In addition to artificially-induced experimental models for stroke such as thrombo-embolism and ligation of cerebral arteries, a unique model, the stroke-prone spontaneously hypertensive rat (SHRSP) developing hemorrhagic or thrombotic stroke spontaneously without exception nowadays, was established in Japan and has been extensively studied world-wide for the pathogenesis and prevention of stroke. Moreover, in search of“a model for lacular stroke”which is preponderant cause of cerebrovascular dementia in aged populations such as Japan, we recently succeeded in establishing the model by cross breeding between SHRSP and OM strain with accelerated platelet aggregavility.
Studies on the pathogenesis showed that genetic factors (consisting of more than 5 major genes) and their interaction with environmental, especially nutritional factors such as salt, protein and lipid intakes were important ; Cultured smooth muscle cells obtained from SHRSP are more vulnerable to hypoxia than those from normotensive control rats, offering the evidence suggestive of cellular desposition to cerebrovascular damages. Severe hypertension, through cerebral blood flow reduction in the brain region fed by recurrent arteries, induces initially medial necrosis of small intracerebral arteries which is the basic vascular lesions for either cerebral hemorrhage or infarction. Dietary conditions with high Na/K ratio accelerate the development of such arterionecrothrombogenic strokes. Not only K but also dietary fibers, especially seaweed or alginic acid obtained from it, counteract the adverse effect of salt and the preventive mechanism of stroke by dietary fibers has been proven to be due to the reduction of gastro-intestinal Na absorption. In contrast, protein-rich diets decrease the incidence of stroke through the attenuation of hypertension, the acceleration of urinary Na excretion and the preservation of vascular wall distensibility. Further, taurine rich in fish protein has been proven to prevent stroke by the neurogenic attenuation of severe hypertension. Among several fatty acids chronically given to SHRSP palmitoleic acids (POA) are proven to be effective for stroke-prevention even in SHRSP on high salt diets.
Some indices such as early development of hypertension, tendency toward cardiovascular hypertrophy and biomembrane abnormalities resulting in intracellular Na retention, are possibly related to predisposition to hypertension and stroke and may be utilized for the scientific prediction of stroke. Thus, stroke is experimentally predictable, and evidently preventable by the control of blood pressure, nutritional conditions and also of thrombogenesis.
The most important message obtained from our experimental models of stroke is“stroke can be prevented by non-pharmacological dietary intervention even in animal models strongly predisposed to cerebrovascular diseases.”Accumulating evidence obtained by our epidemiological and clinical studies support that these experimental findings in animal models can be applied to humans.

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Comparative studies were performed on four SHR substrains; SHR C, SHR B2, SHRSP and M-SHRSP. When the blood pressure increased to 210 mmHg and above in SHRSP and MSHRSP, the plasma renin concentration, cerebrospinal fluid pressure and brain water content began to increase alongside of these elevations in blood pressure and showed significantly positive correlations. High molecular weight renin was also recognized after the blood pressure elevation above 210 mmHg. Blood pressures for SHR C and SHR B2 were below 210 mmHg.
Continued successive selective brother-sister breeding of SHC and M-SHRSP hybrids produced a colony with severe hypertension (blood pressure above 210 mmHg) and marked hyper-cholesterolemia (plasma cholesterol level above 400 mg/dl) .

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The vasoconstrictor responses to electrical nerve stimulation or exogenous norepinephrine as well as endogenous norepinephrine overflow from the adrenergic nerve endings in the isolated mesenteric vasculatures were significantly increased in young SHR (7-8W) compared with their age-matched WKY. On the other hand, basal plasma norepinephrine and its responses to 20 minutes-standing were significantly increased in WHO stage I essential hypertension than those in the normotensive controls and in WHO stage II essential hypertension.
Ca-induced reduction of osmotic fragility of erythrocyte, determined by Coil Planet Centrifuge System, and of erythrocyte membrane fluidity, determined by electron spin resonance method, was more prominent in essential hypertension and SHR compared with that in their normotensive controls.
These results suggest that genetic abnormalities of physicochemical properties or Ca-sensitivity of the membranes could induce an exaggerated sympathetic nerve activity and vascular reactivity and contribute, at least partially, to the pathogenesis of hypertension in SHR and essential hypertension.

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Spontaneously hypertensive rats (SHR) have been regarded as a useful animal model for human essential hypertension. In 1974, stroke-prone SHR (SHRSP) rats were successfully separated from the substrains of SHR. At the prehypertensive phase, the ability of SHR and SHRSP to excrete sodium and water was reduced. Moreover, a reduction of renal perfusion pressure at the hypertensive phase resulted in a marked decrease in sodium and water excretion in both strains of SHR. These results support the hypothesis that kidneys of SHRs require a higher arterial pressure than kidneys of normal rats to excrete a given amount of sodium and water. In addition, reduction in renal perfusion pressure, which seems to be due to the hypertensive arteriolar changes in renal cortex, is likely to be related to the pathogenesis of stroke in SHRSP. From these findings, we speculated that the drugs improving renal hemodynamics in SHR and SHRSP may be useful for the treatment of hypertension and useful for prevention of stroke. In view of our speculation, we successfully developed a new calcium antagonist with a potent inhibitory action against the onset of stroke in SHRSP.
SHRSP rats are also useful for evaluation of the therapeutic drugs for neurological signs in patients with stroke. In the present report, idebenone is introduced as an example of a drug for treating the neurological deficits. The drug showed beneficial effects on post-stroke symptoms, such as emotional changes and decreased spontaneous activity, observed in SHRSP.
The results indicate usefulness of SHR and SHRSP for the development of new cardiovascular drugs.

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A strain of spontaneously hypertensive rats (SHR) has been established as an animal model for human essential hypertension by Okamoto and Aoki. Recently, we found that these SHR have various immunological abnormalities. The percentages of rosette-forming cells and of Thyl.1-positive cells in the thymus of the SHR were lower than those of age-matched W rats. T cell functions such as antibody responses to sheep red blood cells and blastogenic responses to PHA and Con A of SHR spleen cells were significantly suppressed in comparison with those of W rats and progressively decreased with increasing age. The T cell depression of SHR was closely associated with a deficiency in the production of thymic factors relating to T cell differentiation. Furthermore, SHR were found to produce a natural cytotoxic autoantibody (NTA) against thymocytes after they were 1 month old and throughout their lives thereafter. The SHR which showed high blood pressure were always also accompanied with high NTA titers and a high incidence of arterial lesions. Transplantation of neonatal W thymus tissues into 1-week-old SHR showed a long-lasting recovery of T cell numbers and functions. The development of NTA and arterial lesions was also prevented by the neonatal W thymus grafts. When histological examinations were performed on 8-month-old SHR grafted with neonatal W thymus, none of the SHR showed arterial lesions by either macroscopic or microscopic examinations. The blood pressure of the grafted SHR remained at lower levels and was not elevated by aging. These results suggest that there is a close correlation between immunologic abnormalities and the development of hypertension in SHR.

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Preservation of embryos by freezing is useful technique to maintain laboratory animals. In order to establish a SHR embryo bank, we examined the effect of superovulation treatment for SHR females and the suitable preimplantation stages for frozen storage. In addition, the blood pressures of the young derived from frozen embryos were examined.
Mature SHR females were induced to superovulate by gonadotropins (PMSG and hCG) and were mated. Embryos at the two-cell, eight-cell and blastocyst stage were collected from the females. The gonadotropin treatment was effective in increasing the number of embryos except at the blastocyst stage. Two-cell and eight-cell embryos in the presence of 1.5M-DMSO were cooled slowly to - 30°C, plunged into liquid nitrogen and stored. They were thawed rapidly. When the embryos were transferred to pseudopregnant recipients of Wistar strain, approximately 70% developed to live young. All young were suckeled by their foster mothers. The young developed blood pressures as high as naturally delivered SHR.
These results suggest that SHR embryos can be cryopreserved effectively to maintain the strain without losing their genetic characters.

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SHR is a suitable model for human essential hypertension. Stroke-prone SHR (SHRSP) separated from SHR develop severe hypertension and over 95% of them die of stroke under normal dietary condition. However, they develop hypercholesteroremia and ring like fat deposition in small arteries when fed on a high fat cholesterol diet for 2-3 weeks. Many results suppose that development of hypertension and stroke in these models results from the interaction of genetic and environmental factors. It might contribute greatly to prevention of hypertensive diseases that genetic phenotypes related with them can be predicted.
As environmental factors can be controlled in cell cultured system, characteristics of cultured smooth muscle cells from the aorta of SHRSP, SHRSR and Wistar-Kyoto rats (WKY) were examined. Activity of cellular proliferation were increased in SMC of SHRSP and SHRSR than that of WKY and doubling time was shortened in those of SHRSP and SHRSR. SMC from SHRSR and SHRSP showed greater Na⁺ influx and K⁺ efflux than that from WKY under 1mM ouabain treatment. Calcium antagonist, diltiazem, reduced [³H] thymidine incorporation in SMC significantly so that the strain difference in the proliferative activity disappeared completely. There was no obvious effect of diltiazem on sodium influx after blocking Na⁺, K⁺-ATPase with 1mM ouabain. This result indicates that the effect of diltiazem on sodium influx into the cells, but rather due to its direct effect on Ca influx.
The strain difference in the proliferative activity is partly due to difference in transferable rate from Gl stage to S stage of cell cycle. SMC utilize various amino acids when they proliferate. A larger amount of arginine was consumed and formated ornithine in SMC of SHRSP compared to that of WKY. Also, SMC of SHRSP showed abnormalities in lipid metabolism and increased lipid incorporation. These results may contribute to the cellular mechanisms of hypertension and related diseases.

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