Renal Arachidonic Acid Research Articles

Effects of rhein on renal arachidonic acid metabolism and renal function in patients with congestive heart failure

In a randomized double-blind cross-over study the effects of rhein (administered as diacetyl-rhein 50 mg b.d. for 5 days) and placebo on renal arachidonic acid metabolism and renal function have been compared in 12 elderly patients (mean age 75.2 years) with congestive heart failure, whose renal function was known to be dependent on the integrity of the renal prostaglandin system. Rhein like placebo, did not induce any change in the urinary excretion of prostaglandin (PG) E2, 6-keto-PGF1 alpha and thromboxane (TX) B2, nor did it affect creatinine clearance, blood urea, urine output, natriuresis, body weight, plasma renin activity or plasma aldosterone concentration. Separate analysis of the results obtained in the 5 patients receiving diuretic treatment did not show any significant effect of rhein as compared with placebo on the parameters investigated. Serum TXB2 concentration during whole blood clotting, as an index of platelet arachidonic acid metabolism, also showed no significant difference when DAR and placebo were compared. It is concluded that in patients with congestive heart failure rhein does not inhibit renal or platelet eicosanoid metabolism, nor does it modify renal function, sodium excretion or the renal response to diuretics.

The role of eicosanoids in human glomerular disease.

A large body of evidence suggests a complex role of renal prostaglandins (PGs) in the control of cortical and medullary functions, i.e., renal blood flow (RBF), glomerular filtration rate (GFR), renin release, urinary concentrating mechanism, and electrolyte and water excretion. Several reviews of renal arachidonic acid metabolism can be recommended (1–4). PGs are considered to be ubiquitous autacoids, that is, substances that act predominantly at their production site. Since different nephron segments have highly specialized structures and functions, the synthesis and functions of the various PGs should be considered separately for each segment, rather than globally for the whole kidney (3). Although no direct evidence exists for a differential origin of any given urinary eicosanoid from a well-defined nephron segment in human kidney, a working hypothesis can be put forward relating cellular sites of biosynthesis and potential actions of eicosanoids to urinary excretion (Table 1). Prostacyclin (PGI2) and prostaglandin (PG)En are powerful vasodilators and appear to serve an important role in maintaining renal vasodilation. However, in the human kidney, PGI2 is primarily involved in the modulation of cortical events, i.e., RBF, GFR and renin release, while PGE2 is largely confined to controlling medullary functions, i.e., medullary blood flow, sodium and chloride reabsorption in the loop of Henle, and collecting-tubule response to vasopressin.KeywordsNephrotic SyndromeLupus NephritisRenal Blood FlowRenal Plasma FlowRenal ProstaglandinThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Increased urinary excretion of thromboxane B2 and 2,3-dinor-TxB2 in cyclosporin A nephrotoxicity

Cyclosporin A (CsA) administration to rats is associated with a selective increase in urinary excretion of immunoreactive thromboxane B2 (i-TxB2), the stable breakdown product of TxA2. The exaggerated synthesis of TxA2 may play a role in the reduction of glomerular filtration rate (GFR) observed both in animals and humans undergoing CsA treatment. The present study was designed to get further insight into the origin of the abnormal i-TxB2 urinary excretion. Rats given orally CsA (50 mg/kg/day) for 30 days had a significant increase in the urinary excretion of both 2,3-dinor-TxB2 and TxB2 measured by technique of capillary column gas chromatography-negative ion chemical ionization mass spectrometry (HRGC-NICIMS). Urinary TxB2 is more likely to reflect the renal synthesis of the parent compound, whereas 2,3-dinor-TxB2 is considered to reflect the amount of TxB2 formed in the circulation. Experiments in isolated perfused kidney (IPK) taken from animals given CsA for 30 days showed a lower percentage increase in urinary TxB2 over vehicle treated animals. Moreover in IPK the ratio 2,3-dinor-TxB2/TxB2 was lower than in vivo. The amount of i-TxB2 detectable in serum of animals given CsA was not different from that of control animals. In contrast, isolated glomeruli taken from rats given CsA had an increase in their TxA2 synthesis measured as i-TxB2 in the supernatants. Ultrastructural studies on kidney specimens from animals given CsA showed a focal glomerular endothelial damage together with a marked infiltration of blood borne cells of monocyte-macrophage type in the glomerular tuft. In contrast, kidney specimens taken from IPK preparations were devoid of inflammatory cells. In vitro CsA did not interfere with platelet arachidonic acid (AA) metabolism as shown by a normal i-TxB2 generation in vitro by rat platelet-rich plasma (PRP) exposed to CsA and then challenged with AA or ADP. Similarly isolated glomeruli and isolated proximal tubules from normal rats when challenged with CsA in vitro converted AA into TxA2 normally. It is suggested that the cause of the increased urinary excretion of 2,3-dinor-TxB2 is the consequence of intrarenal platelet and macrophage activation, probably triggered by the endothelial damage. The parallel increase in the urinary excretion of unmetabolized TxB2 is likely to reflect a concomitant activation of resident renal cell AA metabolism induced by CsA.

Effect of dietary fish oil supplementation on eicosanoid production by rat renal allografts.

Acute renal allograft rejection is associated with significant alterations in renal arachidonic acid (AA) metabolism including increased production of the vasoconstrictor eicosanoid thromboxane (TX)A2. TX synthetase inhibition improves function of rejecting rat renal allografts but is difficult to accomplish pharmacologically. Therefore, we evaluated the potential use of dietary fish oil (FO) as a more practical method for reducing renal thromboxane production. We examined the effects of dietary FO supplementation on eicosanoid production and hemodynamic function of rat renal allografts. Donor and recipient rats were fed a fat-free diet supplemented with beef tallow (BT) or FO. After 6 weeks on the experimental diet, kidneys from ACI donors were transplanted into PVG recipients. Six days after transplant, renal function and eicosanoid production were measured. FO feeding resulted in significant alterations in eicosanoid production by renal allografts. There was a marked and generalized reduction in prostaglandin (PG) and TX production by allografts when both donor and recipient were fed a diet supplemented with fish oil. An intermediate reduction in production of PGE2 (and perhaps PGI2), but not TXB2, was observed when only the recipient was fed FO. Feeding FO to the donor alone had no effect on renal PG or TX production. These data suggest that both donor and recipient fatty acid pools contribute to AA metabolism during rejection. Unlike specific TX inhibition, the generalized inhibition of AA metabolism that occurred with FO feeding was not associated with improvement in function or morphology of allografts. However, potential benefit at earlier stages or in milder forms of rejection is possible and was not evaluated.

Effects of Ciclosporin on Plasma Renin Activity, Catecholamines and Prostaglandins in Patients with Idiopathic Uveitis

Animals and humans undergoing treatment with ciclosporin (CS) show a reversible increase in renal vascular resistance and a decrease in glomerular filtration rate. The causes of these abnormalities have not yet been established. We evaluated the effects of a 1-week treatment with CS on creatinine clearance, renal arachidonic acid metabolites, plasma renin activity (PRA), plasma aldosterone levels, urinary excretion and plasma levels of catecholamines in 7 patients with idiopathic uveitis. We show that CS treatment induces a significant (p less than 0.05) decrease in creatinine clearance (from 132 +/- 0.5 to 108 +/- 8 ml/min); urinary 6-keto-PGF1 excretion (from 17.8 +/- 4.9 to 10.9 +/- 3.3 ng/mmol creatinine), urinary thromboxane B2 excretion (from 7.0 +/- 1.0 to 3.6 +/- 0.9 ng/mmol creatinine), upright PRA (from 4.2 +/- 0.9 to 2.3 +/- 0.8) and supine PRA (from 2.0 +/- 0.5 to 1.1 +/- 0.3). We found no change in plasma aldosterone levels and plasma levels and urinary excretion of catecholamines. We suggest that the reversible renal vasoconstriction observed in patients treated with CS may be induced by inhibition of renal prostacyclin synthesis. In this setting inhibition of PRA and angiotensin II formation may impair autoregulation of effective filtration pressure and therefore glomerular filtration rate.

Evidence that renal prostaglandin and thromboxane production is stimulated in chronic cyclosporine nephrotoxicity.

Previous reports suggest that cyclosporine (CsA) may have direct effects on arachidonic acid (AA) metabolism in several different tissues. However, the effects of CsA on renal eicosanoid production are unclear. Furthermore, the potential role of changes in renal prostaglandin and thromboxane metabolism in mediating CsA nephrotoxicity is not known. Therefore, in this study, we evaluated the effects of CsA toxicity on the production of AA metabolites by the kidney. In a postischemic, denervated rat model, CsA (50 mg/kg/day) administered for 12-14 days resulted in significant nephrotoxicity with marked decreases in both glomerular filtration rate and renal blood flow. This reduction in renal function was associated with an increase in the renal production of TXB2, PGE2, and 6-PGF1 alpha in vitro. Arachidonic acid significantly stimulated renal eicosanoid production above control values. Increased urinary excretion of TXB2, 2,3-dinorTXB2 (a major TXB2 metabolite), and 6-keto-PGF1 alpha also occurred in rats with CsA nephrotoxicity and reflected the increase in renal production of these eicosanoid products. In contrast, urinary PGE2 excretion was not increased in CsA toxic rats. Thus, CsA nephrotoxicity is associated with specific alterations in renal AA metabolism. Furthermore, alterations in AA metabolism may be important in modulating renal hemodynamics and excretory function in this model. These studies suggest that specific inhibition of vasoconstrictor products of AA metabolism might ameliorate the nephrotoxic effects of CsA.

Functional significance of exaggerated renal thromboxane A2 synthesis induced by cyclosporin A

Animals and humans undergoing a chronic treatment with cyclosporin A (CyA) show a reduction in glomerular filtration rate (GFR). The cause of this abnormality has not been established. Since CyA interferes with arachidonic acid (AA) metabolism in various cells, we wished to determine whether alterations in renal AA metabolites contribute to deteriorating renal function in rats on CyA. We show that chronic CyA treatment induces a progressive increase in the renal synthesis of thromboxane (TX) A2. This is a selective abnormality in that CyA does not influence the renal synthesis of prostaglandin E2 (PGE2) and prostacyclin (PGI2). A significant negative correlation has been found between TXB2 urinary excretion rate and inulin clearance. No correlation has been observed between TXB2 excretion and p-aminohippuric acid clearance. The withdrawal of CyA is followed by a normalization of both TXB2 urinary excretion rate and GFR. The administration of a selective TXA2 inhibitor, UK-38,485, resulted in a significant reduction in urinary excretion of TXB2 accompanied by a significant increase in GFR. We conclude that chronic treatment with CyA in rats is associated with a selective increase in renal TXA2 synthesis and suggest that this abnormality may play a role in the reduction of GFR.

Renal prostaglandin efflux induced by vasopressin, dDAVP and arachidonic acid: contrasting profile and sites of release.

Prostaglandin (PG) efflux into ureteral (UE) and venous effluents (VE) of rabbit isolated perfused kidneys was determined by superfusion bioassay and radioimmunoassay (RIA), in response to injections of arginine-vasopressin (AVP), the non-pressor vasopressin analogue 1-deamino-8-D-arginine vasopressin (dDAVP) and arachidonic acid (AA). dDAVP (10-1000 ng) failed to stimulate renal PG release, whereas AVP (10-100 ng) and AA (10-50 micrograms) caused a dose-dependent release of PG. AVP evoked PG release into both effluents with release into the VE greater than UE at high doses. In contrast, PG release by AA was almost exclusively into the VE. Indomethacin (2.8 X 10(-6) mol/l) abolished AVP- and AA-induced PG efflux in both effluents, and vasodepressor responses to AA. PGE2 was the predominant PG released in response to AVP in both effluents whereas AA released primarily 6-keto-PGF1 alpha. The contrasting sites and profile of released PG suggest that exogenous AA and AVP stimulate the release of PG from different regions/cell types within the kidney.

The effect of sodium intake on renal prostaglandin production.

The effect of chronic alterations in dietary sodium intake on renal arachidonic acid (AA) metabolism was studied in male Wistar rats who were maintained for 14 days on a diet consisting of sodium-deficient food and either deionized water (low salt intake, LSI), 1% saline (normal salt intake, NSI), or 2% saline (high salt intake, HSI). 24 h Urinary Sodium (UNaV) and plasma renin activity (PRA) measurements were shown to validate the dietary protocol. Microsomal preparations from the cortices and medullae were incubated with radiolabeled exogenous AA, and endogenous urinary prostaglandin (PG) levels were assayed by RIA to quantify renal PG synthesis. Cortical PGF2 alpha and PGE2 synthesis was found to be the greatest following LSI. In contrast, medullary PGF2 alpha was shown to be the least following LSI and to increase with increased sodium intake. Likewise, urinary PGF2 alpha levels significantly increased with increasing sodium intake. Changes in urinary PGE2 levels showed the same trend as PGF2 alpha but did not achieve statistical significance. These data show that dietary sodium differentially affects renal cortical and medullary PG synthesis and may reflect physiological differences in the regulation of cyclooxygenase in these zones. These data further suggest that the major source of urinary PGs is the renal medulla since the relationship of urinary levels to sodium intake mimics that described for the synthesis of PGs by the medullary tissue.

Biochemistry and pharmacology of renal arachidonic acid metabolism

Arachidonic acid is metabolized to prostaglandin, lipoxygenase products, and products of the microsomal cytochrome P-450 enzymes of the kidney. The distribution of the metabolizing enzymes and their regulation and pharmacologic manipulation are reviewed. The mechanisms of release of arachidonic acid from membrane lipids through a surface-mediated receptor mechanism are also discussed. The localization of the various enzymes and product formation may have profound effects on glomerular filtration, renal blood flow, and electrolyte excretion. Therefore, an understanding of the potential sites of inhibition of the nonsteroidal anti-inflammatory drugs is important in assessing their effects on renal function.

Renal arachidonic acid metabolism. The third pathway.

Cells were isolated from the outer medulla of the rabbit kidney, primarily from the thick ascending limb of Henle's loop (mTALH). These mTALH cells are heavily invested with a cytochrome P450-linked monooxygenase that represents the third pathway by which arachidonic acid is metabolized. After cell separation, approximately 80% of the cells proved to be mTALH in origin, based on electron microscopic criteria and immunofluorescent localization of Tamm-Horsfall protein, a specific marker for mTALH cells. The specific activity of alkaline phosphatase, a marker for proximal tubular cells, decreased threefold after separation of mTALH cells from outer medullary cells, associated with a fourfold increase in the capacity of the separated mTALH cells to metabolize arachidonic acid. Incubation of mTALH cells with 14C-arachidonic acid resulted in formation of oxygenated metabolites, identified as two peaks (P1 and P2), which accounted for 30 to 40% of the recovered radioactivity. Formation of prostaglandin E2 and F2 alpha accounted for only 3 to 5%. The chromatographic retention times of P1 and P2 were different from products of lipoxygenases. An inhibitor of cytochrome P450-dependent enzymes, SKF-525A (50 microM), reduced product formation by mTALH cells by more than 70%, while induction of cytochrome P450 increased product formation. Formation of P1 and P2 by cell-free homogenates of mTALH was totally dependent on the presence of nicotinamide adenine dinucleotide phosphate, reduced form (NADPH), which suggests a NADPH-dependent cytochrome P450-linked monooxygenase pathway. Vasopressin and calcitonin (10(-10) M to 10(-7) M) stimulated release of arachidonic acid metabolites from mTALH cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal arachidonic acid metabolism.

Renal prostaglandin synthesis has been the focus of extensive investigation during the past decade. This chapter reviews the cellular localization of renal arachidonic acid metabolism in view of the substantial recent progress in the isolation and culture of renal cell types. The study of such specific renal cell types permits a more exact correlation of arachidonic acid metabolism, hor­ monal recognition, and specific kidney functions. In addition, we have chosen to describe a pathophysiological model that exhibits exaggerated prostaglandin (PG) synthesis and demonstrates the impact of nonrenal cells on kidney arachidonate metabolism and function.

Renal arachidonic acid metabolism and cellular changes in the rabbit renal vein constricted kidney: Inflammation as a common process in renal injury models

The process of renal inflammation was examined using the partial renal vein constricted rabbit kidney (RVC) as a model. Forty eight hours of partial renal vein constriction in the rabbit was associated with an increase in prostaglandin (PG) and thromboxane (Tx) production. The ex vivo perfused RVC kidney showed an enhanced time-dependent increase in PG and Tx production in response to bradykinin stimulation when compared with the unlatered contralateral (CLK) or normal kidney. At 6 hrs of perfusion bradykinin stimulation lateral (CLK) or normal kidney. At 6 hrs of perfusion bradykinin stimulation released 2950±350 ng PGE 2, 61±15 ng TxB 2 from the RVC, and 225±85 ng PGE 2 and undetectable TxB 2 from the CLK. Histological examination of the RVC cortex showed an increase in fibroblast-like cells, a modest increase in the interstitial space and an appearance of macrophages and lymphocytes not seen in the normal of CLK. Endotoxin has been reported to stimulate macrophages in culture to produce PGE 2 and TxB 2. Endotoxin (100 ng)_stimulation of the perfused RVC kidney caused an immediate, followed by a chronically increasing, release of PGs and Tx. Two hours after endotoxin injection 50 ml of effluent fromt the RVC contained 1450±107 ng PGE 2 and 15.0±4.5 ng TxB 2. Other models of renal inflammation (e.g., the hydronephrotic kidney, chronic glomerulonephritis) also show the histological appearance of macrophages. In addition, hydronephrotic kidneys undergo fibroblast proliferation and changes in arachidonic acid metabolism similar to what we observed in the RVC. This work suggests that the inflammatory process (mononuclear cell infiltration), fibroblast-like cell proliferation, and accompanying changes in arachidonate metabolism) is common among different forms of renal injury.

Metabolic and cellular alterations underlying the exaggerated renal prostaglandin and thromboxane synthesis in ureter obstruction in rabbits. Inflammatory response involving fibroblasts and mononuclear cells.

Unilateral ureter obstruction in rabbits produced profound changes in endogenous and exogenous renal arachidonic acid metabolism. Isolated perfused hydronephrotic kidneys (removed after 3 or 10 d of ureter obstruction) responded to bradykinin stimulation with a markedly enhanced release of prostaglandin E2 and thromboxane A2. Reversal (3 or 10 d) of the ureter obstruction resulted in a reduction in the vasoactive peptide-induced release of prostaglandin E2 and thromboxane A2 from the perfused hydronephrotic kidney. However, postobstruction reversal of prostaglandin production by the agonist-stimulated perfused kidney was not reflected in the cortical microsomal cyclooxygenase activity, which is greatly enhanced during ureter obstruction and does not decrease after removal of the obstruction. Histological analysis of the renal cortex in rabbits with ureteral obstruction revealed a proliferation of fibroblast-like cells and the presence of mononuclear cells; removal of the obstruction did not result in a disappearance of cortical fibroblasts but did result in a decrease of monocytes. The critical involvement of mononuclear cells in the exaggerated arachidonate metabolism that occurs during hydronephrosis was exhibited by the demonstration that: (a) only the perfused hydronephrotic rabbit kidney responded to administration of endotoxin with a sustained release of prostaglandin E2 and thromboxane A2; (b) the contralateral rabbit kidney, which is devoid of mononuclear cells, did not respond to endotoxin; and (c) the hydronephrotic cat kidney, which exhibits a fibroblast proliferation with a low number of mononuclear cells, did not respond to endotoxin. Thus, proliferation of fibroblast-like cells and the presence of mononuclear cells appear to be involved in the exaggerated prostaglandin and thromboxane production underlying hydronephrosis. The increase in microsomal cyclooxygenase activity is apparently most closely correlated with the increased fibroblastic activation and cellularity, whereas mononuclear cells (possibly via monokines) seem to be critical for the markedly enhanced prostaglandin and thromboxane release induced by endotoxin and bradykinin.