Afferent Arteriolar Vasoconstriction Research Articles

Tailored Pharmacotherapy - Clinical Pharmacological Fine-Tuning to Optimize Acute and Long-Term PatientʼS and TransplantʼS Outcome

Background: There is essential demand for fine-tuning drug therapy esp. in the comorbid transplant patient (P) to contribute to improvement of long-term outcome after transplantation (TX). Tailored pharmacotherapy (TPHT) is a most important step of further post-transplant risk factor management because acute and chronic even vital risk resulting from polypharmacy reveals severely pronounced in this P group. Procedure: Synoptic scale of clinical pharmacology and internal medicine to elucidate acute and long-term pharmacological aspects for further algorithm in TPHT after TX. Advices are drawn from daily involvement in a broad spectrum of post-transplant challenges as in the severely acute intensive care unit P as well as in the cardiovascular and infectious disease and neoplasm affected long-term transplant P. A decade of experience in the fields of TX including review of updated literature is the basis of our attempt to define algorithm for TPHT. Resulting clinical pharmacological advices: Restriction to the essential therapeutic drug monitoring itself to avoid immunosuppressive risks does not sufficiently cover the individual P's requirements resulting from his comorbidities and organ status. All kind of comedication, transplant function, acute and chronic comorbidities, age of P, renal, hepatic and bone marrow function have to be focussed on pharmacologically to treat the individual P instead of addressing his diseases separately. Adverse drug effects and less familiar drug-drug interactions may become lifethreatening and thus have to be aware of continuously. We predominantly focus on adverse effects resulting from standard therapeutic regimen after TX. They themselves require accurate institution of further finetuned TPHT. Myelosuppressive prophylactic regimens are addressed. More agents shown result in alterations of trough levels of calcineurin inhibitors (CNI) than we are familiar with. Being aware that incidence of hypertension is 60-80% in renal TX and knowing from literature that 50-60% of P die from cardiovascular deaths requires more effective and optimal antihypertensive posttransplant treatment. As Ca-channel blockers promote vasodilatation of afferent arterioles which may counteract CNI induced afferent arteriolar vasoconstriction this might be appropriate in the early hypertensive stage and preserve kidney as well. We focus on ACE inhibitors and on angiotensin-II receptor blockers reducing proteinuria, serum TGF-Ãβ1 and endothelin in P with CNI associated nephropathy. Switching to mTOR inhibitors or MMF may be beneficial in this context. HMG-CoA reductase inhibitors always need dose adaption to renal function. Coadministration of azoles, macrolides plus sirolimus has to be avoided because of additive hepatotoxicity. Additive nephrotoxicity may result from CNI plus carbapenems or ACE-inhibitor plus NSAIDs. Switching from CNI to mTOR inhibitor with its antiproliferative effect may offer a potent option to overcome post-transplant lymphoproliferative disorder avoiding life-threatening chemotherapy. Challenging responsibility: Tailored pharmacotherapy means 1) to know the individual P with his entire comorbidities and organ functions 2) to be familiar with the spectrum of different strategic options of pharmacotherapy including all associated interactive and adverse effects 3) to consider both continuously in synoptic view to achieve optimal treatment in each individual P. To afford adequate protection of P and transplant tailored pharmacotherapy should be guaranteed as an important treatment concept.

Abstract 491: Activation of Voltage-dependent L-type Calcium Channels and the Rho Kinase Pathway Are Involved in S1P-induced Vasoconstriction in Rat Juxtamedullary Afferent Arterioles

Sphingosine-1-phosphate (S1P) is an important regulator of resistance vessels and is implicated in pathological processes including diabetic nephropathy and acute kidney injury. Previous studies show that S1P causes marked vasoconstriction in preglomerular but not postglomerular microvessels, however, the intracellular signaling pathways underlying S1P-mediated vasoconstriction are undefined. We postulated that S1P-mediated afferent arteriolar (AA) vasoconstriction involves activation of voltage-dependent L-type Ca 2+ channels (L-VDCC) and the rho kinase pathway. Studies were conducted in vitro using the blood-perfused juxtamedullary nephron preparation. Superfusion of S1P (n=6) evoked concentration-dependent AA vasoconstriction. Control diameter averaged 13.8±0.9 μm and declined to 95±2, 85±4, 75±6, 60±5 and 46±5% of control diameter in response to S1P (from 10 -9 to 10 -5 M), respectively. Superfusion with nifedipine (10 -6 M, n=6), a L-VDCC inhibitor, increased basal diameter by 39±18% (p<0.05) and significantly attenuated the S1P-induced vasoconstriction. S1P reduced AA diameter to 99±1, 98±1, 90±3, 79±3 and 52±5% of control (p<0.05 vs. S1P alone at 10 -8 to 10 -6 M) during nifedipine treatment. Superfusion with the rho kinase inhibitor, Y27632 (10 -5 M, n=6), increased diameter by 60±12% and inhibited S1P-induced vasoconstriction. AA diameter averaged 103±1, 101±1, 93±3, 72±9 and 56±10% of control in response to increasing concentrations of S1P (p<0.05 vs. S1P alone at 10 -9 to 10 -7 M). In contrast, S1P-induced vasoconstriction was unaltered by Ca 2+ store depletion using the Ca 2+ -ATPase inhibitors, thapsigargin (Tg) and cyclopiazonic acid (CPA). Although Tg (10 -6 M, n=4) increased diameter by 38±16%, the S1P-induced vasoconstrictor profile was not different from that without Tg. AA declined to 99±3, 95±5, 85±10, 60±13 and 32±8% (p>0.05), respectively. Similar results were obtained with CPA (10 -5 M, n=6). S1P reduced diameter to 97±2, 93±2, 75±5, 53±6 and 36±5% of the control (p>0.05), respectively, suggesting that mobilization of intracellular Ca 2+ store is not required for S1P-induced vasoconstriction. Overall, these data indicate that both L-VDCC and rho kinase pathways contribute to S1P-mediated AA vasoconstriction.

Abstract 393: Chymase: Augmented Renal Tubular Protein Expression in Diabetes

Chymase is a serine protease that is predominantly found stored in granules of mast cells. Chymase expression is increased in mesangial cells, podocytes, vascular smooth muscle cells, and cardiomyocytes under hyperglycemic culture conditions. We have presented evidence for enhanced intrarenal chymase-dependent Ang II formation on afferent arteriole vasoconstriction in the diabetic kidney ( Hypertension. 56: e55, 2010). We tested the hypothesis that renal chymase protein expression is enhanced in the diabetic kidney. Kidneys were harvested from littermate control ( db/m ) and type II diabetic ( db/db ) mice at 36-wk-old. Body weight was significantly lower (33 ± 3 vs 40 ± 2 g), while plasma glucose was significantly higher (552 ± 40 vs 220 ± 21 mg/dl) in diabetic (n=6) compared to control (n=7) mice. However, plasma insulin levels were not different from controls (0.77 ± 0.10 vs 0.43 ± 0.11 mg/L), indicating that the diabetic mice are transitioning from type II to type I diabetic disease. Immunohistochemistry was performed using primary rabbit polyclonal antibodies [mMCP-4 (chymase, A Husain), anti-malignant T cell amplified sequence 1 (chymase, Abcam), H + -ATPase (D Brown), and aquaporin 2 (AQP2, J Sands)]. Renal subcapsular and adipose tissue mast cell granules stained positively for toluidine blue and chymase. Chymase protein expression was localized to the apical membrane of connecting tubules and cortical collecting ducts of both control and diabetic kidneys. Chymase was co-localized to AQP2 positive and H + -ATPase negative cells indicating that chymase is specifically expressed in the principal cells of the distal nephron. Prominent chymase immunostaining was observed in the renal papilla and endothelial cells of renal arteries and arterioles in all diabetic, but none of the control kidneys. Chymase was not localized to renal vascular smooth muscle cells. The renal localization of chymase was identical using custom and commercially available polyclonal antibodies. These are the first studies to identify chymase protein expression in the principal cells of the distal nephron. These novel data suggest that the serine protease actions of chymase may be linked to altered epithelial transport mechanisms in the distal nephron in diabetic kidney disease.

Role of Purinoceptors in Regulating Afferent Arteriolar Diameter in DOCA‐salt Hypertensive Rats

Afferent arterioles (AA) are highly responsive to purinoceptor stimulation. ATP contributes importantly to renal autoregulation by activating P2 receptors. Pressure‐mediated, and ATP and β,γ‐methylene ATP (P2X1 receptor agonist)‐induced AA vasoconstriction is impaired in DOCA‐salt hypertensive rats (DOCA rats) while the function of other purinoceptors is unclear. We postulated that P2Y and A1 receptor reactivity is intact in AA from DOCA rats. AA responses to UTP (a P2Y2 receptor agonist) and adenosine were studied in juxtamedullary nephrons. Systolic blood pressure averaged 125±5 and 180±7 mmHg in uninephrectomized (UNx) and DOCA rats, respectively. UTP (10−8 to 10−4 mol/L, n=5) evoked concentration‐dependent vasoconstriction of AA in UNx rats. Diameter declined by 2±1, 6±2, 12±2, 23±5 and 49±9%, respectively. UTP‐induced vasoconstriction was unchanged in DOCA rats (n=5, p>0.05) leading to declines of 3±2, 10±2, 14±3, 29±5 and 43±9%, respectively. The AA response to adenosine was also indistinguishable between two groups (n=6 each, p>0.05). Preglomerular microvascular P2X1 receptor protein expression was unaltered in DOCA rats. These data demonstrate that AA P2Y2 and A1 receptor signaling is preserved in DOCA rats. This study suggests that impairment of AA autoregulatory behavior in DOCA‐salt hypertension cannot be attributed to impaired P2Y2 and A1 receptor signaling.

Mechanism of hyperfiltration

Abstract Glomerular hyperfiltration associated with early diabetes mellitus (DM) is a risk factor for the development of progressive diabetic nephropathy. Though pathogenesis of hyperfiltration is not well-known but has been attributed to glomerular hemodynamic and tubular factors. Renal hyperfiltration due to hemodynamic function abnormalities are seen in animal models of DM, including increased intraglomerular capillary pressure and glomerular hyperfiltration and thought to be due to afferent arteriolar vasodilatation, efferent vasoconstriction, and suppression of tubuloglomerular feedback.

Annexin A1 protein attenuates cyclosporine-induced renal hemodynamics changes and macrophage infiltration in rats

Cyclosporine (CsA) remains an important immunosuppressant for transplantation and for treatment of autoimmune diseases. The most troublesome side effect of CsA is renal injury. Acute CsA-induced nephrotoxicity is characterized by reduced renal blood flow (RBF) and glomerular filtration rate (GFR) due to afferent arteriole vasoconstriction. Annexin A1 (ANXA1) is a potent anti-inflammatory protein with protective effect in renal ischemia/reperfusion injury. Here we study the effects of ANXA1 treatment in an experimental model of acute CsA nephrotoxicity. Salt-depleted rats were randomized to treatment with VH (vehicles 1mL/kg body weight/day), ANXA1 (Ac2-26 peptide 1mg/kg body weight/day intraperitoneally), CsA (20mg/kg body weight/day subcutaneously) and CsA+ANXA1 (combination) for seven days. We compared renal function and hemodynamics, renal histopathology, renal tissue macrophage infiltration and renal ANXA1 expression between the four groups. CsA significantly impaired GFR and RBF, caused tubular dilation and macrophage infiltration and increased ANXA1 renal tissue expression. Treatment with ANXA1 attenuated CSA-induced hemodynamic changes, tubular injury and macrophage infiltration. ANXA1 treatment attenuated renal hemodynamic injury and inflammation in an acute CsA nephrotoxicity model.

The Renal Protective Effect of Erythropoietin on Acute Ischemic Injury in Kidney Transplantation

During kidney transplantation, renal ischemia, and reperfusion injury (IRI) is associated with reduced microvasculature blood flow and loss of glycocalyx integrity, thereby leading to a reduction in glomerular filtration rate, afferent arteriolar vasoconstriction, and transtubular back leak of ultrafiltrate [ 1 Snoeijs M.G. van Heurn L.E. Buurman W.A. Biological modulation of renal ischemia-reperfusion injury. Current Opinion in Organ Transplantation. 2010; 15: 190 Crossref PubMed Scopus (49) Google Scholar , 2 Snoeijs M.G. Vink H. Voesten N. et al. Acute ischemic injury to the renal microvasculature in human kidney transplantation. Am J Physiol Renal Physiol. 2010; 299: F1134 Crossref PubMed Scopus (86) Google Scholar ]. Moreover, IRI propagates inflammation by up-regulating genes involved with toll-like receptors, components of the complement pathway, chemokines, and adhesion molecules, and promotes graft rejection by up-regulating genes involved with oxidative stress and apoptosis [ 1 Snoeijs M.G. van Heurn L.E. Buurman W.A. Biological modulation of renal ischemia-reperfusion injury. Current Opinion in Organ Transplantation. 2010; 15: 190 Crossref PubMed Scopus (49) Google Scholar , 2 Snoeijs M.G. Vink H. Voesten N. et al. Acute ischemic injury to the renal microvasculature in human kidney transplantation. Am J Physiol Renal Physiol. 2010; 299: F1134 Crossref PubMed Scopus (86) Google Scholar , 3 Furuichi K. Kaneko S. Wada T. Chemokine/chemokine receptor-mediated inflammation regulates pathologic changes from acute kidney injury to chronic kidney disease. Clin Exp Nephrol. 2009; 13: 9 Crossref PubMed Scopus (71) Google Scholar ]. Collectively, these processes are responsible for a high rate of graft rejection, morbidity, and mortality following kidney transplantation. Therefore, it is clinically relevant and important to determine renal protective mechanisms to modulate this acute ischemic injury [ 4 Toledo-Pereyra LH. Leukocyte depletion, ischemic injury, and organ preservation. J Surg Res (in press, corrected proof). Google Scholar ]. Erythropoietin Ameliorates Renal Ischemia and Reperfusion Injury via Inhibiting Tubulointerstitial InflammationJournal of Surgical ResearchVol. 176Issue 1PreviewTubulointerstitial inflammation is the characteristics of renal ischemia reperfusion injury (IRI) that is inevitable in kidney transplantation. Erythropoietin (EPO) has recently been shown to have protective effects on renal IRI by anti-apoptosis and anti-oxidation. Here, the effect and mechanism of EPO on renal IRI were further investigated, with a focus on tubulointerstitial inflammation. Full-Text PDF

Arterial imaging in patients with lower extremity ischemia and diabetes mellitus

Precise, comprehensive imaging of the arterial circulation is the cornerstone of successful revascularization of the ischemic extremity in patients with diabetes mellitus. Arterial imaging is challenging in these patients because the disease is often multisegmental with a predilection for the distal tibial and peroneal arteries. Occlusive lesions and the arterial wall itself are often calcified and patients presenting with ischemic complications frequently have underlying renal insufficiency. Intra-arterial digital subtraction angiography (DSA), contrast enhanced magnetic resonance angiography (MRA), and more recently, computerized tomographic angiography (CTA) have been used as imaging modalities in lower extremity ischemia. Each has specific advantages and shortcomings in this patient population, which will be summarized and contrasted in this review. DSA is an invasive technique most often performed from a femoral arterial puncture and requires the injection of arterial contrast, which can occasionally cause allergic reactions. In patients with pre-existing renal insufficiency, contrast infusion can result in worsening renal failure; although usually self-limited, it may occasionally require hemodialysis, especially in patients with diabetes. However, DSA provides the highest degree of spatial resolution and image quality. It is also the only modality in which the diagnosis and treatment of arterial disease can be performed simultaneously. MRA is noninvasive, and when enhanced with gadolinium contrast injection provides arterial images of comparable quality to DSA and in some circumstances may uncover distal arterial targets not visualized on DSA. However, spatial resolution is inferior to DSA and erroneous interpretations due to acquisition artifacts are common. Specialized equipment and imaging techniques are necessary to minimize their occurrence in the distal lower extremity. In addition, due to the risk of inducing nephrogenic systemic fibrosis, gadolinium-enhanced MRA cannot be used in patients with renal insufficiency. CTA is noninvasive and rapidly performed, with better spatial resolution than MRA, but requires the largest volume of contrast infusion, exposes patients to high-doses of radiation, and is subject to interpretive error due to reconstruction artifacts especially in heavily calcified arteries, limiting its usefulness in many patients with diabetes. For patients in whom the planned intervention is a surgical bypass, DSA and MRA will provide high quality images of the lower extremity arterial anatomy. For patients in whom a catheter-based intervention is the likely treatment, a diagnostic DSA immediately followed by a catheter-based treatment in the same procedure is the preferred approach. In patients with pre-existing renal dysfunction, in which gadolinium-enhanced MRA is contraindicated, DSA or CTA can be performed. However, patients should have an infusion of intravenous normal saline solution or sodium bicarbonate before the procedure to reduce the incidence of contrast-induced nephropathy.

Direct demonstration of tubular fluid flow sensing by macula densa cells

Macula densa (MD) cells in the cortical thick ascending limb (cTAL) detect variations in tubular fluid composition and transmit signals to the afferent arteriole (AA) that control glomerular filtration rate [tubuloglomerular feedback (TGF)]. Increases in tubular salt at the MD that normally parallel elevations in tubular fluid flow rate are well accepted as the trigger of TGF. The present study aimed to test whether MD cells can detect variations in tubular fluid flow rate per se. Calcium imaging of the in vitro microperfused isolated JGA-glomerulus complex dissected from mice was performed using fluo-4 and fluorescence microscopy. Increasing cTAL flow from 2 to 20 nl/min (80 mM [NaCl]) rapidly produced significant elevations in cytosolic Ca(2+) concentration ([Ca(2+)](i)) in AA smooth muscle cells [evidenced by changes in fluo-4 intensity (F); F/F(0) = 1.45 ± 0.11] and AA vasoconstriction. Complete removal of the cTAL around the MD plaque and application of laminar flow through a perfusion pipette directly to the MD apical surface essentially produced the same results even when low (10 mM) or zero NaCl solutions were used. Acetylated α-tubulin immunohistochemistry identified the presence of primary cilia in mouse MD cells. Under no flow conditions, bending MD cilia directly with a micropipette rapidly caused significant [Ca(2+)](i) elevations in AA smooth muscle cells (fluo-4 F/F(0): 1.60 ± 0.12) and vasoconstriction. P2 receptor blockade with suramin significantly reduced the flow-induced TGF, whereas scavenging superoxide with tempol did not. In conclusion, MD cells are equipped with a tubular flow-sensing mechanism that may contribute to MD cell function and TGF.

Tubular reabsorption and diabetes‐induced glomerular hyperfiltration

Elevated glomerular filtration rate (GFR) is a common observation in early diabetes mellitus and closely correlates with the progression of diabetic nephropathy. Hyperfiltration has been explained to be the result of a reduced load of sodium and chloride passing macula densa, secondarily to an increased proximal reabsorption of glucose and sodium by the sodium-glucose co-transporters. This results in an inactivation of the tubuloglomerular feedback (TGF), leading to a reduced afferent arteriolar vasoconstriction and subsequently an increase in GFR. This hypothesis has recently been questioned due to the observation that adenosine A(1)-receptor knockout mice, previously shown to lack a functional TGF mechanism, still display a pronounced hyperfiltration when diabetes is induced. Leyssac demonstrated in the 1960s (Acta Physiol Scand58, 1963:236) that GFR and proximal reabsorption can work independently of each other. Furthermore, by the use of micropuncture technique a reduced hydrostatic pressure in Bowman's space or in the proximal tubule of diabetic rats has been observed. A reduced pressure in Bowman's space will increase the pressure gradient over the filtration barrier and can contribute to the development of diabetic hyperfiltration. When inhibiting proximal reabsorption with a carbonic anhydrase inhibitor, GFR decreases and proximal tubular pressure increases. Measuring intratubular pressure allows a sufficient time resolution to reveal that net filtration pressure decreases before TGF is activated which highlights the importance of intratubular pressure as a regulator of GFR. Taken together, these results imply that the reduced intratubular pressure observed in diabetes might be crucial for the development of glomerular hyperfiltration.

Adenosine A 1 Receptor Antagonists at a Fork in the Road

If you come to a fork in the road, take it. — —Yogi Berra, US baseball player, coach, and manager (1925–) Up to one half of patients with chronic heart failure and two thirds of patients with acute heart failure have concomitant renal dysfunction—a clinicopathophysiologic association that has been termed the cardiorenal syndrome.1 When present, baseline renal impairment contributes to adverse outcomes in both ambulatory and hospital settings, and the attributable risk increases with progression of both cardiac and renal disease. Although independent risk factors have been identified (eg, hypertension and diabetes) and underlying mechanisms described, targeted therapies are lacking. Complicating in-hospital management further, renal function may worsen, leading to diuretic resistance and an inability or unwillingness to administer life-prolonging therapies due to a circulatory-renal limitation. Against this backdrop, a novel class of drugs, the adenosine A1 receptor antagonists, has emerged as potential agents that can oppose an important pathway in the cardiorenal syndrome.2 Article see p 519 Adenosine, an endogenous purine nucleoside formed from the breakdown of adenosine triphosphate, exerts pleiotropic effects throughout the human body through its interaction with at least 4 receptor subtypes (A1, A2A, A2B, and A3).2,3 In the cardiovascular system, activation of A1 receptors reduces cardiac contractility, inhibits norepinephrine release from sympathetic nerve terminals, and suppresses sinoatrial and atrioventricular conduction. Recent data from transgenic mice also suggest that myocardial A1 receptors may contribute to adverse ventricular remodeling.4 The renal actions of adenosine mediated by A1 receptors are equally complex5 and include vasoconstriction of afferent arterioles, sodium reabsorption in the proximal tubules, and stimulation of tubuloglomerular feedback in the macula densa. The effect of adenosine on tubuloglomerular feedback is particularly relevant in states of volume overload such as heart …

Major role for ACE-independent intrarenal ANG II formation in type II diabetes

Combination therapy of angiotensin-converting enzyme (ACE) inhibition and AT(1) receptor blockade has been shown to provide greater renoprotection than ACE inhibitor alone in human diabetic nephropathy, suggesting that ACE-independent pathways for ANG II formation are of major significance in disease progression. Studies were performed to determine the magnitude of intrarenal ACE-independent formation of ANG II in type II diabetes. Although renal cortical ACE protein activity [2.1 +/- 0.8 vs. 9.2 +/- 2.1 arbitrary fluorescence units (AFU) x mg(-1) x min(-1)] and intensity of immunohistochemical staining were significantly reduced and ACE2 protein activity (16.7 +/- 3.2 vs. 7.2 +/- 2.4 AFU x mg(-1) x min(-1)) and intensity elevated, kidney ANG I (113 +/- 24 vs. 110 +/- 45 fmol/g) and ANG II (1,017 +/- 165 vs. 788 +/- 99 fmol/g) levels were not different between diabetic and control mice. Afferent arteriole vasoconstriction due to conversion of ANG I to ANG II was similar in magnitude in kidneys of diabetic (-28 +/- 3% at 1 microM) and control (-23 +/- 3% at 1 microM) mice; a response completely inhibited by AT(1) receptor blockade. In control kidneys, afferent arteriole vasoconstriction produced by ANG I was significantly attenuated by ACE inhibition, but not by serine protease inhibition. In contrast, afferent arteriole vasoconstriction produced by intrarenal conversion of ANG I to ANG II was significantly attenuated by serine protease inhibition, but not by ACE inhibition in diabetic kidneys. In conclusion, there is a switch from ACE-dependent to serine protease-dependent ANG II formation in the type II diabetic kidney. Pharmacological targeting of these serine protease-dependent pathways may provide further protection from diabetic renal vascular disease.

Yes, No, Maybe So

The myogenic response is an intrinsic vascular response characterized by vasoconstriction in response to increases and vasodilation to decreases in perfusion pressure. Recent studies suggest this response may play a significant role in the protection of the renal microcirculation from pressure dependent injury, especially with concomitant renal disease.1 Although the myogenic response was first described more than 100 years ago, the molecular mechanisms underlying the transduction of pressure into a cellular event (ie, vasoconstriction) in vascular smooth muscle cells (VSMCs) has remained elusive. Several potential molecules with a connection to mechanosensitive responses have been identified including integrins, transient receptor potential (TRP) channels, and members of the Epithelial Na+ Channel (ENaC) family.2 Members of the ENaC family have received recent attention because of evidence for and against a role for ENaC proteins in myogenic constriction in the renal circulation. The current study by Guan et al helps to address the controversial role of ENaC in afferent arteriolar myogenic constriction.3 ENaC and closely related Acid Sensing Ion Channel (ASIC) proteins are considered potential mechanotransducers in VSMCs because of their strong evolutionary connection to a group of proteins termed degenerins. The degenerins are a group of nematode Caenorhabditis elegans proteins that comprise the ion channel pore in a large multimeric mechanosensory complex.4 Genetic evidence suggests the pore forming degenerins are tethered to the cytoskeleton and extracellular matrix and aid in force transduction.4 The …

Established and Newly Proposed Mechanisms of Chronic Cyclosporine Nephropathy

Cyclosporine (CsA) has improved patient and graft survival rates following solid-organ transplantation and has shown significant clinical benefits in the management of autoimmune diseases. However, the clinical use of CsA is often limited by acute or chronic nephropathy, which remains a major problem. Acute nephropathy depends on the dosage of CsA and appears to be caused by a reduction in renal blood flow related to afferent arteriolar vasoconstriction. However, the mechanisms underlying chronic CsA nephropathy are not completely understood. Activation of the intrarenal renin-angiotensin system (RAS), increased release of endothelin-1, dysregulation of nitric oxide (NO) and NO synthase, up-regulation of transforming growth factor-beta1 (TGF-β1), inappropriate apoptosis, stimulation of inflammatory mediators, enhanced innate immunity, endoplasmic reticulum stress, and autophagy have all been implicated in the pathogenesis of chronic CsA nephropathy. Reducing the CsA dosage or using other renoprotective drugs (angiotensin II receptor antagonist, mycophenolate mofetil, and statins, etc.) may ameliorate chronic CsA-induced renal injury. This review discusses old and new concepts in CsA nephropathy and preventive strategies for this clinical dilemma.

Mysteries of Renal Autoregulation

Autoregulation is an important renal regulatory mechanism that provides an important protective role in glomerular hemodynamics.1 The phenomenon of autoregulatory behavior has been recognized for many decades, but a clear understanding of the underlying mechanisms involved in mediating resistance adjustments remains to be established. As illustrated in Figure 1, pressure-mediated autoregulatory behavior operates by recognizing a change in renal perfusion pressure, or renal vascular transmural pressure, and initiating induction of autoregulatory resistance changes intended to stabilize renal blood flow and/or glomerular filtration rate. The net result is that renal blood flow and glomerular filtration rate remain relatively stable over a wide range of renal perfusion pressures. Autoregulatory behavior is exquisitely sensitive in healthy normal kidneys but significantly less efficient in pathological settings, such as diabetes mellitus and some forms of hypertension. Complete loss of autoregulatory control results in renal blood flow that behaves more passively, thus increasing variability and rendering flow more directly determined by renal perfusion pressure.1,2 Figure 1. Outline of the pressure-induced signaling scheme postulated for autoregulatory adjustments in the afferent arteriolar diameter. Whole kidney autoregulation reflects regulatory input from the myogenic and tubuloglomerular feedback mechanisms at the very least.3–7 Recently, contributions of a third and possibly a fourth mechanism have been suggested.6,7 Consequently, with continued study, the mechanisms underlying renal autoregulatory resistance adjustments are slowly becoming clearer. In addition to the P1 receptor hypothesis put forth to explain signal transduction for tubuloglomerular feedback-dependent resistance changes, we have explored the hypothesis that pressure-mediated autoregulatory responses are mediated by extracellular ATP and activation of P2X1 receptors.3,8,9 Conclusive identification of the signaling molecules that mediate autoregulation remains a point of debate, but it appears clear that the release of a purine-based moiety is required for autoregulatory adjustments in afferent arteriolar resistance.3–5,8–10 This …

Factors associated with proteinuria in renal transplant recipients treated with sirolimus

Although sirolimus (SRL) use in renal allograft recipients (RTX) is associated with improved renal function, proteinuria develops in a significant proportion. 48 SRL-treated RTX were evaluated for development of proteinuria and stratified by level of proteinuria after SRL therapy. The Proteinuria Group (n = 25, 52.1%) had new-onset proteinuria or >25% increase in proteinuria following SRL conversion; the Nonproteinuria Group had stable proteinuria <0.5 g/day throughout. There was a higher proportion of male RTX and female donors to male recipients in the Proteinuria Group, (24% vs. 10%, P = 0.008). Calcineurin inhibitor- and statin usage were significantly higher in the Nonproteinuria Group (8% vs. 17%, P = 0.046; 28% vs. 83%, P < 0.001 respectively) whereas biopsy-proven acute rejection was higher in the Proteinuria Group (68% vs. 33%, P = 0.037). SDS-PAGE analysis of urine from 23 RTX in the Proteinuria Group demonstrated glomerular proteinuria in 100% and tubular proteinuria in 87%. While male gender and gender mismatch may impact on glomerular proteinuria through inadequate nephron dose and subsequent hyperfiltration, concurrent cyclosporine use may mitigate the development of proteinuria in SRL-treated patients, through afferent arteriolar vasoconstriction. Glomerular injury occurring following acute rejection may further contribute to glomerular proteinuria. Statins, through their anti-inflammatory and anti-fibrotic effects, may protect against development of proteinuria.

Intact renal afferent arteriolar autoregulatory responsiveness in db/db mice

The db/db mouse is a genetic model of type 2 diabetes that exhibits progressive renal disease. Obesity, hyperglycemia, and albuminuria (822 +/- 365 vs. 28 +/- 8 microg/day) are evident in 18-wk-old db/db compared with db/m (lean littermate control) mice. Our goal was to determine the blood pressure (BP) phenotype of the db/db mouse. Mean arterial BP measured in conscious mice by radiotelemetry was not different between db/db (n = 9) and db/m (n = 12) mice, averaging 113 +/- 3 and 112 +/- 2 mmHg, respectively. The circadian BP profile of db/db mice was shifted to the left and exhibited a significant reduction in amplitude compared with db/m mice. Heart rate (487 +/- 9 vs. 542 +/- 7 beats/min; P < 0.05) and locomotor activity were significantly reduced in db/db compared with db/m mice. We tested the hypothesis that intact afferent arteriole (AA) responsiveness to increases in renal artery pressure (RAP) and angiotensin (ANG) II sensitivity contributes to normal BP in this diabetic model. AA diameters of in vitro blood-perfused juxtamedullary nephrons of db/db mice (15.7 +/- 0.5 microm; n = 38) were significantly larger than those of db/m mice (12.5 +/- 0.4 microm; n = 37). AA responses to increases in RAP and ANG II were not different between kidneys of db/db and db/m mice. Significant AA vasoconstriction to 1 nM ANG II was observed in kidneys of db/db mice (-11 +/- 4%), while 10 nM ANG II decreased AA diameter in both groups [db/db, -20 +/- 4%, (n = 12); db/m, -26 +/- 4% (n = 12)]. In summary, AA responses to increases in renal perfusion pressure and ANG II remain intact in db/db mice. Diabetic renal disease occurs in db/db mice independently of elevated BP.

Effect of ET‐1 on afferent arterioles of ETB‐receptor deficient mice

Endothelins (ET‐1, ET‐2, ET‐3) are a family of regulatory peptides that have long‐lasting vasoconstrictor and pressor effects. Among them ET‐1 is the most active form that plays an important role in the regulation of renal function. The aim of the study was to investigate the contribution of ETA‐ and ETB‐receptors in ET‐1 induced effects on afferent arterioles. Adult ETB‐receptor deficient rescued mice and wild‐type mice were used. Cumulative dose‐response‐curves to ET‐1 (10−12 to 10−7 mol/L) were measured in isolated, perfused afferent arterioles, as well as during ETA‐receptor blockade. Further, dose‐response curves to the ETB‐receptor agonist 4‐Ala‐ET‐1 were measured. Results: 1. Afferent arterioles of wild‐type mice and ETB‐receptor deficient mice showed a concentration‐dependent constriction to ET‐1. 2. The dose‐response curves did not differ between ETB‐receptor deficient mice and wild‐type mice. 3. There was no response to ET‐1 during ETA‐receptor blockade in afferent arterioles of both groups. 4. Afferent arterioles of wild‐type mice and ETB‐receptor deficient mice did not significantly respond to 4‐Ala‐ET‐1. Conclusions: Results indicate that the ETA‐receptor mediates vasoconstriction in afferent arterioles and that there is a lack of functional ETB‐receptor activity in afferent arterioles.

Direct demonstration of tubular fluid flow sensing by macula densa cells

Macula densa (MD) cells in the cortical thick ascending limb (cTAL) detect variations in tubular fluid composition and transmit signals to the afferent arteriole (AA) that control glomerular filtration rate (tubuloglomerular feedback, TGF). Increases in tubular salt at the MD, which normally parallel elevations in tubular fluid flow rate, is a well accepted stimulus of TGF. The present study aimed to determine if MD cells can detect variations in tubular flow per se. Ratiometric calcium imaging of the in vitro microperfused isolated JGA‐glomerulus complex dissected from mice was performed using Fluo‐4/Fura‐Red and confocal microscopy. Increasing cTAL flow from 2 to 20 nl/min (constant 10 mM [NaCl]) rapidly produced a significant elevation in [Ca2+]i in AA smooth muscle cells (delta[Ca2+]i = 223.7 ± 21.5 nM) and AA vasoconstriction. Complete removal of the cTAL from around the MD plaque and application of laminar flow through a perfusion pipette directly to the MD apical surface essentially produced the same results even using low (10mM) salt solutions. Acetylated a‐tubulin immunohistochemistry identified single primary cilia in MD cells. Under no flow conditions, bending MD cilia directly with a micropipette rapidly caused significant elevations in AA smooth muscle [Ca2+]i and vasoconstriction. MD cells are equipped with a tubular flow sensing mechanism which may contribute to MD cell function and TGF.

Intracellular signalling pathways in the vasoconstrictor response of mouse afferent arterioles to adenosine

Adenosine causes vasoconstriction of afferent arterioles of the mouse kidney through activation of adenosine A(1) receptors and Gi-mediated stimulation of phospholipase C. In the present study, we further explored the signalling pathways by which adenosine causes arteriolar vasoconstriction. Adenosine (10(-7) M) significantly increased the intracellular calcium concentration in mouse isolated afferent arterioles measured by fura-2 fluorescence. Pre-treatment with thapsigargin (2 microM) blocked the vasoconstrictor action of adenosine (10(-7) M) indicating that release of calcium from the sarcoplasmic reticulum (SR), stimulated presumably by IP(3), is involved in the adenosine contraction mechanism of the afferent arteriole. In agreement with this notion is the observation that 2 aminoethoxydiphenyl borate (100 microM) blocked the adenosine-induced constriction whereas the protein kinase C inhibitor calphostin C had no effect. The calcium-activated chloride channel inhibitor IAA-94 (30 microM) inhibited the adenosine-mediated constriction. Patch clamp experiments showed that adenosine treatment induced a depolarizing current in preglomerular smooth muscle cells which was abolished by IAA-94. Furthermore, the vasoconstriction caused by adenosine was significantly inhibited by 5 microM nifedipine (control 8.3 +/- 0.2 microM, ado 3.6 +/- 0.6 microM, ado + nifedipine 6.8 +/- 0.2 microM) suggesting involvement of voltage-dependent calcium channels. We conclude that adenosine mediates vasoconstriction of afferent arterioles through an increase in intracellular calcium concentration resulting from release of calcium from the SR followed by activation of Ca(2+)-activated chloride channels leading to depolarization and influx of calcium through voltage-dependent calcium channels.