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.     

Parathyroid hypertensive factor secretion from subcultured spontaneously hypertensive rat parathyroid cells

Cells dissociated from spontaneously hypertensive rat (SHR) parathyroid glands were grown in culture. Media harvested from the cell cultures were analyzed for parathyroid hypertensive factor (PHF) using the blood pressure bioassay. Cells raised in DMEM containing normal (1.8 mmol/L) CaCl₂ secreted a negligible amount of PHF, while cells cultured in Ham’s F-12 medium containing low (0.3 mmol/L) CaCl₂ secreted higher amounts of PHF. The PHF secretion in Ham’s F-12 medium was highest in early passage cells, and was maintained for approximately 12 to 15 passages. PHF purified from the cell culture medium exhibited chromatographic properties identical to those previously described for PHF isolated from SHR plasma or SHR parathyroid gland organ culture medium. These results support the parathyroid gland as the organ of origin of PHF.     

Increased dietary calcium lowers blood pressure in the spontaneously hypertensive rat

The effect of increased dietary calcium on the development of hypertension in spontaneously hypertensive (SH) rats was investigated by feeding lab chow fortified with calcium carbonate (2.5% calcium, hCa) beginning at 4 wk of age. A control SH group was fed regular lab chow (1.2% calcium, rCa). Two groups of age-matched Wistar-Kyoto (WKY) rats were treated in parallel. Systolic blood pressure (BP) was measured weekly until the age of 18 wk using a tail cuff method. The hCa diet significantly attenuated the time course of hypertension in SH rats even though both SH groups eventually developed hypertension. The hCa also lowered BP in WKYs, but to a lesser extent. Urine output (24-hr volumes) was not affected by hCa, but in both SH and WKY groups fed the hCa diet, the excretion of Na+, K+ and Ca++ was markedly elevated at 11, 15, and 19 wk of age. Urine osmolality was also elevated. Plasma Na+, Ca++ and osmolality were not significantly altered by the diet in either SH or WKY rats; plasma potassium was significantly lower in the SH group fed the hCa diet than in the group given rCa. The hCa diet did not significantly affect the body or heart, kidney, adrenal, or thymus weights. The results suggest that hCa diet may attenuate genetic hypertension by inducing an osmotic diuresis.     

Tunica media changes in the spontaneously hypertensive rat (SHR)

The histological, ultrastructural, morphometrical and histochemical aspects of the arterial media were studied in young and aged SHR, and compared to normotensive Wistar Kyoto rats. The diffuse thickening was the most characteristic feature of the hypertensive media. It seems due to three processes: Early generalized hypertrophy of smooth muscle cells (smc); connective matrix neogenesis and smc proliferation, more evident in peripheral vasculature. The present paper discusses the following hypertensive tunica media changes in relation to the atherosclerotic process: the decrease in lipolytic esterase and cholinesterase activities; the activation of some lysosomal enzymes; the increase in collagen, glycosaminoglycan and elastin content; the increased media thickness; the modified smc behavior (migration, secretion, proliferation). These alterations might positively influence arterial susceptibility to atherosclerosis through reduced smc lipolytic activity; slowed transmural diffusion; perturbed efflux and aggravated media hypoxia.     

Free cytosolic calcium in renal proximal tubules from the spontaneously hypertensive rat

Free intracellular calcium was measured in renal proximal tubules obtained from spontaneously hypertensive rats (SHR) and from age-matched Wistar-Kyoto rats (WKY) ingesting a normal diet. Experiments were performed on renal proximal tubule suspensions using fura-2 to monitor cytosolic calcium. In 4-week-old rats, when systolic blood pressure was not significantly different between the two groups, renal proximal tubule cytosolic calcium was similar (143 +/- 28 and 144 +/- 15 nM, respectively). By the age of 5 weeks, cytosolic calcium increased significantly in both SHR and WKY (214 +/- 24 and 262 +/- 34 nM, respectively, p less than 0.05). Calcium, however, was not significantly different between the two groups, even though at this age blood pressure was higher in SHR than in WKY. As compared with values in 4-week-old rats, cytosolic calcium was also found increased in tubules from both SHR and WKY aged 10 to 12 weeks (261 +/- 42 and 279 +/- 30 nM, respectively) and 20 to 24 weeks (263 +/- 42 and 308 +/- 28 nM, respectively). However, no significant differences in cytosolic calcium were found between SHR and WKY even though at these ages systolic blood pressure increased markedly in the SHR. Moreover, regression analysis failed to reveal a correlation between cytosolic calcium and blood pressure when data from either group of rats of all ages studied were pooled. Exposure to ouabain (10(-3) M) to inhibit Na+,K+-adenosine triphosphatase and increase intracellular sodium had no significant effect on cytosolic calcium in tubules from either SHR or WKY (260 +/- 69 and 250 +/- 45 nM, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute effect of calcium blocker on renal hemodynamics in diabetic spontaneously hypertensive rat

This study was done to examine the acute effect of a calcium channel blocker on renal hemodynamics in the diabetic spontaneously hypertensive rat (SHR). Streptozortocin was used to induce diabetes, and barnidipine (B) was used as a calcium blocker. Renal blood flow (RBF) and glomerular filtration rate (GFR) were measured by a clearance method with paraaminohypurate (PAH) and inulin, respectively. Rats were divided into two groups: nondiabetic SHR, N-SHR; diabetic SHR, DM-SHR. B increased RBF in N-SHR (7.44 ± 1.99 versus 8.50 ± 1.97 mL/min/g·kw) while there was no change in DM-SHR. B reduced renovascular resistance (RVR) in DM-SHR and N-SHR. B increased GFR in N-SHR (1.15 ± 0.24 versus 1.34 ± 0.25 mL/min/g·kw), in spite of no changes in DM-SHR. B did not modify filtration fraction (FF) in both groups. These results indicate (1) in SHR, B exerts beneficial effects on hypertensive renal damage by reducing mean arterial pressure (MAP), RVR, RBF, and GFR; (2) in diabetic SHR, B is less effective in restoring renal hyperfiltration in spite of reducing RVR.     

Changes in central catecholaminergic neurons in the spontaneously (genetic) hypertensive rat

Catecholamines and catecholamine-synthesizing enzymes have been examined in specific brain areas during the development of spontaneously (genetic) hypertensive (SH) rats. Changes in catecholamine metabolism were localized to regions of the brain implicated in the regulation of blood pressure. Norepinephrine levels and dopamine-beta-hydroxylase (DBH) activities were decreased in specific nuclei of the hypothalamus and in the nucleus interstitialis striae terminalis ventralis, in both young and adult rats. The decrease in the formation of norepinephrine can result in a reduced activation of central alpha-adrenergic receptors which may be related causally to the onset of hypertension. The activity of the epinephrine-forming enzyme, phenylethanolamine-N-methyltransferase (PNMT), was increased in the A1 and A2 areas of the brainstem in young SH rats, but it was normal in adult hypertensive animals. These results implicate adrenergic neurons in the brainstem and noradrenergic neurons in the hypothalamus in the development of spontaneous (genetic) hypertension in rats.     

Disturbed sodium and calcium exchange in erythrocytes of spontaneously hypertensive rats (author's transl)

In red blood cells of spontaneously hypertensive and of normotensive rats intracellular NA+ activity (a(i)Na), Na+ concentration (]Na+]i) and Ca2+ activity (a(i)Ca) were measured by ion-selective electrodes and flame photometry, respectively. In spontaneously hypertensive rats a(i)Na and a(i)Ca were significantly elevated as compared with the normotensive controls, whereas [Na+]i showed no significant difference. In hypertensive animals a(i)Na, exceeded [Na+]i, in normotensive rats a(i)Na was lower than [Naf+]i. From these results it can be concluded: (1) In spontaneously hypertensive rats the transmembraneous distribution of free Na+ is altered, (2) an elevation of intracellular free Ca2+ may contribute to increased vascular contractility in hypertensive animals, (3) the described abnormalities may be caused by a lowered binding-capacity of intracellular macromolecules for Na+ and Ca2+.     

Regulation of parathyroid hypertensive factor secretion by Ca2+ in spontaneously hypertensive rat parathyroid cells

BACKGROUND: Elevated parathyroid hypertensive factor (PHF) has been suggested to play a causal role in the pathogenesis of hypertension. Previous studies have indicated that PHF secretion is stimulated by low extracellular (EC) Ca2+. Therefore, we hypothesized that the calcium-sensing receptor (CaR) is involved in regulation of PHF release. METHODS: Parathyroid gland (PTG) organ and cell cultures derived from spontaneously hypertensive rats (SHR) or Wistar-Kyoto (WKY) rats were exposed to low and normal EC Ca2+ and PHF release measured by ELISA. Expression of CaR protein was assessed by Western blot. RESULTS: Low EC Ca2+ stimulated both SHR and WKY PTG organ cultures to secrete more PHF, first observable after 60 min incubation. After 4 h, PHF secretion was stimulated (66-fold v 24-fold stimulation for SHR and WKY, respectively). Cultured SHR and WKY parathyroid cells were also stimulated, but to a lesser extent (2.63-fold v 3.75-fold stimulation for SHR and WKY respectively). After 24 h the stimulation by low EC Ca2+ was no longer apparent. Expression of CaR is elevated in the SHR relative to WKY PTG. In both strains expression is higher under conditions of normal (1.5 mmol/L) EC Ca2+ and it increases with incubation time. The apparent suppression of PHF release by normal (1.5 mmol/L) EC Ca2+ is blocked by pre-exposure of the PTG cells to anti-CaR antibody. CONCLUSIONS: Low EC Ca2+ stimulated rapid PHF release from both SHR and WKY PTG. Changes in CaR expression may account for different sensitivity to EC Ca2+ of the two strains and over time.     

Cytosolic calcium in platelets of spontaneously hypertensive rats

To test the hypothesis that an abnormality of the intracellular concentration of ionized calcium, [Ca2+]i, is associated with high blood pressure, we measured [Ca2+]i in the platelets of spontaneously hypertensive (SHR) and Wistar-Kyoto control (WKY) rats using the Quin 2 technique after separation of the platelets in calcium-free medium, during calcium repletion, and upon exposure to agonists which increase platelet [Ca2+]i (thrombin, adenosine diphosphate, serotonin and ionomycin). Despite clear-cut changes in [Ca2+]i during these manipulations, there were no differences between the SHR and WKY rats in baseline levels of [Ca2+]i or in the kinetics of changes in [Ca2+]i. These results do not support the hypothesis that high levels of [Ca2+]i at rest or abnormal kinetics of changes in [Ca2+]i play a pathophysiological role in the hypertension of SHR.     

Developmental change of kidney receptor for atrial natriuretic factor in spontaneously hypertensive rat

Properties of human atrial natriuretic factor (ANF) binding to the crude membrane fraction of rat kidney were studied using the ANF-radiolabeled receptor assay; the developmental change of renal ANF receptors in three age groups of spontaneously hypertensive rats (SHR) was also investigated with the methods of radiolabeled receptor assay and the quantitative approach of in vitro macro-autoradiography. Temperature and incubation time greatly influenced ANF binding capacities because of the degradation of radiolabeled ligand. Addition of 5 mM MgCl2 to assay buffer was useful for the stabilization of ANF specific binding. Scatchard analysis suggested that the crude membrane fraction of rat's kidney had a single binding site with the apparent dissociation constant of 0.55 nM. In the study of the developmental change of renal ANF receptor in SHR, systolic blood pressure of the SHR at the age of 5 weeks and 12 weeks was significantly higher than that of age-matched Wistar-Kyoto (WKY) rats, but there was no significant difference in blood pressure between SHR and WKY rats at the age of 3 weeks. Concerning the radiolabeled receptor assay of ANF, the apparent dissociation constant and maximum binding capacity in SHR were low in all age groups when compared with those of WKY rats. In the in vitro macro-autoradiographic observation, the specific binding of ANF was localized mainly in the renal cortex, and these binding patterns of SHR and WKY rats were the same in all age groups.(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.     

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.     

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.     

Environmental factor(s) during suckling exert long-term effects upon blood pressure and body weight in spontaneously hypertensive and normotensive rats.

To define the role of nurse and pup strain (spontaneously hypertensive rats: SHR nurses and shr pups; Wistar-Kyoto rats: WKY nurses and wky pups) and dietary salt in nurses and pups (low salt and high salt) upon pup body weight and blood pressure, we cross-suckled shr and wky from birth. Nurses after delivery and pups after weaning received either low- or high-salt diets. Pup body weight indicated that WKY dams were better nurses than SHR and that high salt in the nurse and pup diet decreased body weight. At 7 days, WKY on a low-salt diet normalized shr blood pressure and SHR on a high-salt diet reduced shr blood pressure. At 100 days, pup strain strongly affected blood pressure. Salt sensitivity to pup dietary salt depended upon a low-salt diet in nurses. Salt resistance depended on a high-salt diet in WKY nurses. At 300 days, pup strain accounted for most of the blood pressure variability. However, pup diet, pup strain and nurse diet also affected blood pressure (P = 0.05); the blood pressure of shr on a high-salt diet was higher if the nurse was an SHR on a high-salt diet. Thus, nurse environment modulated shr hypertension throughout life.