Brain corticotropin releasing factor in the spontaneously hypertensive rat

Corticotropin releasing factor and vasopressin were measured in major brain regions including the neurohypophysis in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) during development of hypertension. The highest concentration of corticotropin releasing factor was found in the hypothalamus in both strains. Corticotropin releasing factor was decreased in most major brain regions of SHR. In the hypothalamus, corticotropin releasing factor was lower in 3- and 6-week-old SHR than in age-matched WKY (p less than 0.01), but was similar at 12 and 24 weeks of age. The content of corticotropin releasing factor did not differ in the neurohypophysis in 3-week-old rats but began to decrease at 6 weeks of age (p less than 0.01) and continued to decrease during the development of hypertension (p less than 0.01). Brain vasopressin concentration did not differ between SHR and WKY except in the hypothalamus. The level of hypothalamic vasopressin was consistently lower in SHR than in WKY (p less than 0.01). These peptides are thought to be associated with autonomic nervous regulation, and our results may further strengthen the possibility that the deficit of the peptides may be involved in the development of spontaneous hypertension.     

Brain angiotensin II and baroreceptor reflex function in spontaneously hypertensive rats

We examined whether the increase in baroreceptor reflex function previously reported in lifetime - captopril-treated spontaneously hypertensive rats (SHR) was due to an inhibition of brain angiotensin II mechanisms. Pregnant and lactating SHR were given oral captopril (100 mg/kg/day). After weaning, pups were maintained on captopril (50 mg/kg/day) until the study (19-21 weeks). Control rats received tap water. One week before study captopril-treated and control SHR were given an intracerebroventricular infusion of angiotensin II (7.5 ng/hr, osmotic pump) or vehicle (artificial cerebrospinal fluid). Baroreceptor reflex control of heart rate was assessed by the slope of the relation between the change in mean arterial pressure (mm Hg) versus the change in pulse interval (msec beat-1). Arterial pressure was raised or lowered by intravenous bolus injections of phenylephrine or nitroprusside, respectively. Central infusion of angiotensin II had no significant effect on mean arterial pressure in captopril or control SHR (captopril-angiotensin II 125 +/- 4 vs. captopril-vehicle 121 +/- 2; control-angiotensin II 169 +/- 5 vs. control-vehicle 173 +/- 7 mm Hg), but it produced a significant rise in basal heart rate (captopril-angiotensin II 371 +/- 10 vs. captopril-vehicle 323 +/- 8, p less than 0.0002; control-angiotensin II 338 +/- 7 vs. control-vehicle 312 +/- 8 beats/min, p less than 0.0183) and in daily water intake (captopril-angiotensin II 20.7 +/- 2.2 vs. captopril-vehicle 9.8 +/- 0.7, p less than 0.0426; control-angiotensin II 33.1 +/- 3.8 vs. control-vehicle 9.0 +/- 0.6 ml/100 g body wt, p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Hyperprolactinaemia in the spontaneously hypertensive rat.

A hypothalamic role in the aetiology of hypertension in the spontaneously hypertensive rat (SHR) has been suggested by prior observations. In an attempt to determine whether the central control of prolactin (PRL) release is altered in the SHR we have compared the PRL response to immobilization stress, thyrotrophin releasing hormone (TRH), haloperidol, and L-DOPA in the SHR and in normotensive Wistar control rats. Carotid artery catheters were inserted 48 h prior to the PRL response studies and the catheters were maintained patent with heparinized saline. Timed blood samples were obtained in SHR and control rats weighing 180-225 g. The SHR demonstrated elevated basal serum levels of PRL and greater PRL responses to stress. However, administration of L-DOPA resulted in a similar suppression of serum PRL in the SHR and in the normotensive controls. These findings suggest alteration in the central control of PRL release in the SHR. Observations of elevated basal PRL, exaggerated PRL in response to L-DOPA in SHR are consistent with normal pituitary responsiveness to dopamine suppression of PRL release, but defective hypothalamic metabolism of dopamine. Alterations in central dopamine control mechanisms in the SHR may play a role in the pathogenesis of essential hypertension 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.     

A comparison of CNS angiotensin II binding in the hypothalamus-thalamus-septum-midbrain region of developing spontaneously hypertensive and normotensive rats

Specific angiotensin II (ANG II) binding was studied in brain homogenates from the hypothalamus-thalamus-septum-midbrain (HTSM) region of age-matched 4-, 8-, 12- and 16-week spontaneously hypertensive rats (SHR) and their normotensive controls, Wistar-Kyoto (WKY) rats, using 125I-angiotensin II. Scatchard analysis revealed that the dissociation constants (Kd) ranged from 0.36 to 0.73 nmol/l, although these values were not significantly different at any given age period between the SHR and WKY rats. In contrast, a statistically significant increase in ANG II receptor binding was seen between the SHR and WKY rat at 4 weeks of age. However, this difference was not observed at older age periods. Furthermore, both the SHR and WKY rat showed a decrease in ANG II receptor levels during development, with the most marked reductions occurring between 12 and 16 weeks of age for both strains. These findings suggest that ANG II receptors in the HTSM region of both the SHR and WKY rat are down-regulated during development, that receptor loss is more significant in the SHR than in its normotensive control and that binding capacity differences between the two strains are only seen before the onset of measureable increases in the arterial pressure of the SHR. We conclude that there is a significant difference in the ANG II binding capacity during the development of hypertension in the SHR as compared with the WKY rat and therefore it may play a role in the pathogenesis of this disorder.     

Immunisation of spontaneously hypertensive rats against angiotensin I

Immunization against angiotensin I has been considered in comparison with the immunization against renin, in the spontaneously hypertensive rat (SHR). Among 6 different methods of immunization, two (AI-gluta-LPH and AI-Carbo-LPH coupling) permitted to obtain high levels of antibodies against angiotensin I (higher than 1/10,000), after four injections of 50 micrograms AI at three weeks interval. The titration of the antibodies was realized in radio-immuno-assay (RIA), with the determination of the cross-reactivity with AII by the same method. Characterization of the isotypes and the affinity calculation were realized with the ELISA method. The average level of antibodies is about 1/10,000 to 1/100,000, and the cross reactivity of the antibodies for AII is about 0.1 p. 100 in RIA. In ELISA, the study of the different isotypes shows a good maturation of the immune system with a sharp elevation of the IgG1 and IgG2 alpha isotypes, after 2 or 3 injections. The affinity of the antibodies purified by affinity chromatography is about 10.3 10(-9) M. The weekly measure of the arterial pressure during 6 months does not reveal at any moment a fall of pressure during the immunizations. The average pressure of the immunized group (209.4 +/- 23.8 mmHg, n = 40) is non significantly different from the average pressure of the mock group (208.5 +/- 22.6 mmHg, n = 10).     

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.     

Systemic and regional hemodynamic profile of five angiotensin I converting enzyme inhibitors in the spontaneously hypertensive rat

The effects of 5 angiotensin I converting enzyme (ACE) inhibitors--captopril, enalapril, perindopril, trandolapril and ramipril--on general and regional hemodynamics were investigated (using radioactive microspheres) and compared in anesthetized adult spontaneously hypertensive rats. The 5 treatments were administered daily by gavage for 8 days in doses inducing identical decreases in arterial blood pressure. This effect was entirely due to a decrease in total peripheral resistance inasmuch as cardiac index was not affected by the 5 ACE inhibitors. In addition, despite their different chemical structures, all exhibited the same regional vasodilator pattern, which thus appears to be related only to ACE inhibition. The vascular beds resistances were decreased in the following order: renal greater than splenic = liver greater than skin greater than total peripheral greater than muscle = brain. Simultaneously, and despite the decrease in perfusion pressure, most regional blood flows and especially renal blood flow were increased. Finally, renal vasodilator effects of ACE inhibitors were observed even after doses that lacked any effect on total peripheral resistance.     

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 sarcoplasmic reticulum and angiotensin II receptor type 1 gene expression in spontaneously hypertensive rat hearts

Left ventricular hypertrophy (LVH) is an adaptive change in response to hypertensive pressure overload. Some evidence indicates that the decrease in sarcoplasmic reticulum (SR) Ca2+-ATPase mRNA expression, which may contribute to a diastolic dysfunction of the heart, occurs in the experimental pressure overload model. Also, recent studies have demonstrated that angiotensin II (Ang II) and angiotensin II receptor type 1 (AT1) play important roles in LVH. The purpose of this study was to investigate the function of the SR and the role of AT1 in genetic hypertension in spontaneously hypertensive rats (SHR) at ages 10 and 18 weeks. In SHR, cardiac hypertrophy has already developed at 10 weeks of age. SR Ca2+-ATPase activity and mRNA expression were significantly lower in SHR than in Wistar-Kyoto rats (WKY). Plasma renin activity in SHR was unchanged compared with WKY, whereas the Ang II concentration in SHR was significantly higher than that in WKY. AT1 mRNA expression in SHR was similar to that in WKY. These results suggest that in the early stage of hypertension in SHR Ang II may stimulate hypertrophy in the cardiomyocytes through the AT1, which is not downregulated by a high concentration of Ang II.     

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.     

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.