Humoral and neurohormonal aspects of blood pressure regulation: focus on angiotensin

Sodium Intake and Disease: Another Relationship to Consider

Sodium (Na+) is crucial for numerous homeostatic processes in the body and, consequentially, its levels are tightly regulated by multiple organ systems. Sodium is acquired from the diet, commonly in the form of NaCl (table salt), and substances that contain sodium taste salty and are innately palatable at concentrations that are advantageous to physiological homeostasis. The importance of sodium homeostasis is reflected by sodium appetite, an "all-hands-on-deck" response involving the brain, multiple peripheral organ systems, and endocrine factors, to increase sodium intake and replenish sodium levels in times of depletion. Visceral sensory information and endocrine signals are integrated by the brain to regulate sodium intake. Dysregulation of the systems involved can lead to sodium overconsumption, which numerous studies have considered causal for the development of diseases, such as hypertension. The purpose here is to consider the inverse-how disease impacts sodium intake, with a focus on stress-related and cardiometabolic diseases. Our proposition is that such diseases contribute to an increase in sodium intake, potentially eliciting a vicious cycle toward disease exacerbation. First, we describe the mechanism(s) that regulate each of these processes independently. Then, we highlight the points of overlap and integration of these processes. We propose that the analogous neural circuitry involved in regulating sodium intake and blood pressure, at least in part, underlies the reciprocal relationship between neural control of these functions. Finally, we conclude with a discussion on how stress-related and cardiometabolic diseases influence these circuitries to alter the consumption of sodium.

Artemisinin Improves Acetylcholine-Induced Vasodilatation in Rats with Primary Hypertension

Purpose

Endothelial dysfunction and the subsequent decrease in endothelium-dependent vascular relaxation of small arteries are major features of hypertension. Artemisinin, a well-known antimalarial drug, has been shown to exert protecting roles against endothelial cell injury in cardiac and pulmonary vascular diseases. The current study aimed to investigate the effects of artemisinin on endothelium-dependent vascular relaxation and arterial blood pressure, as well as the potential signalling pathways in spontaneously hypertensive rats (SHRs).

Methods

In this study, acetylcholine (ACh)-induced dose-dependent relaxation assays were performed to evaluate vascular endothelial function after treatment with artemisinin. Artemisinin was administered to the rats by intravenous injection or to arteries by incubation for the acute exposure experiments, and it was administered to rats by intraperitoneal injection for 28 days for the chronic experiments.

Results

Both acute and chronic administration of artemisinin decreased the heart rate and improved ACh-induced endothelium-dependent relaxation but negligibly affected the arterial blood pressure in SHRs. Incubation with artemisinin decreased basal vascular tension, NAD(P)H oxidase activity and reactive oxygen species (ROS) levels, but it also increased endothelial nitric oxide (NO) synthase (eNOS) activity and NO levels in the mesenteric artery, coronary artery, and pulmonary artery of SHRs. Artemisinin chronic administration to SHRs increased the protein expression of eNOS and decreased the protein expression of the NAD(P)H oxidase subunits NOX-2 and NOX-4 in the mesenteric artery.

Conclusion

These results indicate that treatment with artemisinin has beneficial effects on reducing the heart rate and basal vascular tension and improving endothelium-dependent vascular relaxation in hypertension, which might occur by increasing eNOS activation and NO release and inhibiting NAD(P)H oxidase derived ROS production.

Simulating Dynamics of Circulation in the Awake State and Different Stages of Sleep Using Non-autonomous Mathematical Model With Time Delay

We propose a mathematical model of the human cardiovascular system. The model allows one to simulate the main heart rate, its variability under the influence of the autonomic nervous system, breathing process, and oscillations of blood pressure. For the first time, the model takes into account the activity of the cerebral cortex structures that modulate the autonomic control loops of blood circulation in the awake state and in various stages of sleep. The adequacy of the model is demonstrated by comparing its time series with experimental records of healthy subjects in the SIESTA database. The proposed model can become a useful tool for studying the characteristics of the cardiovascular system dynamics during sleep.

Strategies for investigation of CNS mechanisms of phenotypic variation in blood pressure and salt appetite in genetic hypertensive rats

Several characteristics of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY), make these homozygous strains particularly well suited for investigating the interactions of salt appetite, blood pressure control, and their neuroendocrine substrates. Appropriate genetic and developmental investigations of sources of variation in salt appetite, blood pressure, and their putative neuroendocrine substrates in these homozygous strains can provide valuable insights into fundamental mechanisms of disease, as well as factors controlling homeostatic behavioral and physiological processes. However, inappropriate use of these strains can produce misleading, although seductively plausible, conclusions regarding mechanisms. Selective inbreeding for hypertension has concentrated in SHR the "high pressure" allele for several genes that influence blood pressure, whereas breeding for normal blood pressure has left WKY with the "normal pressure" allele for all or most of these genes. In principle, inbred hypertensive strains could provide information about specific genetic alterations that mediate the hypertensive phenotype. The benefits of work with these strains are discussed, but several false assumptions and logical pitfalls are described that might cause misleading or erroneous interpretations of results from work with such strains. These problems illustrate the importance of the research strategy in elucidating the particular information that can be provided by these inbred animal models of hypertension. Two strategic approaches for studying hypertension and other genetically determined or influenced characteristics in inbred animal models such as SHR are discussed: cosegregation analysis for identifying or rejecting genetic linkage, and brain graft techniques for identifying brain specific genetic influences on cardiovascular or behavioral phenotypes. Examples of each approach are described.

Developmental differences of angiotensinogen mRNA in the preoptic area between spontaneously hypertensive and age-matched Wistar-Kyoto rats

In order to know the possible involvement of the central angiotensin system in hypertension, angiotensinogen mRNA (AomRNA) levels of eight discrete brain areas were measured by Northern blot hybridization analysis in the spontaneously hypertensive rats (SHR), compared with those in age-matched normotensive Wistar-Kyoto rats (WKY). In 16-week-old SHR (hypertensive stage), AomRNA levels in the preoptic area (POA), but not in the ventromedial hypothalamus, lateral hypothalamus and mammillary body, among the hypothalamic nuclei, were higher (approximate 50%) than in WKY. There were no differences in other brain areas, such as the striatum, septum, amygdala and cerebellum between both the strains. The AomRNA levels in POA were already higher (38%) in 4-week-old SHR (prehypertensive stage) without significance, and the difference was augmented (82%) in 7-week-old SHR (evolving stage). These results suggest that the developmental changes of AomRNA levels at POA may be related in some aspect to hypertension process.

Antagonism of central pressor response to angiotensin II by alpha-human atrial natriuretic polypeptide at the preoptic area and posterior hypothalamus in rats

Effects of alpha-human atrial natriuretic polypeptides (alpha-hANP) on pressor responses to angiotensin II (AII) were assessed at the preoptic area, posterior hypothalamus and central amygdaloid nucleus (ACE) in spontaneously hypertensive (SHR) and control normotensive Wistar-Kyoto (WKY) rats. Angiotensin II, administered intracerebroventricularly, at a dose of 100 ng produced a marked pressor response in hypertensive, as well as in normotensive rats and the response was potentiated in hypertensive rats. The response was antagonized in a dose-dependent manner by administration of alpha-hANP into the preoptic area and posterior hypothalamus but not to the amygdaloid nucleus. The antagonism was more marked in hypertensive than in normotensive rats. Angiotensin II, when injected directly to the preoptic area at a small dose of 10 ng similarly evoked a marked pressor response, which was augmented in hypertensive rats. This response was also antagonized by coadministration of alpha-hANP to the preoptic area in hypertensive but not in normotensive rats. The results suggest that the antagonistic relationship between ANP and AII exists at the preoptic area and posterior hypothalamus, probably implying that the activity of the ANP and AII systems in brain play a role in centrally controlling the cardiovascular system and is altered at these areas in genetically hypertensive rats.

Brain and liver angiotensinogen messenger RNA in genetic hypertensive and normotensive rats

The brain's renin-angiotensin system in integrally involved in the regulation of blood pressure and fluid/mineral metabolism. Enhanced activity of the angiotensin system in the brain has been implicated as a possible source of the hypertension and the elevated salt appetite of the spontaneously hypertensive rat, as compared with the Wistar-Kyoto rat. This study tested whether these inbred strains of hypertensive and normotensive rats differ in central or peripheral expression of the gene coding for angiotensinogen, the prohormone for the angiotensin peptides. Angiotensinogen messenger RNA was measured in the brain by in situ hybridization and in the liver by Northern blot analysis, using a synthetic oligonucleotide. There was a 28% greater expression of the angiotensinogen gene in the region of the anteroventral hypothalamus, preoptic area, and medial septum of the hypertensive strain. There were no differences between strains in liver angiotensinogen gene expression. These results are consistent with the possibility that enhanced elaboration of the angiotensin prohormone in the brain contributes, in part, to the hypertension or the elevated salt appetite of the spontaneously hypertensive rat.

Immunocytochemical localization of angiotensinogen in the rat brain

The distribution of angiotensinogen-like immunoreactivity in the rat brain was investigated using specific antisera against pure rat plasma angiotensinogen in conjunction with the sensitive streptavidin-biotin peroxidase method. Angiotensinogen antisera were shown by radioimmunoassay and Western blotting to recognize angiotensinogen from both rat plasma and cerebrospinal fluid, and to cross-react with des-AI-angiotensinogen (100%) but not with angiotensin I and II, tetradecapeptide, luteinizing hormone-releasing hormone, rat albumin and angiotensinogen from eight other species. Angiotensinogen-like immunoreactivity was detected throughout the rat brain in both neuroglia and neurons. The highest concentration of neuroglial angiotensinogen-like immunoreactivity was in the hypothalamus and preoptic areas, with moderate to heavy concentrations in the mesencephalon and myelencephalon. The cerebellum demonstrated neuroglial staining in the granular layer and fibre tracts. Very little neuroglial staining was noted in the cerebral cortex or olfactory bulbs. Neuronal immunostaining was observed throughout the globus pallidus and the caudate putamen, in various parts of the thalamus and the supraoptic nucleus of the hypothalamus. In the midbrain moderate immunostaining was observed in periaquaductal central gray, the deep mesencephalic nucleus, the inferior colliculus and in scattered cells in the anterior mesencephalon. In the medulla, neuronal staining was localized to the vestibular nuclei and to other cell bodies mainly in the dorsolateral regions. In the cerebellum, staining was noted mainly in the deeper cerebellar nuclei and in the Purkinje cells. Immunostaining in the cerebral cortex was localized to the cingulate cortex and the primary olfactory cortex. Light staining was present in the endopiriform cortex and in scattered neurons adjacent to the external capsule. In the olfactory bulbs light neuronal staining was mainly associated with the mitral cell layer. The widespread distribution of angiotensinogen-like immunoreactivity supports the view that it is synthesized in the central nervous system and forms part of a brain renin-angiotensin system. In addition, its presence at sites other than those normally associated with the control of blood pressure and fluid and electrolyte homeostasis suggests that its involvement may not be limited to these regulatory functions.

Potentiation of pressor response to angiotensin II at the preoptic area in spontaneously hypertensive rat

Cardiovascular responses to angiotensin II(AII) at the preoptic area (POA) were compared between normotensive Wistar Kyoto rat (WKY) and spontaneously hypertensive rat(SHR) by measuring blood pressure and heart rate under unrestrained, conscious state via a catheter implanted chronically into the abdominal aorta and by injection of drugs into POA through a chronic guide cannula. AII injected into POA at doses of 0.3 ng and 1 ng produced a dose-dependent pressor response, accompanied with a slight decrease of heart rate, in both WKY and SHR. However, in SHR, the pressor response to AII was more than 2 times greater than that in WKY and was quick in onset and lasted about 30 min. When AII in combination with [Sar1, Ile8]-angiotensin II (0.5 microgram), an AII receptor antagonist, were simultaneously administered to POA, the pressor response to AII was strongly inhibited in both WKY and SHR. The results suggest that the pressor response to AII due to its receptor stimulation at POA is markedly potentiated in SHR.

Differential effects of central angiotensin II and substance P on sympathetic nerve activity in conscious rats. Implications for cardiovascular adaptation to behavioral responses

The centrally induced effects of angiotensin II and substance P on the cardiovascular system and on neuronal efferent activity of the splanchnic, renal, and adrenal nerves were investigated in chronically instrumented conscious rats. The pressor responses to substance P injected into the lateral brain ventricle were accompanied by marked and short latency increases in heart rate, cardiac output, splanchnic, renal, and adrenal nerve activity, and a rise in plasma noradrenaline and adrenaline. Behaviorally, an arousal-type reaction was observed. In contrast, the pressor responses to intracerebroventricular angiotensin II were associated with initial decreases in heart rate, cardiac output, splanchnic, renal, and adrenal nerve activity, and a fall in plasma noradrenaline at the time of the maximal blood pressure increase. In some but not all animals, a second blood pressure peak associated with increases in heart rate and splanchnic nerve activity was observed after several minutes. Incomplete chronic sinoaortic baroreceptor deafferentiation prevented the angiotensin II-induced fall in heart rate but not the initial fall in splanchnic nerve activity. The decreases in splanchnic nerve activity also occurred in diabetes insipidus rats and persisted in Long Evans rats after vascular vasopressin receptor blockade with d(CH2)5AVP, despite marked reductions of the pressor responses in both groups. Peripheral alpha-adrenoceptor blockade with prazosin or ganglion blockade with hexamethonium inhibited the central angiotensin II pressor responses only in combination with vasopressin receptor blockade. On the other hand, either sympatholytic drug, alone, abolished the pressor responses in the diabetes insipidus rats. This indicates that in intact conscious rats the central pressor effects of angiotensin II are initiated by vasopressin release but become dependent on the sympathetic nervous system when vasopressin is absent or not effective. When rats were allowed to drink in response to angiotensin II, a further sharp rise in blood pressure occurred, together with increases in heart rate and splanchnic nerve activity. The results demonstrate fundamental differences in the mechanisms by which central pressor peptides can influence cardiovascular and autonomic function. It is conceivable that the distinct sympathetic response patterns to central angiotensin II and substance P receptor stimulation form part of a specific cardiovascular adjustment to the individual behavioral reactions, such as drinking, as in the case of angiotensin II, or arousal within the central processing of pain, as in the case of substance P.

The tissue renin-angiotensin systems: focus on brain angiotensin, adrenal gland and arterial wall

The enzyme renin has long been considered an exclusively renal enzyme responsible for the generation of angiotensin in the plasma, and angiotensin was considered a peptide hormone with specific target organs. Since renin has been discovered to occur not only in the kidney but in addition in a large number of other tissues, e.g. the salivary gland, the uterus, the blood vessels, the adrenal gland and the brain, this concept had to be revised. The possibility of local generation of angiotensin in these target tissues, where it has marked biological effects casts a new light on angiotensin as a more ubiquitous tissue factor or even a neurohormone.

Central effects of converting enzyme inhibitors

Evidence has accumulated that systemic administration of converting enzyme inhibitors (CEI) such as captopril, MK 421 or SA 446 not only produces an inhibition of the plasma renin angiotensin system (RAS), but also of the RAS in various target organs which are relevant for blood pressure (BP) regulation. A potential target organ is the brain, where a local CE inhibition could contribute to the BP lowering action of CEI. CE in the brain can be inhibited by intracerebroventricular (i.c.v.) injection of CEI as evidenced by an inhibition of the pressor and drinking responses to i.c.v. angiotensin I (ANG I) or renin and by potentiation of the pressor responses to i.c.v. bradykinin. Site of the inhibition is not only the cerebrospinal fluid but also periventricular brain tissue such as the hypothalamus. I.c.v. injection of captopril at doses which inhibit brain CE but do not leak into the peripheral blood were shown to lower BP in conscious stroke-prone spontaneously hypertensive rats (SHRSP), but not in normotensive Wistar Kyoto (WKY) controls. Acute peripheral administration of CEI can produce an inhibition of brain CE. This was shown by an attenuation of the drinking responses to i.c.v. ANG I and renin and by direct measurements of CE activity in brain tissue. Chronic oral treatment with CEI produces changes of brain RAS parameters which suggest an inhibition of ANG II formation in the brain.

The localization of converting enzyme in kidney vessels of the rat

An antibody against pure rabbit lung converting enzyme (CE) showing cross-reaction with CE from other species was used for immunocytochemical studies in the kidney of rats. Using the indirect labelling PAP-technique, specific immunostaining was found in the endothelial layer of all arteries and arterioles of kidney cortex and in some descending vasa recta. CE-positive reactions were also seen in most glomeruli, the reaction product being confined to only a few capillary loops in connection with the glomerular stalk. A few immunostained capillaries in the cortical labyrinth were suspected to belong to the first ramifications of the efferent arteriole. The bulk of all other of the glomerular and peritubula capillaries as well as all veins of the kidney showed no obvious immunostaining. The functional significance of this specific localization pattern of CE in the endothelium of kidney vessels is discussed with respect to the actions of the systemic and the local, intrarenal renin-angiotensin-system on kidney functions.

Angiotensin-converting enzyme is present in the subfornical organ and other circumventricular organs of the rat

Angiotensin-converting enzyme (ACE, kininase II, EC 3.4.15.1) activity was measured by a radiochemical assay in isolated circumventricular organs of the rat. ACE activity was unevenly distributed, with a 100-fold difference between the lowest (subcommisural) and the highest (subfornical organ) activities. Our results suggest that angiotensin II could be locally formed in circumventricular organs and especially in the subfornical organ. The high angiotensin-converting enzyme activity in the subfornical organ could indicate a physiological role of endogenous angiotensin II in this structure.

Effects of [D-Ala2]-methionine-enkephalin on blood pressure, heart rate, and baroreceptor reflex sensitivity in conscious cats

Effects of intracerebroventricular (i.c.v) injection of [D-Ala2]-methionine-enkephalinamide (DAME) on blood pressure (BP), heart rate, and baroreceptor reflex sensitivity were studied in conscious cats. DAME was administered at doses between 5 and 100 nmoles. Blood pressure and heart rate increased dose dependently. The sensitivity of the baroreceptor reflex was attenuated for 15 to 60 minutes after DAME administration; this was independent of the BP changes. The effects of enkephalin on BP and baroreceptor reflex were abolished by i.c.v. naloxone. DAME caused pathological changes in the electroencephalogram (EEG) characterized by sharp waves in the hippocampus recordings and a loss of theta activity in the electrocorticogram. Behavioral changes were characterized by decreased physical mobility and anxiousness. These behavioral and EEG changes lasted for a longer period of time than the cardiovascular changes; they were also counteracted by naloxone. It is concluded that DAME produces a centrally mediated vasopressor response and a baroreceptor reflex attenuation and that, with respect to the time course, the effects on the baroreceptor reflex are separated from those on BP behavior and EEG, but not on heart rate. The fact that all effects of enkephalin on the parameters tested in the present experiment were completely antagonized by naloxone suggests that they are mediated by naloxone-sensitive enkephalin brain receptors.

Enkephalin effects on blood pressure, heart rate, and baroreceptor reflex

The cardiovascular effects of opioid peptides have been studied. Leucine-enkephalin (Leu-ENK) produced blood pressure (BP) increases following administration into the lateral brain ventricles (i.v.t.), into the cisterna magna (i.c.i.), and following intravenous (i.v.) administration. Heart rate (HR) increases were observed following all routes of administration (threshold for BP and HR effects at 0.3 nmole, maximum at 360 nmoles). The cardiovascular effects were independent of generalized seizures, which may occur at higher doses of enkephalins (ENK). D-alanine-enkephalin (D-Ala-ENK) attenuated the vagal component of the baroreceptor reflex in cats. This was indicated by the findings that HR did not decrease following D-Ala-ENK-induced BP increases and that the compensatory decreases in HR following i.v. pressor doses of angiotensin II (ANG II) were markedly attenuated in cats treated with i.v.t. D-Ala-ENK. Naloxone inhibited the BP and HR effects following i.c.i. and i.v., but not following i.v.t., administration of Leu-ENK. The i.v.t. Leu-ENK effect were inhibited by beta-adrenergic receptor blockade. Bratteboro rats homozygous for hereditary diabetes insipidus with total absence of antidiuretic hormone (ADH) synthesis responded with BP decreases following i.v.t. Leu-ENK, while BP increases were observed in control Long-Evans rats. Blood pressure increases to i.v.t. Leu-ENK were markedly greater in spontaneously hypertensive rats of the stroke-prone strain (SHR-sp) than in normotensive control rats; SHR-sp exhibit a humoral pattern of increased ADH, ACTH, and catecholamines, presumably due to central peptidergic stimulation. The known effects of opioid peptides on these hormones and the observed cardiovascular responses suggest a possible participation of this peptide system in the maintenance of high BP in the SHR-sp.