Afferent arteriolar reactivity to angiotensin II is enhanced during the early phase of angiotensin II hypertension

Interaction of Angiotensin II AT1 Receptors with Purinergic P2X Receptors in Regulating Renal Afferent Arterioles in Angiotensin II-Dependent Hypertension

In angiotensin II (Ang II)-dependent hypertension, Ang II activates angiotensin II type 1 receptors (AT1R) on renal vascular smooth muscle cells, leading to renal vasoconstriction with eventual glomerular and tubular injury and interstitial inflammation. While afferent arteriolar vasoconstriction is initiated by the increased intrarenal levels of Ang II activating AT1R, the progressive increases in arterial pressure stimulate the paracrine secretion of adenosine triphosphate (ATP), leading to the purinergic P2X receptor (P2XR)-mediated constriction of afferent arterioles. Thus, the afferent arteriolar tone is maintained by two powerful systems eliciting the co-existing activation of P2XR and AT1R. This raises the conundrum of how the AT1R and P2XR can both be responsible for most of the increased renal afferent vascular resistance existing in angiotensin-dependent hypertension. Its resolution implies that AT1R and P2XR share common receptor or post receptor signaling mechanisms which converge to maintain renal vasoconstriction in Ang II-dependent hypertension. In this review, we briefly discuss (1) the regulation of renal afferent arterioles in Ang II-dependent hypertension, (2) the interaction of AT1R and P2XR activation in regulating renal afferent arterioles in a setting of hypertension, (3) mechanisms regulating ATP release and effect of angiotensin II on ATP release, and (4) the possible intracellular pathways involved in AT1R and P2XR interactions. Emerging evidence supports the hypothesis that P2X1R, P2X7R, and AT1R actions converge at receptor or post-receptor signaling pathways but that P2XR exerts a dominant influence abrogating the actions of AT1R on renal afferent arterioles in Ang II-dependent hypertension. This finding raises clinical implications for the design of therapeutic interventions that will prevent the impairment of kidney function and subsequent tissue injury.

Purinergic P2X1 receptor, purinergic P2X7 receptor, and angiotensin II type 1 receptor interactions in the regulation of renal afferent arterioles in angiotensin II-dependent hypertension

In ANG II-dependent hypertension, ANG II activates ANG II type 1 receptors (AT₁Rs), elevating blood pressure and increasing renal afferent arteriolar resistance (AAR). The increased arterial pressure augments interstitial ATP concentrations activating purinergic P2X receptors (P2XRs) also increasing AAR. Interestingly, P2X₁R and P2X₇R inhibition reduces AAR to the normal range, raising the conundrum regarding the apparent disappearance of AT₁R influence. To evaluate the interactions between P2XRs and AT₁Rs in mediating the increased AAR elicited by chronic ANG II infusions, experiments using the isolated blood perfused juxtamedullary nephron preparation allowed visualization of afferent arteriolar diameters (AAD). Normotensive and ANG II-infused hypertensive rats showed AAD responses to increases in renal perfusion pressure from 100 to 140 mmHg by decreasing AAD by 26 ± 10% and 19 ± 4%. Superfusion with the inhibitor P2X₁Ri (NF4490; 1 μM) increased AAD. In normotensive kidneys, superfusion with ANG II (1 nM) decreased AAD by 16 ± 4% and decreased further by 19 ± 5% with an increase in renal perfusion pressure. Treatment with P2X₁Ri increased AAD by 30 ± 6% to values higher than those at 100 mmHg plus ANG II. In hypertensive kidneys, the inhibitor AT₁Ri (SML1394; 1 μM) increased AAD by 10 ± 7%. In contrast, treatment with P2X₁Ri increased AAD by 21 ± 14%; combination with P2X₁Ri plus P2X₇Ri (A438079; 1 μM) increased AAD further by 25 ± 8%. The results indicate that P2X₁R, P2X₇R, and AT₁R actions converge at receptor or postreceptor signaling pathways, but P2XR exerts a dominant influence abrogating the actions of AT₁Rs on AAR in ANG II-dependent hypertension.

Cyp2j5-Gene Deletion Affects on Acetylcholine and Adenosine-Induced Relaxation in Mice: Role of Angiotensin-II and CYP-Epoxygenase Inhibitor

Previously, we showed vascular endothelial overexpression of human-CYP2J2 enhances coronary reactive hyperemia in Tie2-CYP2J2 Tr mice, and eNOS^−/− mice had overexpression of CYP2J-epoxygenase with adenosine A_2A receptor-induced enhance relaxation, but we did not see the response in CYP2J-epoxygenase knockout mice. Therefore, we hypothesized that Cyp2j5-gene deletion affects acetylcholine- and 5'-N-ethylcarboxamidoadenosine (NECA) (adenosine)-induced relaxation and their response is partially inhibited by angiotensin-II (Ang-II) in mice. Acetylcholine (Ach)-induced response was tested with N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide (MS-PPOH, CYP-epoxygenase inhibitor; 10⁻⁵M) and Ang-II (10⁻⁶M). In Cyp2j5^−/− mice, ACh-induced relaxation was different from C57Bl/6 mice, at 10⁻⁵ M (76.1 ± 3.3 vs. 58.3 ± 5.2, P < 0.05). However, ACh-induced relaxation was not blocked by MS-PPOH in Cyp2j5^−/−: 58.5 ± 5.0%, P > 0.05, but blocked in C57Bl/6: 52.3 ± 7.5%, P < 0.05, and Ang-II reduces ACh-induced relaxation in both Cyp2j5^−/− and C57Bl/6 mice (38.8 ± 3.9% and 45.9 ± 7.8, P <0.05). In addition, NECA-induced response was tested with Ang-II. In Cyp2j5^−/− mice, NECA-induced response was not different from C57Bl/6 mice at 10⁻⁵M (23.1 ± 2.1 vs. 21.1 ± 3.8, P > 0.05). However, NECA-induced response was reduced by Ang-II in both Cyp2j5^−/− and C57Bl/6 mice (−10.8 ± 2.3% and 3.2 ± 2.7, P < 0.05). Data suggest that ACh-induced relaxation in Cyp2j5^−/− mice depends on nitric oxide (NO) but not CYP-epoxygenases, and the NECA-induced different response in male vs. female Cyp2j5^−/− mice when Ang-II treated.

Mechanisms of sphingosine-1-phosphate-mediated vasoconstriction of rat afferent arterioles

AIM

Sphingosine-1-phosphate (S1P) influences resistance vessel function and is implicated in renal pathological processes. Previous studies revealed that S1P evoked potent vasoconstriction of the pre-glomerular microvasculature, but the underlying mechanisms remain incompletely defined. We postulated that S1P-mediated pre-glomerular microvascular vasoconstriction involves activation of voltage-dependent L-type calcium channels (L-VDCC) and the rho/rho kinase pathway.

METHODS

Afferent arteriolar reactivity was assessed in vitro using the blood-perfused rat juxtamedullary nephron preparation, and diameter was measured during exposure to physiological and pharmacological agents.

RESULTS

Exogenous S1P (10^-9 -10^-5 mol L^-1 ) evoked concentration-dependent vasoconstriction of afferent arterioles. Superfusion with nifedipine, a L-VDCC blocker, increased arteriolar diameter by 39 ± 18% of baseline and significantly attenuated the S1P-induced vasoconstriction. Superfusion with the rho kinase inhibitor, Y-27632, increased diameter by 60 ± 12% of baseline and also significantly blunted vasoconstriction by S1P. Combined nifedipine and Y-27632 treatment significantly inhibited S1P-induced vasoconstriction over the entire concentration range tested. In contrast, depletion of intracellular Ca²⁺ stores with the Ca²⁺ -ATPase inhibitors, thapsigargin or cyclopiazonic acid, did not alter the S1P-mediated vasoconstrictor profile. Scavenging reactive oxygen species (ROS) or inhibition of nicotinamide adenine dinucleotide phosphate oxidase activity significantly attenuated S1P-mediated vasoconstriction.

CONCLUSION

Exogenous S1P elicits potent vasoconstriction of rat afferent arterioles. These data also demonstrate that S1P-mediated pre-glomerular vasoconstriction involves activation of L-VDCC, the rho/rho kinase pathway and ROS. Mobilization of Ca²⁺ from intracellular stores is not required for S1P-mediated vasoconstriction. These studies reveal a potential role for S1P in the modulation of renal microvascular tone.

p66Shc regulates renal vascular tone in hypertension-induced nephropathy

Renal preglomerular arterioles regulate vascular tone to ensure a large pressure gradient over short distances, a function that is extremely important for maintaining renal microcirculation. Regulation of renal microvascular tone is impaired in salt-sensitive (SS) hypertension-induced nephropathy, but the molecular mechanisms contributing to this impairment remain elusive. Here, we assessed the contribution of the SH2 adaptor protein p66Shc (encoded by Shc1) in regulating renal vascular tone and the development of renal vascular dysfunction associated with hypertension-induced nephropathy. We generated a panel of mutant rat strains in which specific modifications of Shc1 were introduced into the Dahl SS rats. In SS rats, overexpression of p66Shc was linked to increased renal damage. Conversely, deletion of p66Shc from these rats restored the myogenic responsiveness of renal preglomerular arterioles ex vivo and promoted cellular contraction in primary vascular smooth muscle cells (SMCs) that were isolated from renal vessels. In primary SMCs, p66Shc restricted the activation of transient receptor potential cation channels to attenuate cytosolic Ca2+ influx, implicating a mechanism by which overexpression of p66Shc impairs renal vascular reactivity. These results establish the adaptor protein p66Shc as a regulator of renal vascular tone and a driver of impaired renal vascular function in hypertension-induced nephropathy.

Radiation-induced afferent arteriolar endothelial-dependent dysfunction involves decreased epoxygenase metabolites

Chronic kidney disease is a known complication of hematopoietic stem cell transplant (HSCT) and can be caused by irradiation at the time of the HSCT. In our rat model there is a 6- to 8-wk latent period after irradiation that leads to the development of proteinuria, azotemia, and hypertension. The current study tested the hypothesis that decreased endothelial-derived factors contribute to impaired afferent arteriolar function in rats exposed to total body irradiation (TBI). WAG/RijCmcr rats underwent 11 Gy TBI, and afferent arteriolar responses to acetylcholine were determined at 1, 3, and 6 wk. Blood pressure and blood urea nitrogen were not different between control and irradiated rats. Afferent arteriolar diameters were not altered in irradiated rats. Impaired endothelial-dependent responses to acetylcholine were evident at 3 and 6 wk following TBI. Nitric oxide synthase (NOS), cyclooxygenase (COX), and epoxygenase (EPOX) contribution to acetylcholine dilator responses were evaluated. NOS inhibition with N(G)-nitro-l-arginine methyl ester (l-NAME) reduced acetylcholine responses by 50% in controls and 90% in 3-wk TBI rats. COX inhibition with indomethacin did not significantly alter the acetylcholine response in the presence or absence of l-NAME. EPOX inhibition with N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide significantly decreased acetylcholine responses (35%) in controls but did not significantly alter acetylcholine responses (4%) in TBI rats. Biochemical analysis revealed decreased urinary EPOX metabolites but no change in COX, NOS, or reactive oxygen species at 3 wk TBI. Taken together, these results indicate that afferent arteriolar endothelial dysfunction involves a decrease in EPOX metabolites that precedes the development of proteinuria, azotemia, and hypertension in irradiated rats.

Renovascular remodeling and renal injury after extended angiotensin II infusion

Chronic angiotensin II (ANG II) infusion for 1 or 2 wk leads to progressive hypertension and induces inward hypertrophic remodeling in preglomerular vessels, which is associated with increased renal vascular resistance (RVR) and decreased glomerular perfusion. Considering the ability of preglomerular vessels to exhibit adaptive responses, the present study was performed to evaluate glomerular perfusion and renal function after 6 wk of ANG II infusion. To address this study, male Wistar rats were submitted to sham surgery (control) or osmotic minipump insertion (ANG II 200 ng·kg(-1)·min(-1), 42 days). A group of animals was treated or cotreated with losartan (10 mg·kg(-1)·day(-1)), an AT1 receptor antagonist, between days 28 and 42 Chronic ANG II infusion increased systolic blood pressure to 185 ± 4 compared with 108 ± 2 mmHg in control rats. Concomitantly, ANG II-induced hypertension increased intrarenal ANG II level and consequently, preglomerular and glomerular injury. Under this condition, ANG II enhanced the total renal plasma flow (RPF), glomerular filtration rate (GFR), urine flow and induced pressure natriuresis. These changes were accompanied by lower RVR and enlargement of the lumen of interlobular arteries and afferent arterioles, consistent with impairment of renal autoregulatory capability and outward preglomerular remodeling. The glomerular injury culminated with podocyte effacement, albuminuria, tubulointerstitial macrophage infiltration and intrarenal extracellular matrix accumulation. Losartan attenuated most of the effects of ANG II. Our findings provide new information regarding the contribution of ANG II infusion over 2 wk to renal hemodynamics and function via the AT1 receptor.

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.

Epoxyeicosatrienoic acid analogue lowers blood pressure through vasodilation and sodium channel inhibition

Epoxyeicosatrienoic acids (EETs) contribute to haemodynamics, electrolyte homoeostasis and blood pressure regulation, leading to the concept that EETs can be therapeutically targeted for hypertension. In the present study, multiple structural EET analogues were synthesized based on the EET pharmacophore and vasodilator structure-activity studies. Four EET analogues with 91-119% vasodilatory activity in the isolated bovine coronary artery (EC50: 0.18-1.6 μM) were identified and studied for blood-pressure-lowering in hypertension. Two EET analogues in which the COOH group at carbon 1 of the EET pharmacophore was replaced with either an aspartic acid (EET-A) or a heterocyclic surrogate (EET-X) were administered for 14 days [10 mg/kg per day intraperitoneally (i.p.)]. Both EET-A and EET-X lowered blood pressure in spontaneously hypertensive rats (SHRs) and in angiotensin II (AngII) hypertension. On day 14, the mean arterial pressures in EET analogue-treated AngII-hypertensive and SHRs were 30-50 mmHg (EET-A) and 15-20 mmHg (EET-X) lower than those in vehicle-treated controls. These EET analogues (10 mg/kg per day) were further tested in AngII hypertension by administering orally in drinking water for 14 days and EET-A lowered blood pressure. Additional experiments demonstrated that EET-A inhibits epithelial sodium channel (ENaC) activity in cultured cortical collecting duct cells and reduced renal expression of ENaC subunits in AngII hypertension. In conclusion, we have characterized EET-A as an orally active antihypertensive EET analogue that protects vascular endothelial function and has ENaC inhibitory activity in AngII hypertension.

ROCK/NF-κB axis-dependent augmentation of angiotensinogen by angiotensin II in primary-cultured preglomerular vascular smooth muscle cells

In angiotensin II (ANG II)-dependent hypertension, the augmented intrarenal ANG II constricts the renal microvasculature and stimulates Rho kinase (ROCK), which modulates vascular contractile responses. Rho may also stimulate angiotensinogen (AGT) expression in preglomerular vascular smooth muscle cells (VSMCs), but this has not been established. Therefore, the aims of this study were to determine the direct interactions between Rho and ANG II in regulating AGT and other renin-angiotensin system (RAS) components and to elucidate the roles of the ROCK/NF-κB axis in the ANG II-induced AGT augmentation in primary cultures of preglomerular VSMCs. We first demonstrated that these preglomerular VSMCs express renin, AGT, angiotensin-converting enzyme, and ANG II type 1 (AT1) receptors. Furthermore, incubation with ANG II (100 pmol/l for 24 h) increased AGT mRNA (1.42 ± 0.03, ratio to control) and protein (1.68 ± 0.05, ratio to control) expression levels, intracellular ANG II levels, and NF-κB activity. In contrast, the ANG II treatment did not alter AT1a and AT1b mRNA levels in the cells. Treatment with H-1152 (ROCK inhibitor, 10 nmol/l) and ROCK1 small interfering (si) RNA suppressed the ANG II-induced AGT augmentation and the upregulation and translocalization of p65 into nuclei. Functional studies showed that ROCK exerted a greater influence on afferent arteriole responses to ANG II in rats subjected to chronic ANG II infusions. These results indicate that ROCK is involved in NF-κB activation and the ROCK/NF-κB axis contributes to ANG II-induced AGT upregulation, leading to intracellular ANG II augmentation.

Neurohormonal interactions on the renal oxygen delivery and consumption in haemorrhagic shock-induced acute kidney injury

Haemorrhagic shock is a common cause of acute kidney injury (AKI), which is a major risk factor for developing chronic kidney disease. The mechanism is superficially straightforward. An arterial pressure below the kidney's autoregulatory region leads to a direct reduction in filtration pressure and perfusion, which in turn cause renal failure with reduced glomerular filtration rate and AKI because of hypoxia. However, the kidney's situation is further worsened by the hormonal and neural reactions to reduced perfusion pressure. There are three major systems working to maintain arterial pressure in shock: sympathetic signalling, the renin-angiotensin system and vasopressin. These work to retain electrolytes and water and to increase peripheral resistance and cardiac output. In the kidney, the increased electrolyte reabsorption consumes oxygen. At the same time, at the signalling level seen in shock, all of these hormones reduce renal perfusion and thereby oxygen delivery. This creates an exaggerated hypoxic situation that is liable to worsen the AKI. The present review will examine this mechanistic background and identify a number of areas that require further studies. At this time, the ideal treatment of haemorrhagic shock appears to be slow fluid resuscitation, possibly with hyperosmolar sodium, low chloride and no artificial colloids. From the standpoint of the kidney, renin-angiotensin system inhibitors appear fruitful for further study.

Renal oxygen content is increased in healthy subjects after angiotensin-converting enzyme inhibition

OBJECTIVE

The association between renal hypoxia and the development of renal injury is well established. However, no adequate method currently exists to non-invasively measure functional changes in renal oxygenation in normal and injured patients.

METHOD

R2* quantification was performed using renal blood oxygen level-dependent properties. Five healthy normotensive women (50 ± 5.3 years) underwent magnetic resonance imaging in a 1.5T Signa Excite HDx scanner (GE Healthcare, Waukesha, WI). A multiple fast gradient-echo sequence was used to acquire R2*/T2* images (sixteen echoes from 2.1 ms/slice to 49.6 ms/slice in a single breath hold per location). The images were post-processed to generate R2* maps for quantification. Data were recorded before and at 30 minutes after the oral administration of an angiotensin II-converting enzyme inhibitor (captopril, 25 mg). The results were compared using an ANOVA for repeated measurements (mean + standard deviation) followed by the Tukey test. ClinicalTrials.gov: NCT01545479.

RESULTS

A significant difference (p<0.001) in renal oxygenation (R2*) was observed in the cortex and medulla before and after captopril administration: right kidney, cortex = 11.08 ± 0.56 ms, medulla = 17.21 ± 1.47 ms and cortex = 10.30 ± 0.44 ms, medulla = 16.06 ± 1.74 ms, respectively; and left kidney, cortex= 11.79 ± 1.85 ms, medulla = 17.03 ± 0.88 ms and cortex = 10.89 ± 0.91 ms, medulla = 16.43 ± 1.49 ms, respectively.

CONCLUSIONS

This result suggests that the technique efficiently measured alterations in renal blood oxygenation after angiotensin II-converting enzyme inhibition and that it may provide a new strategy for identifying the early stages of renal disease and perhaps new therapeutic targets.

Epoxides and soluble epoxide hydrolase in cardiovascular physiology

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites that importantly contribute to vascular and cardiac physiology. The contribution of EETs to vascular and cardiac function is further influenced by soluble epoxide hydrolase (sEH) that degrades EETs to diols. Vascular actions of EETs include dilation and angiogenesis. EETs also decrease inflammation and platelet aggregation and in general act to maintain vascular homeostasis. Myocyte contraction and increased coronary blood flow are the two primary EET actions in the heart. EET cell signaling mechanisms are tissue and organ specific and provide significant evidence for the existence of EET receptors. Additionally, pharmacological and genetic manipulations of EETs and sEH have demonstrated a contribution for this metabolic pathway to cardiovascular diseases. Given the impact of EETs to cardiovascular physiology, there is emerging evidence that development of EET-based therapeutics will be beneficial for cardiovascular diseases.

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.

Sex differences in acute ANG II-mediated hemodynamic responses in mice

Male sex is associated with higher blood pressure and greater renal injury, perhaps related to greater sensitivity to ANG II. In anesthetized male and female C57BLK/6 mice, we assessed responses of mean arterial pressure (MAP) and renal vascular resistance (RVR; Transonic flow probe) to acute bolus injections of ANG II (0.3-3.0 microg/kg iv) and phenylephrine (PE; 30-300 microg/kg) during low-, normal-, and high-sodium diets. The role of reactive oxygen species was determined by coadministration of tempol. ANG II type 1 and type 2 (AT1 and AT2) receptor and endothelial nitric oxide synthase (NOS3) expression were determined in dissected kidney vessels. While no difference was found on the low-sodium (LS) diet, MAP and RVR responses to ANG II were greater in males during the normal-sodium (NS) and high-sodium (HS) diets (e.g., RVR response at ANG II 3.0 microg/kg during NS: +329 +/- 22 vs. +271 +/- 28 mmHg.ml(-1).min, P = 0.029, effect size = 0.75). Tempol had no effect on the sex-dependent responses on any of the diets. On the LS diet, AT1 and AT2 receptor expression was higher in males. No sex differences were found on the NS diet. On the HS diet, AT1 was higher, and NOS3 expression was lower in males. Acute responses to ANG II are greater in male mice during NS and HS diets, which is, in part, related to differences in AT1, AT2, and NOS3 expression in kidney vessels. Mouse models will be useful to study the role of sex differences in ANG II sensitivity for cardiovascular and renal disease.

ATP, P2 receptors and the renal microcirculation

Purinoceptors are rapidly becoming recognised as important regulators of tissue and organ function. Renal expression of P2 receptors is broad and diverse, as reflected by the fact that P2 receptors have been identified in virtually every major tubular/vascular element. While P2 receptor expression by these renal structures is recognised, the physiological functions that they serve remains to be clarified. Renal vascular P2 receptor expression is complex and poorly understood. Evidence suggests that different complements of P2 receptors are expressed by individual renal vascular segments. This unique distribution has given rise to the postulate that P2 receptors are important for renal vascular function, including regulation of preglomerular resistance and autoregulatory behaviour. More recent studies have also uncovered evidence that hypertension reduces renal vascular reactivity to P2 receptor stimulation in concert with compromised autoregulatory capability. This review will consolidate findings related to the role of P2 receptors in regulating renal microvascular function and will present areas of controversy related to the respective roles of ATP and adenosine in autoregulatory resistance adjustments.

Increased blood pressure in mice lacking cytochrome P450 2J5

The cytochrome P450 (CYP) enzymes participate in a wide range of biochemical functions, including metabolism of arachidonic acid and steroid hormones. Mouse CYP2J5 is abundant in the kidney where its products, the cis-epoxyeicosatrienoic acids (EETs), modulate sodium transport and vascular tone. To define the physiological role of CYP2J5 in the kidney, knockout mice were generated using a conventional gene targeting approach. Cyp2j5 (-/-) mice develop normally and exhibit no overt renal pathology. While renal EET biosynthesis was apparently unaffected by the absence of CYP2J5, deficiency of this CYP in female mice was associated with increased blood pressure, enhanced proximal tubular transport rates, and exaggerated afferent arteriolar responses to angiotensin II and endothelin I. Interestingly, plasma 17beta-estradiol levels were reduced in female Cyp2j5 (-/-) mice and estrogen replacement restored blood pressure and vascular responsiveness to normal levels. There was no evidence of enhanced estrogen metabolism, or altered expression or activities of steroidogenic enzymes in female Cyp2j5 (-/-) mice, but their plasma levels of luteinizing hormone and follicle stimulating hormone were inappropriately low. Together, our findings illustrate a sex-specific role for CYP2J5 in regulation of blood pressure, proximal tubular transport, and afferent arteriolar responsiveness via an estrogen-dependent mechanism.

Chemokine receptor 2b inhibition provides renal protection in angiotensin II - salt hypertension

The present study was designed to determine whether chemokine receptor 2b (CCR2b) contributes to the development of renal injury in salt-sensitive angiotensin II (ANG) hypertension. Rats were infused with ANG and fed a high-salt diet (HS) for 14 days. Rats were divided into 4 groups: HS; HS administered the CCR2b antagonist, RS102895; Ang/HS hypertensive; and Ang/HS hypertensive administered RS102895. CCR2b inhibition slowed the progression of blood pressure elevation during the first week of ANG/HS hypertension; however, it did not alter blood pressure in the HS group. At 2 weeks, arterial pressure was not significantly different between ANG/HS and ANG/HS hypertensive rats administered RS102895. Renal cortical nuclear factor kappaB activity increased in ANG/HS hypertension compared with the HS group (0.11+/-0.006 versus 0.08+/-0.003 ng of activated nuclear factor kappaB per microgram of protein), and RS102895 treatment lowered nuclear factor kappaB activity in ANG/HS hypertension (0.08+/-0.005 ng of activated nuclear factor kappaB per microgram of protein). Renal tumor necrosis factor-alpha and intercellular adhesion molecule-1 expression increased, and Cyp2c23 expression decreased in ANG/HS hypertension compared with the HS group, and CCR2b inhibition reduced tumor necrosis factor-alpha and intercellular adhesion molecule-1 and increased Cyp2c23 expression. Histological immunostaining revealed increased renal monocyte and macrophage infiltration in ANG/HS hypertensive rats with decreased infiltration in rats receiving RS102895 treatment. Albuminuria and cortical collagen staining also increased in ANG/HS hypertensive rats, and RS102895 treatment lowered these effects. Afferent arteriolar autoregulatory responses to increasing renal perfusion pressure were blunted in ANG/HS hypertension, and RS102895 treatment improved this response. These data suggest that CCR2b inhibition protects the kidney in hypertension by reducing inflammation and delaying the progression of hypertension.

Renal responses to acute reflex activation of renal sympathetic nerve activity and renal denervation in secondary hypertension

We tested whether the responsiveness of the kidney to basal renal sympathetic nerve activity (RSNA) or hypoxia-induced reflex increases in RSNA, is enhanced in angiotensin-dependent hypertension in rabbits. Mean arterial pressure, measured in conscious rabbits, was similarly increased (+16 ± 3 mmHg) 4 wk after clipping the left ( n = 6) or right ( n = 5) renal artery or commencing a subcutaneous ANG II infusion ( n = 9) but was not increased after sham surgery ( n = 10). Under pentobarbital sodium anesthesia, reflex increases in RSNA (51 ± 7%) and whole body norepinephrine spillover (90 ± 17%), and the reductions in glomerular filtration rate (−27 ± 5%), urine flow (−43 ± 7%), sodium excretion (−40 ± 7%), and renal cortical perfusion (−7 ± 3%) produced by hypoxia were similar in normotensive and hypertensive groups. Hypoxia-induced increases in renal norepinephrine spillover tended to be less in hypertensive (1.1 ± 0.5 ng/min) than normotensive (3.7 ± 1.2 ng/min) rabbits, but basal overflow of endogenous and exogenous dihydroxyphenolglycol was greater. Renal plasma renin activity (PRA) overflow increased less in hypertensive (22 ± 29 ng/min) than normotensive rabbits (253 ± 88 ng/min) during hypoxia. Acute renal denervation did not alter renal hemodynamics or excretory function but reduced renal PRA overflow. Renal vascular and excretory responses to reflex increases in RSNA induced by hypoxia are relatively normal in angiotensin-dependent hypertension, possibly due to the combined effects of reduced neural norepinephrine release and increased postjunctional reactivity. In contrast, neurally mediated renin release is attenuated. These findings do not support the hypothesis that enhanced neural control of renal function contributes to maintenance of hypertension associated with activation of the renin-angiotensin system.

The Roles of Intrarenal 20-Hydroxyeicosatetraenoic and Epoxyeicosatrienoic Acids in the Regulation of Renal Function in Hypertensive Ren-2 Transgenic Rats

BACKGROUND

The present study was performed in hypertensive Ren-2 transgenic rats (TGR) and in normotensive Hannover Sprague-Dawley (HanSD) rats. First, the intrarenal protein expression of CYP4A, the enzyme catalyzing the formation of 20-hydroxyeicosatetraenoic acid (20-HETE), and of CYP2C23, the enzyme responsible for epoxyeicosatrienoic acid (EET) production, was evaluated. Second, the renal functional responses to inhibition of the intrarenal formation of 20-HETE and EETs were investigated.

METHODS

Renal hemodynamics and electrolyte excretion were evaluated in response to the administration of inhibitors of 20-HETE and EET formation into the renal artery. In renal cortical tissue, CYP4A and CYP2C23 protein expression was assessed by Western blot analysis. Urinary concentrations of 20-HETE and EETs were measured using a fluorescent HPLC assay.

RESULTS

TGR have higher kidney CYP4A protein expression and urinary 20-HETE excretion but significantly lower CYP2C23 protein expression and urinary EET excretion than HanSD. Intrarenal inhibition of 20-HETE and EET formation decreased sodium excretion in HanSD, whereas inhibition of 20-HETE increased urinary excretion of sodium in TGR without altering renal hemodynamics.

CONCLUSIONS

Our data suggest that in TGR, deficient intrarenal synthesis of EETs combined with increased synthesis of 20-HETE with its stimulation of tubular sodium absorption may contribute to the development of hypertension in TGR.

Effects of chronic cytochrome P-450 inhibition on the course of hypertension and end-organ damage in Ren-2 transgenic rats

The aim of the present study was to evaluate the effects of inhibition of cytochrome P-450 (CYP) activity by 1-aminobenzotriazole (ABT) and by CoCl(2), first, on the development of hypertension when treatment was started in young male heterozygous Ren-2 transgenic rats (TGR) and, second, on blood pressure (BP) when treatment was started in adult TGR with established hypertension. Normotensive Hannover Sprague-Dawley (HanSD) rats served as controls. In addition, the renal cortical activities of omega-hydroxylase, the enzyme catalyzing the formation of 20-hydroxyeicosatetraenoic acid (20-HETE), and of epoxygenase, the enzyme responsible for epoxyeicosatrienoic acids (EETs) production, and urinary excretion of 20-HETE and EETs in TGR and HanSD rats were assessed. TGR have higher renal tissue omega-hydroxylase activity and urinary excretion of 20-HETE but have significantly lower renal epoxygenase activity and urinary excretion of EETs than HanSD rats. Treatment of young TGR with ABT and CoCl(2) attenuated the development of hypertension and cardiac hypertrophy and prevented glomerulosclerosis. Administration of ABT and CoCl(2) in adult TGR decreased BP, cardiac hypertrophy, but did not reduce glomerulosclerosis. Our data suggest that altered production and/or action of CYP-derived metabolites play a permissive role in the development and maintenance of hypertension in TGR by enhancing ANG II-induced vasoconstriction.

Cardiovascular therapeutic aspects of soluble epoxide hydrolase inhibitors

Soluble epoxide hydrolase (sEH) is an enzyme responsible for the conversion of lipid epoxides to diols by the addition of water. Biological actions on the cardiovascular system that are attributed to epoxides include vasodilation, antiinflammatory actions and vascular smooth muscle cell antimigratory actions. Conversion of arachidonic acid epoxides to diols by sEH diminishes the beneficial cardiovascular properties of these epoxyeicosano-ids. Cardiovascular diseases in animal models and humans have been associated with decreased epoxygenase activity or increased sEH activity and these changes are responsible for the progression of the disease state. More recently, sEH gene polymorphisms in the human population have been associated with increased risk for cardiovascular diseases. Thus the biological actions of epoxyeicosanoids and the sEH enzyme are ideal therapeutic targets for cardiovascular diseases. The rapid development of 1,3-disubstituted urea based sEH inhibitors over the past five years has resulted in a number of studies demonstrating cardiovascular protection. sEH inhibitors have antihypertensive and antiinflammatory actions and have been demonstrated to decrease cerebral ischemic and renal injury in rat models of hypertension. These findings of beneficial actions in animal models of disease position the sEH enzyme as a promising therapeutic target for cardiovascular diseases.

Role of Extracellular Superoxide Dismutase in the Mouse Angiotensin Slow Pressor Response

Low rates of angiotensin II (Ang II) infusion raise blood pressure, renal vascular resistance (RVR), NADPH oxidase activity, and superoxide. We tested the hypothesis that these effects are ameliorated by extracellular superoxide dismutase (EC-SOD). EC-SOD knockout (-/-) and wild type (+/+) mice were equipped with blood pressure telemeters and infused subcutaneously with Ang II (400 ng/kg per minute) or vehicle for 2 weeks. During vehicle infusion, EC-SOD -/- mice had significantly (P<0.05) higher MAP (+/+: 107+/-3 mm Hg versus -/-: 114+/-2 mm Hg; n=11 to 14), RVR, lipid peroxidation, renal cortical p22(phox) expression, and NADPH oxidase activity. Ang II infusion in EC-SOD +/+ mice significantly (P<0.05) increased MAP, RVR, p22(phox), NADPH oxidase activity, and lipid peroxidation. Ang II reduced SOD activity in plasma, aorta, and kidney accompanied by reduced renal EC-SOD expression. During Ang II infusion, both groups had similar values for MAP (+/+ Ang II: 125+/-3 versus -/- Ang II: 124+/-3 mmHg; P value not significant), RVR, NADPH oxidase activity, and lipid peroxidation. SOD activity in the kidneys of Ang II-infused mice was paradoxically higher in EC-SOD -/- mice (+/+: 8.8+/-1.2 U/mg protein(-1) versus -/-: 13.7+/-1.6 U/mg protein(-1); P<0.05) accompanied by a significant upregulation of mRNA and protein for Cu/Zn-SOD. In conclusion, EC-SOD protects normal mice against oxidative stress by attenuating renal p22(phox) expression, NADPH oxidase activation, and the accompanying renal vasoconstriction and hypertension. However, during an Ang II slow pressor response, renal EC-SOD expression is reduced and, in its absence, renal Cu/Zn-SOD is upregulated and may prevent excessive Ang II-induced renal oxidative stress, renal vasoconstriction, and hypertension.

Angiotensin II decreases the renal MRI blood oxygenation level-dependent signal

Acute experimental reduction of renal blood flow decreases the renal blood oxygenation level-dependent (BOLD) MRI signal in animals. Angiotensin II also reduces renal blood flow, but the ability of BOLD MRI to dynamically detect this response has not yet been investigated in humans. Six healthy male volunteers underwent an individual dose-finding study to identify the intravenous doses of angiotensin II, norepinephrine, and sodium nitroprusside necessary to induce a 15-mm Hg peak mean arterial blood pressure change. MRI studies followed within 3 weeks, when angiotensin II (8.8+/-1.4 ng/kg), norepinephrine (52+/-12 ng/kg), and sodium nitroprusside (2.0+/-0.3 microg/kg) were given twice in an unblocked, randomized sequence while imaging experiments were performed on a 1.5-T Siemens Sonata. A multiecho echo-planar imaging sequence was used to acquire T2* maps with a temporal resolution of 1 respiratory cycle. Averaged over a renal cortex dominated region of interest, angiotensin II caused a shortening of T2* between 6% and 10%. Sodium nitroprusside and norepinephrine, although of equal potency concerning blood pressure responses, did not alter the renal BOLD signal. The renal BOLD response to angiotensin II appeared with short onset latency (as early as 10 seconds after peripheral intravenous angiotensin II bolus administration) suggesting that this response is a consequence of altered perfusion rather than increased renal oxygen consumption. The methods described here are suitable to assess renal responsiveness to angiotensin II and may, thus, be of great value in human hypertension research.

Acute effects of the superoxide dismutase mimetic tempol on split kidney function in two-kidney one-clip hypertensive rats

OBJECTIVE

To investigate the acute effects of the superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (tempol) on split kidney function, and renal haemodynamics, in two-kidney, one-clip (2K1C) hypertensive rats.

METHODS

Three weeks after clipping, or the sham procedure, the effects of intravenous tempol (200 micromol/kg per h) were evaluated on thiobutabarbital anaesthetized Sprague-Dawley rats.

RESULTS

Mean arterial pressure (MAP; 152 +/- 3 versus 122 +/- 3 mmHg, P < 0.001), plasma renin activity (28.7 +/- 3.0 versus 9.5 +/- 0.6 ng/ml per h, P < 0.001) and urinary 8-iso-prostaglandin F2alpha excretion (124 +/- 4 versus 92 +/- 10 pmol/24 h, P = 0.003) were significantly elevated in 2K1C rats compared with sham. Tempol reduced MAP by 15 +/- 1% compared with baseline (P < 0.001) in 2K1C rats. In clipped kidneys, tempol increased the glomerular filtration rate (GFR; +50 +/- 15% from baseline) and the effective renal plasma flow (ERPF; +37 +/- 13%, from baseline), and reduced renal vascular resistance (RVR; -32 +/- 6% from baseline) compared with saline-treated controls (P < 0.05). In non-clipped kidneys, tempol reduced RVR (-24 +/- 5% from baseline) compared with saline-treated controls (P = 0.001). In sham-operated rats, tempol produced a modest reduction in MAP (-8 +/- 2% from baseline, P = 0.003), but did not significantly affect renal haemodynamics or function.

CONCLUSION

Tempol reduced MAP and RVR in both clipped and non-clipped kidneys of 2K1C hypertensive rats. In addition, tempol increased ERPF and GFR in the clipped kidney. These findings suggest important roles for superoxide in the regulation of renal haemodynamics during the early maintenance phase of renovascular hypertension.

RNA silencing in vivo reveals role of p22phox in rat angiotensin slow pressor response

The angiotensin II (Ang II) slow-pressor response entails an increase in mean arterial pressure and reactive oxygen species. We used double-stranded interfering RNAs (siRNAs) in Sprague Dawley rats in vivo to test the hypothesis that an increase in the p22phox component of NADPH oxidase is required for this response. Reactive oxygen species were assessed from excretion of 8-isoprostane prostaglandin F2alpha and blood pressure by telemetry. Two siRNA sequences to p22phox (sip22phox) reduced mRNA >85% in cultured vascular smooth muscle cells. Rats received rapid (10 second) IV injections (50 to 100 microg) of 1 of 2 different sip22phox, control siRNA, or vehicle (TransIt in saline) during 14 day SC infusions of Ang II (200 ng.kg(-1).min(-1)) or sham infusions. In both groups, sip22phox, relative to control siRNA, led to significant (P<0.001; approximately 50%) reductions in expression of p22phox mRNA and protein and of NADPH oxidase activity in the kidney cortex. In Ang II-infused rats, sip22phox decreased protein expression for Nox-1, -2, and -4 but increased p47phox. Three days after sip22phox, conscious rats infused with Ang II had a reduced excretion of 8-isoprostane (10+/-1 versus 19+/-2 pg.24 h(-1); P<0.01) and a reduced mean arterial pressure (142+/-5 versus 168+/-4 mm Hg; P<0.005). An increase in p22phox is required for increased renal NADPH oxidase activity, expression of Nox proteins and oxidative stress, and contributes < or =50% to hypertension during an Ang II slow-pressor response.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of hypertension.

Impaired Ca2+ signaling attenuates P2X receptor-mediated vasoconstriction of afferent arterioles in angiotensin II hypertension

This study tested the hypothesis that afferent arteriolar responses to purinoceptor activation are attenuated, and Ca2+ signaling mechanisms are responsible for the blunted preglomerular vascular reactivity in angiotensin II (Ang II) hypertension. Experiments determined the effects of ATP, the P2X1 agonist beta,gamma-methylene ATP or the P2Y agonist UTP on arteriolar diameter using the juxtamedullary nephron technique and on renal myocyte intracellular Ca2+ concentration ([Ca2+]i) using single cell fluorescence microscopy. Six or 13 days of Ang II infusion significantly attenuated the vasoconstrictor responses to ATP and beta,gamma-methylene ATP (P<0.05). During exposure to ATP (1, 10, and 100 micromol/L), afferent diameter declined by 17+/-2%, 29+/-3%, and 30+/-2% in normal control rats and 8+/-3%, 7+/-3%, and 22+/-3% in kidneys of Ang II-infused rats (13 days). Renal myocyte intracellular calcium responses to ATP or beta,gamma-methylene ATP were also decreased in Ang II hypertensive rats. In myocytes of control rats, peak increases in [Ca2+]i averaged 107+/-21, 170+/-38, and 478+/-79 nmol/L at ATP concentrations of 1, 10, and 100 micromol/L, respectively. Ang II infusion for 13 days decreased the peak responses to ATP (1, 10, and 100 micromol/L) to 65+/-13, 102+/-20, and 367+/-73 nmol/L, respectively. The peak increases in [Ca2+]i in response to beta,gamma-methylene ATP were also reduced in Ang II hypertensive rats. However, angiotensin hypertension did not change the UTP-mediated vasoconstrictor responses or the myocyte calcium responses to UTP. These results indicate that the impaired autoregulatory response observed in Ang II-dependent hypertension can be attributed to impairment of P2X1 receptor-mediated signal transduction.

The role of intrarenal angiotensin II in the development of hypertension in Ren-2 transgenic rats

OBJECTIVE

We investigated the responses of mean arterial pressure and renal blood flow to intravenous and intrarenal angiotensin II, plasma and kidney angiotensin II concentrations and renal angiotensin receptor subtype 1 protein expression, and renal functional responses to intravenous and intrarenal angiotensin receptor 1 blockade with candesartan.

METHODS

In male anaesthetized transgenic rats and Hannover Sprague-Dawley rats aged 36-38 days mean arterial pressure and renal blood flow were determined after intravenous and intrarenal boluses of angiotensin II. Mean arterial pressure, renal blood flow and sodium excretion after intravenous or intrarenal candesartan were studied. Plasma and kidney angiotensin II concentrations were determined by radioimmunoassay. Renal angiotensin receptor subtype 1 protein levels were analysed by immunoblotting.

RESULTS

The responses of mean arterial pressure and renal blood flow to angiotensin II were significantly greater in transgenic than in Hannover Sprague-Dawley rats. The administration of candesartan resulted in comparable decreases in mean arterial pressure and increases in renal blood flow and sodium excretion in both groups of rats. Renal angiotensin receptor subtype 1 protein levels were no different between Hannover Sprague-Dawley and transgenic rats.

CONCLUSIONS

Plasma and kidney angiotensin II levels were lower in anaesthetized transgenic rats but, in contrast, were higher in decapitated transgenic rats when compared with Hannover Sprague-Dawley rats, suggesting that the kidney function of prehypertensive transgenic rats is under inappropriately high angiotensin II-dependent influence.

Chronic ANG II infusion increases NO generation by rat descending vasa recta

We tested whether chronic ANG II infusion into rats affects descending vasa recta (DVR) contractility, synthesis of superoxide, or synthesis of nitric oxide (NO). Rats were infused with ANG II at 250 ng·kg−1·min−1 for 11–13 days. DVR were loaded with dihydroethidium (DHE) to measure superoxide and 3-amino-4-aminomethyl-2′,7′-difluorofluorescein (DAFFM) to measure NO. Acute constriction of DVR by ANG II (0.1, 1, and 10 nM) was diminished, and NO generation rate was raised by chronic ANG II infusion. DHE oxidation by DVR from ANG II-infused rats was similar to controls and was significantly higher when NO synthesis was prevented with Nω-nitro-l-arginine methyl ester (l-NAME). The superoxide dismutase mimetic Tempol (1 mM) increased NO generation compared with controls. The increased synthesis of NO by chronic ANG II-treated vessels persisted in the presence of Tempol. DVR endothelial cytoplasmic Ca2+ response to ACh was diminished by chronic ANG II treatment, but the capacity of ACh to increase NO generation was unaltered. We conclude that DVR generation of superoxide is not affected by chronic ANG II exposure but that basal NO synthesis is increased. DVR superoxide is unlikely to be an important mediator of chronic ANG II slow pressor hypertension in rats.

Enhanced Contractility of Renal Afferent Arterioles From Angiotensin-Infused Rabbits

We tested the hypothesis that cyclooxygenase (COX), thromboxane A 2 synthase (TxA 2 -S), thromboxane prostanoid receptors (TP-Rs), or superoxide anion (O 2 − ) mediates enhanced contractions of renal afferent arterioles (Aff) of angiotensin II (Ang II)-infused rabbits. Rabbits were infused with vehicle (sham), Ang II 60 ng·kg −1 ·min −1 (Ang II 60) or 200 ng·kg −1 ·min −1 (Ang II 200). There was a selective enhanced vasoconstriction of Affs from Ang II 60 rabbits to Ang II (Δdiameter−78±8% versus −43±9%; P <0.01) that was normalized by a TP-R antagonist but not by a superoxide dismutase (SOD) mimetic. Affs from Ang II 200 rabbits had increased ( P <0.01) mRNA for COX-2 and enhanced vasoconstriction to Ang II, U-46 619 (TP-R mimetic), and endothelin-1 that was normalized by ifetroban plus tempol together. Endothelium removal enhanced Ang II responses of Affs from sham rabbits but blunted responses from Ang II 200 rabbits and abolished responses to ifetroban. Affs from Ang II 200 rabbits had an endothelium-dependent contraction factor (EDCF) response to that was blunted ( P <0.001) by a SOD mimetic or antagonists of COX-1 or TxA 2 -S but normalized by antagonists of COX-2 or TP-R. Thus, enhanced Ang II responses in Affs from rabbits infused with slow pressor Ang II are mediated independently by O 2 − in the vascular smooth muscle cells and by an EDCF that is principally a vasoconstrictor prostaglandin generated by COX-2 >−1 activating TP-Rs, whereas enhanced responses in rabbits infused with a lower Ang II dose are dependent on TP-R but not O 2 − .

Selective increase in renal arcuate innervation density and neurogenic constriction in chronic angiotensin II-infused rats

This study investigated the effects of angiotensin II "slow pressor" hypertension on structure and function of nerves supplying the renal vasculature. Low-dose angiotensin II (10 ng/kg per minute, initially sub-pressor) or saline vehicle was infused intravenously for 21 days in rats, and the effects were compared in renal and mesenteric arteries. Mean arterial pressure averaged 12+/-2 mm Hg higher than in vehicle-infused rats at 21 days. Using electron microscopy, the innervation density of renal arcuate, but not mesenteric arteries of equivalent size, was significantly higher in angiotensin II-infused than in vehicle-infused rats. Functional testing on a pressure myograph revealed that constrictions evoked by nerve stimulation in arcuate arteries were 2.3+/-0.7-fold greater in vessels from angiotensin II-infused compared with vehicle-infused rats (P<0.0001), whereas there was no significant difference in nerve-induced constrictions in mesenteric arteries. Sensitivity to and maximum amplitude of constrictions evoked by phenylephrine were not different in renal or mesenteric arteries between groups, suggesting that the increased neurally evoked constriction in renal arcuate arteries was not caused by postsynaptic changes. Endothelium-dependent vasorelaxation and the vessel wall physical properties were not different between the two groups in either artery. Thus, angiotensin II infusion appeared to evoke renal-specific increases in vessel innervation and increased vasoconstriction to nerve stimulation. These changes appear early and occur before changes in renal endothelial function are apparent. Thus, "slow pressor" angiotensin II hypertension is associated with increased renal innervation, compatible with a pathogenetic role.

Increased renovascular response to angiotensin II in persons genetically predisposed to arterial hypertension disappears after chronic angiotensin-converting enzyme inhibition

OBJECTIVE AND METHODS

Functional changes in the kidneys of healthy men with (FH+) (n = 15) and without (FH-) (n = 15) family history of primary arterial hypertension were examined during administration of low-dose exogenous angiotensin II (A2) (1 ng/kg per min) before and after acute (1 mg intravenous enalaprilat) and chronic (7 days oral enalapril, 30 mg/day) angiotensin-converting enzyme (ACE) inhibition.

RESULTS

Before chronic ACE inhibition, A2 increased mean arterial blood pressure (FH+, 8.7 +/- 0.8 mmHg; FH-, 8.9 +/- 0.9 mmHg), plasma immunoreactive A2 (FH+, 21 +/- 2 pg/ml; FH-, 18 +/- 3 pg/ml) and plasma aldosterone (FH+, 64 +/- 7 pg/ml; FH-, 56 +/- 6 pg/ml) to a similar degree in both groups. Chronic ACE inhibition had no impact on A2 blood pressure, plasma A2, or plasma aldosterone effects. A2 significantly increased renal vascular resistance in both groups (FH+, 3956 +/- 462 dyne s cm(-5); FH-, 2219 +/- 550 dyne s cm(-5)), but the effect was more pronounced in FH+ (P = 0.02). Glomerular hemodynamics, estimated by a modified Gomez model, revealed increased afferent and efferent responsiveness to A2 in FH+ subjects. These differences disappeared after chronic ACE inhibition when total, afferent and efferent sensitivities to A2 were similar in both groups.

CONCLUSIONS

Systemic blood pressure and plasma aldosterone responses to A2 were similar in men with or without a genetic disposition to primary arterial hypertension. However, our data demonstrate that men with a family history of hypertension have increased renovascular sensitivity to A2, and that chronic ACE inhibition normalizes their sensitivity.

Role of Oxidative Stress in Endothelial Dysfunction and Enhanced Responses to Angiotensin II of Afferent Arterioles from Rabbits Infused with Angiotensin II

ABSTRACT. The hypothesis that O2·− enhances angiotensin II (AngII)-induced vasoconstriction and impairs acetylcholine-induced vasodilation of afferent arterioles (Aff) in AngII–induced hypertension was investigated. Rabbits (n = 6 per group) received 12 to 14 d of 0.154 M NaCl (Sham), subpressor AngII (60 ng/kg per min; AngII 60) or slow pressor AngII (200 ng/kg per min; AngII 200). Individual Aff were perfused in vitro at 60 mmHg. AngII 200 increased mean arterial pressure (mean ± SD; 103 ± 9 versus 73 ± 6 mmHg; P < 0.01), plasma lipid peroxides (2.6 ± 0.3 versus 2.0 ± 0.3 nM; P < 0.05), renal cortical NADPH- and NADH-dependent O2·− generation, and Aff mRNA for p22phox 5-fold (P < 0.001) but decreased that for AT1-receptor 2.4-fold (P < 0.01). AngII 60 increased only NADH-dependent O2·− generation by renal cortex. Aff from AngII 200 rabbits had diminished acetylcholine relaxations (+50 ± 4 versus +85 ± 6%; P < 0.001), but these became similar in the presence of nitro-l-arginine (10−4 M). Aff from AngII 60 and AngII 200 rabbits had unchanged norepinephrine contractions (10−7 M) but significantly (P < 0.05) enhanced AngII contractions (10−8 M: Sham −52 ± 5 versus AngII 60 to 77 ± 5 versus AngII 200 to 110 ± 10%). The superoxide dismutase mimetic tempol (10−4 M) moderated the AngII responses of Aff from AngII 200 rabbits to levels of AngII 60 rabbits (−64 ± 7%). The AngII slow pressor response enhances renal cortical O2·− and p22phox expression. Increased O2·− generation in Aff mediates an impaired nitric oxide synthase–dependent endothelium-derived relaxing factor response and paradoxically enhances contractions to AngII despite downregulation of the mRNA for AT1 receptors. A subpressor dose of AngII enhances Aff responses to AngII independent of O2·−. E-mail: wilcoxch@georgetown.edu

Neural control of the renal vasculature in angiotensin II-induced hypertension

1. Chronic administration of angiotensin (Ang) II causes an increase in blood pressure via a multitude of actions, including direct vasoconstriction, hypertrophy and increased sympathetic nerve activity. In the present study, we assessed whether the hypertension resulting from chronic Ang II alters the ability of the renal vasculature to respond to sympathetic activity. 2. Angiotensin II was administered for 7 weeks via an osmotic minipump at a dose of 50 ng/kg per min, i.v., to a group of six rabbits. Blood pressure, measured at 0, 1, 2 and 6 weeks after insertion of the pump, increased from 76 +/- 2 to 104 +/- 6 mmHg at the end of 6 weeks, without any significant change in heart rate. The blood pressure in the control group remained constant at 76 +/- 2 mmHg. 3. After 7 weeks, rabbits were anaesthetized and the renal nerves were stimulated at 0.5, 1, 1.5, 2, 3, 5 or 8 Hz for 3 min at their supramaximal voltage (5.5 +/- 1.0 V in the normotensive group and 6.5 +/- 1.5 V in the hypertensive group) while the renal blood flow (RBF) response was recorded. Under anaesthesia, there was no difference in mean arterial pressure between the normotensive and hypertensive animals (77 +/- 2 and 80 +/- 7 mmHg, respectively). The resting RBF under these conditions was not significantly different in the hypertensive group (30 +/- 4 vs 26 +/- 5 mL/min in the normotensive vs hypertensive group, respectively). 4. Stimulation at increasing frequencies was associated with increasing reductions in RBF (e.g. 36 +/- 8% at 2 Hz in normotensive rabbits and 48 +/- 7% at 2 Hz in hypertensive rabbits). However, there were no significant differences between RBF responses in normotensive and hypertensive rabbits. 5. We conclude that hypertension associated with chronic Ang II administration does not alter the response in RBF to electrical stimulation of the nerves.

Soluble epoxide hydrolase inhibition lowers arterial blood pressure in angiotensin II hypertension

Epoxyeicosatrienoic acids (EETs) have antihypertensive properties and play a part in the maintenance of renal microvascular function. A novel approach to increase EET levels is to inhibit epoxide hydrolase enzymes that are responsible for conversion of biologically active EETs to dihydroxyeicosatrienoic acids (DHETs) that are void of effects on the preglomerular vasculature. We hypothesized that inhibition of soluble epoxide hydrolase (sEH) would lower blood pressure in angiotensin II (Ang II) hypertension. Rat renal cortical tissue was harvested and urine collected 2 weeks following implantation of an osmotic minipump containing Ang II (60 ng/min). Renal cortical sEH protein expression was significantly higher in Ang II hypertension compared with normotensive animals. Likewise, urinary 14,15-DHET levels were significantly increased in hypertensive compared with normotensive animals and averaged 8.1 +/- 1.3 and 2.7 +/- 1.1 ng/d; respectively. In additional experiments, the sEH inhibitor N-cyclohexyl-N-dodecyl urea (NCND; 3 mg/d) or vehicle (corn oil, 0.5 mL) was administered daily by intraperitoneal injection starting on day 10. Administration of NCND for 4 days lowered systolic blood pressure by 30 mm Hg in Ang II hypertensive animals, whereas the corn oil vehicle had no effect on blood pressure in normotensive or Ang II hypertensive animals. Measurement of blood pressure by indwelling arterial catheters in conscious animals with free movement in their cages confirmed that NCND had antihypertensive properties. Arterial blood pressure averaged 119 +/- 5 mm Hg in normotensive, 170 +/- 3 mm Hg in hypertensive and 149 +/- 10 mm Hg in NCND-treated, Ang II-infused animals. Administration of the potential metabolite of NCND, N-cyclohexylformamide to Ang II hypertensive rats did not lower the systolic blood pressure. These studies demonstrate that increased sEH expression in the Ang II hypertensive kidney leads to increased EET hydration. Moreover, sEH plays a role in the regulation of blood pressure, and inhibition of sEH during Ang II hypertension is antihypertensive.

Role of nitric oxide in regional blood flow in angiotensin II-induced hypertensive rats

The present study was designed to evaluate the contribution of nitric oxide (NO) to regional hemodynamics during the early phase of angiotensin II (Ang II)-induced hypertension. The responses of regional blood flow to chronic NO synthase inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME) were assessed using radioactive microspheres in conscious Ang II-infused hypertensive rats. Ang II-infused rats (270 ng/kg/min, subcutaneously for 12 days: n=11) showed higher mean arterial pressure (MAP: 153+/-4 mmHg) and total peripheral resistance (TPR: 1.61+/-0.06 mmHg/min/ml), and lower cardiac output (CO: 102+/-3 ml/min) than vehicle-infused normotensive rats (115+/-2 mmHg, 0.96+/-0.05 mmHg/min/ml and 130+/-7 ml/min, n=11, respectively). The blood flow rates in the brain, spleen, large intestine and skin were significantly reduced in Ang III-infused rats compared with vehicle-infused rats, while those in the lung, heart, liver, kidney, adrenal gland, small intestine, and skeletal muscle were similar. Treating Ang II-infused rats with L-NAME (75 mg/l in drinking water for 10 days, n=11) resulted in higher MAP (166+/-6 mmHg) and TPR (1.89+/-0.18 mmHg/min/ml) and lower CO (87+/-7 m/min) than untreated Ang II-infused rats. L-NAME-treated Ang II-infused rats showed widespread increases in regional vascular resistance and reduced blood flow rates in the kidney (3.81+/-0.27 ml/min/g) and skeletal muscle (0.20+/-0.03 ml/min/g) compared with untreated Ang II-infused rats (6.88+/-0.27 and 0.33+/-0.04 ml/min/g, respectively). However, there were no significant differences in the flow rates of other organs investigated between these animals. An NO donor, (+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (FK409: 30 microg/kg/min, i.v.), significantly decreased MAP (110+/-6 mmHg) and TPR (1.23+/-0.18 mmHg/min/ml) without significant changes in CO (89+/-9 ml/min) in L-NAME-treated Ang II-infused rats. Furthermore, FK409 partially reversed blood flow rates in the kidney (4.72+/-0.40 ml/min/g) and skeletal muscle (0.25+/-0.02 ml/min/g)in these animals. These results suggest that NO counteracts, at least in part, the vasoconstrictor effects of elevated Ang II levels in renal and skeletal muscle vascular beds, and is an important modulator in the regulation of blood flow to these organs during the development of Ang II-induced hypertension.

Enhanced renal microvascular reactivity to angiotensin II in hypertension is ameliorated by the sulfonimide analog of 11,12-epoxyeicosatrienoic acid

OBJECTIVES

Epoxygenase metabolites produced by the kidney affect renal blood flow and tubular transport function and 11,12-epoxyeicosatrienoic acid (11,12-EET) has been putatively identified as an endothelium-derived hyperpolarizing factor. The current studies were performed to determine the influence of 11,12-EET on the regulation of afferent arteriolar diameter in angiotensin II-infused hypertensive rats.

MATERIALS AND METHODS

Male Sprague-Dawley rats received angiotensin II (60 ng/min) or vehicle via an osmotic minipump. Angiotensin II-infused hypertensive and vehicle-infused normotensive rats were studied for 2 weeks following implantation of the minipump. Renal microvascular responses to the sulfonimide analog of 11,12-EET (11,12-EET-SI) and angiotensin II were observed utilizing the in-vitro juxtamedullary nephron preparation. Renal cortical epoxygenase enzyme protein levels were quantified by Western blot analysis. Renal microvessels were also isolated and epoxygenase metabolite levels measured by negative ion chemical ionization (NICI)/gas chromatography-mass spectroscopy.

RESULTS

Systolic blood pressure averaged 118 +/- 2 mmHg prior to pump implantation and increased to 185 +/- 7 mmHg in rats infused with angiotensin II for 2 weeks. Afferent arteriolar diameters of 2-week normotensive animals averaged 22 +/- 1 microm. Diameters of the afferent arterioles were 17% smaller in hypertensive rats (P< 0.05); however, arterioles from both groups responded to 11,12-EET-SI (100 nmol) with similar 15-17% increases in diameter. As we previously demonstrated, the afferent arteriolar reactivity to angiotensin II was enhanced in angiotensin II-infused animals. Interestingly, elevation of 11,12-EET-SI levels to 100 nmol reversed the enhanced vascular reactivity to angiotensin II associated with angiotensin II hypertension. Renal microvascular EET levels were not different between groups and averaged 81 +/- 9 and 87 +/- 13 pg/mg per 30 min in normotensive and hypertensive animals, respectively. Renal cortical microsomal levels of the epoxygenase CYP2C23 and CYP2C11 proteins were also similar in normotensive and angiotensin II hypertensive rats.

CONCLUSIONS

Taken together, these data support the concept that renal microvascular 11,12-EET activity and levels may not properly offset the enhanced angiotensin II renal vasoconstriction during angiotensin II hypertension.