Megakaryocytopoiesis in spontaneously hypertensive rats (SHR)

Bone marrow megakaryocytes and their progenitors were studied in SHR in order to obtain more information about megakaryocytopoiesis in hypertension since it is known that various anomalies of platelet function occur in hypertension. Megakaryocytopoiesis under steady state conditions and following stimulated erythropoiesis and thrombocytopenia was not found to be significantly different in SHR from that in normotensive Wistar controls.     

Sex difference of barbiturate-protection in experimental cerebral ischemia in spontaneously hypertensive rats (SHR)

Survival time following bilateral carotid occlusion was significantly longer in spontaneously hypertensive rats (SHR) anesthetized with barbiturate than in those with ether. Under barbiturate anesthesia, female SHR survived longer (16.6 h) than did males (10.9 h), whereas such sex difference was not found in those with ether anesthesia. The present results indicate sex difference of barbiturate-protection in cerebral ischemia.     

Regional distribution of renin and angiotensinogen in the brain of normotensive (WKY) and spontaneously hypertensive (SHR) rats

The distributions of angiotensinogen and specific renin activity were examined in the brains of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto controls (WKY). Specific renin activity was markedly elevated in the pituitary of SHR compared to WKY. Renin levels in other regions of SHR brain were either significantly lower or similar compared to WKY. In contrast, angiotensinogen was significantly elevated in several regions of SHR compared to WKY brain. These results indicate involvement of a brain renin-angiotensin system in the development of genetic hypertension.     

Altered brainstem structure of spontaneously hypertensive (SHR) rats

By use of an antiserum raised against Corticotropin Releasing Factor (CRF 1-41), nerve fibers can be stained in the medulla oblongata and spinal cord of rats. A dense plexus of CRF-immunoreactive (CRFi) nerve fibers is present in the nucleus tractus spinalis nervi trigemini from which fiber bundles enter the tractus spinalis nervi trigemini. Large numbers of CRFi fibers are present in the substantia gelatinosa of the spinal cord, while the tractus solitarius, the nucleus tractus solitarius and the nucleus ambiguus contain a low number of CRFi fibers. In rats treated with colchicine, CRFi cell bodies are found in the hypothalamus and occasionally in the nucleus tractus solitarius and the nucleus olivaris inferior. Posterolateral deafferentation of the hypothalamus did not result in a disappearance of the CRFi fibers in the medulla oblongata and spinal cord 7 days after surgery. These results indicate that CRFi fibers present in the spinal cord and medulla oblongata are part of a novel peptidergic neuronal system, which is different from the hypothalamo-infundibular CRF system.     

Experimental cerebral ischemia in spontaneously hypertensive rats (SHR): Importance of degree of hypertension

To study the relationship between the degree of hypertension and experimentally-induced cerebral ischemia, brain metabolites, including lactate, pyruvate and adenosine triphosphate (ATP) were determined one hour after bilateral carotid occlusion in 119 spontaneously hypertensive rats (SHR) with a variety of mean arterial pressures (MAP). Of these, 36 SHR were given antihypertensive agents for 10 weeks to reduce blood pressure prior to the experiment. There was a significant linear correlation between MAP before and either supratentorial lactate (r = 0.482, p less than 0.001) or the lactate/pyruvate ratio (r = 0.388, p less than 0.001) in the brain after carotid occlusion. An inverse correlation was observed between supratentorial lactate and either ATP (r = -0.627, p less than 0.001) or arterial PCO2 (r = -0.477, p less than 0.001) after carotid occlusion. The changes suggest that the animals with a higher MAP had a greater increase in ischemic metabolites with a decrease in ATP and a more pronounced hypocapnia after carotid occlusion. This hypocapnia is believed to be due to hyperventilation induced by cerebral ischemia. It is concluded that hypertensive rats are more susceptible to cerebral ischemia and the susceptibility is related to the degree of hypertension. By long-term lowering of the blood pressure prior to carotid occlusion, the ischemic changes are lessened in this experimental model.     

Altered CNS neuroanatomical organization of spontaneously hypertensive (SHR) rats

Compared to Wistar-Kyoto (WKY) normotensive control rats, spontaneously hypertensive (SHR) rats have significantly reduced brain weights(−10.6%) and brain volumes (−11.8%). Computerized morphometric analysis of soma cross-sections areas of single neurons in 12 selected hypothalamic regions revealed significant differences between SHR and WKY animals. Neurons from the periventricular, medial and lateral preoptic nuclei and ventromedial hypothalamus show significantly increased soma cross-sectional areas in SHR animals when compared to normotensive controls. Cells located in the two circumventricular organs, organ vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), also showed significantly greater cross-sectional areas in the SHR. In contrast, neurons in the paraventricular and arcuate nuclei and dorsomedial hypothalamus were significantly smaller spontaneously hypertensive rats when compared to normotensive controls. Only neurons in supraoptic nucleus, lateral and anterior hypothalamus have equivalent cross-sectional areas in WKY and SHR animals. Differences also exist in the number of cells in certain nuclei in SHR animals. Cell densities in periventricular preoptic nucleus, paraventricular nucleus, arcuate nucleus, ventromedial and anterior hypothalamus, organ vasculosum lamina terminalis and subfornical organ were reduced in SHR animals compared to WKY controls. Because of decreased brain weight and volume along with observed morphometric differences in individual neuronal soma size and cell densities, it is suggested that the SHR brain differs significantly from normotensive control rats. The differences may underlie some of the abnormalities in cardiovascular and endocrine regulation associated with neurogenic hypertension.     

Experimental focal cerebral ischemia produced by embolization with silicone cylinder in normotensive (NTR) and spontaneously hypertensive rats (SHR): comparison of neurological and pathological findings

A focal cerebral ischemic model was produced by occlusion of the intracranial main cerebral artery with a silicone cylinder in normotensive (NTR) and spontaneously hypertensive rats (SHR). Main cerebral artery could be successfully occluded in approximately 90%. The most frequent embolized site was the distal part of the internal cerebral artery (ICb) and less frequently the horizontal segment of the anterior cerebral artery (Al). Mortality rate of NTR with ICb occlusion (NTR-ICb) was 43% at 72 hours after embolization and that of SHR with ICb occlusion (SHR-ICb) was 67% at 24 hours after embolization. NTR-ICb showed neurological signs (i.e. circling movement, hemiparesis, poor response to pain stimuli) and histologically, showed infarction in the deep cerebral structures (i.e. thalamus, hypothalamus, hippocampus, and internal capsule) accompanied with mild disruption of blood-brain barrier (BBB). SHR-ICb showed more serious neurological signs and more severe cerebral infarction in the deep cerebral structures with severe disruption of BBB. In SHR-ICb, ischemic cerebral edema was more prominent which may deteriorate symptoms and pathological findings compared to NTR-ICb. This embolization model is proposed to be useful for studying the pathophysiology of focal cerebral ischemia, especially, early ischemic edema.     

Significant changes in neuropeptide concentrations in the brain of normotensive (WKY) and spontaneously hypertensive (SHR) rats following knee joint monoarthritis

Changes induced by chronic monoarthritis in the nervous system was studied by measuring concentrations of substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-like immunoreactivities in the brain and in the knee joints of control and monoarthritic normotensive (WKY) and spontaneously hypertensive (SHR) rats on day 21 after the induction of monoarthritis. Knee joint monoarthritis was induced by intra-articular injection of Freund's adjuvant into the right knee joint. The severity of arthritis was examined by measuring knee volumes and scratching behaviour and by X-ray. The right knee of both WKY and SHR monoarthritic rats had an increased volume and osteoporosis. SHR rats had more severe arthritis and increased scratching behaviour compared to the WKY. Tachykinins were significantly decreased in the hypothalamus of arthritic rats. In the pituitary higher concentrations of tachykinins and CGRP were found in the arthritic and/or control SHR rats than in the WKY. In the occipital cortex, striatum and hippocampus NPY was increased in monoarthritic rats. No correlation was found between neuropeptide concentrations in the brain and knee joints. Decrease of tachykinins and increase of CGRP to different degree in the hypothalamus and/or pituitary of the arthritic WKY and SHR rats indicates that these changes were selectively associated with the basal level of sympathetic tone and possibly related to the greater severity seen in SHR rats. The increase of NPY in the brain, not influenced by sympathetic tone, may be part of a general defence reaction to inflammation.     

Cerebral vasomotor reactivity in normotensive and spontaneously hypertensive rats

Intravenously injected metaraminol induced a larger blood pressure increase in spontaneously hypertensive rats (SHR) than in normotensive controls (NR) when the pressure was raised from the same starting level. Cerebral blood flow (CBF) response in NR was either perfect autoregulation, partial autoregulation or "break-through." When present, the autoregulatory response was very rapid, i.e. the flow returned to the initial value within 10-15 sec. All SHR showed an initial prompt vasoconstrictor response which was followed after 30-40 sec by a gradual flow increase. The blood pressure elevation was highest in SHR when hypertension was induced by compression of the aorta, which supports the hypothesis that the enhanced response is, at least in part, a consequence of an increased vessel wall to lumen ratio. The characteristic CBF pattern observed in SHR after a metaraminol-induced rise in blood pressure was not seen when the blood pressure was increased by aortic compression, which suggests an effect of the drug separate from its pressor effect. During maximum vasodilatation the cerebrovascular resistance (CVR) was considerably higher in SHR than in NR. Assuming an equivalent vessel density in the 2 groups, our results suggest that structural changes in resistance vessels in SHR encroach on the lumen.     

The morphometry of consecutive segments in cerebral arteries of normotensive and spontaneously hypertensive rats

The media cross-sectional area, the media thickness, the internal radius and the ratio between media thickness and internal radius were determined in consecutive sections of extraparenchymal cerebral arteries of 7- and 12-month-old normotensive and spontaneously hypertensive rats. The study included intracranial pial and basal arteries as well as extracranial cervical arteries. In the chronically hypertensive rats the media to radius ratio was consistently higher than in normotensive rats over the entire calibre spectrum investigated (radius 5-400 micron). The increase of the ratio in the extracranial arteries of the hypertensive rats was exclusively due to a thicker media. In the basal intracranial arteries the increase of ratio was due to a thicker media and/or a smaller internal radius in 7- and 12-month-old rats with moderate hypertension (mean arterial pressure, MAP 171 +/- 8 and 177 +/- 7 mm Hg respectively). In 7-month-old rats with severe hypertension (MAP 204 +/- 11 mm Hg) the increase of ratio was mainly due to a smaller internal radius. The observed structural alterations are likely to be of hemodynamic importance.     

Differential effects of V2 vasopressin agonist and antagonists on blood pressure regulation in normotensive (WKY) and spontaneously hypertensive (SHR) rats.

The effects of arginine vasopressin analogs with V2 agonistic and antagonistic properties on blood pressure (BP) and heart rate (HR) were compared in conscious, spontaneously hypertensive (SHR) and normotensive (WKY) rats under resting conditions and after administration of phenylephrine (Phe) and sodium nitroprusside (SN). In WKY rats, resting BP and HR were not significantly affected during intravenous (i.v.) infusion of dVDAVP, (V2 agonist; 200 pg/kg/min), d(CH2)5 (D-Ile2,Abu4]AVP (V2 antagonist 1; weak V1 antagonist; V2/V1 ratio = 29; 0.6 microgram/kg/min), d(CH2)5[D-Ile2,Ile4,AlaNH2]AVP (V2 antagonist 2; very weak V1 antagonist; V2/V1 ratio = 83; 0.6 microgram/kg/min) and combined infusion of V2 agonist and V2 antagonist 2. Under resting conditions BP and HR were not affected in WKY by any of the treatments. In SHR rats BP and HR were significantly decreased by V2 antagonist 2 infused alone or in combination with V2 agonist. In WKY but not in SHR V2 agonist without and with prior V2 receptors blockade significantly augmented bradycardia associated with a maximum increase of the systolic blood pressure after Phe administration. Significant differences were found between SHR and WKY in SN-induced changes of HR and BP after administration of V2 agonist and antagonists. The results suggest that circulating vasopressin may modify the baroreflex by interaction with receptors which are stimulated by V2 agonist but are different from the classical V2 receptors. The study supports evidence for differential effects of vasopressin analogs on blood pressure and blood pressure-heart rate relations in WKY and SHR.     

Effects of long-term antihypertensive treatment on cerebral, thalamic and cerebellar blood flow in spontaneously hypertensive rats (SHR)

Cerebral blood flows (CBF) were measured in the parietal cortex, the thalamus and the cerebellum by the hydrogen clearance technique in anesthetized spontaneously hypertensive rats, of which hypertension was treated for 16 weeks (long-term) or 8 weeks (short-term) with antihypertensive agents of hydralazine and guanethidine. As compared to non-treated control animals, CBF in the three regions were significantly increased while the calculated cerebrovascular resistances (CVR) were decreased in hypertension-treated animals. Such CBF and CVR changes were greater in SHR with long-term than short-term therapy. Both an increase in CBF and a decrease in CVR were closely related to a fall in the blood pressure. From the present results, it was concluded that earlier and longer treatment of hypertension could lessen or even prevent the increased CVR due to the hypertensive vascular changes, and increase CBF as a result.     

Mortality and cerebral metabolism after bilateral carotid artery ligation in normotensive and spontaneously hypertensive rats.

Mortality and cerebral glycolytic metabolism were studied after bilateral ligation of the common carotid artery in normotensive Wistar rats (NTR), and spontaneously hypertensive rats (SHR) derived from Wistar strain. In the first 24 hours after occlusion of carotid arteries, 72 per cent of 108 SHR died, whereas it was fatal in only 16 per cent of 43 NTR. In SHR, cerebral lactate and cerebral lactate/pyruvate ratio (L/P ratio) increased by 12.4 and 12.1 times the control, respectively at five to six hours after ligation, and remained raised even in rats surviving for two to three days thereafter. Changes in cerebral lactate and L/P ratio were minimal in NTR. Cerebral ATP decreased markedly at five to six hours after ligation in SHR studied. These results indicate that bilateral carotid artery ligation causes severe brain damage in SHR but not in NTR, suggesting hypertension per se to be operative for the development of cerebral ischaemia.     

Effects of hyperventilation on cerebral blood flow and brain tissue metabolism in normotensive and spontaneously hypertensive rats

Cerebral vascular carbon dioxide (CO2) reactivities were compared in normotensive (NTR) and hypertensive (SHR) rats. Cerebral blood flow (CBF) in cortex and thalamus were evaluated before and during one hour of hyperventilation. After one hour of hyperventilation brain lactate, pyruvate, and ATP concentrations were also determined. Significant and similar reductions of CBF due to hyperventilation induce hypocapnia were found in both NTR and SHR groups. In contrast the percent increase in cerebrovascular resistance (CVR) per unit decrease in paCO2 was significant, indicating that hypocapnia induced vasoconstriction is greater in NTR than in SHR groups. During hyperventilation the average value for lactate in the NTR group was 3.98 mM/kg. In contrast it was 3.15 mM/kg in the SHR group, a significant difference (p less than 0.05). When paCO2 fell below 15 mm Hg the cerebral lactate increased strikingly in the NTR group and cortical CVR was reduced suggesting that an accumulation of the ischemic metabolites caused dilatation of the constricted cerebral vessels. In contrast the SHR group disclosed no such changes. The increase CVR characteristic of SHR appeared to diminish the cerebral vasoconstrictive response to hypocapnia. As a result ischemic metabolites in the brain do not increase in this group to the degree that they do in NTR.     

The CBF threshold and dynamics for focal cerebral infarction in spontaneously hypertensive rats.

Two strategies were used to estimate the blood flow threshold for focal cerebral infarction in spontaneously hypertensive rats (SHRs) subjected to permanent middle cerebral artery and common carotid artery occlusion (MCA/CCAO). The first compared the volume of cortical infarction (24 h after ischemia onset) to the volumes of ischemic cortex (image analysis of [14C]iodoantipyrine CBF autoradiographs) perfused below CBF values less than 50 (VIC50) and less than 25 ml 100 g-1 min-1 (VIC25) at serial intervals during the first 3 h of ischemia. The infarct process becomes irreversible within 3 h in this model. In the second, measurements of CBF at the border separating normal from infarcted cortex at 24 h after ischemia onset were used as an index of the threshold. During the first 3 h of ischemia, VIC50 increased slightly to reach a maximum size at 3 h that closely matched the 24 h infarct volume. VIC25, in contrast, consistently underestimated the infarct volume by a factor of 2-3. CBF at the 24 h infarct border averaged 50 ml 100 g-1 min -1. Taken together, the results indicate that the CBF threshold for infarction in SHRs approaches 50 ml 100 g-1 min-1 when ischemia persists for greater than or equal to 3 h. This threshold value is approximately three times higher than in primates. Since cortical neuronal density is also threefold greater in rats than in primates, the higher injury threshold in the rat may reflect a neuronal primacy in determining the brain's susceptibility to partial ischemia.     

Carotid chemoreceptor discharge responses to hypoxia and hypercapnia in normotensive and spontaneously hypertensive rats

The carotid chemoreceptor discharge responses to hypoxia and hypercapnia were quantitatively compared between normotensive (NTR) and spontaneously hypertensive rats (SHR). For this purpose we recorded afferent mass discharges from the carotid sinus nerve (CSN) at various levels of end-tidal O2 and CO2 concentrations (FetO2, FetCO2 (%)) in the urethane-anesthetized, vagotomized and artificially ventilated rats. The CSN chemoreceptor discharge was evaluated by subtracting the small activity remaining in acute hyperoxia (chemoreceptor inactivation), which was estimated as baroreceptor in origin, from the large total CSN activity. The CSN chemoreceptor discharges at various levels of FetO2 or FetCO2 were expressed as the percent of control activity measured in normoxic and normocapnic conditions (FetO2, 15-16%; FetCO2, 4.5-5.1%). There was an exponential increase in the CSN chemoreceptor discharge as FetO2 was decreased from hyperoxic to various hypoxic levels (maximally 6%) at a maintained FetCO2 (normocapnia). The relationship between the CSN chemoreceptor discharge and the hypoxic stimulus was quantitatively assessed by the regression analysis using an exponential function. Exponential increases in the CSN chemoreceptor discharge by hypoxia and the parameters in the exponential function reflecting the sensitivity to hypoxia were significantly higher in the SHR than in the NTR, which indicated a high carotid chemoreceptor discharge response to hypoxia in the SHR. The CSN chemoreceptor discharge was increased linearly by increasing the FetCO2 from the normocapnic level up to about 10% at a maintained FetO2 (normoxia). Increases in discharge produced by severe hypercapnia were, however, much smaller than that caused by hypoxia. The slope of the CO2 stimulus-CSN chemoreceptor discharge response line was almost the same in NTR and SHR. The results demonstrated that the responsiveness of rat carotid chemoreceptor to hypoxia is augmented in the SHR. The role of carotid chemoreceptor afferents in ventilatory reflex responses to hypoxia and their alterations in the SHR are discussed.     

Changes in catecholamine neuronal uptake and receptor binding in the brains of spontaneously hypertensive rats (SHR)

In these studies we have characterized differences between spontaneously hypertensive rats (SHR) and normotensive Wistar/Kyoto (WKY) rats with respect to rates of neuronal uptake of norepinephrine (NE) and dopamine (DA), and β-adrenergic receptor (dihydroalprenolol; [³H]DHA) binding in the central nervous system. We find that SHR have greater rates of NE uptake in the frontal cortex, cerebellum, hypothalamus and pons-medulla during early development, and that these changes are accounted for, at least in the cerebral cortex, by an increased V_max of the NE uptake mechanism. In addition, we find a decrease in the B_max for [³H]DHA binding, suggestive of down-regulation of beta-adrenergic receptors of this region. In contrast to the results for NE uptake, we have measured significant decreases in DA uptake in the frontal cortex of the SHR at several postnatal ages. Decreases in DA uptake were also observed in the striatum of SHR although these changes were found only in animals approximately 6 weeks of age. From these results we have suggested that NE neurons projecting to a number of brain regions have elevated functional activity, while more regionally selective decreases in dopaminergic functional activity are characteristic of the SHR. We have further proposed that these changes in catecholamine neurons of the central nervous system may play an important role in the development of both the hypertension and behavioral hyperactivity exhibited by these animals.