Regulation of renal renin release

Shared genetic etiology and antagonistic relationship of plasma renin activity and systolic blood pressure in a Korean cohorts

Despite extensive studies on blood pressure, its genetic risk factors remain uncertain. Even one of the most researched blood pressure-related traits - renin - is not fully understood genetically. Here, we determine the genetic relationship and associated predisposition between blood pressure and baseline renin. In 8840 Korean individuals, we observed a strong negative genome-wide genetic correlation (r_g = -0.484) between systolic blood pressure (SBP) and plasma renin activity (PRA), suggesting that antagonistic genetic signals explain the variance in the two traits. We found 51 significant pleiotropic SNPs affecting the two traits, which could contribute to the Renin-Angiotensin-Aldosterone System (RAAS). Our findings provide insight into studies on RAAS by identifying the genome-wide relationship and susceptibility loci of SBP and PRA.

Angiotensin II Short-Loop Feedback: Is There a Role of Ang II for the Regulation of the Renin System In Vivo?

The activity of the renin-angiotensin-aldosterone system is triggered by the release of the protease renin from the kidneys, which in turn is controlled in the sense of negative feedback loops. It is widely assumed that Ang II (angiotensin II) directly inhibits renin expression and secretion via a short-loop feedback by an effect on renin-producing cells (RPCs) mediated by AT₁ (Ang II type 1) receptors. Because the concept of such a direct short-loop negative feedback control, which originates mostly from in vitro experiments, has not yet been systematically proven in vivo, we aimed to test the validity of this concept by studying the regulation of renin synthesis and secretion in mice lacking Ang II-AT₁ receptors on RPCs. We found that RPCs of the kidney express Ang II-AT₁ receptors. Mice with conditional deletion of Ang II-AT₁ receptors in RPCs were normal with regard to the number of renin cells, renal renin mRNA, and plasma renin concentrations. Renin expression and secretion of these mice responded to Ang I (angiotensin I)-converting enzyme inhibition and to Ang II infusion like in wild-type (WT) controls. In summary, we did not obtain evidence that Ang II-AT₁ receptors on RPCs are of major relevance for the normal regulation of renin expression and secretion in mice. Therefore, we doubt the existence of a direct negative feedback function of Ang II on RPCs.

Interaction between cyclooxygenase and the renin-angiotensin-aldosterone system: rationale and clinical relevance

Increased understanding of pathophysiological mechanisms of cardiovascular diseases has shown that the renin-angiotensin-aldosterone system (RAAS) is activated in this setting and suggests a central role for the angiotensin-converting enzyme (ACE). ACE transforms angiotensin I (Ang I) to angiotensin II (Ang II), and also promotes the degradation of bradykinin into inactive metabolites. These bradykinins stimulate nitric oxide synthesis and vasodilatator prostaglandin synthesis via a cyclooxygenase (COX) pathway.

COX inhibitors may therefore be deleterious in cardiovascular disease and/or counteract part of ACE inhibitor (ACE-I) efficacy. This has been clearly demonstrated with non-steroidal anti-inflammatory drugs (NSAIDs), including high-dose aspirin, in avoiding their use in such patients. hypertension, coronary artery disease and chronic heart failure (CHF); most guidelines recommend avoiding their use in such patients.

Theoretically, this effect is dose-mediated and the existence of an identical deleterious effect with low-dose aspirin has been an area of intense debate. In this article, we review studies, most of them conducted in CHF, that pointed out such a possible deleterious effect and a counteraction of ACE-Is with low-dose aspirin , using various criteria of assessment.

However, there are no prospective long-term studies that have validated such an effect, and the role of other anti-aggregating agents has not been evaluated. Until such studies are published, the use of low-dose aspirin (100 mg/day) in such patients can be recommended.

Plasticity of renal endocrine function

The kidneys are important endocrine organs. They secrete humoral factors, such as calcitriol, erythropoietin, klotho, and renin into the circulation, and therefore, they are essentially involved in the regulation of a variety of processes ranging from bone formation to erythropoiesis. The endocrine functions are established by cells, such as proximal or distal tubular cells, renocortical interstitial cells, or mural cells of afferent arterioles. These endocrine cells are either fixed in number, such as tubular cells, which individually and gradually upregulate or downregulate hormone production, or they belong to a pool of cells, which display a recruitment behavior, such as erythropoietin- and renin-producing cells. In the latter case, regulation of humoral function occurs via (de)recruitment of active endocrine cells. As a consequence renin- and erythropoietin-producing cells in the kidney show a high degree of plasticity by reversibly switching between distinct cell states. In this review, we will focus on the characteristics of renin- and of erythropoietin-producing cells, especially on their origin and localization, their reversible transformations, and the mediators, which are responsible for transformation. Finally, we will discuss a possible interconversion of renin and erythropoietin expression.

Regulation of renin release via cyclic ADP-ribose-mediated signaling: evidence from mice lacking CD38 gene

BACKGROUND/AIMS

Despite extensive studies, the intracellular regulatory mechanism of renin production and release is still poorly understood. The present study was designed to test whether CD38-ADP-ribosylcyclase signaling pathway contributes to the regulation of renin production and release, and to examine whether CD38 gene knockout (CD38(-/-)) can change this important renal endocrinal function.

METHODS

ADP-ribosylcyclase activity was estimated utilizing HPLC, cADPR levels from western blot, plasma renin activity from RIA kit, urinary sodium and potassium excretion from fame photometry.

RESULTS

The expression of CD38 and the activity of ADP-ribosylcyclase to produce cyclic ADP-ribose (cADPR) were nearly abolished in the kidney from CD38(-/-) mice, indicating that CD38 gene is a major enzyme responsible for the generation of cADPR in vivo. Mice lacking CD38 gene showed increased plasma renin activity (PRA) in either conscious or anesthetized status (P<0.05). Low salt intake significantly increased, but high salt intake significantly decreased renin release in both CD38(+/+) and CD38(-/-) mice. In acute experiments, it was demonstrated that plasma renin activity (PRA) significantly increased upon isoprenaline infusion in CD38(-/-) mice compared to CD38(+/+) mice. Accompanied with such increase in PRA, glomerular filtration rate (GFR), renal blood flow (RBF), urine volume (UV) and sodium excretion (UNaV) more significantly decreased in CD38(-/-) than CD38(+/+) mice. Similarly, more increases in PRA but more decreases in GFR, RBF, UV and UNaV were observed in CD38(-/-) than CD38(+/+) mice when they had a low renal perfusion pressure (RPP).

CONCLUSION

CD38-cADPR-mediated signaling may importantly contribute to the maintenance of low PRA and participate in the regulation of renal hemodynamics and excretory function in mice.

Role of blood pressure in mediating the influence of salt intake on renin expression in the kidney

The salt intake of an organism controls the number of renin-producing cells in the kidney by yet undefined mechanisms. This study aimed to assess a possible mediator role of preglomerular blood pressure in the control of renin expression by oral salt intake. We used wild-type (WT) mice and mice lacking angiotensin II type 1a receptors (AT1a−/−) displaying an enhanced salt sensitivity to renin expression. In WT kidneys, we found renin-expressing cells at the ends of all afferent arterioles. A low-salt diet (0.02%) led to a moderate twofold increase in renin-expressing cells along afferent arterioles. In AT1a−/− mice, lowering of salt content led to a 12-fold increase in renin expression. Here, the renin-expressing cells were distributed along the preglomerular vascular tree in a typical distal-to-proximal distribution gradient which was most prominent at high salt intake and was obliterated at low salt intake by the appearance of renin-expressing cells in proximal parts of the preglomerular vasculature. While lowering of salt intake produced only a small drop in blood pressure in WT mice, the marked reduction of systolic blood pressure in AT1a−/− mice was accompanied by the disappearance of the distribution gradient from afferent arterioles to arcuate arteries. Unilateral renal artery stenosis in AT1a−/− mice on a normal salt intake produced a similar distribution pattern of renin-expressing cells as did low salt intake. Conversely, increasing blood pressure by administration of the NOS inhibitor N-nitro-l-arginine methyl ester or of the adrenergic agonist phenylephrine in AT1a−/− mice kept on low salt intake produced a similar distribution pattern of renin-producing cells as did normal salt intake alone. These findings suggest that changes in preglomerular blood pressure may be an important mediator of the influence of salt intake on the number and distribution of renin-producing cells in the kidney.

Genetic modifiers of hypertension in soluble guanylate cyclase α1-deficient mice

Nitric oxide (NO) plays an essential role in regulating hypertension and blood flow by inducing relaxation of vascular smooth muscle. Male mice deficient in a NO receptor component, the α1 subunit of soluble guanylate cyclase (sGCα1), are prone to hypertension in some, but not all, mouse strains, suggesting that additional genetic factors contribute to the onset of hypertension. Using linkage analyses, we discovered a quantitative trait locus (QTL) on chromosome 1 that was linked to mean arterial pressure (MAP) in the context of sGCα1 deficiency. This region is syntenic with previously identified blood pressure-related QTLs in the human and rat genome and contains the genes coding for renin. Hypertension was associated with increased activity of the renin-angiotensin-aldosterone system (RAAS). Further, we found that RAAS inhibition normalized MAP and improved endothelium-dependent vasorelaxation in sGCα1-deficient mice. These data identify the RAAS as a blood pressure-modifying mechanism in a setting of impaired NO/cGMP signaling.

Pressure induces intracellular calcium changes in juxtaglomerular cells in perfused afferent arterioles

Calcium (Ca(2+)) has an important role in nearly all types of cellular secretion, with a particularly novel role in the juxtaglomerular (JG) cells in the kidney. In JG cells, Ca(2+) inhibits renin secretion, which is a major regulator of blood pressure and renal hemodynamics. However, whether alterations in afferent arteriolar (Af-Art) pressure change intracellular Ca(2+) concentration ([Ca(2+)](i)) in JG cells and whether [Ca(2+)](i) comes from extracellular or intracellular sources remains unknown. We hypothesize that increases in perfusion pressure in the Af-Art result in elevations in [Ca(2+)](i) in JG cells. We isolated and perfused Af-Art of C57BL6 mice and measured changes in [Ca(2+)](i) in JG cells in response to perfusion pressure changes. The JG cells' [Ca(2+)](i) was 93.3±2.2 nM at 60 mm Hg perfusion pressure and increased to 111.3±13.4, 119.6±7.3, 130.3±2.9 and 140.8±12.1 nM at 80, 100, 120 and 140 mm Hg, respectively. At 120 mm Hg, increases in [Ca(2+)](i) were reduced in mice receiving the following treatments: (1) the mechanosensitive cation channel blocker, gadolinium (94.6±7.5 nM); (2) L-type calcium channel blocker, nifedipine (105.8±7.5 nM); and (3) calcium-free solution plus ethylene glycol tetraacetic acid (96.0±5.8 nM). Meanwhile, the phospholipase C inhibitor, inositol triphosphate receptor inhibitor, T-type calcium channel blocker, N-type calcium channel blocker and Ca(2+)-ATPase inhibitor did not influence changes in [Ca(2+)](i) in JG cells. In summary, JG cell [Ca(2+)](i) rise as perfusion pressure increases; furthermore, the calcium comes from extracellular sources, specifically mechanosensitive cation channels and L-type calcium channels.

A computational model of the circulating renin-angiotensin system and blood pressure regulation

The renin-angiotensin system (RAS) is critical in sodium and blood pressure (BP) regulation, and in cardiovascular-renal (CVR) diseases and therapeutics. As a contribution to SAPHIR project, we present a realistic computer model of renin production and circulating RAS, integrated into Guyton's circulatory model (GCM). Juxtaglomerular apparatus, JGA, and Plasma modules were implemented in C ++/M2SL (Multi-formalism Multi-resolution Simulation Library) for fusion with GCM. Matlab optimization toolboxes were used for parameter identification. In JGA, renin production and granular cells recruitment (GCR) are controlled by perfusion pressure (PP), macula densa (MD), angiotensin II (Ang II), and renal sympathetic activity (RSNA). In Plasma, renin and ACE (angiotensin-converting enzyme) activities are integrated to yield Ang I and II. Model vs. data deviation is given as normalized root mean squared error (nRMSE; n points).

IDENTIFICATION

JGA and Plasma parameters were identified against selected experimental data. After fusion with GCM: (1) GCR parameters were identified against Laragh's PRA-natriuresis nomogram; (2) Renin production parameters were identified against two sets of data ([renin] transients vs. ACE or renin inhibition). Finally, GCR parameters were re-identified vs. Laragh's nomogram (nRMSE 8%, n = 9).

VALIDATION

(1) model BP, PRA and [Ang II] are within reported ranges, and respond physiologically to sodium intake; (2) short-term Ang II infusion induces reported rise in BP and PRA. The modeled circulating RAS, in interaction with an integrated CVR, exhibits a realistic response to BP control maneuvers. This construction will allow for modelling hypertensive and CVR patients, including salt-sensitivity, polymorphisms, and pharmacotherapeutics.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Substitution of connexin40 with connexin45 prevents hyperreninemia and attenuates hypertension

Connexins (Cxs) are a family of transmembrane proteins that form gap junctions with unique and redundant biophysical functions. Juxtaglomerular cells express Cx40, which is crucial to the control of renin secretion by blood pressure and angiotensin II, and mice that lack Cx40 have high plasma renin and hypertension. To examine whether normal juxtaglomerular cell function depends on the unique properties of Cx40, we measured renin release in mice where the coding sequence for Cx40 was replaced by that for Cx45, using the knock-in method. We first found that the knock-in strategy indeed resulted in expression of Cx45 but not Cx40 in the juxtaglomerular cells of these mice. The plasma renin concentration of the knock-in mice was similar to that in wild-type mice. The high blood pressure of the Cx40 knockout mice was significantly reduced when Cx45 was knocked into the locus but remained mildly elevated compared to wild-type mice. Blockade of angiotensin II formation by enalapril increased the plasma renin concentration in wild-type and the Cx45 knock-in mice but not in the Cx40 knockout mice. Infusion of angiotensin II into isolated perfused kidneys results in decreased renin release, a phenomenon that was attenuated in the Cx40 knockout mice. However, in the Cx45 knock-in mice, angiotensin II suppressed renin release similar to its effect in wild type mice. Unilateral renal artery stenosis increased the plasma renin concentration and blood pressure in both the wild-type and the Cx45 knock-in mice but not in the Cx40 knockout mice. Since Cx40 can be replaced by Cx45, a connexin with a significantly lower conductivity, we suggest that the regulation of renin release is not dependent on the unique electrical properties of these channel proteins.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

Normotensive sodium loading in normal man: regulation of renin secretion during beta-receptor blockade

Saline administration may change renin-angiotensin-aldosterone system (RAAS) activity and sodium excretion at constant mean arterial pressure (MAP). We hypothesized that such responses are elicited mainly by renal sympathetic nerve activity by beta1-receptors (beta1-RSNA), and tested the hypothesis by studying RAAS and renal excretion during slow saline loading at constant plasma sodium concentration (Na+ loading; 12 micromol Na+.kg(-1).min(-1) for 4 h). Normal subjects were studied on low-sodium intake with and without beta1-adrenergic blockade by metoprolol. Metoprolol per se reduced RAAS activity as expected. Na+ loading decreased plasma renin concentration (PRC) by one-third, plasma ANG II by one-half, and plasma aldosterone by two-thirds (all P < 0.05); surprisingly, these changes were found without, as well as during, acute metoprolol administration. Concomitantly, sodium excretion increased indistinguishably with and without metoprolol (16 +/- 2 to 71 +/- 14 micromol/min; 13 +/- 2 to 55 +/- 13 micromol/min, respectively). Na+ loading did not increase plasma atrial natriuretic peptide, glomerular filtration rate (GFR by 51Cr-EDTA), MAP, or cardiac output (CO by impedance cardiography), but increased central venous pressure (CVP) by approximately 2.0 mmHg (P < 0.05). During Na+ loading, sodium excretion increased with CVP at an average slope of 7 micromol.min(-1).mmHg(-1). Concomitantly, plasma vasopressin decreased by 30-40% (P < 0.05). In conclusion, beta1-adrenoceptor blockade affects neither the acute saline-mediated deactivation of RAAS nor the associated natriuretic response, and the RAAS response to modest saline loading seems independent of changes in MAP, CO, GFR, beta1-mediated effects of norepinephrine, and ANP. Unexpectedly, the results do not allow assessment of the relative importance of RAAS-dependent and -independent regulation of renal sodium excretion. The results are compatible with the notion that at constant arterial pressure, a volume receptor elicited reduction in RSNA via receptors other than beta1-adrenoceptors, decreases renal tubular sodium reabsorption proximal to the macula densa leading to increased NaCl concentration at the macula densa, and subsequent inhibition of renin secretion.

Connexin40 is essential for the pressure control of renin synthesis and secretion

Renin secretion and synthesis in renal juxtaglomerular cells are controlled by short feed back loops involving angiotensin II and the intrarenal blood pressure. The operating mechanisms of these negative feed back regulators are widely unknown, except for the fact that both require calcium to exert their inhibitory action. We here show that in the absence of connexin40 (Cx40), which form gap junctions between juxtaglomerular and endothelial cells, the negative control of renin secretion and synthesis by angiotensin II and by intravasal pressure is abrogated, while the regulation by salt intake and beta-adrenergic stimulation is maintained. Renin secretion from Cx40-deficient kidneys or wild-type kidneys treated with the nonselective gap junction blocker 18alpha-glycyrrhetinic acid (10 micromol/L) resembles the situation in wild-type kidneys in the absence of extracellular calcium. This disturbed regulation is reflected by an enhanced plasma renin concentration despite an elevated blood pressure in Cx40-deficient mice. These findings indicate that Cx40 connexins and likely intercellular communication via Cx40-dependent gap junctions mediate the calcium-dependent inhibitor effects of angiotensin II and of intrarenal pressure on renin secretion and synthesis. Because Cx40 gap junctions are also formed between renin producing cells and endothelial cells our finding could provide additional information to suggest that the endothelium may be strongly involved in the control of the renin system.

Regulation of thick ascending limb transport: role of nitric oxide

Nitric oxide (NO) plays a role in many physiological and pathophysiological processes. In the kidney, NO reduces renal vascular resistance, increases glomerular filtration rate, alters renin release, and inhibits transport along the nephron. The thick ascending limb is responsible for absorbing 20-30% of the filtered load of NaCl, much of the bicarbonate that escapes the proximal nephron, and a significant fraction of the divalent cations reclaimed from the forming urine. Additionally, this nephron segment plays a role in K+ homeostasis. This article will review recent advances in our understanding of the role NO plays in regulating the transport processes of the thick ascending limb. NO has been shown to inhibit NaCl absorption primarily by reducing Na+-K+-2Cl- cotransport activity. NO also inhibits bicarbonate absorption by reducing Na+/H+ exchange activity. It has also been reported to enhance luminal K+ channel activity and thus is likely to alter K+ secretion. The source of NO may be vascular structures such as the afferent arteriole or vasa recta, or the thick ascending limb itself. NO is produced by NO synthase 3 in this segment, and several factors that regulate its activity both acutely and chronically have recently been identified. Although the effects of NO on thick ascending limb transport have received a great deal of attention recently, its effects on divalent ion absorption and many other issues remain unexplored.

Nitric oxide stimulates cyclooxygenase-2 in cultured cTAL cells through a p38-dependent pathway

To examine the interaction of nitric oxide (NO) and cyclooxygenase (COX-2) and the signaling pathway involved, primary cultured rabbit cortical thick ascending limb (cTAL) were used. In these cells, immunoreactive COX-2 and vasodilatory prostaglandins were increased by a NO donor, S-nitros- N-acetylpenicillamine (SNAP; 2.5 ± 0.3-fold control, n = 6, P < 0.01). SNAP increased expression of phosphorylated p38 (pp38; 2.4 ± 0.3-fold control; n = 5; P < 0.01), which was inhibited by the p38 inhibitor SB-203580 (1.3 ± 0.1-fold control, n = 5, P < 0.01). SB-203580 inhibited SNAP-induced COX-2 expression [1.4 ± 0.2-fold control, n = 6, not significant (NS) vs. control] and levels of PGE2significantly. In cTAL cells transfected with a luciferase reporter driven by the wild-type mouse COX-2 promoter, SNAP stimulated luciferase activity, which was reversed by SB-203580 (control vs. SNAP vs. SNAP + SB-203580: 1.4 ± 0.2-, 8.3 ± 1.4-, and 0.4 ± 0.1-fold control, respectively, n = 4, P < 0.01). Electrophoretic mobility shift assay indicated that SNAP stimulated nuclear factor (NF)-κB binding activity in cTAL that was also inhibited by the p38 inhibitor. SNAP was not able to stimulate a mutant COX-2 promoter construct that is not activated by NF-κB (0.9 ± 0.1, 1.2 ± 0.1, and 1.0 ± 0.2 respectively, n = 4, NS). Low chloride increased COX-2 expression (2.7 ± 0.4-fold control, n = 6, P < 0.01) and pp38 expression (2.8 ± 0.3-fold; n = 5, P < 0.01), which were reversed by the specific NO synthase (NOS) inhibitor 7-nitroindazole. Administration of a low-salt diet increased immunoreactive COX-2 and neuronal NOS (nNOS) in the macula densa and surrounding cTAL of kidneys of wild-type mice but did not significantly elevate COX-2 expression in nNOS−/−mice. In summary, these studies indicate that, in cTAL, NO can increase COX-2 expression in cTAL and macula densa through p38-dependent signaling pathways via activation of NF-κB.

Role of CREB1 and NFκB-p65 in the Down-regulation of Renin Gene Expression by Tumor Necrosis Factor α

Tumor necrosis factor-alpha (TNFalpha) is a potent inhibitor of renin gene expression in renal juxtaglomerular cells. We have found that TNFalpha suppresses renin transcription via transcription factor NFkappaB, which targets a cAMP responsive element (CRE) in the renin promoter. Here we aimed to further clarify the role of NFkappaB and the canonical CRE-binding proteins of the CRE-binding protein/activating transcription factor (CREB/ATF) family in the inhibition of renin gene expression by TNFalpha in the juxtaglomerular cell line As4.1. TNFalpha caused a moderate decrease in the binding of CREB1 to its cognate CRE DNA binding site. On the other hand, NFkappaB-p65 transcriptional activity was substantially reduced by TNFalpha, which targeted a trans-activation domain at the very C terminus of the p65 molecule. Our results suggest that TNFalpha inhibits renin gene expression by decreasing the transactivating capacity of NFkappaB-p65 and partially by attenuating CREB1 binding to CRE.

Tumor necrosis factor-alpha activates NFkappaB to inhibit renin transcription by targeting cAMP-responsive element

Tumor necrosis factor-alpha (TNFalpha) is known to inhibit renin gene expression in juxtaglomerular cells, which are the main source of renin in vivo. In the present study we aimed to characterize the intracellular mechanisms of TNFalpha signaling to renin gene in the mouse juxtaglomerular cell line As4.1. TNFalpha was found to activate NFkappaB, which is one of the principal intracellular mediators of TNFalpha signal transduction. Constitutive activation of NFkappaB suppressed renin gene transcription, but NFkappaB appeared not to target the NFkappaB binding sites in the renin promoter. Thus, NFkappaB, but not the canonical NFkappaB binding sequences in the renin promoter, seemed to be involved in the suppression of renin transcription by TNFalpha. Deletion/mutation analysis revealed that the effect of TNFalpha on renin gene is transmitted by a cAMP-responsive element (CRE) located at -2697 to -2690. Mobility shift/supershift assays evidenced for the presence of NFkappaB proteins in the complex that binds to mouse renin CRE. Our results strongly suggest that NFkappaB mediates the effect of TNFalpha on renin transcription targeting a CRE in the mouse renin promoter.

Renal Extraction of Angiotensin II

Angiotensin II regulates many aspects of renal function and thereby influences long-term blood pressure. The effects of angiotensin II on the kidney have been exhaustively studied; however, the converse (i.e., effects of the kidney on angiotensin II) has received little attention. Accordingly, the focus of this study was to determine whether renal degradation of angiotensin II is regulated by chronic levels of angiotensin II or long-term levels of blood pressure. Twenty hypertensive rats and 22 normotensive rats were treated for 1 week with either vehicle, angiotensin II (50 ng/kg/min, subcutaneously) or captopril (100 mg/kg/day, orally). Right kidney vascular resistance was measured during infusions of angiotensin II into the left renal artery or vena cava at the level of left renal vein. Dose-response data were curve-fitted, and the extraction of angiotensin II by the left kidney was calculated by comparing the doses of angiotensin II required to elicit equal increases in right renal vascular resistance during intravenous versus left intrarenal artery infusions. Renal extraction of angiotensin II was high (mean, 81%) and demonstrated little animal-to-animal variation (coefficient of variation, 23%; standard deviation, 19%). Renal extraction of angiotensin II was independent of hypertension (P = 0.257) or previous chronic exposure to angiotensin II or captopril (P = 0.270), and there was no interaction between hypertension and chronic exposure to angiotensin II or captopril (P = 0.950). We conclude that renal degradation of angiotensin II is constitutively high, is unaffected by chronic levels of arterial blood pressure, and is independent of long-term changes in levels of angiotensin II.

Preserved macula densa-dependent renin secretion in A1 adenosine receptor knockout mice

Recent studies demonstrated that the influence of the macula densa on glomerular filtration is abolished in adenosine A(1) receptor (A(1)AR) knockout mice. Inasmuch as the macula densa not only regulates glomerular filtration but also controls the activity of the renin system, the present study aimed to determine the role of the A(1)AR in macula densa control of renin synthesis and secretion. Although a high-salt diet over 1 wk suppressed renin mRNA expression and renal renin content to similar degrees in A(1)AR(+/+), A(1)AR(+/-), and A(1)AR(-/-) mice, stimulation of Ren-1 mRNA expression and renal renin content by salt restriction was markedly enhanced in A(1)AR(-/-) compared with wild-type mice. Pharmacological blockade of macula densa salt transport with loop diuretics stimulated renin expression in vivo (treatment with furosemide at 1.2 mg/day for 6 days) and renin secretion in isolated perfused mouse kidneys (treatment with 100 microM bumetanide) in all three genotypes to the same extent. Taken together, our data are consistent with the concept of a tonic inhibitory role of the A(1)AR in the renin system, whereas they indicate that the A(1)AR is not indispensable in macula densa control of the renin system.

Renal dysfunction after chronic blockade of nitric oxide synthesis

The effects of the chronic inhibition of nitric oxide (NO) on renal hemodynamics and tubular function were studied in rats treated for 8 weeks with the NO synthesis inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 40 mg/kg/day). In addition, the effect of L-NAME administration on vasoactive systems (renin-angiotensin system, aldosterone, catecholamines, endothelin, and thromboxane A(2)) was evaluated. Chronic inhibition of NO significantly elevated blood pressure, reduced glomerular filtration rate and renal blood flow, blunted the pressure-diuresis-natriuresis response, and increased protein urine excretion. All these changes were associated with blunted nitrite production in response to acetylcholine in glomeruli. No changes were observed in the plasma levels of either renin activity, aldosterone, or endothelin in L-NAME-treated rats. Similarly, no differences were observed in the urinary excretion of thromboxane B(2) between both group of animals. By contrast, plasma concentrations of both epinephrine and norepinephrine were elevated in rats treated with L-NAME. In summary, the results show that chronic blockade of NO produced not only alterations in renal function, but also renal damage, suggesting an important renoprotective role of NO. An activation of sympathoadrenal system could participate in these renal alterations.

Tumor necrosis factor-alpha inhibits renin gene expression

Renin, produced in renal juxtaglomerular (JG) cells, is a fundamental regulator of blood pressure. Accumulating evidence suggests that cytokines may directly influence renin production in the JG cells. TNF-alpha, which is one of the key mediators in immunity and inflammation, is known to participate in the control of vascular proliferation and contraction and hence in the pathogenesis of cardiovascular diseases. Thus TNF-alpha may exert its effects on the cardiovascular system through modulation of renal renin synthesis. Therefore we have tested the effect of TNF-alpha on renin transcription in As4.1 cells, which represent transformed mouse JG cells, and in native mouse JG cells in culture. Renin gene expression was also determined in mice lacking the gene for TNF-alpha (TNF-alpha knockout mice). TNF-alpha inhibited renin gene expression via an inhibition of the transcriptional activity, targeting the proximal 4.1 kb of the renin promoter in As4.1 cells. TNF-alpha also attenuated forskolin-stimulated renin gene expression in primary cultures of mouse JG cells. Mice lacking the TNF-alpha gene had almost threefold higher basal renal renin mRNA abundance relative to the control strain. The general physiological regulation of renin expression by salt was not disturbed in TNF-alpha knockout mice. Our data suggest that TNF-alpha inhibits renin gene transcription at the cellular level and thus may act as a modulator of renin synthesis in (physio)pathological situations.

Prostaglandins that increase renin production in response to ACE inhibition are not derived from cyclooxygenase-1

It is well known that nonselective, nonsteroidal anti-inflammatory drugs inhibit renal renin production. Our previous studies indicated that angiotensin-converting enzyme inhibitor (ACEI)-mediated renin increases were absent in rats treated with a cyclooxygenase (COX)-2-selective inhibitor and in COX-2 -/- mice. The current study examined further whether COX-1 is also involved in mediating ACEI-induced renin production. Because renin increases are mediated by cAMP, we also examined whether increased renin is mediated by the prostaglandin E(2) receptor EP(2) subtype, which is coupled to G(s) and increases cAMP. Therefore, we investigated if genetic deletion of COX-1 or EP(2) prevents increased ACEI-induced renin expression. Age- and gender-matched wild-type (+/+) and homozygous null mice (-/-) were administered captopril for 7 days, and plasma and renal renin levels and renal renin mRNA expression were measured. There were no significant differences in the basal level of renal renin activity from plasma or renal tissue in COX-1 +/+ and -/- mice. Captopril administration increased renin equally [plasma renin activity (PRA): +/+ 9.3 +/- 2.2 vs. 50.1 +/- 10.9; -/- 13.7 +/- 1.5 vs. 43.9 +/- 6.6 ng ANG I x ml(-1) x h(-1); renal renin concentration: +/+ 11.8 +/- 1.7 vs. 35.3 +/- 3.9; -/- 13.0 +/- 3.0 vs. 27.8 +/- 2.7 ng ANG I x mg protein(-1) x h(-1); n = 6; P < 0.05 with or without captopril]. ACEI also increased renin mRNA expression (+/+ 2.4 +/- 0.2; -/- 2.1 +/- 0.2 fold control; n = 6-10; P < 0.05). Captopril led to similar increases in EP(2) -/- compared with +/+. The COX-2 inhibitor SC-58236 blocked ACEI-induced elevation in renal renin concentration in EP(2) null mice (+/+ 24.7 +/- 1.7 vs. 9.8 +/- 0.4; -/- 21.1 +/- 3.2 vs. 9.3 +/- 0.4 ng ANG I x mg protein(-1) x h(-1); n = 5) as well as in COX-1 -/- mice (SC-58236-treated PRA: +/+ 7.3 +/- 0.6; -/- 8.0 +/- 0.9 ng ANG I x ml(-1) x h(-1); renal renin: +/+ 9.1 +/- 0.9; -/- 9.6 +/- 0.5 ng ANG I x mg protein(-1) x h(-1); n = 6-7; P < 0.05 compared with no treatment). Immunohistochemical analysis of renin expression confirmed the above results. This study provides definitive evidence that metabolites of COX-2 rather than COX-1 mediate ACEI-induced renin increases. The persistent response in EP(2) nulls suggests involvement of prostaglandin E(2) receptor subtype 4 and/or prostacyclin receptor (IP).

Cyclic mechanical distension regulates renin gene transcription in As4.1 cells

The renin-producing and -secreting juxtaglomerular (JG) cells are thought to function as the baroreceptor of the kidney. The mechanism by which changes in pressure, or mechanical force, regulate renin at the molecular level has not been elucidated. The renin gene-expressing and -secreting clonal cell line As4.1 was derived from transgene-targeted oncogenesis in mice and was used as a cellular model for JG cells. As4.1 cells subjected to cyclic mechanical distension for a period of 24 h at various frequencies (0. 05 or 0.5 Hz) and magnitudes (12 or 24% elongation) were analyzed via Northern analysis for renin mRNA levels. Results indicate that renin gene expression is decreased by 50-85% and returns to basal levels after a 24-h recovery period. Renin gene expression was attenuated independently of elevated cell growth or changes in renin message decay, suggesting that renin gene transcription is directly modulated by mechanical distension. Transient transfection of As4.1 cells with renin 5' flanking sequence-luciferase reporter gene constructs confirmed the role of mechanical stimulation in regulating renin gene transcription. A 43% inhibition of luciferase activity, by stretch, was observed in cells transfected with a 4,000 base pair 5' flanking sequence to the renin proximal promoter. These results demonstrate for the first time that changes in mechanical force can result in the regulation of renin gene transcription and thus provide further insight into the baroreceptor properties of renin-expressing cells.

The mechanisms of aquaporin control in the renal collecting duct

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Central to its antidiuretic action in mammals is the exocytotic insertion of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the apical membrane of principal cells, an event initiated by an increase in cAMP and activation of protein kinase A. Water is then reabsorbed from the hypotonic urine of the collecting duct. The water channels aquaporin-3 (AQP3) and aquaporin-4 (AQP4), which are constitutively present in the basolateral membrane, allow the exit of water from the cell into the hypertonic interstitium. Withdrawal of the hormone leads to endocytotic retrieval of AQP2 from the cell membrane. The hormone-induced rapid redistribution between the interior of the cell and the cell membrane establishes the basis for the short term regulation of water permeability. In addition water channels (AQP2 and 3) of principal cells are regulated at the level of expression (long term regulation). This review summarizes the current knowledge on the molecular mechanisms underlying the short and long term regulation of water channels in principal cells. In the first part special emphasis is placed on the proteins involved in short term regulation of AQP2 (SNARE proteins, Rab proteins, cytoskeletal proteins, G proteins, protein kinase A anchoring proteins and endocytotic proteins). In the second part, physiological and pathophysiological stimuli determining the long term regulation are discussed.

Constitutive nitric oxide synthesis in the kidney--functions at the juxtaglomerular apparatus

Tubulo-vascular information transfer at the renal juxtaglomerular apparatus (JGA) serves to adjust the biosynthesis and release of renin, the key enzyme of the renin angiotensin system, and to regulate glomerular arteriolar muscle tone. The macula densa serves as a sensor of tubular NaCl. Concentration-dependent salt uptake through the Na-K-2Cl cotransporter located in the apical membrane of macula densa cells triggers a signal transduction cascade that involves the synthesis of nitric oxide (NO) through a type 1 NO synthase (NOS1) which is described with respect to its complex mRNA structure and regulatory aspects. The anatomical and functional targets of the NO-soluble guanylyl cyclase-cGMP pathway at the JGA are reviewed.

Smooth-muscle contraction without smooth-muscle myosin

Here we have used gene-targeting to eliminate expression of smooth-muscle myosin heavy chain. Elimination of this gene does not affect expression of non-muscle myosin heavy chain, and knockout individuals typically survive for three days. Prolonged activation, by KCl depolarisation, of intact bladder preparations from wild-type neonatal mice produces an initial transient state (phase 1) of high force generation and maximal shortening velocity, which is followed by a sustained state (phase 2) characterized by low force generation and maximal shortening velocity. Similar preparations from knockout neonatal mice do not undergo phase 1, but exhibit a normal phase 2. We propose that, in neonatal smooth muscle phase 1 is generated by recruitment of smooth-muscle myosin heavy chain, whereas phase 2 can be generated by activation of non-muscle myosin heavy chain. We conclude that phase 1 becomes indispensable for survival and normal growth soon after birth, particularly for functions such as homeostasis and circulation.