemed to improve renal function by altering the balance of renal TXA2 and PGI2 biosynthesis and subsequently caused a delay in the initiation of hypertension. The present findings lend support to the idea that an imbalance in the renal TXA2-PGI2 biosynthesis may be involved in the initiation of hypertension in SHR.     

PLASMA AND PITUITARY PROLACTIN AND BLOOD PRESSURE IN BROMOCRIPTINE-TREATED SPONTANEOUSLY HYPERTENSIVE AND WISTAR-KYOTO RATS

1. The effects on blood pressure (BP) and plasma and pituitary prolactin (PRL) of a 13 day intraperitoneal infusion of bromocriptine delivered by osmotic minipump were investigated in spontaneously hypertensive rats (SHR) and their normotensive controls, the Wistar-Kyoto rats (WKY). 2. In the SHR, a fall in BP which was steepest over the initial few days and sustained up to day 12 was observed in the bromocriptine-treated group compared with the lack of a change in BP observed in the vehicle-treated group. The plasma PRL level taken on day 13 was found to be significantly lower in the bromocriptine-treated group than in the vehicle-treated group. 3. In the WKY, bromocriptine had no significant effect on either BP or plasma PRL. 4. Pituitary PRL content was significantly lower in the SHR than in the WKY. The suppression by bromocriptine treatment was greater in the SHR than in the WKY. 5. These results provide further evidence for a central dopaminergic insufficiency in the SHR and raise the possibility that PRL may, either directly or indirectly by interacting with other factors in the SHR, influence BP.     

Increased central cholinergic activity in rat models of hypertension

The role of brain acetylcholine (ACh) in the development and maintenance of experimental hypertension was evaluated. Single, acute injections of hemicholinium-3 (HC-3), 20 micrograms, were made into the lateral cerebral ventricle of unanesthetized rats during development of hypertension in the spontaneously hypertensive rat (SHR) or following induction of deoxycorticosterone-salt (DOCA), aortic coarctation (AoCo), and Grollman hypertension. HC-3 caused nonsignificant reductions in blood pressure (BP) when injected in SHR and Wistar-Kyoto (WKY) controls at 5 and 8 weeks of age. At 12 weeks in SHR, depressor responsiveness increased concomitantly with the development of hypertension and reached maximal stable effect during established hypertension (18-60 weeks). Intravenous (i.v.) infusion of sodium nitroprusside (NaNP) normalized BP of adult SHR, yet did not prevent the hypotensive response to HC-3. In DOCA hypertension, the HC-3 hypotensive effect (1 week postinduction) preceded establishment of significantly elevated BP (3 weeks), but then remained constant despite a continual rise in BP (2-6 weeks). In the AoCo model, onset of severe hypertension (day 2 postinduction) preceded the appearance of significant depressor responsiveness to HC-3 (day 10). Although BP remained uniformly elevated from 10 to 40 days, the magnitude of the depressor response was stable between 10 and 28 days, then increased further at day 40. At the only time studied, 4 weeks after induction of hypertension, HC-3 also lowered BP of Grollman rats. Marked bradycardia occurred in all models and controls. The results suggest that enhancement of central cholinergic neurotransmission is a common feature of experimental hypertension regardless of its etiology. Increased brain cholinergic function appears to be involved in the development (but not the initiation) and maintenance of elevated BP in the SHR, whereas it may play a role in the initiation of DOCA hypertension and in the maintenance of AoCo hypertension.     

Thromboxane A2 and the development of hypertension in spontaneously hypertensive rats

Osmotic minipumps containing OKY-046 (15-20 mg/kg per day), to inhibit thromboxane (TX) A2 synthase, were implanted into 43-day-old SHR and age-matched Wistar-Kyoto rats (WKY) to study the role of TXA2 in the development of hypertension in SHR. Inhibition of TXA2 synthase with OKY-046 did not affect urine volume, sodium excretion, potassium excretion, food and water intake or body weight in either WKY or SHR during the two weeks of study. In the first week systolic blood pressure (SBP) was significantly lower in SHR receiving OKY-046 in comparison to SHR which received no OKY-046 (127 +/- 3 vs. 110 +/- 4 mm Hg, P less than 0.01). OKY-046 did not affect SBP in WKY. By the second week SBP in SHR and WKY receiving OKY-046 did not differ from their respective controls despite an 85% reduction in serum immunoreactive TXB2 (iTXB2; the stable hydrolysis product of TXA2) and a 45% reduction in urinary iTXB2 excretion. These results support a possible role for TXA2 in the developmental stage of hypertension in SHR and other factors in the sustained elevation of blood pressure.     

Kidney denervation combined with elimination of adrenal-renal portal circulation prevents the development of hypertension in spontaneously hypertensive rats

1. Kidney denervation in spontaneously hypertensive rats (SHR) during the prehypertensive stage delays and attenuates the development of hypertension. The same results have been obtained after elimination of the adrenal-renal portal circulation (ARPC). The aim of the present study was to investigate the influence of concomitant kidney denervation and elimination of ARPC on hypertension in SHR. 2. Experiments were performed on 6-week-old male SHR and Wistar-Kyoto (WKY) rats. In the first group of animals (group I), the ARPC was eliminated by removing the left adrenal gland and the right kidney. In the second group of rats (group II), the right kidney and the right adrenal gland were removed and the left kidney was denervated. In the third group of rats (group III), the right adrenal gland and the left kidney were removed and the right kidney was denervated. In the fourth group of rats (group IV), the right adrenal gland and the right kidney were removed. Group IV served as the control group. Denervations were repeated every 3 weeks. Systolic blood pressure was measured indirectly. 3. Elimination of ARPC (group I) and kidney denervation (group II) delayed and attenuated hypertension to the same degree (163 +/- 5 and 157 +/- 4 mmHg, respectively). Application of the these two methods concomitantly (group III) prevented the development of hypertension (130 +/- 6 mmHg). 4. We conclude that both intact efferent sympathetic renal nerves and adrenal hormones reaching the kidney through the ARPC may be mandatory factors for the development of arterial hypertension in SHR.     

Alterations of renal vasopressin V1A and V2 receptors in spontaneously hypertensive rats

To elucidate the role of arginine vasopressin (AVP) in a hypertensive state, the characteristics of renal cortex V(1A) and medulla V(2) receptors in young spontaneously hypertensive rats (SHR) during the developmental phase of hypertension were compared with those of age-matched Wistar-Kyoto (WKY) rats using the radioligand receptor assay technique. The systolic blood pressure of 8-week-old SHR was statistically significantly higher than that of WKY rats (142 +/- 1 vs. 125 +/- 2 mm Hg). The plasma AVP levels were also significantly higher in SHR than in WKY rats (3.20 +/- 0.41 vs. 1.96 +/- 0.34 pg/ml). In SHR, the maximum capacity of (3)H-d(CH(2))(5)Tyr(Me)AVP binding to cortical V(1A) receptors (B(max)) was statistically significantly higher than that of WKY rats (39.7 +/- 2.7 vs. 22.4 +/- 0.9 fmol/mg protein). Furthermore, the B(max) values of (3)H-AVP binding to medullary V(2) receptors in SHR were also significantly higher than in WKY rats (40.2 +/- 1.9 vs. 28.3 +/- 1.3 fmol/mg protein). However, the apparent dissociation constant (K(d)) values of renal cortex V(1A) and medulla V(2) receptors in SHR and WKY rats were not significantly different. These results indicate that increased amounts of renal cortex V(1A) and medulla V(2) receptors in SHR play an important role in the pathophysiology of hypertension.     

Upregulated function of phosphatidylinositol-3-kinase in genetically hypertensive rats: a moderator of arterial hypercontractility

1. The growth enzyme phosphatidylinositol 3-kinase (PI3K) was recently implicated in the mediation of arterial spontaneous tone, an event observed in arteries from hypertensive, but not normotensive, subjects that contributes to changes in total peripheral resistance in the hypertensive state. We have shown this occurrence in experimentally induced models of hypertension. However, because the majority of hypertension is genetically based, it is important to demonstrate a similar change in genetically hypertensive animals. 2. Aorta from spontaneously hypertensive rats (SHR; systolic blood pressure = 183 +/- 4 mmHg) and Wistar Kyoto (WKY) rats (115 +/- 2 mmHg) were isolated for the measurement of isometric contractile force. Aorta from SHR displayed small increases (approximately 5% maximum phenylephrine (PE)-induced contraction) in spontaneous tone, whereas aorta from WKY rats displayed none. The non-selective PI3K inhibitor LY294002 (20 micromol/L) and the selective inhibitor of the p110delta catalytic subunit of PI3K IC87114 (20 micromol/L) caused a fall of basal tone in SHR aorta (20 +/- 7 and 24 +/- 6% of the initial PE contraction, respectively), but did not alter tone in arteries from WKY rats. LY294002, but not IC87114, normalized the increased potency of noradrenaline (NA) observed in aorta from SHR (-log EC50 values for NA in the presence of vehicle in WKY rats and SHR 7.5 +/- 0.1 and 7.8 +/- 0.1, respectively (P < 0.05); -log EC(50) values for NA in the presence of LY294002 in WKY rats and SHR 7.0 +/- 0.1 and 7.0 +/- 0.1, respectively). 3. Biochemical expression of the p110 catalytic and p85 regulator subunits of PI3K in western analyses revealed no difference in expression of the regulatory p85alpha or p110alpha protein subunits between WKY rats and SHR; p110gamma was not detected. In contrast, p110delta expression was increased greater than 30% in aorta from SHR compared with WKY rats (827.6 +/- 88.5 vs 576.8 +/- 53.4 arbitrary densitometry units, respectively). Immunohistochemical analyses revealed expression of the p110delta isoform in the smooth muscle of arteries. 4. These data underscore the relevance of an enzyme historically classified as one committed to growth/anti-apoptosis in modifying contractility and supports involvement of PI3K in genetically based hypertension.     

Angiotensin-II-stimulated release of thromboxane A2 and prostacyclin (PGI2) in isolated, perfused kidneys of spontaneously hypertensive rats.

Angiotensin-II(A-II)-stimulated release of rabbit aorta-contracting substance (RCS), prostaglandin-like substance (PLS) and dog coronary-relaxing substance (DRS) was studied in isolated, perfused kidneys of spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). When the perfusion pressure was set at 60–70 mmHg, the dose-related pressor response of the kidney to A-II did not differ significantly between the strains, or between the ages of 6, 12 and 18 weeks. The A-II-induced release of RCS from the kidney was most remarkable in 6-week SHR, slight in 12- and 18-week SHR, but negligible for all ages of WKY. The DRS release was remarkable in 12- and 18-week SHR, but was hardly detectable in 6-week SHR and all ages of WKY. The PLS release was much more remarkable in 18-week SHR than in 6- and 12-week SHR and all ages of WKY. The RCS was confirmed to be thromboxane A₂ (TXA₂) by: (1) its half-life of 38 sec at 37°, (2) a concomitant release of radio-immunoassayable TXB₂: (a stable metabolite of TXA₂) into the perfusate and (3) specific inhibition of its release by L-8027 (a TXA₂ synthetase inhibitor). The DRS was considered to be prostacyclin (PGI₂) by: (1) its half-life of 5.2 min at 37° and (2) its anti-platelet aggregation activity. It is speculated that an enhanced TXA₂ synthesis in the 6-week SHR kidney may be involved in increasing the renal vascular resistance and in initiating the process of hypertension.     

Changes in autonomic activity and baroreflex sensitivity with the hypertension process and age in rats

1. Autonomic activity and baroreflex sensitivity (BRS) were compared in age-matched conscious groups of Wistar Kyoto (WKY) rats, spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive rats (SHRSP). 2. Male WKY rats, SHR and SHRSP aged 4-30 weeks were used. Autonomic activity and BRS were estimated by power spectral and cross-spectral analysis of systolic blood pressure (SBP) and SBP-SBP (SS) interval fluctuations, respectively. 3. The time-course of heart rate (HR), SBP, the amplitude of the low-frequency component of SBP fluctuation (SBP-LF; prazosin-sensitive index) and the amplitude of the high-frequency component of the SS interval fluctuation (SS-HF; atropine-sensitive index) consisted of two periods. In the first period (up to 10 or 15 weeks of age), BP, SBP-LF and SS-HF increased with age. The order of SBP-LF was SHRSP > SHR > WKY rats throughout this period. During the second period, BP was sustained at certain levels in all strains, but changes in SBP-LF and SS-HF with age were different among strains. In particular, in SHRSP, SBP-LF markedly decreased with age after 10 weeks. Baroreflex sensitivity in WKY rats increased gradually with age, whereas the BRS in SHR and SHRSP decreased before 6 weeks of age and remained lower than that in WKY rats. 4. In conclusion, the present study shows that both prazosin-sensitive and atropine-sensitive indices are associated with the elevation of BP in all strains studied. However, hypertension after 15 weeks of age in SHRSP is sustained despite a paradoxical reduction in sympathetic activity with an abnormal control of BRS. Therefore, the contribution of autonomic activity to hypertension may be discussed separately as a developmental period and a sustained period.     

The role of endogenous bradykinin in blood pressure homeostasis in spontaneously hypertensive rats.

1. The role of endogenous bradykinin in mean arterial blood pressure (BP) homeostasis was studied in spontaneously hypertensive (SHR) and normotensive (WKY) rats by the use of a bradykinin B2-receptor antagonist (BKant; Hoe 140, 11.6 micrograms kg-1) and converting enzyme (kininase II) inhibitor (captopril, 10 mg). To obtain a response to captopril that was induced through inhibition of kinin-degradation only and not through inhibition of angiotensin II-formation, the studies were performed on binephrectomized male rats to eliminate the renin-angiotensin system. 2. The role of the nitric oxide (NO) and the adrenergic systems were evaluated by the use of NO-synthase inhibitor (L-NAME, 0.3 g kg-1) and phentolamine (2 mg kg-1), respectively. 3. The rats were anaesthetized and pretreated with two injections of vehicle (PBS) or drugs spaced 5 min apart: PBS + PBS; BKant + PBS; PBS + L-NAME; BKant + L-NAME; or phentolamine + L-NAME. All rats were given captopril 15 min later. Time-control groups were treated with L-NAME but not captopril. 4. In WKY rats, captopril did not significantly alter BP in any of the groups. In the SHR-PBS + PBS group, on the other hand, captopril induced an immediate fall in BP (delta BP = -23 +/- 4 mmHg, P < 0.0017) which was completely blocked by BKant (delta BP = 2 +/- 2 mmHg) (P < 0.0011). L-NAME did not significantly alter the immediate hypotensive response to captopril but disclosed a later hypertensive reaction. In L-NAME + BKant-treated rats, both the hypotensive response and the late hypertension was abolished. In rats treated with phentolamine + L-NAME, the immediate fall in BP was not different from the controls whereas the late hypertension was absent. 5. BKant itself had no effect on basal BP in either WKY or SHR even when a 10 times higher dose was tested in a separate set of experiments. This was true also for conscious, nonnephrectomized SHR rats. 6. It was concluded that endogenous production of bradykinin was demonstrable through kininase II-inhibition in hypertensive but not in normotensive rats. However, this endogenous bradykinin did not play a role in basal BP homeostasis. The captopril-induced hypotension depended on kinin but, under the present conditions, not on NO as a mediator. The fall in BP induced a compensatory adrenergic hypertensive response which was revealed when the continuous NO-synthesis was blocked by L-NAME.