Participation of neural factor in the pathogenesis of hypertension in the spontaneously hypertensive rat

Recent Advances in Pharmacological and Non-Pharmacological Strategies of Cardioprotection

Ischemic heart diseases (IHD) are the leading cause of death worldwide. Although the principal form of treatment of IHD is myocardial reperfusion, the recovery of coronary blood flow after ischemia can cause severe and fatal cardiac dysfunctions, mainly due to the abrupt entry of oxygen and ionic deregulation in cardiac cells. The ability of these cells to protect themselves against injury including ischemia and reperfusion (I/R), has been termed “cardioprotection”. This protective response can be stimulated by pharmacological agents (adenosine, catecholamines and others) and non-pharmacological procedures (conditioning, hypoxia and others). Several intracellular signaling pathways mediated by chemical messengers (enzymes, protein kinases, transcription factors and others) and cytoplasmic organelles (mitochondria, sarcoplasmic reticulum, nucleus and sarcolemma) are involved in cardioprotective responses. Therefore, advancement in understanding the cellular and molecular mechanisms involved in the cardioprotective response can lead to the development of new pharmacological and non-pharmacological strategies for cardioprotection, thus contributing to increasing the efficacy of IHD treatment. In this work, we analyze the recent advances in pharmacological and non-pharmacological strategies of cardioprotection.

Pharmacological modulation of b-adrenoceptors as a new cardioprotective strategy for therapy of myocardial dysfunction induced by ischemia and reperfusion

Purpose:

To evaluate the cardioprotective response of the pharmacological modulation of β-adrenergic receptors (β-AR) in animal model of cardiac ischemia and reperfusion (CIR), in spontaneously hypertensive (SHR) and normotensive (NWR) rats.

Methods:

CIR was induced by the occlusion of left anterior descendent coronary artery (10 min) and reperfusion (75 min). The SHR was treated with β-AR antagonist atenolol (AT, 10 mg/kg, IV) 5 min before CIR, and NWR were treated with β-AR agonist isoproterenol (ISO, 0.5 mg/kg, IV) 5 min before CIR.

Results:

The treatment with AT increased the incidence of VA, AVB and LET in SHR, suggesting that spontaneous cardioprotection in hypertensive animals was abolished by blockade of β-AR. In contrast, the treatment with ISO significantly reduced the incidence of ventricular arrhythmia, atrioventricular blockade and lethality in NWR (30%, 20% and 20%, respectively), suggesting that the activation of β-AR stimulate cardioprotection in normotensive animals. Serum CK-MB were higher in SHR/CIR and NWR/CIR compared to respective SHAM group (not altered by treatment with AT or ISO).

Conclusion:

The pharmacological modulation of β-AR could be a new cardioprotective strategy for the therapy of myocardial dysfunctions induced by CIR related to cardiac surgery and cardiovascular diseases.

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Aging Male Spontaneously Hypertensive Rat as an Animal Model for the Evaluation of the Interplay between Contrast-Induced Acute Kidney Injury and Cardiorenal Syndrome in Humans

BACKGROUND

Although there are some animal models for biomarkers of contrast-induced acute kidney injury (CI-AKI), for cardiorenal syndrome (CRS) and for acute renal failure, the interplay between CI-AKI and CRS has yet to be evaluated. Insight into the pathogenesis of CRS is urgently needed from animal models in order to foster the discovery and implementation of novel biomarkers for this disease. Specially designed animal models for type 1 and 3 CRS, particularly CI-AKI, have not yet emerged.

SUMMARY

We hypothesize that the aging male spontaneously hypertensive rat (SHR) is likely to be a suitable model. The SHR model is able to mimic risk factors for preclinical CRS that appears in the clinical setting, specifically hypertension, age, preexisting damage and dysfunction of the heart and kidney, endothelial dysfunction, increased level of reactive oxygen species, decreased level and bioavailability of nitric oxide (NO), impairment of the L-arginine-NO pathway, and insulin resistance. In the SHR, CI-AKI results in a different profile of AKI biomarkers than is seen with preexisting chronic kidney injury.

KEY MESSAGES

The SHR model can be used to evaluate the interaction between CI-AKI and CRS type 1 and 3 and to verify neutrophil gelatinase-associated lipocalin (NGAL) as a reliable CI-AKI biomarker for clinical application. Further research is warranted with a large number of aging male SHRs to prove NGAL as a sensitive, specific, highly predictive, early biomarker for CI-AKI.

Urinary catecholamines and the relationship with blood pressure and pharmacological therapy

OBJECTIVES

Our aim was to assess the importance of the sympathetic nervous system as assessed by urinary catecholamine measurement in the aetiology of essential hypertension and the importance of antihypertensive therapy in the excretion of urinary catecholamines.

METHODS

Twenty-four-hour urinary catecholamine measurement was performed in 1925 patients who were referred for treatment of hypertension and grouped according to the Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure classification: of the 655 untreated patients, 59 were normotensive individuals (SBP < 140 and DBP < 90 mmHg), n = 219 stage 1 (SBP 140-159 or DBP 90-99 mmHg), n = 236 stage 2 (SBP 160-179 or DBP 100-109 mmHg) and n = 141 stage 3 (SBP ≥ 180 or DBP ≥ 110 mmHg).

RESULTS

There was a statistically significant positive relationship between 24-h urinary norepinephrine excretion and the severity of hypertension, such that the higher the blood pressure the higher the urinary norepinephrine excretion (mean ± standard error of mean): normotensive group, 221 ± 13 nmol/24 h; stage 1, 254 ± 8 nmol/24 h; stage 2, 263 ± 7 nmol/24 h and stage 3, 296 ± 12 nmol/24 h (P < 0.001). The above relationship remained highly significant when corrected for urinary creatinine, weight, age and sex. No differences were found with urinary epinephrine or dopamine excretion. Urinary norepinephrine excretion was increased in those patients taking single-drug therapy with either a long-acting dihydropyridine calcium antagonist or a β-blocker.

CONCLUSION

Our results demonstrate that in untreated hypertensive patients, urinary norepinephrine excretion is increased in proportion to the severity of blood pressure rise and also in patients taking a long-acting dihydropyridine calcium antagonist or a β-blocker. Sympathetic overactivity may play a role in the aetiology and maintenance of essential hypertension.

Cerium Dioxide Nanoparticle Exposure Improves Microvascular Dysfunction and Reduces Oxidative Stress in Spontaneously Hypertensive Rats

The elevated production of reactive oxygen species (ROS) in the vascular wall is associated with cardiovascular diseases such as hypertension. This increase in oxidative stress contributes to various mechanisms of vascular dysfunction, such as decreased nitric oxide bioavailability. Therefore, anti-oxidants are being researched to decrease the high levels of ROS, which could improve the microvascular dysfunction associated with various cardiovascular diseases. From a therapeutic perspective, cerium dioxide nanoparticles (CeO₂ NP) hold great anti-oxidant potential, but their in vivo activity is unclear. Due to this potential anti-oxidant action, we hypothesize that injected CeO₂ NP would decrease microvascular dysfunction and oxidative stress associated with hypertension. In order to simulate a therapeutic application, spontaneously hypertensive (SH) and Wistar-Kyoto (WKY) rats were intravenously injected with either saline or CeO₂ NP (100 μg suspended in saline). Twenty-four hours post-exposure mesenteric arteriolar reactivity was assessed via intravital microscopy. Endothelium-dependent and –independent function was assessed via acetylcholine and sodium nitroprusside. Microvascular oxidative stress was analyzed using fluorescent staining in isolated mesenteric arterioles. Finally, systemic inflammation was examined using a multiplex analysis and venular leukocyte flux was counted. Endothelium-dependent dilation was significantly decreased in the SH rats (29.68 ± 3.28%, maximal response) and this microvascular dysfunction was significantly improved following CeO₂ NP exposure (43.76 ± 4.33%, maximal response). There was also an increase in oxidative stress in the SH rats, which was abolished following CeO₂ NP treatment. These results provided evidence that CeO₂ NP act as an anti-oxidant in vivo. There were also changes in the inflammatory profile in the WKY and SH rats. In WKY rats, IL-10 and TNF-α were increased following CeO₂ NP treatment. Finally, leukocyte flux was increased in the SH rats (34 ± 4 vs. 17 ± 3 cells/min in the normotensive controls), but this activation was decreased following exposure (15 ± 2 vs. 34 ± 4 cells/min). These results indicated that CeO₂ NP may alter the inflammatory response in both SH and WKY rats. Taken together, these results provide evidence that CeO₂ NP act as an anti-oxidant in vivo and may improve microvascular reactivity in a model of hypertension.

Neurogenic hypertension: revelations from genome-wide gene expression profiling

There is now good evidence for a role of the sympathetic nervous system in the etiology of essential hypertension in humans. Although genetic variation is expected to underlie the elevated sympathetic outflow in this complex polygenic condition, only limited information has emerged from classic molecular genetic studies. Recently, progress has been made in understanding neurogenic aspects by determination of global alterations in gene expression in key brain regions of animal models of neurogenic hypertension. Such genome-wide expression studies in the hypothalamus and brainstem support roles for factors such as neuronal nitric oxide synthase, inflammation and reactive oxygen species. A role for non-coding RNAs such as microRNAs, and epigenetic alterations await exploration. Ongoing novel approaches should provide a better understanding of the processes responsible for the increased sympathetic outflow in animal models, as well as essential hypertension in humans. Such information may lead to better therapies for neurogenic hypertension in humans.

Fasudil attenuates sympathetic nervous activity in the adrenal medulla of spontaneously hypertensive rats

We investigated the effects of fasudil, a Rho kinase inhibitor, on hypertension in spontaneously hypertensive rats and on the catecholamine synthetic pathway. Ten-week-old male SHR and Wistar-Kyoto rats were administered fasudil (10 mg/kg/day s.c.) for 4 days. Systolic blood pressure was measured using the tail-cuff method. Catecholamine levels were measured with high-performance liquid chromatography-ECD methods. Tyrosine hydroxylase protein levels were measured in Western blot analysis. The tyrosine hydroxylase mRNA level was measured using real-time PCR methods. Fasudil significantly decreased systolic blood pressure in spontaneously hypertensive rats, but not in Wistar-Kyoto rats. Fasudil also significantly decreased catecholamine, tyrosine hydroxylase protein, and tyrosine hydroxylase mRNA levels in the adrenal medulla of spontaneously hypertensive rats. These results suggest that the depressor effects of fasudil on hypertension in spontaneously hypertensive rats may be related to inhibition of the catecholamine synthetic pathway.

Effects of high-sucrose feeding on insulin resistance and hemodynamic responses to insulin in spontaneously hypertensive rats

This study was designed to investigate the effects of a sucrose diet on vascular and metabolic actions of insulin in spontaneously hypertensive rats (SHR). Male SHR were randomized to receive a sucrose or regular chow diet for 4 wk. Age-matched, chow-fed Wistar-Kyoto (WKY) rats were used as normotensive control. In a first series of experiments, the three groups of rats had pulsed Doppler flow probes and intravascular catheters implanted to determine blood pressure, heart rate, and blood flows. Insulin sensitivity was assessed during a euglycemic hyperinsulinemic clamp performed in conscious rats. In a second series of experiments, new groups of rats were used to examine glucose transport activity in isolated muscles and to determine endothelial nitric oxide synthase (eNOS) protein expression in muscles and endothelin content in vascular tissues. Sucrose feeding was shown to markedly enhance the pressor response to insulin and its hindquarter vasoconstrictor effect when compared with chow-fed SHR. A reduction in eNOS protein content in muscle, but no change in vascular endothelin-1 protein, was noted in sucrose-fed SHR when compared with WKY rats, but these changes were not different from those noted in chow-fed SHR. Similar reductions in insulin-stimulated glucose transport were observed in soleus muscles from both groups of SHR when compared with WKY rats. In extensor digitorum longus muscles, a significant reduction in insulin-stimulated glucose transport was only seen in sucrose-fed rats when compared with the other two groups. Environmental factors, that is, high intake of simple sugars, could possibly potentiate the genetic predisposition in SHR to endothelial dysfunction and insulin resistance.

Physical exercise decreases neuronal activity in the posterior hypothalamic area of spontaneously hypertensive rats

Recently, physical exercise has been shown to significantly alter neurochemistry and neuronal function and to increase neurogenesis in discrete brain regions. Although we have documented that physical exercise leads to molecular changes in the posterior hypothalamic area (PHA), the impact on neuronal activity is unknown. The purpose of the present study was to determine whether neuronal activity in the PHA is altered by physical exercise. Spontaneously hypertensive rats (SHR) were allowed free access to running wheels for a period of 10 wk (exercised group) or no wheel access at all (nonexercised group). Single-unit extracellular recordings were made in anesthetized in vivo whole animal preparations or in vitro brain slice preparations. The spontaneous firing rates of PHA neurons in exercised SHR in vivo were significantly lower (8.5 ± 1.6 Hz, n = 31 neurons) compared with that of nonexercised SHR in vivo (13.7 ± 1.8 Hz, n = 38 neurons; P < 0.05). In addition, PHA neurons that possessed a cardiac-related rhythm in exercised SHR fired significantly lower (6.0 ± 1.8 Hz, n = 11 neurons) compared with nonexercised SHR (12.1 ± 2.4 Hz, n = 18 neurons; P < 0.05). Similarly, the spontaneous in vitro firing rates of PHA neurons from exercised SHR were significantly lower (3.5 ± 0.3 Hz, n = 67 neurons) compared with those of nonexercised SHR (5.6 ± 0.5 Hz, n = 58 neurons; P < 0.001). Both the in vivo and in vitro findings support the hypothesis that physical exercise can lower spontaneous activity of neurons in a cardiovascular regulatory region of the brain. Thus physical exercise may alter central neural control of cardiovascular function by inducing lasting changes in neuronal activity.

Biochemical and morphological characterization of spontaneously hypertensive hyperlipidaemic rats

1. To develop and characterize a new animal model of hypertension and hyperlipidaemia, we cross-bred spontaneously hypertensive rats (SHR) with spontaneously hyperlipidaemic rats (HLR). 2. A new strain of spontaneously hypertensive hyperlipidaemic rats (SHHR) was established at generation 10 through selective mating of brothers and sisters (systolic blood pressure > 150 mmHg, plasma cholesterol > 150 mg/dL). Cross-bred Wistar-Kyoto (WKY) rats and Sprague-Dawley (SD) rats (SDWKY rats) were used as a control. 3. Adrenaline and noradrenaline levels in the plasma and adrenal medulla of male SHHR were significantly higher than those of male SDWKY rats. The tyrosine hydroxylase activity in the adrenal medulla of male SHHR was significantly higher than that of male SDWKY rats. Low-density lipoprotein expression was found in the plasma of male and female SHHR and HLR. Cholesterol 7alpha-hydroxylase mRNA expression in the liver of male SHHR was lower than that of male SDWKY rats. Endothelium lesions and lipid deposition under the endothelium were observed in the aorta of 24-month-old SHHR, especially female SHHR, but not in age-matched HLR and SDWKY rats. 4. The hypertension of this new animal model of hypertension and hyperlipidaemia may be related to increased catecholamine activity and the hyperlipidaemia may be related to changes in the expression of cholesterol 7alpha-hydroxylase mRNA and lipoprotein profiles. The SHHR may be valuable in the study of mechanisms of atherosclerosis and the evaluation of anti-atherosclerosis drugs as a new strain of cardiovascular disease.

Increased cardiac norepinephrine release in spontaneously hypertensive rats: role of presynaptic alpha-2A adrenoceptors

OBJECTIVES AND DESIGN

An increased sympathoadrenergic activation is thought to contribute to the maintenance of elevated blood pressure levels in hypertension. Therefore, the regulation of cardiac presynaptic sympathetic neurotransmission was investigated in spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY).

METHODS AND RESULTS

Electrical field stimulation (1 min, 4 Hz) evoked a higher norepinephrine (NE) overflow from isolated perfused SHR than from WKY hearts (171 +/- 78 versus 111 +/- 27 nmol/g; means +/- SD, n = 7, P < 0.05). The difference in stimulation-evoked NE overflow was neither due to increased NE stores nor to a higher density of sympathetic nerve endings in SHR hearts. Furthermore, impairment of cardiac NE re-uptake was ruled out, as pharmacological inhibition of NE re-uptake by desipramine (300 nmol/l) similarly increased NE overflow from SHR (+ 54 +/- 17%) and WKY hearts (+ 59 +/- 18%). However, inhibition of presynaptic alpha-2 adrenoceptors (alpha-2R) with yohimbine (1 micromol/l) resulted in a significantly larger increase in NE overflow from WKY (+ 244 +/- 42%) than from SHR hearts (+ 162 +/- 47%, P < 0.05 versus WKY), indicating impairment of presynaptic inhibitory effect of alpha-2R in SHR. Supporting this notion, mRNA concentrations of alpha-2(A), the predominant presynaptic alpha-2R subtype, were reduced in SHR compared with WKY (738 +/- 251 versus 1468 +/- 518 mRNA molecules/10 ng, n = 7, P < 0.01), as quantified by competitive reverse transcription-polymerase chain reaction derived from left stellate ganglia.

CONCLUSIONS

The impairment of the alpha-2R mediated presynaptic negative feedback mechanism by a reduced expression of the alpha-2R subtype A may increase cardiac net secretion of NE in SHR and could therefore contribute to their hypertensive phenotype.

Exercise and hypertension: a model for central neural plasticity

1. Physical movement is accompanied by coordinated changes in respiratory and cardiovascular activity proportional to the metabolic demands of the locomotor task. Cardiorespiratory changes include increases in ventilation, blood pressure and heart rate, as well as altered regional sympathetic nerve activity and blood flow. 2. The posterior hypothalamic area, a periventricular region in the caudal-most diencephalon, has been shown to play a role in mediating the coupling of locomotion and cardiorespiratory activity. Stimulation of this brain region produces locomotor behaviour and simultaneous increases in cardiorespiratory activity that are independent of peripheral feedback from contracting muscles. Posterior hypothalamic neurons are also activated by exercise and exercise-related stimuli, such as muscle contraction. 3. In spontaneously hypertensive rats (SHR), a deficiency in the inhibitory GABA neurotransmitter system within the posterior hypothalamic area contributes to tonically elevated levels of arterial blood pressure. We previously identified a reduction in the GABA synthesizing enzyme glutamic acid decarboxylase (GAD) within the posterior hypothalamus of SHR. 4. We have recently demonstrated that exercise can upregulate GABA-mediated caudal hypothalamic control of cardiovascular function in SHR. Similarly, exercise increases GAD gene transcript levels in the posterior hypothalamus. Thus, we have identified a model to study exercise-related central neural plasticity in GABAergic neurotransmitter function. Moreover, we suggest that exercise may increase cardiovascular health through changing central neural regulation of blood pressure.

Spontaneously hypertensive rats exhibit altered cardiovascular and neuronal responses to muscle contraction

We examined the cardiovascular and ventrolateral medullary neuronal responses to muscle contraction in the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto rat (WKY) control. Cardiovascular, respiratory and ventrolateral medullary neuronal responses to muscle contraction evoked by tibial nerve stimulation were recorded. SHRs exhibited significantly larger drops in arterial pressure compared to WKYs in response to muscle contraction (P < 0.05). Basal ventrolateral medulla neuronal discharge rates were similar between the SHR and the WKY groups. A majority of neurons recorded responded to muscle contraction in both the WKY (77 %; n = 53) and the SHR groups (68 %; n = 62). There was no difference in the percentage of neurons that responded with an increase (approximately 60 %) or decrease (approximately 40 %) in firing rate between hypertensive and normotensive rats. Pulse wave-triggered averaging techniques showed that most neurons that responded to muscle contraction also possessed a basal firing rhythm temporally related to the cardiac cycle (85 % in WKYs, 83 % in SHRs). However, decreases in neuronal firing rates in response to muscle contraction were significantly greater in SHRs than WKYs. Therefore, we conclude that muscle contraction unmasks a hyperexcitability of neurons in the ventrolateral medulla of SHRs that parallels the heightened blood pressure responses.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Tyrosine hydroxylase antisense gene therapy causes hypotensive effects in the spontaneously hypertensive rats

We investigated the effect of antisense oligodeoxynucleotides (AS ODN) against tyrosine hydroxylase (TH) on hypertension and sympathetic nervous system activity in spontaneously hypertensive rats (SHR). Systolic blood pressure (SBP) in SHR treated with TH AS ODN (50, 200 microg/rat, i.v.) was significantly lower than that of control SHR. Epinephrine and norepinephrine levels, TH activity, and TH protein levels in the adrenal medulla of SHR were reduced concomitant with TH AS ODN treatment-induced changes in SBP. In contrast, TH AS ODN (200 microg/rat) had no effect on SBP in Wistar-Kyoto rats (WKY), despite significantly decreased catecholamine levels, TH activity, and TH protein levels. These findings suggest that peripheral systemic injection of TH AS ODN may be effective as hypotensive therapy in SHR.

Chronic exercise alters caudal hypothalamic regulation of the cardiovascular system in hypertensive rats

Previous studies have documented a deficit in the GABA neurotransmitter system within the caudal hypothalamus (CH) of spontaneously hypertensive rats (SHR). The reduction in inhibitory influence on this cardiovascular excitatory brain region is associated with an increased neuronal activity and resting blood pressure. The purpose of this study was to determine if chronic treadmill and wheel-running activities alter the ability of the CH to regulate cardiovascular function. SHR were exercised on a treadmill (5 times/wk) at moderate intensity or allowed free access to running wheels (7 days/wk) for a period of 10 wk. Resting blood pressures were obtained before and after the exercise training periods. After the exercise period, rats were anesthetized and microinjection experiments were performed. Treadmill-trained SHR exhibited a significantly blunted developmental rise in resting blood pressure after 10 wk of exercise. A similar yet less marked effect was observed in wheel-run rats. Microinjection of the GABA synthesis inhibitor 3-mercaptopropionic acid (3-MP) into the CH of nonexercised SHR did not produce any change in arterial pressure. In contrast, microinjection of 3-MP into the CH produced significant increases in blood pressure and heart rate in exercised SHR. These results demonstrate that exercise training can alter CH cardiovascular regulation in hypertensive rats and therefore may play a role in increasing cardiovascular health.

Review of studies establishing the aging male spontaneously hypertensive rat as a detector and quantifier of the kidney toxicity of radiocontrast media and other chemicals

RATIONALE AND OBJECTIVES

There is a need for practical and sensitive preclinical tests for detecting the kidney toxicity of chemicals. The spontaneously hypertensive rat (SHR), as it ages, develops renal and cardiovascular changes similar to those considered as human risk factors for radiocontrast-induced renal damage. Age, male gender, and uncontrolled hypertension make these animals susceptible to the volume and osmolality of the administered contrast agent and the effect of repeated contrast administration after a brief interval. This article reviews studies in which the role of these and other factors were evaluated to validate the male SHR as a small animal model for renal damage induced by contrast and other agents.

METHODS

Systolic blood pressure was measured with a tail cuff before and after the administration of the experimental substances, and the left kidney and heart were studied histologically to determine the influence of age, dose of contrast repeated at a short interval, gender and strain, the role of the sympathetic adrenergic nervous system, osmolality, and apoptosis.

RESULTS

As the animals aged and the systolic blood pressure remained elevated, the animals developed progressive renal lesions that worsened after the administration of contrast. The most advanced renal lesions occurred in adult male SHRs that received two doses of contrast 6 hours apart. Female SHR rats and male Wistar Kyoto rats showed no effect or only minimal changes in heart and kidneys after the administration of contrast compared with age-matched male SHRs. Adrenergic blockade allowed only a small elevation in systolic blood pressure after contrast administration but did not protect the kidneys against renal damage by contrast. Hypaque, Omnipaque, and mannitol caused renal damage in proportion to their osmolality. Apoptosis with Hypaque, Omnipaque, and mannitol was observed in the kidney and heart.

CONCLUSION

The results indicate that the aging male SHR develops spontaneous renal lesions that progress with age, increasing the susceptibility to the renal-damaging effects of contrast. Thus, the aging male SHR provides a laboratory tool for detecting the risk of renal damage of new contrast media as well as other pharmaceuticals and assessing methods to protect the kidneys and possible mechanisms of renal damage.

Implication of hypertensive rat models for primordial nutritional prevention of cardiovascular diseases

1. Various substrains maintained during selective sib-mating contributed to the establishment of spontaneously hypertensive rats (SHR) with a variety of clinical features. 2. Stroke-prone SHR (SHRSP), developing haemorrhagic and/or ischaemic stroke spontaneously, are regarded as a model for osteoporosis. 3. The genetic mechanisms of spontaneous hypertension have been attributed pathophysiologically to neural and structural vascular alterations. 4. The mechanisms of stroke are ascribed to the limited regional oxygen and nutrient supplies to the brain areas fed by perforating arteries. 5. The genome-wide linkage analysis on the F2 obtained by crosses of SHRSP with normotensive strains has demonstrated different gene loci contributing to the development and maintenance of hypertension during the ageing process and also genes influencing the susceptibility to stroke without any effect on blood pressure. 6. Experimental studies in SHRSP revealed that stroke could be prevented by protein, Ca- or Mg-supplemented diets, particularly if given in the early stage, indicating the importance of primordial nutritional prevention of cardiovascular diseases (CVD). 7. Experimental findings in SHRSP as well as epidemiological studies on nutrition and CVD indicate the future avenue towards 'predictive-preventive medicine' for CVD.

Repressive/defensive coping, endogenous opioids and health: how a life so perfect can make you sick

Hyperactivity of endogenous opioid systems has been postulated to mediate the associations between defensive/repressive coping styles, enhanced stress responsivity, and reduced immunocompetence. Study 1 examined whether repressive/defensive coping would be associated with greater sensitivity to opioid antagonism. Judgments of the painfulness of ascending series of electrocutaneous stimulation applied to the forearm were determined before and after the administration of naloxone and placebo in 38 men and 42 women. All subjects were healthy with a mean age of 32.9 years. Naloxone (10 mg i.v.) and placebo were administered in double-blind fashion and counterbalanced. Subjects were classified as High- and Low-defensive and repressive copers on the basis of scores on the Marlowe-Crowne Social Desirability Scale and the Balanced Inventory of Desirable Responding, respectively. High Self-Deception was associated with naloxone-induced hyperalgesia, whereas no effects of naloxone on pain ratings were observed in low-Self-Deceptive subjects. In Study 2, resting plasma beta-endorphin levels were found to be positively correlated with defensiveness in men (n = 26), but not women (n = 44). Study 3 examined 82 healthy subjects (mean age = 28.7 years). Beta-endorphin/defensiveness correlations were found to be greater following, compared to prior to, electrical nociceptive stimulation in men (n = 49), but unrelated in women (n = 33). These findings are consistent with the hypothesized endorphinergic dysregulation associated with repressive/defensive coping styles and are discussed in terms of the immuno-regulatory implications of such a dysregulation.

The determinants of long-term blood pressure stability: control of trough blood pressure during sleep

Long-term blood pressure stability in humans is not affected by prolonged hypotensive drug therapy. After withdrawal of long-term hypotensive therapy from essential hypertensives, their blood pressures soon return to pre-treatment levels. Since the well-studied stabilizing systems all adapt to prevailing blood pressure levels, they cannot account for long-term stability of blood pressure. There is a parallel between trough blood pressure and the lower limit of cerebral autoregulation. Blood pressure goes down to a trough level once sleep has been established. This level is maintained for 3-4 h, with minor perturbations. For normal and hypertensive humans and rats, the level is almost the same as the lower limit of cerebral autoregulation. In essential hypertension the nocturnal fall of blood pressure and the lower limit for cerebral autoregulation could be set by atheromatous narrowing or occlusion of large cerebral arteries, especially those supplying the brain stem, from which the Cushing response is initiated. Constriction and maldevelopment of smaller cerebral arteries are probably the main causes of increased cerebrovascular resistance in spontaneously hypertensive rats (SHR). The structural difference between the two situations might explain the failure of prolonged hypotensive therapy to reduce the lower limit of cerebral autoregulation in humans, which it is able to achieve in young SHR. If trough blood pressure stability were the anchor which prevents casual blood pressures from drifting, a primary increase of cerebrovascular resistance would be a plausible explanation for essential hypertension. It could also make a major causal neurogenic contribution to hypertension in rat models, especially SHR and stroke-prone SHR.

Decrease in glutamic acid decarboxylase level in the hypothalamus of spontaneously hypertensive rats

BACKGROUND

A reduction in gamma-aminobutyric (GABA)-mediated inhibition of pressor sites in the caudal hypothalamus of spontaneously hypertensive rats compared with that of normotensive Wistar-Kyoto rats has recently been demonstrated.

OBJECTIVE

To determine whether the reduction in GABA-mediated inhibition of the caudal hypothalamus of the spontaneously hypertensive rats results from reductions both in the number of GABA-synthesizing neurons and in the amount of the GABA-synthesizing enzyme, glutamic acid decarboxylase messenger RNA (mRNA).

DESIGN AND METHODS

A polyclonal antibody (Chemicon) for the 67 kDa isoform of glutamic acid decarboxylase (GAD67) was used to immunocytochemically label GABAergic neurons in the caudal hypothalamus of spontaneously hypertensive and Wistar-Kyoto rats that had been treated beforehand with colchicine. The labeled cells were counted for both strains by a blinded analysis and compared. Caudal hypothalamic tissues from spontaneously hypertensive and Wistar-Kyoto rats were analysed for GAD67 mRNA by Northern blotting. The signal intensities of the radioactive probe specific for GAD67 for the two strains were analyzed by using a phosphorimager and compared. Control areas for the immunocytochemical (zona incerta) and Northern blotting (cortex, midbrain, cerebellum, and brain stem) experiments were used to determine regional differences in expression of GAD67.

RESULTS

Both the hypothalamus and cerebellum of spontaneously hypertensive and Wistar-Kyoto rats contained GAD67-immunoreactive neurons; however, there were 42% fewer GAD67 neurons in the caudal hypothalamus of spontaneously hypertensive rats than there were in that of Wistar-Kyoto rats. Furthermore, a 33% reduction in the amount of GAD67 messenger RNA in the caudal hypothalamus of spontaneously hypertensive rats compared with that for Wistar-Kyoto rats was demonstrated. Analysis of the expression of GAD67 in the cortex, midbrain, cerebellum, brain stem, and total brain revealed no difference between spontaneously hypertensive and Wistar-Kyoto rats.

CONCLUSIONS

Our findings demonstrate that the spontaneously hypertensive rat has fewer neurons synthesizing GABA and less GAD67 mRNA in the caudal hypothalamus than do Wistar-Kyoto rats. This deficit in the GABAergic system in the caudal hypothalamus, a well-known cardiovascular regulatory site, could contribute to the essential hypertension in this animal model.

Effects of radiocontrast and endothelin administration on systolic blood pressure and renal damage in male spontaneously hypertensive and Wistar Kyoto rats with phentolamine-induced adrenergic blockade

RATIONALE AND OBJECTIVES

The systemic administration of hypertonic solutions may activate the adrenergic system, thus triggering vasomotor reactions that may result in renal failure. In this study, the effects of diatrizoate meglumine sodium radiocontrast agent Hypaque-76 on systolic blood pressure (BP) and renal damage were determined in male spontaneously hypertensive (SH) rats and Wistar Kyoto (WKY) rats under adrenergic blockade.

METHODS

The systolic BP was measured in ketamine-anesthetized male SH and WKY rats after administration of saline solution, radiocontrast, or endothelin during adrenergic blockade with phentolamine. Then the left kidney was removed and examined histologically.

RESULTS

The fall in systolic BP after phentolamine was not influenced by saline solution or radiocontrast in WKY rats but was restored partially by radiocontrast in SH rats. Endothelin produced an elevation in BP toward baseline levels in both strains. Only moderate renal damage was observed in the kidneys of WKY rats given radiocontrast or endothelin, but very severe damage was produced by these agents in SH rats.

CONCLUSIONS

Adrenergic blockade with phentolamine did not prevent the elevation in systolic BP in SH rats by radiocontrast or by endothelin in SH and WKY rats, nor did it protect against renal damage by radiocontrast or by endothelin in SH rats.

The SHR as a small animal model for radiocontrast renal failure. Relation of nephrotoxicity to animal's age, gender, strain, and dose of radiocontrast

The male spontaneously hypertensive rat (SHR), as it ages, suffers many of the renal and cardiovascular complications that are recognized in humans as risk factors for radiocontrast (RC) agent induced renal failure (RF). Knowledge of this led us to test this strain of rats as a small animal model for RC-induced renal failure (RC-RF). Functional studies demonstrated a significant fall in GFR in the recovery period after RC administration. In addition, histopathologic evaluation of the kidneys was done in this study. Our results are based on assigning separate scale values to the histopathological evaluation of the (a) glomeruli, (b) tubules, (c) interstitium, and (d) arteries and arterioles of the kidneys. Saline (S) was administered to one group and the RC agent Hypaque-76 (diatrizoate meglumine sodium) to paired groups of 5-, 8-, 10-, 12-, and 14-month-old male SHR. The results indicated that younger animals (5 and 8 months old) were resistant to the nephrotoxic effects of the RC, but developed susceptibility at 10 months of age, when spontaneous renal pathology became manifest. Both spontaneous renal pathology and RC-induced renal damage (RC-RD) increased as the animals aged. In addition, when the administered dose of RC was repeated after a short interval of only 6 h, the degree of RC-RD increased greatly. In parallel control studies of the influence of gender and strain on the response to RC in 12-month-old rats, neither hypertensive female SHR nor male normotensive Wistar-Kyoto (WKY) rats demonstrated significant spontaneous renal pathology or the marked susceptibility to RC nephrotoxicity shown by their male SHR counterparts. This small animal model for RC-RD, the mature male SHR, has the distinct advantage that risk factors for RC-RD, similar to those characterized in humans for RC-RF, develop spontaneously without requiring any special treatment or surgical intervention.

Does the neuropeptide Y Y1 receptor contribute to blood pressure control in the spontaneously hypertensive rat?

OBJECTIVE

To study the effects of the selective neuropeptide Y (NPY) Y1 receptor antagonist BIBP 3226 in spontaneously hypertensive rats (SHR) in order to elucidate whether NPY function may be altered in the SHR and whether the NPY Y1 receptor plays a specific role in the maintenance of high blood pressure in this genetic form of hypertension.

METHODS

Pithed and conscious SHR were studied after intravenous administration of 0.125-1 mg/kg BIBP 3226. The cardiovascular effects were evaluated under baseline conditions, under acute stress and after exogenous administration of 20 microg/kg NPY. The potentiating effects of NPY on pressor responses to phenylephrine and tyramine were studied in the SHR.

RESULTS

Intravenous administration of 0.125-1 mg/kg BIBP 3226 dose-dependently inhibited the pressor response to exogenous NPY in pithed SHR. At higher doses BIBP 3226 had an effect duration of 20-40 min. In the pithed SHR, a 0.5 mg/kg bolus injection of BIBP 3226 shifted the pressor response curve for exogenous NPY significantly to the right It also inhibited significantly the potentiating effects of NPY on pressor responses to phenylephrine and tyramine. In conscious SHR, 0.125-1 mg/kg BIBP 3226 did not reduce the basal blood pressure. In combination with a hypotensive dose of prazosin, administration of 0.5 mg/kg BIBP 3226 had no added effects lowering the basal blood pressure. A stressful stimulus, namely an air jet, caused a brief increase in blood pressure and heart rate in the conscious SHR. In this model, 0.5 mg/kg BIBP inhibited the heart rate response slightly but had no effect on the blood pressure response.

CONCLUSIONS

Our results demonstrate that, although the selective NPY Y1 receptor antagonist BIBP 3226 may shift the pressor response to exogenous NPY potently, it does not influence basal blood pressure significantly in the SHR.

Long-term effects of high calorie sucrose-enriched diet and streptozotocin-induced diabetes on insulin resistance in spontaneously hypertensive rats

1. The effects of two experimental manipulations on insulin resistance were compared in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. Rats were fed a high calorie sucrose-enriched diet (high calorie diet) or were made diabetic by the injection of streptozotocin (STZ). 2. After treatment with the high calorie diet for 8 weeks, blood pressure increased in SHR, but not in WKY rats. In contrast, STZ treatment decreased blood pressure in SHR, but increased it in WKY rats. 3. Plasma glucose levels during oral glucose loading were higher in SHR than in WKY rats. Glucose tolerance was impaired to a greater extent by both the high calorie diet and STZ in SHR than in WKY rats. Hyperinsulinaemia induced by the high calorie diet was severe in SHR compared with WKY rats. 4. Abnormalities in lipid metabolism induced by a high calorie diet or STZ-induced diabetes were more marked in SHR than in WKY rats. 5. Steady-state plasma glucose levels in the insulin suppression test were higher in SHR than in WKY rats, both of which were treated by either the high calorie diet or STZ. These findings indicate that insulin-stimulated glucose uptake by high calorie diet or STZ-induced diabetes was impaired to a greater extent in SHR than in WKY rats. 6. It is concluded, therefore, that SHR fed on high calorie diet or SHR with STZ-induced diabetes are suitable models to study the effects of antihypertensive treatment on glucose tolerance, insulin resistance or lipid metabolism as well as blood pressure.

Altered c-fos in rostral medulla and spinal cord of spontaneously hypertensive rats

Neurons immunoreactive for Fos, the protein product of the immediate early gene c-fos, have been compared in the rostral ventral medulla and spinal cord of conscious normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) after baroreceptor unloading. Hypotension induced by a 60-minute intravenous infusion of sodium nitroprusside reduced baroreceptor activity; controls received intravenous saline. In WKY, 474 +/- 56 (n=6) Fos-positive neurons were identified in the rostral ventral medulla after nitroprusside infusion, a fivefold increase from controls; 50% of the tyrosine hydroxylase-containing neurons in the rostral ventral medulla were activated by this hypotension. Sympathetic preganglionic neurons, mainly sympathoadrenal neurons, were Fos positive after nitroprusside, but Fos-positive sympathetic preganglionic neurons were not observed in control WKY. In SHR, Fos immunoreactivity in the rostral ventral medulla was elevated in the control group compared with the WKY controls (236 +/- 31 and 93 +/- 15, respectively, n=6 for both). Nitroprusside hypotension did not further increase Fos immunoreactivity in the rostral ventral medulla, although the number of Fos-positive spinal sympathetic neurons increased. Our results have identified different neuronal activities between WKY and SHR in sites that are critical to sympathetic outflow. In WKY, nitroprusside effects are consistent with an activation of rostral ventral medulla neurons, including bulbospinal neurons, that are normally inhibited by baroreceptor activity. In SHR, basal nerve activity is increased, so even at rest, rostral ventral medulla neurons and sympathetic preganglionic neurons, mainly sympathoadrenal neurons, are Fos immunoreactive. These activated neurons are likely to contribute to the elevated blood pressure in this rat strain.

Elevated tyrosine hydroxylase mRNA levels in medulla oblongata of spontaneously hypertensive rats

The present study has investigated the expression of tyrosine hydroxylase (TH) mRNA and its activity in medulla oblongata of spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). The TH mRNA levels were determined by Northern blot and dot blot analyses. The TH activity and the expression of TH mRNA in medulla oblongata of SHR were significantly higher than those of WKY. These results suggest that the hypertension of SHR may be related to the high activity of TH due to the high level of TH mRNA which increases norepinephrine levels in the medulla oblongata.

GABAergic and glutaminergic modulation of centrally evoked arrhythmias in rats

A standard electrical stimulus applied to the posterior hypothalamus evoked cardiac arrhythmogenic responses in the spontaneously hypertensive rat. Isolated premature ventricular beats or doublets and nonsustained ventricular tachycardic salvos were observed. This effect was associated with a large rise in blood pressure (79 +/- 3 mm Hg). The same stimulus in normotensive Wistar-Kyoto rats produced no significant cardiac arrhythmias, and the rise in blood pressure was smaller (36 +/- 2 mm Hg). We investigated the influence of baclofen, a GABAB receptor agonist, and two N-methyl-D-aspartate receptor antagonists on the arrhythmogenic response to hypothalamic stimulation. Intravenous baclofen (3 mg/kg) had no effect in the normotensive Wistar-Kyoto rats, but in the spontaneously hypertensive rats it enhanced the adjusted mean value of the number of extrasystoles from 0.5 +/- 0.5 to 18 +/- 1 (P < .001). This value was also increased (from 3 +/- 1 to 17 +/- 1, P < .001) by an intracisternal injection of baclofen (1 micrograms/kg). This facilitatory effect of baclofen was prevented by treatment with atenolol (0.5 mg/kg). Two glutamate receptor antagonists, ketamine (7.5 mg/kg IV) and kynurenic acid (200 micrograms/kg intracerebroventricularly), prevented both the arrhythmogenic response to the hypothalamic stimulation and its facilitation by baclofen. The study confirms that hypothalamic stimulation facilitates the development of arrhythmias through a sympathetic drive and that these arrhythmias are easier to induce in spontaneously hypertensive rats than in normotensive Wistar-Kyoto rats. Both the central GABAergic and the glutamatergic systems are implicated in the development of these ventricular arrhythmias, since baclofen could disinhibit the glutamatergic central pathway. These results could account for the ability of the spontaneously hypertensive rats to develop ventricular arrhythmias of central origin.

Influence of androgen on tyrosine hydroxylase mRNA in adrenal medulla of spontaneously hypertensive rats

We investigated the effects of castration and testosterone propionate on tyrosine hydroxylase mRNA, its activity, and catecholamine synthesis in the adrenal medulla of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Four-week-old male rats were castrated. Testosterone propionate (500 micrograms per rat) was administered subcutaneously twice a week to castrated rats (between 14 and 25 weeks of age). Systolic pressure was measured at the age of 25 weeks, and rats were decapitated. The systolic pressure of castrated SHR was significantly lower than that of control and testosterone-replaced SHR. Epinephrine and norepinephrine levels, tyrosine hydroxylase activity, and tyrosine hydroxylase mRNA in the adrenal medulla of castrated SHR were significantly lower than in control and testosterone-replaced SHR. Systolic pressure and epinephrine and norepinephrine levels, tyrosine hydroxylase activity, and tyrosine hydroxylase mRNA levels in the adrenal medulla of WKY showed no significant differences among the control, castrated, and testosterone-replaced groups. These results suggest that androgens contribute to the development and maintenance of hypertension in SHR via sustained enhancement of tyrosine hydroxylase synthesis in the adrenal medulla, leading to increased epinephrine and norepinephrine levels.

Elevated tyrosine hydroxylase mRNA levels in the adrenal medulla of spontaneously hypertensive rats

We investigated the expression of tyrosine hydroxylase (TH) mRNA and its activity in the adrenal medulla of spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). The TH mRNA levels were determined by Northern blot and dot blot analyses. The TH activity and the expression of TH mRNA in the adrenal medulla of SHR were significantly higher than those of WKY (P < 0.01). These results suggested that the hypertension of SHR may be related to the high activity of TH due to the high level of TH mRNA, which increases epinephrine and norepinephrine levels in the adrenal medulla.

Rostral ventrolateral medulla as a site for the central hypertensive action of kinins

In the present study, we focused on the rostral ventrolateral medulla as a possible site of action for kinins because of its established importance in the central regulation of the cardiovascular system. Unilateral microinjections of 100 pmol to 4 nmol bradykinin into the rostral ventrolateral medulla produced dose-dependent increases in mean arterial pressure in Sprague-Dawley (SD) rats, Wistar-Kyoto (WKY) rats, and spontaneously hypertensive rats (SHR). The dose-response curves for the hypertensive responses to bradykinin in SD and WKY rats were essentially the same, whereas the hypertensive effect of bradykinin was significantly greater in SHR than in either SD or WKY rats. The kinin B2 receptor antagonists D-Arg0,Hyp3,Thi5,8,D-Phe7-bradykinin and Hoe 140 inhibited the hypertensive responses to bradykinin in both SHR and WKY rats. The hypertensive effect of 500 pmol bradykinin was reduced 65 +/- 5% after 4 nmol of D-Arg0, Hyp3,Thi5,8,D-Phe7-bradykinin in SHR and 50 +/- 16% in WKY rats, whereas 1 nmol Hoe 140 abolished the hypertensive effect of 500 pmol bradykinin injected into the rostral ventrolateral medulla. Microinjection of D-Arg0,Hyp3,Thi5,8,D-Phe7-bradykinin produced prolonged dose-dependent decreases in mean arterial pressure and heart rate. Blood pressure decreased 70 +/- 8 mm Hg and heart rate decreased 49 +/- 9 beats per minute in SHR, whereas in WKY rats mean arterial pressure decreased 12 +/- 4 mm Hg, with no change in heart rate. In a similar fashion, Hoe 140 caused a 51 +/- 7 and 17 +/- 3 mm Hg reduction in blood pressure in SHR and WKY rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal alpha-adrenergic receptors and genetic hypertension

The hypothesis has been proposed that an increase in the number of renal alpha-adrenergic receptors may contribute to the pathogenesis of genetic hypertension. Herein we review recent findings regarding expression of renal alpha 1 (alpha 1a, alpha 1b)- and alpha 2 (alpha 2a, alpha 2b)-adrenergic subtypes and we provide an updated revision of the above-stated hypothesis. Enhancement in receptor number or in post-receptor components responsible for alpha 1- and alpha 2-adrenergic-mediated sodium reabsorption in proximal tubule may contribute to sodium retention and an elevation in blood pressure. Perhaps such changes contribute to the increase in blood pressure in genetically determined hypertension in humans, although direct tests of this notion have not yet been performed.

Modulation of vascular function by neuropeptide Y during development of hypertension in spontaneously hypertensive rats

Neuropeptide Y (NPY) is a sympathetic cotransmitter and a platelet-derived factor which causes vasoconstriction, potentiation of norepinephrine (NE) action, and vascular mitogenic effects. Reciprocally, NE markedly enhances the actions of NPY. We studied vasopressor effects of NPY and sources of peptide release during the development of hypertension in spontaneously hypertensive rats (SHR). Conscious SHR (4 and 16 weeks old) had higher resting plasma levels of NE and epinephrine than age-matched Wistar-Kyoto (WKY) rats, but similar NPY immunoreactivity (NPY-ir) levels in platelet-poor plasmas (PPP). In both strains, NPY-ir levels in PPP were higher in 4-week-old than in older rats. However, at all ages (4-24 weeks) SHR had markedly elevated NPY-ir content in platelet-rich-plasmas than WKY rats, although levels declined with age and hypertension. In the superior mesenteric artery, NPY-ir content (per mg) was significantly higher in 4-week-old but lower in 16-week-old SHR than in WKY rats, suggesting greater sympatho-neural NPY stores and release (leading to depletion) during the development of hypertension. Four-week-old SHR also tended to have higher NPY-ir content in the adrenal medullae and coeliac ganglia but a lower content in the kidney than WKY rats; these differences disappeared with age. Pressor responsiveness to alpha-agonists and NPY were similar in both strains at 4 weeks. While unchanged by age in WKY rats, adrenergic and NPY-mediated vasopressor responses became augmented in 16- to 24-week-old SHR (compared with WKY rats); this hyperresponsiveness was not completely abolished by ganglionic blockade and not observed with vasopressin.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease of central dopamine level in the adult spontaneously hypertensive rats related to the calcium metabolism disorder

The metabolism of calcium and brain dopamine in spontaneously hypertensive rats (SHR) after the development of hypertension was investigated as a possible model for the hypertension mechanism. Serum calcium level in SHR was lower than that in the normotensive control. Wistar Kyoto rats (WKY, the parent strain of SHR). Conversely, bone calcification of SHR was higher than that in WKY. Possible mechanisms for the lower serum calcium level seen in SHR include a decrease in the availability of calcium from bone. The immunohistochemical dopamine levels in the neostriatum and nucleus accumbens in SHR were lower than those in WKY. In these regions, the dopamine level was increased by the intraventricular administration of CaCl2 through a central, calmodulin-dependent system. This study suggests, based upon previous pharmacological studies, that the decrease of the serum calcium level in SHR causes a decrease in central, calcium-calmodulin-dependent dopamine synthesis and a subsequent low level of dopamine in the brain that produces an increase in blood pressure through functions of cerebral dopaminergic neurons and peripheral sympathetic nerves. Our results suggest that this could be one of the mechanisms of hypertension in SHR.

Plasma catecholamine, renin activity, and ACTH responses to the serotonin receptor agonist DOI in juvenile spontaneously hypertensive rats

Mean arterial blood pressure (MAP), heart rate (HR) and arterial plasma levels of corticotropin (ACTH), renin activity (PRA) and catechols [norepinephrine (NE), epinephrine (EPI), and the intraneuronal NE metabolite dihydroxyphenylglycol (DHPG)] at baseline and in response to the serotonin-1C/2 (5-HT1C/5-HT2) agonist 1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane (DOI, 1.0 mg/kg i.a.) in conscious, freely-moving, juvenile (4 week old) spontaneously hypertensive rats (SHR's) and age-matched Wistar-Kyoto (WKY) normotensive control rats were measured simultaneously. Baseline levels of MAP, NE, DHPG, and EPI all were significantly higher in the SHR's. There was a similar trend for PRA, but ACTH did not differ between the two strains. DOI produced marked increases in levels of MAP, ACTH, EPI, and also PRA but did not affect NE or DHPG concentrations. HR decreased only in the WKY group after administration of DOI. The magnitudes of the EPI and ACTH responses did not differ between the rat strains. Responses of MAP and PRA were significantly larger in SHR's. These results suggest that there is a selective hyperresponsiveness of PRA and blood pressure to 5-HT2 receptor stimulation parallel to a deficient baroreceptor reflex in juvenile SHR's.

Haemodynamic and humoral effects of omega-conotoxin GVIA in normotensive and spontaneously hypertensive rats

Omega-conotoxin GVIA, a 27-amino acid peptide, has been shown to be a potent and selective inhibitor of N-type voltage-operated calcium channels (VOCCs). A single intravenous dose of 10 micrograms/kg conotoxin slowly lowered blood pressure by 41.3 +/- 4.4 and 73.3 +/- 4.6 mm Hg in conscious Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) respectively without changing the heart rate. Plasma renin activity was significantly increased after conotoxin. In anaesthetized rats, conotoxin (3 and 10 micrograms/kg) lowered blood pressure and heart rate and produced a marked increase in renal vascular conductances. Baroreceptor heart rate reflex experiments using methoxamine and sodium nitroprusside before and after treatment with conotoxin showed that conotoxin almost totally abolished the sympathetic component of the reflex without affecting the vagal tone to the heart in both rat strains. Because conotoxin does not affect directly the vasculature and heart contractile properties, we suggest that the control of presynaptic calcium influx and of neurotransmitter release mostly depends on conotoxin-sensitive N-type VOCCs in the peripheral sympathetic system of the rat.

Chronic lesion of rostral ventrolateral medulla in spontaneously hypertensive rats

We studied the effects of chronic selective neuronal lesion of rostral ventrolateral medulla on mean arterial pressure, heart rate, and neurogenic tone in conscious, unrestrained spontaneously hypertensive rats. The lesions were placed via bilateral microinjections of 30 nmol/200 nl N-methyl-D-aspartic acid. The restimulation of this area with N-methyl-D-aspartic acid 15 days postlesion failed to produce a pressor response. One day postlesion, the resting mean arterial pressure was significantly decreased in lesioned rats when compared with sham rats (100 +/- 7 versus 173 +/- 4 mm Hg, p less than 0.05). Fifteen days later, the lesioned group still showed values significantly lower than the sham group (150 +/- 6 versus 167 +/- 5 mm Hg, p less than 0.05). No significant heart rate differences were observed between the sham and lesioned groups. The ganglionic blocker trimethaphan (5 mg/kg i.v.) caused similar reductions in mean arterial pressure in both lesioned and sham groups. The trimethaphan-induced hypotension was accompanied by a significant bradycardia in lesioned rats (-32 +/- 13 beats per minute) but a tachycardia in sham rats (+33 +/- 12 beats per minute) 1 day postlesion. Therefore, rostral ventrolateral medulla neurons appear to play a significant role in maintaining hypertension in conscious spontaneously hypertensive rats. Spinal or suprabulbar structures could be responsible for the gradual recovery of the hypertension in the lesioned rats.

Cardiovascular regulation by cholinergic mechanisms in rostral ventrolateral medulla of spontaneously hypertensive rats

This study aimed to demonstrate the role of acetylcholine receptors in the rostral ventrolateral medulla (RVL) in the central regulation of the cardiovascular system in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The effects of cholinergic drugs, microinjected into the rostral ventrolateral medullary vasopressor area, on blood pressure and heart rate in anesthetized and artificially ventilated rats were investigated. Unilateral microinjection of carbachol (1 nmol/site), physostigmine (300 pmol/site) or 3,4-diaminopyridine (500 pmol/site) into the RVL elicited a pressor and tachycardiac response, of which only the pressor response was significantly greater in SHR than in WKY. Bilateral microinjection of atropine (1 nmol/site) caused a depressor and bradycardiac response. The depressor response produced by atropine injected in the RVL was also significantly greater in SHR than in WKY. These results suggest that there are tonic cholinergic mechanisms in the RVL of the rats, which exert an excitatory cardiovascular action, and that the enhanced responsiveness to acetylcholine receptor stimulation in the RVL may contribute to the sustained elevation of blood pressure in the SHR.

Pharmacological blockade of blood pressure and heart rate increases following milk ingestion in 15-day-old SHR and WKY rat pups

The effects of hexamethonium, a ganglionic blocker, on blood pressure (BP) and heart rate (HR) responses to milk ingestion were assessed in awake, 15-day-old spontaneously hypertensive rats (SHR) and their normotensive progenitor strain, Wistar-Kyoto rats (WKY) using two methods of milk delivery. SHRs had larger increases in BP compared to WKYs, but WKYs exhibited larger increases in HR following milk ingestion from an anesthetized dam. BP responses to milk ingestion from a tongue cannula were also larger in SHRs. Administration of hexamethonium prior to milk delivery resulted in a drop in BP following milk ingestion in both milk delivery situations for each strain. The results suggest that SHRs exhibit exaggerated sympathetic activation to milk ingestion compared to WKYs, and that in both strains, cardiovascular responses to feeding are modulated by the presence of the dam.

Kinin receptors of the central nervous system of spontaneously hypertensive rats related to the pressor response to bradykinin

1. Kinin analogues bradykinin (BK), T-kinin, Met-Lys-BK, Lys-Lys-BK, Des-Arg9-BK with agonist activity and D-Arg0-Hyp3-Thi5,8-D-Phe7-BK (DAHTDBK) and Arg9-Leu8-BK with antagonist activity were injected into the posterior portion of the fourth cerebral ventricle of unanaesthetized rats implanted with permanent cannulae and arterial pressure was measured directly from the abdominal aorta. 2. The spontaneously hypertensive rats (SHR) were more sensitive than normotensive Wistar rats (NWR) to the pressor effect of BK and other kinin analogues. The SHR did not differ in sensitivity of the pressor response to centrally administered angiotensin II or endothelin-1. 3. Experiments with selective kinin agonists and antagonists revealed that in the SHR, as in the NWR, the receptors which mediated the central pressor response are of the BK2 subtype. 4. Measurements of the pressor activity of kinins with different degrees of susceptibility to degradation, as well as experiments with kininase inhibitors, enalaprilat and CPP-Ala-Ala-Phe-pAB, suggest that the kininase activity in the central nervous system of SHR is reduced in comparison to that of NWR. 5. The SHR also showed increased sensitivity to BK and Lys-Lys-BK, compared with the NWR, when the kinins were injected following the administration of a mixture of the kininase inhibitors, suggesting that mechanisms other than kininase activity may play a role in the increased sensitivity of the SHR to the central pressor action of kinins. 6. An in vivo characterization of the kinin receptors which mediate the central pressor response showed that the interaction with DAHTDBK was reversible and of competitive nature. The pA2 in vivo estimated for the kinin receptors of the SHR was 0.7 logarithm units larger than that obtained in the NWR. 7. The kinin receptors which mediate the central BK pressor effect in the SHR are of the BK2 subtype and are similar to receptors in the NWR. The increased sensitivity to kinins in the SHR may be explained, at least in part, by their decreased kininase activity. At present it is impossible to conclude whether the difference observed in the pA2 represents an increased affinity of the kinin receptors or can be attributed to differences amongst strains in the enzymatic degradation of the antagonist.

Presynaptic regulation of neurotransmitter release in hypertension

1. The present review discusses evidence in support of the concept that alterations in sympathetic neurotransmitter release might contribute to the pathogenesis of hypertension. 2. Studies suggest that changes in sympathetic nervous activity in both the central and peripheral nervous systems participate in blood pressure control. 3. In the periphery increased release of norepinephrine from vascular adrenergic neurons might lead to the enhanced vasoconstrictor responses and thus to an elevation in systemic blood pressure. 4. The amount of neurotransmitter release from sympathetic nerve endings can be regulated by autoregulatory systems by presynaptic receptors located on nerve terminals. 5. It has been proposed that alterations to sympathetic nervous activity of hypertension might be partially due to abnormalities in presynaptic modulation of neurotransmitter release in central and peripheral tissues. 6. This article summarizes the results of studies to evaluate presynaptic receptor functions and sympathetic neurotransmitter release in hypertension.

Sympathetic inhibition and attenuation of spontaneous hypertension by PVN lesions in rats

To determine whether the paraventricular nucleus (PVN) contributes to the development of hypertension in spontaneously hypertensive rats (SHR), we compared cardiovascular responses to ganglionic blockade with hexamethonium or vasopressin antagonism with dPVAVP in sham-operated or PVN lesioned SHR and Wistar-Kyoto rats (WKY). Lesions were produced electrolytically when the rats were 5 weeks old. During the next 3 weeks, tail-cuff measurements showed that the development of hypertension in SHR was inhibited, while systolic pressure in WKY was unaffected. Mean pressures recorded directly from the femoral artery at 8 weeks of age were lower in lesioned than in sham-operated SHR (141 +/- 5 vs 110 +/- 3 mm Hg, P less than 0.05), but did not differ in corresponding WKY groups (110 +/- 4 vs 112 +/- 5 mm Hg). Depressor responses to ganglionic blockade induced by i.v. injection of hexamethonium (25 mg/kg) were significantly larger in sham-operated than in lesioned SHR (-41 +/- 4% vs -28 +/- 3%, P less than 0.05). By contrast, vasopressin antagonism with dPVAVP did not alter blood pressure in all rat groups. In 24-h urine samples, excretion of vasopressin was unaffected, but that of norepinephrine was significantly reduced in lesioned SHR. These findings suggest that the PVN contributes to the development of spontaneous hypertension by sympathetic activation without increasing vasopressin secretion.

The juxtaglomerular apparatus of the spontaneously hypertensive rat

Spontaneously hypertensive rats (SHR) are used to study the pathogenesis of essential hypertension. This study investigates the role of the renin-angiotensin system (RAS) in the pathogenesis of hypertension in SHRs and the morphometry of the JGA by a three-dimensional computer reconstruction program "GLOM" and electron microscopy. Systolic blood pressure (SBP) (tail cuff method) was higher in SHRs compared to controls (P less than 0.001). Plasma renin concentration (PRC) was lower in SHRs than in controls (P less than 0.001). Reconstruction of the JGA revealed granulated JG cells in the afferent and efferent arterioles and in the vascular tree away from the JGA area. Electron microscopy showed granulated JG cells in the afferent and efferent arterioles. The percentage volume of the granulated JG cells in SHR was significantly higher than in controls (P less than 0.01). A relationship was found between the percentage volume of granulated JG cells and the SBP in SHRs (r = 0.933, P less than 0.05). The wall/lumen perimeter ratio was also significantly higher in the SHRs compared to the controls (P less than 0.05). Low PRC in SHRs has been reported by several workers. The apparent hyperactivity of the JGA may indicate failure of renin release or an abnormal synthesis/secretion rate.

Neuropeptide Y in rat sympathetic neurons is altered by genetic hypertension and by age

We have used immunocytochemistry to quantitate neuronal neuropeptide Y in superior cervical ganglia of a strain of normotensive Wistar-Otago rats and a related genetically hypertensive strain over the age range 1-60 weeks. The numbers of neuropeptide Y-immunoreactive cells and total ganglionic cell numbers were both greater in ganglia of young normotensive than in those of hypertensive rats. Between 10 and 60 weeks of age, peptide immunoreactivity and total cell numbers both fell in normotensive rat ganglia but remained constant in ganglia from hypertensive rats. Densitometric analysis showed that the concentrations of neuropeptide Y were similar in neurons of age-matched individuals of both strains, but during aging there was a substantial decline in neuronal peptide content that was similar in both strains and that was not accompanied by any decline in neuronal immunoreactivity for tyrosine hydroxylase. Our results suggest that there is a developmental abnormality of neuropeptide Y in sympathetic neurons of this strain of genetically hypertensive rat and that, furthermore, the aging process is accompanied by a selective loss of neuronal neuropeptide Y that is independent of blood pressure status.

Neural influences on synovial mast cell density in rat

The mast cell, an immunocompetent cell that contributes to neurogenic inflammation in a variety of tissues, including synovium, is found in close proximity to peripheral terminals of unmyelinated primary afferents and sympathetic postganglionic nerve terminals. In this study we evaluated the hypothesis that the density of mast cells in synovial tissue is dependent on the neural innervation. In normal rats, we found that the density of mast cells in the ankle joint capsule, which is heavily innervated, is greater than in the capsule of the knee, which is less densely innervated. Selective lesions of unmyelinated primary afferents with capsaicin, or of sympathetic postganglionic neurons with guanethidine, produced a significant decrease in mast cells; surgical removal of the parasympathetic ganglia did not significantly affect mast cell density. Finally, the number of mast cells in the synovial joint of spontaneously hypertensive rats, which have increased sympathetic activity, was significantly greater than in normotensive control rats. These observations support the hypothesis that the innervation in the synovial joint of the rat, by unmyelinated afferents and sympathetic efferents, exerts a trophic effect on mast cell density.

The rostral and caudal ventrolateral medulla in young spontaneously hypertensive rats

This study was designed to compare the cardiovascular influences of the rostral ventrolateral medulla (RVLM) and the caudal ventrolateral medulla (CVLM) in young (5-6 weeks) spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats. SH and WKY groups had similar pressor and depressor responses to microinjection of L-glutamate into the RVLM and the CVLM, respectively. In addition the results of this study indicate a reduced tonic sympathoinhibitory function of the CVLM in young SH rats, which may contribute to the development of hypertension in the spontaneously hypertensive rat.

Supersensitivity of beta-adrenoceptor coupled adenylate cyclase in pulmonary tissue of the spontaneously hypertensive rat

Basal adenylate cyclase activity was similar in plasma membranes prepared from the lungs of 12 week old spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). However, sensitivity to Gpp[NH]p, isoproterenol plus GTP or Gpp[NH]p was significantly greater in the SHR. Beta-receptor density measured by [3H]DHA binding was unaltered. The dissociation constant, Kd, revealed a significantly greater binding affinity of the radioligand in the SHR (6.23 +/- 0.45 nM) compared with the WKY (8.53 +/- 0.82 nM). Activity of Gs was assessed by complementing S49 cyc- acceptor membranes with lung cholate extract. Basal activity of the reconstituted system was decreased 43% in the SHR. However, sensitivity to NaF, Gpp[NH]p, and isoproterenol plus Gpp[NH]p was significantly elevated. These data suggest that desensitization of the adenylate cyclase complex is not a generalized response to chronic hypertension. A tissue specific increase in sympathetic drive appears to be responsible for the lowered concentration of cardiac beta-adrenoceptors in the SHR. In contrast, both indirect and direct evidence indicate an enhanced functional sensitivity of pulmonary Gs in the hypertensive rats.

Differential sensitivity to cocaine in spontaneously hypertensive and Wistar-Kyoto rats

Physiological, pharmacological and toxicological responses to two regimens of cocaine administration were compared between spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. An initial experiment examined renal excretory and hemodynamic function in response to an acute volume load in anesthetized SHR and WKY following subacute cocaine treatment (20 mg/kg, s.c., twice a day for 9 days). Anticipated renal responses to volume loading were obtained but the responses of cocaine-treated SHR and WKY did not differ from vehicle-treated rats. A second group of experiments compared responses to continuous i.v. infusions of cocaine (1.25 mg/kg.min). In freely moving animals, no differences were noted between SHR and WKY in the increases in mean blood pressure (MBP) and heart rate (HR) produced during cocaine infusion. The elapsed time-to-onset of convulsions (Tc) elicited by cocaine was similar in both strains. However, when rats were subjected to restraint during the infusion period, pressor and tachycardic responses were observed to be significantly less in WKY than in SHR or in freely moving rats of either strain. Restraint also differentially affected rectal temperature (RT) responses to cocaine. Hypothermic responses to cocaine were observed in all WKY. Both hypothermic and hyperthermic responses were observed in SHR. A significant correlation was demonstrated between the Tc and the maximal change in RT produced during cocaine infusion. Division of SHR into two arbitrary groups was made, based on the direction of cocaine-induced change in RT. A significant (p less than 0.01) shortening of the Tc was obvious in SHR (8 of 15) in whom cocaine produced a hyperthermia. These animals were designated SHRH. The mean value for Tc in those SHR which demonstrated a lowering in RT (SHRL; 7 of 15) in response to cocaine was similar to that for WKY. Moreover, the SHRH evidenced significantly greater increases in HR, but not MBP, to cocaine infusion than did SHRL. The results indicate that restraint stress causes expression of a significant heterogeneity in the RT response of SHR to cocaine. The magnitude and direction of the RT responses are negatively correlated with sensitivity to the convulsive effects of cocaine in SHR. Stress may modify toxic responses to cocaine by interactions with body temperature homeostasis.