Chronic low-dose angiotensin II infusion increases venomotor tone by neurogenic mechanisms

Forgotten Circulation: Reduced Mesenteric Venous Capacitance in Hypertensive Rats Is Improved by Decreasing Sympathetic Activity

BACKGROUND

The mesenteric venous reservoir plays a vital role in mediating blood volume and pressure changes and is richly innervated by sympathetic nerves; however, the precise nature of venous sympathetic regulation and its role during hypertension remains unclear. We hypothesized that sympathetic drive to mesenteric veins in spontaneously hypertensive (SH) rats is raised, increasing mean circulatory filling pressure (MCFP), and impairing mesenteric capacitance.

METHODS

Arterial pressure, central venous pressure, mesenteric arterial, and venous blood flow were measured simultaneously in conscious male Wistar and SH rats. MCFP was assessed using an intraatrial balloon. Hemodynamic responses to volume changes (±20%) were measured before and after ganglionic blockade and carotid body denervation. Sympathetic venoconstrictor activity was measured in situ.

RESULTS

MCFP in vivo (10.8±1.6 versus 8.0±2.1 mm Hg; P=0.0005) and sympathetic venoconstrictor drive in situ (18±1 versus 10±2 µV; P<0.0001) were higher in SH rats; MCFP decreased in SH rats after hexamethonium and carotid body denervation (7.6±1.4; P<0.0001 and 8.5±1.0 mm Hg; P=0.0045). During volume changes, arterial pressure remained stable. With blood loss, net efflux of blood from the mesenteric bed was measured in both strains. However, during volume infusion, we observed net influx in Wistar (+2.3±2.6 mL/min) but efflux in SH rats (-1.0±1.0 mL/min; P=0.0032); this counterintuitive efflux was abolished by hexamethonium and carotid body denervation (+0.3±1.7 and 0.5±1.6 mL/min, respectively).

CONCLUSIONS

In SH rats, excessive sympathetic venoconstriction elevates MCFP and reduces capacitance, impairing volume buffering by mesenteric veins. We propose selective targeting of mesenteric veins through sympathetic drive reduction as a novel therapeutic opportunity for hypertension.

Controlled Mechanical Ventilation in Critically Ill Patients and the Potential Role of Venous Bagging in Acute Kidney Injury

A very low incidence of acute kidney injury (AKI) has been observed in COVID-19 patients purposefully treated with early pressure support ventilation (PSV) compared to those receiving mainly controlled ventilation. The prevention of subdiaphragmatic venous congestion through limited fluid intake and the lowering of intrathoracic pressure is a possible and attractive explanation for this observed phenomenon. Both venous congestion, or "venous bagging", and a positive fluid balance correlate with the occurrence of AKI. The impact of PSV on venous return, in addition to the effects of limiting intravenous fluids, may, at least in part, explain this even more clearly when there is no primary kidney disease or the presence of nephrotoxins. Optimizing the patient-ventilator interaction in PSV is challenging, in part because of the need for the ongoing titration of sedatives and opioids. The known benefits include improved ventilation/perfusion matching and reduced ventilator time. Furthermore, conservative fluid management positively influences cognitive and psychiatric morbidities in ICU patients and survivors. Here, it is hypothesized that cranial lymphatic congestion in relation to a more positive intrathoracic pressure, i.e., in patients predominantly treated with controlled mechanical ventilation (CMV), is a contributing risk factor for ICU delirium. No studies have addressed the question of how PSV can limit AKI, nor are there studies providing high-level evidence relating controlled mechanical ventilation to AKI. For this perspective article, we discuss studies in the literature demonstrating the effects of venous congestion leading to AKI. We aim to shed light on early PSV as a preventive measure, especially for the development of AKI and ICU delirium and emphasize the need for further research in this domain.

Rodent models of hypertension

Elevated blood pressure (BP), or hypertension, is the main risk factor for cardiovascular disease. As a multifactorial and systemic disease that involves multiple organs and systems, hypertension remains a challenging disease to study. Models of hypertension are invaluable to support the discovery of the specific genetic, cellular and molecular mechanisms underlying essential hypertension, as well as to test new possible treatments to lower BP. Rodent models have proven to be an invaluable tool for advancing the field. In this review, we discuss the strengths and weaknesses of rodent models of hypertension through a systems approach. We highlight the ways how target organs and systems including the kidneys, vasculature, the sympathetic nervous system (SNS), immune system and the gut microbiota influence BP in each rodent model. We also discuss often overlooked hypertensive conditions such as pulmonary hypertension and hypertensive-pregnancy disorders, providing an important resource for researchers.

High dietary salt amplifies osmoresponsiveness in vasopressin-releasing neurons

High dietary salt increases arterial pressure partly through activation of magnocellular neurosecretory cells (MNCVP) that secrete the antidiuretic and vasoconstrictor hormone vasopressin (VP) into the circulation. Here, we show that the intrinsic and synaptic excitation of MNCVP caused by hypertonicity are differentially potentiated in two models of salt-dependent hypertension in rats. One model combined salty chow with a chronic subpressor dose of angiotensin II (AngII-salt), the other involved replacing drinking water with 2% NaCl (salt loading, SL). In both models, we observed a significant increase in the quantal amplitude of EPSCs on MNCVP. However, model-specific changes were also observed. AngII-salt increased the probability of glutamate release by osmoreceptor afferents and increased overall excitatory network drive. In contrast, SL specifically increased membrane stiffness and the intrinsic osmosensitivity of MNCVP. These results reveal that dietary salt increases the excitability of MNCVP through effects on the cell-autonomous and synaptic osmoresponsiveness of MNCVP.

Central angiotensin II-Protein inhibitor of neuronal nitric oxide synthase (PIN) axis contribute to neurogenic hypertension

Activation of renin-angiotensin- system, nitric oxide (NO•) bioavailability and subsequent sympathoexcitation plays a pivotal role in the pathogenesis of many cardiovascular diseases, including hypertension. Previously we have shown increased protein expression of PIN (a protein inhibitor of nNOS: neuronal nitric oxide synthase, known to dissociate nNOS dimers into monomers) with concomitantly reduced levels of catalytically active dimers of nNOS in the PVN of rats with heart failure. To elucidate the molecular mechanism by which Angiotensin II (Ang II) increases PIN expression, we used Sprague-Dawley rats (250-300 g) subjected to intracerebroventricular infusion of Ang II (20 ng/min, 0.5 μl/h) or saline as vehicle (Veh) for 14 days through osmotic mini-pumps and NG108-15 hybrid neuronal cell line treated with Ang II as an in vitro model. Ang II infusion significantly increased baseline renal sympathetic nerve activity and mean arterial pressure. Ang II infusion increased the expression of PIN (1.24 ± 0.04* Ang II vs. 0.65 ± 0.07 Veh) with a concomitant 50% decrease in dimeric nNOS and PIN-Ub conjugates (0.73 ± 0.04* Ang II vs. 1.00 ± 0.03 Veh) in the PVN. Substrate-dependent ligase assay in cells transfected with pCMV-(HA-Ub)8 vector revealed a reduction of HA-Ub-PIN conjugates after Ang II and a proteasome inhibitor, Lactacystin (LC), treatment (4.5 ± 0.7* LC Ang II vs. 9.2 ± 2.5 LC). TUBE (Tandem Ubiquitin-Binding Entities) assay showed decrease PIN-Ub conjugates in Ang II-treated cells (0.82 ± 0.12* LC Ang II vs. 1.21 ± 0.06 LC) while AT₁R blocker, Losartan (Los) treatment diminished the Ang II-mediated stabilization of PIN (1.21 ± 0.07 LC Los vs. 1.16 ± 0.04* LC Ang II Los). Taken together, our studies suggest that increased central levels of Ang II contribute to the enhanced expression of PIN leading to reduced expression of the dimeric form of nNOS, thus diminishing the inhibitory action of NO• on pre-autonomic neurons in the PVN resulting in increased sympathetic outflow.

Brain Prostaglandin D2 Increases Neurogenic Pressor Activity and Mean Arterial Pressure in Angiotensin II-Salt Hypertensive Rats

This study tested whether brain L-PGDS (lipocalin-type prostaglandin [PG] D synthase), through prostanoid signaling, might increase neurogenic pressor activity and thereby cause hypertension. Sprague Dawley rats on high-salt diet received either vehicle or Ang II (angiotensin II) infusion. On day 4, the developmental stage of hypertension, brains from different sets of control and Ang II-treated rats were collected for measuring L-PGDS expression, PGD2 levels, and DP1R (type 1 PGD2 receptor) expression. In a different set of 14-day Ang II-salt-treated rats, mini-osmotic pumps were used to infuse either a nonselective COX (cyclooxygenase) inhibitor ketorolac, L-PGDS inhibitor AT56, or DP1R inhibitor BWA868C to test the role of brain COX-PGD2-DP1R signaling in Ang II-salt hypertension. The acute depressor response to ganglion blockade with hexamethonium was used to quantify neurogenic pressor activity. During the developmental stage of Ang II-salt hypertension, L-PGDS expression was higher in cerebrospinal fluid, and PGD2 levels were increased in the choroid plexus, cerebrospinal fluid, and the cardioregulatory brain region rostral ventrolateral medulla. DP1R expression was decreased in rostral ventrolateral medulla. Both brain COX inhibition with ketorolac and L-PGDS inhibition with AT56 lowered mean arterial pressure by altering neurogenic pressor activity compared with vehicle controls. Blockade of DP1R with BWA868C, however, increased the magnitude of Ang II-salt hypertension and significantly increased neurogenic pressor activity. In summary, we establish that the development of Ang II-salt hypertension requires increased COX- and L-PGDS-derived PGD2 production in the brain, making L-PGDS a possible target for treating neurogenic hypertension.

Central and peripheral slow-pressor mechanisms contributing to Angiotensin II-salt hypertension in rats

Aims

High salt intake markedly enhances hypertension induced by angiotensin II (Ang II). We explored central and peripheral slow-pressor mechanisms which may be activated by Ang II and salt.

Methods and results

In protocol I, Wistar rats were infused subcutaneously with low-dose Ang II (150 ng/kg/min) and fed regular (0.4%) or high salt (2%) diet for 14 days. In protocol II, Ang II-high salt was combined with intracerebroventricular infusion of mineralocorticoid receptor (MR) blockers (eplerenone, spironolactone), epithelial sodium channel (ENaC) blocker (benzamil), angiotensin II type 1 receptor (AT1R) blocker (losartan) or vehicles. Ang II alone raised mean arterial pressure (MAP) ∼10 mmHg, but Ang II-high salt increased MAP ∼50 mmHg. Ang II-high salt elevated plasma corticosterone, aldosterone and endogenous ouabain but not Ang II alone. Both Ang II alone and Ang II-high salt increased mRNA and protein expression of CYP11B2 (aldosterone synthase gene) in the adrenal cortex but not of CYP11B1 (11-β-hydroxylase gene). In the aorta, Ang II-high salt increased sodium-calcium exchanger-1 (NCX1) protein. The Ang II-high salt induced increase in MAP was largely prevented by central infusion of MR blockers, benzamil or losartan. Central blockades significantly lowered plasma aldosterone and endogenous ouabain and markedly decreased Ang II-high salt induced CYP11B2 mRNA expression in the adrenal cortex and NCX1 protein in the aorta.

Conclusion

These results suggest that in Ang II-high salt hypertension, MR-ENaC-AT1R signalling in the brain increases circulating aldosterone and endogenous ouabain, and arterial NCX1. These factors can amplify blood pressure responses to centrally-induced sympatho-excitation and thereby contribute to severe hypertension.

Burst patterning of hypothalamic paraventricular nucleus-driven sympathetic nerve activity in ANG II-salt hypertension

ANG II-salt hypertension selectively increases splanchnic sympathetic nerve activity (sSNA), but the extent to which this reflects increased respiratory versus cardiac rhythmic bursting is unknown. Here, integrated sSNA was elevated in ANG II-infused rats fed a high-salt (2% NaCl) diet (ANG II-HSD) compared with vehicle-infused rats fed a normal-salt (0.4% NaCl) diet (Veh-NSD; P < 0.01). Increased sSNA was not accompanied by increased inspiratory or expiratory bursting, consistent with no group difference in central inspiratory drive. Consistent with preserved inhibitory baroreflex entrainment of elevated sSNA in ANG II-HSD rats, the time integral ( P < 0.05) and amplitude ( P < 0.01) of cardiac rhythmic sSNA were increased. Consistent with activity of hypothalamic paraventricular nucleus (PVN) neurons supporting basal SNA in ANG II-salt hypertension, inhibition of PVN with the GABA-A receptor agonist muscimol reduced mean arterial pressure (MAP) and integrated sSNA only in the ANG II-HSD group ( P < 0.001). PVN inhibition had no effect on respiratory rhythmic sSNA bursting in either group but reduced cardiac rhythmic sSNA in ANG II-HSD rats only ( P < 0.01). The latter likely reflected reduced inhibitory baroreflex entrainment subsequent to the fall of MAP. Of note is that MAP as well as integrated and rhythmic burst patterns of sSNA were similar in vehicle-infused rats whether they were fed a normal or high-salt diet. Findings indicate that PVN neurons support elevated sSNA in ANG II-HSD rats by driving a tonic component of activity without altering respiratory or cardiac rhythmic bursting. Because sSNA was unchanged in Veh-HSD rats, activation of PVN-driven tonic sSNA appears to require central actions of ANG II. NEW & NOTEWORTHY ANG II-salt hypertension is strongly neurogenic and depends on hypothalamic paraventricular nucleus (PVN)-driven splanchnic sympathetic nerve activity (sSNA). Here, respiratory and cardiac bursts of sSNA were preserved in ANG II-salt rats and unaltered by PVN inhibition, suggesting that PVN neurons drive a tonic component of sSNA rather than modulating dominant patterns of burst discharge.

Update on angiotensin II: new endocrine connections between the brain, adrenal glands and the cardiovascular system

In the brain, angiotensinergic pathways play a major role in chronic regulation of cardiovascular and electrolyte homeostasis. Increases in plasma angiotensin II (Ang II), aldosterone, [Na⁺] and cytokines can directly activate these pathways. Chronically, these stimuli also activate a slow neuromodulatory pathway involving local aldosterone, mineralocorticoid receptors (MRs), epithelial sodium channels and endogenous ouabain (EO). This pathway increases AT₁R and NADPH oxidase subunits and maintains/further increases the activity of angiotensinergic pathways. These brain pathways not only increase the setpoint of sympathetic activity per se, but also enhance its effectiveness by increasing plasma EO and EO-dependent reprogramming of arterial and cardiac function. Blockade of any step in this slow pathway or of AT₁R prevents Ang II-, aldosterone- or salt and renal injury-induced forms of hypertension. MR/AT₁R activation in the CNS also contributes to the activation of sympathetic activity, the circulatory and cardiac RAAS and increase in circulating cytokines in HF post MI. Chronic central infusion of an aldosterone synthase inhibitor, MR blocker or AT₁R blocker prevents a major part of the structural remodeling of the heart and the decrease in LV function post MI, indicating that MR activation in the CNS post MI depends on aldosterone, locally produced in the CNS. Thus, Ang II, aldosterone and EO are not simply circulating hormones that act on the CNS but rather they are also paracrine neurohormones, locally produced in the CNS, that exert powerful effects in key CNS pathways involved in the long-term control of sympathetic and neuro-endocrine function and cardiovascular homeostasis.

Chronic high-sodium diet intake after weaning lead to neurogenic hypertension in adult Wistar rats

In this study, we investigated some mechanisms involved in sodium-dependent hypertension of rats exposed to chronic salt (NaCl) intake from weaning until adult age. Weaned male Wistar rats were placed under high (0.90% w/w, HS) or regular (0.27% w/w, Cont) sodium diets for 12 weeks. Water consumption, urine output and sodium excretion were higher in HS rats compared to control. Blood pressure (BP) was directly measured by the arterial catheter and found 13.8% higher in HS vs Cont rats. Ganglionic blockade with hexamethonium caused greater fall in the BP of HS rats (33%), and central antagonism of AT₁ receptors (losartan) microinjected into the lateral ventricle reduced BP level of HS, but not of Cont group. Heart rate variability analysis revealed sympathetic prevalence on modulation of the systolic interval. HS diet did not affect creatinine clearance. Kidney histological analysis revealed no significant change in renal corpuscle structure. Sodium and potassium concentrations in CSF were found higher in HS rats despite no change in plasma concentration of these ions. Taken together, data suggest that animals exposed to chronic salt intake to a level close to that reported for human' diet since weaning lead to hypertension, which appears to rely on sodium-driven neurogenic mechanisms.

Sympathoexcitation in ANG II-salt hypertension involves reduced SK channel function in the hypothalamic paraventricular nucleus

Hypertension (HTN) resulting from subcutaneous infusion of ANG II and dietary high salt (HS) intake involves sympathoexcitation. Recently, we reported reduced small-conductance Ca(2+)-activated K(+) (SK) current and increased excitability of presympathetic neurons in the paraventricular nucleus (PVN) in ANG II-salt HTN. Here, we hypothesized that ANG II-salt HTN would be accompanied by altered PVN SK channel activity, which may contribute to sympathoexcitation in vivo. In anesthetized rats with normal salt (NS) intake, bilateral PVN microinjection of apamin (12.5 pmol/50 nl each), the SK channel blocker, remarkably elevated splanchnic sympathetic nerve activity (SSNA), renal sympathetic nerve activity (RSNA), and mean arterial pressure (MAP). In contrast, rats with ANG II-salt HTN demonstrated significantly attenuated SSNA, RSNA, and MAP (P < 0.05) responses to PVN-injected apamin compared with NS control rats. Next, we sought to examine the individual contributions of HS and subcutaneous infusion of ANG II on PVN SK channel function. SSNA, RSNA, and MAP responses to PVN-injected apamin in rats with HS alone were significantly attenuated compared with NS-fed rats. In contrast, sympathetic nerve activity responses to PVN-injected apamin in ANG II-treated rats were slightly attenuated with SSNA, demonstrating no statistical difference compared with NS-fed rats, whereas MAP responses to PVN-injected apamin were similar to NS-fed rats. Finally, Western blot analysis showed no statistical difference in SK1-SK3 expression in the PVN between NS and ANG II-salt HTN. We conclude that reduced SK channel function in the PVN is involved in the sympathoexcitation associated with ANG II-salt HTN. Dietary HS may play a dominant role in reducing SK channel function, thus contributing to sympathoexcitation in ANG II-salt HTN.

A mathematical model of salt-sensitive hypertension: the neurogenic hypothesis

Salt sensitivity of arterial pressure (salt-sensitive hypertension) is a serious global health issue. The causes of salt-sensitive hypertension are extremely complex and mathematical models can elucidate potential mechanisms that are experimentally inaccessible. Until recently, the only mathematical model for long-term control of arterial pressure was the model of Guyton and Coleman; referred to as the G-C model. The core of this model is the assumption that sodium excretion is driven by renal perfusion pressure, the so-called 'renal function curve'. Thus, the G-C model dictates that all forms of hypertension are due to a primary shift of the renal function curve to a higher operating pressure. However, several recent experimental studies in a model of hypertension produced by the combination of a high salt intake and administration of angiotensin II, the AngII-salt model, are inconsistent with the G-C model. We developed a new mathematical model that does not limit the cause of salt-sensitive hypertension solely to primary renal dysfunction. The model is the first known mathematical counterexample to the assumption that all salt-sensitive forms of hypertension require a primary shift of renal function: we show that in at least one salt-sensitive form of hypertension the requirement is not necessary. We will refer to this computational model as the 'neurogenic model'. In this Symposium Review we discuss how, despite fundamental differences between the G-C model and the neurogenic model regarding mechanisms regulating sodium excretion and vascular resistance, they generate similar haemodynamic profiles of AngII-salt hypertension. In addition, the steady-state relationships between arterial pressure and sodium excretion, a correlation that is often erroneously presented as the 'renal function curve', are also similar in both models. Our findings suggest that salt-sensitive hypertension is not due solely to renal dysfunction, as predicted by the G-C model, but may also result from neurogenic dysfunction.

Comparison of arterial pressure and plasma ANG II responses to three methods of subcutaneous ANG II administration

Angiotensin II (ANG II)-induced hypertension is a commonly studied model of experimental hypertension, particularly in rodents, and is often generated by subcutaneous delivery of ANG II using Alzet osmotic minipumps chronically implanted under the skin. We have observed that, in a subset of animals subjected to this protocol, mean arterial pressure (MAP) begins to decline gradually starting the second week of ANG II infusion, resulting in a blunting of the slow pressor response and reduced final MAP. We hypothesized that this variability in the slow pressor response to ANG II was mainly due to factors unique to Alzet pumps. To test this, we compared the pressure profile and changes in plasma ANG II levels during subcutaneous ANG II administration (150 ng·kg(-1)·min(-1)) using either Alzet minipumps, iPrecio implantable pumps, or a Harvard external infusion pump. At the end of 14 days of ANG II, MAP was highest in the iPrecio group (156 ± 3 mmHg) followed by Harvard (140 ± 3 mmHg) and Alzet (122 ± 3 mmHg) groups. The rate of the slow pressor response, measured as daily increases in pressure averaged over days 2-14 of ANG II, was similar between iPrecio and Harvard groups (2.7 ± 0.4 and 2.2 ± 0.4 mmHg/day) but was significantly blunted in the Alzet group (0.4 ± 0.4 mmHg/day) due to a gradual decline in MAP in a subset of rats. We also found differences in the temporal profile of plasma ANG II between infusion groups. We conclude that the gradual decline in MAP observed in a subset of rats during ANG II infusion using Alzet pumps is mainly due to pump-dependent factors when applied in this particular context.

Knockdown of mineralocorticoid or angiotensin II type 1 receptor gene expression in the paraventricular nucleus prevents angiotensin II hypertension in rats

Circulating Ang II activates an aldosterone-mineralocorticoid receptor (MR) - angiotensin II (Ang II) - angiotensin type 1 receptor (AT1R) pathway in the hypothalamus. To obtain insights into the actual neuronal projections involved, adeno-associated virus carrying small interfering RNA against either AT1aR (AAV-AT1aR-siRNA) or MR (AAV-MR-siRNA) were infused into the paraventricular nucleus (PVN) in Wistar rats. Intra-PVN infusion of AAV-AT1aR-siRNA or AAV-MR-siRNA decreased AT1R or MR expression in the PVN but not in the subfornical organ (SFO) or supraoptic nucleus (SON). Subcutaneous infusion of Ang II at 500 ng kg(-1) min(-1) for 2 weeks increased mean arterial pressure by 60-70 mmHg, and increased AT1R and MR expression in the SFO, SON and PVN. Intra-PVN AT1aR-siRNA prevented the Ang II-induced increase in AT1R but not MR expression in the PVN, and MR-siRNA prevented MR but not AT1R expression in the PVN. The increases in AT1R and MR expression in both the SFO and the SON were not changed by the two AAV-siRNAs. Specific knockdown of AT1R or MR in the PVN by AAV-siRNA each prevented most of the Ang II-induced hypertension. Prevention of the subcutaneous Ang II-induced increase in MR but not the increase in AT1R by knockdown of MR and vice versa suggests an independent regulation of MR and AT1R expression in the PVN. Both AT1R and MR activation in the PVN play a critical role in Ang II-induced hypertension in rats.

CNS neuroplasticity and salt-sensitive hypertension induced by prior treatment with subpressor doses of ANG II or aldosterone

Although sensitivity to high dietary NaCl is regarded to be a risk factor for cardiovascular disease, the causes of salt-sensitive hypertension remain elusive. Previously, we have shown that rats pretreated with subpressor doses of either ANG II or aldosterone (Aldo) show sensitized hypertensive responses to a mild pressor dose of ANG II when tested after an intervening delay. The current studies investigated whether such treatments will induce salt sensitivity. In studies employing an induction-delay-expression experimental design, male rats were instrumented for chronic mean arterial pressure (MAP) recording. In separate experiments, ANG II, Aldo, or vehicle was delivered either subcutaneously or intracerebroventricularly during the induction. There were no sustained differences in BP during the delay prior to being given 2% saline. While consuming 2% saline during the expression, both ANG II- and Aldo-pretreated rats showed significantly greater hypertension. When hexamethonium was used to assess autonomic control of MAP, no differences in the decrease of MAP in response to ganglionic blockade were detected during the induction. However, during the expression, the fall was greater in sensitized rats. In separate experiments, brain tissue that was collected at the end of delay showed increases in message or activation of putative markers of neuroplasticity (i.e., brain-derived neurotrophic factor, p38 mitogen-activated protein kinase, and cAMP response element-binding protein). These experiments demonstrate that prior administration of nonpressor doses of either ANG II or Aldo will induce salt sensitivity. Collectively, our findings indicate that treatment with subpressor doses of ANG II and Aldo initiate central neuroplastic changes that are involved in hypertension of different etiologies.

The neurogenic phase of angiotensin II-salt hypertension is prevented by chronic intracerebroventricular administration of benzamil

Hypertension induced by chronic administration of angiotensin II (AngII) is exacerbated by high‐salt intake. Previous studies have demonstrated that this salt‐sensitive component is due to increased activity of the sympathetic nervous system, suggesting an interaction of plasma AngII with sodium‐sensitive regions of the brain. This study tested the hypothesis that the salt‐sensitive component of AngII‐induced hypertension would be prevented by intracerebroventricular (ICV) administration of the sodium channel/transporter blocker benzamil. Male Sprague Dawley rats were instrumented to measure mean arterial pressure (MAP) by radio telemetry and for ICV administration of benzamil or vehicle and placed in metabolic cages for measurement of sodium and water intake and excretion. In rats consuming a high‐salt diet (2.0% NaCl) and treated with ICV vehicle, administration of AngII (150 ng/kg/min, sc) for 13 days increased MAP by ~30 mmHg. ICV administration of benzamil (16 nmol/day) had no effect during the first 5 days of AngII, but returned MAP to control levels by Day 13. There were minimal or no differences between ICV vehicle or benzamil groups in regards to sodium and water balance. A lower dose of ICV benzamil administered ICV at 8 nmol/day had no effect on the MAP response to AngII in rats on a high‐salt diet. Finally, in contrast to rats on a high‐salt diet, AngII had negligible effects on MAP in rats consuming a low‐salt diet (0.1% NaCl) and there were no differences in any variable between ICV benzamil (16 nmol/day) and ICV vehicle‐treated groups. We conclude that the salt‐sensitive component of AngII‐induced hypertension is dependent on benzamil blockable sodium channels or transporters in the brain.

Chronic intracerebroventricular infusion of benzamil at 16 nmol/day attenuates AngII–salt hypertension. This effect is not observed at a dose of 8 nmol/day.

Do intravenous and subcutaneous angiotensin II increase blood pressure by different mechanisms?

Angiotensin (Ang) II plays a key role in blood pressure regulation. Mechanisms of the pressor effect of chronic intravenous AngII administration include vasoconstriction, stimulation of the sympathetic nervous system and aldosterone production, as well as direct effects on renal excretion of sodium and water.  Chronic AngII administration by subcutaneous minipump at doses higher than required to increase blood pressure by the intravenous route has identified additional pressor mechanisms, including the immune system, cytokines and matrix metalloproteinases. However, pressor doses of subcutaneous AngII may exceed the angiotensinogen synthesis rate and produce inflammation, fibrosis and necrosis of skin overlying the minipump.  Evidence that chronic subcutaneous and intravenous AngII increase blood pressure by different mechanisms includes the prevention of the pressor effects of subcutaneous, but not intravenous, AngII by angiotensin-converting enzyme inhibition. Furthermore, low doses of subcutaneous AngII reduce blood pressure of female, but not male, rodents and higher doses are less pressor in females than in males, whereas intravenous AngII is equally pressor in males and females.  Pressor doses of chronic subcutaneous AngII produce greater weight loss, anorexia and reduced kidney weight and cause greater vascular, cardiac and renal pathology than equally pressor doses of chronic intravenous AngII.  The different effects of chronic intravenous and subcutaneous AngII suggest that these two models of hypertension give different information and may differ in their relevance to blood pressure regulation in physiological and pathological states such as hypertension in humans.

Central mineralocorticoid receptors and the role of angiotensin II and glutamate in the paraventricular nucleus of rats with angiotensin II-induced hypertension

A chronic increase in circulating angiotensin II (Ang II) activates an aldosterone-mineralocorticoid receptor-ouabain neuromodulatory pathway in the brain that increases neuronal activation in hypothalamic nuclei, such as the paraventricular nucleus (PVN) and causes progressive hypertension. Several models of chronic sympathetic hyperactivity are associated with an increase in AT1 and glutamate receptor activation in the PVN. The current study evaluated whether increased angiotensin type 1 (AT1) and glutamate receptor-dependent signaling in the PVN contributes to the maintenance of blood pressure (BP) in Ang II-hypertensive Wistar rats, and the role of aldosterone-mineralocorticoid receptor pathway in this enhanced signaling. After subcutaneous infusion of Ang II for 2 weeks, in conscious rats BP and heart rate were recorded after (1) 10-minute bilateral infusions of candesartan and kynurenate in the PVN; (2) 1 hour intracerebroventricular infusion of eplerenone, and (3) candesartan and kynurenate after eplerenone. Candesartan or kynurenate in the PVN fully reversed the increase in BP from circulating Ang II. Kynurenate after candesartan or candesartan after kynurenate did not further lower BP. Intracerebroventricular infusion of eplerenone at 16 hours after its infusion fully reversed the increase in BP from circulating Ang II. After eplerenone, candesartan and kynurenate in the PVN did not further decrease BP. These findings suggest that increased mineralocorticoid receptor activation in the brain activates a slow neuromodulatory pathway that maintains enhanced AT1 and glutamate receptor-dependent signaling in the PVN, and thereby the hypertension from a chronic increase in circulating Ang II.

A new conceptual paradigm for the haemodynamics of salt-sensitive hypertension: a mathematical modelling approach

A conceptually novel mathematical model of neurogenic angiotensin II-salt hypertension is developed and analysed. The model consists of a lumped parameter circulatory model with two parallel vascular beds; two distinct control mechanisms for both natriuresis and arterial resistances can be implemented, resulting in four versions of the model. In contrast with the classical Guyton-Coleman model (GC model) of hypertension, in the standard version of our new model natriuresis is assumed to be independent of arterial pressure and instead driven solely by sodium intake; arterial resistances are driven by increased sympathetic nervous system activity in response to the elevated plasma angiotensin II and increased salt intake (AngII-salt). We compare the standard version of our new model against a simplified Guyton-Coleman model in which natriuresis is a function of arterial pressure via the pressure-natriuresis mechanism, and arterial resistances are controlled via the whole-body autoregulation mechanism. We show that the simplified GC model and the new model correctly predict haemodynamic and renal excretory responses to induced changes in angiotensin II and sodium inputs. Importantly, the new model reproduces the pressure-natriuresis relationship--the correlation between arterial pressure and sodium excretion--despite the assumption of pressure-independent natriuresis. These results show that our model provides a conceptually new alternative to Guyton's theory without contradicting observed haemodynamic changes or pressure-natriuresis relationships. Furthermore, the new model supports the view that hypertension need not necessarily have a renal aetiology and that long-term arterial pressure could be determined by sympathetic nervous system activity without involving the renal sympathetic nerves.

Differential control of efferent sympathetic activity revisited

This article reviews 40 years of research (1970-2010) into the capability of the efferent sympathetic nervous system to display differential responsiveness. Discovered first were antagonistic changes of activity in sympathetic filaments innervating functionally different sections of the cardiovascular system in response to thermal stimulation. During the subsequent four decades of investigation, a multitude of differential sympathetic efferent response patterns were identified, ranging from opposing activity changes at the level of multi-fiber filaments innervating different organs to the level of single fibers controlling functionally different structures in the same organ. Differential sympathetic responsiveness was shown to be displayed in response to exogenous or artificial stimulation of afferent sensory fibers transmitting particular exogenous stimuli, especially those activating peripheral nociceptors. Moreover, sympathetic differentiation was found to be characteristic of autonomic responses to environmental changes by which homeostasis in the broadest sense would be challenged. Heat or cold loads or their experimental equivalents, altered composition of inspired air or changes in blood gas composition, imbalances of body fluid control, and exposure to agents challenging the immune system were shown to elicit differential efferent sympathetic response patterns which often displayed a high degree of specificity. In summary, autonomic adjustments to changes of biometeorological parameters may be considered as representative of the capability of the sympathetic nervous system to exert highly specific efferent control of organ functions by which bodily homeostasis is maintained.

Requirement for functional BK channels in maintaining oscillation in venomotor tone revealed by species differences in expression of the β1 accessory subunits

We determined the possible role of large-conductance Ca2+-activated K (BK) channels in regulation of venous tone in small capacitance veins and blood pressure. In rat mesenteric venous smooth muscle cells (MV SMC), BK channel α- and β1-subunits were coexpressed, unitary BK currents were detected, and single-channel currents were sensitive to voltage and [Ca2+]i. Rat MV SMCs displayed Ca sparks and iberiotoxin-sensitive spontaneous transient outward currents. Under resting conditions in vitro, rat MV exhibited nifedipine-sensitive spontaneous oscillatory constrictions. Blockade of BK channels by paxilline and Ca2+ sparks by ryanodine constricted rat MV. Nifedipine caused venodilation and blocked paxilline-induced, KCl-induced (20 mM), and BayK8644-induced contraction. Acute inhibition of BK channels with iberiotoxin in vivo increased blood pressure and reduced venous capacitance, measured as an increase in mean circulatory filling pressure in conscious rats. BK channel α-subunits and L-type Ca2+ channel α1-C subunits are expressed in murine MV. However, these channels are not functional because murine MV lack nifedipine-sensitive basal tone and rhythmic constrictions. Murine MV were also insensitive to paxilline, ryanodine, KCl, and BayK8644, consistent with our previous studies showing that murine MV do not have BK β1-subunits. These data show that not only there are species-dependent properties in ion channel control of venomotor tone but also BK channels are required for rhythmic oscillations in venous tone.

The role of the subfornical organ in angiotensin II-salt hypertension in the rat

Hypertension caused by chronic infusion of angiotensin II (Ang II) in experimental animals is dependent, in part, on increased activity of the sympathetic nervous system. This chronic sympathoexcitatory response is amplified by a high-salt diet, suggesting an interaction of circulating Ang II and dietary salt on sympathetic regulatory pathways in the brain. The present study tested the hypothesis that the subfornical organ (SFO), a forebrain circumventricular organ known to be activated by circulating Ang II, is crucial to the pathogenesis of hypertension induced by chronic Ang II administration in rats on a high-salt diet (Ang II-salt model). Rats were randomly selected to undergo either subfornical organ lesion (SFOx) or sham surgery (Sham) and then placed on a high-salt (2% NaCl) diet. One week later, rats were instrumented for radiotelemetric measurement of mean arterial pressure (MAP) and heart rate (HR) and placed in metabolic cages to measure sodium and water balance. Baseline MAP was slightly (but not statistically) lower in SFOx compared with Sham rats during the 5 day control period. During the subsequent 10 days of Ang II administration, MAP was statistically lower in SFOx rats. However, when MAP responses to Ang II were analysed by comparing the change from the 5 day baseline period, only on the fifth day of Ang II was MAP significantly different between groups. There were no differences between groups for water or sodium balance throughout the protocol. We conclude that, although the SFO is required for the complete expression of Ang II-salt hypertension in the rat, other brain sites are also involved.

Time-dependent changes in autonomic control of splanchnic vascular resistance and heart rate in ANG II-salt hypertension

Previous studies suggest that ANG II-induced hypertension in rats fed a high-salt (HS) diet (ANG II-salt hypertension) has a neurogenic component dependent on an enhanced sympathetic tone to the splanchnic veins and independent from changes in sympathetic nerve activity to the kidney or hind limb. The purpose of this study was to extend these findings and test whether altered autonomic control of splanchnic resistance arteries and the heart also contributes to the neurogenic component. Mean arterial pressure (MAP), heart rate (HR), superior mesenteric artery blood flow, and mesenteric vascular resistance (MVR) were measured during 4 control days, 14 days of ANG II delivered subcutaneously (150 ng·kg(-1)·min(-1)), and 4 days of recovery in conscious rats fed a HS (2% NaCl) or low-salt (LS; 0.1% NaCl) diet. Autonomic effects on MAP, HR, and MVR were assessed by acute ganglionic blockade with hexamethonium (20 mg/kg iv) on day 3 of control, days 1, 3, 5, 7, 10, and 13 of ANG II, and day 4 of recovery. MVR increased during ANG II infusion in HS and LS rats but remained elevated only in HS rats. Additionally, the MVR response to hexamethonium was enhanced on days 10 and 13 of ANG II selectively in HS rats. Compared with LS rats, HR in HS rats was higher during the 2nd wk of ANG II, and its response to hexamethonium was greater on days 7, 10, and 13 of ANG II. These results suggest that ANG II-salt hypertension is associated with delayed changes in autonomic control of splanchnic resistance arteries and the heart.

Neural mechanisms of angiotensin II-salt hypertension: implications for therapies targeting neural control of the splanchnic circulation

Chronically elevated plasma angiotensin II (AngII) causes a salt-sensitive form of hypertension that is associated with a differential pattern of peripheral sympathetic outflow. This "AngII-salt sympathetic signature" is characterized by a transient reduction in sympathetic nervous system activity (SNA) to the kidneys, no change in SNA to skeletal muscle, and a delayed activation of SNA to the splanchnic circulation. Studies suggest that the augmented sympathetic influence on the splanchnic vascular bed increases vascular resistance and decreases vascular capacitance, leading to hypertension via translocation of blood volume from the venous to the arterial circulation. This unique sympathetic signature is hypothesized to be generated by a balance of central excitatory inputs and differential baroreceptor inhibitory inputs to sympathetic premotor neurons in the rostral ventrolateral medulla. The relevance of these findings to human hypertension and the future development of targeted sympatholytic therapies are discussed.

Chronic angiotensin II infusion modulates angiotensin II type I receptor expression in the subfornical organ and the rostral ventrolateral medulla in hypertensive rats

Blood-borne angiotensin II (Ang II) has profound effects on the central nervous system, including regulation of vasopressin secretion and modulation of sympathetic outflow. However, the mechanism by which circulating Ang II affects the central nervous system remains largely unknown. We tested the hypothesis that increased circulating levels of Ang II activate angiotensin type I (AT1) receptors in the subfornical organ (SFO), increasing the Ang II signalling in the rostral ventrolateral medulla (RVLM). Male Wistar rats were subcutaneously implanted with two 14-day osmotic minipumps filled with Ang II (150 ng/kg/minute), Losartan (10mg/kg/day), or saline. In addition, AT1 receptor mRNA levels in the SFO and RVLM were detected by reverse transcription polymerase chain reaction (RT-PCR). Infusion of Ang II-induced hypertension (134 ± 10 mmHg vs 98 ± 9 mmHg, n = 9, p < 0.05), which was blunted by concomitant infusion of Losartan (105 ± 8 vs 134 ± 10 mmHg, n = 9, p < 0.05). In addition, hexamethonium produced a greater decrease in blood pressure in Ang II-infused rats. Real time PCR revealed that chronic Ang II infusion induced an increase in AT1 receptor mRNA levels in the RVLM and a decrease in the SFO. Taken together, using combined in vivo and molecular biology approaches, our data suggest that Ang II-induced hypertension is mediated by an increase in sympathetic nerve activity, which seems to involve up-regulation of AT1 receptors in the RVLM and down-regulation of AT1 receptors in the SFO.

Involvement of the AT1 receptor in the venoconstriction induced by angiotensin II in both the inferior vena cava and femoral vein

Although angiotensin II-induced venoconstriction has been demonstrated in the rat vena cava and femoral vein, the angiotensin II receptor subtypes (AT(1) or AT(2)) that mediate this phenomenon have not been precisely characterized. Therefore, the present study aimed to characterize the pharmacological receptors involved in the angiotensin II-induced constriction of rat venae cavae and femoral veins, as well as the opposing effects exerted by locally produced prostanoids and NO upon induction of these vasomotor responses. The obtained results suggest that both AT(1) and AT(2) angiotensin II receptors are expressed in both veins. Angiotensin II concentration-response curves were shifted toward the right by losartan but not by PD 123319 in both the vena cava and femoral vein. Moreover, it was observed that both 10(-5)M indomethacin and 10(-4)M L-NAME improve the angiotensin II responses in the vena cava and femoral vein. In conclusion, in the rat vena cava and femoral vein, angiotensin II stimulates AT(1) but not AT(2) to induce venoconstriction, which is blunted by vasodilator prostanoids and NO.

Large-conductance Ca2+-activated K+ channel beta1-subunit knockout mice are not hypertensive

Large-conductance Ca2+-activated K+ (BK) channels are composed of pore-forming α-subunits and accessory β1-subunits that modulate Ca2+ sensitivity. BK channels regulate arterial myogenic tone and renal Na+ clearance/K+ reabsorption. Previous studies using indirect or short-term blood pressure measurements found that BK channel β1-subunit knockout (BK β1-KO) mice were hypertensive. We evaluated 24-h mean arterial pressure (MAP) and heart rate in BK β1-KO mice using radiotelemetry. BK β1-KO mice did not have a higher 24-h average MAP when compared with wild-type (WT) mice, although MAP was ∼10 mmHg higher at night. The dose-dependent peak declines in MAP by nifedipine were only slightly larger in BK β1-KO mice. In BK β1-KO mice, giving 1% NaCl to mice to drink for 7 days caused a transient (5 days) elevation of MAP (∼5 mmHg); MAP returned to pre-saline levels by day 6. BK β1-KO mesenteric arteries in vitro demonstrated diminished contractile responses to paxilline, increased reactivity to Bay K 8644 and norepinephrine (NE), and maintained relaxation to isoproterenol. Paxilline and Bay K 8644 did not constrict WT or BK β1-KO mesenteric veins (MV). BK β1-subunits are not expressed in MV. The results indicate that BK β1-KO mice are not hypertensive on normal or high-salt intake. BK channel deficiency increases arterial reactivity to NE and L-type Ca2+ channel function in vitro, but the L-type Ca2+ channel modulation of MAP is not altered in BK β1-KO mice. BK and L-type Ca(2+) channels do not modulate murine venous tone. It appears that selective loss of BK channel function in arteries only is not sufficient to cause sustained hypertension.

Adrenergic mechanisms do not contribute to age-related decreases in calf venous compliance

Limb venous compliance decreases with advancing age, even in healthy humans. To test the hypothesis that adrenergic mechanisms contribute to age-associated reductions in limb venous compliance, we measured calf venous compliance before and during acute systemic α- and β-adrenergic blockade in eight young (27 ± 1 yr old, mean ± SE) and eight older healthy men (67 ± 2 yr old). Calf venous compliance was determined in supine subjects by inflating a thigh-collecting cuff to 60 mmHg for 8 min and then decreasing it (1 mmHg/s) to 0 mmHg while calf volume was indexed with a strain gauge. The slope (·10⁻³) of the pressure-compliance relation (compliance= β₁ + 2·β₂·cuff pressure), which is the first derivative of the quadratic pressure-volume relation [(Δlimb volume) = β₀+ β₁·(cuff pressure) + β₂·(cuff pressure)²] during reductions in cuff pressure, was used to quantify calf venous compliance. Calf venous compliance was ∼30% lower (P < 0.01) in older compared with young men before adrenergic blockade. In response to adrenergic blockade calf venous compliance did not increase in young (-2.62 ± 0.14 and -2.29 ± 0.18 ml·dl⁻¹·mmHg⁻¹, before and during blockade, respectively) or older men (-1.78 ± 0.27 and -1.68 ± 0.21 ml·dl⁻¹ ·mmHg⁻¹). Moreover, during adrenergic blockade differences in calf venous compliance between young and older men observed before adrenergic blockade persisted. Collectively, these data strongly suggest that adrenergic mechanisms neither directly restrain calf venous compliance in young or older men nor do they contribute to age-associated reductions in calf venous compliance in healthy men.

Hypertension induced by angiotensin II and a high salt diet involves reduced SK current and increased excitability of RVLM projecting PVN neurons

Although evidence indicates that activation of presympathetic paraventricular nucleus (PVN) neurons contributes to the pathogenesis of salt-sensitive hypertension, the underlying cellular mechanisms are not fully understood. Recent evidence indicates that small conductance Ca(2+)-activated K(+) (SK) channels play a significant role in regulating the excitability of a key group of sympathetic regulatory PVN neurons, those with axonal projections to the rostral ventrolateral medulla (RVLM; i.e., PVN-RVLM neurons). In the present study, rats consuming a high salt (2% NaCl) diet were made hypertensive by systemic infusion of angiotensin II (AngII), and whole cell patch-clamp recordings were made in brain slice from retrogradely labeled PVN-RVLM neurons. To determine if the amplitude of SK current was altered in neurons from hypertensive rats, voltage-clamp recordings were performed to isolate SK current. Results indicate that SK current amplitude (P < 0.05) and density (P < 0.01) were significantly smaller in the hypertensive group. To investigate the impact of this on intrinsic excitability, current-clamp recordings were performed in separate groups of PVN-RVLM neurons. Results indicate that the frequency of spikes evoked by current injection was significantly higher in the hypertensive group (P < 0.05-0.01). Whereas bath application of the SK channel blocker apamin significantly increased discharge of neurons from normotensive rats (P < 0.05-0.01), no effect was observed in the hypertensive group. In response to ramp current injections, subthreshold depolarizing input resistance was greater in the hypertensive group compared with the normotensive group (P < 0.05). Blockade of SK channels increased depolarizing input resistance in normotensive controls (P < 0.05) but had no effect in the hypertensive group. On termination of current pulses, a medium afterhyperpolarization potential (mAHP) was observed in most neurons of the normotensive group. In the hypertensive group, the mAHP was either small or absent. In the latter case, an afterdepolarization potential (ADP) was observed that was unaffected by apamin. Apamin treatment in the normotensive group blocked the mAHP and revealed an ADP resembling that seen in the hypertensive group. We conclude that diminished SK current likely underlies the absence of mAHPs in PVN-RVLM neurons from hypertensive rats. Both the ADP and greater depolarizing input resistance likely contribute to increased excitability of PVN-RVLM neurons from rats with AngII-Salt hypertension.

Brain microglial cytokines in neurogenic hypertension

Accumulating evidence indicates a key role of inflammation in hypertension and cardiovascular disorders. However, the role of inflammatory processes in neurogenic hypertension remains to be determined. Thus, our objective in the present study was to test the hypothesis that activation of microglial cells and the generation of proinflammatory cytokines in the paraventricular nucleus (PVN) contribute to neurogenic hypertension. Intracerebroventricular infusion of minocycline, an anti-inflammatory antibiotic, caused a significant attenuation of mean arterial pressure, cardiac hypertrophy, and plasma norepinephrine induced by chronic angiotensin II infusion. This was associated with decreases in the numbers of activated microglia and mRNAs for interleukin (IL) 1beta, IL-6, and tumor necrosis factor-alpha, and an increase in the mRNA for IL-10 in the PVN. Overexpression of IL-10 induced by recombinant adenoassociated virus-mediated gene transfer in the PVN mimicked the antihypertensive effects of minocycline. Furthermore, acute application of a proinflammatory cytokine, IL-1beta, into the left ventricle or the PVN in normal rats resulted in a significant increase in mean arterial pressure. Collectively, this indicates that angiotensin II induced hypertension involves activation of microglia and increases in proinflammatory cytokines in the PVN. These data have significant implications on the development of innovative therapeutic strategies for the control of neurogenic hypertension.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Does enhanced respiratory-sympathetic coupling contribute to peripheral neural mechanisms of angiotensin II-salt hypertension?

Hypertension caused by chronic infusion of angiotensin II (Ang II) in experimental animals is likely to be mediated, at least in part, by an elevation of ongoing sympathetic nerve activity (SNA). However, the contribution of SNA relative to non-neural mechanisms in mediating Ang II-induced hypertension is an area of intense debate and remains unresolved. We hypothesize that sympathoexcitatory actions of Ang II are directly related to the level of dietary salt intake. To test this hypothesis, chronically instrumented rats were placed on a 0.1 (low), 0.4 (normal) or 2.0% NaCl diet (high) and, following a control period, administered Ang II (150 ng kg(1) min(1), s.c.) for 10-14 days. The hypertensive response to Ang II was greatest in rats on the high-salt diet (Ang II-salt hypertension), which was associated with increased 'whole body' sympathetic activity as measured by noradrenaline spillover and ganglionic blockade. Indirect and direct measures of organ-specific SNA revealed a distinct 'sympathetic signature' in Ang II-salt rats characterized by increased SNA to the splanchnic vascular bed, transiently reduced renal SNA and no change in SNA to the hindlimbs. Electrophysiological experiments indicate that increased sympathetic outflow in Ang II-salt rats is unlikely to involve activation of rostral ventrolateral medulla (RVLM) vasomotor neurons with barosensitive cardiac rhythmic discharge. Instead, another set of RVLM neurons that discharge in discrete bursts have exaggerated spontaneous activity in rats with Ang II-salt hypertension. Although their discharge is not cardiac rhythmic at resting levels of arterial pressure, it nevertheless appears to be barosensitive. Therefore, these burst-firing RVLM neurons presumably serve a vasomotor function, consistent with their having axonal projections to the spinal cord. Bursting discharge of these neurons is respiratory rhythmic and driven by the respiratory network. Given that splanchnic SNA is strongly coupled to respiration, we hypothesize that enhanced central respiratory-vasomotor neuron coupling in the RVLM could be an important mechanism that contributes to exaggerated splanchnic sympathetic outflow in Ang II-salt hypertension. This hypothesis remains to be tested directly in future investigations.

Chronic angiotensin II infusion causes differential responses in regional sympathetic nerve activity in rats

Angiotensin II (AngII)-induced hypertension in experimental animals has been proposed to be attributed in part to activation of the sympathetic nervous system. This sympathetic activation appears to be accentuated in animals consuming a high-salt diet (AngII-salt hypertension). However, accurate quantification of sympathetic activity is difficult, and controversy remains. It is particularly important to ask which are the critical vascular beds targeted by increased sympathetic nerve activity (SNA) in AngII-salt hypertension. To address this issue, mean arterial pressure and renal SNA or lumbar SNA were continuously recorded during a 5-day control period, 11 days of AngII (150 ng/kg per minute, SC), and a 5-day recovery period in conscious rats on a high-salt (2% NaCl) diet. Although mean arterial pressure reached a new steady-state level of 30 to 35 mm Hg above control levels by the end of the AngII period, renal SNA decreased by 40% during the first 7 days of AngII and then returned toward control levels by day 10 of AngII. In contrast, lumbar SNA remained at control levels throughout the AngII period. In another experiment we measured hindlimb norepinephrine spillover in conscious rats on normal (0.4%) or high- (2.0%) salt diets before and during 14 days of AngII administration. AngII had no significant affect on hindlimb norepinephrine spillover in either group. We conclude that chronic AngII modulates renal and lumbar SNAs differentially in rats consuming a high-salt diet and that AngII-salt hypertension in the rat is not caused by increased SNA to the renal or hindlimb vascular beds.

Region-specific changes in sympathetic nerve activity in angiotensin II-salt hypertension in the rat

It is now well accepted that many forms of experimental hypertension and human essential hypertension are caused by increased activity of the sympathetic nervous system. However, the role of region-specific changes in sympathetic nerve activity (SNA) in the pathogenesis of hypertension has been difficult to determine because methods for chronic measurement of SNA in conscious animals have not been available. We have recently combined indirect, and continuous and chronic direct, assessment of region-specific SNA to characterize hypertension produced by administration of angiotensin II (Ang II) to rats consuming a high-salt diet (Ang II-salt hypertension). Angiotensin II increases whole-body noradrenaline (NA) spillover and depressor responses to ganglionic blockade in rats consuming a high-salt diet, but not in rats on a normal-salt diet. Despite this evidence for increased 'whole-body SNA' in Ang II-salt hypertensive rats, renal SNA is decreased in this model and renal denervation does not attenuate the steady-state level of arterial pressure. In addition, neither lumbar SNA, which largely targets skeletal muscle, nor hindlimb NA spillover is changed from control levels in Ang II-salt hypertensive rats. However, surgical denervation of the splanchnic vascular bed attenuates/abolishes the increase in arterial pressure and total peripheral resistance, as well as the decrease in vascular capacitance, observed in Ang II-salt hypertensive rats. We hypothesize that the 'sympathetic signature' of Ang II-salt hypertension is characterized by increased splanchnic SNA, no change in skeletal muscle SNA and decreased renal SNA, and this sympathetic signature creates unique haemodynamic changes capable of producing sustained hypertension.

A high-salt diet does not influence renal sympathetic nerve activity: a direct telemetric investigation

The importance of dietary salt in the development of hypertension has long been a source of controversy. Recent studies suggest a combination of high-salt and ANG II infusion may increase sympathetic drive; however, the effect of a change in dietary salt alone is unclear. Using telemetry, we recorded renal sympathetic nerve activity (RSNA), arterial pressure (MAP), and heart rate (HR) in seven New Zealand white rabbits before and during a 6-day period of increased salt intake (normal NaCl 0.5 g·kg−1·day−1, high NaCl 2.5 g·kg−1·day−1) and a second group of seven rabbits with normal salt intake throughout. The responses to stressful stimuli encountered in the laboratory were recorded and compared with rest in control and high-salt groups. Resting MAP, HR, and RSNA were not significantly altered with high salt intake [88 ± 5 vs. 91 ± 6 mmHg; 251 ± 8 vs. 244 ± 9 beats per minute (bpm); 9.7 ± and 1.2 vs. 10.8 ± 1.7 normalized units (nu)] despite significant reductions in plasma renin activity (1.88 ± 0.18 vs. 1.27 ± 0.15 nmol ANG I·l−1·h−1; P < 0.05) and ANG II (7.5 ± 1.2 vs. 4.3 ± 0.8 pmol/l). Increasing levels of stressful stimuli (resting in home cage, containment in box, handling, and nasopharyngeal activation) in animals on a normal salt diet caused graded increases in MAP (89 ± 2 mmHg, 95 ± 2 mmHg, 107 ± 4 mmHg, and 122 ± 5 mmHg, respectively) and RSNA (9.7 ± 0.9 nu; 11.8 ± 2.7 nu; 31.4 ± 3.7 nu; 100 nu) but not HR (245 ± 8 bpm; 234 ± 8 bpm; 262 ± 9 bpm; 36 ± 5 bpm). High dietary salt did not significantly alter the responses to stress. We conclude that a 6-day period of high salt intake does not alter the level of RSNA, with non-neural mechanisms primarily responsible for the observed renin-angiotensin system suppression.

Limb venous tone and responsiveness in hypertensive humans

Hypertensive (HTN) animal models demonstrate lower venous compliance as well as increased venous tone and responsiveness compared with normotensive (NTN) controls. However, the extent to which findings in experimental animals can be extended to humans is unknown. Forearm and calf venous compliance were quantified in 9 NTN (23 +/- 1 yr) and 9 HTN (24 +/- 1 yr) men at baseline, after administration of nitroglycerin (NTG), during a cold pressor test (CP), and post-handgrip exercise ischemia (PEI). Individual pressure-volume relationships from a cuff deflation protocol (1 mmHg/s) were modeled with a quadratic regression. Regression parameters beta(1) and beta(2) were used to calculate compliance. A one-way ANOVA was used to compare the beta parameters and a repeated-measures ANOVA was used to compare volumes across all pressures (between groups at baseline and within groups during perturbations). Limb venous compliance was similar between groups (forearm: NTN beta(1) = 0.11 +/- 0.01 and beta(2) = -0.00097 +/- 0.0001, HTN beta(1) = 0.10 +/- 0.01 and beta(2) = -0.00088 +/- 0.0001; calf: NTN beta(1) = 0.12 +/- 0.01 and beta(2) = -0.00102 +/- 0.0001, HTN beta(1) = 0.11 +/- 0.01 and beta(2) = -0.00090 +/- 0.0001). However, at baseline, volume across all pressures (i.e., capacitance) was lower in the forearm (P < or = 0.01) and tended to be lower in the calf (P = 0.08) in HTN subjects. Venous compliance was not altered by any perturbation in either group. Forearm volume was increased during NTG in HTN subjects only. While venous compliance was similar between NTN and HTN adults, HTN adults have lower forearm venous capacitance (volume) which is increased with NTG. These data suggest that young HTN adults may have augmented venous smooth muscle tone compared with NTN controls.

Whole body norepinephrine kinetics in ANG II-salt hypertension in the rat

The purpose of this study was to investigate total body norepinephrine (NE) kinetics as an index of global sympathetic nervous system (SNS) outflow in a rat model of chronic ANG II-salt hypertension. Male Sprague-Dawley rats fed a 0.4% (normal salt, NS) or 2% (HS) NaCl diet were instrumented with arterial and venous catheters. After 5 days of recovery and a 3-day control period, ANG II (150 ng.kg(-1).min(-1)) was given subcutaneously by minipump for 14 days. Plasma NE levels and total body NE spillover and clearance were determined on control day 3 and ANG II infusion days 7 and 14 using radioisotope dilution principles. To perform this analysis, 3H-NE and NE were measured in arterial plasma after a 90-min infusion of tracer amounts of 3H-NE. Mean arterial pressure (MAP) was similar during the control period in NS and HS rats; however, MAP increased to a higher level in HS rats. During the control period, plasma NE tended to be lower in rats on HS, whereas NE clearance tended to be higher in HS rats. As a result NE spillover was similar in NS and HS rats during the control period. In NS rats, plasma NE, NE spillover, and NE clearance were unchanged by ANG II. In contrast, in rats on the HS diet, plasma NE and NE spillover increased during ANG II infusion, whereas NE clearance was unchanged. In conclusion, a HS diet alone or ANG II infusion in animals fed NS do not affect global sympathetic outflow. However, the additional hypertensive response to ANG II in animals fed HS is accompanied by SNS activation.

Circulating angiotensin II and dietary salt: converging signals for neurogenic hypertension

Circulating angiotensin II (Ang II) combined with high salt intake increases sympathetic nerve activity (SNA) in some forms of hypertension. Ang II-induced increases in SNA are modest, delayed, and specific to certain vascular beds. The brain targets for circulating Ang II are neurons in the area postrema (AP), subfornical organ (SFO), and possibly other circumventricular organs. Ang II signaling is integrated with sodium-sensitive neurons in the SFO and/or organum vasculosum of the lamina terminalis (OVLT) and drives sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM) via the paraventricular nucleus (PVN). It is likely that, over time, new patterns of gene expression emerge within neurons of the SFO-PVN-RVLM pathway that transform their signaling properties. This transformation is critical in maintaining increased SNA. Identification of a novel gene supporting this process may provide new targets for treatment of neurogenic hypertension.

Splanchnic Circulation Is a Critical Neural Target in Angiotensin II Salt Hypertension in Rats

Chronic angiotensin II (Ang II) infusion, in rats fed high salt, engages the sympathetic nervous system to increase venomotor tone. The splanchnic sympathetic nervous system is the most important regulator of venous tone, indicating that splanchnic sympathetic nervous system activity may be increased in Ang II salt hypertension. We hypothesized that celiac ganglionectomy (CGx), to selectively disrupt sympathetic innervation to the splanchnic circulation, would attenuate arterial pressure (AP), and venous tone increases in Ang II salt hypertension. Rats fed 2% or 0.4% NaCl were instrumented to allow AP measurement by radiotelemetry at the same time as surgical CGx or sham operation. Ang II was delivered by minipump (150 ng/kg per minute) for 14 days. CGx reduced AP independent of salt diet during control. CGx markedly attenuated Ang II hypertension in rats on 2% NaCl but had little effect in rats fed 0.4% NaCl. To test the possibility that CGx exerted its effects via renal denervation, rats were subjected to the same protocol but received selective bilateral renal denervation. Renal denervation decreased AP during control but had no protective effect on Ang II hypertension and actually tended to exacerbate the pressor response. Finally, separate groups of rats underwent CGx or sham operation and were instrumented to allow repeated measures of mean circulatory filling pressure, an index of venous tone. In addition to attenuating Ang II salt hypertension, CGx completely prevented Ang II salt-induced increases in mean circulatory filling pressure and substantially attenuated depressor responses to acute ganglion blockade. We conclude that, in the presence of high salt, Ang II activates the splanchnic sympathetic nervous system to increase venomotor tone and AP.

Angiotensin II-based hypertension and the sympathetic nervous system: the role of dose and increased dietary salt in rabbits

There is accumulating evidence that angiotensin II may exert its hypertensive effect through increasing sympathetic drive. However, this action may be dependent on the dose of angiotensin II as well as salt intake. We determined the effect of different doses of angiotensin II and different levels of salt intake on neurogenic pressor activity. We also examined the effect of renal denervation. New Zealand White rabbits were instrumented to continuously measure arterial pressure. The depressor response to the ganglionic blocker pentolinium tartrate (5 mg kg(-1)) was used to assess pressor sympathetic drive on days 0, 7 and 21 of a 20 or 50 ng kg(-1) min(-1) continuous i.v. angiotensin II infusion. A 50 ng kg(-1) min(-1) infusion caused an immediate increase in pressure (23 +/- 5 mmHg), whereas a 20 ng kg(-1) min(-1) infusion caused a slow increase in pressure, peaking by day 12 (17 +/- 4 mmHg). The ganglionic blockade profiles indicated sympathoinhibition in the 50 ng kg(-1) min(-1) group by day 7 and sympathoinhibition in the 20 ng kg(-1) min(-1) group at day 21, corresponding to the development of hypertension. Animals receiving increased dietary salt (0.9% NaCl in drinking water), however, showed a similar slow increase in pressure with 20 ng kg(-1) min(-1) angiotensin II (16 +/- 5 mmHg) but no sympathoinhibition at day 21. Bilateral renal denervation delayed the onset but not the extent of hypertension in this group. We conclude that different doses of angiotensin II produce distinct profiles of hypertension and associated changes in pressor sympathetic drive and that increased dietary salt intake disrupts the normal sympathoinhibitory response to angiotensin II-based hypertension.

Investigation of the mechanisms by which chronic infusion of an acutely subpressor dose of angiotensin II induces hypertension

The mechanisms by which chronic infusion of an initially subpressor low dose of angiotensin II (ANG II) causes a progressive and sustained hypertension remain unclear. In conscious sheep ( n = 6), intravenous infusion of ANG II (2 μg/h) gradually increased mean arterial pressure (MAP) from 82 ± 3 to 96 ± 5 mmHg over 7 days ( P < 0.001). This was accompanied by peripheral vasoconstriction; total peripheral conductance decreased from 44.6 ± 6.4 to 38.2 ± 6.7 ml·min−1·mmHg−1 ( P < 0.001). Cardiac output and heart rate were unchanged. In the regional circulation, mesenteric, renal, and iliac conductances decreased but blood flows were unchanged. There was no coronary vasoconstriction, and coronary blood flow increased. Ganglion blockade (125 mg/h hexamethonium for 4 h) reduced MAP by 13 ± 1 mmHg in the control period and by 7 ± 2 mmHg on day 8 of ANG II treatment. Inhibition of central AT1 receptors by intracerebroventricular infusion of losartan (1 mg/h for 3 h) had no effect on MAP in the control period or after 7 days of ANG II infusion. Pressor responsiveness to incremental doses of intravenous ANG II (5, 10, 20 μg/h, each for 15 min) was unchanged after 7 days of ANG II infusion. ANG II caused no sodium or water retention. In summary, hypertension due to infusion of a low dose of ANG II was accompanied by generalized peripheral vasoconstriction. Indirect evidence suggested that the hypertension was not neurogenic, but measurement of sympathetic nerve activity is required to confirm this conclusion. There was no evidence for a role for central angiotensinergic mechanisms, increased pressor responsiveness to ANG II, or sodium and fluid retention.

Increased sympathetic venoconstriction and reactivity to norepinephrine in mesenteric veins in anesthetized DOCA-salt hypertensive rats

Increased sympathetic nervous activity (SNA) elevates venomotor tone in deoxycorticosterone acetate (DOCA)-salt hypertension. We studied the mechanisms by which the SNA increases venomotor tone in DOCA-salt hypertension by making in situ intracellular recordings of venous smooth muscle cell (VSMC) membrane potential (E(m)) and measurement of outside diameter (OD) in mesenteric veins (MV) and mesenteric arteries (MA) of anesthetized rats. We also studied norepinephrine (NE)- and endothelin-1 (ET-1)-induced increases in MA or MV perfusion pressure (PP) in vitro. E(m) in DOCA-salt MV was depolarized compared with sham MV. Prazosin hyperpolarized VSMC E(m) in DOCA-salt but not in sham MV. NE concentration-response curves (CRCs) for OD decreases in MV from DOCA-salt rats were left-shifted with an increased maximum response (E(max)) compared with sham MV. NE CRCs for OD decreases in MA were right-shifted with reduced E(max) in DOCA-salt compared with sham rats. ET-1 CRCs were similar in DOCA-salt and sham MV but were right-shifted with reduced E(max) in DOCA-salt MA. NE CRCs for MAPP increases were left-shifted without a change in E(max) in DOCA-salt rats. NE did not change MVPP. MAPP and MVPP for ET-1 CRCs were similar in sham and DOCA-salt rats, but E(max) for MAPP was reduced in DOCA-salt rats. Hematoxylin staining revealed hypertrophy in DOCA-salt MA but not in MV. We conclude that there is increased reactivity to NE released from the sympathetic nervous system in DOCA-salt MV that causes VSMC depolarization and increased venomotor tone. In DOCA-salt rats, in vivo ET-1 reactivity is maintained in MV, but reduced in MA.