Thromboxane synthase and TP receptor mRNA in rat kidney and brain: effects of salt intake and ANG II

Thromboxane A2 receptor antagonist (ONO-8809) attenuates renal disorders caused by salt overload in stroke-prone spontaneously hypertensive rats

Epidemiological and clinical studies have demonstrated that excessive salt intake causes severe hypertension and exacerbates organ derangement, such as in chronic kidney disease (CKD). In this study, we focused on evaluating the histological and gene expression effects in the kidneys of stroke‐prone spontaneously hypertensive rats (SHRSP) with a high salt intake and the thromboxane A₂/ prostaglandin H₂ receptor (TPR) blocker ONO‐8809. Six‐week‐old SHRSPs were divided into three groups and were fed normal chow containing 0.4% NaCl, 2.0%NaCl or 2.0%NaCl + ONO‐8809 (0.6 mg/kg p.o. daily). Histological analyses with immunohistochemistry and a gene expression assay with a DNA kidney microarray were performed after 8 weeks. The following changes were observed in SHRSPs with the high salt intake. Glomerular sclerotic changes were remarkably observed in the juxtamedullary cortex areas. The ED1, monocyte chemoattractant protein‐1 (MCP‐1), nitrotyrosine and hypoxia inducible factor 1α (HIF‐1α) staining areas were increased in the glomeruli and interstitial portion of the kidneys. The genes Tbxa2r (that encodes TPR), Prcp and Car7 were significantly underexpressed in the kidneys. The plasma 8‐isoprostane level was significantly elevated and was attenuated with the ONO‐8809 treatment. Thromboxane A₂ (TXA₂) and oxidative stress exaggerated renal dysfunction in the salt‐loaded SHRSPs, and ONO‐8809 as a TPR blocker suppressed these changes. Therefore, ONO‐8809 is a candidate drug to prevent CKD in hypertensive patients when CKD is associated with a high salt intake.

Cyclooxygenase metabolites in the kidney

In the mammalian kidney, prostaglandins (PGs) are important mediators of physiologic processes, including modulation of vascular tone and salt and water. PGs arise from enzymatic metabolism of free arachidonic acid (AA), which is cleaved from membrane phospholipids by phospholipase A2 activity. The cyclooxygenase (COX) enzyme system is a major pathway for metabolism of AA in the kidney. COX are the enzymes responsible for the initial conversion of AA to PGG2 and subsequently to PGH2, which serves as the precursor for subsequent metabolism by PG and thromboxane synthases. In addition to high levels of expression of the "constitutive" rate-limiting enzyme responsible for prostanoid production, COX-1, the "inducible" isoform of cyclooxygenase, COX-2, is also constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. PGs and thromboxane A2 exert their biological functions predominantly through activation of specific 7-transmembrane G-protein-coupled receptors. COX metabolites have been shown to exert important physiologic functions in maintenance of renal blood flow, mediation of renin release and regulation of sodium excretion. In addition to physiologic regulation of prostanoid production in the kidney, increases in prostanoid production are also seen in a variety of inflammatory renal injuries, and COX metabolites may serve as mediators of inflammatory injury in renal disease.

Augmented cyclooxygenase-2 effects on renal function during varying states of angiotensin II

Nonsteroidal anti-inflammatory drug usage has long revealed renoprotective prostaglandin actions on the renal microvasculature during increased pressor hormone influence, but whether increased cyclooxygenase (COX)-2 expression supports prostaglandin vasodilatory influence by interfering with the actions of ANG II remains unresolved. Therefore, we tested the hypothesis that COX-2 inhibition causes hemodynamic and excretory effects that are increased in proportion to ANG II activity. In anesthetized Sprague-Dawley rats having augmented cortical COX-2 expression but different ANG II activity, we conducted renal clearance experiments during acute inhibition of COX-2 with nimesulide (NMSLD) and inhibition of COX-1 with SC-560. In one series of experiments, acute captopril [acute angiotensin-converting enzyme (ACE) inhibitor (aACEi)] was administered alone (n = 13) or in combination with chronic captopril [chronic ACEi (cACEi)] pretreatment (n = 19). In another series of experiments, rats were fed a normal-sodium [0.4% (NS), n = 12] or a low-sodium [0.03% (LS), n = 18] diet. NMSLD did not alter mean arterial blood pressure in any group but, in the LS and cACEi groups, decreased renal plasma flow (from 3.99 ± 0.33 to 2.85 ± 0.26 and from 4.30 ± 0.19 to 3.22 ± 0.21 ml·min(-1)·g(-1)), cortical blood flow (-12 ± 8% and -13 ± 4%), and glomerular filtration rate (from 0.88 ± 0.04 to 0.65 ± 0.05 and from 0.95 ± 0.07 to 0.70 ± 0.05 ml·min(-1)·g(-1)). In contrast, medullary blood flow (MBF) was significantly decreased by COX-2 inhibition in NS (-24 ± 5%), LS (-27 ± 8%), aACEi (-16 ± 3.8%), and cACEi (-24 ± 4.2%) groups. Absolute and fractional sodium excretion rates were unchanged by NMSLD, except in the LS group (0.75 ± 0.05 μeq/min and 0.43 ± 0.15% and 0.51 ± 0.06 μeq/min and 0.26 ± 0.10%). SC-560 did not augment the effects of NMSLD. These results demonstrate an augmented COX-2-mediated vasodilation that is not contingent on ANG II, in contrast to COX-2-mediated augmented sodium excretion, where ANG II activity is requisite. Furthermore, the COX-2 effects on MBF are not contingent on ANG II or changes in cortical microvascular responses. These results reflect COX-2 continual regulation of MBF and adaptive opposition to ANG II prohypertensinogenic effects on renal plasma flow, cortical blood flow, glomerular filtration rate, and absolute and fractional sodium excretion.

Emerging roles for eicosanoids in renal diseases

PURPOSE OF REVIEW

Eicosanoids are products of arachidonic acid metabolism which have important roles in renal homeostasis and disease. In recent years the development of genetically modified animals and new drugs targeting eicosanoids producing enzymes and receptors has unveiled new roles for eicosanoids in kidney function. This review provides an overview of eicosanoid biosynthesis and receptors and discusses recent findings on their role in acute and chronic renal diseases and in renal transplantation.

RECENT FINDINGS

Products of the cyclooxygenases, 5-lipoxygenase, and cytochrome P450 pathways of arachidonic acid metabolism act through distinct receptors presented at different segment of the nephron. Apart from its role in renal physiology and hemodynamic, eicosanoids actively participate in the pathogenesis of acute and chronic renal diseases and have immunoregulatory role in kidney transplantation.

SUMMARY

The new discoveries on the role of eicosanoids in kidney functions and the development of drugs targeting eicosanoids synthesis or action should help to envisage novel therapeutic approaches for patients suffering from renal diseases.

Cyclooxygenase 2 inhibition suppresses tubuloglomerular feedback: roles of thromboxane receptors and nitric oxide

Thromboxane (TxA(2)) and nitric oxide (NO) are potent vasoactive autocoids that modulate tubuloglomerular feedback (TGF). Each is produced in the macula densa (MD) by cyclooxygenase-2 (COX-2) and neuronal nitric oxide synthase (nNOS), respectively. Both enzymes are similarly regulated in the MD and their interaction may be an important factor in the regulation of TGF and glomerular filtration rate. We tested the hypothesis that TGF is modified by the balance between MD nNOS-dependent NO and MD COX-2-dependent TxA(2). We measured maximal TGF during perfusion of the loop of Henle (LH) by continuous recording of the proximal tubule stopped flow pressure response to LH perfusion of artificial tubular fluid (ATF) at 0 and 40 nl/min. The response to inhibitors of COX-1 (SC-560), COX-2 [parecoxib (Pxb)], and nNOS (l-NPA) added to the ATF solution was measured in separate nephrons. COX-2 inhibition with Pxb reduced TGF by 46% (ATF + vehicle vs. ATF + Pxb), whereas COX-1 inhibition with SC-560 reduced TGF by only 23%. Pretreatment with intravenous infusion of SQ-29,548, a selective thromboxone/PGH(2) receptor (TPR) antagonist, blocked all of the SC-560 effect on TGF, suggesting that this effect was due to activation of TPR. However, SQ-29,548 only partially diminished the effect of Pxb (-66%). Specific inhibition of nNOS with l-NPA increased TGF, as expected. However, the ability of Pxb to reduce TGF was significantly impaired with comicroperfusion of l-NPA. These data suggest that COX-2 modulates TGF by two proconstrictive actions: generation of TxA(2) acting on TPR and by simultaneous reduction of NO.

Prostaglandins in the kidney: developments since Y2K

There are five major PGs (prostaglandins/prostanoids) produced from arachidonic acid via the COX (cyclo-oxygenase) pathway: PGE(2), PGI(2) (prostacyclin), PGD(2), PGF(2alpha) and TXA(2) (thromboxane A(2)). They exert many biological effects through specific G-protein-coupled membrane receptors, namely EP (PGE(2) receptor), IP (PGI(2) receptor), DP (PGD(2) receptor), FP (PGF(2alpha) receptor) and TP (TXA(2) receptor) respectively. PGs are implicated in physiological and pathological processes in all major organ systems, including cardiovascular function, gastrointestinal responses, reproductive processes, renal effects etc. This review highlights recent insights into the role of each prostanoid in regulating various aspects of renal function, including haemodynamics, renin secretion, growth responses, tubular transport processes and cell fate. A thorough review of the literature since Y2K (year 2000) is provided, with a general overview of PGs and their synthesis enzymes, and then specific considerations of each PG/prostanoid receptor system in the kidney.

Renal effects of S18886 (Terutroban), a TP receptor antagonist, in an experimental model of type 2 diabetes

Thromboxane A(2) (TxA(2)) is assumed to contribute to the development of diabetes complications, including nephropathy. We investigated whether the selective thromboxane-prostanoid endoperoxide receptor antagonist, S18886, ameliorates renal damage in uninephrectomized (UNX) obese Zucker rats (OZR). S18886, at doses of 10 (S18886-10) and 30 (S18886-30) mg x kg(-1) x day(-1), was administered to UNX-OZR by gavage over 8 weeks (n = 8 each group). UNX lean rats (n = 12) and OZR rats that received placebo (OZR-PLAC, n = 8) served as controls. As compared with the OZR-PLAC, S18886 had no significant effect on the elevated blood pressure and the enhanced creatinine clearance, while augmented proteinuria was partially prevented (-12 and -37%, low and high dose, respectively; NS). The increased excretion of transforming growth factor beta(1) (TGF-beta(1)) and of the thromboxane metabolite 2,3-dinor thromboxane B(2) (TxB(2)) was lowered (P < 0.05). S18886 prevented both the enhanced mesangiolysis (P < 0.01) in the OZR-PLAC as well as enlargement and degeneration of podocytes. In the blood, S18886-30 augmented the antioxidant enzymes (P < 0.01) and lessened the increase of plasma advanced oxidation protein products (-25%, NS). Body weight, hyperglycemia, and dyslipidemia remained uninfluenced under both doses of treatment. S18886 has renoprotective properties in the model of UNX-OZR. It prevents mesangiolysis, reduces urinary TGF-beta(1) and 2,3-dinor-TxB(2) excretion, and enhances the antioxidative defense.

Differentiation of Cyclooxygenase 1- and 2–Derived Prostanoids in Mouse Kidney and Aorta

Accumulating evidence indicates cyclooxygenase (COX) 1 and COX2 differentially regulate cardiovascular and renal function. We have demonstrated previously in mice that COX2 inhibition enhances angiotensin II-induced hypertension, and COX1 inhibition attenuates the pressor effect of angiotensin II. To further elucidate the mechanism underlying the functional difference of COX1 versus COX2 inhibition, the present studies examined the prostaglandin (PG) profiles derived in COX1- or COX2-inhibited mouse kidney and aorta using gas chromatographic/mass spectrometric assays. PGE 2 is the most abundant prostanoid in both renal cortex and medulla in normal C57BL/6J mice, followed by PGI 2 , PGF 2α and thromboxane A 2 . In contrast PGI 2 was most abundant in aorta followed by thromboxane A 2 , PGE 2 , and PGF 2α . PGD 2 was undetectable in control kidney or aorta. At baseline, inhibition of COX1 decreased total prostaglandins in renal cortex, medulla, and aorta, whereas COX2 inhibition decreased total prostaglandins only in renal medulla. Angiotensin II infusion significantly increased COX2-dependent/COX1-independent PGE 2 and PGI 2 in renal cortex and medulla. Angiotensin II also significantly increased renal PGF 2α in cortex, but not in medulla, through both COX1- and COX2-dependent mechanisms. These studies demonstrate that although COX1 primarily contributes to basal prostanoid production in the kidney and aorta, angiotensin II increases renal vasodilator prostanoids predominately via COX2 activity. These effects may contribute to the specific effect of COX2 inhibitors to increase blood pressure.

Eicosanoids and renal vascular function in diseases

Arachidonic acid metabolites are vital for the proper control of renal haemodynamics and, when not properly controlled, can contribute to renal vascular injury and end-stage renal disease. Three major enzymatic pathways, COX (cyclo-oxygenase), CYP450 (cytochrome P450) and LOX (lipoxygenase), are responsible for the metabolism of arachidonic acid metabolites to bioactive eicosanoids. These eicosanoids can dilate or constrict the renal vasculature and maintain vascular resistance in the face of changing vasoactive hormones. Renal vascular generation of eicosanoids is altered in pathophysiological conditions such as hypertension, diabetes, metabolic syndrome and acute renal failure. Experimental evidence supports the concept that altered eicosanoid metabolism contributes to renal haemodynamic alterations and the development and progression of nephropathy. The possible beneficial renal vascular actions of enzymatic inhibitors, eicosanoid analogues and receptor antagonists have been examined in hypertension, diabetes and metabolic syndrome. This review highlights the roles of renal vascular eicosanoids in the pathogenesis of nephropathy and therapeutic targets for renal disease related to hypertension, diabetes, metabolic syndrome and acute renal failure.

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 − .