A novel effect of angiotensin on renal sympathetic nerve activity in mice

Effect of Angiotensin receptor blockade on Plasma Osmolality and Neurohumoral Responses to High Environmental Temperature in Rats Fed a High Salt Diet

Plasma osmolality (pOsmol) and neurohumoral signals play important roles in the pathophysiology of cardiovascular diseases. Our study investigated the effect of high environmental temperature (HET) on neurohumoral responses and pOsmol in rats fed a high salt diet (HSD), with and without angiotensin II receptor blockade (ARB), using telmisartan.  Fifty-six male 8-week old Sprague-Dawley rats (95-110g) were randomly assigned into seven groups of 8 rats. These included control rats (I) fed with 0.3% NaCl diet (normal diet, ND); salt-loaded rats (II) fed with 8% NaCl (high salt) diet; ND rats (III) exposed to HET (38.5±0.5oC ) 4 hours daily per week; rats (IV) fed with 8% NaCl diet and exposed to HET daily. Others included rats (V) fed with 8% NaCl diet and treated with telmisartan (30mg/kg); ND rats (VI) exposed to HET and treated with telmisartan; rats (VI) fed with 8% NaCl diet, exposed to HET and treated with telmisartan. Plasma angiotensin II, aldosterone, vasopressin and norepinephrine (NE) concentrations were determined by ELISA technique; pOsmol from plasma K+, Na+ and Urea. HSD combined with HET in rats synergistically increased pOsmol (P<0.001) with an associated non-synergistic rise in fluid intake (P<0.001), fluid balance (P<0.001), plasma angiotensin II (P<0.01) and aldosterone (P<0.05), NE (P<0.001) and vasopressin (P<0.05) concentrations compared to control. Telmisartan did not alter pOsmol in all the treated-rats, but normalized fluid intake levels and plasma vasopressin in the rats exposed to either HSD or HEt alone. Prolonged exposure of rats to hot environment exacerbated the effect of excess dietary salt on pOsmol, with no effect on angiotensin II-mediated neurohumoral responses.

A Focused Review of Neural Recording and Stimulation Techniques With Immune-Modulatory Targets

The complex interactions established between the nervous and immune systems have been investigated for a long time. With the advent of small and portable devices to record and stimulate nerve activity, researchers from many fields began to be interested in how nervous activity can elicit immune responses and whether this activity can be manipulated to trigger specific immune responses. Pioneering works demonstrated the existence of a cholinergic inflammatory reflex, capable of controlling the systemic inflammatory response through a vagus nerve-mediated modulation of the spleen. This work inspired many different areas of technological and conceptual advancement, which are here reviewed to provide a concise reference for the main works expanding the knowledge on vagus nerve immune-modulatory capabilities. In these works the enabling technologies of peripheral nervous activity recordings were implemented and embody the current efforts aimed at controlling neural activity with modulating functions in immune response, both in experimental and clinical contexts.

Angiotensin receptor blockade with Losartan attenuates pressor response to handgrip contraction and enhances natriuresis in salt loaded hypertensive subjects: a quasi-experimental study among Nigerian adults

INTRODUCTION

Sympathetic and Renin-Angiotensin-Aldosterone systems play crucial roles in blood pressure response to increased salt intake. This study investigated the effects of angiotensin receptor blocker (ARB) and sympathetic excitation on the responses of blood pressure (BP) and peripheral vascular resistance (PVR) in salt loaded normotensive (NT) and hypertensive (HT) Nigerian subjects.

METHODS

16 NT and 14 HT participants, that were age-matched [39.9 ± 1.3 vs 44.1±2.1yrs (P= 0.10)], underwent 5 days each of oral administration of 200mmol NaCl, and 200mmol NaCl + 50mg Losartan, preceded by a baseline control condition. BP and PVR responses to 30% Maximum Voluntary Contraction (MVC) of handgrip (HG) for one minute were determined at baseline, after salt load and after salt + Losartan. Data were presented as Mean ± SEM, and analyzed with two-way ANOVA and paired t-test, with P<0.05 accepted as significant.

RESULTS

BP and PVR were significantly increased by HG at baseline, after salt load and after salt + Losartan in NT and HT. Salt load augmented the HG-induced SBP (P=0.04) and MABP responses (P=0.02) in HT. While Losartan attenuated the HG- induced Systolic Blood Pressure (SBP) SBP response (P=0.007) and DBP response (P=0.003) in HT and NT respectively after salt + Losartan. HG-induced PVR response was significantly accentuated after salt load in HT (P=0.005), but it was not significant in NT (P=0.38).

CONCLUSION

The implication of our finding is that angiotensin II receptor blockade possibly attenuates salt-induced sympathetic nerve excitation in black hypertensive patients.

An in vitro experimental model for analysis of central control of sympathetic nerve activity

Newborn rat brainstem-spinal cord preparations are useful for in vitro analysis of various brainstem functions including respiratory activity. When studying the central control of sympathetic nerve activity (SNA), it is important to record peripheral outputs of the SNA. We developed an in vitro preparation in which neuronal connections between the cardiovascular center in the medulla and SNA peripheral outputs are preserved. Zero- to 1-day-old rats were deeply anesthetized with isoflurane, and the brainstem and spinal cord were isolated with a partial right thoracic cage to record sympathetic nerve discharge from the right thoracic sympathetic nerve trunk (T9-T11). SNA in this preparation was strongly modulated by inspiratory activity. Single-shot electrical stimulation of the ipsilateral rostral ventrolateral medulla (RVLM) induced a transient increase of SNA. Bath application of angiotensin II induced an increase of SNA, and local ipsilateral microinjection of angiotensin II to the RVLM induced a transient increase of SNA. This preparation allows analysis of the central control of the SNA in vitro.

Characteristics of renal sympathetic nerve single units in rabbits with angiotensin-induced hypertension

We examined the effect of chronic angiotensin (Ang II)-induced hypertension on activity of postganglionic renal sympathetic units to determine whether altered whole renal nerve activity is due to recruitment or changes in firing frequency. Rabbits were treated with a low (20 ng kg(-1) min(-1), 8 weeks) or high dose (50 ng kg(-1) min(-1), 4 weeks) of Ang II before the experiment under chloralose-urethane anaesthesia. Spontaneously active units were detected from multiunit recordings using an algorithm that separated units by action potential shape using templates that matched spikes within a prescribed standard deviation. Multiunit sympathetic nerve activity was 40% higher in rabbits treated with low-dose Ang II than in sham (P = 0.012) but not different in high-dose Ang II. Resting firing frequency was similar in sham rabbits (1.00 ± 0.09 spikes s(-1), n = 144) and in those treated with high-dose Ang II (1.10 ± 0.08 spikes s(-1), n = 112) but was lower with low-dose Ang II (0.65 ± 0.08 spikes s(-1), n = 149, P < 0.05). Unit firing rhythmicity was linked to the cardiac cycle and was similar in sham and low-dose Ang II groups but 29-32% lower in rabbits treated with high-dose Ang II (P < 0.001). Cardiac linkage followed a similar pattern during hypoxia. All units showed baroreceptor dependency. Baroreflex gain and range were reduced and curves shifted to the right in Ang II groups. Firing frequency during hypoxia increased by +39% in low-dose Ang II and +82% in shams, but the greatest increase was in the high-dose Ang II group (+103%, P(dose) = 0.001). Responses to hypercapnia were similar in all groups. Increases in sympathetic outflow in hypertension caused by low-dose chronic Ang II administration are due to recruitment of neurons, but high-dose Ang II increases firing frequency in response to chemoreceptor stimuli independently of the arterial baroreceptors.

Dual agonist occupancy of AT1-R-α2C-AR heterodimers results in atypical Gs-PKA signaling

Hypersecretion of norepinephrine (NE) and angiotensin II (AngII) is a hallmark of major prevalent cardiovascular diseases that contribute to cardiac pathophysiology and morbidity. Herein, we explore whether heterodimerization of presynaptic AngII AT1 receptor (AT1-R) and NE α2C-adrenergic receptor (α2C-AR) could underlie their functional cross-talk to control NE secretion. Multiple bioluminescence resonance energy transfer and protein complementation assays allowed us to accurately probe the structures and functions of the α2C-AR-AT1-R dimer promoted by ligand binding to individual protomers. We found that dual agonist occupancy resulted in a conformation of the heterodimer different from that induced by active individual protomers and triggered atypical Gs-cAMP-PKA signaling. This specific pharmacological signaling unit was identified in vivo to promote not only NE hypersecretion in sympathetic neurons but also sympathetic hyperactivity in mice. Thus, we uncovered a new process by which GPCR heterodimerization creates an original functional pharmacological entity and that could constitute a promising new target in cardiovascular therapeutics.

Brain-selective overexpression of angiotensin-converting enzyme 2 attenuates sympathetic nerve activity and enhances baroreflex function in chronic heart failure

Angiotensin-converting enzyme 2 (ACE2) has been suggested to be involved in the central regulation of autonomic function. During chronic heart failure (CHF), elevated central angiotensin II signaling contributes to the sustained increase of sympathetic outflow. This is accompanied by a downregulation of ACE2 in the brain. We hypothesized that central overexpression of ACE2 decreases sympathetic outflow and enhances baroreflex function in CHF. Transgenic mice overexpressing human ACE2 selectively in the brain (SYN-hACE2 [SA]) and wild-type littermates (WT) were used. CHF was induced by permanent coronary artery ligation. Four weeks after coronary artery ligation, both WT and SA mice exhibited a significant decrease in left ventricular ejection fraction (<40%). A slight decrease in mean arterial pressure was found only in SA mice. Compared with WT mice with CHF, brain-selective ACE2 overexpression attenuated left ventricular end-diastolic pressure; decreased urinary norepinephrine excretion; baseline renal sympathetic nerve activity (WT CHF: 71.6±7.6% max versus SA CHF: 49.3±6.1% max); and enhanced baroreflex sensitivity (maximum slope: WT sham: 1.61±0.16%/mm Hg versus SA CHF: 1.51±0.17%/mm Hg). Chronic subcutaneous blockade of mas receptor increased renal sympathetic nerve activity in SA mice with CHF (A779: 67.3±5.8% versus vehicle: 46.4±3.6% of max). An upregulation in angiotensin II type 1 receptor expression was detected in medullary nuclei in WT CHF mice, which was significantly attenuated in SA mice with CHF. These data suggest that central ACE2 overexpression exerts a potential protective effect in CHF through attenuating sympathetic outflow. The mechanism for this effect involves angiotensin (1-7) mas signaling, as well as a decrease in angiotensin II type 1 receptor signaling in the medulla.

Sympathetic and angiotensinergic responses mediated by paradoxical sleep loss in rats

Introduction: Recent investigations over the past decade have linked the development of hypertension to sleep loss, although the mechanisms underlying this association are still under scrutiny. To determine the relationship between sleep deprivation and cardiovascular dysfunction, we examined the effects of paradoxical sleep deprivation on heart rate, blood pressure, sympathetic nerve activity (SNA) and their consequences in the blood renin—angiotensin system.

Materials and methods: Wistar-Hannover male rats were randomly assigned to three experimental groups: 1) control, 2) paradoxical sleep deprivation for 24 h and 3) paradoxical sleep deprivation for 96 h. Blood pressure and heart rate were recorded in awake, freely moving rats.

Results: Heart rate was higher in the 96 h paradoxical sleep deprivation group compared with the control group. Renal SNA was increased in all deprived groups. However, no significant statistical differences were observed in blood pressure or splanchnic SNA among groups. Paradoxical sleep deprivation (24 and 96 h) reduced plasma angiotensin II (Ang II) concentrations.

Conclusions: The results suggest that selective sleep deprivation produces an increase in SNA, preferentially in the kidney. Thus, alterations in the sympathetic system in response to sleep loss may be an important pathway through which hypertension develops.

Hypertension in obstructive sleep apnea: risk and therapy

Obstructive sleep apnea (OSA) is a common form of sleep-disordered breathing that occurs due to recurrent collapse of the upper airway with inspiration. Large epidemiologic studies have established that OSA is a risk factor for developing hypertension. The pathophysiologic mechanism of this relationship is due to the distinctive pattern of intermittent hypoxia seen in OSA. This pattern increases sympathetic tone, oxidative stress, inflammation and endothelial dysfunction. These processes can all lead to persistent elevation of blood pressure beyond the obstructive events. OSA should be considered as part of the workup of patients with hypertension. Treatment of OSA with continuous positive airway pressure has an effect on hypertension control and risk reduction of cardiovascular diseases. This review discusses the pathophysiology and causal relationship between OSA and hypertension, along with the cardiovascular effects of treatment of OSA.

Obstructive sleep apnea: the new cardiovascular disease. Part I: Obstructive sleep apnea and the pathogenesis of vascular disease

Obstructive sleep apnea (OSA) is increasingly recognized as a novel cardiovascular risk factor. OSA is implicated in the pathogenesis of hypertension, left ventricular dysfunction, coronary artery disease and stroke. OSA exerts its negative cardiovascular consequences through its unique pattern of intermittent hypoxia. Endothelial dysfunction, oxidative stress, and inflammation are all consequences of OSA directly linked to intermittent hypoxia and critical pathways in the pathogenesis of cardiovascular disease in patients with OSA. This review will discuss the known mechanisms of vascular dysfunction in patients with OSA and their implications for cardiovascular disease.

Regulator of G protein signalling 2 ameliorates angiotensin II-induced hypertension in mice

Angiotensin II (Ang II) activates signalling pathways predominantly through the G-protein-coupled Ang II type 1 receptor (AT(1)R). The regulator of G protein signalling 2 (RGS2) is a negative G protein regulator. We hypothesized that RGS2 deletion changes blood pressure regulation by increasing the response to Ang II. To address this issue, we infused Ang II (0.5 mg kg(-1) day(-1)) chronically into conscious RGS2-deleted (RGS2(-/-)) and wild-type (RGS2(+/+)) mice, measured mean arterial blood pressure and heart rate (HR) with telemetry and assessed vasoreactivity and gene expression of AT(1A), AT(1B) and AT(2) receptors. Angiotensin II infusion increased blood pressure more in RGS2(-/-) than in RGS2(+/+) mice, while HR was not different between the groups, indicating a resetting of the baroreceptor reflex. Urinary catecholamine excretion was similar in Ang II-infused RGS2(-/-) and RGS2(+/+) mice, indicating a minor role of sympathetic tone for blood pressure differences. Myogenic tone and vasoreactivity in response to Ang II, endothelin-1 and phenylephrine were increased in isolated renal interlobar arterioles of RGS2(-/-) mice compared with RGS2(+/+) mice. The AT(1A), AT(1B) and AT(2) receptor gene expression was not different between RGS2(-/-) and RGS2(+/+) mice. Our findings suggest that RGS2 deletion promotes Ang II-dependent hypertension primarily through an increase of myogenic tone and vasoreactivity, probably by sensitization of AT(1) receptors.

Wavelet methods for spike detection in mouse renal sympathetic nerve activity

Abnormal autonomic nerve traffic has been associated with a number of peripheral neuropathies and cardiovascular disorders prompting the development of genetically altered mice to study the genetic and molecular components of these diseases. Autonomic function in mice can be assessed by directly recording sympathetic nerve activity. However, murine sympathetic spikes are typically detected using a manually adjusted voltage threshold and no unsupervised detection methods have been developed for the mouse. Therefore, we tested the performance of several unsupervised spike detection algorithms on simulated murine renal sympathetic nerve recordings, including an automated amplitude discriminator and wavelet-based detection methods which used both the discrete wavelet transform (DWT) and the stationary wavelet transform (SWT) and several wavelet threshold rules. The parameters of the wavelet methods were optimized by comparing basal sympathetic activity to postmortem recordings and recordings made during pharmacological suppression and enhancement of sympathetic activity. In general, SWT methods were found to outperform amplitude discriminators and DWT methods with similar wavelet coefficient thresholding algorithms when presented with simulations with varied mean spike rates and signal-to-noise ratios. A SWT method which estimates the noise level using a "noise-only" wavelet scale and then selectively thresholds scales containing the physiologically important signal information was found to have the most robust spike detection. The proposed noise-level estimation method was also successfully validated during pharmacological interventions.

Dual mechanisms of angiotensin-induced activation of mouse sympathetic neurones

Ang II directly activates neurones in sympathetic ganglia. Our goal was to define the electrophysiological basis of this activation. Neurones from mouse aortic-renal and coeliac ganglia were identified as either 'tonic' or 'phasic'. With injections of depolarizing currents, action potentials (APs) were abundant and sustained in tonic neurones (TNs) and scarce or absent in phasic neurones (PNs). Resting membrane potentials were equivalent in TNs (-48 +/- 2 mV, n = 18) and PNs (-48 +/- 1 mV, n = 23) while membrane resistance was significantly higher in TNs. Ang II depolarized and increased membrane resistance equally in both TNs (n = 8) and PNs (n = 8) but it induced APs only in TNs, and enhanced current-evoked APs much more markedly in TNs (P < 0.05). The AT1 receptor antagonist losartan (2 microm, n = 6) abolished all responses to Ang II, whereas the AT2 receptor blocker PD123,319 had no effect. The transient K+ current (IA), which was more than twice as large in TNs as in PNs, was significantly inhibited by Ang II in TNs only whereas the delayed sustained K+ current (IK), which was comparable in both TNs and PNs, was not inhibited. M currents were more prominent in PNs and were inhibited by Ang II. The IA channel blocker 4-aminopyridine triggered AP generation in TNs and prevented the Ang II-induced APs but not the depolarization. Blockade of M currents by oxotremorine M or linopirdine prevented the depolarizing action of Ang II. The protein kinase C (PKC) inhibitor H7 (10 microm, n = 9) also prevented the Ang II-induced inhibition of IA and the generation APs but not the depolarization nor the inhibition of M currents. Conversely, the PKC agonist phorbol 12-myristate 13-acetate mimicked the Ang II effects by triggering APs. The results indicate that Ang II may increase AP generation in sympathetic neurones by inducing a PKC-dependent inhibition of IA currents, and a PKC-independent depolarization through inhibition of M currents. The differential expression of various K+ channels and their sensitivity to phosphorylation by PKC may determine the degree of activation of sympathetic neurones and hence may influence the severity of the hypertensive response.

Cardiovascular autonomic control in mice lacking angiotensin AT1a receptors

Studies examined the role of angiotensin (ANG) AT1a receptors in cardiovascular autonomic control by measuring arterial pressure (AP) and heart rate (HR) variability and the effect of autonomic blockade in mice lacking AT1a receptors (AT1a -/-). Using radiotelemetry in conscious AT1a +/+ and AT1a -/- mice, we determined 1) AP and pulse interval (PI) variability in time and frequency (spectral analysis) domains, 2) AP response to alpha(1)-adrenergic and ganglionic blockade, and 3) intrinsic HR after ganglionic blockade. Pulsatile AP was recorded (5 kHz) for measurement of AP and PI and respective variability. Steady-state AP responses to prazosin (1 microg/g ip) and hexamethonium (30 microg/g ip) were also measured. AP was lower in AT1a -/- vs. AT1a +/+, whereas HR was not changed. Prazosin and hexamethonium produced greater decreases in mean AP in AT1a -/- than in AT1a +/+. The blood pressure difference was marked after ganglionic blockade (change in mean AP of -44 +/- 10 vs. -18 +/- 2 mmHg, AT1a -/- vs. AT1a +/+ mice). Intrinsic HR was also lower in AT1a -/- mice (431 +/- 32 vs. 524 +/- 22 beats/min, AT1a -/- vs. AT1a +/+). Beat-by-beat series of systolic AP and PI were submitted to autoregressive spectral estimation with variability quantified in low-frequency (LF: 0.1-1 Hz) and high-frequency (HF: 1-5 Hz) ranges. AT1a -/- mice showed a reduction in systolic AP LF variability (4.3 +/- 0.8 vs. 9.8 +/- 1.3 mmHg(2)), with no change in HF (2.7 +/- 0.3 vs. 3.3 +/- 0.6 mmHg(2)). There was a reduction in PI variability of AT1a -/- in both LF (18.7 +/- 3.7 vs. 32.1 +/- 4.2 ms(2)) and HF (17.7 +/- 1.9 vs. 40.3 +/- 7.3 ms(2)) ranges. The association of lower AP and PI variability in AT1a -/- mice with enhanced AP response to alpha(1)-adrenergic and ganglionic blockade suggests that removal of the ANG AT1a receptor produces autonomic imbalance. This is seen as enhanced sympathetic drive to compensate for the lack of ANG signaling.

Predominant postglomerular vascular resistance response to reflex renal sympathetic nerve activation during ANG II clamp in rabbits

We have shown previously that a moderate reflex increase in renal sympathetic nerve activity (RSNA) elevated glomerular capillary pressure, whereas a more severe increase in RSNA decreased glomerular capillary pressure. This suggested that the nerves innervating the glomerular afferent and efferent arterioles could be selectively activated, allowing differential control of glomerular capillary pressure. A caveat to this conclusion was that intrarenal actions of neurally stimulated ANG II might have contributed to the increase in postglomerular resistance. This has now been investigated. Anesthetized rabbits were prepared for renal micropuncture and RSNA recording. One group (ANG II clamp) received an infusion of an angiotensin-converting enzyme inhibitor (enalaprilat, 2 mg/kg bolus plus 2 mg·kg−1·h−1) plus ANG II (∼20 ng·kg−1·min−1), the other vehicle. Measurements were made before (room air) and during 14% O2. Renal blood flow decreased less during ANG II clamp compared with vehicle [9 ± 1% vs. 20 ± 4%, interaction term (PGT) < 0.05], despite a similar increase in RSNA in response to 14% O2in the two groups. Arterial pressure and glomerular filtration rate were unaffected by 14% O2in both groups. Glomerular capillary pressure increased from 33 ± 1 to 37 ± 1 mmHg during ANG II clamp and from 33 ± 2 to 35 ± 1 mmHg in the vehicle group before and during 14% O2, respectively (PGT< 0.05). During ANG II clamp, postglomerular vascular resistance was still increased in response to RSNA during 14% O2, demonstrating that the action of the renal nerves on the postglomerular vasculature was independent of the renin-angiotensin system. This further supports our hypothesis that increases in RSNA can selectively control pre- and postglomerular vascular resistance and therefore glomerular ultrafiltration.

A pharmacological differentiation between postjunctional (AT1A) and prejunctional (AT1B) angiotensin II receptors in the rabbit aorta

The effects of angiotensin II and angiotensin III were compared at prejunctional and postjunctional AT(1) receptors of the rabbit thoracic aorta. Furthermore, the influence of PD123319, losartan and eprosartan on these effects was also compared. To study prejunctional effects, the tissues were preincubated with ((3)H)-noradrenaline, superfused and electrically stimulated (1 Hz, 2 ms, 50 mA, 5 min). To study postjunctional effects, non-cumulative concentration-response curves were determined. Both angiotensin II and angiotensin III were more potent prejunctionally than postjunctionally. In the case of angiotensin II, the EC(50) was 12 times lower at the prejunctional than at the postjunctional level, while that of angiotensin III was 30 times lower prejunctionally. Furthermore, whereas angiotensin II was about 33 times more potent than angiotensin III postjunctionally, it was only 12 times more potent than angiotensin III prejunctionally. Eprosartan did not differentiate between prejunctional and postjunctional effects of both angiotensins. In contrast, PD123319 and losartan did differentiate; however, whereas PD123319 concentration-dependently antagonised the facilitation of tritium release caused by angiotensin II and angiotensin III and had no influence on the contraction of the aortic rings elicited by the peptides, losartan did the opposite: it concentration-dependently antagonised the contractions caused by the peptides on the aortic rings and exerted no influence on the facilitatory effect of angiotensin II and angiotensin III. These results show that prejunctional and postjunctional receptors for angiotensin II and angiotensin III are different and underline the hypothesis that postjunctional AT(1) receptors belong to the AT(1A) subtype, while prejunctional AT(1) receptors belong to the AT(1B) subtype.

Reduction of Vascular Noradrenaline Sensitivity by AT 1 Antagonists Depends on Functional Sympathetic Innervation

Blockade of angiotensin II type-1 (AT 1 ) receptors has been shown to reduce the magnitude of the blood pressure response to noradrenaline in pithed rats via an unidentified mechanism. Dose-response curves were established for the noradrenaline-induced (10 −12 to 10 −7 mol/kg) increase of diastolic blood pressure in pithed rats treated with tubocurarine, propranolol, and atropine. Candesartan (1 mg/kg) increased the ED 50 of the noradrenaline response (1.3±0.1 nmol/kg) up to 20-fold. Vasopressor responsiveness to noradrenaline was attenuated specifically, whereas the vasopressin-induced increase in diastolic blood pressure was maintained. Specific involvement of AT 1 receptors was confirmed by equivalent actions of losartan. Blockade of norepinephrine transporter or α 2 -adrenoceptors using desipramine or rauwolscine reduced the losartan-induced shifts in the ED 50 values of noradrenaline by 63% and 21%, respectively. Combined blockade of norepinephrine transporter and α 2 -adrenoceptors eliminated the influence of losartan on noradrenaline sensitivity ( ED 50 5.5±1.3 versus 5.6±1.2 nmol/kg), a result also observed after sympathetic denervation by reserpine ( ED 50 7.1±0.8 versus 7.8±0.8 nmol/kg). Our experiments show that the reduction of vascular noradrenaline sensitivity by AT 1 blockade is dependent on the intact functioning of both neuronal noradrenaline uptake via norepinephrine transporter and presynaptic α 2 -mediated autoinhibition, exclusively provided by the sympathetic innervation. These newly identified mechanisms may contribute to the antihypertensive and protective actions of AT 1 blockers.

NO and endogenous angiotensin II interact in the generation of renal sympathetic nerve activity in conscious rats

Nitric oxide (NO) appears to inhibit sympathetic tone in anesthetized rats. However, whether NO tonically inhibits sympathetic outflow, or whether endogenous angiotensin II (ANG II) promotes NO-mediated sympathoinhibition in conscious rats is unknown. To address these questions, we determined the effects of NO synthase (NOS) inhibition on renal sympathetic nerve activity (RSNA) and heart rate (HR) in conscious, unrestrained rats on normal (NS), high-(HS), and low-sodium (LS) diets, in the presence and absence of an ANG II receptor antagonist (AIIRA). When arterial pressure was kept at baseline with intravenous hydralazine, NOS inhibition with l-NAME (10 mg/kg iv) resulted in a profound decline in RSNA, to 42 ± 11% of control ( P < 0.01), in NS animals. This effect was not sustained, and RSNA returned to control levels by 45 min postinfusion. l-NAME also caused bradycardia, from 432 ± 23 to 372 ± 11 beats/min postinfusion ( P < 0.01), an effect, which, in contrast, was sustained 60 min postdrug. The effects of NOS inhibition on RSNA and HR did not differ between NS, HS, and LS rats. However, when LS and HS rats were pretreated with AIIRA, the initial decrease in RSNA after l-NAME infusion was absent in the LS rats, while the response in the HS group was unchanged by AIIRA. These findings indicate that, in contrast to our hypotheses, NOS activity provides a stimulatory input to RSNA in conscious rats, and that in LS animals, but not HS animals, this sympathoexcitatory effect of NO is dependent on the action of endogenous ANG II.

Ganglionic Action of Angiotensin Contributes to Sympathetic Activity in Renin-Angiotensinogen Transgenic Mice

In addition to central nervous system actions, angiotensin (Ang) II may increase sympathetic nerve activity (SNA) via a direct action on sympathetic ganglia. We hypothesized that sympathetic ganglionic actions of endogenous Ang II contribute to SNA in transgenic mice that overexpress renin and angiotensinogen (R + A + mice). Renal SNA and arterial pressure were recorded in anesthetized R + A + and littermate control mice before and after ganglionic blockade, and after additional blockade of angiotensin type 1 (AT 1 ) receptors with losartan. Ganglionic blockade essentially abolished SNA in control mice, but only reduced SNA to 47±18% of baseline in R + A + mice. The residual SNA remaining after ganglionic blockade in R + A + mice was reduced from 47±18% to 8±6% of baseline by losartan ( P <0.05). The sympathoinhibitory response to losartan was accompanied by an enhanced decrease in arterial pressure in R + A + mice compared with that observed in control mice. AT 1 receptor expression in sympathetic ganglia, as measured by real-time reverse transcription–polymerase chain reaction, was increased ≈3-fold in R + A + versus control mice. The results demonstrate that, as anticipated, essentially all of the renal postganglionic SNA in control mice is driven by preganglionic input. The major new finding is that Ang II–evoked ganglionic activity accounts for ≈40% of total SNA in R + A + mice. The significant contribution of the direct ganglionic action of Ang II in R + A + mice likely reflects both increased levels of Ang II and upregulation of AT 1 receptors in sympathetic ganglia.

Different AT1 Receptor Subtypes at Pre- and Postjunctional Sites: AT1A versus AT1B Receptors

Angiotensin (AT) II is known to enhance responses to electrical field stimulation (EFS) via AT1 receptors located on sympathetic nerve terminals. Differences in potency exist between AT1 receptor antagonists regarding the inhibition of the prejunctional and postjunctional AT1 receptors. It is hypothesized that prejunctional AT1 receptors might belong to the AT1B receptor subtype. Accordingly, the authors investigated whether AT1B receptor inhibition by high concentrations of PD123319 could suppress ATII-augmented noradrenergic transmission (prejunctional) in the rabbit thoracic aorta by means of a noradrenaline spillover model. Additionally, the influence of PD123319 on ATII-enhanced constrictor responses to electrical field stimulation was investigated in the isolated rabbit mesenteric artery. Furthermore, the authors investigated whether PD123319 could influence the constrictor responses (postjunctional) to ATII in both preparations. In the thoracic aorta, ATII (10 nM) caused a significant enhancement of EFS-evoked [3H]-noradrenaline release by a factor of 2.0 +/- 0.1. This reinforcement could be inhibited by PD123319 (0.1, 1, and 10 microM). The constrictor response to ATII was unaffected by PD123319. In the mesenteric artery, ATII (0.5 nM) caused a significant enhancement of constrictor responses to EFS by factors of 2.9 +/- 0.3, 2.3 +/- 0.3, and 1.6 +/- 0.1 at 1, 2, and 4 Hz, respectively. This enhancement could be attenuated by PD123319 (1 and 10 microM). The constrictor response to ATII was unaffected by PD123319. It is concluded that the prejunctional AT1 receptors belong to the AT1B subtype whereas postjunctional AT1 receptors do not.

Neurocardiovascular regulation in mice: Experimental approaches and novel findings

1. Neural mechanisms are of major importance in the regulation of arterial blood pressure, blood volume and other aspects of cardiovascular function. The recent explosion in gene discovery and advances in molecular technologies now provide the opportunity to define the molecular and cellular mechanisms essential to integrative neurocardiovascular regulation. The unique susceptibility of mice to genetic manipulation makes this species an attractive model for such investigation. 2. We provide here a brief overview of: (i) experimental approaches used to assess autonomic and reflex control of the circulation in mice; (ii) novel mechanisms of neurocardiovascular regulation revealed using these approaches; and (iii) findings from recent studies involving mouse models of cardiovascular disease.

AT1-receptor blockade and sympathetic neurotransmission in cardiovascular disease

1. The present survey is dealing with the interactions between the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS) in various organs and tissues, with an emphasis on the angiotensin AT-receptors located at the sympathetic nerve endings. 2. Angiotensin II, the main effector of the RAAS is known to stimulate sympathetic nerve traffic and its sequelae in numerous organs and tissues, such as the central nervous system, the adrenal medulla, the sympathetic ganglia and the sympathetic nerve endings. These stimulatory effects are mediated by AT(1)-receptors and counteracted by AT(1)-receptor antagonists. 3. Sympatho-inhibition at the level of the sympathetic nerve ending appears to be a class effect of the AT(1)-receptor blockers, mediated by presynaptic AT(1)-receptors. With respect to the ratio pre-/postsynaptic AT(1)-receptor antagonism important quantitative differences between the various compounds were found. 4. Both the pre- and postjunctional receptors at the sympathetic nerve endings belong to the AT(1)-receptor population. However, the presynaptic receptors belong to the AT(1B)-subtype, whereas the postjunctional receptors probably belong to a different AT(1)-receptor subpopulation. 5. Sympatho-inhibition is a class effect of the AT(1)-receptor antagonists. In conditions in which the SNS plays a pathophysiological role, such as hypertension and congestive heart failure, this property may well be of therapeutic relevance.

Pre- and postsynaptic inhibitory potencies of the angiotensin AT1 receptor antagonists eprosartan and candesartan

The aim of the present study was to determine the inhibitory potency of two selective angiotensin AT(1) receptor antagonists, eprosartan and candesartan, at the level of the sympathetic nerve terminal and the vascular smooth muscle. Male New Zealand White rabbits, weighing 2100-2550 g, were used. To study eprosartan and candesartan at the neuronal angiotensin AT(1) receptor, we investigated their influence on the angiotensin II-enhanced, electrical field stimulation-evoked sympathetic transmission in the rabbit isolated thoracic aorta in a noradrenaline spillover model. To study both antagonists at the vascular angiotensin AT(1) receptor, concentration-response curves for angiotensin II were constructed in the presence or absence of the two angiotensin AT(1) receptor antagonists. Angiotensin II (10 nM) caused a significant increase by 107+/-11.1% of the stimulation-evoked sympathetic outflow, which was concentration-dependently inhibited by both eprosartan (pIC(50) 7.91+/-0.12) and candesartan (pIC(50) 10.76+/-0.13). Angiotensin II (1 nM-0.3 microM) caused a concentration-dependent increase in contractile force (E(max) 20.62+/-2.24 mN, pD(2) 8.16+/-0.04). Both eprosartan (pA(2) 8.90+/-0.11, pIC(50) 8.87+/-0.12 (10 nM angiotensin II)) and candesartan (pD(2)' 10.80+/-0.13) counteracted the contractions evoked by cumulative concentrations of angiotensin II. Candesartan proved a more potent antagonist than eprosartan at both the pre- and postjunctional angiotensin AT(1) receptor. For eprosartan, vascular inhibitory concentrations were 10-fold lower than sympatho-inhibitory concentrations, whereas for candesartan, inhibitory concentrations at both sites were similar. The results may be explained by differences between the pre- and postjunctional angiotensin AT(1) receptor subtype.

Hypertension and prolonged vasoconstrictor signaling in RGS2-deficient mice

Signaling by hormones and neurotransmitters that activate G protein-coupled receptors (GPCRs) maintains blood pressure within the normal range despite large changes in cardiac output that can occur within seconds. This implies that blood pressure regulation requires precise kinetic control of GPCR signaling. To test this hypothesis, we analyzed mice deficient in RGS2, a GTPase-activating protein that greatly accelerates the deactivation rate of heterotrimeric G proteins in vitro. Both rgs2+/- and rgs2-/- mice exhibited a strong hypertensive phenotype, renovascular abnormalities, persistent constriction of the resistance vasculature, and prolonged response of the vasculature to vasoconstrictors in vivo. Analysis of P2Y receptor-mediated Ca2+ signaling in vascular smooth muscle cells in vitro indicated that loss of RGS2 increased agonist potency and efficacy and slowed the kinetics of signal termination. These results establish that abnormally prolonged signaling by G protein-coupled vasoconstrictor receptors can contribute to the onset of hypertension, and they suggest that genetic defects affecting the function or expression of RGS2 may be novel risk factors for development of hypertension in humans.

Qualitative and quantitative morphology of renal nerves in C57BL/6J mice

The detailed morphology of the renal nerves in mice has not been reported previously. The aims of this study were to describe the general morphology of the extrinsic renal nerve in C57BL/6 mice, and determine its morphometric parameters. The major renal nerve innervating the left kidney was isolated in five mice. Thin sections of the nerve segments were then examined by transmission electron microscopy. The renal nerve averaged 35.4 +/- 3.6 (S.E.M.) microm in diameter and 741 +/- 104 microm in area. The renal nerve contained an average of 830 +/- 169 unmyelinated fibers and only 4.6 +/- 1.7 myelinated fibers. The axon diameter of myelinated and unmyelinated fibers averaged 2.2 +/- 0.3 microm and 0.76 +/- 0.02 microm, respectively. The diameter of the unmyelinated fibers ranged from 0.3 to 2.0 microm, and the distribution histogram was unimodal. The majority of fibers (85%) had diameters of 0.6-1.0 microm. These results are similar to those obtained for renal nerves of rats with respect to the predominance of unmyelinated fibers. However, the diameter of unmyelinated fibers is larger in rats and the distribution histogram of rat unmyelinated fibers is bimodal, in contrast to the unimodal distribution in mice. The morphological description of the renal nerves in mice provides baseline data for further investigations of the structural basis of altered autonomic reflexes. The results will be useful in analyses of genes that influence the development and structure of sympathetic and sensory innervation of the kidney in genetically manipulated mice.

Analysis of afferent, central, and efferent components of the baroreceptor reflex in mice

Studies of genetically modified mice provide a powerful approach to investigate consequences of altered gene expression in physiological and pathological states. The goal of the present study was to characterize afferent, central, and efferent components of the baroreceptor reflex in anesthetized Webster 4 mice. Baroreflex and baroreceptor afferent functions were characterized by measuring changes in renal sympathetic nerve activity (RSNA) and aortic depressor nerve activity (ADNA) in response to nitroprusside- and phenylephrine-induced changes in arterial pressure. The data were fit to a sigmoidal logistic function curve. Baroreflex diastolic pressure threshold (P(th)), the pressure at 50% inhibition of RSNA (P(mid)), and baroreflex gain (maximum slope) averaged 74 +/- 5 mmHg, 101 +/- 3 mmHg, and 2.30 +/- 0.54%/mmHg, respectively (n = 6). The P(th), P(mid), and gain for the diastolic pressure-ADNA relation (baroreceptor afferents) were similar to that observed for the overall reflex averaging 79 +/- 9 mmHg, 101 +/- 4 mmHg, and 2.92 +/- 0.53%/mmHg, respectively (n = 5). The central nervous system mediation of the baroreflex and the chronotropic responsiveness of the heart to vagal efferent activity were independently assessed by recording responses to electrical stimulation of the left ADN and the peripheral end of the right vagus nerve, respectively. Both ADN and vagal efferent stimulation induced frequency-dependent decreases in heart rate and arterial pressure. The heart rate response to ADN stimulation was nearly abolished in mice anesthetized with pentobarbital sodium (n = 4) compared with mice anesthetized with ketamine-acepromazine (n = 4), whereas the response to vagal efferent stimulation was equivalent under both types of anesthesia. Application of these techniques to studies of genetically manipulated mice can be used to identify molecular mechanisms of baroreflex function and to localize altered function to afferent, central, or efferent sites.

Angiotensin II induces catecholamine release by direct ganglionic excitation

Angiotensin II (ANG) is known to facilitate catecholamine release from peripheral sympathetic neurons by enhancing depolarization-dependent exocytosis. In addition, a direct excitation by ANG of peripheral sympathetic nerve activity has recently been described. This study determined the significance of the latter mechanism for angiotensin-induced catecholamine release in the pithed rat. Rats were anesthetized and instrumented for measuring either hemodynamics and renal sympathetic nerve activity or plasma catecholamine concentrations in response to successively increasing doses of angiotensin infusions. Even during ganglionic blockade by hexamethonium (20 mg/kg), angiotensin dose-dependently elevated sympathetic nerve activity, whereas blood pressure-equivalent doses of phenylephrine were ineffective. Independently of central nervous sympathetic activity and ganglionic transmission, angiotensin (0.1 to 1 microg/kg) also induced an up-to 27-fold increase in plasma norepinephrine levels, reaching 2.65 ng/mL. Preganglionic electrical stimulation (0.5 Hz) raised basal norepinephrine levels 11-fold and further enhanced the angiotensin-induced increase in norepinephrine (4.04 ng/mL at 1 microg/kg ANG). Stimulation of sympathetic nerve activity and norepinephrine release were suppressed by candesartan (1 mg/kg) or tetrodotoxin (100 microg/kg), respectively. Angiotensin enhanced plasma norepinephrine, heart rate, and sympathetic nerve activity at similar threshold doses (0.3 to 1 microg/kg), but raised blood pressure at a significantly lower dose (0.01 microg/kg). It is concluded that direct stimulation of ganglionic angiotensin type 1 (AT(1)) receptors arouses electrical activity in sympathetic neurons, leading to exocytotic junctional catecholamine release. In both the absence and presence of preganglionic sympathetic activity, this mechanism contributes significantly to ANG-induced enhancement of catecholamine release.

Acute sympathoexcitatory action of angiotensin II in conscious baroreceptor-denervated rats

Angiotensin II (ANG II) has complex actions on the cardiovascular system. ANG II may act to increase sympathetic vasomotor outflow, but acutely the sympathoexcitatory actions of exogenous ANG II may be opposed by ANG II-induced increases in arterial pressure (AP), evoking baroreceptor-mediated decreases in sympathetic nerve activity (SNA). To examine this hypothesis, the effect of ANG II infusion on lumbar SNA was measured in unanesthetized chronic sinoaortic-denervated rats. Chronic sinoaortic-denervated rats had no reflex heart rate (HR) responses to pharmacologically evoked increases or decreases in AP. Similarly, in these denervated rats, nitroprusside-induced hypotension had no effect on lumbar SNA; however, phenylephrine-induced increases in AP were still associated with transient decreases in SNA. In control rats, infusion of ANG II (100 ng x kg(-1) x min(-1) iv) increased AP and decreased HR and SNA. In contrast, ANG II infusion increased lumbar SNA and HR in sinoaortic-denervated rats. In rats that underwent sinoaortic denervation surgery but still had residual baroreceptor reflex-evoked changes in HR, the effect of ANG II on HR and SNA was variable and correlated to the extent of baroreceptor reflex impairment. The present data suggest that pressor concentrations of ANG II in rats act rapidly to increase lumbar SNA and HR, although baroreceptor reflexes normally mask these effects of ANG II. Furthermore, these studies highlight the importance of fully characterizing sinoaortic-denervated rats used in experiments examining the role of baroreceptor reflexes.

Autonomic control of blood pressure in mice: basic physiology and effects of genetic modification

Control of blood pressure and of blood flow is essential for maintenance of homeostasis. The hemodynamic state is adjusted by intrinsic, neural, and hormonal mechanisms to optimize adaptation to internal and environmental challenges. In the last decade, many studies showed that modification of the mouse genome may alter the capacity of cardiovascular control systems to respond to homeostatic challenges or even bring about a permanent pathophysiological state. This review discusses the progress that has been made in understanding of autonomic cardiovascular control mechanisms from studies in genetically modified mice. First, from a physiological perspective, we describe how basic hemodynamic function can be measured in conscious conditions in mice. Second, we focus on the integrative role of autonomic nerves in control of blood pressure in the mouse, and finally, we depict the opportunities and insights provided by genetic modification in this area.

Downregulation of neuronal nitric oxide synthase and interleukin-1beta mediates angiotensin II-dependent stimulation of sympathetic nerve activity

There is substantial evidence that angiotensin II (Ang II) enhances sympathetic nervous system (SNS) activity. We recently observed that nitric oxide and interleukin-1beta (IL-1beta) exert a tonic inhibitory action on central SNS activity. Moreover, in 2 rat models of neurogenic hypertension, one caused by intrarenal injection of phenol and the other by 5/6 nephrectomy, we observed that losartan, an Ang II type 1 receptor blocker, inhibits SNS activity and increases the abundance of IL-1beta and the neuronal isoform of nitric oxide synthase (nNOS) in the posterior hypothalamic nuclei (PH), paraventricular nuclei (PVN), and locus ceruleus (LC). This raises the possibility that the stimulatory effects of Ang II on central SNS activity may be mediated by inhibition of nNOS and IL-1beta. To test this hypothesis, we studied the effect of an intracerebroventricular (ICV) infusion of Ang II on blood pressure (BP), norepinephrine (NE) secretion from the PH, renal SNS activity (RSNA), and abundance of IL-1beta and nNOS mRNA in the PH, PVN, and LC of normal Sprague-Dawley rats. Finally, we measured the concentration of nitrite/nitrate in the dialysate collected from the PH after Ang II or vehicle. ICV infusion of Ang II (100 ng/kg body wt dissolved in 10 microL of artificial cerebrospinal fluid) raised BP, RSNA, and NE secretion from the PH compared with control rats. Ang II reduced the abundance of IL-1beta and nNOS mRNA in the PH, PVN, and LC. Pretreatment with losartan (10 microg/kg body wt dissolved in 10 microL of aCSF) given ICV 20 minutes before Ang II abolished the effects of Ang II on BP, RSNA, and NE secretion from the PH and IL-1beta and nNOS mRNA. Ang II also decreased the secretion of NO from the PH. In conclusion, these studies suggest that Ang II inhibits the expression of IL-1beta and nNOS in the brain. Because locally produced NO exerts a tonic inhibitory action on SNS activity, the decrease in NO expression caused by Ang II results in greater SNS activity.

Mechanisms determining sensitivity of baroreceptor afferents in health and disease

Baroreceptors sense and signal the central nervous system of changes in arterial pressure through a series of sensory processes. An increase in arterial pressure causes vascular distension and baroreceptor deformation, the magnitude of which depends on the mechanical viscoelastic properties of the vessel wall. Classic methods (e.g., isolated carotid sinus preparation) and new approaches, including studies of isolated baroreceptor neurons in culture, gene transfer using viral vectors, and genetically modified mice have been used to define the cellular and molecular mechanisms that determine baroreceptor sensitivity. Deformation depolarizes the nerve endings by opening a new class of mechanosensitive Ion channel. This depolarization triggers action potential discharge through opening of voltage-dependent sodium (Na+) and potassium (K+) channels at the "spike initiating zone" (SIZ) near the sensory terminals. The resulting baroreceptor activity and its sensitivity to changes in pressure are modulated through a variety of mechanisms that influence these sensory processes. Modulation of voltage-dependent Na+ and K+ channels and the Na+ pump at the SIZ by membrance potential, action potential discharge, and chemical autocrine and paracrine factors are important mechanisms contributing to changes in baroreceptor sensitivity during sustained increases in arterial pressure and in pathological states associated with endothelial dysfunction, oxidative stress, and platelet activation.

Angiotensin selectively activates a subpopulation of postganglionic sympathetic neurons in mice

Angiotensin II (Ang II) increases renal sympathetic nerve activity in anesthetized mice before and after ganglionic blockade, suggesting that Ang II may directly activate postganglionic sympathetic neurons. The present study directly tested this hypothesis in vitro. Neurons were dissociated from aortic-renal and celiac ganglia of C57BL/6J mice. Cytosolic Ca(2+) concentration ([Ca(2+)](i)) was measured with ratio imaging using fura 2. Ang II increased [Ca(2+)](i) in a subpopulation of sympathetic neurons. At a concentration of 200 nmol/L, 14 (67%) of 21 neurons responded with a rise in [Ca(2+)](i). The Ang II type 1 (AT(1)) receptor blocker (losartan, 2 micromol/L) but not the Ang II type 2 (AT(2)) receptor blocker (PD123,319, 4 micromol/L) blocked this effect. The Ang II-induced [Ca(2+)](i) increase was abolished by removal of extracellular Ca(2+) but not altered by depletion of intracellular Ca(2+) stores with thapsigargin. Ang II no longer elicited a [Ca(2+)](i) increase in the presence of lanthanum (25 micromol/L). The specific N-type and L-type Ca(2+) channel blockers, omega-conotoxin GVIA and nifedipine, respectively, significantly inhibited the Ang II-induced [Ca(2+)](i) increase. The protein kinase C inhibitor H7 but not the protein kinase A inhibitor H89 blocked the response to Ang II. These results demonstrate that Ang II selectively activates a subpopulation of postganglionic sympathetic neurons in aortic-renal and celiac ganglia, triggering Ca(2+) influx through voltage-gated Ca(2+) channels. This effect is mediated through AT(1) receptors and requires the activation of protein kinase C. The activation of a subgroup of sympathetic neurons by Ang II may exert unique effects on kidney function in pathological states associated with elevated Ang II.