Dietary sodium intake modulates renal excretory responses to intrarenal angiotensin (1-7) administration in anesthetized rats

Effects of intrarenal angiotensin 1-7 infusion on renal haemodynamic and excretory function in anaesthetised two-kidney one-clip and deoxycorticosterone acetate-salt hypertensive rats

NEW FINDINGS

What is the central question of this study? Are renal functional responses to intrarenal angiotensin 1-7 (Ang (1-7)) infusion dependent on the level of the endogenous renin-angiotensin system (RAS) in the two-kidney one-clip (2K1C) and deoxycorticosterone acetate (DOCA)-salt animal models of hypertension? What is the main finding and its importance? The renal actions of Ang (1-7) are dependent on the relative endogenous levels of each arm of the classical angiotensin II-angiotensin II type 1 receptor (AT1 R) axis and those of the Ang (1-7)-Mas receptor axis. These findings support the hypothesis that a balance exists between the intrarenal classical and novel arms of the RAS, and in particular the relative abundance of AT1 R to Mas receptor, which may to a large extent determine the renal excretory response to Ang (1-7) infusion.

ABSTRACT

This study investigated the action of angiotensin 1-7 (Ang (1-7)) on renal haemodynamic and excretory function in the two-kidney one-clip (2K1C) and deoxycorticosterone acetate (DOCA)-salt rat models of hypertension, in which the endogenous renin-angiotensin system (RAS) activity was likely to be raised or lowered, respectively. Rats were anaesthetised and prepared for the measurement of mean arterial pressure and kidney function during renal interstitial infusion of Ang (1-7) or saline. Kidney tissue concentrations of angiotensin II (Ang II) and Ang (1-7) were determined. Intrarenal infusion of Ang (1-7) into the clipped kidney of 2K1C rats increased urine flow (UV), absolute (UNa V) and fractional sodium (FENa ) excretions by 110%, 214% and 147%, respectively. Renal Ang II concentrations of the clipped kidney were increased with no major changes in Ang (1-7) concentration. By contrast, Ang (1-7) infusion decreased UV, UNa V, and FENa by 27%, 24% and 21%, respectively in the non-clipped kidney in which tissue Ang (1-7) concentrations were increased, but renal Ang II concentrations were unchanged compared to sham animals. Ang (1-7) infusion in DOCA-salt rats had minimal effects on glomerular filtration rate but significantly decreased UV, UNa V and FENa by ∼30%. Renal Ang (1-7) concentrations were higher and Ang II concentrations were lower in DOCA-salt rats compared to sham rats. These findings demonstrate that the intrarenal infusion of exogenous Ang (1-7) elicits different renal excretory responses the magnitude of which is dependent on the balance between the endogenous renal Ang II-AT1 receptor axis and Ang (1-7)-Mas receptor axis.

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

Excessive dietary salt promotes neuroinflammation to worsen retinopathy in mice with streptozotocin-induced diabetes

OBJECTIVE

Diabetic retinopathy (DR) includes vascular and neural tissue injury. Persistent low-grade inflammation may contribute to DR. Increased salt intake has been shown to promote autoimmunity in the brain. This study determined the role of salt intake in DR development.

METHODS

Eight-week-old C57BL/6 J male mice received streptozotocin to induce diabetes. Diabetic or non-diabetic mice were fed a diet containing normal, low and high amounts of salt. The retinal function, structure and inflammatory response were determined 8 weeks after the establishment of diabetes. Interleukin (IL)-1β or a NLR family pyrin domain containing 3 (NLRP3) inhibitor was injected intravitreally and the retinal changes were evaluated.

RESULTS

A high salt diet worsened the functional and structural damage of retinal cells and increased IL-1β in the retina of diabetic mice. IL-1β injection impaired the function of photoreceptors and retinal structure in the diabetic mice. Blocking NLRP3 inhibited IL-1β increase in the mouse bone marrow macrophages cultured in high sodium medium. NLRP3 inhibition attenuated retinal injury of diabetic mice on high salt diet. A low-salt diet also triggered inflammation and cell damage in the retina of diabetic mice but at a lower grade than those induced by high salt diet. A low or high salt diet for 8 weeks did not induce inflammation or cell injury in the retina of mice without diabetes.

CONCLUSION

These results indicate that high salt intake has deleterious effects in DR development through NLRP3 inflammasome activation and the subsequent production of IL-1β. Limiting salt intake may not attenuate DR development.

The renal excretory responses to acute renal interstitial angiotensin (1-7) infusion in anaesthetised spontaneously hypertensive rats

This study investigated the impact of intrarenal angiotensin 1-7 (Ang [1-7]) infusion on renal excretory function in a rat model of hypertension. Eleven-week-old spontaneously hypertensive rats (SHRs, n = 7) and Han Wistar controls (NCR, n = 7) were anaesthetised with sodium pentobarbital (60 mg/kg i.p.) and prepared for the measurement of mean arterial pressure (MAP) and left renal function during renal interstitial infusion of Ang (1-7) (50 ng/min). The kidneys were harvested, the renal cortex and medulla separated, prepared for measurement of Ang II and Ang (1-7) and Western blot determination of AT1 and Mas receptor protein expression. MAP, glomerular filtration rate (GFR), urine flow (UF) and absolute sodium excretion (UNaV) were 109 ± 16 mmHg, 4.4 ± 1.0 mL/min/kg, 102 ± 16 µL/min/kg and 16 ± 3 µmol/min/kg, respectively in the NCR and 172 ± 24 mmHg, 3.4 ± 0.7 mL/min/kg, 58 ± 30 μL/min/kg and 8.6 ± 4.8 μmol/min/kg respectively in the SHR. Ang (1-7) increased UF (31%), UNa V (50%) and fractional sodium excretion (FENa+ ) (22%) in the NCR group (all p < 0.05) but had no effect on GFR in either group. The magnitudes of the Ang (1-7)-induced increases in UF and UNa V were significantly blunted in the SHR group (model × drug p < 0.05). The renal cortical AT1: Mas receptor expression ratio was significantly higher in the SHR group (p < 0.05) but renal Ang II and Ang (1-7) levels were not statistically different between groups. The Ang (1-7)-induced increases in sodium and water excretion were impaired in the SHR group in the context of an unstimulated RAS. The decrease in responsiveness of the SHR kidney to Ang (1-7) appears to be associated with higher levels of AT1 receptor expression in the renal cortex.

A Low-Sodium Diet Boosts Ang (1-7) Production and NO-cGMP Bioavailability to Reduce Edema and Enhance Survival in Experimental Heart Failure

Sodium restriction is often recommended in heart failure (HF) to block symptomatic edema, despite limited evidence for benefit. However, a low-sodium diet (LSD) activates the classical renin-angiotensin-aldosterone system (RAAS), which may adversely affect HF progression and mortality in patients with dilated cardiomyopathy (DCM). We performed a randomized, blinded pre-clinical trial to compare the effects of a normal (human-equivalent) sodium diet and a LSD on HF progression in a normotensive model of DCM in mice that has translational relevance to human HF. The LSD reduced HF progression by suppressing the development of pleural effusions (p < 0.01), blocking pathological increases in systemic extracellular water (p < 0.001) and prolonging median survival (15%, p < 0.01). The LSD activated the classical RAAS by increasing plasma renin activity, angiotensin II and aldosterone levels. However, the LSD also significantly up-elevated the counter-regulatory RAAS by boosting plasma angiotensin converting enzyme 2 (ACE2) and angiotensin (1-7) levels, promoting nitric oxide bioavailability and stimulating 3'-5'-cyclic guanosine monophosphate (cGMP) production. Plasma HF biomarkers associated with poor outcomes, such as B-type natriuretic peptide and neprilysin were decreased by a LSD. Cardiac systolic function, blood pressure and renal function were not affected. Although a LSD activates the classical RAAS system, we conclude that the LSD delayed HF progression and mortality in experimental DCM, in part through protective stimulation of the counter-regulatory RAAS to increase plasma ACE2 and angiotensin (1-7) levels, nitric oxide bioavailability and cGMP production.

Determinants of renal oxygen metabolism during low Na+ diet: effect of angiotensin II AT1 and aldosterone receptor blockade

KEY POINTS

Reducing Na⁺ intake reduces the partial pressure of oxygen in the renal cortex and activates the renin-angiotensin-aldosterone system. In the absence of high blood pressure, these consequences of dietary Na⁺ reduction may be detrimental for the kidney. In a normotensive animal experimental model, reducing Na⁺ intake for 2 weeks increased renal oxygen consumption, which was normalized by mineralocorticoid receptor blockade. Furthermore, blockade of the angiotensin II AT₁ receptor restored cortical partial pressure of oxygen by improving oxygen delivery. This shows that increased activity of the renin-angiotensin-aldosterone system contributes to increased oxygen metabolism in the kidney after 2 weeks of a low Na⁺ diet. The results provide insights into dietary Na⁺ restriction in the absence of high blood pressure, and its consequences for the kidney.

ABSTRACT

Reduced Na⁺ intake reduces the P O 2 (partial pressure of oxygen) in the renal cortex. Upon reduced Na⁺ intake, reabsorption along the nephron is adjusted with activation of the renin-angiotensin-aldosterone system (RAAS). Thus, we studied the effect of reduced Na⁺ intake on renal oxygen homeostasis and function in rats, and the impact of intrarenal angiotensin II AT₁ receptor blockade using candesartan and mineralocorticoid receptor blockade using canrenoic acid potassium salt (CAP). Male Sprague-Dawley rats were fed standard rat chow containing normal (0.25%) and low (0.025%) Na⁺ for 2 weeks. The animals were anaesthetized (thiobutabarbital 120 mg kg^-1 ) and surgically prepared for kidney oxygen metabolism and function studies before and after acute intrarenal arterial infusion of candesartan (4.2 μg kg^-1 ) or intravenous infusion of CAP (20 mg kg^-1 ). Baseline mean arterial pressure and renal blood flow were similar in both dietary groups. Fractional Na⁺ excretion and cortical oxygen tension were lower and renal oxygen consumption was higher in low Na⁺ groups. Neither candesartan nor CAP affected arterial pressure. Renal blood flow and cortical oxygen tension increased in both groups after candesartan in the low Na⁺ group. Fractional Na⁺ excretion was increased and oxygen consumption reduced in the low Na⁺ group after CAP. These results suggest that blockade of angiotensin II AT₁ receptors has a major impact upon oxygen delivery during normal and low Na⁺ conditions, while aldosterone receptors mainly affect oxygen metabolism following 2 weeks of a low Na⁺ diet.

Kidney dopamine D1-like receptors and angiotensin 1-7 interaction inhibits renal Na+ transporters

The role of dopamine D₁-like receptors (DR) in the regulation of renal Na⁺ transporters, natriuresis, and blood pressure is well established. However, the involvement of the angiotensin 1-7 (ANG 1-7)-Mas receptor in the regulation of Na⁺ balance and blood pressure is not clear. The present study aimed to investigate the hypothesis that ANG 1-7 can regulate Na⁺ homeostasis by modulating the renal dopamine system. Sprague-Dawley rats were infused with saline alone (vehicle) or saline with ANG 1-7, ANG 1-7 antagonist A-779, DR agonist SKF38393, and antagonist SCH23390. Infusion of ANG 1-7 caused significant natriuresis and diuresis compared with saline alone. Both natriuresis and diuresis were blocked by A-779 and SCH23390. SKF38393 caused a significant, SCH23390-sensitive natriuresis and diuresis, and A-779 had no effect on the SKF38393 response. Concomitant infusion of ANG 1-7 and SKF38393 did not show a cumulative effect compared with either agonist alone. Treatment of renal proximal tubules with ANG 1-7 or SKF38393 caused a significant decrease in Na⁺-K⁺-ATPase and Na⁺/H⁺ exchanger isoform 3 activity. While SCH23390 blocked both ANG 1-7- and SKF38393-induced inhibition, the DR response was not sensitive to A-779. Additionally, ANG 1-7 activated PKG, enhanced tyrosine hydroxylase activity via Ser⁴⁰ phosphorylation, and increased renal dopamine production. These data suggest that ANG 1-7, via PKG, enhances tyrosine hydroxylase activity, which increases renal dopamine production and activation of DR and subsequent natriuresis. This study provides evidence for a unidirectional functional interaction between two G protein-coupled receptors to regulate renal Na⁺ transporters and induce natriuresis.

Acute intrarenal angiotensin (1-7) infusion decreases diabetes-induced glomerular hyperfiltration but increases kidney oxygen consumption in the rat

AIM

Common kidney alterations early after the onset of insulinopenic diabetes include glomerular hyperfiltration, increased oxygen consumption and tissue hypoxia. Increased activity of the renin-angiotensin-aldosterone system (RAAS) has been implicated in most of these early alterations. The RAAS peptide angiotensin (1-7) has the potential to modulate RAAS-mediated alterations in kidney function. Thus, the aim of the present study was to determine the acute effects of angiotensin (1-7) in the kidney of insulinopenic type 1 diabetic rat and the results compared to that of normoglycaemic controls.

METHODS

Renal haemodynamics and oxygen homeostasis were measured 3 weeks after administration of streptozotocin before and after acute intrarenal infusion of angiotensin (1-7) at a dose of 400 ng min^-1 .

RESULTS

Arterial pressure and renal blood flow were similar between groups and not affected by exogenous angiotensin (1-7). Diabetics presented with glomerular hyperfiltration, increased urinary sodium excretion and elevated kidney oxygen consumption. Angiotensin (1-7) infusion normalized glomerular filtration, increased urinary sodium excretion, decreased proximal tubular reabsorption, and elevated kidney oxygen consumption even further. The latter resulting in tubular electrolyte transport inefficiency. Angiotensin (1-7) did not affect tissue oxygen tension and had no significant effects in controls on any of the measured parameters.

CONCLUSION

Diabetes results in increased responsiveness to elevated levels of angiotensin (1-7) which is manifested as inhibition of tubular sodium transport and normalization of glomerular filtration. Furthermore, elevated angiotensin (1-7) levels increase kidney oxygen consumption in the diabetic kidney even further which affects tubular electrolyte transport efficiency negatively.

Inappropriate activity of local renin-angiotensin-aldosterone system during high salt intake: impact on the cardio-renal axis

Although there is a general agreement on the recommendation for reduced salt intake as a public health issue, the mechanism by which high salt intake triggers pathological effects on the cardio-renal axis is not completely understood. Emerging evidence indicates that the renin-angiotensin-aldosterone system (RAAS) is the main target of high Na⁺ intake. An inappropriate activation of tissue RAAS may lead to hypertension and organ damage. We reviewed the impact of high salt intake on the RAAS on the cardio-renal axis highlighting the molecular pathways that leads to injury effects. We also provide an assessment of recent observational studies related to the consequences of non-osmotically active Na⁺ accumulation, breaking the paradigm that high salt intake necessarily increases plasma Na⁺ concentration promoting water retention

Modulating Role of Ang1-7 in Control of Blood Pressure and Renal Function in AngII-infused Hypertensive Rats

BACKGROUND

Indirect evidence suggests that angiotensin 1-7 (Ang1-7) may counterbalance prohypertensive actions of angiotensin II (AngII), via activation of vascular and/or renal tubular receptors to cause vasodilation and natriuresis/diuresis. We examined if Ang1-7 would attenuate the development of hypertension, renal vasoconstriction, and decreased natriuresis in AngII-infused rats and evaluated the mechanisms involved.

METHODS

AngII, alone or with Ang1-7, was infused to conscious Sprague-Dawley rats for 13 days and systolic blood pressure (SBP) and renal excretion were repeatedly determined. In anesthetized rats, acute actions of Ang1-7 and effects of blockade of angiotensin AT1 or Mas receptors (candesartan or A-779) were studied.

RESULTS

Chronic AngII infusion increased SBP from 143 ± 4 to 195 ± 6 mm Hg. With Ang1-7 co-infused, SBP increased from 133 ± 5 to 161 ± 5 mm Hg (increase reduced, P < 0.002); concurrent increases in urine flow (V) and sodium excretion (UNaV) were greater. In anesthetized normotensive or AngII-induced hypertensive rats, Ang1-7 infusion transiently increased mean arterial pressure (MABP), transiently decreased renal blood flow (RBF), and caused increases in UNaV and V. In normotensive rats, candesartan prevented the Ang1-7-induced increases in MABP and UNaV and the decrease in RBF. In anesthetized normotensive, rats intravenous A-779 increased MABP (114 ± 5 to 120 ± 5 mm Hg, P < 0.03) and urine flow. Surprisingly, these changes were not observed with A-779 applied during background Ang1-7 infusion.

CONCLUSIONS

The results suggest that in AngII-dependent hypertension, Ang1-7 deficit contributes to sodium and fluid retention and thereby to BP elevation; a correction by Ang1-7 infusion seems mediated by AT1 and not Mas receptors.

The ACE2/Angiotensin-(1-7)/MAS Axis of the Renin-Angiotensin System: Focus on Angiotensin-(1-7)

The renin-angiotensin system (RAS) is a key player in the control of the cardiovascular system and hydroelectrolyte balance, with an influence on organs and functions throughout the body. The classical view of this system saw it as a sequence of many enzymatic steps that culminate in the production of a single biologically active metabolite, the octapeptide angiotensin (ANG) II, by the angiotensin converting enzyme (ACE). The past two decades have revealed new functions for some of the intermediate products, beyond their roles as substrates along the classical route. They may be processed in alternative ways by enzymes such as the ACE homolog ACE2. One effect is to establish a second axis through ACE2/ANG-(1-7)/MAS, whose end point is the metabolite ANG-(1-7). ACE2 and other enzymes can form ANG-(1-7) directly or indirectly from either the decapeptide ANG I or from ANG II. In many cases, this second axis appears to counteract or modulate the effects of the classical axis. ANG-(1-7) itself acts on the receptor MAS to influence a range of mechanisms in the heart, kidney, brain, and other tissues. This review highlights the current knowledge about the roles of ANG-(1-7) in physiology and disease, with particular emphasis on the brain.

Intrarenal Mas and AT1 receptors play a role in mediating the excretory actions of renal interstitial angiotensin-(1-7) infusion in anaesthetized rats

NEW FINDINGS

What is the central question of this study? Dietary sodium manipulation alters the magnitude of angiotensin-(1-7) [Ang-(1-7)]-induced natriuresis. The present study sought to determine whether this was related to relative changes in the activity of intrarenal Mas and/or AT₁ receptors. What is the main finding and its importance? Angiotensin-(1-7)-induced diuresis and natriuresis is mediated by intrarenal Mas receptors. However, intrarenal AT₁ receptor blockade also had an inhibitory effect on Ang-(1-7)-induced natriuresis and diuresis. Thus, Ang-(1-7)-induced increases in sodium and water excretion are dependent upon functional Mas and AT₁ receptors. We investigated whether angiotensin-(1-7) [Ang-(1-7)]-induced renal haemodynamic and excretory actions were solely dependent upon intrarenal Mas receptor activation or required functional angiotensin II type 1 (AT₁ ) receptors. The renin-angiotensin system was enhanced in anaesthetized rats by prior manipulation of dietary sodium intake. Angiotensin-(1-7) and AT₁ and Mas receptor antagonists were infused into the kidney at the corticomedullary border. Mas receptor expression was measured in the kidney. Mean arterial pressure, urine flow and fractional sodium excretion were 93 ± 4 mmHg, 46.1 ± 15.7 μl min^-1  kg^-1 and 1.4 ± 0.3%, respectively, in the normal-sodium group and 91 ± 2 mmHg, 19.1 ± 3.3 μl min^-1  kg^-1 and 0.7 ± 0.2%, respectively, in the low-sodium group. Angiotensin-(1-7) infusion had no effect on mean arterial pressure in rats receiving a normal-sodium diet but decreased it by 4 ± 5% in rats receiving a low-sodium diet (P < 0.05). Interstitial Ang-(1-7) infusion increased urine flow twofold and fractional sodium excretion threefold (P < 0.05) in rats receiving a normal-sodium diet and to a greater extent, approximately three- and fourfold, respectively, in rats receiving the low-sodium diet (both P < 0.05). Angiotensin-(1-7)-induced increases in urine flow and fractional sodium excretion were absent in both dietary groups during intrarenal AT₁ or Mas receptor inhibition after either losartan or A-779, respectively. Thus, AT₁ receptor activation, as well as Mas receptor activation, plays an essential role in mediating Ang-(1-7)-induced natriuresis and diuresis. Whether this is because Ang-(1-7) partly antagonizes AT₁ receptors or whether Ang-(1-7)-induced natriuresis is mediated through AT₁ -Mas receptor dimerization remains unclear.

Assessment of renal function in the anaesthetised rat following injection of superparamagnetic iron oxide nanoparticles

A recent study showed that a significant fall in mean arterial pressure (MAP) occurred following intravenous injection of two novel superparamagnetic iron oxide nanoparticles (SPIONs), MF66 and OD15. To assess if this was caused by excessive glomerular clearance, the effect of both particles on renal function was studied. Experiments were performed on sodium pentobarbital anaesthetised male Wistar rats (250–350 g). Twenty-minute urine clearances were taken followed by an i.v. bolus of MF66, OD15 (2 mg·kg–1), or dH2O (0.4 mL·kg–1). MF6 or OD15 injection resulted in a significant transient drop in MAP and renal blood flow by approximately 33% and 50% (P < 0.05). The absolute excretion of sodium was significantly increased (P < 0.05) by almost 80% and 70% following OD15 and MF66, respectively. Similarly, fractional excretion of sodium was increased by almost 80% and 60% following OD15 and MF66, respectively. The glomerular filtration rate was not significantly affected, but urine flow increased nonsignificantly by approximately 50% and 66% following i.v. injection of OD15 and MF66, respectively. SPIONs produce a decrease in blood pressure and a natriuresis; however, the rate of fluid filtration in the kidney was not significantly affected.

The intrarenal renin-angiotensin system in hypertension

The renin-angiotensin system (RAS) is a well-studied hormonal cascade controlling fluid and electrolyte balance and blood pressure through systemic actions. The classical RAS includes renin, an enzyme catalyzing the conversion of angiotensinogen to angiotensin (Ang) I, followed by angiotensin-converting enzyme (ACE) cleavage of Ang I to II, and activation of AT1 receptors, which are responsible for all RAS biologic actions. Recent discoveries have transformed the RAS into a far more complex system with several new pathways: the (des-aspartyl(1))-Ang II (Ang III)/AT2 receptor pathway, the ACE-2/Ang (1-7)/Mas receptor pathway, and the prorenin-renin/prorenin receptor/mitogen-activated protein kinase pathway, among others. Although the classical RAS pathway induces Na(+) reabsorption and increases blood pressure, several new pathways constitute a natriuretic/vasodilator arm of the system, opposing detrimental actions of Ang II through Ang II type 1 receptors. Instead of a simple circulating RAS, several independently functioning tissue RASs exist, the most important of which is the intrarenal RAS. Several physiological characteristics of the intrarenal RAS differ from those of the circulating RAS, autoamplifying the activity of the intrarenal RAS and leading to hypertension. This review will update current knowledge on the RAS with particular attention to the intrarenal RAS and its role in the pathophysiology of hypertension.

Intrarenal bradykinin elicits reno-renal reflex sympatho-excitation and renal nerve-dependent fluid retention

AIMS

The renal sensory nerves are importantly involved in the sympathetic regulation of cardiovascular and renal function. Two reno-renal reflexes are recognized, one in which activation of renal sensory nerves elicits a renal sympatho-inhibition, and one which causes a renal sympatho-excitation and about which little is known. This study investigated the role of bradykinin (BK) in engaging an excitatory reno-renal reflex.

METHODS

Rats were anaesthetized (chloralose/urethane) and prepared for the measurement of renal function or renal sympathetic nerve activity (RSNA). BK was infused into the cortico-medullary border of the ipsilateral kidney and the impact on contralateral renal function and RSNA evaluated.

RESULTS

Intrarenal infusion of BK at 3 × 10(-9) and 6 × 10(-9) g L(-1) had no effect on mean arterial pressure, at 104 ± 5 mmHg or glomerular filtration rate in either the ipsilateral or contralateral kidneys, at 4.31 ± 0.45 mL min(-1) kg(-1) . At the highest dose of BK, fractional sodium excretion (FENa) was 1.47% in the ipsilateral kidney and was significantly lower, at 0.64% (P < 0.05) in the contralateral kidney but this difference did not occur following ipsilateral renal denervation. Ipsilateral intrarenal infusion of BK at 3 × 10(-9) , 6 × 10(-9) and 1.2 × 10(-8) g L(-1) elicited dose-related increases (P < 0.05) in contralateral RSNA, reaching some 78% at the highest dose, but these responses were prevented by ipsilateral renal denervation.

CONCLUSIONS

Intrarenal infusion of BK produced an excitatory reno-renal reflex which was expressed as a renal nerve-dependent antinatriuresis in the contralateral kidney. The findings suggest that inflammatory mediators such as BK may be important in initiating a sympatho-excitation associated with renal and cardiovascular diseases.

The adrenergic regulation of proximal tubular Na⁺/H⁺ exchanger 3 in the rat

AIM

This study in the anaesthetized rat investigated how renal sympathetic nerve activity and catecholamine release influenced NHE3 abundance and activity in proximal tubular brush border membranes using both in vivo and in vitro approaches.

METHODS

Renal excretory function and brush border NHE3 abundance and activity were measured in rat kidneys which underwent renal denervation, renal nerve electrical stimulation and renal infusion of phenylephrine and the NHE3 inhibitor S1661. NHE3 activity and cell surface abundance were also measured in primary cultures of proximal tubular cells treated with noradrenaline and prazosin.

RESULTS

Acute renal denervation caused a natriuresis and diuresis, which occurred with a reduction in NHE3 abundance and activity in the brush border membranes. By contrast, low-level electrical stimulation of the renal innervation causing an antinatriuresis and antidiuresis increased NHE3 activity in the brush border membranes. Intrarenal infusion of phenylephrine caused an antinatriuresis and antidiuresis, while blockade of NHE3 activity, using local infusion of the blocker S1661, caused a natriuresis and diuresis. Exposure of primary cultures of proximal tubular cells to noradrenaline increased brush border NHE3 abundance and activity which was blocked by prior exposure to prazosin, indicating it as an α1 -adrenoceptor-mediated mechanism.

CONCLUSION

Together, these findings demonstrate that the renal sympathetic nerves not only have a direct action to modulate tubular sodium reabsorption via stimulation of the NHE transporter, but also have an indirect effect, whereby NHE3 abundance is increased within the brush border membrane, thereby increasing the capacity for fluid reabsorption.