Streptozotocin-induced diabetes in the spontaneously hypertensive rat.

Spontaneously hypertensive rats (SHR) were more sensitive to the diabetogenic effects of streptozotocin than normotensive Wistar-Kyoto (WKY) rats. Thus, 10 days after intravenous administration of 25 mg/kg streptozotocin in SHR, mean pancreatic insulin content was decreased by 42% (p less than 0.05), and mean plasma glucose concentration was increased from 85 to 215 mg/dl (p less than 0.001), whereas between 37.5 and 50 mg/kg of streptozotocin was required to produce similar effects in normotensive WKY rats. Also, there was a progressive decrease in blood pressure in SHR injected with 25, 35.7, or 50 mg/kg of streptozotocin, whereas blood pressure was progressively increased after streptozotocin in normotensive WKY rats. The opposite effects of streptozotocin-induced diabetes on blood pressure in SHR and WKY rats could be observed at similar degrees of hyperglycemia and are presently unexplained.     

Vascular, ganglia and cardiac catecholamine disposition in the spontaneously hypertensive rat and in the stroke-prone spontaneously hypertensive rat.

We have examined the disposition of catecholamines in cardiac tissue, mesenteric arteries and ganglia from normotensive Wistar-Kyoto rats (WKY), spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive rats (SHR-SP). The norepinephrine (NE) contents of mesenteric arteries from all three strains of rats adhered to a pattern which was characterized by the largest concentrations of the catecholamine in arteries from SHR and SHR-SP rats, and the smallest values present in mesenteric arteries from WKY rats. This pattern of NE disposition was not present in either ganglia or cardiac tissue from the three strains of rats. The results highlight two features of the hypernoradrenergic hypothesis in the SHR. Firstly, the enhanced NE contents observed in the blood vessels of the two hypertensive strains are not consistently increased in sympathetic cell bodies or cardiac tissue. Secondly, the significantly enhanced concentrations of NE in the vasculature parallel the elevated direct arterial blood pressure in the two strains of hypertensive rat when compared with the normotensive strain.     

Pulmonary vascular reactivity in the spontaneously hypertensive rat.

We examined the pulmonary vascular reactivity of normotensive rats (NR) and spontaneously hypertensive rats (SHR) to acute and chronic pressor stimuli. In rats kept at low altitude (1,520 m), SHR had a slight degree of right ventricular hypertrophy, but there was no difference between SHR and NR in either right ventricular systolic pressure or pulmonary artery wall thickness. When compared to blood-perfused lungs from low altitude NR, lungs from low altitude SHR were normoresponsive to acute airway hypoxia, hyporesponsive to intra-arterial angiotensin II, and hyperresponsive to intra-arterial prostaglandin F2alpha. After exposing rats to simulated high altitude (4--6 weeks at 4,270 m) to induce hypoxic pulmonary hypertension, SHR had a higher right ventricular systolic pressure, a greater degree of right ventricular hypertrophy, and more pulmonary artery medial thickening than did NR. The results indicate that although the pulmonary vasculature of SHR does not become hypertensive spontaneously, it might have an increased tendency to develop hypertension when exposed to an appropriate stimulus, i.e., chronic airway hypoxia.     

Adrenocortical activity in the newborn spontaneously hypertensive rat.

Blood pressure is reportedly elevated in the spontaneously hypertensive rat (SHR) neonate, the etiology of which remains unclear. Aberrations in the hypothalamic-pituitary-adrenal axis have been implicated, as it is well accepted that excess corticosteroids are associated with hypertension. We examined aspects of adrenocortical activity in the neonatal SHR 1 to 21 days old and its normotensive genetic control, the Wistar-Kyoto rat (WKY). We found a fourfold greater abundance of P450scc mRNA in adrenals of SHR versus WKY day 1 neonates, and increasing but comparable abundance of adrenal P450c11B mRNA on neonatal days 1 to 21. The pattern of P450c11AS mRNA expression was distinctly different in the adrenals of SHR and WKY neonates; the relative abundance of this mRNA in SHR increased 15-fold over the 21-day period examined, whereas that in WKY remained fairly stable. RT-PCR for the presence/abundance of adrenal P450c11B3 mRNA showed absence in day 1 SHR and WKY, comparable abundances on neonatal days 7 and 14, and a distinctly greater abundance in the day 21 SHR adrenals. Peripheral corticosterone levels were threefold greater in the day 1 SHR neonate; aldosterone levels were elevated in both the SHR and WKY day 1 neonate. Thereafter, corticosterone and aldosterone levels were comparable on days 7, 14, and 21, although the anticipated depression in circulating corticosterone levels typical of the stress hyporesponsive period was noted in both SHR and WKY neonates. Although patterns of adrenocortical activity differ in the newborn SHR and WKY rat, our findings do not support an etiologic role for corticosteroids in the reported hypertension of the SHR. However, observed differences in corticosteroid profiles may augment or have a permissive effect upon the etiologic factor(s).     

Neurotrophin 3 is increased in the spontaneously hypertensive rat.

OBJECTIVE: To determine whether the noradrenergic sympathetic hyperinnervation in the spontaneously hypertensive rat (SHR), a genetic model of essential hypertension, is associated with changes in neurotrophin 3 (NT3) concentrations. METHODS: NT3 levels were measured using a sensitive enzyme-linked immunosorbent assay (ELISA) in the superior cervical ganglia (SCG), heart, mesenteric artery (MA) and blood of postnatal and mature SHR and normotensive Wistar-Kyoto (WKY) rats. RESULTS AND CONCLUSIONS: NT3 levels in SHR are significantly higher in the SCG during the first 4 postnatal weeks, and in the heart and MA from 2 to 10 weeks of age, compared with levels in WKY rats. The elevated NT3 found in the sympathetic ganglia and hyperinnervated organs of SHR indicates that NT3 may play an important role in the development of hyperinnervation, possibly by enhancing the survival and/or nerve sprouting of sympathetic neurons.     

Control of aldosterone secretion in the spontaneously hypertensive rat.

Adrenal secretion rates of aldosterone, corticosterone, and deoxycorticosterone were studied sequentially in the spontaneously hypertensive rat and the normotensive Kyoto Wistar rat. Steroid secretion was studied at three different ages: 7-8, 11-13, and 22-25 weeks. Also, peripheral plasma levels of aldosterone and plasma renin activity were determined in both the spontaneously hypertensive and the normotensive rats at 7-8 weeks of age. Aldosterone secretion was elevated markedly in dexamethasone-morphine-treated spontaneously hypertensive rats at both 7-8 and 11-13 weeks of age but was not significantly different from control in 22-25-week-old spontaneously hypertensive rats. No statistically significant differences in corticosterone or deoxycorticosterone secretion rates were observed between the spontaneously hypertensive rats and the normotensive Kyoto Wistar controls; however, the data suggested that dexamethasone did not suppress adrenocorticotropic hormone in the 7-8- and 11-13-week-old spontaneously hypertensive rats to the same extent that it did in the normotensive Kyoto Wistar rats. Therefore, aldosterone secretion was reexamined in acutely hypophysectomized 7-8-week-old rats to eliminate completely the influence of the anterior pituitary; no differences in aldosterone, corticosterone, or deoxycorticosterone secretion rates were observed between hypophysectomized spontaneously hypertensive rats and normotensive Kyoto Wistar rats. Moreover, aldosterone secretion in the hypophysectomized 7-8-week-old spontaneously hypertensive rats was reduced markedly compared with that in the intact 7-8-week old spontaneously hypertensive rats, thus confirming the importance of the pituitary in these animals. Determinations of peripheral plasma aldosterone concentration and plasma renin activity in unstressed 7-8-week-old spontaneously hypertensive and normotensive rats revealed that both parameters were depressed significantly in the spontaneously hypertensive rats. Thus, the present data indicate that the renin-angiotensin-aldosterone system is suppressed in the spontaneously hypertensive rat but do not suggest that the system is critically involved in the hypertensive process in these animals     

Insulin resistance in the spontaneously hypertensive rat

To determine experimentally if insulin resistance is associated with spontaneously occurring hypertension, insulin-stimulated glucose metabolism was studied in an animal model of genetic hypertension. The spontaneously hypertensive rat (SHR) and its genetic control, the Wistar-Kyoto strain (WKY) were studied with the euglycemic hyperinsulinemic clamp technique. Clamp studies demonstrated reduced insulin-stimulated glucose uptake in SHR (P less than .001). These data indicate that SHR is insulin-resistant when compared with WKY. A reduction of insulin-stimulated glucose metabolism occurred in older animals of both strains, providing evidence of an aging effect on insulin-stimulated glucose metabolism. However, the reduction of insulin-stimulated glucose metabolism was more pronounced in the hypertensive animals. This study demonstrates the presence of peripheral (skeletal muscle) insulin resistance in the SHR.     

Genetic divergence between the Wistar-Kyoto rat and the spontaneously hypertensive rat.

A method of restriction fragment length polymorphism (RFLP) analysis was used to estimate the amount of genetic divergence between the spontaneously hypertensive rat (SHR) strain and the Wistar-Kyoto (WKY) strain. DNA from each strain was digested with eight restriction endonucleases and hybridized with six single copy gene sequences. The number of hybridization bands in each digestion was used to estimate the total number of bases analyzed and RFLPs were scored as single mutations. Divergence was then estimated by dividing the number of mutations by the number of bases analyzed. In a total of 808 bases analyzed in WKY rats, a minimum of 13 mutations were scored in SHR, which yields a nucleotide divergence of 1 change per 62 bp. This is an extremely high amount of divergence given the known origin of these two strains and is comparable to the maximum divergence possible between unrelated humans.     

Brain angiotensin in the developing spontaneously hypertensive rat

There are several factors in the manifestation of high blood pressure in spontaneously hypertensive rats (SHR) which implicate a central role for brain angiotensin II (Ang II). We have measured levels of angiotensin in the brain of SHR and rats of the Wistar-Kyoto strain (WKY). The experiments were carried out in 2-, 4-, 14- and 20-week-old rats. Areas of brain from rats were homogenized and purified with SepPak C-18 cartridges. The levels were measured by radio-immunoassay whose detection limit was 1.95 pg/tube. Significant differences were found between the different age groups and between SHR and controls. In the hypothalamus, there was a consistent elevation of brain Ang II in SHR as compared to WKY in all age groups. Cerebellum also had higher levels in SHR, especially in rats at 2 and 4 weeks of age. Brainstem levels were significantly higher in SHR only in the 14-week-old age group. Plasma levels during these time periods did not differ significantly between the strains. The results demonstrate changes in brain Ang II with development. At an early age, there are high levels of Ang II in the hypothalamus and cerebellum which do not correlate with hypertension but may be important for the development of hypertension. The higher levels of brain Ang II in SHR support the hypothesis that hypertension in SHR is related to brain Ang II activity.     

Low-dose streptozotocin-induced diabetes in the spontaneously hypertensive rat

Streptozotocin (STZ, 35 mg/kg body weight) was injected into spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats, and plasma glucose and triglyceride concentrations measured 10 days later. Neither mean (+/- SEM) plasma glucose (130 +/- 3 v 136 +/- 3 mg/dL) nor triglyceride (93 +/- 6 v 108 +/- 7 mg/dL) concentrations increased in WKY rats. In contrast, both plasma glucose (141 +/- 3 v 262 +/- 36) and triglyceride (121 +/- 8 v 196 +/- 7 mg/dL) concentrations increased significantly (P less than .01) following administration of STZ in SHR. Furthermore, when SHR previously injected with STZ were fed a diet enriched in fructose, they had a further increase (P less than .01) in both plasma glucose (343 +/- 38 mg/dL) and triglyceride (774 +/- 57 mg/dL) concentrations. Plasma triglyceride concentration also increased significantly (P less than .05) when STZ-injected WKY rats ingested the fructose-enriched diet, but plasma glucose levels still remained within the normal range (152 +/- 5 mg/dL). These results indicate that SHR were more sensitive to the effects of a decrease in pancreatic beta-cell function (STZ) and an increase in insulin resistance (fructose feeding) than WKY rats.     

Genetic heterogeneity of the spontaneously hypertensive rat

We examined DNA fingerprints of the spontaneously hypertensive rat from Shimane Institute of Health Science, Izumo, Japan, including seven substrains that were separated in the early stages of the establishment of the stroke-prone spontaneously hypertensive rat, and compared their fingerprints with those of rats from other sources. Obtained DNA fingerprints revealed that, in both the stroke-resistant spontaneously hypertensive rat and the Wistar-Kyoto rat, there is a substantial genetic difference between the rats from the National Institutes of health and from Shimane Institute of Health Science. By contrast, only a small genetic difference was observed either between the rats from the National Institutes of Health and Charles River Laboratories or among the substrains of the spontaneously hypertensive rat in the Shimane Institute of Health Science. Further, in the strains from the Shimane Institute of Health Science, there were fingerprinting bands that could distinguish either the Wistar-Kyoto rat from all the substrains of the spontaneously hypertensive rat or the stroke-prone from the stroke-resistant spontaneously hypertensive rat in spite of their close genetic backgrounds. From the observations above, we concluded 1) that there is substantial genetic variance of the spontaneously hypertensive rat between the two major sources in the world, the National Institutes of Health and the Shimane Institute of Health Science and 2) that by DNA fingerprinting analysis, it is possible to identify the restriction fragment length polymorphisms that are specific for the spontaneously hypertensive rat or the stroke-prone spontaneously hypertensive rat. These polymorphisms can be applied in the segregation study of the F2 generation.     

Alterations in renal endothelin-1 production in the spontaneously hypertensive rat.

Endothelin-1 inhibits sodium and water transport systems in the inner medullary collecting duct. Endothelin-1 levels are reduced in the medulla of spontaneously hypertensive rats (SHR), raising the possibility that decreased inner medullary collecting duct production of endothelin-1 could contribute to inappropriate sodium and water retention. In the current study, immunoreactive endothelin-1 was measured in the urine, blood, and eluates from cortex and outer and inner medulla of SHR before (age 3-4 weeks) and after (age 8-9 weeks) the development of hypertension and in age-matched Wistar-Kyoto (WKY) controls. There was no difference in endothelin-1 levels between prehypertensive SHR and WKY rats. In contrast, 8-9-week-old SHR had significantly reduced endothelin-1 in the urine and outer and inner medulla, but not in the cortex or serum compared with those of WKY controls. Furthermore, inner medullary collecting duct cells from 8-9-week-old SHR, either acutely isolated or cultured, released less endothelin-1 than did those from WKY rats. Finally, the level of endothelin-1 messenger RNA was only reduced in the inner medulla and in inner medullary collecting duct cells from 8-9-week-old SHR. In summary, renal medullary, and in particular terminal collecting duct, endothelin-1 production is reduced in SHR only after the development of hypertension. Such decreases in inner medullary collecting duct endothelin-1 production may contribute to the hypertensive state in SHR.     

Pathogenesis of hypertension in the sinoaortic-denervated spontaneously hypertensive rat

The present study was performed to examine the relation between the gain of the baroreceptor reflex and the pathogenesis of hypertension in the spontaneously hypertensive rat. Spontaneously hypertensive or Wistar-Kyoto rats underwent either sinoaortic baroreceptor denervation or sham denervation at 28-35 days of age. Four months later these rats were chronically instrumented for measurements of arterial pressure and heart rate. Sixty-minute computerized measurements of arterial pressure showed no difference between spontaneously hypertensive sham (163 +/- 5 mm Hg) and spontaneously hypertensive baroreceptor-denervated (166 +/- 5 mm Hg) rats, or Wistar-Kyoto sham (114 +/- 3 mm Hg) and Wistar-Kyoto baroreceptor-denervated (121 +/- 4 mm Hg) rats. The gain of baroreceptor reflex control of heart rate was assessed by measuring maximal heart rate responses to changes in arterial pressure elicited by bolus injection of phenylephrine and nitroprusside (gain = slope of linear regression equation of change in heart rate versus change in arterial pressure). Baroreceptor reflex gain was significantly higher in Wistar-Kyoto sham rats (-2.10 beats/min/mm Hg) than spontaneously hypertensive sham rats (-0.94 beats/min/mm Hg). Baroreceptor denervation significantly decreased baroreceptor reflex gain in both Wistar-Kyoto (-0.26 mm Hg) and spontaneously hypertensive (-0.22 beats/min/mm Hg) groups. Since baroreceptor denervation did not exacerbate the development of hypertension in adult spontaneously hypertensive rats or lead to hypertension in Wistar-Kyoto rats, we conclude that a primary dysfunction in the baroreceptor reflex alone is not responsible for the development of hypertension in this model.     

Exercise-induced myocardial capillary growth in the spontaneously hypertensive rat

Spontaneously hypertensive rats (SHR) were studied to test the hypothesis that endurance exercise training can stimulate capillary growth and offset the decrement associated with the development of myocardial hypertrophy. The exercise group (SHR-T) was trained on a treadmill for 10 weeks at 70-90% maximum VO2 and compared to nontrained SHR and normotensive Wistar-Kyoto (WKY) at 16 weeks of age. Thus, the training program coincided with the development of hypertension and hypertrophy in SHR. Image analysis was used to study capillaries in one micron thick left ventricular tissue samples from perfuse-fixed hearts. Training did not affect left ventricular mass or blood pressure, but reversed the characteristic decrements in capillary surface area (CSA), volume (CV), and numerical density (CD). CSA and CV were most markedly affected by exercise, as mean values for these parameters increased by 31 and 40%, respectively, compared to SHR. The magnitude of these changes approximated the magnitude of hypertrophy as evidenced by left ventricular weight/body weight ratios (42% in SHR and 37% in SHR-T). Anatomical intercapillary distance was also normalized by training (means +/- SEM): SHR-T, 11.65 +/- 0.31; SHR, 13.97 +/- 0.37; WKY, 11.19 +/- 0.37. These data indicate that exercise stimulates capillary growth in the face of developing hypertension and its related left ventricular hypertrophy.     

Myocardial beta-adrenergic receptors in the stroke-prone spontaneously hypertensive rat

In view of the severity of the hypertension in the stroke-prone spontaneously hypertensive rats (sp-SHR), myocardial beta-adrenergic receptors were investigated by the binding of (?)[³H]-dihydroalprenolol (DHA) to membranes from sp-SHR (9-week-old males, Okamoto-Aoki strain) and age-matched and sex-matched normotensive Wistar-Kyoto rats. Scatchard analysis showed no significant differences in binding parameters between sp-SHR and normal rats. Myocardial membranes from sp-SHR bound 31.8 ± 2.3 fmol DNA per mg protein with a dissociation constant of 3.8 ± 0.9 nm, whereas membranes from normal rats bound 33.4 ± 2.9 fmol DHA per mg protein with a dissociation constant of 3.9 ± 1.0 nm. However, mean arterial pressure and heart rate determined directly via aortic cannulae, while the rats were conscious and unrestrained, were significantly higher in sp-SHR. Plasma norepinephrine concentration was also significantly higher in sp-SHR. The finding that cardiac beta-adrenergic receptors are unchanged, despite evidence of increased sympathetic nerve activity, suggests that in the sp-SHR there may be a failure of catecholamine-induced “down regulation” of beta-adrenergic receptors. This defect could contribute to the increased cardiac drive in these animals and may thus explain the severity of the hypertension in this strain of spontaneously hypertensive rats.     

Central resetting of baroreflex in the spontaneously hypertensive rat

The role of central nervous system in the resetting of baroreflex was investigated in 5-month-old spontaneously hypertensive rats (SHR) of Okamoto strain. Age-matched Wistar-Kyoto (WKY) rats were used as normotensive controls. The aortic nerves, which in the rat, contain few or no chemoreceptor fibers, were stimulated electrically using a wide range of stimulus frequencies. The depressor responses (expressed as percent decrease in blood pressure as compared to its blood pressure value prior to aortic nerve stimulation) produced by these stimulations were significantly smaller in SHR than those in WKY. In another series of experiments, changes in the efferent limb of the baroreflex arc (i.e., greater splanchnic nerve activity) in response to stimulation of the baroreceptor afferents in the aortic nerve were recorded. Inhibition of the greater splanchnic nerve activity due to aortic nerve stimulation was found to be significantly smaller in SHR than in the WKY. Control sympathetic nerve activity was greater in SHR than in WKY. These results suggest that the central bulbospinal nervous system may be another site for resetting of baroreflex in hypertension.     

Alterations of lymphocyte populations during development in the spontaneously hypertensive rat.

OBJECTIVE: Immune system abnormalities have been linked to hypertension in the spontaneously hypertensive rat (SHR). The goal of our study was to examine different lymphocyte subpopulations in the prehypertensive and developmental phases of hypertension in the SHR. DESIGN: Blood samples were obtained from SHR and Wistar-Kyoto (WKY) rats at the following time-points: 2 weeks and 1, 2, 3 and 4 months. Lymphocytes were separated from the whole blood. METHODS: Monoclonal antibodies were used to fluorescently label the following lymphocyte subpopulations; total T cells, T non-helper cells, T helper cells and B cells. Fluorescence-activated cell sorting (FACS) analysis was used to quantify the percentages of the different subpopulations examined. RESULTS: The T non-helper cell population was depressed in SHR from 2 weeks of age. This finding persisted throughout the entire 4-month study period. At the 4-month time-point, the total T cell percentage was also depressed in the SHR. CONCLUSIONS: These results demonstrate that immune system abnormalities are present in the prehypertensive and developmental phases of hypertension in the SHR. This supports the hypothesis that the immune system is involved in the development and maintenance of hypertension in the SHR, preceding not adapting to this state.