Angiotensin II action in isolated microperfused rabbit afferent arterioles is modulated by flow

Purinoceptors, renal microvascular function and hypertension

Proper renal blood flow (RBF) and glomerular filtration rate (GFR) are critical for maintaining normal blood pressure, kidney function and water and electrolyte homeostasis. The renal microvasculature expresses a multitude of receptors mediating vasodilation and vasoconstriction, which can influence glomerular blood flow and capillary pressure. Despite this, RBF and GFR remain quite stable when arterial pressure fluctuates because of the autoregulatory mechanism. ATP and adenosine participate in autoregulatory control of RBF and GFR via activation of two different purinoceptor families (P1 and P2). Purinoceptors are widely expressed in renal microvasculature and tubules. Emerging data show altered purinoceptor signaling in hypertension-associated kidney injury, diabetic nephropathy, sepsis, ischemia-reperfusion induced acute kidney injury and polycystic kidney disease. In this brief review, we highlight recent studies and new insights on purinoceptors regulating renal microvascular function and renal hemodynamics. We also address the mechanisms underlying renal microvascular injury and impaired renal autoregulation, focusing on purinoceptor signaling and hypertension-induced renal microvascular dysfunction. Interested readers are directed to several excellent and comprehensive reviews that recently covered the topics of renal autoregulation, and nucleotides in kidney function under physiological and pathophysiological conditions (Inscho 2009, Navar et al. 2008, Carlstrom et al. 2015, Vallon et al. 2020).

Prostaglandin E2 mediates connecting tubule glomerular feedback

Connecting tubule glomerular feedback (CTGF) is a mechanism in which Na reabsorption in the connecting tubule (CNT) causes afferent arteriole (Af-Art) dilation. CTGF is mediated by eicosanoids, including prostaglandins and epoxyeicosatrienoic acids; however, their exact nature and source remain unknown. We hypothesized that during CTGF, the CNT releases prostaglandin E2, which binds its type 4 receptor (EP4) and dilates the Af-Art. Rabbit Af-Arts with the adherent CNT intact were microdissected, perfused, and preconstricted with norepinephrine. CTGF was elicited by increasing luminal NaCl in the CNT from 10 to 80 mmol/L. We induced CTGF with or without the EP4 receptor blocker ONO-AE3-208 added to the bath in the presence of the epoxyeicosatrienoic acid synthesis inhibitor MS-PPOH. ONO-AE3-208 abolished CTGF (control, 9.4 ± 0.5; MS-PPOH+ONO-AE3-208, -0.6 ± 0.2 μm; P<0.001; n=6). To confirm these results, we used a different, specific EP4 blocker, L161982 (10(-5) mol/L), that also abolished CTGF (control, 8.5 ± 0.9; MS-PPOH+L161982, 0.8 ± 0.4 μm; P<0.001; n=6). To confirm that the eicosanoids that mediate CTGF are released from the CNT rather than the Af-Art, we first disrupted the Af-Art endothelium with an antibody and complement. Endothelial disruption did not affect CTGF (7.9 ± 0.9 versus 8.6 ± 0.6 μm; P=NS; n=7). We then added arachidonic acid to the lumen of the CNT while maintaining zero NaCl in the perfusate. Arachidonic acid caused dose-dependent dilation of the attached Af-Art (from 8.6 ± 1.2 to 15.3 ± 0.7 μm; P<0.001; n=6), and this effect was blocked by ONO-AE3-208 (10(-7) mol/L). We conclude that during CTGF, the CNT releases prostaglandin E2, which acts on EP4 on the Af-Art inducing endothelium-independent dilation.

Salt-resistant blood pressure and salt-sensitive renal autoregulation in chronic streptozotocin diabetes

Hyperfiltration occurs in early type 1 diabetes mellitus in both rats and humans. It results from afferent vasodilation and thus may impair stabilization of glomerular capillary pressure by autoregulation. It is inversely related to dietary salt intake, the "salt paradox." Restoration of normal glomerular filtration rate (GFR) involves increased preglomerular resistance, probably mediated by tubuloglomerular feedback (TGF). To begin to test whether the salt paradox has pathogenic significance, we compared intact vs. diabetic (streptozotocin) Long-Evans rats with normal and increased salt intake, 1 and approximately 3% by weight of food eaten, respectively. Weekly 24-h blood pressure records were acquired by telemetry before and during diabetes. Blood glucose was maintained at approximately 20 mmol/l by insulin implants. GFR was significantly elevated only in diabetic rats on normal salt intake, confirming diabetic hyperfiltration and the salt paradox. Renal blood flow dynamics show strong contributions to autoregulation by both TGF and the myogenic mechanism and were not impaired by diabetes or by increased salt intake. Separately, systolic pressure was not elevated in diabetic rats at any time during 12 wk with normal or high salt intake. Autoregulation was effective in all groups, and the diabetic-normal salt group showed significantly improved autoregulation at low perfusion pressures. Histological examination revealed very minor glomerulosclerosis and modest mesangial expansion, although neither was diagnostic of diabetes. Periodic acid-Schiff-positive droplets found in distal tubules and collecting duct segments were diagnostic of diabetic kidneys. Biologically significant effects attributable to increased salt intake were abrogation of hyperfiltration and of the left shift in autoregulation in diabetic rats.

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

AT1 receptors mediate angiotensin II–induced release of nitric oxide in afferent arterioles

BACKGROUND

Recent studies have indicated that angiotensin II (Ang II) possibly activates the nitric oxide (NO) system. We investigated the role of AT receptor subtypes (AT-R) in mediating the Ang II-induced NO release in afferent arterioles (Af) of mice.

METHODS

Isolated Af of mice were perfused, and the isotonic contraction measured. Further, NO release was determined using DAF-FM, a fluorescence indicator for NO. Moreover, we qualitatively assessed the expression of AT-R at the mRNA level using reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Ang II reduced luminal diameters dose dependently (67.3 +/- 6.3% at 10(-6) mol/L). Inhibition of AT2-R with PD123.319 did not change the Ang II contractile response. AT1-R blockade with ZD7155 inhibited contraction. Stimulation of AT2-R during AT1-R inhibition with ZD7155, and preconstriction with norepinephrine (NE) had no influence on the diameter. Drug application via the perfusion pipette changed flow and pressure, and enhanced NO fluorescence by DeltaF = 4.0 +/- 0.4% (N= 14, background). Luminal application of Ang II (10(-7) mol/L) increased the NO fluorescence by DeltaF = 9.9 +/- 1.2% (N= 8). AT1-R blockade blunted the increase to background levels (DeltaF to 4.0 +/- 0.3%, N= 6, P < 0.05), but AT2-R blockade did not (8.1 +/- 0.9%, N= 9). L-NAME nearly abolished the Ang II effect on the NO fluorescence (DeltaF = 1.6 +/- 0.5% (N= 8). NE did not increase NO release beyond the background levels. RT-PCR showed expression of both AT1-R and AT2-R.

CONCLUSION

The results indicate an Ang II-induced NO release in Af of mice, which is mediated by AT1-R. Thus, Ang II balances its own constrictor action in Af. This control mechanism is very important in view of high renin and angiotensin II concentration in the juxtaglomerular apparatus.

Endothelium-Derived Nitric Oxide Modulates Vascular Action of Aldosterone in Renal Arteriole

We have recently demonstrated that aldosterone causes nongenomic vasoconstriction by activating phospholipase C (PLC) in the preglomerular afferent arteriole (Af-Art). In the present study, we tested the hypothesis that endothelium modulates this vasoconstrictor action by releasing nitric oxide (NO). In addition, to study the post-PLC mechanism, we examined possible contributions of phosphoinositol hydrolysis products. Rabbit Af-Arts were microperfused at 60 mm Hg in vitro, and increasing doses of aldosterone (10 −10 to 10 −8 mol/L) were added to the bath and lumen. Aldosterone caused dose-dependent vasoconstriction (within 10 minutes); significant ( P <0.01) constriction was observed from 5×10 −9 mol/L, and at 10 −8 mol/L, intraluminal diameter decreased by 29%±3% (n=9). Disrupting the endothelium augmented vasoconstriction; significant constriction was observed from 10 −10 mol/L, and at 10 −8 mol/L, the diameter decreased by 38%±2% (n=6). NO synthesis inhibition reproduced this augmentation (n=7). Pretreatment with chelerythrine (10 −6 mol/L), a protein kinase C (PKC) inhibitor, slightly attenuated the constriction; aldosterone at 10 −8 mol/L now decreased the diameter by 18%±3% (n=7). However, in Af-Arts treated with thapsigargin (10 −6 mol/L) or dantrolene (3×10 −5 mol/L), which blocks inositol 1,4,5-triphosphate (IP 3 )-induced intracellular calcium release, aldosterone at 10 −8 mol/L decreased the diameter by only 9%±1% (n=6) or 9%±2% (n=5), respectively. These results demonstrate that in the Af-Art endothelium-derived NO modulates vasoconstrictor actions of aldosterone that are mediated by the activation of both IP 3 and PKC pathways. Such vasoconstrictor actions of aldosterone may contribute to the development or aggravation of hypertension by elevating renal vascular resistance in cardiovascular diseases associated with endothelium dysfunction.

Angiotensin II type 1 receptor blockade restores nitric oxide-dependent renal vascular responses in renovascular hypertension

It was previously reported that enhancement of renal vascular responses to angiotensin II in hypertensive rats is related to decreased release of nitric oxide. Thus, it was suggested that impairment of nitric oxide synthesis during development of hypertension is related to a decreased nitric oxide synthase mRNA expression by an angiotensin II-dependent mechanism. The current study evaluated whether the blockade of angiotensin II type 1 receptor during the development of hypertension restored nitric oxide synthase mRNA expression, nitric oxide synthesis, and nitric oxide-dependent modulation of angiotensin II vasoconstrictor effects. It was shown that losartan treatment prevented increased vascular responses to angiotensin II in hypertensive rats and that this effect was associated with restoration of nitric oxide synthase mRNA expression and nitric oxide synthase activity. Furthermore, angiotensin II-dependent nitric oxide release in hypertensive rats was potentiated by losartan treatment. Angiotensin II (1 microg) released renal nitrites by 485 +/- 178, 470 +/- 150, 185 +/- 45, and 515 +/- 100 nmol/ml/30 s in the kidneys from normotensive, losartan-treated normotensive rats, hypertensive, and losartan-treated hypertensive rats, respectively. The data suggest that during development of hypertension, angiotensin II downregulates nitric oxide synthase mRNA expression, blunting nitric oxide vasodilatory tone and increasing vascular sensitivity to vasoconstrictor agents in the renal circulation.

Effect of exogenous and endogenous angiotensin II on intrarenal distribution of glomerular filtration rate in rats

Different changes in glomerular filtration rates (GFR) in deep and superficial glomeruli have been suggested to influence renal NaCl excretion and concentrating ability. Angiotensin II (AngII) has been implicated in such changes, but the experimental evidence has been conflicting, probably because of the methodological limitation of just one 'snapshot' measurement of local GFR per kidney. We have therefore studied the effect of AngII and AT(1)-receptor blockade on glomerular filtration in outer, middle and inner cortex (OC, MC and IC, respectively) in pentobarbitone-anaesthetised rats using the aprotinin (Ap) method, providing control and experimental measurements in the same kidney. Glomerular filtration rate per gram cortical tissue was measured based on 'free' glomerular filtration of Ap followed by complete tubular uptake and a 20 min sojourn in the proximal tubular cells before breakdown and incipient return to the plasma.(125)I-labelled Ap was injected I.V. to determine control Ap clearance, followed after 13 min by injection of AngII or the A1 type AngII receptor blocker losartan and 2 min thereafter by (131)I-labelled Ap to determine clearance in the experimental period. Tracer activity in frequent blood samples and in tissue samples allowed calculation of GFR in the two periods. Mean GFR control values were: 1.13 ml min(-1) in whole kidney and 1.44, 1.27 and 0.76 ml min(-1) per gram cortical tissue in OC, MC and IC, respectively. The most sensitive and comprehensive measure of altered GFR distribution is the ratio between the relative filtration change in inner versus that in outer cortex, F = (IC(E)/IC(C))/(OC(E)/OC(C)), where subscripts E and C stand for experimental and control, respectively. F values greater than 1.00 directly indicate and quantify a relatively greater increase of filtration rate in inner than in outer cortex. We found in salt-replete rats that at practically unchanged total GFR, intravenous and intra-arterial infusion of AngII increased F to 1.07 and 1.04 (P < 0.05) whereas losartan reduced F to 0.99. After pretreatment with the inhibitor of nitric oxide production L-NAME, losartan increased total GFR by 8 % and F fell to 0.95 (P < 0.05). In salt-depleted rats losartan reduced F to 0.95 (P < 0.05) at unchanged total GFR. All IC/OC changes induced by losartan were significantly different from that obtained by AngII infusions. We conclude that deep nephrons have higher postglomerular AngII tone and also higher AngII sensitivity than superficial nephrons. The better preserved GFR in deep cortex during AngII action may contribute towards maintaining the renal concentrating ability by providing NaCl for reabsorption by the ascending limb of the loop of Henle.

Vasodilator action of angiotensin-(1-7) on isolated rabbit afferent arterioles

Recent studies have shown that angiotensin-(1-7) (Ang-[1-7]), which is generated endogenously from both Ang I and II, is a bioactive component of the renin-angiotensin system and may play an important role in the regulation of blood pressure. However, little is known about its role in regulating the reactivity of the afferent arteriole or the mechanism(s) involved. We hypothesized that Ang-(1-7), acting on specific receptors, participates in the control of afferent arteriole tone. We first examined the direct effect of Ang-(1-7) on rabbit afferent arterioles microperfused in vitro, and we tested whether endothelium-derived relaxing factor/NO and cyclooxygenase products are involved in its actions. To assess the vasodilator effect of Ang-(1-7), afferent arterioles were preconstricted with norepinephrine, and increasing concentrations of Ang-(1-7) were added to the lumen. We found that 10(-10) to 10(-6) mol/L Ang-(1-7) produced dose-dependent vasodilatation, increasing luminal diameter from 8.9+/-1.0 to 16.3+/-1.1 microm (P<0.006). Indomethacin had no effect on Ang-(1-7)-induced dilatation. N(G)-nitro-L-arginine methyl ester, a NO synthesis inhibitor, abolished the dilatation induced by Ang-(1-7). We attempted to determine which angiotensin receptor subtype is involved in this process. We found that 10(-6) mol/L [d-Ala7]-Ang-(1-7), a potent and selective Ang-(1-7) antagonist, abolished the dilatation induced by Ang-(1-7). An angiotensin II type 1 receptor antagonist (L158809) and an angiotensin II type 2 receptor antagonist (PD 123319) at 10(-6) mol/L had no effect on Ang-(1-7)-induced dilatation. Our results show that Ang-(1-7) causes afferent arteriole dilatation. This effect may be due to production of NO, but not the action of cyclooxygenase products. Ang-(1-7) has a receptor-mediated vasodilator effect on the rabbit afferent arteriole. This effect may be mediated by Ang-(1-7) receptors, because angiotensin type 1 and type 2 receptor antagonists could not block Ang-(1-7)-induced dilatation. Thus, our data suggest that Ang-(1-7)opposes the action of Ang II and plays an important role in the regulation of renal hemodynamics.

Role of neuronal nitric oxide synthase in the macula densa

BACKGROUND

There is evidence that macula densa nitric oxide (NO) inhibits tubuloglomerular feedback (TGF). However, TGF response is not altered in mice deficient in neuronal nitric oxide synthase (nNOS) (-/-). Furthermore, nNOS expression in the macula densa is inversely related to salt intake, yet micropuncture studies have shown that NOS inhibition potentiates TGF in rats on high sodium intake but not in rats on a low-salt diet. These inconsistencies may be due to confounding systemic factors, such as changes in circulating renin. To further clarify the role of macula densa nNOS in TGF response, independent of systemic factors, we tested the hypothesis that (1) TGF response is inversely related to sodium intake, and (2) during low sodium intake, NO produced by macula densa nNOS tonically controls the basal diameter of the afferent arteriole (Af-Art).

METHODS

Af-Arts and attached macula densas were simultaneously microperfused in vitro. TGF response was determined by measuring Af-Art diameter before and after increasing NaCl in the macula densa perfusate. TGF response was studied in wild-type (+/+) and nNOS knockout mice (-/-), as well as in juxtaglomerular apparatuses (JGAs) from rabbits fed a low-, normal-, or high-NaCl diet.

RESULTS

TGF responses were similar in nNOS +/+ and -/- mice. However, in nNOS +/+ mice, 7-nitroindazole (7-NI) perfused into the macula densa significantly potentiated the TGF response (P = 0.001), while in nNOS -/- mice, this potentiation was absent. In rabbits on three different sodium diets, TGF responses were similar and were potentiated equally by 7-NI. However, in JGAs from rabbits on a low-NaCl diet, adding 7-NI to the macula densa while perfusing it with low-NaCl fluid caused Af-Art vasoconstriction, decreasing the diameter by 14% (from 21.7 +/- 1.3 to 18.6 +/- 1.5 microm; P < 0.001). This effect was not observed in JGAs from rabbits fed a normal- (19.0 +/- 0.5 vs. 19.3 +/- 0.8 microm after 7-NI) or high-NaCl diet (18.6 +/- 0.7 vs. 18.4 +/- 0.7 microm).

CONCLUSIONS

First, in this in vitro preparation, chronic changes in macula densa nNOS do not play a major role in the regulation of TGF. Compensatory mechanisms may develop during chronic alteration of nNOS that keep TGF relatively constant. Second, nNOS regulates TGF response acutely. Third, the results obtained in the +/+ and -/- mice also confirm that the effect of 7-NI is due to inhibition of macula densa nNOS. Finally, during low sodium intake (without induction of TGF), the regulation of basal Af-Art resistance by macula densa nNOS suggests that NO in the macula densa helps maintain renal blood flow during the high renin secretion caused by low sodium intake.

Interaction between nitric oxide and renal myogenic autoregulation in normotensive and hypertensive rats

Blood pressure fluctuates continuously throughout life and autoregulation is the primary mechanism that isolates the kidney from this fluctuation. Compared with Wistar rats, Brown Norway (B-N) rats display impaired renal myogenic autoregulation when blood pressure fluctuation is increased. They also are very susceptible to hypertension-induced renal injury. Because blockade of nitric oxide augments myogenic autoregulation in Wistar rats, we compared the response of the myogenic system in B-N rats to nitric oxide blockade with that of other strains [Wistar, Sprague-Dawley, Long-Evans, spontaneously hypertensive (SHR)]. Renal blood flow dynamics were assessed in isoflurane anesthetized rats before and after inhibition of nitric oxide synthase by Lω-nitro-arginine methyl-ester (L-NAME, 10 mg/kg, iv). Under control conditions, myogenic autoregulation in the B-N rats was weaker than in the other strains. Myogenic autoregulation was not augmented after L-NAME administration in the SHR, but was augmented in all the normotensive rats. The enhancement was significantly greater in B-N rats so that after L-NAME the efficiency of autoregulation did not differ among the strains. The data suggest that nitric oxide is involved in the impaired myogenic autoregulation seen in B-N rats. Furthermore, the similarity of response in Wistar, Long-Evans, and Sprague-Dawley rats suggests that modulation by nitric oxide is a fundamental property of renal myogenic autoregulation.Key words: renal blood flow, transfer function, dynamics, SHR, Wistar, Long-Evans, Sprague-Dawley, Brown-Norway, L-NAME.

Hemodynamic and renal effects of acute and progressive nitric oxide synthesis inhibition in anesthetized dogs

This study evaluated the effects of progressive nitric oxide (NO) inhibition in the regulation of systemic and regional hemodynamics and renal function in anesthetized dogs. The N(G)-nitro-L-arginine methyl ester group (n = 9) received progressive doses of 0.1, 1, 10, and 50 microg. kg(-1). min(-1). Renal (RBF), mesenteric (MBF), iliac (IBF) blood flows, mean arterial pressure (MAP), pulmonary pressures, cardiac output (CO), and systemic and pulmonary vascular resistances were measured. During N(G)-nitro-L-arginine methyl ester infusion, MAP and systemic vascular resistances increased in a dose-dependent manner. Mean pulmonary pressure and pulmonary vascular resistances increased in both the N(G)-nitro-L-arginine methyl ester and the control group, but the increase was more marked in the N(G)-nitro-L-arginine methyl ester group during the last two infusion periods. CO decreased progressively, before any significant change in blood pressure was noticeable in the N(G)-nitro-L-arginine methyl ester group. IBF decreased significantly from the first N(G)-nitro-L-arginine methyl ester dose, whereas RBF and MBF only decreased significantly during the highest N(G)-nitro-L-arginine methyl ester dose. Urinary volume and sodium excretion only increased significantly in the time control group during the two last time periods. The pulmonary vasculature was more sensitive than the systemic vasculature, whereas skeletal muscle and renal vasculatures showed a greater sensitivity to the inhibition of NO production than the mesenteric vasculature. NO synthesis inhibition induces a progressive antidiuretic and antinatriuretic effect, which is partially offset by the increase in blood pressure.

Eicosanoid regulation of the renal vasculature

Even though it has been recognized that arachidonic acid metabolites, eicosanoids, play an important role in the control of renal blood flow and glomerular filtration, several key observations have been made in the past decade. One major finding was that two distinct cyclooxygenase (COX-1 and COX-2) enzymes exist in the kidney. A renewed interest in the contribution of cyclooxygenase metabolites in tubuloglomerular feedback responses has been sparked by the observation that COX-2 is constitutively expressed in the macula densa area. Arachidonic acid metabolites of the lipoxygenase pathway appear to be significant factors in renal hemodynamic changes that occur during disease states. In particular, 12(S)- hydroxyeicosatetraenoic acid may be important for the full expression of the renal hemodynamic actions in response to angiotensin II. Cytochrome P-450 metabolites have been demonstrated to possess vasoactive properties, act as paracrine modulators, and be a critical component in renal blood flow autoregulatory responses. Last, peroxidation of arachidonic acid metabolites to isoprostanes appears to be involved in renal oxidative stress responses. The recent developments of specific enzymatic inhibitors, stable analogs, and gene-disrupted mice and in antisense technology are enabling investigators to understand the complex interplay by which eicosanoids control renal blood flow.

Prostaglandins buffer ANG II-mediated increases in cytosolic calcium in preglomerular VSMC

In order to exert an appropriate biological effect, the action of the vasoconstrictive hormone angiotensin II (ANG II) is modulated by vasoactive factors such as prostaglandins PGE2 and PGI2. The present study investigates whether prostaglandins alter ANG II-mediated increases in cytosolic calcium concentration ([Ca2+]i) in vascular smooth muscle cells (VSMC) isolated from rat renal preglomerular arterioles. [Ca2+]i was assessed using the calcium-sensitive dye fura 2 and a microscope-based photometer system. ANG II (10(-7) M) caused a biphasic, time-dependent [Ca2+]i response: an initial peak increase from 52 +/- 7 to 264 +/- 25 nM, followed by a sustained plateau of 95 +/- 9 nM in cultured VSMC. Coadministration of PGE2 or PGI2 or synthetic mimetics caused dose-dependent decreases in the peak [Ca2+]i response to ANG II, with attenuation of 40-50%. This degree of inhibition was even more pronounced in individual freshly isolated preglomerular VSMC. Increasing cAMP levels in cultured VSMC, by using either a cell-permeable analog or inhibiting phosphodiesterase activity, mirrored the antagonistic effects of prostaglandins on ANG II-stimulated increases in [Ca2+]i. Radioimmunoassays demonstrate that ANG II (10(-7) M) stimulates production of PGI2 and PGE2; the stable prostacyclin metabolite 6-keto-PGF(1alpha) was released in 10-fold greater concentrations than PGE(2.) Indomethacin blockade of prostaglandin production potentiated both the peak (264 to 337 +/- 26 nM) and sustained [Ca2+]i responses (95 to 181 +/- 22 nM) to ANG II. When prostaglandin analogs were added during indomethacin treatment, the ANG II response was restored to the typical pattern. In conclusion, we demonstrate that modulation of intracellular calcium levels is one mechanism by which prostaglandins can buffer ANG II-mediated constriction in renal preglomerular VSMC. PGI2 is more potent than PGE2 in this regard.

Role of shear stress in nitric oxide-dependent modulation of renal angiotensin II vasoconstriction

1. Renal vasoconstriction in response to angiotensin II (ANGII) is known to be modulated by nitric oxide (NO). Since shear stress stimulates the release of a variety of vasoactive compounds from endothelial cells, we studied the impact of shear stress on the haemodynamic effect of ANGII in isolated perfused kidneys of rats under control conditions and during NO synthase inhibition with L-NAME (100 microM). 2. Kidneys were perfused in the presence of cyclo-oxygenase inhibitor (10 microM indomethacin) with Tyrode's solution of relative viscosity zeta=1 (low viscosity perfusate, LVP) or, in order to augment shear stress, with Tyrode's solution containing 7% Ficoll 70 of relative viscosity zeta=2 (high viscosity perfusate, HVP). 3. Vascular conductance was 3.5+/-0.4 fold larger in HVP as compared with LVP kidneys, associated with an augmentation of overall wall shear stress by 37+/-5%. During NO inhibition, vascular conductance was only 2.5+/-0.2 fold elevated in HVP vs LVP kidneys, demonstrating shear stress-induced vasodilatation by NO and non-NO/non-prostanoid compound(s). 4. ANGII (10 - 100 pM) constricted the vasculature in LVP kidneys, but was without effect in HVP kidneys. During NO inhibition, in contrast, ANGII vasoconstriction was potentiated in HVP as compared with LVP kidneys. 5. The potentiation of ANGII vasoconstriction during NO inhibition has been shown to be mediated by endothelium-derived P450 metabolites and to be sensitive to AT2 receptor blockade in our earlier studies. Accordingly, in HVP kidneys, increasing concentrations of the AT2 receptor antagonist PD123319 (5 and 500 nM) gradually abolished the potentiation of ANGII vasoconstriction during NO inhibition, but did not affect vasoconstriction in response to ANGII in LVP kidneys. 6. Our results demonstrate, that augmentation of shear stress by increasing perfusate viscosity induces vasodilatation in the rat kidney, which is partially mediated by NO. Elevated levels of shear stress attenuate renal ANGII vasoconstriction through enhanced NO production and are required for AT2 sensitive potentiation during NO inhibition.

Losartan attenuates modest but not strong renal vasoconstriction induced by nitric oxide inhibition

Previous studies showed variable success of angiotensin II (ANG II) antagonists to oppose systemic and renal vasoconstriction during long-term nitric oxide synthase (NOS) inhibition. We explored in short-term experiments whether the systemic and renal vasodilatory response to angiotensin II type 1 (AT1)-receptor blockade depends on the extent of NOS blockade. In the first series of experiments, anesthetized rats underwent clearance studies during continuous monitoring of mean arterial pressure (MAP), renal blood flow (RBF, flow probe), and renal vascular resistance (RVR). Compared with control animals, low-dose infusion of the NOS-inhibitor nitro-L-arginine (NLA) increased MAP and RVR, decreased glomerular filtration rate, RBF, and sodium excretion, and had no effect on plasma and kidney ANG II content. High-dose NLA induced stronger effects, did not affect plasma ANG II, and reduced kidney ANG II to approximately 60%. In the second series of experiments, we studied the effect of low- and high-dose NLA on autoregulation of RBF. NLA induced a dose-dependent increase in MAP and decrease in RBF but left autoregulation intact. The AT1-receptor antagonist losartan restored MAP and RBF during low-dose NLA but had no depressor or renal vasodilating effect during high-dose NLA. In summary, short-term NOS blockade causes a dose-dependent pressor and renal vasoconstrictor response, without affecting renal autoregulation, and AT1-receptor blockade restores systemic pressor and renal vasoconstrictive effects of mild NOS inhibition but fails to exert vasorelaxation during strong NOS blockade. Both levels of NOS inhibition did not importantly alter intrarenal ANG II levels. Apparently the functional role of endogenous ANG II as determinant of vascular tone is diminished during strong NOS inhibition.

Role of eicosanoids in renal angiotensin II vasoconstriction during nitric oxide blockade

Nitric oxide (NO) buffers the effect of vasoconstrictors currently active in the renovascular system. Enhancement of the angiotensin II (Ang II)-induced vasoconstriction during NO blockade comprises both AT2-sensitive potentiation, decreasing the half maximal vasoconstriction (EC50) value to the subnanomolar concentration range, and augmentation, increasing the maximal effect (Emax) value in the isolated perfused rat kidney. In this study, we examine whether constrictory prostanoids are involved in Ang II subtype receptor (AT2)-sensitive potentiation of the Ang II effect during NO blockade. Thus, Ang II-induced vasoconstriction (0.1 or 10 nM Ang II) was measured in six series of constant-flow perfused isolated rat kidneys in the presence of indomethacin under control conditions, during NO inhibition, and during combined inhibition of NO and all arachidonic pathways by eicosatetraynoic acid (ETYA), an analog of arachidonic acid. The vasoconstriction elicited by 10 nM Ang II, which is the maximal response, increased about threefold during NO inhibition compared with control. This augmentation was not affected by ETYA. In contrast, the vasoconstriction elicited by 0.1 nM Ang II increased about 20-fold during NO inhibition, reflecting mainly potentiation of the Ang II effect. This increase was abrogated by ETYA. We conclude that vasoconstrictor eicosanoids, which are suppressed by endogenous NO, mediate AT2-sensitive potentiation of the Ang II-induced vasoconstriction in the rat kidney.

The role of nitric oxide in angiotensin II-induced renal vasoconstriction in renovascular hypertension

OBJECTIVE

To evaluate the contribution of nitric oxide to the regulation of angiotensin II-induced renal vasoconstriction in normotensive rats and in rats with aortic coarctation-induced hypertension.

METHODS

We evaluated the renal vascular reactivity of nonischemic kidney to angiotensin II with and without nitric oxide synthesis inhibitor (NG-nitro-L-arginine methyl ester) in the isolated perfused kidney. The nitrite concentration in renal perfusate of nonischemic kidney was measured as an index of nitric oxide released and the activity of nitric oxide synthase in renal tissue was determined by production of [3H]-L-citrulline.

RESULTS

The perfusion of NG-nitro-L-arginine methyl ester potentiated angiotensin II-induced renal vasoconstriction in normotensive rats but had no effect on hypertensive rats. The release of nitrites in kidneys from hypertensive rats was lower than that in kidneys from normotensive rats. The activity of renal nitric oxide synthase was less in the hypertensive rats than it was in the normotensive rats.

CONCLUSIONS

Nitric oxide counteracts the vasoconstrictor effect of angiotensin II in normotensive rats, whereas this protective mechanism is impaired in hypertensive rats. This impairment potentiates effect of angiotensin II on vascular resistance, thereby contributing to the development of high blood pressure.

Possible role of P-450 metabolite of arachidonic acid in vasodilator mechanism of angiotensin II type 2 receptor in the isolated microperfused rabbit afferent arteriole

Although angiotensin II type 2 (AT2) receptor has recently been cloned, its functional role is not well understood. We tested the hypothesis that selective activation of AT2 receptor causes vasodilation in the preglomerular afferent arteriole (Af-Art), a vascular segment that accounts for most of the preglomerular resistance. We microperfused rabbit Af-Arts at 60 mmHg in vitro, and examined the effect of angiotensin II (Ang II; 10(-11)-10(-8) M) on the luminal diameter in the presence or absence of the Ang II type 1 receptor antagonist CV11974 (CV; 10(-8) M). Ang II was added to both the bath and lumen of preconstricted Af-Arts. Ang II further constricted Af-Arts without CV (by 74+/-7% over the preconstricted level at 10(-8) M; P < 0.01, n = 7). In contrast, in the presence of CV, Ang II caused dose-dependent dilation; Ang II at 10(-8) M increased the diameter by 29+/-2% (n = 7, P < 0.01). This dilation was completely abolished by pretreatment with an AT2 receptor antagonist PD123319 (10(-7) M, n = 6), suggesting that activation of AT2 receptor causes vasodilation in Af-Arts. The dilation was unaffected by inhibiting either nitric oxide synthase (n = 7) or cyclooxygenase (n = 7), however, it was abolished by either disrupting the endothelium (n = 10) or inhibiting the cytochrome P-450 pathway, particularly the synthesis of epoxyeicosatrienoic acids (EETs, n = 7). These results suggest that in the Af-Art activation of the AT2 receptor may cause endothelium-dependent vasodilation via a cytochrome P-450 pathway, possibly by EETs.

AT2-antagonist sensitive potentiation of angiotensin II-induced vasoconstrictions by blockade of nitric oxide synthesis in rat renal vasculature

1. Although the actions of angiotensin II (Ang II) on renal haemodynamics appear to be mediated by activation of the AT1 receptor subtype, AT2 binding sites have also been evidenced in the adult kidney vasculature. As NO is known to mask part of the renal effects of vasoconstrictor drugs, we queried whether the Ang II-induced vasoconstrictions could occur via multiple receptor subtypes during inhibition of NO synthesis. We explored the effect of AT1 and AT2 receptor (AT-R) antagonists on Ang II-induced pressure increases during NO synthase or soluble guanylyl cyclase inhibition in rat isolated kidneys perfused in the presence of indomethacin at constant flow in a single-pass circuit. 2. In the absence of NO blockade, the AT1-R antagonist L-158809 (500 nM) antagonized the Ang II-induced vasoconstrictions, while the AT2-R antagonist PD-123319 (500 nM) had no effect. 3. Perfusing kidneys in the presence of either NO synthase inhibitors, L-NAME (100 microM) or L-NOARG (1 mM), or soluble guanylyl cyclase inhibitor, LY-83583 (10 microM), significantly increased both molar pD2 (from 9.40+/-0.25 to 10.36+/-0.11) and Emax values (from 24.9+/-3.1 to 79.9+/-4.9 mmHg) of the concentration-response curve for Ang II-induced vasoconstriction. 4. In the presence of L-NAME, 500 nM L158809 abolished the Ang II-induced vasoconstrictions whatever the concentration tested. On the other hand, 500 nM PD-123319 reversed the left shift of the concentration-response curve for Ang II (molar pD2 value 9.72+/-0.13) leaving Emax value unaffected (91.3+/-7.6 mmHg). 5. In the presence of L-NAME, the potentiated vasoconstriction induced by 0.1 nM and the augmented vasoconstriction induced by 10 nM Ang II were fully inhibited in a concentration-dependent manner by L-158809 (0.05-500 nM). By contrast, PD-123319 (0.5-500 nM) did not affect the 10 nM Ang II-induced vasoconstriction and concentration-dependently decreased the 0.1 nM Ang II-induced vasoconstriction plateauing at 65% inhibition above 5 nM antagonist. 6. Similar to PD-123319, during NO blockade the AT2-R antagonist CGP-42112A at 5 nM decreased by 50% the 0.1 nM Ang II-induced vasoconstriction and at 500 nM had no effect on 10 nM Ang II-induced vasoconstriction. 7. In conclusion, the renal Ang II-induced vasoconstriction, which is antagonized only by AT1-R antagonist in the presence of endogenous NO, becomes sensitive to both AT1- and AT2-R antagonists during NO synthesis inhibition. While AT1-R antagonist inhibited both L-NAME-potentiated and -augmented components of Ang II-induced vasoconstriction, AT2-R antagonists inhibited only the L-NAME-potentiated component.

Abnormal endothelium-dependent responses in early radiation nephropathy

While arterial hypertension and renal dysfunction are well recognized complications of renal irradiation, the mechanisms that trigger the development of these complications are unknown. Recently, it was reported that the endothelium is a major target in radiation injury. Because dysfunction of the endothelial cells may lead or contribute to the development of hypertension and renal dysfunction in radiation nephropathy, we tested the hypothesis that endothelium-dependent vasodilation is impaired in radiated kidneys prior to the onset of hypertension. To test this hypothesis, we used Long-Evans rats that had undergone left nephrectomy (3 weeks earlier) and irradiation (3000 r's) to the right kidney 8 days earlier (mean blood pressures in the irradiated rats were not different than in the controls). We then measured the changes in renal blood flow (RBF) induced by endothelium-dependent (acetylcholine and bradykinin) and -independent (nitroprusside, norepinephrine, and angiotensin II) vasoactive agents. We found that the increases in RBF induced by the endothelium-dependent but not independent vasodilators were markedly impaired in the irradiated kidneys. Blocking nitric oxide synthesis with nitro L-arginine methyl ester in sham rats mimicked the blunted responsiveness of the irradiated rats, whereas indomethacin (an inhibitor of prostaglandin synthesis) had no effect on either sham or irradiated rats. Finally, the RBF responses to the endothelium-independent vasoconstrictors, norepinephrine and angiotensin II, were not altered in the irradiated kidneys. These results suggest that renal irradiation causes endothelial dysfunction (prior to the onset of hypertension) but spares the vascular smooth muscle cells.

Increased blood pressure in rats after long-term inhibition of the neuronal isoform of nitric oxide synthase

In the kidney, nitric oxide synthase (NOS) of the neuronal isoform (nNOS) is predominantly located in the macula densa cells. Unspecific chronic NOS inhibition in rats leads to elevated blood pressure (P(A)), associated with increased renal vascular resistance. This study was designed to examine the effect of chronic selective inhibition of nNOS with 7-nitro indazole (7-NI) on P(A), GFR, and the tubuloglomerular feedback (TGF) system. P(A) was repeatedly measured by a noninvasive tail-cuff technique for 4 wk in rats treated orally with 7-NI, and in control rats. After treatment, the animals were anesthetized and renal excretion rates, GFR, and TGF activity were determined. After 1 wk of 7-NI treatment P(A) was increased from 129+/-4 to 143 2 mmHg. GFR (1.85+/-0.1 vs. 1.97+/-0.2 ml/min in controls) was unchanged, but micropuncture studies revealed a more sensitive TGF than in controls. After 4 wk of 7-NI treatment P(A) was 152+/-4 mmHg, but no change in GFR (1.90+/-0.5 ml/min) or TGF sensitivity was detected. Acute administration of 7-NI to nontreated rats did not affect P(A), but decreased GFR (1.49+/-0.1 ml/min) and increased TGF sensitivity. In conclusion, chronic nNOS inhibition leads to increased P(A). Our results suggest that the elevated P(A) could be caused by an initially increased TGF sensitivity, leading to decreased GFR and an increased body fluid volume.