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

Roles of EP Receptors in the Regulation of Fluid Balance and Blood Pressure

Prostaglandin E2 (PGE2) is an important prostanoid expressing throughout the kidney and cardiovascular system. Despite the diverse effects on fluid metabolism and blood pressure, PGE2 is implicated in sustaining volume and hemodynamics homeostasis. PGE2 works through four distinct E-prostanoid (EP) receptors which are G protein-coupled receptors. To date, pharmacological specific antagonists and agonists of all four subtypes of EP receptors and genetic targeting knockout mice for each subtype have helped in uncoupling the diverse functions of PGE2 and discriminating the respective characteristics of each receptor. In this review, we summarized the functions of individual EP receptor subtypes in the renal and blood vessels and the molecular mechanism of PGE2-induced fluid metabolism and blood pressure homeostasis.

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.

Cyclooxygenase (COX) Inhibitors and the Newborn Kidney

This review summarizes our current understanding of the role of cyclo-oxygenase inhibitors (COXI) in influencing the structural development as well as the function of the developing kidney. COXI administered either during pregnancy or after birth can influence kidney development including nephronogenesis, and can decrease renal perfusion and ultrafiltration potentially leading to acute kidney injury in the newborn period. To date, which COX isoform (COX-1 or COX-2) plays a more important role in during fetal development and influences kidney function early in life is not known, though evidence points to a predominant role for COX-2. Clinical implications of the use of COXI in pregnancy and in the newborn infant are also evaluated herein, with specific reference to the potential effects of COXI on nephronogenesis as well as newborn kidney function.

Inactivation of the E-prostanoid 3 receptor attenuates the angiotensin II pressor response via decreasing arterial contractility

OBJECTIVE

The present studies aimed at elucidating the role of prostaglandin E(2) receptor subtype 3 (E-prostanoid [EP] 3) in regulating blood pressure.

METHODS AND RESULTS

Mice bearing a genetic disruption of the EP3 gene (EP(3)(-/-)) exhibited reduced baseline mean arterial pressure monitored by both tail-cuff and carotid arterial catheterization. The pressor responses induced by EP3 agonists M&B28767 and sulprostone were markedly attenuated in EP3(-/-) mice, whereas the reduction of blood pressure induced by prostaglandin E(2) was comparable in both genotypes. Vasopressor effect of acute or chronic infusion of angiotensin II (Ang II) was attenuated in EP3(-/-) mice. Ang II-induced vasoconstriction in mesenteric arteries decreased in EP3(-/-) group. In mesenteric arteries from wild-type mice, Ang II-induced vasoconstriction was inhibited by EP3 selective antagonist DG-041 or L798106. The expression of Arhgef-1 is attenuated in EP3 deficient mesenteric arteries. EP3 antagonist DG-041 diminished Ang II-induced phosphorylation of myosin light chain 20 and myosin phosphatase target subunit 1 in isolated mesenteric arteries. Furthermore, in vascular smooth muscle cells, Ang II-induced intracellular Ca(2+) increase was potentiated by EP3 agonist sulprostone but inhibited by DG-041.

CONCLUSIONS

Activation of the EP3 receptor raises baseline blood pressure and contributes to Ang II-dependent hypertension at least partially via enhancing Ca(2+) sensitivity and intracellular calcium concentration in vascular smooth muscle cells. Selective targeting of the EP3 receptor may represent a potential therapeutic target for the treatment of hypertension.

Acute inhibition of superoxide formation and Rac1 activation by nitric oxide and iloprost in human vascular smooth muscle cells in response to the thromboxane A2 analogue, U46619

BACKGROUND

The over-production of superoxide (O(2)(-)) derived from NADPH oxidase (NOX) plays a central role in cardiovascular diseases. By contrast, nitric oxide (NO) and prostacyclin (PGI(2)) are vasculoprotective. The effect of the NO donor, NONOate and iloprost on O(2)(-) formation, p47(phox) and Rac(1) activation in human vascular smooth muscle cells (hVSMCs) was investigated.

METHODS

hVSMCs were incubated with 10nM thromboxane A(2) analogue, U46619 for 16h, and then with apocynin (a NOX inhibitor), NONOate or iloprost for 1h and O(2)(-) measured spectrophometrically. The role of cyclic AMP and cyclic GMP was examined by co-incubation of drugs with protein kinase (PK) A and G inhibitors listed above. Rac(1) was studied using pull-down assays.

RESULTS

NONOate and iloprost inhibited O(2)(-) formation, acutely, effects blocked by inhibition of PKG and PKA, respectively. Rac(1) and p47(phox) activation and translocation to the plasma membrane was completely inhibited by NONOate and iloprost, effects again reversed by co-incubation with PKG or PKA inhibitors.

CONCLUSIONS

NO and PGI(2) block the acute activity of NOX in hVSMCs via the cGMP-PKG axis (for NO) and by the cAMP-PKA axis (for iloprost) through inhibition of Rac(1) and p47(phox) translocation. These findings have implications in the pathophysiology and treatment of CVD.

Antihypertensive effects of selective prostaglandin E2 receptor subtype 1 targeting

Clinical use of prostaglandin synthase-inhibiting NSAIDs is associated with the development of hypertension; however, the cardiovascular effects of antagonists for individual prostaglandin receptors remain uncharacterized. The present studies were aimed at elucidating the role of prostaglandin E2 (PGE2) E-prostanoid receptor subtype 1 (EP1) in regulating blood pressure. Oral administration of the EP1 receptor antagonist SC51322 reduced blood pressure in spontaneously hypertensive rats. To define whether this antihypertensive effect was caused by EP1 receptor inhibition, an EP1-null mouse was generated using a "hit-and-run" strategy that disrupted the gene encoding EP1 but spared expression of protein kinase N (PKN) encoded at the EP1 locus on the antiparallel DNA strand. Selective genetic disruption of the EP1 receptor blunted the acute pressor response to Ang II and reduced chronic Ang II-driven hypertension. SC51322 blunted the constricting effect of Ang II on in vitro-perfused preglomerular renal arterioles and mesenteric arteriolar rings. Similarly, the pressor response to EP1-selective agonists sulprostone and 17-phenyltrinor PGE2 were blunted by SC51322 and in EP1-null mice. These data support the possibility of targeting the EP1 receptor for antihypertensive therapy.

Angiotensin II-induced calcium signaling in the afferent arteriole from rats with two-kidney, one-clip hypertension

The aim of this study was to investigate ANG II-induced Ca2+signaling in freshly isolated afferent arterioles (AA) from two-kidney, one-clip hypertensive (2K1C) rats, which have an elevated plasma and renal ANG II level, and different perfusion pressure and vascular tone in the clipped and nonclipped kidney. The Ca2+responses in vessels from 2K1C and control rats were similar in all groups ( P > 0.1). The intracellular Ca2+(Cai2+) response in the afferent arteriole after 10−8M ANG II stimulation was 0.57 ± 0.10, 0.50 ± 0.07, 0.48 ± 0.04, and 0.36 ± 0.05 in the control, sham, nonclipped, and clipped kidney, respectively. These data were consistent with the finding of unchanged AT1aR mRNA levels in AAs from all groups. Although the absolute values were similar, the dose-response curves to ANG II were different. In the control, sham, and nonclipped kidney from 2K1C, the dose-response curve leveled off between 10−8and 10−6M ANG II. In the clipped kidney, the dose-response curve was linear, with a significantly increased response at 10−6M compared with 10−8M ANG II ( P < 0.05). Inhibition of cyclooxygenase-1 (COX-1) with indomethacin enhanced the ANG II response in the nonclipped (Δ0.30 ± 0.09) and clipped (Δ0.30 ± 0.09) kidneys from 2K1C ( P < 0.005), but not in control rats (Δ−0.02 ± 0.11, P > 0.8). Conclusively, the ANG II-induced Cai2+response was reduced by COX-1-derived prostaglandins in 2K1C, in contrast to control animals, where the COX-1 inhibition had no effect. COX-2 inhibition with NS-398 did not increase the ANG II-mediated Cai2+response in any of the groups.

Increased renal vascular reactivity to ANG II after unilateral nephrectomy in the rat involves 20-HETE

This study examined the role of intrarenal ANG II in the renal vascular reactivity changes occurring in the remaining kidney undergoing adaptation following contralateral nephrectomy. Renal blood flow responses to intrarenal injections of ANG II (0.25 to 5 ng) were measured in anesthetized euvolemic male Wistar rats 1, 4, 12, and 24 wk after uninephrectomy (UNX) or sham procedure (SHAM). At week 4, renal vasoconstriction induced by 2 ng ANG II was greater in UNX (69 +/- 5%) than in SHAM rats (50 +/- 3%; P < 0.01). This response was inhibited, by 50 and 66%, and by 20 and 25%, in SHAM and UNX rats, after combined injections of ANG II and losartan, or PD-123319 (P < 0.05), respectively. Characteristics of ANG II receptor binding in isolated preglomerular resistance vessels were similar in the two groups. After prostanoid inhibition with indomethacin, renal vasoconstriction was enhanced by 42 +/- 8% (P < 0.05), only in SHAM rats, whereas after 20-HETE inhibition with HET0016, it was reduced by 53 +/- 16% (P < 0.05), only in UNX rats. These differences vanished after concomitant prostanoid and 20-HETE inhibition in the two groups. After UNX, renal cortical protein expression of cytochrome P-450 2c23 isoform (CYP2c23) and cyclooxygenase-1 (COX-1) was unaltered, but it was decreased for CYP4a and increased for COX-2. In conclusion, renal vascular reactivity to ANG II was significantly increased in the postuninephrectomy adapted kidney, independently of protein expression, but presumably involving interactions between 20-HETE and COX in the renal microvasculature and changes in the paracrine activity of ANG II and 20-HETE.

Calcium and chloride channel activation by angiotensin II-AT1 receptors in preglomerular vascular smooth muscle cells

The pathways responsible for the rapid and sustained increases in [Ca(2+)](i) following activation of ANG II receptors (AT(1)) in renal vascular smooth muscle cells were evaluated using fluorescence microscopy. Resting intracellular calcium concentration [Ca(2+)](i) averaged 75 +/- 9 nM. The response to ANG II (100 nM) was characterized by a rapid initial increase of [Ca(2+)](i) by 74 +/- 6 nM (n = 35) followed by a decrease to a sustained level of 12 +/- 2 nM above baseline. The average time from peak to 50% reduction from the peak value (50% time point) was 32 +/- 4 s. AT(1) receptor blockade with 1 microM candesartan (n = 5) prevented the responses to ANG II. In nominally calcium-free conditions (n = 8), the peak increase in [Ca(2+)](i) averaged 42 +/- 7 nM but the sustained phase was absent and the 50% time point was reduced to 11 +/- 4 s. L-type calcium channel blockade with diltiazem reduced the peak [Ca(2+)](i) to 24 +/- 8 nM and the sustained level to 4 +/- 2 nM (n = 10). In cells preincubated in low Cl(-) (3.0 mM), the peak response to ANG II was suppressed as was the sustained response. Blockade of chloride channels with DIDS eliminated both the peak and sustained responses (n = 11); chloride channel blockade with DPC (n = 17) suppressed the peak increase in [Ca(2+)](i) to 18 +/- 5 and also prevented the sustained response. IP3 receptor blockade by 10 microM TMB-8 (n = 6) reduced the peak to 22 +/- 8 and prevented the sustained response. Exposure to 10 microM TMB-8 in the presence of Ca(2+)-free medium prevented the ANG II response (n = 9). In the presence of 100 microM DPC and 10 microM TMB-8 (n = 7), the ANG II response was also prevented. Thus the rapid initial increase in [Ca(2+)](i) is due not only to release from intracellular stores, but also to Ca(2+) influx from the extracellular fluid. Although Ca(2+) entry via L-type calcium channels is responsible for the major portion of the sustained response, other entry pathways participate. The finding that chloride channel blockers markedly attenuate both rapid and sustained responses indicates that chloride channel activation contributes to, rather than being the consequence of, the initial rapid response.

Effects of chloride channel blockers on rat renal vascular responses to angiotensin II and norepinephrine

The aim of the present study was to investigate the role of Ca2+-activated Cl-channels in the renal vasoconstriction elicited by angiotensin II (ANG II) and norepinephrine (NE). Renal blood flow (RBF) was measured in vivo using electromagnetic flowmetry. Ratiometric photometry of fura 2 fluorescence was used to estimate intracellular free Ca2+concentration ([Ca2+]i) in isolated preglomerular vessels from rat kidneys. Renal arterial injection of ANG II (2-4 ng) and NE (20-40 ng) produced a transient decrease in RBF. Administration of ANG II (10-7M) and NE (5 × 10-6M) to the isolated preglomerular vessels caused a prompt increase in [Ca2+]i. Renal preinfusion of DIDS (0.6 and 1.25 μmol/min) attenuated the ANG II-induced vasoconstriction to ∼35% of the control response, whereas the effects of NE were unaltered. Niflumic acid (0.14 and 0.28 μmol/min) and 2-[(2-cyclopentenyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1 H-inden-5-yl)oxy]acetic acid (IAA-94; 0.045 and 0.09 μmol/min) did not affect the vasoconstrictive responses of these compounds. Pretreatment with niflumic acid (50 μM) or IAA-94 (30 μM) for 2 min decreased baseline [Ca2+]ibut did not change the magnitude of the [Ca2+]iresponse to ANG II and NE in the isolated vessels. The present results do not support the hypothesis that Ca2+-activated Cl-channels play a crucial role in the hemodynamic effects of ANG II and NE in rat renal vasculature.

S100A13 and S100A6 exhibit distinct translocation pathways in endothelial cells

S100 proteins have attracted great interest in recent years because of their cell- and tissue-specific expression and association with various human pathologies. Most S100 proteins are small acidic proteins with calcium-binding domains — the EF hands. It is thought that this group of proteins carry out their cellular functions by interacting with specific target proteins, an interaction that is mainly dependent on exposure of hydrophobic patches, which result from calcium binding. S100A13, one of the most recently identified members of the S100 family, is expressed in various tissues. Interestingly,hydrophobic exposure was not observed upon calcium binding to S100A13 even though the dimeric form displays two high- and two low- affinity sites for calcium. Here, we followed the translocation of S100A13 in response to an increase in intracellular calcium levels, as protein translocation has been implicated in assembly of signaling complexes and signaling cascades, and several other S100 proteins are involved in such events. Translocation of S100A13 was observed in endothelial cells in response to angiotensin II, and the process was dependent on the classic Golgi-ER pathway. By contrast, S100A6 translocation was found to be distinct and dependent on actin-stress fibers. These experiments suggest that different S100 proteins utilize distinct translocation pathways, which might lead them to certain subcellular compartments in order to perform their physiological tasks in the same cellular environment.

Role of 20-hydroxyeicosatetraenoic acid in the renal and vasoconstrictor actions of angiotensin II

The present study examined the effects of ANG II on the renal synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE) and its contribution to the renal vasoconstrictor and the acute and chronic pressor effects of ANG II in rats. ANG II (10(-11) to 10(-7) mol/l) reduced the diameter of renal interlobular arteries treated with inhibitors of nitric oxide synthase and cyclooxygenase, lipoxygenase, and epoxygenase by 81 +/- 8%. Subsequent blockade of the synthesis of 20-HETE with 17-octadecynoic acid (1 micromol/l) increased the ED(50) for ANG II-induced constriction by a factor of 15 and diminished the maximal response by 61%. Graded intravenous infusion of ANG II (5-200 ng/min) dose dependently increased mean arterial pressure (MAP) in thiobutylbarbitol-anesthetized rats by 35 mmHg. Acute blockade of the formation of 20-HETE with dibromododecenyl methylsulfimide (DDMS; 10 mg/kg) attenuated the pressor response to ANG II by 40%. An intravenous infusion of ANG II (50 ng. kg(-1). min(-1)) in rats for 5 days increased the formation of 20-HETE and epoxyeicosatrienoic acids (EETs) in renal cortical microsomes by 60 and 400%, respectively, and increased MAP by 78 mmHg. Chronic blockade of the synthesis of 20-HETE with intravenous infusion of DDMS (1 mg. kg(-1). h(-1)) or EETs and 20-HETE with 1-aminobenzotriazole (ABT; 2.2 mg. kg(-1). h(-1)) attenuated the ANG II-induced rise in MAP by 40%. Control urinary excretion of 20-HETE averaged 350 +/- 23 ng/day and increased to 1,020 +/- 105 ng/day in rats infused with ANG II (50 ng. kg(-1). min(-1)) for 5 days. In contrast, urinary excretion of 20-HETE only rose to 400 +/- 40 and 600 +/- 25 ng/day in rats chronically treated with ANG II and ABT or DDMS respectively. These results suggest that acute and chronic elevations in circulating ANG II levels increase the formation of 20-HETE in the kidney and peripheral vasculature and that 20-HETE contributes to the acute and chronic pressor effects of ANG II.

Cloning and expression of the rabbit prostaglandin EP2 receptor

BACKGROUND

Prostaglandin E2 (PGE2) has multiple physiologic roles mediated by G protein coupled receptors designated E-prostanoid, or "EP" receptors. Evidence supports an important role for the EP2 receptor in regulating fertility, vascular tone and renal function.

RESULTS

The full-length rabbit EP2 receptor cDNA was cloned. The encoded polypeptide contains 361 amino acid residues with seven hydrophobic domains. COS-1 cells expressing the cloned rabbit EP2 exhibited specific [3H]PGE2 binding with a Kd of 19.1 +/- 1.7 nM. [3H]PGE2 was displaced by unlabeled ligands in the following order: PGE2>>PGD2=PGF2alpha=iloprost. Binding of [3H]PGE2 was also displaced by EP receptor subtype selective agonists with a rank order of affinity consistent with the EP2 receptor (butaprost>AH13205>misoprostol>sulprostone). Butaprost free acid produced a concentration-dependent increase in cAMP accumulation in rabbit EP2 transfected COS-1 cells with a half-maximal effective concentration of 480 nM. RNase protection assay revealed high expression in the ileum, spleen, and liver with lower expression in the kidney, lung, heart, uterus, adrenal gland and skeletal muscle. In situ hybridization localized EP2 mRNA to the uterine endometrium, but showed no distinct localization in the kidney. EP2 mRNA expression along the nephron was determined by RT-PCR and its expression was present in glomeruli, MCD, tDL and CCD. In cultured cells EP2 receptor was not detected in collecting ducts but was detected in renal interstitial cells and vascular smooth muscle cells. EP2 mRNA was also detected in arteries, veins, and preglomerular vessels of the kidney.

CONCLUSION

EP2 expression pattern is consistent with the known functional roles for cAMP coupled PGE2 effects in reproductive and vascular tissues and renal interstitial cells. It remains uncertain whether it is also expressed in renal tubules.

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.

EP(1) and EP(4) receptors mediate prostaglandin E(2) actions in the microcirculation of rat kidney

Vasodilator prostaglandin PGE(2) protects the kidney from excessive vasoconstriction during contraction of extracellular fluid volume and pathophysiological states. However, it is not yet clear which of the four known E-prostanoid (EP) receptors is localized to resistance vessels and mediates net vasodilation. In the present study, we assessed the presence, signal transduction, and actions of EP receptor subtypes in preglomerular arterioles of Sprague-Dawley rat kidneys. RNA encoding EP(1), an EP(1)-variant, and EP(4) receptors was identified by RT-PCR in freshly isolated preglomerular microvessels; cultured preglomerular vascular smooth muscle cells (VSMC) had EP(1)-variant and EP(4) RNA but lacked EP(1). EP(2) and EP(3) receptors were undetectable in both vascular preparations. In studies of cell signaling, stimulation of cAMP by various receptor agonists is consistent with primary actions of PGE(2) on the EP(4) receptor, with no inhibition of cAMP by EP(1) receptors. Studies of cytosolic calcium concentration in cultured renal VSMC support an inhibitory role of EP(4) during ANG II stimulation. In vivo renal blood flow (RBF) studies indicate that the EP(4) receptor is the primary receptor mediating sustained renal vasodilation produced by PGE(2), whereas the EP(1) receptor elicits transient vasoconstriction. The EP(1)-variant receptor does not appear to possess any cAMP or cytosolic calcium signaling capable of affecting RBF. Collectively, these studies demonstrate that the EP(4) receptor is the major receptor in preglomerular VSMC. EP(4) mediates PGE(2)-induced vasodilation in the rat kidney and signals through G(s) proteins to stimulate cAMP and inhibit cytosolic calcium concentration.