Long-term blood pressure telemetry in AT2 receptor-disrupted mice

Artificial light at night affects the daily profile of pulse pressure and protein expression in the thoracic aorta of rats

Artificial light at night (ALAN) disrupts 24-h variability of blood pressure, but the molecular mechanisms underlying these effects are unknown. Therefore, we analysed the daily variability of pulse pressure, the maximum value of acceleration rate of aortic pressure (dP/dt(max)) measured by telemetry and protein expression in the thoracic aorta of normotensive male rats exposed to ALAN (1-2 lx) for 3 weeks. Daily, 24-h variability of pulse pressure and dP/dt(max) was observed during a regular light/dark regimen with higher values during the dark compared to the light phase of the day. ALAN suppressed 24-h variability and enhanced ultradian (<12-h) periods of pulse pressure and dP/dt(max) in duration-dependent manners. From beat-to-beat blood pressure variability, ALAN decreased low-frequency bands (a sympathetic marker) and had minimal effects on high-frequency bands. At the molecular level, ALAN decreased angiotensin II receptor type 1 expression and reduced 24-h variability. ALAN caused the appearance of 12-h oscillations in transforming growth factor β1 and fibulin 4. Expression of sarco/endoplasmic reticulum Ca2+-ATPase type 2 was increased in the middle of the light and dark phase of the day, and ALAN did not affect its daily and 12-h variability. In conclusion, ALAN suppressed 24-h variability of pulse pressure and dP/dt(max), decreased the power of low-frequency bands and differentially affected the expression of specific proteins in the rat thoracic aorta. Suppressed 24-h oscillations by ALAN underline the pulsatility of individual endocrine axes with different periods, disrupting the cardiovascular control of central blood pressure.

The contribution of the AT1 receptor to erythropoiesis

The renin-angiotensin system (RAS) comprises a broad set of functional peptides and receptors that play a role in cardiovascular homeostasis and contribute to cardiovascular pathologies. Angiotensin II (Ang II) is the most potent peptide hormone produced by the RAS due to its high abundance and its strong and pleiotropic impact on the cardiovascular system. Formation of Ang II takes place in the bloodstream and additionally in tissues in the so-called local RAS. Of the two Ang II receptors (AT1 and AT2) that Ang II binds to, AT1 is the most expressed throughout the mammalian body. AT1 expression is not restricted to cells of the cardiovascular system but in fact AT1 protein is found in nearly all organs, hence, Ang II takes part in several modulatory physiological processes one of which is erythropoiesis. In this review, we present multiple evidence supporting that Ang II modulates physiological and pathological erythropoiesis processes trough the AT1 receptor. Cumulative evidence indicates that Ang II by three distinct mechanisms influences erythropoiesis: 1) stimulation of renal erythropoietin synthesis; 2) direct action on bone marrow precursor cells; and 3) modulation of sympathetic nerve activity to the bone marrow. The text highlights clinical and preclinical evidence focusing on mechanistic studies using rodent models.

Nondipping Blood Pressure: Predictive or Reactive Failure of Renal Sodium Handling?

Blood pressure follows a daily rhythm, dipping during nocturnal sleep in humans. Attenuation of this dip (nondipping) is associated with increased risk of cardiovascular disease. Renal control of sodium homeostasis is essential for long-term blood pressure control. Sodium reabsorption and excretion have rhythms that rely on predictive/circadian as well as reactive adaptations. We explore how these rhythms might contribute to blood pressure rhythm in health and disease.

Brain Angiotensin Type-1 and Type-2 Receptors in Physiological and Hypertensive Conditions: Focus on Neuroinflammation

PURPOSE OF REVIEW

To review recent data that suggest opposing effects of brain angiotensin type-1 (AT1R) and type-2 (AT2R) receptors on blood pressure (BP). Here, we discuss recent studies that suggest pro-hypertensive and pro-inflammatory actions of AT1R and anti-hypertensive and anti-inflammatory actions of AT2R. Further, we propose mechanisms for the interplay between brain angiotensin receptors and neuroinflammation in hypertension.

RECENT FINDINGS

The renin-angiotensin system (RAS) plays an important role in regulating cardiovascular physiology. This includes brain AT1R and AT2R, both of which are expressed in or adjacent to brain regions that control BP. Activation of AT1R within those brain regions mediate increases in BP and cause neuroinflammation, which augments the BP increase in hypertension. The fact that AT1R and AT2R have opposing actions on BP suggests that AT1R and AT2R may have similar opposing actions on neuroinflammation. However, the mechanisms by which brain AT1R and AT2R mediate neuroinflammatory responses remain unclear. The interplay between brain angiotensin receptor subtypes and neuroinflammation exacerbates or protects against hypertension.

The Renin-Angiotensin System in Hypertension, a Constantly Renewing Classic: Focus on the Angiotensin AT2-Receptor

It is common knowledge that the renin-angiotensin system (RAS), in particular angiotensin II acting through the angiotensin AT₁-receptor (AT₁R), is pivotal for the regulation of blood pressure (BP) and extracellular volume. More recent findings have revealed that the RAS is far more complex than initially thought and that it harbours additional mediators and receptors, which are able to counteract and thereby fine-tune AT₁R-mediated actions. This review will focus on the angiotensin AT₂-receptor (AT₂R), which is one of the "counter-regulatory" receptors within the RAS. It will review and discuss data related to the role of the AT₂R in regulation of BP and focus on the following 3 questions: Do peripheral AT₂R have an impact on BP regulation, and, if so, does this effect become apparent only under certain conditions? Are central nervous system AT₂R involved in regulation of BP, and, if so, which brain areas are involved and what are the mechanisms? Does dysfunction of AT₂R contribute to the pathogenesis of hypertension in preeclampsia?

Circadian regulation of renal function

The kidneys regulate many vital functions that require precise control throughout the day. These functions, such as maintaining sodium balance or regulating arterial pressure, rely on an intrinsic clock mechanism that was commonly believed to be controlled by the central nervous system. Mounting evidence in recent years has unveiled previously underappreciated depth of influence by circadian rhythms and clock genes on renal function, at the molecular and physiological level, independent of other external factors. The impact of circadian rhythms in the kidney also affects individuals from a clinical standpoint, as the loss of rhythmic activity or clock gene expression have been documented in various cardiovascular diseases. Fortunately, the prognostic value of examining circadian rhythms may prove useful in determining the progression of a kidney-related disease, and chronotherapy is a clinical intervention that requires consideration of circadian and diurnal rhythms in the kidney. In this review, we discuss evidence of circadian regulation in the kidney from basic and clinical research in order to provide a foundation on which a great deal of future research is needed to expand our understanding of circadian relevant biology.

Dimerization of AT2 and Mas Receptors in Control of Blood Pressure

PURPOSE OF REVIEW

Angiotensin type 2 receptor (AT2R) and receptor Mas (MasR) are part of the "protective arm" of the renin angiotensin system. Gene and pharmacological manipulation studies reveal that AT2R and MasR are involved in natriuretic, vasodilatory, and anti-inflammatory responses and in lowering blood pressure in various animal models under normal and pathological conditions such as salt-sensitive hypertension, obesity, and diabetes. The scope of this review is to discuss co-localization and heterodimerization as potential molecular mechanisms of AT2R- and MasR-mediated functions including antihypertensive activities.

RECENT FINDINGS

Accumulating evidences show that AT2R and MasR are co-localized, make a heterodimer, and are functionally interdependent in producing their physiological responses. Moreover, ang-(1-7) preferably may be an AT1R-biased agonist while acting as a MasR agonist. The physical interactions of AT2R and MasR appear to be an important mechanism by which these receptors are involved in blood pressure regulation and antihypertensive activity. Whether heteromers of these receptors influence affinity or efficacy of endogenous or synthetic agonists remains a question to be considered.

Protective Angiotensin Type 2 Receptors in the Brain and Hypertension

PURPOSE OF REVIEW

The goal of this review is to assess the evidence that activation of angiotensin type 2 receptors (AT2R) in the brain can lower blood pressure and possibly constitute an endogenous anti-hypertensive mechanism.

RECENT FINDINGS

Recent studies that detail the location of AT2R in the brain, particularly within or near cardiovascular control centers, mesh well with findings from pharmacological and gene transfer studies which demonstrate that activation of central AT2R can influence cardiovascular regulation. Collectively, these studies indicate that selective activation of brain AT2R causes moderate decreases in blood pressure in normal animals and more profound anti-hypertensive effects, along with restoration of baroreflex function, in rodent models of neurogenic hypertension. These findings have opened the door to studies that can (i) assess the role of specific AT2R neuron populations in depressing blood pressure, (ii) determine the relevance of such mechanisms, and (iii) investigate interactions between AT2R and depressor angiotensin-(1-7)/Mas mechanisms in the brain.

Angiotensin II type 2 receptor (AT2R) in renal and cardiovascular disease

Angiotensin II (Ang II) is well-considered to be the principal effector of the renin-angiotensin system (RAS), which binds with strong affinity to the angiotensin II type 1 (AT1R) and type 2 (AT2R) receptor subtype. However, activation of both receptors is likely to stimulate different signalling mechanisms/pathways and produce distinct biological responses. The haemodynamic and non-haemodynamic effects of Ang II, including its ability to regulate blood pressure, maintain water-electrolyte balance and promote vasoconstriction and cellular growth are well-documented to be mediated primarily by the AT1R. However, its biological and functional effects mediated through the AT2R subtype are still poorly understood. Recent studies have emphasized that activation of the AT2R regulates tissue and organ development and provides in certain context a potential counter-regulatory mechanism against AT1R-mediated actions. Thus, this review will focus on providing insights into the biological role of the AT2R, in particular its actions within the renal and cardiovascular system.

AT2 Receptors: Potential Therapeutic Targets for Hypertension

The renin-angiotensin system (RAS) is arguably the most important and best studied hormonal system in the control of blood pressure (BP) and the pathogenesis of hypertension. The RAS features its main effector angiotensin II (Ang II) acting via its 2 major receptors, angiotensin type-1(AT1R) and type-2 (AT2R). In general, AT2Rs oppose the detrimental actions of Ang II via AT1Rs. AT2R activation induces vasodilation and natriuresis, but its effects to lower BP in hypertension have not been as clear as anticipated. Recent studies, however, have demonstrated that acute and chronic AT2R stimulation can induce natriuresis and lower BP in the Ang II infusion model of experimental hypertension. AT2R activation induces receptor recruitment from intracellular sites to the apical plasma membranes of renal proximal tubule cells via a bradykinin, nitric oxide, and cyclic guanosine 3',5' monophosphate signaling pathway that results in internalization and inactivation of sodium (Na+) transporters Na+-H+ exchanger-3 and Na+/K+ATPase. These responses do not require the presence of concurrent AT1R blockade and are effective both in the prevention and reversal of hypertension. This review will address the role of AT2Rs in the control of BP and Na+ excretion and the case for these receptors as potential therapeutic targets for hypertension in humans.

Collecting Duct Renin Does Not Mediate DOCA-Salt Hypertension or Renal Injury

Collecting duct (CD)-derived renin is involved in the hypertensive response to chronic angiotensin-II (Ang-II) administration. However, whether CD renin is involved in Ang-II independent hypertension is currently unknown. To begin to examine this, 12 week old male and female CD-specific renin knock out (KO) mice and their littermate controls were subjected to uni-nephrectomy followed by 2 weeks of deoxycorticosterone acetate (DOCA) infusion combined with a high salt diet. Radiotelemetric blood pressure (BP) was similar between KO and control mice at baseline; BP increased in both groups to a similar degree throughout the 2 weeks of DOCA-salt treatment. Urinary albumin excretion and plasma blood urea nitrogen were comparable between the two groups after DOCA-salt treatment. Fibrosis as assessed by Masson’s Trichrome stain/Sirius Red stain and collagen-1 mRNA expression was similar between control and KO mice. Compared to baseline, DOCA-salt treatment decreased plasma renin concentration (PRC), urinary renin excretion and medullary renin mRNA expression in both floxed and CD renin KO mice with no detectable differences between the two groups. Further, in primary culture of rat inner medullary CD, aldosterone treatment did not change renin activity or total renin content. Taken together, these data suggest that CD derived renin does not play a role in DOCA-salt hypertension.

Suppression of Resting Metabolism by the Angiotensin AT2 Receptor

Activation of the brain renin-angiotensin system (RAS) stimulates energy expenditure through increasing of the resting metabolic rate (RMR), and this effect requires simultaneous suppression of the circulating and/or adipose RAS. To identify the mechanism by which the peripheral RAS opposes RMR control by the brain RAS, we examined mice with transgenic activation of the brain RAS (sRA mice). sRA mice exhibit increased RMR through increased energy flux in the inguinal adipose tissue, and this effect is attenuated by angiotensin II type 2 receptor (AT2) activation. AT2 activation in inguinal adipocytes opposes norepinephrine-induced uncoupling protein-1 (UCP1) production and aspects of cellular respiration, but not lipolysis. AT2 activation also opposes inguinal adipocyte function and differentiation responses to epidermal growth factor (EGF). These results highlight a major, multifaceted role for AT2 within inguinal adipocytes in the control of RMR. The AT2 receptor may therefore contribute to body fat distribution and adipose depot-specific effects upon cardio-metabolic health.

How Does Circadian Rhythm Impact Salt Sensitivity of Blood Pressure in Mice? A Study in Two Close C57Bl/6 Substrains

Background

Mouse transgenesis has provided the unique opportunity to investigate mechanisms underlying sodium kidney reabsorption as well as end organ damage. However, understanding mouse background and the experimental conditions effects on phenotypic readouts of engineered mouse lines such as blood pressure presents a challenge. Despite the ability to generate high sodium and chloride plasma levels during high-salt diet, observed changes in blood pressure are not consistent between wild-type background strains and studies.

Methods

The present work was designed in an attempt to determine guidelines in the field of salt-induced hypertension by recording continuously blood pressure by telemetry in mice submitted to different sodium and potassium loaded diets and changing experimental conditions in both C57BL/6N and C57BL/6J mice strain (Normal salt vs. Low salt vs. High-salt/normal potassium vs. High salt/low potassium, standard vs. modified light cycle, Non-invasive tail cuff blood pressure vs. telemetry).

Results

In this study, we have shown that, despite a strong blood pressure (BP) basal difference between C57BL/6N and C57BL/6J mice, High salt/normal potassium diet increases BP and heart rate during the active phase only (dark period) in the same extent in both strains. On the other hand, while potassium level has no effect on salt-induced hypertension in C57BL/6N mice, high-salt/low potassium diet amplifies the effect of the high-salt challenge only in C57BL/6J mice. Indeed, in this condition, salt-induced hypertension can also be detected during light period even though this BP increase is lower compared to the one occurring during the dark period. Finally, from a methodological perspective, light cycle inversion has no effect on this circadian BP phenotype and tail-cuff method is less sensitive than telemetry to detect BP phenotypes due to salt challenges.

Conclusions

Therefore, to carry investigations on salt-induced hypertension in mice, chronic telemetry and studies in the active phase are essential prerequisites.

The brain renin-angiotensin system plays a crucial role in regulating body weight in diet-induced obesity in rats

BACKGROUND AND PURPOSE

Reduced weight gain after treatment with AT1 receptor antagonists may involve a brain-related mechanism. Here, we investigated the role of the brain renin-angiotensin system on weight regulation and food behaviour, with or without additional treatment with telmisartan.

METHODS

Transgenic rats with a brain-specific deficiency in angiotensinogen (TGR(ASrAOGEN)) and the corresponding wild-type, Sprague Dawley (SD) rats were fed (3 months) with a high-calorie cafeteria diet (CD) or standard chow. SD and TGR(ASrAOGEN) rats on the CD diet were also treated with telmisartan (8 mg·kg(-1) ·d(-1) , 3 months).

RESULTS

Compared with SD rats, TGR(ASrAOGEN) rats (i) had lower weights during chow feeding, (ii) did not become obese during CD feeding, (iii) had normal baseline leptin plasma concentrations independent of the feeding regimen, whereas plasma leptin of SD rats was increased due to CD, (iv) showed a reduced energy intake, (v) had a higher, strain-dependent energy expenditure, which is additionally enhanced during CD feeding, (vi) had enhanced mRNA levels of pro-opiomelanocortin and (vii) showed improved glucose control. Weight gain and energy intake in rats fed the CD diet were markedly reduced by telmisartan in SD rats but only to a minor extent in TGR(ASrAOGEN) rats.

CONCLUSIONS

The brain renin-angiotensin system affects body weight regulation, feeding behaviour and metabolic disorders. When angiotensin II levels are low in brain, rats are protected from developing diet-induced obesity and obesity-related metabolic impairments. We further suggest that telmisartan at least partly lowers body weight via a CNS-driven mechanism.

Brain endogenous angiotensin II receptor type 2 (AT2-R) protects against DOCA/salt-induced hypertension in female rats

Background

Recent studies demonstrate that there are sex differences in the expression of angiotensin receptor type 2 (AT2-R) in the kidney and that AT2-R plays an enhanced role in regulating blood pressure (BP) in females. Also, brain AT2-R activation has been reported to negatively modulate BP and sympathetic outflow. The present study investigated whether the central blockade of endogenous AT2-R augments deoxycorticosterone acetate (DOCA)/salt-induced hypertension in both male and female rats.

Methods

All rats were subcutaneously infused with DOCA combined with 1% NaCl solution as the sole drinking fluid. BP and heart rate (HR) were recorded by telemetric transmitters. To determine the effect of central AT2-R on DOCA/salt-induced hypertension, male and female rats were intracerebroventricularly (icv) infused with AT2-R antagonist, PD123,319, during DOCA/salt treatment. Subsequently, the paraventricular nucleus (PVN) of the hypothalamus, a key cardiovascular regulatory region of the brain, was analyzed by quantitative real-time PCR and Western blot.

Results

DOCA/salt treatment elicited a greater increase in BP in male rats than that in females. Icv infusions of the AT2-R antagonist significantly augmented DOCA/salt pressor effects in females. However, this same treatment had no enhanced effect on DOCA/salt-induced increase in the BP in males. Real-time PCR and Western blot analysis of the female brain revealed that DOCA/salt treatment enhanced the mRNA and protein expression for both antihypertensive components including AT2-R, angiotensin-converting enzyme (ACE)-2, and interleukin (IL)-10 and hypertensive components including angiotensin receptor type 1 (AT1-R), ACE-1, tumor necrosis factor (TNF)-α, and IL-1β, but decreased mRNA expression of renin in the PVN. The central blockade of AT2-R reversed the changes in mRNA and protein expressions of ACE-2, IL-10, and renin, further increased the expressions of TNF-α and IL-1β, and kept higher the expressions of AT1-R, ACE-1, and AT2-R.

Conclusions

These results indicate that endogenous AT2-R activation in the brain plays an important protective role in the development of DOCA/salt-induced hypertension in females, but not in males. The protective effect of AT2-R in females involves regulating the expression of brain renin-angiotensin system components and proinflammatory cytokines.

Angiotensin type 2 receptors: blood pressure regulation and end organ damage

In most situations, the angiotensin AT2-receptor (AT2R) mediates physiological actions opposing those mediated by the AT1-receptor (AT1R), including a vasorelaxant effect. Nevertheless, experimental evidence vastly supports that systemic application of AT2R-agonists is blood pressure neutral. However, stimulation of AT2R locally within the brain or the kidney apparently elicits a systemic blood pressure lowering effect. A systemic effect of AT2R stimulation on blood pressure can also be achieved, when the prevailing effect of continuous background AT1R-stimulation is attenuated by low-dose AT1R blockade. Despite a lack of effect on blood pressure, AT2R stimulation still protects from hypertensive end-organ damage. Current data and evidence therefore suggest that AT2R agonists will not be suitable as future anti-hypertensive drugs, but that they may well be useful for end-organ protection in combination with established anti-hypertensives.

Comparison of simultaneous measurement of mouse locomotor activity by radiotelemetry and photobeam methods

INTRODUCTION

Locomotor activity recordings are widely used in different physiological and pharmacological studies. There are two mainly used methods - radiotelemetry and photobeam recording systems. To our knowledge, these methods have not previously been directly and simultaneously compared.

METHODS

The current study consisted of a comparison of locomotor activity data gained simultaneously from radiotelemetry and photobeam recordings, firstly from a robotic device and secondly from an animal experiment performed with mice.

RESULTS

Data gained from the animal study showed relatively high variation, but overall agreement between the methods was good.

DISCUSSION

The two methods were cross-validated in the current study. The data gained from both methods were in good general agreement. However, in an animal experiment, e.g. when sedative drugs or other behavior-modifying interventions are used, one should interpret the results with caution as alterations in animal behavior (e.g. in grooming) may possibly not be picked up similarly by the two methods.

Increased expression of angiotensin II type 2 receptors in the solitary-vagal complex blunts renovascular hypertension

Angiotensin II increases and decreases arterial pressure by acting at angiotensin type 1 and type 2 receptors, respectively. Renovascular hypertensive rats exhibit a high level of activity of the peripheral and central renin-angiotensin system. Therefore, in the present study, we evaluated the effect of increasing the expression of angiotensin type 2 receptors in the solitary-vagal complex (nucleus of the solitary tract/dorsal motor nucleus of the vagus), a key brain stem region for cardiovascular regulation, on the development of renovascular hypertension. Holtzman normotensive rats were implanted with a silver clip around the left renal artery to induce 2-kidney 1-clip renovascular hypertension. Three weeks later, rats were microinjected in the solitary-vagal complex with either an adenoassociated virus to increase the expression of angiotensin type 2 receptors or with a control vector. We observed that increasing angiotensin type 2 receptor expression in the solitary-vagal complex attenuated the development of renovascular hypertension and also reversed the impairment of the baroreflex and the increase in the low-frequency component of systolic blood pressure observed in renovascular hypertensive rats. Furthermore, an observed decrease in mRNA levels of angiotensin-converting enzyme 2 in the solitary-vagal complex of renovascular hypertensive rats was restored to control levels after viral-mediated increases in angiotensin type 2 receptors at this site. Collectively, these data demonstrate specific and beneficial effects of angiotensin type 2 receptors via the brain of hypertensive rats and suggest that central angiotensin type 2 receptors may be a potential target for therapeutics in renovascular hypertension.

Influence of Fimasartan (a Novel AT(1) Receptor Blocker) on Catecholamine Release in the Adrenal Medulla of Spontaneously Hypertensive Rats

The aim of this study was to determine whether fimasartan, a newly developed AT₁ receptor blocker, can affect the CA release in the isolated perfused model of the adrenal medulla of spontaneously hypertensive rats (SHRs). Fimasartan (5~50 µM) perfused into an adrenal vein for 90 min produced dose- and time-dependently inhibited the CA secretory responses evoked by ACh (5.32 mM), high K⁺ (56 mM, a direct membrane depolarizer), DMPP (100 µM) and McN-A-343 (100 µM). Fimasartan failed to affect basal CA output. Furthermore, in adrenal glands loaded with fimasartan (15 µM), the CA secretory responses evoked by Bay-K-8644 (10 µM, an activator of L-type Ca²⁺ channels), cyclopiazonic acid (10 µM, an inhibitor of cytoplasmic Ca²⁺-ATPase), and veratridine (100 µM, an activator of Na⁺ channels) as well as by angiotensin II (Ang II, 100 nM), were markedly inhibited. In simultaneous presence of fimasartan (15 µM) and L-NAME (30 µM, an inhibitor of NO synthase), the CA secretory responses evoked by ACh, high K⁺, DMPP, Ang II, Bay-K-8644, and veratridine was not affected in comparison of data obtained from treatment with fimasartan (15 µM) alone. Also there was no difference in NO release between before and after treatment with fimasartan (15 µM). Collectively, these experimental results suggest that fimasartan inhibits the CA secretion evoked by Ang II, and cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by membrane depolarization from the rat adrenal medulla. It seems that this inhibitory effect of fimasartan may be mediated by blocking the influx of both Na⁺ and Ca²⁺ through their ion channels into the rat adrenomedullary chromaffin cells as well as by inhibiting the Ca²⁺ release from the cytoplasmic calcium store, which is relevant to AT₁ receptor blockade without NO release.

Impact of AT2 receptor deficiency on postnatal cardiovascular development

BACKGROUND

The angiotensin II receptor subtype 2 (AT2 receptor) is ubiquitously and highly expressed in early postnatal life. However, its role in postnatal cardiac development remained unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Hearts from 1, 7, 14 and 56 days old wild-type (WT) and AT2 receptor-deficient (KO) mice were extracted for histomorphometrical analysis as well as analysis of cardiac signaling and gene expression. Furthermore, heart and body weights of examined animals were recorded and echocardiographic analysis of cardiac function as well as telemetric blood pressure measurements were performed. Moreover, gene expression, sarcomere shortening and calcium transients were examined in ventricular cardiomyocytes isolated from both genotypes. KO mice exhibited an accelerated body weight gain and a reduced heart to body weight ratio as compared to WT mice in the postnatal period. However, in adult KO mice the heart to body weight ratio was significantly increased most likely due to elevated systemic blood pressure. At postnatal day 7 ventricular capillarization index and the density of α-smooth muscle cell actin-positive blood vessels were higher in KO mice as compared to WT mice but normalized during adolescence. Echocardiographic assessment of cardiac systolic function at postnatal day 7 revealed decreased contractility of KO hearts in response to beta-adrenergic stimulation. Moreover, cardiomyocytes from KO mice showed a decreased sarcomere shortening and an increased peak Ca(2+) transient in response to isoprenaline when stimulated concomitantly with angiotensin II.

CONCLUSION

The AT2 receptor affects postnatal cardiac growth possibly via reducing body weight gain and systemic blood pressure. Moreover, it moderately attenuates postnatal vascularization of the heart and modulates the beta adrenergic response of the neonatal heart. These AT2 receptor-mediated effects may be implicated in the physiological maturation process of the heart.

The genetic deletion of Mas abolishes salt induced hypertension in mice

The G protein-coupled receptor Mas is a physiological antagonist of the angiotensin II type 1 receptor and is associated with angiotensin-(1-7) signaling. We investigated the effect of Mas-deficiency on blood pressure regulation under physiological conditions and salt load using radiotelemetry. Mas-knockout mice and their wild-type controls received a telemetry implant in the carotid artery. One week after surgery, animals were monitored for 3 days receiving normal diet (0.6% NaCl) followed by one-week high-salt diet (8% NaCl). Under same high-salt diet, another set of mice was placed in individual metabolic cages for 4 days. Basal mean arterial pressure, heart rate and locomotor activity displayed normal day-night rhythm in Mas-deficient mice. Mas-knockout mice were normotensive. High dietary NaCl ingestion did not alter heart rate or locomotor activity in both groups, but significantly increased night time mean arterial pressure in control mice whereas this increase was blunted in Mas-deficient mice. Baseline food and water intake and urine osmolality were not different between both genotypes. Under high-salt diet, water consumption and food intake were equally increased in wild-type controls and Mas-knockout, but urinary electrolytes and osmolality were significantly higher in Mas-knockout. Taken together, basal hemodynamic parameters are unchanged in Mas-knockout mice. In contrast to wild-type controls, telemetric mean arterial pressure measurement revealed salt resistance in Mas-deficient animals, probably due to their higher urinary NaCl excretion. This is the first direct proof that Mas blockade might be a new option in the treatment of salt-sensitive hypertension.

Sex differences in the pressor and tubuloglomerular feedback response to angiotensin II

Awareness of sex differences in the pathology of cardiovascular disease is increasing. Previously, we have shown a role for the angiotensin type 2 receptor (AT(2)R) in the sex differences in the arterial pressure response to Ang II. Tubuloglomerular feedback (TGF) contributes in setting pressure-natriuresis properties, and its responsiveness is closely coupled to renal Ang II levels. We hypothesize that, in females, the attenuated pressor response to Ang II is mediated via an enhanced AT(2)R mechanism that, in part, offsets Ang II-induced sensitization of the TGF mechanism. Mean arterial pressure was measured via telemetry in male and female wild-type (WT) and AT(2)R knockout (AT(2)R-KO) mice receiving Ang II (600 ng/kg per minute SC). Basal 24-hour mean arterial pressure did not differ among the 4 groups. After 10 days of Ang II infusion, mean arterial pressure increased in the male WT (28±6 mm Hg), male AT(2)R-KO (26±2 mm Hg), and female AT(2)R-KO (26±4 mm Hg) mice, however, the response was attenuated in female WT mice (12±4 mm Hg; P between sex and genotype=0.016). TGF characteristics were determined before and during acute subpressor Ang II infusion (100 ng/kg per minute IV). Basal TGF responses did not differ between groups. The expected increase in maximal change in stop-flow pressure and enhancement of TGF sensitivity in response to Ang II was observed in the male WT, male AT(2)R-KO, and female AT(2)R-KO but not in the female WT mice (P between sex and genotype <0.05; both). In conclusion, these data indicate that an enhanced AT(2)R-mediated pathway counterbalances the hypertensive effects of Ang II and attenuates the Ang II-dependent resetting of TGF activity in females. Thus, the enhancement of the AT(2)R may, in part, underlie the protection that premenopausal women demonstrate against cardiovascular disease.

Circadian Variations in Blood Pressure, Heart Rate, and HR-BP Cross-Correlation Coefficient during Progression of Diabetes Mellitus in Rat

Circadian changes in cardiovascular function during the progression of diabetes mellitus in the diabetes prone rat (BBDP) (n = 8) were studied. Age-matched diabetes-resistant rats (BBDR) served as controls. BP was recorded via telemetry in contiguous 4 hr time periods over 24 hours starting with 12 midnight to 4 am as period zero (P0). Prior to onset of diabetes BP was high at P0, peaked at P2, and then fell again at P3; BP and heart rate (HR) then increased gradually at P4 and leveled off at P5, thereby exhibiting a bipodal rhythm. These patterns changed during long-term diabetes. The cross-correlation coefficient of BP and HR was not significantly different across groups at onset, but it fell significantly at 9 months of duration of diabetes (BBDP: 0.39 ± 0.06; BBDR: 0.65 ± 0.03; P < .05). These results show that changes in circadian cardiovascular rhythms in diabetes mellitus became significant at the late stage of the disease.

New function for an old enzyme: NEP deficient mice develop late-onset obesity

Background

According to the World Health Organization (WHO) there is a pandemic of obesity with approximately 300 million people being obese. Typically, human obesity has a polygenetic causation. Neutral endopeptidase (NEP), also known as neprilysin, is considered to be one of the key enzymes in the metabolism of many active peptide hormones.

Methodology/Principal Findings

An incidental observation in NEP-deficient mice was a late-onset excessive gain in body weight exclusively from a ubiquitous accumulation of fat tissue. In accord with polygenetic human obesity, mice were characterized by deregulation of lipid metabolism, higher blood glucose levels, with impaired glucose tolerance. The key role of NEP in determining body mass was confirmed by the use of the NEP inhibitor candoxatril in wild-type mice that increased body weight due to increased food intake. This is a peripheral and not a central NEP action on the switch for appetite control, since candoxatril cannot cross the blood-brain barrier. Furthermore, we demonstrated that inhibition of NEP in mice with cachexia delayed rapid body weight loss. Thus, lack in NEP activity, genetically or pharmacologically, leads to a gain in body fat.

Conclusions/Significance

In the present study, we have identified NEP to be a crucial player in the development of obesity. NEP-deficient mice start to become obese under a normocaloric diet in an age of 6–7 months and thus are an ideal model for the typical human late-onset obesity. Therefore, the described obesity model is an ideal tool for research on development, molecular mechanisms, diagnosis, and therapy of the pandemic obesity.

Circadian clocks and vascular function

The circadian clock regulates many aspects of physiology, including cardiovascular function. Internal oscillators exist in endothelial, smooth muscle cells, and fibroblasts of the vasculature. Vascular tone and thrombus formation, 2 key elements of vascular function with regard to adverse cardiovascular events, exhibit diurnal rhythmicity. In this review, we describe changes in vascular function that result from genetic disruption of discrete elements of the circadian clock.

Tissue renin-angiotensin-aldosterone systems: Targets for pharmacological therapy

The renin-angiotensin-aldosterone system is one of the most important systems in cardiovascular control and in the pathogenesis of cardiovascular diseases. Therefore, it is already a very successful drug target for the therapy of these diseases. However, angiotensins are generated not only in the plasma but also locally in tissues from precursors and substrates either locally expressed or imported from the circulation. In most areas of the brain, only locally generated angiotensins can exert effects on their receptors owing to the blood-brain barrier. Other tissue renin-angiotensin-aldosterone systems are found in cardiovascular organs such as kidney, heart, and vessels and play important roles in the function of these organs and in the deleterious actions of hypertension and diabetes on these tissues. Novel components with mostly opposite actions to the classical renin-angiotensin-aldosterone systems have been described and need functional characterization to evaluate their suitability as novel drug targets.

Pressed for time: the circadian clock and hypertension

Hypertension is a major risk factor for cardiovascular disease and death. The "silent" rise of blood pressure that occurs over time is largely asymptomatic. However, its impact is deafening-causing and exacerbating cardiovascular disease, end-organ damage, and death. The present article addresses recent observations from human and animal studies that provide new insights into how the circadian clock regulates blood pressure, contributes to hypertension, and ultimately evolves vascular disease. Further, the molecular components of the circadian clock and their relationship with locomotor activity, metabolic control, fluid balance, and vascular resistance are discussed with an emphasis on how these novel, circadian clock-controlled mechanisms contribute to hypertension.

Direct blood pressure monitoring in laboratory rodents via implantable radio telemetry

The ability to monitor and record precise blood pressure fluctuations in research animals is vital to research for human hypertension. Direct measurement of blood pressure via implantable radio telemetry devices is the preferred method for automatic collection of chronic, continuous blood pressure data. Two surgical techniques are described for instrumenting the two most commonly used laboratory rodent species with radiotelemetry devices. The basic rat procedure involves advancing a blood pressure catheter into the abdominal aorta and placing a radio transmitting device in the peritoneal cavity. The mouse technique involves advancing a thin, flexible catheter from the left carotid artery into the aortic arch and placing the telemetry device under the skin along the animal's flank. Both procedures yield a chronically instrumented model to provide accurate blood pressure data from an unrestrained animal in its home cage.

AT2 receptors: functional relevance in cardiovascular disease

The renin angiotensin system (RAS) is intricately involved in normal cardiovascular homeostasis. Excessive stimulation by the octapeptide angiotensin II contributes to a range of cardiovascular pathologies and diseases via angiotensin type 1 receptor (AT1R) activation. On the other hand, tElsevier Inc.he angiotensin type 2 receptor (AT2R) is thought to counter-regulate AT1R function. In this review, we describe the enhanced expression and function of AT2R in various cardiovascular disease settings. In addition, we illustrate that the RAS consists of a family of angiotensin peptides that exert cardiovascular effects that are often distinct from those of Ang II. During cardiovascular disease, there is likely to be an increased functional importance of AT2R, stimulated by Ang II, or even shorter angiotensin peptide fragments, to limit AT1R-mediated overactivity and cardiovascular pathologies.

Naphtho[1,2-d]thiazol-2-ylamine (SKA-31), a new activator of KCa2 and KCa3.1 potassium channels, potentiates the endothelium-derived hyperpolarizing factor response and lowers blood pressure

Small-conductance (KCa2.1-2.3) and intermediate-conductance (KCa3.1) calcium-activated K(+) channels are critically involved in modulating calcium-signaling cascades and membrane potential in both excitable and nonexcitable cells. Activators of these channels constitute useful pharmacological tools and potential new drugs for the treatment of ataxia, epilepsy, and hypertension. Here, we used the neuroprotectant riluzole as a template for the design of KCa2/3 channel activators that are potent enough for in vivo studies. Of a library of 41 benzothiazoles, we identified 2 compounds, anthra[2,1-d]thiazol-2-ylamine (SKA-20) and naphtho[1,2-d]thiazol-2-ylamine (SKA-31), which are 10 to 20 times more potent than riluzole and activate KCa2.1 with EC(50) values of 430 nM and 2.9 microM, KCa2.2 with an EC(50) value of 1.9 microM, KCa2.3 with EC(50) values of 1.2 and 2.9 microM, and KCa3.1 with EC(50) values of 115 and 260 nM. Likewise, SKA-20 and SKA-31 activated native KCa2.3 and KCa3.1 channels in murine endothelial cells, and the more "drug-like" SKA-31 (half-life of 12 h) potentiated endothelium-derived hyperpolarizing factor-mediated dilations of carotid arteries from KCa3.1(+/+) mice but not from KCa3.1(-/-) mice. Administration of 10 and 30 mg/kg SKA-31 lowered mean arterial blood pressure by 4 and 6 mm Hg in normotensive mice and by 12 mm Hg in angiotensin-II-induced hypertension. These effects were absent in KCa3.1-deficient mice. In conclusion, with SKA-31, we have designed a new pharmacological tool to define the functional role of the KCa2/3 channel activation in vivo. The blood pressure-lowering effect of SKA-31 suggests KCa3.1 channel activation as a new therapeutic principle for the treatment of hypertension.

Novel role for inhibitor of differentiation 2 in the genesis of angiotensin II-induced hypertension

BACKGROUND

Angiotensin (Ang) II-induced target-organ damage involves innate and acquired immunity. Mice deficient for the helix-loop-helix transcription factor inhibitor of differentiation (Id2(-/-)) lack Langerhans and splenic CD8a+ dendritic cells, have reduced natural killer cells, and have altered CD8 T-cell memory. We tested the hypothesis that an alteration in the number and quality of circulating blood cells caused by Id2 deletion would ameliorate Ang II-induced target-organ damage.

METHODS AND RESULTS

We used gene-deleted and transgenic mice. We conducted kidney and bone marrow transplants. In contrast to Ang II-infused Id2(+/-), Id2(-/-) mice infused with Ang II remained normotensive and failed to develop albuminuria or renal damage. Bone marrow transplant of Id2(+/-) bone marrow to Id2(-/-) mice did not restore the blunted blood pressure response to Ang II. Transplantation of Id2(-/-) kidneys to Id2(+/-) mice also could not prevent Ang II-induced hypertension and renal damage. We verified the Ang II resistance in Id2(-/-) mice in a model of local tissue Ang II production by crossing hypertensive mice transgenic for rat angiotensinogen with Id2(-/-) or Id2(+/-) mice. Angiotensinogen-transgenic Id2(+/-) mice developed hypertension, albuminuria, and renal injury, whereas angiotensinogen-transgenic Id2(-/-) mice did not. We also found that vascular smooth muscle cells from Id2(-/-) mice showed an antisenescence phenotype.

CONCLUSIONS

Our bone marrow and kidney transplant experiments suggest that alterations in circulating immune cells or Id2 in the kidney are not responsible for Ang II resistance. The present studies identify a previously undefined role for Id2 in the pathogenesis of Ang II-induced hypertension.

Peripheral Circadian Clocks in the Vasculature

Living organisms have adapted to the daily rotation of the earth and regular changes in the light environment. Life forms anticipate environmental transitions, adapt their own physiology, and perform activities at behaviorally advantageous times during the day. This is achieved by means of endogenous circadian clocks that can be synchronized to the daily changes in external cues, most notably light and temperature. For many years it was thought that neurons of the suprachiasmatic nucleus (SCN) uniquely controlled circadian rhythmicity of peripheral tissues via neural and humoral signals. The cloning and characterization of mammalian clock genes revealed that they are expressed in a circadian manner throughout the body. It is now accepted that peripheral cells, including those of the cardiovascular system, contain a circadian clock similar to that in the SCN. Many aspects of cardiovascular physiology are subject to diurnal variation, and serious adverse cardiovascular events including myocardial infarction, sudden cardiac death, and stroke occur with a frequency conditioned by time of day. This has raised the possibility that biological responses under the control of the molecular clock might interact with environmental cues to influence the phenotype of human cardiovascular disease.

Renin enhancer is critical for control of renin gene expression and cardiovascular function

The important cardiovascular regulator renin contains a strong in vitro enhancer 2.7 kb upstream of its gene. Here we tested the in vivo role of the mouse Ren-1c enhancer. In renin-expressing As4.1 cells stably transfected with Ren-1c promoter with or without enhancer, expression of linked beta-geo reporter, stable expression, and colony formation were dependent on the presence of the enhancer. We then generated mice carrying a targeted deletion of the enhancer (REKO mice) and found marked depletion of renin in renal juxtaglomerular and submandibular ductal cells, as well as hyperplasia of macula densa cells. Plasma creatinine was increased, but electrolytes were normal. Male REKO mice implanted with telemetry devices had 9 +/- 1 mm Hg lower mean arterial pressure (p < 0.001), which was partly normalized by a high NaCl diet. Locomotor activity was lower, and baroreflex sensitivity was normal. Markedly reduced mean arterial pressure variability in the midfrequency band indicated a contribution of reduced sympathetic vasomotor tone to the hypotension. In conclusion, the renin enhancer is critical for renin gene expression and physiological sequelae, including response to alteration in salt intake. The REKO mouse may be useful as a low renin expression model.

Impaired endothelium-derived hyperpolarizing factor-mediated dilations and increased blood pressure in mice deficient of the intermediate-conductance Ca2+-activated K+ channel

The endothelium plays a key role in the control of vascular tone and alteration in endothelial cell function contributes to several cardiovascular disease states. Endothelium-dependent dilation is mediated by NO, prostacyclin, and an endothelium-derived hyperpolarizing factor (EDHF). EDHF signaling is thought to be initiated by activation of endothelial Ca(2+)-activated K(+) channels (K(Ca)), leading to hyperpolarization of the endothelium and subsequently to hyperpolarization and relaxation of vascular smooth muscle. In the present study, we tested the functional role of the endothelial intermediate-conductance K(Ca) (IK(Ca)/K(Ca)3.1) in endothelial hyperpolarization, in EDHF-mediated dilation, and in the control of arterial pressure by targeted deletion of K(Ca)3.1. K(Ca)3.1-deficient mice (K(Ca)3.1(-/-)) were generated by conventional gene-targeting strategies. Endothelial K(Ca) currents and EDHF-mediated dilations were characterized by patch-clamp analysis, myography and intravital microscopy. Disruption of the K(Ca)3.1 gene abolished endothelial K(Ca)3.1 currents and significantly diminished overall current through K(Ca) channels. As a consequence, endothelial and smooth muscle hyperpolarization in response to acetylcholine was reduced in K(Ca)3.1(-/-) mice. Acetylcholine-induced dilations were impaired in the carotid artery and in resistance vessels because of a substantial reduction of EDHF-mediated dilation in K(Ca)3.1(-/-) mice. Moreover, the loss of K(Ca)3.1 led to a significant increase in arterial blood pressure and to mild left ventricular hypertrophy. These results indicate that the endothelial K(Ca)3.1 is a fundamental determinant of endothelial hyperpolarization and EDHF signaling and, thereby, a crucial determinant in the control of vascular tone and overall circulatory regulation.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Salt consumption increases blood pressure and abolishes the light/dark rhythm in angiotensin AT1a receptor deficient mice

Experiments were performed to study the role of angiotensin (Ang) AT1a receptors in dietary sodium-induced changes in blood pressure (BP). We measured light/dark rhythms in BP, heart rate (HR) and drinking behavior in Ang AT1a deficient (AT1a -/-) and wild type (AT1a +/+) mice with arterial telemetric catheters. Mice were given ad libitum access to a high salt diet (8% NaCl, HSD for 8 days) and tap water. The major finding was that the Ang AT1a -/- mice showed enhanced sodium sensitivity. This was seen by a greater percentage increase in BP (+21% vs. +12%) and an earlier onset of BP change (increase on day 5 vs. day 8) in AT1a -/- vs. AT1a +/+. The normal light/dark BP rhythm was abolished in AT1a -/- after 5 days of HSD. HSD produced an increase in water intake (drinking activity and volume consumed) in both groups with no difference in the percentage increase or the light/dark drinking rhythm. HSD produced no changes in plasma osmolality, hematocrit or body weight in either group. Evidence shows that a deficiency of Ang AT1a receptors results in an enhancement in sodium sensitivity along with a disruption of the normal light/dark BP rhythm. The data combined with previous findings suggests that activation of other components of the renin angiotensin system and/or sympathetic pathways may be responsible for the cardiovascular changes in AT1a deficient mice.

Animal models of hypertension: an overview

Hypertension is a multifactorial disease involving complex interactions between genetic and environmental factors. Development of experimental models of hypertension allowed dissection and isolation of various factors associated with regulation of blood pressure, inheritance of hypertensive traits, and cellular responses to injury. The phenotype-driven approach is taking advantage of selective breeding of animals (primarily rats) that exhibit a desired phenotype, like the useful SHR. Genotype-driven models include transgenic techniques, in which mice are the most successful for selective deletion or overexpression of target genes. Notably, a combination of comparative genomics strategies and phenotypic correlates enhances the utility of hypertension models and their clinical relevance. Indeed, experimental models enabled development of targeted interventions aimed at decreasing not only blood pressure but also target organ injury. Continued utilization of experimental models simulating human hypertension, particularly those that combine other clinically relevant comorbidities like obesity or hypercholesterolemia, may afford development of effective strategies to address this common disease. Nevertheless, a cautious approach is mandatory when experimental findings in these models are extrapolated to human hypertension.

High-Salt Diet Inhibits Expression of Angiotensin Type 2 Receptor in Resistance Arteries

Recent studies suggested that type 2 angiotensin receptor (AT2R) could contribute to regulation of blood pressure and/or vascular remodeling. A key question relates to the effects of potential modulators of vascular AT2R expression. In the present work, we evaluated if high salt intake (70 mmol/L NaCl in drinking water) could modulate rat mesenteric artery AT2R function and expression. Angiotensin II dose-response curves were studied in rat perfused pressurized small-diameter arteries in the presence of losartan (AT1R antagonist). Arteries were precontracted with phenylephrine, yielding ≈30% decrease in resting diameter. AT2R activation by angiotensin-induced dose-dependent relaxation of precontracted arteries (60.1±9.1% of phenylephrine-induced contraction, P <0.05). In contrast, AT2R-dependent relaxation was not observed in arteries obtained from rats on high-salt diet. Semi-quantitative reverse-transcription polymerase chain reaction experiments demonstrated reduced amount of AT2R mRNA in arteries of rats on high-salt diet (65.5±7.5% of control levels, P <0.05). Western blot studies demonstrated decreased AT2R in mesenteric artery protein fractions of high-salt diet rats (60.0±18.0 of control levels, P <0.05). In a second set of experiments, adrenalectomy (4 days) blunted AT2R-mediated vasorelaxation and decreased AT2R mRNA (72.0±11.0% of control levels, P <0.05). AT2R abundance in protein fractions of mesenteric arteries of ADX rats was also diminished (64.0±13% of control levels, P <0.05). Both, AT2R mRNA and protein downregulation were prevented by mineralocorticoid replacement therapy. Finally, physiological concentrations of aldosterone caused a dose-dependent increase in AT2R mRNA of small diameter mesenteric artery explants. The results are consistent with aldosterone-mediated upregulation AT2R.

Links between dietary salt intake, renal salt handling, blood pressure, and cardiovascular diseases

Epidemiological, migration, intervention, and genetic studies in humans and animals provide very strong evidence of a causal link between high salt intake and high blood pressure. The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt. From an evolutionary viewpoint, the human species is adapted to ingest and excrete <1 g of salt per day, at least 10 times less than the average values currently observed in industrialized and urbanized countries. Independent of the rise in blood pressure, dietary salt also increases cardiac left ventricular mass, arterial thickness and stiffness, the incidence of strokes, and the severity of cardiac failure. Thus chronic exposure to a high-salt diet appears to be a major factor involved in the frequent occurrence of hypertension and cardiovascular diseases in human populations.

Recommendations for Blood Pressure Measurement in Humans and Experimental Animals

In experimental animals, as in humans, techniques for measuring blood pressure (BP) have improved considerably over the past decade. In this document, we present recommendations for measuring BP in experimental animals with the goal of helping investigators select optimal methods for BP monitoring in the research laboratory. The advantages and disadvantages of various BP measurement methods are discussed and specific recommendations are provided for selecting the optimal technique depending on the study objective. Although indirect techniques that permit only sporadic measurements of BP may be suitable for some purposes, methods for directly measuring BP are generally preferred because of their ability to monitor the highly dynamic nature of BP in a comprehensive fashion. Selection of the methods to be used should ultimately be guided by the study objectives to insure that the techniques chosen are appropriate for the experimental questions being explored.

Recommendations for Blood Pressure Measurement in Humans and Experimental Animals

In experimental animals, as in humans, techniques for measuring blood pressure (BP) have improved considerably over the past decade. In this document, we present recommendations for measuring BP in experimental animals with the goal of helping investigators select optimal methods for BP monitoring in the research laboratory. The advantages and disadvantages of various BP measurement methods are discussed and specific recommendations are provided for selecting the optimal technique depending on the study objective. Although indirect techniques that permit only sporadic measurements of BP may be suitable for some purposes, methods for directly measuring BP are generally preferred because of their ability to monitor the highly dynamic nature of BP in a comprehensive fashion. Selection of the methods to be used should ultimately be guided by the study objectives to insure that the techniques chosen are appropriate for the experimental questions being explored.

AT1 receptors mediate angiotensin II–induced release of nitric oxide in afferent arterioles

BACKGROUND

Recent studies have indicated that angiotensin II (Ang II) possibly activates the nitric oxide (NO) system. We investigated the role of AT receptor subtypes (AT-R) in mediating the Ang II-induced NO release in afferent arterioles (Af) of mice.

METHODS

Isolated Af of mice were perfused, and the isotonic contraction measured. Further, NO release was determined using DAF-FM, a fluorescence indicator for NO. Moreover, we qualitatively assessed the expression of AT-R at the mRNA level using reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Ang II reduced luminal diameters dose dependently (67.3 +/- 6.3% at 10(-6) mol/L). Inhibition of AT2-R with PD123.319 did not change the Ang II contractile response. AT1-R blockade with ZD7155 inhibited contraction. Stimulation of AT2-R during AT1-R inhibition with ZD7155, and preconstriction with norepinephrine (NE) had no influence on the diameter. Drug application via the perfusion pipette changed flow and pressure, and enhanced NO fluorescence by DeltaF = 4.0 +/- 0.4% (N= 14, background). Luminal application of Ang II (10(-7) mol/L) increased the NO fluorescence by DeltaF = 9.9 +/- 1.2% (N= 8). AT1-R blockade blunted the increase to background levels (DeltaF to 4.0 +/- 0.3%, N= 6, P < 0.05), but AT2-R blockade did not (8.1 +/- 0.9%, N= 9). L-NAME nearly abolished the Ang II effect on the NO fluorescence (DeltaF = 1.6 +/- 0.5% (N= 8). NE did not increase NO release beyond the background levels. RT-PCR showed expression of both AT1-R and AT2-R.

CONCLUSION

The results indicate an Ang II-induced NO release in Af of mice, which is mediated by AT1-R. Thus, Ang II balances its own constrictor action in Af. This control mechanism is very important in view of high renin and angiotensin II concentration in the juxtaglomerular apparatus.

Insights into angiotensin II receptor function through AT2 receptor knockout mice

Angiotensin II signals via at least two receptors termed AT1 and AT2. The function of the AT1 receptor is well defined, while that of the AT2 receptor is still shrouded in uncertainty. AT2 gene-deficient (-/-) mice have been helpful in unravelling the function of the AT2 receptor. We have studied AT2-/- and AT2+/+ mice with classical physiological techniques developed for the rat. We found that although AT2-/- mice have normal glomerular filtration rate, the pressure-natriuresis relationship in these mice, compared with AT2+/+ mice, is shifted rightward. Moreover, medullary blood flow fails to increase with increased perfusion pressure while the AT1 receptor expression in the kidneys is increased. We used telemetry and found that AT2-/- mice have about 10 mmHg higher blood pressures than AT2+/+ mice and that their circadian rhythm is disturbed. Moreover, their baroreflexes, as measured by spectral analyses, differs from AT2+/+ controls. The cardiac function of AT2-/- mice is remarkably preserved and the differences are subtle. However, if the mice are given l-NAME hypertension, they exhibit an end-systolic pressure-volume relationship that reveals decreased contractility and probable increased vascular stiffness. Furthermore, the hearts of AT2-/- mice hypertrophy more in response to l-NAME than those of AT2+/+ mice and perivascular fibrosis is increased. DOCA-salt treatment also shows a more rightward pressure-natriuresis relationship in AT2-/- compared with AT2+/+ mice. The renal iNOS expression is increased with DOCA-salt treatment. Our findings support the notion that the AT2 receptor signals antiproliferative and antifibrotic effects and that its presence results in lower blood pressures and lesser responses to secondary forms of hypertension. Technical advances that have allowed us to adapt methods for the rat to the much smaller mouse have facilitated our studies.

Cardiac hypertrophy and fibrosis in chronic l-NAME-treated AT2 receptor-deficient mice

BACKGROUND

The role of angiotensin II type 1 (AT1) and type 2 (AT2) receptors in cardiac hypertrophy and fibrosis is incompletely understood. The availability of AT2 receptor-deficient mice (AT2 -/y) makes it possible to study the effects of AT1 receptors without the confounding influence of AT2 receptor activity.

OBJECTIVE

To test the hypothesis that the AT2 receptor affords protection from left ventricular hypertrophy and fibrosis in chronic hypertension induced by N-nitro-L-arginine methyl ester (L-NAME).

DESIGN

Four groups of mice were studied over a period of 3 weeks: AT2 -/y mice with and without L-NAME, and AT2 +/y mice with and without L-NAME.

METHODS

Blood pressure and heart rate were monitored by telemetry in groups of AT2 +/y and AT2 -/y mice for 4 weeks. L-NAME groups received the compound in drinking water for the last 3 weeks. We determined left ventricular AT1 receptor expression, cardiac hypertrophy and fibrosis, with and without L-NAME treatment. We used a miniaturized conductance-manometer system to measure pressure-volume loops at the time when the animals were killed.

RESULTS

AT2 -/y mice treated with L-NAME showed worse left ventricular hypertrophy, more perivascular fibrosis and greater concentrations of brain natriuretic peptide than did AT2 +/y mice treated with L-NAME. The end-systolic pressure-volume relationship, an index of left ventricular contractility, was decreased in AT2 -/y mice treated with L-NAME.

CONCLUSIONS

The AT2 receptor is not essential for development of L-NAME-induced cardiac hypertrophy, fibrosis and concomitant changes in left ventricular performance. In contrast, the AT2 receptor offers a protective effect.

Evaluation and applications of radiotelemetry in small laboratory animals

Radiotelemetry is the "state of the art" for monitoring physiological functions in awake and freely moving laboratory animals, while minimizing stress artifacts. For researchers, especially those in the fields of pharmacology and toxicology, the technique provides a valuable tool for defining the physiological and pathophysiological consequences derived from advances molecular, cellular, and tissue biology and in predicting the effectiveness and safety of new compounds in humans. There is ample evidence that radiotelemetry systems for measuring physiological functions has been sufficiently validated. Today, the technology is an important tool for collection of a growing number of physiological parameters, for contributing to animal welfare (reduction and refinement alternatives), and for reducing overall animal research costs.