Glomerular and tubular adaptive responses to acute nephron loss in the rat. Effect of prostaglandin synthesis inhibition

High serum creatinine nonlinearity: a renal vital sign?

Patients with chronic kidney disease (CKD) may have nonlinear serum creatinine concentration (SC) trajectories, especially as CKD progresses. Variability in SC is associated with renal failure and death. However, present methods for measuring SC variability are unsatisfactory because they blend information about SC slope and variance. We propose an improved method for defining and calculating a patient's SC slope and variance so that they are mathematically distinct, and we test these methods in a large sample of US veterans, examining the correlation of SC slope and SC nonlinearity (SCNL) and the association of SCNL with time to stage 4 CKD (CKD4) and death. We found a strong correlation between SCNL and rate of CKD progression, time to CKD4, and time to death, even in patients with normal renal function. We therefore argue that SCNL may be a measure of renal autoregulatory dysfunction that provides an early warning sign for CKD progression.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Role of COX-2/mPGES-1/prostaglandin E2 cascade in kidney injury

COX-2/mPGES-1/PGE2 cascade plays critical roles in modulating many physiological and pathological actions in different organs. In the kidney, this cascade is of high importance in regulating fluid metabolism, blood pressure, and renal hemodynamics. Under some disease conditions, this cascade displays various actions in response to the different pathological insults. In the present review, the roles of this cascade in the pathogenesis of kidney injuries including diabetic and nondiabetic kidney diseases and acute kidney injuries were introduced and discussed. The new insights from this review not only increase the understanding of the pathological role of the COX-2/mPGES-1/PGE2 pathway in kidney injuries, but also shed new light on the innovation of the strategies for the treatment of kidney diseases.

Antagonism of the prostaglandin E2 EP1 receptor in MDCK cells increases growth through activation of Akt and the epidermal growth factor receptor

The actions of prostaglandin E2 (PGE2) in the kidney are mediated by G protein-coupled E-prostanoid (EP) receptors, which affect renal growth and function. This report examines the role of EP receptors in mediating the effects of PGE2 on Madin-Darby canine kidney (MDCK) cell growth. The results indicate that activation of Gs-coupled EP2 and EP4 by PGE2 results in increased growth, while EP1 activation is growth inhibitory. Indeed, two EP1 antagonists (ONO-8711 and SC51089) stimulate, rather than inhibit, MDCK cell growth, an effect that is lost following an EP1 knockdown. Similar observations were made with M1 collecting duct and rabbit kidney proximal tubule cells. ONO-8711 even stimulates growth in the absence of exogenous PGE2, an effect that is prevented by ibuprofen (indicating a dependence upon endogenous PGE2). The involvement of Akt was indicated by the observation that 1) ONO-8711 and SC51089 increase Akt phosphorylation, and 2) MK2206, an Akt inhibitor, prevents the increased growth caused by ONO-8711. The involvement of the EGF receptor (EGFR) was indicated by 1) the increased phosphorylation of the EGFR caused by SC51089 and 2) the loss of the growth-stimulatory effect of ONO-8711 and SC51089 caused by the EGFR kinase inhibitor AG1478. The growth-stimulatory effect of ONO-8711 was lost following an EGFR knockdown, and transduction of MDCK cells with a dominant negative EGFR. These results support the hypothesis that 1) signaling via the EP1 receptor involves Akt as well as the EGFR, and 2), EP1 receptor pharmacology may be employed to prevent the aberrant growth associated with a number of renal diseases.

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.

Intrarenal dopamine inhibits progression of diabetic nephropathy

The kidney has a local intrarenal dopaminergic system, and in the kidney, dopamine modulates renal hemodynamics, inhibits salt and fluid reabsorption, antagonizes the renin-angiotensin system, and inhibits oxidative stress. The current study examined the effects of alterations in the intrarenal dopaminergic system on kidney structure and function in models of type 1 diabetes. We studied catechol-O-methyl-transferase (COMT)(-/-) mice, which have increased renal dopamine production due to decreased dopamine metabolism, and renal transplantation was used to determine whether the effects seen with COMT deficiency were kidney-specific. To determine the effects of selective inhibition of intrarenal dopamine production, we used mice with proximal tubule deletion of aromatic amino acid decarboxylase (ptAADC(-/-)). Compared with wild-type diabetic mice, COMT(-/-) mice had decreased hyperfiltration, decreased macula densa cyclooxygenase-2 expression, decreased albuminuria, decreased glomerulopathy, and inhibition of expression of markers of inflammation, oxidative stress, and fibrosis. These differences were also seen in diabetic mice with a transplanted kidney from COMT(-/-) mice. In contrast, diabetic ptAADC(-/-) mice had increased nephropathy. Our study demonstrates an important role of the intrarenal dopaminergic system to modulate the development and progression of diabetic kidney injury and indicate that the decreased renal dopamine production may have important consequences in the underlying pathogenesis of diabetic nephropathy.

Microsomal prostaglandin E synthase 1 deletion retards renal disease progression but exacerbates anemia in mice with renal mass reduction

Microsomal prostaglandin E synthase 1 (mPGES-1) is a cytokine-inducible enzyme responsible for generation of prostaglandin E(2) (PGE(2)) during the inflammatory response. In the present study, we investigated the role of mPGES-1 in the development of chronic renal failure in mice with 5/6 nephrectomy (Nx). After 4 weeks of Nx, wild-type mice with renal mass reduction exhibited increased blood urea nitrogen, plasma creatinine and phosphorus concentrations, and defective urine concentrating capability, all of which were significantly attenuated by mPGES-1 deletion. The Nx wild-type mice developed a 2.6-fold increase in urinary albumin excretion, accompanied by glomerulosclerosis and reduction of nephrin and wild-type 1 expression in the remnant kidney. In contrast, the Nx KO mice had normal albuminuria with improvement of glomerular injury. Nx-induced increases in circulating and renal tumor necrosis factor 1α and renal interleukin 1β and monocyte chemoattractant protein 1 expressions were all remarkably attenuated or abolished by mPGES-1 deletion. Paradoxically, the Nx knockout mice developed worsened anemia, accompanied by impaired erythropoietin synthesis. The coinduction of mPGES-1 and cyclooxygenase 2 but not cyclooxygenase 1 mRNA expressions, along with increased PGE(2) synthesis, was demonstrated in the remnant kidney of wild-type mice. mPGES-1 deletion remarkably reduced renal PGE(2) content and urinary PGE(2) excretion after renal ablation but had a limited effect on the baseline PGE(2) production. We conclude that mPGES-1 deletion ameliorates chronic renal failure in the mouse model of renal mass reduction, and mPGES-1 deletion paradoxically exacerbates anemia in this model likely via suppression of erythropoietin synthesis.

Cyclooxygenase-2 and kidney failure

Cyclooxygenase (COX)-dependent prostaglandins are necessary for normal kidney function. These prostaglandins are associated with inflammation, maintenance of sodium and water homeostasis, control of renin release, renal vasodilation, vasoconstriction attenuation, and prenatal renal development. COX-2 expression is regulated by the renin-angiotensin system, glucocorticoids or mineralcorticoids, and aldosterone, supporting a role for COX-2 in kidney function. Indeed, COX-2 mRNA and protein levels as well as enzyme activity are increased, along with PGE2, during kidney failure. In addition, changes in COX-2 expression are associated with increased blood pressure, urinary volume, sodium and protein and decreased urinary osmolarity. Intrarenal mechanisms such as angiotensin II (Ang II) production, increased sodium delivery, glomerular hypertension, and renal tubular inflammation have been suggested to be responsible for the increase in COX-2 expression. Although, specific COX-2 pharmacological inhibition has been related to the prevention of kidney damage, clinical studies have reported that COX-2 inhibition may cause side effects such as edema or a modest elevation in blood pressure and could possibly interfere with antihypertensive drugs and increase the risk of cardiovascular complications. Thus, administration of COX-2 inhibitors requires caution, especially in the presence of underlying cardiovascular disease.

Renal effects of prolonged high protein intake and COX2 inhibition on hypertensive rats with altered renal development

Cyclooxygenase 2 (COX2) is involved in regulating renal hemodynamics after renal ablation. It is also known that high protein intake (HPI) leads to a deterioration of renal function when there is preexisting renal disease and that there are important gender differences in the regulation of renal function. This study tested the hypothesis that the role of COX2 in regulating renal function and the renal hemodynamic effects elicited by HPI are enhanced when nephrogenesis is altered during renal development. It was also expected that the role of COX2 and the effects elicited by HPI are age and sex dependent. Newborn Sprague-Dawley rats were treated with an AT1 ANG II receptor antagonist during the nephrogenic period (ARAnp). Experiments were performed at 3–4 and 10–11 mo of age. Arterial pressure was elevated ( P < 0.05) at both ages and in both sexes of ARAnp-treated rats. Renal COX2 expression was only elevated ( P < 0.05) at 10–11 mo of age in both sexes of ARAnp-treated rats. COX2 inhibition induced greater renal vasoconstriction in male and female hypertensive than in normotensive rats at both ages. HPI did not induce glomerular filtration rate (GFR) in the youngest hypertensive rats and in the oldest female hypertensive rats. However, the GFR decreased during HPI (0.63 ± 0.07 to 0.19 ± 0.05 ml/min) in the oldest male hypertensive rats. The HPI-induced increment in proteinuria was greater ( P < 0.05) in male (99 ± 22 mg/day) than in female (30 ± 8 mg/day) hypertensive rats. These results show that COX2 plays an important role in the regulation of renal function when renal development is altered and that prolonged HPI can lead to a renal insufficiency in males but not in females with reduced nephron endowment.

Reactive oxygen species mediate compensatory glomerular hypertrophy in rat uninephrectomized kidney

Hyperfiltration in glomeruli is the most common pathway to progressive renal dysfunction. Moreover, reduction of renal mass by unilateral nephrectomy results in an immediate increase in glomerular flow to the remnant kidney, followed by compensatory glomerular hypertrophy. Reactive oxygen species (ROS) are involved in renal hypertrophic responses; however, the role of ROS in compensatory glomerular hypertrophy remains unclear. Therefore, this role was investigated in the present study. Wistar rats were randomly placed into two groups: uninephrectomized rats (Nx) and uninephrectomized rats treated with tempol (Nx + TP). The glomerular volume increased in the Nx 1 week after surgery, but was significantly suppressed in the Nx + TP. Levels of phospho-Akt and phospho-ribosomal protein S6, which are critical for cell growth and hypertrophy, were markedly increased in the glomeruli of the Nx, while tempol treatment almost abolished the activation of these proteins. These results suggest that ROS have important roles in compensatory hypertrophy in glomeruli.

Roles of Lipid Mediators in Kidney Injury

Small lipids such as eicosanoids exert diverse and complex functions. In addition to their role in regulating normal kidney function, these lipids also play important roles in the pathogenesis of kidney diseases. Increased glomerular cyclooxygenase (COX)1 or COX2 expression has been reported in patients with nephritis and in animal models of nephritis. COX inhibitors have shown beneficial effects on lupus nephritis and passive Heymann nephritis, but not anti-Thy1.1-induced nephritis. 5-Lipoxygenase-derived leukotrienes are involved in inflammatory glomerular injury. Lipoxygenase product 12-hydroxyeicosatetraenoic acid may mediate angiotensin II and transforming growth factor beta-induced mesangial cell abnormality in diabetic nephropathy. P450 arachidonic acid mono-oxygenase-derived 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids are involved in several forms of kidney injury, including renal injury in metabolic syndrome. Ceramide also has been shown to be an important signaling molecule that is involved in the pathogenesis of acute kidney injury caused by ischemia/reperfusion and toxic insults. Those pathways should provide fruitful targets for intervention in the pharmacologic treatment of renal disease.

Angiotensin II stimulates cyclooxygenase-2 mRNA expression in renal tissue from rats with kidney failure

We have shown increased cyclooxygenase-2 (COX-2) expression in rats with kidney failure. Increased angiotensin II concentration, hypertension, and renal mass reduction have been described during development of kidney failure. Thus we explored each of these mechanisms, because any one of them could be responsible for COX-2 induction. Kidney failure increased systolic blood pressure from 104 +/- 5 to 138 +/- 2 mmHg, urinary PGE(2) from 74 +/- 17 to 185 +/- 25 ng/24 h, and COX-2 expression from 0.06 +/- 0.04 to 0.17 +/- 0.03 arbitraty units (AU). Treatment of the rats with ramipril or losartan prevented the increase in blood pressure, urinary PGE(2), and COX-2 expression in the rats with kidney failure. Infusion of angiotensin II increased blood pressure from 101 +/- 6 to 132 +/- 6 mm Hg, urinary PGE(2) excretion from 62 +/- 15 to 155 +/- 17 ng/24 h, and COX-2 expression from 0.23 +/- 0.01 to 1.6 +/- 0.3 AU. When the angiotensin II-infused rats were treated with nitrendipine, blood pressure decreased from 132 +/- 6 to 115 +/- 2 mm Hg, and urinary PGE(2) excretion decreased from 152 +/- 18 to 97 +/- 12 ng/24 h, whereas COX-2 expression was 1.6 +/- 0.7 and 1.7 +/- 0.5 AU for rats with and without nitrendipine. Blood pressure of the rats with renal pole resection was similar to that in sham rats (97 +/- 7 and 91 +/- 4 mmHg, respectively), whereas COX-2 expression was increased in rats with renal pole resection, from 0.06 +/- 0.04 to 0.12 +/- 0.03 AU. We suggest that in kidney failure, the increase in angiotensin II concentration regulates COX-2 expression, thereby increasing prostaglandin synthesis, which contributes to the development of kidney failure.

Cyclooxygenase 2 and the kidney

Cyclooxygenase metabolizes arachidonic acid to a family of bioactive fatty acids designated prostaglandins. Two isoforms of cyclooxygenase exist, designated COX1 and COX2. These isoforms are expressed in distinct but important areas of the kidney. COX1 predominates in vascular smooth muscle and collecting ducts, whereas COX2 predominates in the macula densa and nearby cells in the cortical thick ascending limb. COX2 is also highly expressed in medullary interstitial cells. Whereas COX1 expression does not exhibit dynamic regulation, COX2 expression is subject to regulation by several environmental conditions, including salt intake, water intake, medullary tonicity, growth factors, cytokines, and adrenal steroids. Recently, COX2-selective non-steroidal anti-inflammatory drugs have become widely available. Many of the renal effects of non-selective non-steroidal anti-inflammatory drugs (including sodium retention, decreased glomerular filtration rate, and effects on renin-angiotensin levels) appear to be mediated by the inhibition of COX2 rather than COX1. Therefore, in contrast to the gastrointestinal-sparing effects of COX2-selective non-steroidal anti-inflammatory drugs, when considering the kidney, the same caution must be applied when using COX2-selective inhibitors as has been used with traditional non-selective non-steroidal anti-inflammatory drugs.

A selective cyclooxygenase-2 inhibitor decreases proteinuria and retards progressive renal injury in rats

BACKGROUND

We have previously shown that cyclooxygenase-2 (COX-2) expression is low in the renal cortex of adult rats, but is increased in macula densa/cortical thick ascending limb and in glomerular podocytes after subtotal renal ablation.

METHODS

To evaluate the functional consequences of this increased COX-2 expression, male rats were subjected to subtotal renal ablation and divided into four groups: (1) treatment with the selective COX-2 inhibitor SC58236, (2) treatment with vehicle, (3) treatment with the angiotensin-converting enzyme inhibitor enalapril, and (4) treatment with enalapril + SC58236. The administration of drugs was begun on the third day after ablation and continued for 6 to 10 weeks.

RESULTS

Within one week after ablation, vehicle-treated rats developed hypertension. Although enalapril led to significant reductions in blood pressure, either alone or in combination with the COX-2 inhibitor, SC58236 alone did not significantly alter ablation-induced hypertension. However, the SC58236-treated animals exhibited levels of proteinuria at six weeks after ablation that were comparable to those seen with enalapril (vehicle, 47 +/- 4; enalapril, 27 +/- 2; SC58236, 30 +/- 2 mg/day; N = 7, P < 0.01, each group compared with vehicle), and continued SC58236 treatment led to persistent reductions in proteinuria at 10 weeks after renal ablation (vehicle, 77 +/- 4; SC58236, 50 +/- 4 mg/day; N = 6, P < 0. 01). SC58236 treatment also significantly reduced the percentage of glomeruli exhibiting segmental or global sclerosis at 10 weeks (32.6 +/- 7.8% vs. 10.9 +/- 2.8%, N = 6, P < 0.03). Furthermore, SC58236 treatment partially inhibited increases in transforming growth factor-beta1 mRNA expression and increases in collagen III and collagen IV mRNA expression.

CONCLUSIONS

These studies indicate that chronic treatment with a specific COX-2 inhibitor may retard the progression of progressive renal injury, and suggest that such compounds can be used in combination with angiotensin-converting enzyme inhibitors. Further studies are required to determine the mechanism by which COX-2 inhibition is renoprotective.

Effect of cyclooxygenase-2 inhibition on renal function after renal ablation

Kidney failure is the common end of hypertension and renal diseases. Several authors have suggested that vasodilatory prostaglandins participate in the hemodynamic mechanism responsible for the development of kidney failure. However, the mechanism by which prostaglandins are increased in renal disease is not clear. Recently, 2 isoforms of the enzyme responsible for prostaglandin synthesis, cyclooxygenase, have been described as cyclooxygenase-1 (COX-1), a constitutive isoform, and cyclooxygenase-2 (COX-2), an inducible isoform. In the present study, we investigated whether COX-2-dependent prostaglandins participate in the evolution of renal functional changes after renal ablation. We inhibited prostaglandin synthesis by COX-1 and COX-2 with indomethacin (3 mg/kg) and prostaglandin synthesis by COX-2 with NS-398 (3 mg/kg) and tested the effect of these inhibitors on the renal functional changes elicited by renal ablation. Renal ablation produced an increase in urinary volume, protein, and prostaglandin E(2), whereas urinary sodium and potassium were not affected and urinary osmolarity decreased; treatment with indomethacin or NS-398 partially prevented the renal functional changes elicited by renal ablation. Immunoblots for COX showed an increase in the expression of COX-2 protein 2 days after renal ablation. Furthermore, COX-2 mRNA expression was increased 1 day after renal ablation. These data suggest that COX-2-dependent prostaglandins participate in the renal mechanisms associated with the development of renal functional changes after renal ablation.

Selective increase of cyclooxygenase-2 expression in a model of renal ablation

Previous studies have suggested a possible role for prostaglandins (PGs) in mediating alterations in nephron structure and function ensuing after renal ablation. Two isoforms of cyclooxygenase (COX) have been described: constitutive (COX-1) and inducible (COX-2). We examined expression of these isoforms following subtotal renal ablation (5/6 ablation, RA) in rats. In renal cortex, COX-2 mRNA and immunoreactive protein (IP) increased progressively compared with sham-operated littermates. In contrast, there were no significant changes in COX-1 mRNA expression. In normal kidney, cortical COX-1 IP was immunolocalized predominantly to mesangial cells and collecting tubules, whereas COX-2 IP was found in a subset of cortical thick ascending limb of Henle's loop (CTAL) cells in the region of the macula densa (MD). Following RA, significantly increased COX-2 IP was detected in the MD and surrounding CTAL cells. In addition, fainter immunoreactive COX-2 was detected in scattered visceral epithelial cells and mesangial cells of the glomerulus. Immunoblotting of isolated glomeruli demonstrated a selective increase of glomerular immunoreactive COX-2 expression following RA. No change of COX-1 expression was seen. To determine COX activity, isolated glomeruli were incubated with arachidonic acid and PGE2 measured by enzyme immunoassay (EIA). Compared with sham, glomeruli from 2 wk RA produced significantly more PGs. SC-58560, a selective COX-1 inhibitor, did not inhibit PG production in the remnant glomeruli at concentrations up to 10(-4) M, whereas SC-58236, a relatively selective COX-2 inhibitor, significantly inhibited PG production by RA glomeruli. In preliminary studies, to define mechanisms of altered expression of glomerular COX-2, rat mesangial cells were incubated with serum from sham or 2 wk RA. There were significant increases in COX-2 expression in response to 2 wk RA serum. In summary, these results indicate selective increases in renal cortical COX-2 expression following renal ablation.

Impaired autoregulation of glomerular capillary hydrostatic pressure in the rat remnant nephron

In the present micropuncture study, the autoregulation of glomerular capillary hydrostatic pressure (PG) in Munich-Wistar rats 24 h after 75% nephrectomy (Nx) or sham operation (Sh) was investigated. The effect of varying renal perfusion pressure (RPP) on paired determinations of directly measured PG was evaluated in glomeruli of nephrons in which distal fluid delivery was present (unblocked). Autoregulation of PG in Sh glomeruli with unblocked tubules occurred at RPP values between 99.5 +/- 1.0 and 132.1 +/- 1.0 mmHg. In contrast, in Nx glomeruli with unblocked tubules PG increased by 0.32 +/- 0.07 mmHg/mmHg increase in RPP over this same range of RPP (P less than 0.0001). To determine whether enhanced prostaglandins synthesis was responsible for the altered regulation of PG in Nx glomeruli, we repeated the micropuncture measurements in a setting of prostaglandin synthesis inhibition. Although prostaglandins synthesis inhibition did not affect the autoregulation of PG in Sh glomeruli, it did normalize the autoregulatory capacity for PG of Nx glomeruli with unblocked tubules. Thus, acute Nx is associated with a significant loss of the autoregulatory capacity for PG and this impairment appears to be related to a prostaglandin-mediated alteration of the responsiveness of the vascular effector site for autoregulation.