Angiotensin II, reactive oxygen species, and Ca2+signaling in afferent arterioles

Reactivity of renal and mesenteric resistance vessels to angiotensin II is mediated by NOXA1/NOX1 and superoxide signaling

Activation of NADPH oxidase (NOX) enzymes and the generation of reactive oxygen species and oxidative stress regulate vascular and renal function and contribute to the pathogenesis of hypertension. The present study examined the role of NOXA1/NOX1 function in vascular reactivity of renal and mesenteric resistance arteries/arterioles of wild-type and Noxa1-/- mice. A major finding was that renal blood flow is less sensitive to acute stimulation by angiotensin II (ANG II) in Noxa1-/- mice compared with wild-type mice, with a direct action on resistance arterioles independent of nitric oxide (NO) bioavailability. These functional results were reinforced by immunofluorescence evidence of NOXA1/NOX1 protein presence in renal arteries, afferent arterioles, and glomeruli as well as their upregulation by ANG II. In contrast, the renal vascular response to the thromboxane mimetic U46619 was effectively blunted by NO and was similar in both mouse genotypes and thus independent of NOXA1/NOX1 signaling. However, phenylephrine- and ANG II-induced contraction of isolated mesenteric arteries was less pronounced and buffering of vasoconstriction after acetylcholine and nitroprusside stimulation was reduced in Noxa1-/- mice, suggesting endothelial NO-dependent mechanisms. An involvement of NOXA1/NOX1/O2•- signaling in response to ANG II was demonstrated with the specific NOXA1/NOX1 assembly inhibitor C25 and the nonspecific NOX inhibitor diphenyleneiodonium chloride in cultured vascular smooth muscle cells and isolated mesenteric resistance arteries. Collectively, our data indicate that the NOX1/NOXA1/O2•- pathway contributes to acute vasoconstriction induced by ANG II in renal and mesenteric vascular beds and may contribute to ANG II-induced hypertension.NEW & NOTEWORTHY Renal reactivity to angiotensin II (ANG II) is mediated by superoxide signaling produced by NADPH oxidase (NOX)A1/NOX1. Acute vasoconstriction of renal arteries by ANG was blunted in Noxa1-/- compared with wild-type mice. NOXA1/NOX1/O2•- signaling was also observed in ANG II stimulation of vascular smooth muscle cells and isolated mesenteric resistance arteries, indicating that it contributes to ANG II-induced hypertension. A NOXA1/NOX1 assembly inhibitor (C25) has been characterized that inhibits superoxide production and ameliorates the effects of ANG II.

NADPH Oxidase 1 Mediates Acute Blood Pressure Response to Angiotensin II by Contributing to Calcium Influx in Vascular Smooth Muscle Cells

BACKGROUND

Reactive oxygen species (ROS) and calcium ions (Ca²⁺) are among the major effectors of Ang II (angiotensin II) in vascular smooth muscle cells. ROS are related to Ca²⁺ signaling or contraction induced by Ang II, but little is known about their detailed functions. Here, NOX (NADPH oxidase), a major ROS source responsive to Ang II, was investigated regarding its contribution to Ca²⁺ signaling.

METHODS

Vascular smooth muscle cells were primary cultured from rat aorta. Ca²⁺ and ROS were monitored mainly using fura-2 and HyPer family probes' respectively. Signals activating NOX were examined with relevant pharmacological inhibitors and genetic manipulation techniques.

RESULTS

Ang II-induced ROS generation was found to be biphasic: the first phase of ROS production, which was mainly mediated by NOX1, was small and transient, preceding a rise in Ca²⁺, and the second phase of ROS generation, mediated by NOX1 and NOX4, was slow but sizeable, continuing over tens of minutes. NOX1-derived superoxide in the first phase is required for Ca²⁺ influx through nonselective cation channels. AT1R (Ang II type 1 receptor)-G_βγ-PI3K_γ (phosphoinositide 3-kinase γ) signaling pathway was responsible for the rapid activation of NOX1 in the first phase, while in the second phase, NOX1 was further activated by a separate AT1R-Gα_q/11-PLC (phospholipase C)-PKC_β (protein kinase C β) signaling axis. Consistent with these observations, aortas from NOX1-knockout mice exhibited reduced contractility in response to Ang II, and thus the acute pressor response to Ang II was also attenuated in NOX1-knockout mice.

CONCLUSIONS

NOX1 mediates Ca²⁺ signal generation and thereby contributes to vascular contraction and blood pressure elevation by Ang II.

Reactive oxygen species in renal vascular function

Reactive oxygen species (ROS) are produced by the aerobic metabolism. The imbalance between production of ROS and antioxidant defence in any cell compartment is associated with cell damage and may play an important role in the pathogenesis of renal disease. NADPH oxidase (NOX) family is the major ROS source in the vasculature and modulates renal perfusion. Upregulation of Ang II and adenosine activates NOX via AT1R and A1R in renal microvessels, leading to superoxide production. Oxidative stress in the kidney prompts renal vascular remodelling and increases preglomerular resistance. These are key elements in hypertension, acute and chronic kidney injury, as well as diabetic nephropathy. Renal afferent arterioles (Af), the primary resistance vessel in the kidney, fine tune renal hemodynamics and impact on blood pressure. Vice versa, ROS increase hypertension and diabetes, resulting in upregulation of Af vasoconstriction, enhancement of myogenic responses and change of tubuloglomerular feedback (TGF), which further promotes hypertension and diabetic nephropathy. In the following, we highlight oxidative stress in the function and dysfunction of renal hemodynamics. The renal microcirculatory alterations brought about by ROS importantly contribute to the pathophysiology of kidney injury, hypertension and diabetes.

NADPH oxidase in the vasculature: Expression, regulation and signalling pathways; role in normal cardiovascular physiology and its dysregulation in hypertension

The last 20-25 years have seen an explosion of interest in the role of NADPH oxidase (NOX) in cardiovascular function and disease. In vascular smooth muscle and endothelium, NOX generates reactive oxygen species (ROS) that act as second messengers, contributing to the control of normal vascular function. NOX activity is altered in response to a variety of stimuli, including G-protein coupled receptor agonists, growth-factors, perfusion pressure, flow and hypoxia. NOX-derived ROS are involved in smooth muscle constriction, endothelium-dependent relaxation and smooth muscle growth, proliferation and migration, thus contributing to the fine-tuning of blood flow, arterial wall thickness and vascular resistance. Through reversible oxidative modification of target proteins, ROS regulate the activity of protein tyrosine phosphatases, kinases, G proteins, ion channels, cytoskeletal proteins and transcription factors. There is now considerable, but somewhat contradictory evidence that NOX contributes to the pathogenesis of hypertension through oxidative stress. Specific NOX isoforms have been implicated in endothelial dysfunction, hyper-contractility and vascular remodelling in various animal models of hypertension, pulmonary hypertension and pulmonary arterial hypertension, but also have potential protective effects, particularly NOX4. This review explores the multiplicity of NOX function in the healthy vasculature and the evidence for and against targeting NOX for antihypertensive therapy.

Oxidative activation of type III CD38 by NADPH oxidase-derived hydrogen peroxide in Ca2+ signaling

Reactive oxygen species (ROS) derived from NADPH oxidase (Nox) has been shown to activate ADP-ribosyl cyclase (ARC), which produces the Ca²⁺ mobilizing second messenger, cyclic ADP-ribose (cADPR). In the present study, we examined how ROS activates cluster of differentiation (CD)38, a mammalian prototype of ARC. CD38 exists in type II and III forms with opposing membrane orientation. This study showed the coexpression of type II and III CD38 in lymphokine-activated killer (LAK) cells. The catalytic site of the constitutively active type II CD38 faces the outside of the cell or the inside of early endosomes (EEs), whereas the basally inactive type III CD38 faces the cytosol. Type III CD38 interacted with Nox4/phosphorylated-p22phox (p-p22phox) in EEs of LAK cells upon IL-8 treatment. H₂O₂ derived from Nox4 activated type III CD38 by forming a disulfide bond between Cys164 and Cys177, resulting in increased cADPR formation. Our study identified the mechanism by which type III CD38 is activated in an immune cell (LAK), in which H₂O₂ generated by Nox4 oxidizes and activates type III CD38 to generate cADPR. These findings provide a novel model of cross-talk between ROS and Ca²⁺ signaling.-Park, D.-R., Nam, T.-S., Kim, Y.-W., Bae, Y. S., Kim, U.-H. Oxidative activation of type III CD38 by NADPH oxidase-derived hydrogen peroxide in Ca²⁺ signaling.

Short-term hypoxia and vasa recta function in kidney slices

BACKGROUND

Descending vasa recta (DVR) supply the inner part of outer renal medulla an area at risk for hypoxic damages.

OBJECTIVE

We hypothesize increased vasoreactivity after hypoxia/re-oxygenation (H/R) in DVR, which might contribute to the reduced medullary perfusion after an ischemic event.

METHODS

Live kidney slices (200μm) from SD rats were used for functional experiments. TUNEL assay and H&E staining were used to estimate slice viability. Kidney slices were treated with carbogen or hypoxia (1% O2) for 60 or 90 min and vasoreactivity to Ang II (10-7 M) was recorded by DIC microscopy after re-oxygenation with carbogen. Expression of NOS and NADPH enzymes mRNA were determined in iron-perfusion isolated VR.

RESULTS

Percentage of apoptotic cells increased in control and H/R after 90 min in the medulla. Ang II- induced constriction of DVR was reduced after 90 min in control (compared to 60 min), but not after H/R. NOS enzymes mRNA expression levels decreased over 90 min hypoxia.

CONCLUSIONS

Increased reactivity of DVR to Ang II after H/R compared to control (90 min) suggest a role of DVR in renal ischemia/reperfusion injury.

Comparison of acute kidney injury of different etiology reveals in-common mechanisms of tissue damage

Acute kidney injury (AKI) is a syndrome of reduced glomerular filtration rate and urine production caused by a number of different diseases. It is associated with renal tissue damage. This tissue damage can cause tubular atrophy and interstitial fibrosis that leads to nephron loss and progression of chronic kidney disease (CKD). This review describes the in-common mechanisms behind tissue damage in AKI caused by different underlying diseases. Comparing six high-quality microarray studies of renal gene expression after AKI in disease models (gram-negative sepsis, gram-positive sepsis, ischemia-reperfusion, malignant hypertension, rhabdomyolysis, and cisplatin toxicity) identified 5,254 differentially expressed genes in at least one of the AKI models; 66% of genes were found only in one model, showing that there are unique features to AKI depending on the underlying disease. There were in-common features in the form of four genes that were differentially expressed in all six models, 49 in at least five, and 215 were found in common between at least four models. Gene ontology enrichment analysis could be broadly categorized into the injurious processes hypoxia, oxidative stress, and inflammation, as well as the cellular outcomes of cell death and tissue remodeling in the form of epithelial-to-mesenchymal transition. Pathway analysis showed that MYC is a central connection in the network of activated genes in-common to AKI, which suggests that it may be a central regulator of renal gene expression in tissue injury during AKI. The outlining of this molecular network may be useful for understanding progression from AKI to CKD.

Reactive oxygen species: key regulators in vascular health and diseases

ROS are a group of small reactive molecules that play critical roles in the regulation of various cell functions and biological processes. In the vascular system, physiological levels of ROS are essential for normal vascular functions including endothelial homeostasis and smooth muscle cell contraction. In contrast, uncontrolled overproduction of ROS resulting from an imbalance of ROS generation and elimination leads to the development of vascular diseases. Excessive ROS cause vascular cell damage, the recruitment of inflammatory cells, lipid peroxidation, activation of metalloproteinases and deposition of extracellular matrix, collectively leading to vascular remodelling. Evidence from a large number of studies has revealed that ROS and oxidative stress are involved in the initiation and progression of numerous vascular diseases including hypertension, atherosclerosis, restenosis and abdominal aortic aneurysm. Furthermore, considerable research has been implemented to explore antioxidants that can reduce ROS production and oxidative stress in order to ameliorate vascular diseases. In this review, we will discuss the nature and sources of ROS, their roles in vascular homeostasis and specific vascular diseases and various antioxidants as well as some of the pharmacological agents that are capable of reducing ROS and oxidative stress. The aim of this review is to provide information for developing promising clinical strategies targeting ROS to decrease cardiovascular risks.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Myoglobin facilitates angiotensin II-induced constriction of renal afferent arterioles

Vasoconstriction plays an important role in the development of acute kidney injury in rhabdomyolysis. We hypothesized that myoglobin enhances the angiotensin II (ANG II) response in afferent arterioles by increasing superoxide and reducing nitric oxide (NO) bioavailability. Afferent arterioles of C57Bl6 mice were isolated perfused, and vasoreactivity was analyzed using video microscopy. NO bioavailability, superoxide concentration in the vessel wall, and changes in cytosolic calcium were measured using fluorescence techniques. Myoglobin treatment (10−5 M) did not change the basal arteriolar diameter during a 20-min period compared with control conditions. NG-nitro-l-arginine methyl ester (l-NAME, 10−4 M) and l-NAME + myoglobin reduced diameters to 94.7 and 97.9% of the initial diameter, respectively. Myoglobin or l-NAME enhanced the ANG II-induced constriction of arterioles compared with control (36.6 and 34.2%, respectively, vs. 65.9%). Norepinephrine responses were not influenced by myoglobin. Combined application of myoglobin and l-NAME further facilitated the ANG II response (7.0%). Myoglobin or l-NAME decreased the NO-related fluorescence in arterioles similarly. Myoglobin enhanced the superoxide-related fluorescence, and tempol prevented this enhancement. Tempol also partly prevented the myoglobin effect on the ANG II response. Myoglobin increased the fura 2 fluorescence ratio (cytosolic calcium) during ANG II application (10−12 to 10−6 M). The results suggest that the enhanced afferent arteriolar reactivity to ANG II is mainly due to a myoglobin-induced increase in superoxide and associated reduction in the NO bioavailability. Signaling pathways for the augmented ANG II response include enhanced cytosolic calcium transients. In conclusion, myoglobin may contribute to the afferent arteriolar vasoconstriction in this rhabdomyolysis model.

Oxidative Stress and Hypertensive Diseases

It has become clear that reactive oxygen species (ROS) contribute to the development of hypertension via myriad effects. ROS are essential for normal cell function; however, they mediate pathologic changes in the brain, the kidney, and blood vessels that contribute to the genesis of chronic hypertension. There is also emerging evidence that ROS contribute to immune activation in hypertension. This article discusses these events and how they coordinate to contribute to hypertension and its consequent end-organ damage.

Adrenomedullin and angiotensin II signaling pathways involved in the effects on cerebellar antioxidant enzymes activity

Human adrenomedullin (AM) is a 52-amino acid peptide involved in cardiovascular control. AM has two specific receptors formed by the calcitonin-receptor-like receptor (CRLR) and receptor activity-modifying protein (RAMP) 2 or 3, known as AM1 and AM2 receptors, respectively. In addition, AM has appreciable affinity for the calcitonin gene-1 related peptide receptor (CGRP1), composed of CRLR/RAMP1. In brain, AM and their receptors are expressed in several localized areas, including the cerebellum. AM has been reported as an antioxidant. Little is known about the role of AM in the regulation of cerebellar reactive oxygen species (ROS) metabolism. We assessed the effect of AM on three antioxidant enzymes activity: catalase (CAT), glutathione peroxidase (GPx) and superoxide dismutase (SOD) and on thiobarbituric acid reactive substances (TBARS) production in rat cerebellar vermis, as well the receptor subtypes involved in AM actions. Additionally, we evaluated the role of angiotensin II (ANG II), protein kinase A (PKA) activity, and protein kinase C/nicotinamide adenine dinucleotide phosphate oxidase (PKC/NAD(P)H) (oxidase) pathway. Sprague-Dawley rats were sacrificed by decapitation and cerebellar vermis was microdissected under stereomicroscopic control. CAT, GPx, SOD activity and TBARS production was determined spectrophotometrically. Our findings demonstrated that in cerebellar vermis, AM decreased and ANG II increased CAT, GPx and SOD activity and TBARS production. Likewise, AM antagonized ANG II-induced increase antioxidant enzyme activity. AM(22-50) and CGRP(8-37) blunted AM-induced decrease of antioxidant enzymes activity and TBARS production indicating that these actions are mediated through AM and CGRP₁ receptors. Further, PKA inhibitor (PKAi) blunted AM action and apocynin and chelerythrine reverted ANG II action, suggesting that AM antioxidant action is mediated through stimulation of PKA activity, while ANG II-induced stimulation through PKC/NAD(P)H oxidase pathway. Our results support the role of AM in the regulation of cerebellar antioxidant enzymes activity and suggest a physiological role for AM in cerebellum.

TRPC6 channel as an emerging determinant of the podocyte injury susceptibility in kidney diseases

Podocytes (terminally differentiated epithelial cells of the glomeruli) play a key role in the maintenance of glomerular structure and permeability and in the incipiency of various renal abnormalities. Injury to podocytes is considered a major contributor to the development of kidney disease as their loss causes proteinuria and progressive glomerulosclerosis. The physiological function of podocytes is critically dependent on proper intracellular calcium handling; excessive calcium influx in these cells may result in the effacement of foot processes, apoptosis, and subsequent glomeruli damage. One of the key proteins responsible for calcium flux in the podocytes is transient receptor potential cation channel, subfamily C, member 6 (TRPC6); a gain-of-function mutation in TRPC6 has been associated with the onset of the familial forms of focal segmental glomerulosclerosis (FSGS). Recent data also revealed a critical role of this channel in the onset of diabetic nephropathy. Therefore, major efforts of the research community have been recently dedicated to unraveling the TRPC6-dependent effects in the initiation of podocyte injury. This mini-review focuses on the TRPC6 channel in podocytes and colligates recent data in an attempt to shed some light on the mechanisms underlying the pathogenesis of TRPC6-mediated glomeruli damage and its potential role as a therapeutic target for the treatment of chronic kidney diseases.

Superoxide enhances Ca2+ entry through L-type channels in the renal afferent arteriole

Reactive oxygen species regulate cardiovascular and renal function in health and disease. Superoxide participates in acute calcium signaling in afferent arterioles and renal vasoconstriction produced by angiotensin II, endothelin, thromboxane, and pressure-induced myogenic tone. Known mechanisms by which superoxide acts include quenching of nitric oxide and increased ADP ribosyl cyclase/ryanodine-mediated calcium mobilization. The effect(s) of superoxide on other calcium signaling pathways in the renal microcirculation is poorly understood. The present experiments examined the acute effect of superoxide generated by paraquat on calcium entry pathways in isolated rat afferent arterioles. The peak increase in cytosolic calcium concentration caused by KCl (40 mmol/L) was 99±14 nmol/L. The response to this membrane depolarization was mediated exclusively by L-type channels because it was abolished by nifedipine but was unaffected by the T-type channel blocker mibefradil. Paraquat increased superoxide production (dihydroethidium fluorescence), tripled the peak response to KCl to 314±68 nmol/L (P<0.001) and doubled the plateau response. These effects were abolished by tempol and nitroblue tetrazolium, but not by catalase, confirming actions of superoxide and not of hydrogen peroxide. Unaffected by paraquat and superoxide was calcium entry through store-operated calcium channels activated by thapsigargin-induced calcium depletion of sarcoplasmic reticular stores. Also unresponsive to paraquat was ryanodine receptor-mediated calcium-induced calcium release from the sarcoplasmic reticulum. Our results provide new evidence that superoxide enhances calcium entry through L-type channels activated by membrane depolarization in rat cortical afferent arterioles, without affecting calcium entry through store-operated entry or ryanodine receptor-mediated calcium mobilization.

Involvement of neutral sphingomyelinase in the angiotensin II signaling pathway

The possibility that angiotensin II (ANG II) exerts its effects through the activation of neutral sphingomyelinase (nSMase) has not been tested in kidneys. The results of the present study provide evidence for the activity and expression of nSMase in rat kidneys. In isolated perfused rat kidney, ANG II-induced renal vasoconstriction was inhibited by GW4869, an inhibitor of nSMase. We used nSMase for investigating the signal transduction downstream of ceramide. nSMase constricted the renal vasculature. An inhibitor of ceramidase (CDase), N-oleoylethanolamine (OEA), enhanced either ANG II- or nSMase-induced renal vasoconstriction. To demonstrate the interaction between the nSMase and cytosolic phospholipase A2 (cPLA2) signal transduction pathways, we evaluated the response to nSMase in the presence and absence of inhibitors of arachidonic acid (AA) metabolism: arachidonyl trifluoromethyl ketone (AACOCF3), an inhibitor of cPLA2; 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of all AA pathways; indomethacin, an inhibitor of cyclooxygenase (COX); furegrelate, a thromboxane A2 (TxA2)-synthase inhibitor; and SQ29548 , a TxA2-receptor antagonist. In these experiments, the nSMase-induced renal vasoconstriction decreased. ANG II or nSMase was associated with an increase in the release of thromboxane B2 (TxB2) in the renal perfusate of isolated perfused rat kidney. In addition, the coexpression of the ceramide with cPLA2, was found in the smooth muscle layer of intrarenal vessels. Our results suggest that ANG II stimulates ceramide formation via the activation of nSMase; thus ceramide may indirectly regulate vasoactive processes that modulate the activity of cPLA2 and the release of TxA2.

CD38 mediates angiotensin II-induced intracellular Ca(2+) release in rat pulmonary arterial smooth muscle cells

CD38 is a multifunctional enzyme that catalyzes the formation of the endogenous Ca(2+)-mobilizing messengers cyclic ADP-ribose (cADPR) and nicotinic acid adenosine dinucleotide phosphate (NAADP) for the activation of ryanodine receptors (RyRs) of sarcoplasmic reticulum and NAADP-sensitive Ca(2+) release channels in endolysosomes, respectively. It plays important roles in systemic vascular functions, but there is little information on CD38 in pulmonary arterial smooth muscle cells (PASMCs). Earlier studies suggested a redox-sensing role of CD38 in hypoxic pulmonary vasoconstriction. This study sought to characterize its roles in angiotensin II (Ang II)-induced Ca(2+) release (AICR) in PASMCs. Examination of CD38 expression in various rat arteries found high levels of CD38 mRNA and protein in pulmonary arteries. The Ang II-elicited Ca(2+) response consisted of extracellular Ca(2+) influx and intracellular Ca(2+) release in PASMCs. AICR activated in the absence of extracellular Ca(2+) was reduced by pharmacological or siRNA inhibition of CD38, by the cADPR antagonist 8-bromo-cADPR or ryanodine, and by the NAADP antagonist Ned-19 or disruption of endolysosomal Ca(2+) stores with the vacuolar H(+)-ATPase inhibitor bafilomycin A1. Suppression of AICR by the inhibitions of cADPR- and NAADP-dependent pathways were nonadditive, indicating interdependence of RyR- and NAADP-gated Ca(2+) release. Furthermore, AICR was inhibited by the protein kinase C inhibitor staurosporine, the nonspecific NADPH oxidase (NOX) inhibitors apocynin and diphenyleneiodonium, the NOX2-specific inhibitor gp91ds-tat, and the scavenger of reactive oxygen species (ROS) tempol. These results provide the first evidence that Ang II activates CD38-dependent Ca(2+) release via the NOX2-ROS pathway in PASMCs.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Renal vasoconstriction by vasopressin V1a receptors is modulated by nitric oxide, prostanoids, and superoxide but not the ADP ribosyl cyclase CD38

Renal blood flow (RBF) responses to arginine vasopressin (AVP) were tested in anesthetized wild-type (WT) and CD38(-/-) mice that lack the major calcium-mobilizing second messenger cyclic ADP ribose. AVP (3-25 ng) injected intravenously produced dose-dependent decreases in RBF, reaching a maximum of 25 ± 2% below basal RBF in WT and 27 ± 2% in CD38(-/-) mice with 25 ng of AVP. Renal vascular resistance (RVR) increased 75 ± 6% and 78 ± 6% in WT and CD38(-/-) mice. Inhibition of nitric oxide (NO) synthase with nitro-L-arginine methyl ester (L-NAME) increased the maximum RVR response to AVP to 308 ± 76% in WT and 388 ± 81% in CD38(-/-) (P < 0.001 for both). Cyclooxygenase inhibition with indomethacin increased the maximum RVR response to 125 ± 15% in WT and 120 ± 14% in CD38(-/-) mice (P < 0.001, <0.05). Superoxide suppression with tempol inhibited the maximum RVR response to AVP by 38% in both strains (P < 0.005) but was ineffective when administered after L-NAME. The rate of RBF recovery (relaxation) after AVP was slowed by L-NAME and indomethacin (P < 0.001, <0.005) but was unchanged by tempol. All vascular responses to AVP were abolished by an AVP V1a receptor antagonist. A V2 receptor agonist or antagonist had no effect on AVP-induced renal vasoconstriction. Taken together, the results indicate that renal vasoconstriction by AVP in the mouse is strongly buffered by vasodilatory actions of NO and prostanoids. The vasoconstriction depends on V1a receptor activation without involvement of CD38 or concomitant vasodilatation by V2 receptors. The role of superoxide is to enhance the contractile response to AVP, most likely by reducing the availability of NO rather than directly stimulating intracellular contraction signaling pathways.

Thromboxane-induced renal vasoconstriction is mediated by the ADP-ribosyl cyclase CD38 and superoxide anion

The present renal hemodynamic study tested the hypothesis that CD38 and superoxide anion (O2(·-)) participate in the vasoconstriction produced by activation of thromboxane prostanoid (TP) receptors in the mouse kidney. CD38 is the major mammalian ADP-ribosyl cyclase contributing to vasomotor tone through the generation of cADP-ribose, a second messenger that activates ryanodine receptors to release Ca(2+) from the sarcoplasmic reticulum in vascular smooth muscle cells. We evaluated whether the stable thromboxane mimetic U-46619 causes less pronounced renal vasoconstriction in CD38-deficient mice and the involvement of O2(·-) in U-46619-induced renal vasoconstriction. Our results indicate that U-46619 activation of TP receptors causes renal vasoconstriction in part by activating cADP-ribose signaling in renal resistance arterioles. Based on maximal renal blood flow and renal vascular resistance responses to bolus injections of U-46619, CD38 contributes 30-40% of the TP receptor-induced vasoconstriction. We also found that the antioxidant SOD mimetic tempol attenuated the magnitude of vasoconstriction by U-46619 in both groups of mice, suggesting mediation by O2(·-). The degree of tempol blockage of U-46619-induced renal vasoconstriction was greater in wild-type mice, attenuating renal vasoconstriction by 40% compared with 30% in CD38-null mice. In other experiments, U-46619 rapidly stimulated O2(·-) production (dihydroethidium fluorescence) in isolated mouse afferent arterioles, an effect abolished by tempol. These observations provide the first in vivo demonstration of CD38 and O2(·-) involvement in the vasoconstrictor effects of TP receptor activation in the kidney and in vitro evidence for TP receptor stimulation of O2(·-) production by the afferent arteriole.

H₂O₂ lowers the cytosolic Ca²⁺ concentration via activation of cGMP-dependent protein kinase Iα

The cGMP-dependent protein kinase I (cGKI) is a key mediator of cGMP signaling, but the specific functions of its two isoforms, cGKIα and cGKIβ, are poorly understood. Recent studies indicated a novel cGMP-independent role for cGKIα in redox sensing. To dissect the effects of oxidative stress on the cGKI isoforms, we used mouse embryonic fibroblasts and vascular smooth muscle cells (VSMCs) expressing both, one, or none of them. In cGKIα-expressing cells, but not in cells expressing only cGKIβ, incubation with H₂O₂ induced the formation of a disulfide bond between the two identical subunits of the dimeric enzyme. Oxidation of cGKIα was associated with increased phosphorylation of its substrate, vasodilator-stimulated phosphoprotein. H₂O₂ did not stimulate cGMP production, indicating that it activates cGKIα directly via oxidation. Interestingly, there was a mutual influence of H₂O₂ and cGMP on cGKI activity and disulfide bond formation, respectively; preoxidation of the kinase with H₂O₂ slightly impaired its activation by cGMP, whereas preactivation of the enzyme with cGMP attenuated its oxidation by H₂O₂. To evaluate the functional relevance of the noncanonical H₂O₂-cGKIα pathway, we studied the regulation of the cytosolic Ca²⁺ concentration ([Ca²⁺](i)). H₂O₂ suppressed norepinephrine-induced Ca²⁺ transients in cGKIα-expressing VSMCs and, to a lower extent, in VSMCs expressing only cGKIβ or none of the isoforms. Thus, H₂O₂ lowers [Ca²⁺](i) mainly via a cGKIα-dependent pathway. These results indicate that oxidative stress selectively targets the cGKIα isoform, which then modulates cellular processes in a cGMP-independent manner. A decrease in [Ca²⁺](i) in VSMCs via activation of cGKIα might be a major mechanism of H₂O₂-induced vasodilation.

Cooperative interaction between reactive oxygen species and Ca2+ signals contributes to angiotensin II-induced hypertrophy in adult rat cardiomyocytes

Reactive oxygen species (ROS) and Ca(2+) signals are closely associated with the pathogenesis of cardiac hypertrophy. However, the cause and effect of the two signals in cardiac hypertrophy remain to be clarified. We extend our recent report by investigating a potential interaction between ROS and Ca(2+) signals utilizing in vitro and in vivo angiotensin II (ANG II)-induced cardiac hypertrophy models. ANG II-induced initial Ca(2+) transients mediated by inositol trisphosphate (IP(3)) triggered initial ROS production in adult rat cardiomyocytes. The ROS generated by activation of the NAD(P)H oxidase complex via Rac1 in concert with Ca(2+) activates ADP-ribosyl cyclase to generate cyclic ADP-ribose (cADPR). This messenger-mediated Ca(2+) signal further augments ROS production, since 2,2'-dihydroxyazobenzene, an ADP-ribosyl cyclase inhibitor, or 8-Br-cADPR, an antagonistic analog of cADPR, abolished further ROS production. Data from short hairpin RNA (shRNA)-mediated knockdown of Akt1 and p47(phox) demonstrated that Akt1 is the upstream key molecule responsible for the initiation of Ca(2+) signal that activates p47(phox) to generate ROS in cardiomyocytes. Nuclear translocation of nuclear factor of activated T-cell in cardiomyocytes was significantly suppressed by treatment with NAD(P)H oxidase inhibitors as well as by shRNA against Akt1 and p47(phox). Our results suggest that in cardiomyocytes Ca(2+) and ROS messengers generated by ANG II amplify the initial signals in a cooperative manner, thereby leading to cardiac hypertrophy.

Superoxide modulates myogenic contractions of mouse afferent arterioles

Reactive oxygen species enhance or impair autoregulation. Because superoxide is a vasoconstrictor, we tested the hypothesis that stretch generates superoxide that mediates myogenic responses. Increasing perfusion pressure of mouse isolated perfused renal afferent arterioles from 40 to 80 mm Hg reduced their diameter by 13.3±1.8% (P<0.001) and increased reactive oxygen species (ethidium: dihydroethidium fluorescence) by 9.8±2.3% (P<0.05). Stretch-induced fluorescence was reduced significantly (P<0.05) by incubation with Tempol (3.7±0.8%), pegylated superoxide dismutase (3.2±1.0%), or apocynin (3.5±0.9%) but not by pegylated catalase, L-nitroarginine methylester, or Ca(2+)-free medium, relating it to Ca(2+)-independent vascular superoxide. Compared with vehicle, basal tone and myogenic contractions were reduced significantly (P<0.05) by pegylated superoxide dismutase (5.4±0.8), Tempol (4.1±1.0%), apocynin (1.0±1.3%), and diphenyleneiodinium (3.9±0.9%) but not by pegylated catalase (10.1±1.6%). L-Nitroarginine methylester enhanced basal tone, but neither it (15.8±3.3%) nor endothelial NO synthase knockout (10.2±1.8%) significantly changed myogenic contractions. Tempol had no further effect after superoxide dismutase but remained effective after catalase. H(2)O(2) >50 μmol/L caused contractions but at 25 μmol/L inhibited myogenic responses (7.4±0.8%; P<0.01). In conclusion, increasing the pressure within afferent arterioles led to Ca(2+)-independent increased vascular superoxide production from nicotinamide adenine dinucleotide phosphate oxidase, which enhanced myogenic contractions largely independent of NO, whereas H(2)O(2) impaired pressure-induced contractions but was not implicated in the normal myogenic response.

Activation of NADPH oxidase 1 increases intracellular calcium and migration of smooth muscle cells

Redox-dependent migration and proliferation of vascular smooth muscle cells (SMCs) are central events in the development of vascular proliferative diseases; however, the underlying intracellular signaling mechanisms are not fully understood. We tested the hypothesis that activation of Nox1 NADPH oxidase modulates intracellular calcium ([Ca(2+)](i)) levels. Using cultured SMCs from wild-type and Nox1 null mice, we confirmed that thrombin-dependent generation of reactive oxygen species requires Nox1. Thrombin rapidly increased [Ca(2+)](i), as measured by fura-2 fluorescence ratio imaging, in wild-type but not Nox1 null SMCs. The increase in [Ca(2+)](i) in wild-type SMCs was inhibited by antisense to Nox1 and restored by expression of Nox1 in Nox1 null SMCs. Investigation into potential mechanisms by which Nox1 modulates [Ca(2+)](i) showed that thrombin-induced inositol triphosphate generation and thapsigargin-induced intracellular calcium mobilization were similar in wild-type and Nox1 null SMCs. To examine the effects of Nox1 on Ca(2+) entry, cells were either bathed in Ca(2+)-free medium or exposed to dihydropyridines to block L-type Ca(2+) channel activity. Treatment with nifedipine or removal of extracellular Ca(2+) reduced the thrombin-mediated increase of [Ca(2+)](i) in wild-type SMCs, whereas the response in Nox1 null SMCs was unchanged. Sodium vanadate, an inhibitor of protein tyrosine phosphatases, restored the thrombin-induced increase of [Ca(2+)](i) in Nox1 null SMCs. Migration of SMCs was impaired with deficiency of Nox1 and restored with expression of Nox1 or the addition of sodium vanadate. In summary, we conclude that Nox1 NADPH oxidase modulates Ca(2+) mobilization in SMCs, in part through regulation of Ca(2+) influx, to thereby promote cell migration.

Effects of Dantrolene on Ischemia-Reperfusion Injury in Animal Models: A Review of Outcomes in Heart, Brain, Liver, and Kidney

Background/Objectives

Ischemia-reperfusion (IR) is the restoration of blood flow to a tissue that was formerly deficient of blood flow. Tissue damage after IR is considered an IR injury (IRI). During IR, there is an increased level of cytosolic calcium ([Ca2+]i) due to the release of calcium from mitochondrial, sarcoendoplasmic reticulum, and nuclear organelles. Dantrolene sodium (dantrolene) is a 1-[[[5-(4-nitrophenol)-2-furanyl]methylene]amino]-2, 4-imidazolidinedione sodium salt with a nonspecific mechanism, inhibiting organelle release of Ca2+ into the cytosol. This work reviews the outcomes of administering dantrolene in brain, heart, liver, and kidney animal models of IRI.

Methods

An extensive PubMed, MEDLINE, and MEDLAR literature review during the last 30 years on the effect of dantrolene in IRI in animal models was analyzed to determine the clinical implications of this important study. Particular attention was given to dantrolene in heart, brain, liver, and kidney IRI.

Results

Heart: Nine studies of heart IRI were reviewed and include an in vivo dog model (n = 1), in vivo rabbit model (n = 1), isolated dog myocardial fibers (n = 1), and isolated rat hearts (n = 6). Four studies showed decreased infarct size and increased cardiac function after IRI. One in vivo rabbit study found no difference in infarct size or cardiac function after IRI versus controls. Dantrolene may be protective or inductive of post-IRI arrhythmias depending on preestablished myocyte cycling times. Brain: Nine studies of brain IRI were reviewed and include an in vivo dog model (n = 1), in vivo gerbil model (n = 2), and in vivo rat models (n = 6). Dantrolene shows protective decreases in apoptotic markers in 6 studies, but it shows no effect on the necrotic core and mixed effects on reduction of infarct volume. One study found increased mortality in the dantrolene group. Liver: One study of in vivo rat liver IRI found that dantrolene decreased liver function tests, tissue necrosis factor α, tissue necrosis, and increased interleukin 10. Kidney: One study of in vivo rat kidney IRI showed that dantrolene had no effect.

Conclusions

Dantrolene shows protective effects in animal models of heart, brain, and potentially liver IRI, reinforcing the importance of calcium homeostasis during IRI. Variations of dose, timing of administration, route of administration, and outcomes between studies make definitive conclusions difficult. The nonspecific mechanism of action of dantrolene may also account for the variation among studies. Lack of studies in the liver and kidney makes any consensus in these organs premature, and thus, emphasis for this review was put on studies of the heart and brain.

Superoxide dismutase 1 limits renal microvascular remodeling and attenuates arteriole and blood pressure responses to angiotensin II via modulation of nitric oxide bioavailability

Oxidative stress is associated with vascular remodeling and increased preglomerular resistance that are both implicated in the pathogenesis of renal and cardiovascular disease. Angiotensin II induces superoxide production, which is metabolized by superoxide dismutase (SOD) or scavenged by NO. We investigated the hypothesis that SOD1 regulates renal microvascular remodeling, blood pressure, and arteriolar responsiveness and sensitivity to angiotensin II using SOD1-transgenic (SOD1-tg) and SOD1-knockout (SOD1-ko) mice. Blood pressure, measured telemetrically, rose more abruptly during prolonged angiotensin II infusion in SOD1-ko mice. The afferent arteriole media:lumen ratios were reduced in SOD1-tg and increased in SOD1-ko mice. Afferent arterioles from nontreated wild types had graded contraction to angiotensin II (sensitivity: 10(-9) mol/L; responsiveness: 40%). Angiotensin II contractions were less sensitive (10(-8) mol/L) and responsive (14%) in SOD1-tg but more sensitive (10(-13) mol/L) and responsive (89%) in SOD1-ko mice. Arterioles from SOD1-ko had 4-fold increased superoxide formation with angiotensin II at 10(-9) mol/L. N(G)-nitro-l-arginine methyl ester reduced arteriole diameter of SOD1-tg and enhanced angiotensin II sensitivity and responsiveness of wild-type and SOD1-tg mice to the level of SOD1-ko mice. SOD mimetic treatment with Tempol increased arteriole diameter and normalized the enhanced sensitivity and responsiveness to angiotensin II of SOD1-ko mice but did not affect wild-type or SOD1-tg mice. Neither SOD1 deficiency nor overexpression was associated with changes in nitrate/nitrite excretion or renal mRNA expression of NO synthase, NADPH oxidase, or SOD2/SOD3 isoforms and angiotensin II receptors. In conclusion, SOD1 limits afferent arteriole remodeling and reduces sensitivity and responsiveness to angiotensin II by reducing superoxide and maintaining NO bioavailability. This may prevent an early and exaggerated blood pressure response to angiotensin II.

Enhanced myogenic response in the afferent arteriole of spontaneously hypertensive rats

Spontaneously hypertensive rats (SHRs) have normal glomerular capillary pressure even though renal perfusion pressure is higher, suggesting that preglomerular vessels exhibit abnormally high resistance. This may be due to increased superoxide (O(2)(-)) production, which contributes to the vasoconstriction in hypertension. We tested the hypothesis that the myogenic response of the afferent arteriole (Af-Art) is exaggerated in SHRs because of increased levels of reactive oxygen species (ROS). Single Af-Arts were microdissected from kidneys of SHRs and Wistar-Kyoto (WKY) rats and microperfused in vitro. When perfusion pressure in the Af-Art was increased stepwise from 60 to 140 mmHg, the luminal diameter decreased by 8.4 + or - 2.9% in WKY Af-Arts but fell by 29.3 + or - 5.6% in SHR Af-Arts. To test whether ROS production is enhanced during myogenic response in SHRs, we measured chloromethyl-dichlorodihydrofluorescein diacetate acetyl ester (CM-H(2)DCFDA) florescence before and after increasing intraluminal pressure from 60 to 140 mmHg. Pressure-induced increases in ROS were fourfold greater in SHR Af-Arts compared with WKY Af-Arts (SHR, 48.0 + or - 2.2%; and WKY, 12.2 + or - 0.3%). To test whether O(2)(-) contributes to the myogenic response in SHRs, either the membrane-permeant O(2)(-) scavenger Tempol or the nox2-based NADPH oxidase (NOX2) inhibitor gp91ds-tat were added to the Af-Art lumen and bath and the myogenic response was tested before and after treatment. Both Tempol (10(-4) M) and gp91ds-tat (10(-5) M) significantly attenuated the pressure-induced constriction in SHR Af-Arts but not in WKY Af-Arts. We conclude that 1) pressure-induced constriction is exaggerated in SHR Af-Arts, 2) NOX2-derived O(2)(-) may contribute to the enhanced myogenic response, and 3) O(2)(-) exerts little influence on the myogenic response under normotensive conditions.

Induction of hypertension and peripheral inflammation by reduction of extracellular superoxide dismutase in the central nervous system

The circumventricular organs (CVOs) lack a well-formed blood-brain barrier and produce superoxide in response to angiotensin II and other hypertensive stimuli. This increase in central superoxide has been implicated in the regulation of blood pressure. The extracellular superoxide dismutase (SOD3) is highly expressed in cells associated with CVOs and particularly with tanycytes lining this region. To understand the role of SOD3 in the CVOs in blood pressure regulation, we performed intracerebroventricular injection an adenovirus encoding Cre-recombinase (5x10(8) particles per milliliter) in mice with loxP sites flanking the SOD3 coding region (SOD3(loxp/loxp) mice). An adenovirus encoding red-fluorescent protein was injected as a control. Deletion of CVO SOD3 increased baseline blood pressure modestly and markedly augmented the hypertensive response to low-dose angiotensin II (140 ng/kg per day), whereas intracerebroventricular injection of adenovirus encoding red-fluorescent protein had minimal effects on these parameters. Adenovirus encoding Cre-recombinase-treated mice exhibited increased sympathetic modulation of heart rate and blood pressure variability, increased vascular superoxide production, and T-cell activation as characterized by increased circulating CD69(+)/CD3(+) cells. Deletion of CVO SOD3 also markedly increased vascular T-cell and leukocyte infiltration caused by angiotensin II. We conclude that SOD3 in the CVO plays a critical role in the regulation of blood pressure, and its loss promotes T-cell activation and vascular inflammation, in part by modulating sympathetic outflow. These findings provide insight into how central signals produce vascular inflammation in response to hypertensive stimuli, such as angiotensin II.

Complex interactions of NO/cGMP/PKG systems on Ca2+ signaling in afferent arteriolar vascular smooth muscle

Little is known about the effects of nitric oxide (NO) and the cyclic GMP (cGMP)/protein kinase G (PKG) system on Ca(2+) signaling in vascular smooth muscle cells (VSMC) of resistance vessels in general and afferent arterioles in particular. We tested the hypotheses that cGMP-, Ca(2+)-dependent big potassium channels (BK(Ca(2+))) buffer the Ca(2+) response to depolarization by high extracellular KCl and that NO inhibits adenosine diphosphoribose (ADPR) cyclase, thereby reducing the Ca(2+)-induced Ca(2+) release. We isolated rat afferent arterioles, utilizing the magnetized microsphere method, and measured cytosolic Ca(2+) concentration ([Ca(2+)](i)) with fura-2, a preparation in which endothelial cells do not participate in [Ca(2+)](i) responses. KCl (50 mM)-induced depolarization causes an immediate increase in [Ca(2+)](i) of 151 nM. The blockers N(omega)-nitro-L-arginine methyl ester (of nitric oxide synthase), 1,2,4-oxodiazolo-[4,3-a]quinoxalin-1-one (ODQ, of guanylyl cyclase), KT-5823 (of PKG activation), and iberiotoxin (IBX, of BK(Ca(2+)) activity) do not alter the [Ca(2+)](i) response to KCl, suggesting no discernible endogenous NO production under basal conditions. The NO donor sodium nitroprusside (SNP) reduces the [Ca(2+)](i) response to 77 nM; IBX restores the response to control values. These data show that activation of BK(Ca(2+)) in the presence of NO/cGMP provides a brake on KCl-induced [Ca(2+)](i) responses. Experiments with the inhibitor of cyclic ADPR 8-bromo-cyclic ADPR (8-Br-cADPR) and SNP + downstream inhibitors of PKG and BK(Ca(2+)) suggest that NO inhibits ADPR cyclase in intact arterioles. When we pretreat afferent arterioles with 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP; 10 muM), the response to KCl is 143 nM. However, in the presence of both IBX and 8-Br-cGMP, we observe a surprising doubling of the [Ca(2+)](i) response to KCl. In summary, we present evidence for effects of the NO/cGMP/PKG system to reduce [Ca(2+)](i), via activation of BK(Ca(2+)) and possibly by inhibition of ADPR cyclase, and to increase [Ca(2+)](i), by a mechanism(s) yet to be defined.

NADPH oxidase 1 deficiency alters caveolin phosphorylation and angiotensin II-receptor localization in vascular smooth muscle

The superoxide-generating NADPH oxidase NOX1 is thought to be involved in signaling by the angiotensin II-receptor AT1R. However, underlying signaling steps are poorly understood. In this study, we investigated the effect of AngII on aortic smooth muscle from wild-type and NOX1-deficient mice. NOX1-deficient cells showed decreased basal ROS generation and did not produce ROS in response to AngII. Unexpectedly, AngII-dependent Ca(2+) signaling was markedly decreased in NOX1-deficient cells. Immunostaining demonstrated that AT1R was localized on the plasma membrane in wild-type, but intracellularly in NOX1-deficient cells. Immunohistochemistry and immunoblotting showed a decreased expression of AT1R in the aorta of NOX1-deficient mice. To investigate the basis of the abnormal AT1R targeting, we studied caveolin expression and phosphorylation. The amounts of total caveolin and of caveolae were not different in NOX1-deficient mice, but a marked decrease occurred in the phosphorylated form of caveolin. Exogenous H(2)O(2) or transfection of a NOX1 plasmid restored AngII responses in NOX1-deficient cells. Based on these findings, we propose that NOX1-derived reactive oxygen species regulate cell-surface expression of AT1R through mechanisms including caveolin phosphorylation. The lack cell-surface AT1R expression in smooth muscle could be involved in the decreased blood pressure in NOX1-deficient mice.

Rac1 modulates stimulus-evoked Ca(2+) release in neuronal growth cones via parallel effects on microtubule/endoplasmic reticulum dynamics and reactive oxygen species production

The small G protein Rac regulates cytoskeletal protein dynamics in neuronal growth cones and has been implicated in axon growth, guidance, and branching. Intracellular Ca(2+) is another well known regulator of growth cone function; however, effects of Rac activity on intracellular Ca(2+) metabolism have not been well characterized. Here, we investigate how Rac1 activity affects release of Ca(2+) from intracellular endoplasmic reticulum (ER) stores stimulated by application of serotonin (5-hydroxytriptamine). We also address how Rac1 effects on microtubule assembly dynamics affect distribution of Ca(2+) release sites. Multimode fluorescent microscopy was used to correlate microtubule and ER behavior, and ratiometric imaging was used to assess intracellular Ca(2+) dynamics. We report that Rac1 activity both promotes Ca(2+) release and affects its spatial distribution in neuronal growth cones. The underlying mechanism involves synergistic Rac1 effects on microtubule assembly and reactive oxygen species (ROS) production. Rac1 activity modulates Ca(2+) by 1) enhancing microtubule assembly which in turn promotes spread of the ER-based Ca(2+) release machinery into the growth cone periphery, and 2) by increasing ROS production which facilitated inositol 1,4,5-trisphosphate-dependent Ca(2+) release. These results cast Rac1 as a key modulator of intracellular Ca(2+) function in the neuronal growth cone.

Angiotensin II-induced hypertension regulates AT1 receptor subtypes and extracellular matrix turnover in mouse retinal pigment epithelium

Accumulation of specific deposits and extracellular molecules under the retinal pigment epithelium (RPE) has been previously observed in eyes with age-related macular degeneration (AMD) and may play a role in the pathogenesis of AMD. Even though age is the major determinant for developing AMD, clinical studies have revealed hypertension (HTN) as another systemic risk factor. Angiotensin II (Ang II) is considered the most important hormone associated with HTN. To evaluate the relationship of Ang II to AMD, we studied whether mouse RPE expresses functional Ang II receptor subtypes and whether HTN-induced Ang II regulates expression of these receptors as well as critical ECM molecules (MMP-2 and type IV collagen) involved in ECM turnover in RPE. We used 9-month-old C57BL/6 male mice infused with Ang II alone or Ang II in combination with the AT1 receptor antagonist candesartan or the AT2 receptor antagonist PD123319 for 4 weeks to determine whether HTN-associated Ang II was important for ECM regulation in RPE. We found that mouse RPE expressed both Ang II receptor subtypes at the mRNA and protein levels. Infusion with Ang II induced HTN and elevated plasma and ocular Ang II levels. Ang II also regulated AT1a and AT1b receptor mRNA expression, the intracellular concentration of calcium [Ca(2+)](i), MMP-2 activity, and type IV collagen accumulation. Concurrent administration of Ang II with the AT1 receptor blocker prevented the increase in blood pressure and rise in ocular Ang II levels, as well as the calcium and MMP-2 responses. In contrast, the type IV collagen response to Ang II was prevented by blockade of AT2 receptors, but not AT1 receptors. Plasma Ang II levels were not modified by the AT1 or AT2 receptor blockade. Since the effects of Ang II on MMP-2 and type IV collagen require inhibition of both Ang II receptor subtypes, these receptors may play a role as a potential therapeutic targets to prevent ECM turnover dysregulation in the RPE basement membrane, suggesting a pathogenic mechanism to explain the link between HTN and AMD.

Oxidative stress and hypertension

This review has summarized some of the data supporting a role of ROS and oxidant stress in the genesis of hypertension. There is evidence that hypertensive stimuli, such as high salt and angiotensin II, promote the production of ROS in the brain, the kidney, and the vasculature and that each of these sites contributes either to hypertension or to the untoward sequelae of this disease. Although the NADPH oxidase in these various organs is a predominant source, other enzymes likely contribute to ROS production and signaling in these tissues. A major clinical challenge is that the routinely used antioxidants are ineffective in preventing or treating cardiovascular disease and hypertension. This is likely because these drugs are either ineffective or act in a non-targeted fashion, such that they remove not only injurious ROS Fig. 5. Proposed role of T cells in the genesis of hypertension and the role of the NADPH oxidase in multiple cells/organs in modulating this effect. In this scenario, angiotensin II stimulates an NADPH oxidase in the CVOs of the brain, increasing sympathetic outflow. Sympathetic nerve terminals in lymph nodes activate T cells, and angiotensin II also directly activates T cells. These stimuli also activate expression of homing signals in the vessel and likely the kidney, which attract T cells to these organs. T cells release cytokines that stimulate the vessel and kidney NADPH oxidases, promoting vasoconstriction and sodium retention. SFO, subfornical organ. 630 Harrison & Gongora but also those involved in normal cell signaling. A potentially important and relatively new direction is the concept that inflammatory cells such as T cells contribute to hypertension. Future studies are needed to understand the interaction of T cells with the CNS, the kidney, and the vasculature and how this might be interrupted to provide therapeutic benefit.

Regulation of angiotensin II receptors and extracellular matrix turnover in human retinal pigment epithelium: role of angiotensin II

The early stage of age-related macular degeneration (AMD) is characterized by the formation of subretinal pigment epithelium (RPE) deposits as a result of the dysregulation in the turnover of extracellular matrix (ECM) molecules. However, the mechanism involved remains unclear. Hypertension (HTN) is an important risk factor for AMD, and angiotensin II (ANG II) is the most important hormone associated with HTN. However, the relevance of ANG II receptors and ANG II effects on RPE have not been investigated yet. Therefore, the expression and regulation of ANG II receptors as well as the ECM turnover were studied in human RPE. ANG II receptors were expressed and upregulated by ANG II in human RPE. This regulation resulted in functional receptor expression, since an increase in intracellular concentration of calcium was observed upon ANG II stimulation. ANG II also increased matrix metalloproteinase (MMP)-2 activity and MMP-14 at the mRNA and protein levels as well as type IV collagen degradation. These ANG II effects were abolished in the presence of the ANG II receptor subtype 1 (AT1) receptor antagonist candesartan. In contrast, ANG II decreased type IV collagen via both AT1 and AT2 receptors, suggesting a synergistic effect of the two receptor subtypes. In conclusion, we have confirmed the presence of ANG II receptors in human RPE and their regulation by ANG II as well as the regulation of ECM molecules via ANG II receptors. Our data support the hypothesis that ANG II may exert biological function in RPE through ANG II receptors and that ANG II may cause dysregulation of molecules that play a major role in the turnover of ECM in RPE basement membrane and Bruch's membrane, suggesting a pathogenic mechanism to explain the link between HTN and AMD.

Role of NOX2 in the regulation of afferent arteriole responsiveness

NADPH oxidases (NOX) are the major source of reactive oxygen species (ROS) in the vasculature and contribute to the control of renal perfusion. The role of NOX2 in the regulation of blood pressure and afferent arteriole responsiveness was investigated in NOX2(-/-) and wild-type mice. Arteriole constrictions to ANG II (10(-14)-10(-6) mol/l) were weaker in NOX2(-/-) compared with wild types. N(omega)-nitro-l-arginine methyl ester (l-NAME; 10(-4) mol/l) treatment reduced basal diameters significantly more in NOX2(-/-) (-18%) than in wild types (-6%) and augmented ANG II responses. Adenosine (10(-11)-10(-4) mol/l) constricted arterioles of wild types but not of NOX2(-/-). However, simultaneous inhibition of adenosine type-2 receptors induced vasoconstriction, which was stronger in NOX2(-/-). Adenosine (10(-8) mol/l) enhanced the ANG II response in wild type, but not in NOX2(-/-). This sensitizing effect by adenosine was abolished by apocynin. Chronic ANG II pretreatment (14 days) did not change the ANG II responses in NOX2(-/-), but strengthened the response in wild types. ANG II pretreatment augmented the l-NAME response in NOX2(-/-) (-33%), but not in wild types. Simultaneous application of l-NAME and ANG II caused a stronger constriction in the NOX2(-/-) (-64%) than in wild types (-46%). Basal blood pressures were similar in both genotypes, however, chronic ANG II infusion elevated blood pressure to a greater extent in wild-type (15 +/- 1%) than in NOX2(-/-) (8 +/- 1%) mice. In conclusion, NOX2 plays an important role in the control of afferent arteriole tone and is involved in the contractile responses to ANG II and/or adenosine. NOX2 can be activated by elevated ANG II and may play an important role in ANG II-induced hypertension. NOX2-derived ROS scavenges nitric oxide, causing subsequent nitric oxide-deficiency.

Regulation of angiotensin II receptors and extracellular matrix turnover in human retinal pigment epithelium: role of angiotensin II

The early stage of age-related macular degeneration (AMD) is characterized by the formation of subretinal pigment epithelium (RPE) deposits as a result of the dysregulation in the turnover of extracellular matrix (ECM) molecules. However, the mechanism involved remains unclear. Hypertension (HTN) is an important risk factor for AMD, and angiotensin II (ANG II) is the most important hormone associated with HTN. However, the relevance of ANG II receptors and ANG II effects on RPE have not been investigated yet. Therefore, the expression and regulation of ANG II receptors as well as the ECM turnover were studied in human RPE. ANG II receptors were expressed and upregulated by ANG II in human RPE. This regulation resulted in functional receptor expression, since an increase in intracellular concentration of calcium was observed upon ANG II stimulation. ANG II also increased matrix metalloproteinase (MMP)-2 activity and MMP-14 at the mRNA and protein levels as well as type IV collagen degradation. These ANG II effects were abolished in the presence of the ANG II receptor subtype 1 (AT1) receptor antagonist candesartan. In contrast, ANG II decreased type IV collagen via both AT1 and AT2 receptors, suggesting a synergistic effect of the two receptor subtypes. In conclusion, we have confirmed the presence of ANG II receptors in human RPE and their regulation by ANG II as well as the regulation of ECM molecules via ANG II receptors. Our data support the hypothesis that ANG II may exert biological function in RPE through ANG II receptors and that ANG II may cause dysregulation of molecules that play a major role in the turnover of ECM in RPE basement membrane and Bruch's membrane, suggesting a pathogenic mechanism to explain the link between HTN and AMD.

ADP-ribosyl cyclase and ryanodine receptors mediate endothelin ETA and ETB receptor-induced renal vasoconstriction in vivo

ADP-ribosyl cyclase (ADPR cyclase) and ryanodine receptors (RyR) participate in calcium transduction in isolated afferent arterioles. We hypothesized that this signaling pathway is activated by ETA and ETB receptors in the renal vasculature to mediate vasoconstriction in vivo. To test this, we measured acute renal blood flow (RBF) responses to ET-1 in anesthetized rats and mice in the presence and absence of functional ADPR cyclase and/or RyR. Inhibitors of ADPR cyclase (nicotinamide) or RyR (ruthenium red) reduced RBF responses to ET-1 by 44% (P < 0.04 for both) in Sprague-Dawley rats. Mice lacking the predominant form of ADPR cyclase (CD38-/-) had RBF responses to ET-1 that were 47% weaker than those seen in wild-type mice (P = 0.01). Selective ETA receptor stimulation (ET-1+BQ788) produced decreases in RBF that were attenuated by 43 and 56% by nicotinamide or ruthenium red, respectively (P < 0.02 for both). ADPR cyclase or RyR inhibition also reduced vasoconstrictor effects of the ETB receptor agonist sarafotoxin 6c (S6c; 77 and 54%, respectively, P < 0.02 for both). ETB receptor stimulation by ET-1 + the ETA receptor antagonist BQ123 elicited responses that were attenuated by 59 and 60% by nicotinamide and ruthenium red, respectively (P < 0.01 for both). Nicotinamide attenuated RBF responses to S6c by 54% during inhibition of nitric oxide synthesis (P = 0.001). We conclude that in the renal microcirculation in vivo 1) ET-1-induced vasoconstriction is mediated by ADPR cyclase and RyR; 2) both ETA and ETB receptors activate this pathway; and 3) ADPR cyclase participates in ETB receptor signaling independently of NO.

Reactive oxygen species participate in acute renal vasoconstrictor responses induced by ETAand ETBreceptors

Reactive oxygen species (ROS) play important roles in renal vasoconstrictor responses to acute and chronic stimulation by angiotensin II and norepinephrine, as well as in long-term effects of endothelin-1 (ET-1). Little is known about participation of ROS in acute vasoconstriction produced by ET-1. We tested the influence of NAD(P)H oxidase inhibition by apocynin [4 mg·kg−1·min−1, infused into the renal artery (ira)] on ETAand ETBreceptor signaling in the renal microcirculation. Both receptors were stimulated by ET-1, ETAreceptors by ET-1 during ETBantagonist BQ-788, and ETBby ETBagonist sarafotoxin 6C. ET-1 (1.5 pmol injected ira) reduced renal blood flow (RBF) 17 ± 4%. Apocynin raised baseline RBF (+10 ± 1%, P < 0.001) and attenuated the ET-1 response to 10 ± 2%, i.e., 35 ± 9% inhibition ( P < 0.05). Apocynin reduced ETA-induced vasoconstriction by 42 ± 12% ( P < 0.05) and that of ETBstimulation by 50 ± 8% ( P < 0.001). During nitric oxide (NO) synthase inhibition ( Nω-nitro-l-arginine methyl ester), apocynin blunted ETA-mediated vasoconstriction by 60 ± 8% ( P < 0.01), whereas its effect on the ETBresponse (by 87 ± 8%, P < 0.001) was even larger without than with NO present ( P < 0.05). The cell-permeable superoxide dismutase mimetic tempol (5 mg·kg−1·min−1ira), which reduces O2−and may elevate H2O2, attenuated ET-1 responses similar to apocynin (by 38 ± 6%, P < 0.01). We conclude that ROS, O2−rather than H2O2, contribute substantially to acute renal vasoconstriction elicited by both ETAand ETBreceptors and to basal renal vasomotor tone in vivo. This physiological constrictor action of ROS does not depend on scavenging of NO. In contrast, scavenging of O2−by NO seems to be more important during ETBstimulation.

Effects on kidney filtration rate by agmatine requires activation of ryanodine channels for nitric oxide generation

Agmatine, decarboxylated arginine, is produced in the kidney and can increase nephron and kidney filtration rate via renal vasodilatation and increases in plasma flow. This increase in filtration rate after agmatine is prevented by administration of nitric oxide synthase (NOS) inhibitors. In endothelial cells, agmatine-stimulated nitrite production is accompanied by induction of cytosolic calcium. NOS activity requires calcium for activation; however, the source of this calcium remains unknown. Ryanodine receptor (RyR) calcium-activated calcium release channels are present in the kidney cortex, and we evaluated if RyR contributes to the agmatine response. Agmatine microperfused into Bowman's space reversibly increases nephron filtration rate (SNGFR) by approximately 30%. cADP-ribose (cADPR) regulates RyR channel activity. Concurrent infusion of agmatine with the cADPR blocker 8-bromo-cADPR (2 microM) prevents the increase in filtration rate. Furthermore, direct activation of the RyR channel with ryanodine at agonist concentrations (5 microM) increases SNGFR, and, like agmatine, this increase is prevented by administration of N(G)-monomethyl-l-arginine, a nonselective NOS blocker. We demonstrate that agmatine does not elicit ADPR cyclase activity in vascular smooth muscle membranes and does not directly affect RyR calcium channel responses using sea urchin egg homogenates. These results imply interplay between endothelial cell cADPR/RyR/Ca(2+)/NO and the cADPR/RyR/Ca(2+) pathways in vascular smooth muscle cells in arterioles in the regulation of kidney filtration rate. In conclusion, we show that agmatine-induced effects require activation of cADPR and RyR calcium release channels for NO generation, vasodilation, and increased filtration rate.

Angiotensin II-stimulated Ca2+ entry mechanisms in afferent arterioles: role of transient receptor potential canonical channels and reverse Na+/Ca2+ exchange

In afferent arterioles, the signaling events that lead to an increase in cytosolic Ca2+ concentration ([Ca2+]i) and initiation of vascular contraction are increasingly being delineated. We have recently studied angiotensin II (ANG II)-mediated effects on sarcoplasmic reticulum (SR) mobilization of Ca2+ and the role of superoxide and cyclic adenosine diphosphoribose in these processes. In the current study we investigated the participation of transient receptor potential canonical channels (TRPC) and a Na+/Ca2+ exchanger (NCX) in Ca2+ entry mechanisms. Afferent arterioles, isolated with the magnetized polystyrene bead method, were loaded with fura-2 to measure [Ca2+]i ratiometrically. We observed that the Ca2+-dependent chloride channel blocker niflumic acid (10 and 50 μ M) affects neither the peak nor plateau [Ca2+]i response to ANG II. Arterioles were pretreated with ryanodine (100 μM) and TMB-8 to block SR mobilization via the ryanodine receptor and inositol trisphosphate receptor, respectively. The peak [Ca2+]i response to ANG II was reduced by 40%. Addition of 2-aminoethoxydiphenyl borane to block TRPC-mediated Ca2+ entry inhibited the peak [Ca2+]i ANG II response by 80% and the plateau by 74%. Flufenamic acid (FFA; 50 μM), which stimulates TRPC6, caused a sustained increase of [Ca2+]i of 146 nM. This response was unaffected by diltiazem or nifedipine. KB-R7943 (at the low concentration of 10 μM) inhibits reverse (but not forward) mode NCX. KB-R7943 decreased the peak [Ca2+]i response to ANG II by 48% and to FFA by 38%. We conclude that TRPC6 and reverse-mode NCX may be important Ca2+ entry pathways in afferent arterioles.

ADP-ribosyl cyclase and ryanodine receptor activity contribute to basal renal vasomotor tone and agonist-induced renal vasoconstriction in vivo

An important role for the enzyme ADP-ribosyl cyclase (ADPR cyclase) and its downstream targets, the ryanodine receptors (RyR), is emerging for a variety of vascular systems. We hypothesized that the ADPR cyclase/RyR pathway contributes to regulation of renal vasomotor tone in vivo. To test this, we continuously measured renal blood flow (RBF) in anesthetized Sprague-Dawley rats. Infusion of the ADPR cyclase inhibitor nicotinamide intrarenally at low doses inhibits angiotensin II (ANG II)- and norepinephrine (NE)-induced vasoconstriction by 72 and 67%, respectively ( P < 0.001). RBF studies in rats were extended to mice lacking the predominant form of ADPR cyclase (CD38). Acute renal vasoconstrictor responses to ANG II and NE are impaired by 59 and 52%, respectively, in anesthetized CD38−/− mice compared with wild-type controls ( P < 0.05). Intrarenal injection of the RyR activator FK506 decreases RBF by 22% ( P > 0.03). Furthermore, RyR inhibition with ruthenium red attenuates ANG II and NE responses by 50 and 59%, respectively ( P ≤ 0.01). Given at higher doses, nicotinamide increases basal RBF by 22% ( P > 0.001). Non-receptor-mediated renal vasoconstriction by L-type voltage-gated Ca2+channels is also dependent on ADPR cyclase and RyRs. Nicotinamide and ruthenium red inhibit constriction by the L-type channel agonist BAY K 8644 by 59% ( P > 0.02) and 63% ( P > 0.001). We conclude that 1) ADPR cyclase activity contributes to regulation of renal vasomotor tone under resting conditions, 2) renal vasoconstriction induced by G protein-coupled receptor agonists ANG II and NE is mediated in part by ADPR cyclase and RyRs, and 3) ADPR cyclase and RyRs participate in L-type channel-mediated renal vasoconstriction in vivo.

The elevated blood pressure of human GRK4gamma A142V transgenic mice is not associated with increased ROS production

G protein-coupled receptor (GPCR) kinases (GRKs) regulate the sensitivity of GPCRs, including dopamine receptors. The GRK4 locus is linked to, and some of its polymorphisms are associated with, human essential hypertension. Transgenic mice overexpressing human (h) GRK4gamma A142V on a mixed genetic background (C57BL/6J and SJL/J) have impaired renal D(1)-dopamine receptor (D(1)R) function and increased blood pressure. We now report that hGRK4gamma A142V transgenic mice, in C57BL/6J background, are hypertensive and have higher blood pressures than hGRK4gamma wild-type transgenic and nontransgenic mice. The hypertensive phenotype is stable because blood pressures in transgenic founders and F6 offspring are similarly increased. To determine whether the hypertension is associated with increased production of reactive oxygen species (ROS), we measured renal NADPH oxidase (Nox2 and Nox4) and heme oxygenase (HO-1 and HO-2) protein expressions and urinary excretion of 8-isoprostane and compared the effect of Tempol on blood pressure in hGRK4gamma A142V transgenic mice and D(5)R knockout (D(5)(-/-)) mice in which hypertension is mediated by increased ROS. The expressions of Nox isoforms and HO-2 and the urinary excretion of 8-isoprostane were similar in hGRK4gamma A142V transgenic mice and their controls. HO-1 expression was increased in hGRK4gamma A142V relative to hGRK4gamma wild-type transgenic mice. In contrast with the hypotensive effect of Tempol in D(5)(-/-) mice, it had no effect in hGRK4gamma A142V transgenic mice. We conclude that the elevated blood pressure of hGRK4gamma A142V transgenic mice is due mainly to the effect of hGRK4gamma A142V transgene acting via D(1)R and increased ROS production is not a contributor.

Superoxide mediates acute renal vasoconstriction produced by angiotensin II and catecholamines by a mechanism independent of nitric oxide

NAD(P)H oxidases (NOX) and reactive oxygen species (ROS) are involved in vasoconstriction and vascular remodeling during hypertension produced by chronic angiotensin II (ANG II) infusion. These effects are thought to be mediated largely through superoxide anion (O2−) scavenging of nitric oxide (NO). Little is known about the role of ROS in acute vasoconstrictor responses to agonists. We investigated renal blood flow (RBF) reactivity to ANG II (4 ng), norepinephrine (NE, 20 ng), and α1-adrenergic agonist phenylephrine (PE, 200 ng) injected into the renal artery (ira) of anesthetized Sprague-Dawley rats. The NOX inhibitor apocynin (1–4 mg·kg−1·min−1 ira, 2 min) or the superoxide dismutase mimetic Tempol (1.5–5 mg·kg−1·min−1 ira, 2 min) rapidly increased resting RBF by 8 ± 1% ( P < 0.001) or 3 ± 1% ( P < 0.05), respectively. During NO synthase (NOS) inhibition ( Nω-nitro-l-arginine methyl ester, 25 mg/kg iv), the vasodilation tended to increase (apocynin 13 ± 4%, Tempol 10 ± 1%). During control conditions, both ANG II and NE reduced RBF by 24 ± 4%. Apocynin dose dependently reduced the constriction by up to 44% ( P < 0.05). Similarly, Tempol blocked the acute actions of ANG II and NE by up to 48–49% ( P < 0.05). In other animals, apocynin (4 mg·kg−1·min−1 ira) attenuated vasoconstriction to ANG II, NE, and PE by 46–62% ( P < 0.01). During NOS inhibition, apocynin reduced the reactivity to ANG II and NE by 60–72% ( P < 0.01), and Tempol reduced it by 58–66% ( P < 0.001). We conclude that NOX-derived ROS substantially contribute to basal RBF as well as to signaling of acute renal vasoconstrictor responses to ANG II, NE, and PE in normal rats. These effects are due to O2− rather than H2O2, occur rapidly, and are independent of scavenging of NO.

Voltage-gated Ca2+ entry and ryanodine receptor Ca2+-induced Ca2+ release in preglomerular arterioles

We have previously shown that in afferent arterioles, angiotensin II (ANG II) involves activation of the inositol trisphosphate receptor (IP(3)R), activation of adenine diphosphoribose (ADPR) cyclase, and amplification of the initial IP(3)R-stimulated release of cytosolic Ca(2+) ([Ca(2+)](i)) from the sarcoplasmic reticulum (SR) (Fellner SK, Arendshorst WJ. Am J Physiol Renal Physiol 288: F785-F791, 2004). The response of the ryanodine receptor (RyR) to local increases in [Ca(2+)](i) is defined as calcium-induced calcium release (CICR). To investigate whether Ca(2+) entry via voltage-gated channels (VGCC) can stimulate CICR, we treated fura 2-loaded, freshly isolated afferent arterioles with KCl (40 mM; high KCl). In control arterioles, peak [Ca(2+)](i) increased by 165 +/- 10 nM. Locking the RyR in the closed position with ryanodine (100 microM) inhibited the [Ca(2+)](i) response by 59% (P < 0.01). 8-Br cADPR, a specific blocker of the ability of cyclic ADPR (cADPR) to sensitize the RyR to Ca(2+), caused a 43% inhibition. We suggest that the lower inhibition by 8-Br cADPR (P = 0.02, ryanodine vs. 8-Br cADPR) represents endogenously active ADPR cyclase. Depletion of SR Ca(2+) stores by inhibiting the SR Ca(2+)-ATPase with cyclopiazonic acid or thapsigargin blocked the [Ca(2+)](i) responses to KCl by 51% (P not significant vs. ryanodine or 8-Br cADPR). These data suggest that about half of the increase in [Ca(2+)](i) induced by high KCl is accomplished by activation of CICR through the ability of entered Ca(2+) to expose the RyR to high local concentrations of Ca(2+) and that endogenous cADPR contributes to the process.

Endothelin-A and -B receptors, superoxide, and Ca2+ signaling in afferent arterioles

It is unknown if endothelin-A and -B receptors (ET(A)R and ET(B)R) activate the production of superoxide via NAD(P)H oxidase and subsequently stimulate the formation of cyclic adenine diphosphate ribose (cADPR) in afferent arterioles. Vessels were isolated from rat kidney and loaded with fura 2. Endothelin-1 (ET-1) rapidly increased cytosolic Ca(2+) concentration ([Ca(2+)](i)) by 303 nM. The superoxide dismutase mimetic tempol, the NAD(P)H oxidase inhibitor apocynin, and nicotinamide, an inhibitor of ADPR cyclase, diminished the response by approximately 60%. The ET(B)R agonist sarafotoxin 6c (S6c) increased peak [Ca(2+)](i) by 117 nM. Subsequent addition of ET-1 in the continued presence of S6c caused an additional [Ca(2+)](i) peak of 225 nM. Neither nicotinamide or 8-bromo- (8-Br) cADPR nor apocynin decreased the [Ca(2+)](i) response to S6c, but inhibited the subsequent [Ca(2+)](i) response to ET-1. The ET(B)R blockers BQ-788 and A-192621 prevented the S6c [Ca(2+)](i) peak and reduced the ET-1 response by more than one-half, suggesting an ET(B)R/ET(A)R interaction. In contrast, the ET(A)R blocker BQ-123 had no effect on the S6c [Ca(2+)](i) peak and obliterated the subsequent ET-1 response. ET-1 immediately stimulated superoxide formation (measured with TEMPO-9-AC, 68 arbitrary units) that was inhibited 95% by apocynin or diphenyl iodonium. S6c or IRL-1620 increased superoxide by 8% of that caused by subsequent ET-1 addition. We conclude that ET(A)R activation of afferent arterioles increases the formation of superoxide that accounts for approximately 60% of subsequent Ca(2+) signaling. ET(B)R activation appears to result in only minor increases in superoxide production. Nicotinamide and 8-Br-cADPR results suggest that ET-1 (and primarily ET(A)R) causes the activation of vascular smooth muscle cell-ADPR cyclase.