PROTECTIVE EFFECTS OF ME3221 ON HYPERTENSIVE COMPLICATIONS AND LIFESPAN IN SALT-LOADED STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. A comparison was made on the protective effects of the following: ME3221, a competitive angiotensin AT₁ receptor antagonist; losartan, in which a major active metabolite is a non-competitive angiotensin AT₁ receptor antagonist; and enalapril, an angiotensin-converting enzyme inhibitor, using the salt-loaded stroke-prone spontaneously hypertensive rats (SHRSP). 2. SHRSP received orally ME3221 (3 and 10mg/kg per day), losartan (10mg/kg per day) and enalapril (10mg/kg per day) from the 6th to the 20th week of age. All the control rats showed rapid elevation of systolic blood pressure (SBP), accompanied by hypertensive complications, and died by 15 weeks of age. 3. ME3221, losartan and enalapril suppressed the elevation of SBP in the salt-loaded SHRSP to a comparable degree. ME3221 and losartan increased the survival rate to >90%, and diminished hypertensive complications such as cerebral apoplexy (stroke), renal injury (increased proteinuria, and total N-acetyl-β-D-glucosaminidase activity) and heart failure (cardiac hypertrophy and pleural effusion). 4. Competitive (ME3221) and non-competitive (losartan) angiotensin AT₁ receptor antagonists showed comparable efficacy against the complications and mortality of the salt-loaded SHRSP; both were more potent than enalapril in the protective effect.     

EFFECTS OF NEONATAL SYMPATHECTOMY BY 6-HYDROXYDOPAMINE ON BLOOD PRESSURE AND INTRAVASCULAR VOLUME IN YOUNG STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. Stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar Kyoto rats (WKY) were 'chemically sympathectomized' immediately after birth with 6-hydroxydopamine (6-OHDA, 100 mg/kg s.c. daily) for the first 10 days of life. 2. Body weight gain was diminished in both groups as compared with sham-treated controls. Blood pressure was reduced in 'sympathectomized' SHRSP, and also WKY rats had a slightly lower blood pressure than control rats. 3. Plasma concentration of angiotensin II and renin content of the kidney were not influenced by 6-OHDA. 4. 'Sympathectomized' SHRSP retained similar amounts of sodium than sham-treated SHRSP when sodium retention is expressed per body weight gained. Plasma and blood volumes were increased in both SHRSP and WKY rats, whereas packed cell volume was significantly decreased. 5. These results demonstrate the significance of an intact sympathetic nervous system for the development of hypertension in SHRSP. The expanded plasma and blood volume in 'sympathectomized' rats indicate an important role of the sympathetic nervous system and/or the arterial blood pressure for the regulation of intravascular volume.     

COMPARISON OF VASOPRESSOR EFFECTS OF NITRO ARGININE IN STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS AND WISTAR-KYOTO RATS

1. NG-nitro-L-arginine (NO2Arg) is a guanidine nitro arginine derivative and an inhibitor of endothelium-dependent vascular relaxation. Significant rise of the systolic blood pressure was observed after 1 week administration of NO2Arg in food (0.023% in weight, about 2.8 mg of NO2Arg/rat per day) in female rats of stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY). The rises were not different between SHRSP (21 mmHg) and WKY (23 mmHg). 2. In ring preparations of the thoracic aorta of NO2Arg-administered rats of both strains, relaxation by acetylcholine decreased markedly compared with those of the control rats (to 43-44%). On the contrary, glyceryltrinitrate-induced relaxation was slightly but significantly increased in the aorta of WKY after NO2Arg administration and the same tendency was observed in SHRSP. 3. The rise of blood pressure and the decrease of acetylcholine-induced relaxation suggested that NO2Arg inhibited the endothelium-dependent relaxation not only in WKY but also in SHRSP. The relaxation of the thoracic aorta preparation of SHRSP by acetylcholine was much less (ca 38%) than that of WKY; however, that of SHRSP by glyceryltrinitrate was slightly less (ca 74%), indicating that endothelium-dependent relaxation declined in vascular preparation of SHRSP. 4. The present results suggest that endothelium-dependent relaxation has some contribution on blood pressure regulation in the hypertensive state, although a decline of endothelium-dependent relaxation is evident in vascular preparation of SHRSP compared with WKY.     

PREVENTATIVE AND THERAPEUTIC EFFECTS OF AE0047 ON RENAL INJURY IN STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. The present study was designed to investigate the preventative and therapeutic effects of AE0047 on renal injury compared with those of nitrendipine in stroke-prone spontaneously hypertensive rats (SHRSP). 2. In the preventative study, drug administration was started before the appearance of renal injury, such as proteinuria. Treatment for 6 weeks with AE0047 (1 and 3 mg/kg, p.o.) led to a dose-related reduction in systolic blood pressure (SBP). Nitrendipine, at doses of 10 and 30 mg/kg, also lowered SBP to a similar degree to that seen with AE0047 at 1 and 3 mg/kg, respectively. 3. In the vehicle-administered SHRSP group, urinary excretion of protei. (U_proteinV) increased progressively from 14 weeks of age for another 6 weeks. AE0047 at both doses maintained U_proteinV within normal levels throughout the experimental period. However, the elevation of U_proteinV was only inhibited in the 30 mg/kg nitrendipine-treated group. Urinary N-acetyl-β-D-glucosaminide (NAG) activity in the vehicle-treated SHRSP group was elevated. Urinary NAG activity remained at a low level only in AE0047-treated groups. 4. Histopathological examination revealed severe lesion. (i.e. fibrinoid necrosis, proliferative vasculitis and glomerular lesions) of the kidney in SHRSP. AE0047 treatment at each dose attenuated the development of renal lesions in SHRSP. In contrast, nitrendipine, at 10 mg/kg, was ineffective against the development of renal lesions. Although nitrendipine at 30 mg/kg suppressed the development of renal lesions, this effect was still weaker than that seen with AE0047 at 1 mg/kg. 5. In the therapeutic study, drugs were administered to 17-week-old SHRSP with moderate renal damage for 10 days. Treatment with AE0047 (1 and 3 mg/kg) produced dose-dependent decreases in U_proteinV. In the nitrendipine-treated group, U_proteinV tended to decrease but the changes were not significant. 6. Histopathological studies revealed that 3 mg/kg AE0047 improved renal lesions, such as fibrinoid necrosis, proliferative vasculitis and glomerular lesions, whereas 30 mg/kg nitrendipine di. not. 7. Taken together, the results indicate that AE0047 is capable of preventing proteinuria as well as renal lesions, in part via a mechanism independent of its depressor action on SBP. Furthermore, AE0047 improves proteinuria and renal lesions in proteinuria-established SHRSP. Thus, AE0047 may have therapeutic potential in suppressing either the development or the progression of renal disease in hypertensive patients.     

INTRALYMPHOCYTIC FREE CALCIUM AND MAGNESIUM IN STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS AND EFFECTS OF BLOOD PRESSURE AND VARIOUS ANTIHYPERTENSIVE AGENTS

1. Free Ca²⁺ ([Ca²⁺]i) and Mg²⁺ ([Mg²⁺]i) were measured in peripheral lymphocytes from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY) at the age of 5, 7 and 17 weeks, from various antihypertensive agents-treated SHRSP, and from secondary hypertensive WKY. 2. At the age of 5 weeks, no difference was observed in systolic blood pressure (SBP), or lymphocyte [Ca²⁺]i and [Mg²⁺]i between SHRSP and WKY. At the age of 7 or 17 weeks, SBP and [Ca²⁺]i of SHRSP were significantly higher than in WKY, and at the age of 17 weeks, [Mg²⁺]i of SHRSP was significantly lower than in WKY. Further, [Ca²⁺]i or [Mg²⁺]i was positively or negatively correlated to SBP, and [Mg²⁺]i was negatively correlated to [Ca²⁺]i. 3. SBP of SHRSP fell significantly after antihypertensive treatment with calcium antagonist, angiotensin-converting enzyme (ACE) inhibitor or hydralazine for 40 days. [Ca²⁺]i was significantly lower in calcium antagonist and hydralazine groups, and tended to be low in ACE inhibitor group. These four groups showed no difference in [Mg²⁺]i. 4. After 40-day administration of N^G-nitro-l-arginine (l-NNA), WKY developed severe hypertension, but there were no significant differences in lymphocyte [Ca²⁺]i and [Mg²⁺]i between the l-NNA treated and non-treated groups. 5. These results suggested that increased lymphocyte [Ca²⁺]i and decreased [Mg²⁺]i observed in SHRSP are not only secondary to hypertension but possibly related to a basic genetic abnormality of divalent cation handling.     

RELATIONSHIP BETWEEN NIFEDIPINE SENSITIVITY OF AORTAE AND BLOOD PRESSURE OF STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. We have previously described an increased sensitivity to inhibition by nifedipine of noradrenaline-induced contractures of blood vessels in hypertension. In this study we have investigated whether changes in blood pressure (BP) change the sensitivity to nifedipine and K⁺ of aortic rings from normotensive (Wistar-Kyoto rats, WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). 2. SHRSP were treated with: hydralazine plus hydrochloro-thiazide; captopril plus hydrochlorothiazide; hydralazine plus guanethidine; or captopril alone. WKY rats were treated with deoxycorticosterone acetate (DOCA) and NaCl. Treatment commenced from 5 weeks of age and continued until 13–15 weeks. 3. The SHRSP treatments produced similar reductions in BP, and the BP of all the treated groups were significantly lower than the mean BP of untreated SHRSP (201.0 ± 7.7 mmHg). The mean BP of the treated WKY rats (134.2 ± 7.6 mmHg) was significantly higher than the mean BP of the untreated WKY rats (86.8 ± 7.4 mmHg). 4. An area-under-curve (AUC) analysis of the inhibitory effects of nifedipine on responses of aortae to noradrenaline showed no differences between treated and untreated SHRSP groups (overall mean 40.6 ± 1.9% and 43.4 ± 3.4% inhibition of control AUC, respectively), or between DOCA-salt treated WKY and untreated WKY groups (58.8 ± 5.9 and 64.8 ± 2.3, respectively). Noradrenaline-induced contractures of aortae from all SHRSP groups were significantly more sensitive to inhibition by nifedipine than aortae from both WKY groups. 5. The molar concentration of agonist required to evoke 50% of the maximum response (EC₅₀) values for potassium chloride (KCI) were significantly increased in the aortae of all treated SHRSP groups in comparison to those from untreated SHRSP (treated SHRSP groups, 15.53 ± 0.68 mmol/L vs untreated SHRSP group, 11.36 ± 1.10 mmol/L). The EC₅₀ values for KC1 for the aortae from the DOCA-treated WKY rats were significantly less than those from aortae of the untreated WKY (11.80 ± 0.80 and 17.08 ± 1.50 mmol/L, respectively). 6. We conclude that reduction (in SHRSP) or increase (in WKY) of the BP has no effect on the sensitivity of aortic smooth muscle to the inhibitory effects of nifedipine on responses to noradrenaline, suggesting that alterations in voltage-dependent Ca²⁺ mechanisms may be a primary phenomenon in the SHRSP. In contrast, the fact that sensitivity to KC1 changes in the treated SHRSP and WKY aortae suggests such sensitivity is secondary to the BP and thus a separate phenomenon from voltage-dependent Ca²⁺ mechanisms.     

CEREBROVASCULAR PROTECTION BY SEQUENTIAL BILATERAL CAROTID ARTERY LIGATION IN AGED SPONTANEOUSLY HYPERTENSIVE RATS

1. Sequential bilateral carotid artery ligation (BCL) separated by a 1 week interval was performed on 5 month spontaneously hypertensive rats (SHR) (i.e. SHRSR-B₁/Izm) and the developmental course of hypertension and cerebrovascular lesions in advanced age were analysed as compared with those in age-matched sham-operated controls. 2. Behavioural activity and behavioural reaction to light were also investigated in the above-mentioned SHR, young and adult stroke-prone SHR (i.e. SHRSP-A₃/Izm), SHR (i.e. B₁/Izm) and Wistar-Kyoto rats (i.e. WKY/Izm). 3. All of the control SHR developed severe hypertension resulting in cerebral stroke with focal oedema due to cerebral haemorrhage and infarction as a result of arterionecrosis 18 months after birth. 4. SHR usually die within a few days of BCL. In the present study, however, they successfully survived without cerebrovascular damage for a long time, although they developed a similar severe hypertension in a significantly shorter period of time (P < 0.05) and showed behavioural abnormalities that were probably due to severe cerebral ischaemia. 5. These experimental results suggest an ischaemic tolerance phenomenon in a hypertensive model that was exposed to mild ischaemic stress by unilateral carotid artery ligation (UCL) before the subsequent induction of severe ischaemia by BCL. The results also suggest that an aggravation of hypertensive cerebrovascular changes due to long-lasting ischaemia after BCL was prevented through a possible cumulative effect of ischaemic stress.     

RELAXANT EFFECTS OF VASODILATOR PEPTIDES ON ISOLATED BASILAR ARTERIES FROM STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. The relaxant effects of vasodilator peptides were examined in ring preparations of basilar arteries from stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto (WKY) rats. 2. Vasoactive intestinal peptide and peptide histidine iso-leucine produced similar endothelium-independent relaxations in basilar arteries from WKY rats and SHRSP. Calcitonin gene-related peptide (CGRP) elicited endothelium-independent relaxations in both groups and the CGRP-induced relaxation was greater in SHRSP than in WKY rats. Substance P and neurokinin A did not relax basilar arteries from either group. 3. Both WKY rat and SHRSP basilar arteries were relatively insensitive to atrial natriuretic peptide, brain natriuretic peptide and C-type natriuretic peptide. 4. Bradykinin (BK) potently relaxed basilar arteries with endothelium, but BK produced contractions in endotheliumrubbed arteries in both WKY rats and in SHRSP. There was no significant difference in the relaxant response to BK between WKY rat and SHRSP arteries. 5. Adrenomedullin (AM) produced endothelium-independent relaxations in both groups and the relaxant response to AM was significantly greater in SHRSP than in WKY rats. 6. Human CGRP(8–37; 1 μmol/L), a CGRP receptor antagonist, significantly inhibited both relaxant responses induced by CGRP and AM in WKY rats and in SHRSP arteries. 7. Among various peptides tested, the responses to CGRP and AM were higher in basilar arteries from SHRSP than in those from WKY rats. The relaxation produced by AM may be via CGRP receptors in WKY rat and SHRSP basilar arteries.     

INTRACELLULAR pH AND SODIUM-PROTON EXCHANGE ACTIVITY OF LYMPHOCYTES IN STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. At the age of 20 weeks, intracellular pH (pHi) of circulating lymphocytes suspended in HCO(3-)-free NaCl media was not significantly different between stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY). 2. The initial recovery rate of pHi in lymphocytes tended to be greater in SHRSP than in WKY after the addition of 60 mmol/L or 120 mmol/L of NaCl, but there was no statistically significant difference. 3. The H+ equivalent efflux rate, which was a true reflection of Na(+)-H(+) activity, was significantly greater in SHRSP than in WKY (P less than 0.05). The difference in H+ equivalent efflux rate was not due to the difference in cellular buffering power between the two groups (P greater than 0.05). An increased Na(+)-H(+) exchange activity may play a partial role in the pathogenesis of hypertension.     

CHLORIDE ION INGESTED WITH SODIUM AFFECTS THE DEVELOPMENT OF CEREBRAL LESIONS IN STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. To determine the effect of chloride ion on the development of hypertension and the incidence of cerebral lesions in stroke-prone spontaneously hypertensive rats (SHRSP), groups of 10 rats were administered chronically with either 171 mmol/L sodium chloride or equimolar sodium provided as sodium citrate in the drinking water from the age of 12 weeks. 2. The life span was significantly extended in SHRSP given sodium citrate (336 +/- 28 vs 246 +/- 26 days, mean +/- s.e.m., P less than 0.05) but their development of hypertension was not different from SHRSP given sodium chloride. 3. In order to determine the role of calcium homeostasis, calcium in urine was collected. Urinary calcium in SHRSP given sodium citrate was significantly decreased (1.0 +/- 0.12 vs 1.8 +/- 0.18 mg/24 h urine, P less than 0.05). 4. If the normal life span is 320 +/- 35 days, this suggests that chloride ion ingested with sodium accelerates the development of cerebrovascular diseases, and that increased urinary calcium excretion may be related to this adverse chloride effect on the development of hypertension in SHRSP.     

EFFECT OF DIETARY UREA ON BLOOD PRESSURE IN SPONTANEOUSLY HYPERTENSIVE RATS

The feeding of a normal diet containing 13.5% urea (in place of protein in a high protein diet) attenuated the development of severe hypertension and decreased the incidence of stroke in spontaneously hypertensive rats (SHR), when 1% NaCl solution was given to them. The urea not only increased urine volume, but also increased urinary sodium excretion in SHR given 1% NaCl for drinking. Although there was no obvious difference in erythrocyte size between the urea and the control groups, there was a significant inverse correlation between plasma urea level and erythrocyte size. These results suggest that a high protein diet reduced blood pressure partly through the diuretic effect of urea, the common metabolite of various proteins.     

THE EFFECT OF DIETARY FISH OIL AND LONG-TERM SALT LOADING ON BLOOD PRESSURE AND EICOSANOID METABOLISM IN SPONTANEOUSLY HYPERTENSIVE RATS

1. Dietary suppression of prostanoid synthesis with fish oils has had little effect on blood pressure in models of experimental hypertension in rats. However, a pressor effect of dietary fish oils was observed in spontaneously hypertensive rats (SHR) subject to 1 week of salt loading. 2. Animals were allocated to semisynthetic diets containing either 10% by weight Max EPA fish oil or a control diet of coconut oil, and studied after receiving 1.5% saline for 4 weeks. 3. Within the first week of salt loading, SHR-fed fish oil showed an increase in blood pressure (mean = 9 mmHg) relative to controls. This effect was transient, and after the first week of salt loading there was little difference in blood pressure between the two dietary groups. 4. Following dietary treatment there were substantial changes in plasma fatty acid composition with a 48% decrease in arachidonic acid content of fish oil-fed rats compared with control animals. Rats on the fish oil diet showed a threefold decrease in serum thromboxane generation. Prostacyclin production by incubated segments of aorta was reduced by more than 50% compared with the coconut oil-fed control group. 5. SHR on the fish oil diet showed increased urine volume and sodium excretion, presumably due to increased fluid and salt intake. 6. This study shows that dietary suppression of prostacyclin synthesis is associated with only a minor effect on blood pressure in long-term salt loading of SHR.