Tubule-vascular feedback in renal autoregulation.

Afferent arteriole (Af-Art) diameter regulates pressure and flow into the glomerulus, which are the main determinants of the glomerular filtration rate. Thus, Af-Art resistance is crucial for Na+ filtration. Af-Arts play a role as integrative centers, where systemic and local systems interact to determine the final degree of resistance. The tubule of a single nephron contacts an Af-Art of the same nephron at two locations: in the transition of the thick ascending limb to the distal tubule (macula densa) and again in the connecting tubule. These two sites are the anatomic basis of two intrinsic feedback mechanisms: tubule-glomerular feedback and connecting tubule-glomerular feedback. The cross communications between the tubules and Af-Arts integrate tubular Na+ and water processing with the hemodynamic conditions of the kidneys. Tubule-glomerular feedback provides negative feedback that tends to avoid salt loss, and connecting tubule-glomerular feedback provides positive feedback that favors salt excretion by modulating tubule-glomerular feedback (resetting it) and increasing glomerular filtration rate. These feedback mechanisms are also exposed to systemic modulators (hormones and the nervous system); however, they can work in isolated kidneys or nephrons. The exaggerated activation or absence of any of these mechanisms may lead to disequilibrium in salt and water homeostasis, especially in extreme conditions (e.g., high-salt diet/low-salt diet) and may be part of the pathogenesis of some diseases. In this review, we focus on molecular signaling, feedback interactions, and the physiological roles of these two feedback mechanisms.

Renal release of N-acetyl-seryl-aspartyl-lysyl-proline is part of an antifibrotic peptidergic system in the kidney.

The antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is released from thymosin-ß4 (Tß4) by the meprin-α and prolyl oligopeptidase (POP) enzymes and is hydrolyzed by angiotensin-converting enzyme (ACE). Ac-SDKP is present in urine; however, it is not clear whether de novo tubular release occurs or if glomerular filtration is the main source. We hypothesized that Ac-SDKP is released into the lumen of the nephrons and that it exerts an antifibrotic effect. We determined the presence of Tß4, meprin-α, and POP in the kidneys of Sprague-Dawley rats. The stop-flow technique was used to evaluate Ac-SDKP formation in different nephron segments. Finally, we decreased Ac-SDKP formation by inhibiting the POP enzyme and evaluated the long-term effect in renal fibrosis. The Tß4 precursor and the releasing enzymes meprin-α and POP were expressed in the kidneys. POP enzyme activity was almost double that in the renal medulla compared with the renal cortex. With the use of the stop-flow technique, we detected the highest Ac-SDKP concentrations in the distal nephron. The infusion of a POP inhibitor into the kidney decreased the amount of Ac-SDKP in distal nephron segments and in the proximal nephron to a minor extent. An ACE inhibitor increased the Ac-SDKP content in all nephron segments, but the increase was highest in the distal portion. The chronic infusion of a POP inhibitor increased kidney medullary fibrosis, which was prevented by Ac-SDKP. We conclude that Ac-SDKP is released by the nephron and is part of an important antifibrotic system in the kidney.

A Novel Mechanism of Renal Microcirculation Regulation: Connecting Tubule-Glomerular Feedback.

PURPOSE OF REVIEW: In this review, we summarized the current knowledge of connecting tubule-glomerular feedback (CTGF), a novel mechanism of renal microcirculation regulation that integrates sodium handling in the connecting tubule (CNT) with kidney hemodynamics. RECENT FINDINGS: Connecting tubule-glomerular feedback is a crosstalk communication between the CNT and the afferent arteriole (Af-Art), initiated by sodium chloride through the epithelial sodium channel (ENaC). High sodium in the CNT induces Af-Art vasodilation, increasing glomerular pressure and the glomerular filtration rate and favoring sodium excretion. CTGF antagonized and reset tubuloglomerular feedback and thus increased sodium excretion. CTGF is absent in spontaneous hypertensive rats and is overactivated in Dahl salt-sensitive rats. CTGF is also modulated by angiotensin II and aldosterone. CTGF is a feedback mechanism that integrates sodium handling in the CNT with glomerular hemodynamics. Lack of CTGF could promote hypertension, and CTGF overactivation may favor glomerular damage and proteinuria. More studies are needed to explore the alterations in renal microcirculation and the role of these alterations in the genesis of hypertension and glomerular damage in animals and humans. KEY POINTS: • CTGF is a vasodilator mechanism that regulates afferent arteriole resistance. • CTGF is absent in spontaneous hypertensive rats and overactivated in Dahl salt-sensitive rats. • CTGF in excess may promote glomerular damage and proteinuria, while the absence may participate in sodium retention and hypertension.

Role of connecting tubule glomerular feedback in obesity related renal damage.

Zucker obese rats (ZOR) have higher glomerular capillary pressure (PGC) that can cause renal damage. PGC is controlled by afferent (Af-Art) and efferent arteriole (Ef-Art) resistance. Af-Art resistance is regulated by factors that regulate other arterioles, such as myogenic response. In addition, it is also regulated by 2 intrinsic feedback mechanisms: 1) tubuloglomerular feedback (TGF) that causes Af-Art constriction in response to increased NaCl in the macula densa and 2) connecting tubule glomerular feedback (CTGF) that causes Af-Art dilatation in response to an increase in NaCl transport in the connecting tubule via the epithelial sodium channel. Since CTGF is an Af-Art dilatory mechanism, we hypothesized that increased CTGF contributes to TGF attenuation, which in turn increases PGC in ZOR. We performed a renal micropuncture experiment and measured stop-flow pressure (PSF), which is an indirect measurement of PGC in ZOR. Maximal TGF response at 40 nl/min was attenuated in ZOR (4.47 ± 0.60 mmHg) in comparison to the Zucker lean rats (ZLR; 8.54 ± 0.73 mmHg, P < 0.05), and CTGF was elevated in ZOR (5.34 ± 0.87 mmHg) compared with ZLR (1.12 ± 1.28 mmHg, P < 0.05). CTGF inhibition with epithelial sodium channel blocker normalized the maximum PSF change in ZOR indicating that CTGF plays a significant role in TGF attenuation (ZOR, 10.67 ± 1.07 mmHg vs. ZLR, 9.5 ± 1.53 mmHg). We conclude that enhanced CTGF contributes to TGF attenuation in ZOR and potentially contribute to progressive renal damage.

Noninvasive measurement of renal blood flow by magnetic resonance imaging in rats.

Renal blood flow (RBF) provides important information regarding renal physiology and nephropathies. Arterial spin labeling-magnetic resonance imaging (ASL-MRI) is a noninvasive method of measuring blood flow without exogenous contrast media. However, low signal-to-noise ratio and respiratory motion artifacts are challenges for RBF measurements in small animals. Our objective was to evaluate the feasibility and reproducibility of RBF measurements by ASL-MRI using respiratory-gating and navigator correction methods to reduce motion artifacts. ASL-MRI images were obtained from the kidneys of Sprague-Dawley (SD) rats on a 7-Tesla Varian MRI system with a spin-echo imaging sequence. After 4 days, the study was repeated to evaluate its reproducibility. RBF was also measured in animals under unilateral nephrectomy and in renal artery stenosis (RST) to evaluate the sensitivity in high and low RBF models, respectively. RBF was also evaluated in Dahl salt-sensitive (SS) rats and spontaneous hypertensive rats (SHR). In SD rats, the cortical RBFs (cRBF) were 305 ± 59 and 271.8 ± 39 ml·min-1·100 g tissue-1 in the right and left kidneys, respectively. Retest analysis revealed no differences ( P = 0.2). The test-retest reliability coefficient was 92 ± 5%. The cRBFs before and after the nephrectomy were 296.8 ± 30 and 428.2 ± 45 ml·min-1·100 g tissue-1 ( P = 0.02), respectively. The kidneys with RST exhibited a cRBF decrease compared with sham animals (86 ± 17.6 vs. 198 ± 33.7 ml·min-1·100 g tissue-1; P < 0.01). The cRBFs in SD, Dahl-SS, and SHR rats were not different ( P = 0.35). We conclude that ASL-MRI performed with navigator correction and respiratory gating is a feasible and reliable noninvasive method for measuring RBF in rats.

Connecting tubule glomerular feedback mediates tubuloglomerular feedback resetting after unilateral nephrectomy.

Unilaterally nephrectomized rats (UNx) have higher glomerular capillary pressure (PGC) that can cause significant glomerular injury in the remnant kidney. PGC is controlled by the ratio of afferent (Af-Art) and efferent arteriole resistance. Af-Art resistance in turn is regulated by two intrinsic feedback mechanisms: 1) tubuloglomerular feedback (TGF) that causes Af-Art constriction in response to increased NaCl in the macula densa; and 2) connecting tubule glomerular feedback (CTGF) that causes Af-Art dilatation in response to an increase in NaCl transport in the connecting tubule via the epithelial sodium channel (ENaC). Resetting of TGF post-UNx can allow systemic pressure to be transmitted to the glomerulus and cause renal damage, but the mechanism behind this resetting is unclear. Since CTGF is an Af-Art dilatory mechanism, we hypothesized that CTGF is increased after UNx and contributes to TGF resetting. To test this hypothesis, we performed UNx in Sprague-Dawley (8) rats. Twenty-four hours after surgery, we performed micropuncture of individual nephrons and measured stop-flow pressure (PSF). PSF is an indirect measurement of PGC. Maximal TGF response at 40 nl/min was 8.9 ± 1.24 mmHg in sham-UNx rats and 1.39 ± 1.02 mmHg in UNx rats, indicating TGF resetting after UNx. When CTGF was inhibited with the ENaC blocker benzamil (1 µM/l), the TGF response was 12.29 ± 2.01 mmHg in UNx rats and 13.03 ± 1.25 mmHg in sham-UNx rats, indicating restoration of the TGF responses in UNx. We conclude that enhanced CTGF contributes to TGF resetting after UNx.

Effect of salt intake on afferent arteriolar dilatation: role of connecting tubule glomerular feedback (CTGF).

Afferent arteriole (Af-Art) resistance is modulated by two intrinsic nephron feedbacks: 1) the vasoconstrictor tubuloglomerular feedback (TGF) mediated by Na+-K+-2Cl- cotransporters (NKCC2) in the macula densa and blocked by furosemide and 2) the vasodilator connecting tubule glomerular feedback (CTGF), mediated by epithelial Na+ channels (ENaC) in the connecting tubule and blocked by benzamil. High salt intake reduces Af-Art vasoconstrictor ability in Dahl salt-sensitive rats (Dahl SS). Previously, we measured CTGF indirectly, by differences between TGF responses with and without CTGF inhibition. We recently developed a new method to measure CTGF more directly by simultaneously inhibiting NKCC2 and the Na+/H+ exchanger (NHE). We hypothesize that in vivo during simultaneous inhibition of NKCC2 and NHE, CTGF causes an Af-Art dilatation revealed by an increase in stop-flow pressure (PSF) in Dahl SS and that is enhanced with a high salt intake. In the presence of furosemide alone, increasing nephron perfusion did not change the PSF in either Dahl salt-resistant rats (Dahl SR) or Dahl SS. When furosemide and an NHE inhibitor, dimethylamiloride, were perfused simultaneously, an increase in tubular flow caused Af-Art dilatation that was demonstrated by an increase in PSF. This increase was greater in Dahl SS [4.5 ± 0.4 (SE) mmHg] than in Dahl SR (2.5 ± 0.3 mmHg; P < 0.01). We confirmed that CTGF causes this vasodilation, since benzamil completely blocked this effect. However, a high salt intake did not augment the Af-Art dilatation. We conclude that during simultaneous inhibition of NKCC2 and NHE in the nephron, CTGF induces Af-Art dilatation and a high salt intake failed to enhance this effect.

Mechanisms of connecting tubule glomerular feedback enhancement by aldosterone.

Connecting tubule glomerular feedback (CTGF) is a mechanism where an increase in sodium (Na) concentration in the connecting tubule (CNT) causes the afferent arteriole (Af-Art) to dilate. We recently reported that aldosterone within the CNT lumen enhances CTGF via a nongenomic effect involving GPR30 receptors and sodium/hydrogen exchanger (NHE), but the signaling pathways of this mechanism are unknown. We hypothesize that aldosterone enhances CTGF via cAMP/protein kinase A (PKA) pathway that activates protein kinase C (PKC) and stimulates superoxide (O2-) production. Rabbit Af-Arts and their adherent CNTs were microdissected and simultaneously perfused. Two consecutive CTGF curves were elicited by increasing the CNT luminal NaCl. We found that the main effect of aldosterone was to sensitize CTGF and we analyzed data by comparing NaCl concentration in the CNT perfusate needed to achieve half of the maximal response (EC50). During the control period, the NaCl concentration that elicited a half-maximal response (EC50) was 37.0 ± 2.0 mmol/l; addition of aldosterone (10-8 mol/l) to the CNT lumen decreased EC50 to 19.3 ± 1.3 mmol/l (P ≤ 0.001 vs. Control). The specific adenylyl cyclase inhibitor 2',3'-dideoxyadenosine (ddA; 2 × 10-4 mol/l) and the PKA inhibitor H-89 dihydrochloride hydrate (H-89; 2 × 10-6 mol/l) prevented the aldosterone effect. The selective PKC inhibitor GF109203X (10-8 mol/l) also prevented EC50 reduction caused by aldosterone. CNT intraluminal addition of O2- scavenger tempol (10-4 mol/l) blocked the aldosterone effect. We conclude that aldosterone inside the CNT lumen enhances CTGF via a cAMP/PKA/PKC pathway and stimulates O2- generation and this process may contribute to renal damage by increasing glomerular capillary pressure.

Angiotensin II-mediated hypertension impairs nitric oxide-induced NKCC2 inhibition in thick ascending limbs.

In thick ascending limbs (THALs), nitric oxide (NO) decreases NaCl reabsorption via cGMP-mediated inhibition of Na-K-2Cl cotransporter (NKCC2). In angiotensin (ANG II)-induced hypertension, endothelin-1 (ET-1)-induced NO production by THALs is impaired. However, whether this alters NO's natriuretic effects and the mechanisms involved are unknown. In other cell types, ANG II augments phosphodiesterase 5 (PDE5)-mediated cGMP degradation. We hypothesized that NO-mediated inhibition of NKCC2 activity and stimulation of cGMP synthesis are blunted via PDE5 in ANG II-induced hypertension. Sprague-Dawley rats were infused with vehicle or ANG II (200 ng·kg-1·min-1) for 5 days. ET-1 reduced NKCC2 activity by 38 ± 13% (P < 0.05) in THALs from vehicle-treated rats but not from ANG II-hypertensive rats (Δ: -9 ± 13%). A NO donor yielded similar results as ET-1. In contrast, dibutyryl-cGMP significantly decreased NKCC2 activity in both vehicle-treated and ANG II-hypertensive rats (control: Δ-44 ± 15% vs. ANG II: Δ-41 ± 10%). NO increased cGMP by 2.08 ± 0.36 fmol/µg protein in THALs from vehicle-treated rats but only 1.06 ± 0.25 fmol/µg protein in ANG II-hypertensive rats (P < 0.04). Vardenafil (25 nM), a PDE5 inhibitor, restored NO's ability to inhibit NKCC2 activity in THALs from ANG II-hypertensive rats (Δ: -60 ± 9%, P < 0.003). Similarly, NO's stimulation of cGMP was also restored by vardenafil (vehicle-treated: 1.89 ± 0.71 vs. ANG II-hypertensive: 2.02 ± 0.32 fmol/µg protein). PDE5 expression did not differ between vehicle-treated and ANG II-hypertensive rats. We conclude that NO-induced inhibition of NKCC2 and increases in cGMP are blunted in ANG II-hypertensive rats due to PDE5 activation. Defects in the response of THALs to NO may enhance NaCl retention in ANG II-induced hypertension.

The anti-inflammatory peptide Ac-SDKP is released from thymosin-ß4 by renal meprin-α and prolyl oligopeptidase.

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a natural tetrapeptide with anti-inflammatory and antifibrotic properties. Previously, we have shown that prolyl oligopeptidase (POP) is involved in the Ac-SDKP release from thymosin-ß4 (Tß4). However, POP can only hydrolyze peptides shorter than 30 amino acids, and Tß4 is 43 amino acids long. This indicates that before POP hydrolysis takes place, Tß4 is hydrolyzed by another peptidase that releases NH2-terminal intermediate peptide(s) with fewer than 30 amino acids. Our peptidase database search pointed out meprin-α metalloprotease as a potential candidate. Therefore, we hypothesized that, prior to POP hydrolysis, Tß4 is hydrolyzed by meprin-α. In vitro, we found that the incubation of Tß4 with both meprin-α and POP released Ac-SDKP, whereas no Ac-SDKP was released when Tß4 was incubated with either meprin-α or POP alone. Incubation of Tß4 with rat kidney homogenates significantly released Ac-SDKP, which was blocked by the meprin-α inhibitor actinonin. In addition, kidneys from meprin-α knockout (KO) mice showed significantly lower basal Ac-SDKP amount, compared with wild-type mice. Kidney homogenates from meprin-α KO mice failed to release Ac-SDKP from Tß4. In vivo, we observed that rats treated with the ACE inhibitor captopril increased plasma concentrations of Ac-SDKP, which was inhibited by the coadministration of actinonin (vehicle, 3.1 ± 0.2 nmol/l; captopril, 15.1 ± 0.7 nmol/l; captopril + actinonin, 6.1 ± 0.3 nmol/l; P < 0.005). Similar results were obtained with urinary Ac-SDKP after actinonin treatment. We conclude that release of Ac-SDKP from Tß4 is mediated by successive hydrolysis involving meprin-α and POP.

Ac-SDKP suppresses TNF-α-induced ICAM-1 expression in endothelial cells via inhibition of IκB kinase and NF-κB activation.

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a naturally occurring tetrapeptide that prevents inflammation and fibrosis in hypertension and other cardiovascular diseases. We previously showed that, in angiotensin II-induced hypertension, Ac-SDKP decreased the activation of nuclear transcription factor NF-κB, whereas, in experimental autoimmune myocarditis and hypertension animal models, it also reduced the expression of endothelial leukocyte adhesion molecule ICAM-1. However, the mechanisms by which Ac-SDKP downregulated ICAM-1 expression are still unclear. TNF-α is a proinflammatory cytokine that induces ICAM-1 expression in various cell types via TNF receptor 1 and activation of the classical NF-κB pathway. We hypothesized that in endothelial cells Ac-SDKP suppresses TNF-α-induced ICAM-1 expression by decreasing IKK phosphorylation that as a consequence leads to a decrease of IκB phosphorylation and NF-κB activation. To test this hypothesis, human coronary artery endothelial cells were treated with Ac-SDKP and then stimulated with TNF-α. We found that TNF-α-induced ICAM-1 expression was significantly decreased by Ac-SDKP in a dose-dependent manner. Ac-SDKP also decreased TNF-α-induced NF-κB translocation from cytosol to nucleus, as assessed by electrophoretic mobility shift assay, which correlated with a decrease in IκB phosphorylation. In addition, we found that Ac-SDKP decreased TNF-α-induced IKK phosphorylation and IKK-ß expression. However, Ac-SDKP had no effect on TNF-α-induced phosphorylation of p38 MAP kinase or ERK. Thus we conclude that Ac-SDKP inhibition of TNF-α activation of canonical, i.e., IKK-ß-dependent, NF-κB pathway and subsequent decrease in ICAM-1 expression is achieved via inhibition of IKK-ß.

Cardiac-deleterious role of galectin-3 in chronic angiotensin II-induced hypertension.

Galectin-3 (Gal-3), a member of the ß-galactoside lectin family, has an important role in immune regulation. In hypertensive rats and heart failure patients, Gal-3 is considered a marker for an unfavorable prognosis. Nevertheless, the role and mechanism of Gal-3 action in hypertension-induced target organ damage are unknown. We hypothesized that, in angiotensin II (ANG II)-induced hypertension, genetic deletion of Gal-3 prevents left ventricular (LV) adverse remodeling and LV dysfunction by reducing the innate immune responses and myocardial fibrosis. To induce hypertension, male C57BL/6J and Gal-3 knockout (KO) mice were infused with ANG II (3 µg·min-1·kg-1 sc) for 8 wk. We assessed: 1) systolic blood pressure by plethysmography, 2) LV function and remodeling by echocardiography, 3) myocardial fibrosis by histology, 4) cardiac CD68+ macrophage infiltration by histology, 5) ICAM-1 and VCAM-1 expression by Western blotting, 6) plasma cytokines, including interleukin-6 (IL-6), by enzyme-linked immunosorbent assay, and 7) regulatory T (Treg) cells by flow cytometry as detected by their combined expression of CD4, CD25, and FOXP3. Systolic blood pressure and cardiac hypertrophy increased similarly in both mouse strains when infused with ANG II. However, hypertensive C57BL/6J mice suffered impaired ejection and shortening fractions. In these mice, the extent of myocardial fibrosis and macrophage infiltration was greater in histological sections, and cardiac ICAM-1, as well as plasma IL-6, expression was higher as assessed by Western blotting. However, all these parameters were blunted in Gal-3 KO mice. Hypertensive Gal-3 KO mice also had a higher number of splenic Treg lymphocytes. In conclusion, in ANG II-induced hypertension, genetic deletion of Gal-3 prevented LV dysfunction without affecting blood pressure or LV hypertrophy. This study indicates that the ANG II effects are, in part, mediated or triggered by Gal-3 together with the related intercellular signaling (ICAM-1 and IL-6), leading to cardiac inflammation and fibrosis.

Angiotensin II-induced hypertension blunts thick ascending limb NO production by reducing NO synthase 3 expression and enhancing threonine 495 phosphorylation.

Thick ascending limbs reabsorb 30% of the filtered NaCl load. Nitric oxide (NO) produced by NO synthase 3 (NOS3) inhibits NaCl transport by this segment. In contrast, chronic angiotensin II (ANG II) infusion increases net thick ascending limb transport. NOS3 activity is regulated by changes in expression and phosphorylation at threonine 495 (T495) and serine 1177 (S1177), inhibitory and stimulatory sites, respectively. We hypothesized that NO production by thick ascending limbs is impaired by chronic ANG II infusion, due to reduced NOS3 expression, increased phosphorylation of T495, and decreased phosphorylation of S1177. Rats were infused with 200 ng·kg(-1)·min(-1) ANG II or vehicle for 1 and 5 days. ANG II infusion for 5 days decreased NOS3 expression by 40 ± 12% (P < 0.007; n = 6) and increased T495 phosphorylation by 147 ± 26% (P < 0.008; n = 6). One-day ANG II infusion had no significant effect. NO production in response to endothelin-1 was blunted in thick ascending limbs from ANG II-infused animals [ANG II -0.01 ± 0.06 arbitrary fluorescence units (AFU)/min vs. 0.17 ± 0.02 AFU/min in controls; P < 0.01]. This was not due to reduced endothelin-1 receptor expression. Phosphatidylinositol 3,4,5-triphosphate (PIP3)-induced NO production was also reduced in ANG II-infused rats (ANG II -0.07 ± 0.06 vs. 0.13 ± 0.04 AFU/min in controls; P < 0.03), and this correlated with an impaired ability of PIP3 to increase S1177 phosphorylation. We conclude that in ANG II-induced hypertension NO production by thick ascending limbs is impaired due to decreased NOS3 expression and altered phosphorylation.

N-Acetyl-Seryl-Aspartyl-Lysyl-Proline: mechanisms of renal protection in mouse model of systemic lupus erythematosus.

Systemic lupus erythematosus is an autoimmune disease characterized by the development of auto antibodies against a variety of self-antigens and deposition of immune complexes that lead to inflammation, fibrosis, and end-organ damage. Up to 60% of lupus patients develop nephritis and renal dysfunction leading to kidney failure. N-acetyl-seryl-aspartyl-lysyl-proline, i.e., Ac-SDKP, is a natural tetrapeptide that in hypertension prevents inflammation and fibrosis in heart, kidney, and vasculature. In experimental autoimmune myocarditis, Ac-SDKP prevents cardiac dysfunction by decreasing innate and adaptive immunity. It has also been reported that Ac-SDKP ameliorates lupus nephritis in mice. We hypothesize that Ac-SDKP prevents lupus nephritis in mice by decreasing complement C5-9, proinflammatory cytokines, and immune cell infiltration. Lupus mice treated with Ac-SDKP for 20 wk had significantly lower renal levels of macrophage and T cell infiltration and proinflammatory chemokine/cytokines. In addition, our data demonstrate for the first time that in lupus mouse Ac-SDKP prevented the increase in complement C5-9, RANTES, MCP-5, and ICAM-1 kidney expression and it prevented the decline of glomerular filtration rate. Ac-SDKP-treated lupus mice had a significant improvement in renal function and lower levels of glomerular damage. Ac-SDKP had no effect on the production of autoantibodies. The protective Ac-SDKP effect is most likely achieved by targeting the expression of proinflammatory chemokines/cytokines, ICAM-1, and immune cell infiltration in the kidney, either directly or via C5-9 proinflammatory arm of complement system.

Tubuloglomerular and connecting tubuloglomerular feedback during inhibition of various Na transporters in the nephron.

Afferent (Af-Art) and efferent arterioles resistance regulate glomerular capillary pressure. The nephron regulates Af-Art resistance via: 1) vasoconstrictor tubuloglomerular feedback (TGF), initiated in the macula densa via Na-K-2Cl cotransporters (NKCC2) and 2) vasodilator connecting tubuloglomerular feedback (CTGF), initiated in connecting tubules via epithelial Na channels (ENaC). Furosemide inhibits NKCC2 and TGF. Benzamil inhibits ENaC and CTGF. In vitro, CTGF dilates preconstricted Af-Arts. In vivo, benzamil decreases stop-flow pressure (PSF), suggesting that CTGF antagonizes TGF; however, even when TGF is blocked, CTGF does not increase PSF, suggesting there is another mechanism antagonizing CTGF. We hypothesize that in addition to NKCC2, activation of Na/H exchanger (NHE) antagonizes CTGF, and when both are blocked CTGF dilates Af-Arts and this effect is blocked by a CTGF inhibitor benzamil. Using micropuncture, we studied the effects of transport inhibitors on TGF responses by measuring PSF while increasing nephron perfusion from 0 to 40 nl/min. Control TGF response (-7.9 ± 0.2 mmHg) was blocked by furosemide (-0.4 ± 0.2 mmHg; P < 0.001). Benzamil restored TGF in the presence of furosemide (furosemide: -0.2 ± 0.1 vs. furosemide+benzamil: -4.3 ± 0.3 mmHg; P < 0.001). With furosemide and NHE inhibitor, dimethylamiloride (DMA), increase in tubular flow increased PSF (furosemide+DMA: 2.7 ± 0.5 mmHg, n = 6), and benzamil blocked this (furosemide+DMA+benzamil: -1.1 ± 0.2 mmHg; P < 0.01, n = 6). We conclude that NHE in the nephron decreases PSF (Af-Art constriction) when NKCC2 and ENaC are inhibited, suggesting that in the absence of NKCC2, NHE causes a TGF response and that CTGF dilates the Af-Art when TGF is blocked with NKCC2 and NHE inhibitors.

Activation of angiotensin II type 2 receptor suppresses TNF-α-induced ICAM-1 via NF-кB: possible role of ACE2.

ANG II type 2 receptor (AT2) and ANG I-converting enzyme 2 (ACE2) are important components of the renin-ANG system. Activation of AT2 and ACE2 reportedly counteracts proinflammatory effects of ANG II. However, the possible interaction between AT2 and ACE2 has never been established. We hypothesized that activation of AT2 increases ACE2 activity, thereby preventing TNF-α-stimulated ICAM-1 expression via inhibition of NF-κB signaling. Human coronary artery endothelial cells were pretreated with AT2 antagonist PD123319 (PD) or ACE2 inhibitor DX600 and then stimulated with TNF-α in the presence or absence of AT2 agonist CGP42112 (CGP). We found that AT2 agonist CGP increased both ACE2 protein expression and activity. This effect was blunted by AT2 antagonist PD. ICAM-1 expression was very low in untreated cells but greatly increased by TNF-α. Activation of AT2 with agonist CGP or with ANG II under concomitant AT1 antagonist reduced TNF-α-induced ICAM-1 expression, which was reversed by AT2 antagonist PD or ACE2 inhibitor DX600 or knockdown of ACE2 with small interfering RNA. AT2 activation also suppressed TNF-α-stimulated phosphorylation of inhibitory κB (p-IκB) and NF-κB activity. Inhibition of ACE2 reversed the inhibitory effect of AT2 on TNF-α-stimulated p-IκB and NF-κB activity. Our findings suggest that stimulation of AT2 reduces TNF-α-stimulated ICAM-1 expression, which is partly through ACE2-mediated inhibition of NF-κB signaling.

Mechanism of impaired afferent arteriole myogenic response in Dahl salt-sensitive rats: role of 20-HETE.

The afferent arteriole (Af-Art) controls glomerular capillary pressure, an important determinant of glomerular injury. Af-Art myogenic response is mediated by ATP, and ATP signaling is in turn mediated by 20-HETE. Dahl salt-sensitive rats (Dahl SS) have decreased renal 20-HETE production. We hypothesized that Dahl SS have an impaired myogenic response and constrictor response to ATP, due to decreased 20-HETE. Af-Arts from Dahl SS or Dahl salt-resistant rats (Dahl SR) were microdissected and perfused. When myogenic response was induced by increasing Af-Art perfusion pressure from 60 to 140 mmHg, luminal Af-Art diameter decreased in Dahl SR but not in Dahl SS (-3.1 ± 0.8 vs. 0.5 ± 0.8 µm, P < 0.01). The 20-HETE antagonist 20-HEDE (10(-6) M) blocked the myogenic response in Dahl SR but had no effect in Dahl SS. Addition of a subconstrictor concentration of 20-HETE (but not a subconstrictor concentration of norepinephrine) restored the myogenic response in Dahl SS. We then perfused Af-Arts at 60 mmHg and tested the effects of the ATP analog α,ß-methylene-ATP (10(-6) M). Maximum ATP-induced constriction was attenuated in Dahl SS compared with Dahl SR (1.5 ± 0.5 vs. 7.4 ± 0.8 µm, P < 0.001). 20-HEDE attenuated ATP-induced Af-Art constriction in Dahl SR but not in Dahl SS, and consequently, ATP-induced constriction was no longer different between strains. In conclusion, Dahl SS have an impaired myogenic response and ATP-induced Af-Art constriction due to a decrease in Af-Art 20-HETE. The impaired myogenic responses may contribute to the nephrosclerosis that develops in Dahl SS.

Aldosterone sensitizes connecting tubule glomerular feedback via the aldosterone receptor GPR30.

Increasing Na delivery to epithelial Na channels (ENaC) in the connecting tubule (CNT) dilates the afferent arteriole (Af-Art), a process we call connecting tubule glomerular feedback (CTGF). We hypothesize that aldosterone sensitizes CTGF via a nongenomic mechanism that stimulates CNT ENaC via the aldosterone receptor GPR30. Rabbit Af-Arts and their adherent CNTs were microdissected and simultaneously perfused. Two consecutive CTGF curves were elicited by increasing luminal NaCl in the CNT. During the control period, the concentration of NaCl that elicited a half-maximal response (EC50) was 37.0 ± 2.0 mmol/l; addition of aldosterone 10(-8) mol/l to the CNT lumen caused a left-shift (decrease) in EC50 to 19.3 ± 1.3 mmol/l (P = 0.001 vs. control; n = 6). Neither the transcription inhibitor actinomycin D nor the translation inhibitor cycloheximide prevented the effect of aldosterone (control EC50 = 34.7 ± 1.9 mmol/l; aldosterone+actinomycin D EC50 = 22.6 ± 1.6 mmol/l; P < 0.001 and control EC50 = 32.4 ± 4.3 mmol/l; aldosterone+cycloheximide EC50 = 17.4 ± 3.3 mmol/l; P < 0.001). The aldosterone antagonist eplerenone prevented the sensitization of CTGF by aldosterone (control EC50 = 33.2 ± 1.7 mmol/l; aldosterone+eplerenone EC50 = 33.5 ± 1.3 mmol/l; n = 7). The GPR30 receptor blocker G-36 blocked the sensitization of CTGF by aldosterone (aldosterone EC50 = 16.5 ± 1.9 mmol/l; aldosterone+G-36 EC50 = 29.0 ± 2.1 mmol/l; n = 7; P < 0.001). Finally, we found that the sensitization of CTGF by aldosterone was mediated, at least in part, by the sodium/hydrogen exchanger (NHE). We conclude that aldosterone in the CNT lumen sensitizes CTGF via a nongenomic effect involving GPR30 receptors and NHE. Sensitized CTGF induced by aldosterone may contribute to renal damage by increasing Af-Art dilation and glomerular capillary pressure (glomerular barotrauma).

Thymosin-ß4 prevents cardiac rupture and improves cardiac function in mice with myocardial infarction.

Thymosin-ß4 (Tß4) promotes cell survival, angiogenesis, and tissue regeneration and reduces inflammation. Cardiac rupture after myocardial infarction (MI) is mainly the consequence of excessive regional inflammation, whereas cardiac dysfunction after MI results from a massive cardiomyocyte loss and cardiac fibrosis. It is possible that Tß4 reduces the incidence of cardiac rupture post-MI via anti-inflammatory actions and that it decreases adverse cardiac remodeling and improves cardiac function by promoting cardiac cell survival and cardiac repair. C57BL/6 mice were subjected to MI and treated with either vehicle or Tß4 (1.6 mg·kg(-1)·day(-1) ip via osmotic minipump) for 7 days or 5 wk. Mice were assessed for 1) cardiac remodeling and function by echocardiography; 2) inflammatory cell infiltration, capillary density, myocyte apoptosis, and interstitial collagen fraction histopathologically; 3) gelatinolytic activity by in situ zymography; and 4) expression of ICAM-1 and p53 by immunoblot analysis. Tß4 reduced cardiac rupture that was associated with a decrease in the numbers of infiltrating inflammatory cells and apoptotic myocytes, a decrease in gelatinolytic activity and ICAM-1 and p53 expression, and an increase in the numbers of CD31-positive cells. Five-week treatment with Tß4 ameliorated left ventricular dilation, improved cardiac function, markedly reduced interstitial collagen fraction, and increased capillary density. In a murine model of acute MI, Tß4 not only decreased mortality rate as a result of cardiac rupture but also significantly improved cardiac function after MI. Thus, the use of Tß4 could be explored as an alternative therapy in preventing cardiac rupture and restoring cardiac function in patients with MI.

N-acetyl-seryl-aspartyl-lysyl-proline reduces cardiac collagen cross-linking and inflammation in angiotensin II-induced hypertensive rats.

We have reported previously that Ac-SDKP (N-acetyl-seryl-aspartyl-lysyl-proline) reduces fibrosis and inflammation (in macrophages and mast cells). However, it is not known whether Ac-SDKP decreases collagen cross-linking and lymphocyte infiltration; lymphocytes modulate both collagen cross-linking and ECM (extracellular matrix) formation in hypertension. Thus we hypothesized that (i) in AngII (angiotensin II)-induced hypertension, Ac-SDKP prevents increases in cross-linked and total collagen by down-regulating LOX (lysyl oxidase), the enzyme responsible for cross-linking, and (ii) these effects are associated with decreased pro-fibrotic cytokine TGFß (transforming growth factor ß) and the pro-inflammatory transcription factor NF-κB (nuclear factor κB) and CD4+/CD8+ lymphocyte infiltration. We induced hypertension in rats by infusing AngII either alone or combined with Ac-SDKP for 3 weeks. Whereas Ac-SDKP failed to lower BP (blood pressure) or LV (left ventricular) hypertrophy, it did prevent AngII-induced increases in (i) cross-linked and total collagen, (ii) LOX mRNA expression and LOXL1 (LOX-like 1) protein, (iii) TGFß expression, (iv) nuclear translocation of NF-κB, (v) CD4+/CD8+ lymphocyte infiltration, and (vi) CD68+ macrophages infiltration. In addition, we found a positive correlation between CD4+ infiltration and LOXL1 expression. In conclusion, the effect of Ac-SDKP on collagen cross-linking and total collagen may be due to reduced TGFß1, LOXL1, and lymphocyte and macrophage infiltration, and its effect on inflammation could be due to lower NF-κB.