Differential regulation of renal prostaglandin receptor mRNAs by dietary salt intake in the rat

Distinct cell types along thick ascending limb express pathways for monovalent and divalent cation transport

Kidney thick ascending limb cells reabsorb sodium, potassium, calcium, and magnesium and contribute to urinary concentration. These cells are typically viewed as of a single type that recycles potassium across the apical membrane and generates a lumen-positive transepithelial voltage driving calcium and magnesium reabsorption, although variability in potassium channel expression has been reported. Additionally, recent transcriptomic analyses suggest that different cell types exist along this segment, but classifications have varied and have not led to a new consensus model. We used immunolocalization, electrophysiology and enriched single nucleus RNA-Seq to identify thick ascending limb cell types in rat, mouse and human. We identified three major TAL cell types defined by expression of potassium channels and claudins. One has apical potassium channels, low basolateral potassium conductance, and is bordered by a sodium-permeable claudin. A second lacks apical potassium channels, has high basolateral potassium conductance and is bordered by calcium- and magnesium-permeable claudins. A third type also lacks apical potassium channels and has a high basolateral potassium conductance, but these cells are ringed by sodium-permeable claudins. The recognition of diverse cell types resolves longstanding questions about how solute transport can be modulated selectively and how disruption of these cells leads to human disease.

Inducible deletion of the prostaglandin EP3 receptor in kidney tubules of male and female mice has no major effect on water homeostasis

The prostaglandin E2 (PGE2) receptor EP3 has been detected in the thick ascending limb (TAL) and the collecting duct of the kidney, where its actions are proposed to inhibit water reabsorption. However, EP3 is also expressed in other cell types, including vascular endothelial cells. The aim here was to determine the contribution of EP3 in renal water handling in male and female adult mice by phenotyping a novel mouse model with doxycycline-dependent deletion of EP3 throughout the kidney tubule (EP3-/- mice). RNAscope demonstrated that EP3 was highly expressed in the cortical and medullary TAL of adult mice. Compared with controls EP3 mRNA expression was reduced by >80% in whole kidney (RT-qPCR) and nondetectable (RNAscope) in renal tubules of EP3-/- mice. Under basal conditions, there were no significant differences in control and EP3-/- mice of both sexes in food and water intake, body weight, urinary output, or clinical biochemistries. No differences were detectable between genotypes in handling of an acute water load or in their response to the vasopressin analog 1-deamino-8-d-arginine-vasopressin (dDAVP). No differences in water handling were observed when PGE2 production was enhanced using 1% NaCl load. Expression of proteins involved in kidney water handling was not different between genotypes. This study demonstrates that renal tubular EP3 is not essential for body fluid homeostasis in males or females, even when PGE2 levels are high. The mouse model is a novel tool for examining the role of EP3 in kidney function independently of potential developmental abnormalities or systemic effects.NEW & NOTEWORTHY The prostanoid EP3 receptor is proposed to play a key role in the kidney tubule and antagonize the effects of vasopressin on aquaporin-mediated water reabsorption. Here, we phenotyped a kidney tubule-specific inducible knockout mouse model of the EP3 receptor. Our major finding is that, even under physiological stress, tubular EP3 plays no detectable role in renal water or solute handling. This suggests that other EP receptors must be important for renal salt and water handling.

Associations of E-proteinoid 3 receptor genetic polymorphisms with salt sensitivity, longitudinal blood pressure changes, and hypertension incidence in Chinese adults

The E-proteinoid 3 receptor (PTGER3), a member of the prostaglandin E2 (PGE2) subtype receptor, belongs to the G-protein-coupled superfamily of receptors. Animal studies have demonstrated its involvement in salt sensitivity by regulating sodium reabsorption. This study aimed to investigate the association between genetic variants of PTGER3 and salt sensitivity, longitudinal blood pressure (BP) changes, and the incidence of hypertension in Chinese adults. A chronic salt intake intervention was conducted involving 514 adults from 124 families in the 2004 Baoji Salt-Sensitivity Study Cohort in northern China. These participants followed a 3-day regular baseline diet, followed by a 7-day low-salt diet (3.0 g/d) and a 7-day high-salt diet (18 g/d), and were subsequently followed for 14 years. The findings revealed a significant relationship between the single nucleotide polymorphism (SNP) rs17482751 of PTGER3 and diastolic blood pressure (DBP) response to high salt intervention. Additionally, SNPs rs11209733, rs3765894, and rs2268062 were significantly associated with longitudinal changes in systolic blood pressure (SBP), DBP, and mean arterial pressure (MAP) during the 14-year follow-up period. SNP rs6424414 was significantly associated with longitudinal changes in DBP over 14 years. Finally, SNP rs17482751 showed a significant correlation with the incidence of hypertension over 14 years. These results emphasize the significant role of PTGER3 gene polymorphism in salt sensitivity, longitudinal BP changes, and the development of hypertension in the Chinese population.

Anthocyanin attenuates high salt-induced hypertension via inhibiting the hyperactivity of the sympathetic nervous system

BACKGROUND

Anthocyanin plays a protective role in cardiovascular disease through antioxidant effect. Whether anthocyanin can reduce salt-induced hypertension and the related mechanisms remain unclear.

METHODS

Chronic infusion of vehicle (artificial cerebrospinal fluid, aCSF, 0.4 μL/h) or anthocyanin (10 mg/kg, 0.4 μL/h) into bilateral paraventricular nucleus (PVN) of Sprague-Dawley rats was performed. Then, the rats were fed a high salt diet (8% NaCl, HS) or normal salt diet (0.9%, NaCl, NS) for 4 weeks.

RESULTS

High salt diet induced an increase in blood pressure and peripheral sympathetic nerve activity (increased LF/HF and decreased SDNN and RMSSD), which was accompanied by increased reactive oxygen species (ROS) production and angiotensin II type-1 receptor (AT₁R) expression and function in the PVN. Moreover, the NOD-like receptor protein 3 (NLRP3) and related inflammatory proteins (caspase-1) expression, the pro-inflammatory cytokine levels including IL-1β and TNF-α were higher in PVN of rats with a high salt diet. Bilateral PVN infusion of anthocyanin attenuated NLRP3-dependent inflammation (NLRP3, caspase-1, IL-1β and TNF-α) and ROS production, reduced AT₁R expression and function in PVN and lowered peripheral sympathetic nerve activity and blood pressure in rats with salt-induced hypertension.

CONCLUSIONS

Excessive salt intake activates NLRP3-dependent inflammation and oxidative stress and increased AT₁R expression and function in the PVN. Bilateral PVN infusion of anthocyanin lowers peripheral sympathetic nerve activity and blood pressure in rats with salt-induced hypertension by improvement of expression and function of AT₁R in the PVN through inhibiting NLRP3 related inflammatory and oxidative stress.

Arachidonic Acid Metabolism and Kidney Inflammation

As a major component of cell membrane lipids, Arachidonic acid (AA), being a major component of the cell membrane lipid content, is mainly metabolized by three kinds of enzymes: cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP450) enzymes. Based on these three metabolic pathways, AA could be converted into various metabolites that trigger different inflammatory responses. In the kidney, prostaglandins (PG), thromboxane (Tx), leukotrienes (LTs) and hydroxyeicosatetraenoic acids (HETEs) are the major metabolites generated from AA. An increased level of prostaglandins (PGs), TxA₂ and leukotriene B4 (LTB₄) results in inflammatory damage to the kidney. Moreover, the LTB₄-leukotriene B4 receptor 1 (BLT1) axis participates in the acute kidney injury via mediating the recruitment of renal neutrophils. In addition, AA can regulate renal ion transport through 19-hydroxystilbenetetraenoic acid (19-HETE) and 20-HETE, both of which are produced by cytochrome P450 monooxygenase. Epoxyeicosatrienoic acids (EETs) generated by the CYP450 enzyme also plays a paramount role in the kidney damage during the inflammation process. For example, 14 and 15-EET mitigated ischemia/reperfusion-caused renal tubular epithelial cell damage. Many drug candidates that target the AA metabolism pathways are being developed to treat kidney inflammation. These observations support an extraordinary interest in a wide range of studies on drug interventions aiming to control AA metabolism and kidney inflammation.

Sequential and synchronized hypertonicity-induced activation of Rel-family transcription factors is required for osmoprotection in renal cells

NF-κB and TonEBP belong to the Rel-superfamily of transcription factors. Several specific stimuli, including hypertonicity which is a key factor for renal physiology, are able to activate them. It has been reported that, after hypertonic challenge, NF-κB activity can be modulated by TonEBP, considered as the master regulator of transcriptional activity in the presence of changes in environmental tonicity. In the present work we evaluated whether hypertonicity-induced gene transcription mediated by p65/RelA and TonEBP occurs by an independent action of each transcription factor or by acting together. To do this, we evaluated the expression of their specific target genes and cyclooxygenase-2 (COX-2), a common target of both transcription factors, in the renal epithelial cell line Madin-Darby canine kidney (MDCK) subjected to hypertonic environment. The results herein indicate that hypertonicity activates the Rel-family transcription factors p65/RelA and TonEBP in MDCK cells, and that both are required for hypertonic induction of COX-2 and of their specific target genes. In addition, present data show that p65/RelA modulates TonEBP expression and both colocalize in nuclei of hypertonic cultures of MDCK cells. Thus, a sequential and synchronized action p65/RelA → TonEBP would be necessary for the expression of hypertonicity-induced protective genes.

The EP3 receptor regulates water excretion in response to high salt intake

The mechanisms by which prostanoids contribute to the maintenance of whole body water homeostasis are complex and not fully understood. The present study demonstrates that an EP3-dependent feedback mechanism contributes to the regulation of water homeostasis under high-salt conditions. Rats on a normal diet and tap water were placed in metabolic cages and given either sulprostone (20 μg·kg^-1·day^-1) or vehicle for 3 days to activate EP3 receptors in the thick ascending limb (TAL). Treatment was continued for another 3 days in rats given either 1% NaCl in the drinking water or tap water. Sulprostone decreased expression of cyclooxygenase 2 (COX-2) expression by ∼75% in TAL tubules from rats given 1% NaCl concomitant with a ∼60% inhibition of COX-2-dependent PGE₂ levels in the kidney. Urine volume increased after ingestion of 1% NaCl but was reduced ∼40% by sulprostone. In contrast, the highly selective EP3 receptor antagonist L-798106 (100 μg·kg^-1·day^-1), which increased COX-2 expression and renal PGE₂ production, increased urine volume in rats given 1% NaCl. Sulprostone increased expression of aquaporin-2 (AQP2) in the inner medullary collecting duct plasma membrane in association with an increase in phosphorylation at Ser269 and decrease in Ser261 phosphorylation; antagonism of EP3 with L-798106 reduced AQP2 expression. Thus, although acute activation of EP3 by PGE₂ in the TAL and collecting duct inhibits the Na-K-2Cl cotransporter and AQP2 activity, respectively, chronic activation of EP3 in vivo limits the extent of COX-2-derived PGE₂ synthesis, thereby mitigating the inhibitory effects of PGE₂ on these transporters and decreasing urine volume.

PGE(2) EP(3) receptor downregulates COX-2 expression in the medullary thick ascending limb induced by hypertonic NaCl

We tested the hypothesis that inhibition of EP3 receptors enhances cyclooxygenase (COX)-2 expression in the thick ascending limb (TAL) induced by hypertonic stimuli. COX-2 protein expression in the outer medulla increased approximately twofold in mice given free access to 1% NaCl in the drinking water for 3 days. The increase was associated with an approximate threefold elevation in COX-2 mRNA accumulation and an increase in PGE2 production by isolated medullary (m)TAL tubules from 77.3 ± 8.4 to 165.7 ± 10.8 pg/mg protein. Moreover, administration of NS-398 abolished the increase in PGE2 production induced by 1% NaCl. EP3 receptor mRNA levels also increased approximately twofold in the outer medulla of mice that ingested 1% NaCl. The selective EP3 receptor antagonist L-798106 increased COX-2 mRNA by twofold in mTAL tubules, and the elevation in COX-2 protein induced by 1% NaCl increased an additional 50% in mice given L-798106. COX-2 mRNA in primary mTAL cells increased twofold in response to media made hypertonic by the addition of NaCl (400 mosmol/kg H2O). L-798106 increased COX-2 mRNA twofold in isotonic media and fourfold in cells exposed to 400 mosmol/kg H2O. PGE2 production by mTAL cells increased from 79.3 ± 4.6 to 286.7 ± 6.3 pg/mg protein after challenge with 400 mosmol/kg H2O and was inhibited in cells transiently transfected with a lentivirus short hairpin RNA construct targeting exon 5 of COX-2 to silence COX-2. Collectively, the data suggest that local hypertonicity in the mTAL is associated with an increase in COX-2 expression concomitant with elevated EP3 receptor expression, which limits COX-2 activity in this segment of the nephron.

The renoprotective effects of berberine via the EP4-Gαs-cAMP signaling pathway in different stages of diabetes in rats

AIMS

To investigate the renoprotective roles of berberine (BBR) in different stages of diabetic nephropathy (DN) in streptozotocin (STZ)-induced diabetic rats fed a high-sugar and high-fat diet.

METHODS

Diabetes was induced in mice by intraperitoneal injection of STZ, and the mice were then randomly divided into groups: normal, diabetes, high-sugar and high-fat and BBR (high, median and low dose) groups. The body weight (BW), kidney weight to body weight (KW/BW), blood urea nitrogen, urine total protein to urine creatinine ratio and serum creatinine were measured on different weeks throughout the study. The protein levels of E prostanoid receptor 4 (EP4), Gαs and content of cAMP in the kidney were, respectively, detected by western blot analysis and RIA analysis.

RESULTS

In the DN rats, there was remarkable renal damage. BBR restored renal functional parameters, suppressed alterations in histological and ultrastructural changes in the kidney tissues and increased EP4, Gαs and cAMP levels compared with those of the DN model group. In addition, BBR has different therapeutic effects during the different stages of the development of DN, and it works best in the sixth week.

CONCLUSION

These studies demonstrate, for the first time, that BBR exerts renoprotective effects in different stages of DN via EP4- Gαs- AC-cAMP signaling pathway in STZ-induced DN rats fed a high-sugar and high-fat diet.

The prostanoid EP4 receptor and its signaling pathway

The EP4 prostanoid receptor is one of four receptor subtypes for prostaglandin E2. It belongs to the family of G protein-coupled receptors. It was originally identified, similar to the EP2 receptor as a G(s)α-coupled, adenylyl cyclase-stimulating receptor. EP4 signaling plays a variety of roles through cAMP effectors, i.e., protein kinase A and exchange protein activated by cAMP. However, emerging evidence from studies using pharmacological approaches and genetically modified mice suggests that EP4, unlike EP2, can also be coupled to G(i)α, phosphatidylinositol 3-kinase, β-arrestin, or β-catenin. These signaling pathways constitute unique roles for the EP4 receptor. EP4 is widely distributed in the body and thus plays various physiologic and pathophysiologic roles. In particular, EP4 signaling is closely related to carcinogenesis, cardiac hypertrophy, vasodilation, vascular remodeling, bone remodeling, gastrointestinal homeostasis, renal function, and female reproductive function. In addition to the classic anti-inflammatory action of EP4 on mononuclear cells and T cells, recent evidence has shown that EP4 signaling contributes to proinflammatory action as well. The aim of this review is to present current findings on the biologic functions of the EP4 receptor. In particular, we will discuss its diversity from the standpoint of EP4-mediated signaling.

E-type prostanoid receptor 4 (EP4) in disease and therapy

The large variety of biological functions governed by prostaglandin (PG) E2 is mediated by signaling through four distinct E-type prostanoid (EP) receptors. The availability of mouse strains with genetic ablation of each EP receptor subtype and the development of selective EP agonists and antagonists have tremendously advanced our understanding of PGE2 as a physiologically and clinically relevant mediator. Moreover, studies using disease models revealed numerous conditions in which distinct EP receptors might be exploited therapeutically. In this context, the EP4 receptor is currently emerging as most versatile and promising among PGE2 receptors. Anti-inflammatory, anti-thrombotic and vasoprotective effects have been proposed for the EP4 receptor, along with its recently described unfavorable tumor-promoting and pro-angiogenic roles. A possible explanation for the diverse biological functions of EP4 might be the multiple signaling pathways switched on upon EP4 activation. The present review attempts to summarize the EP4 receptor-triggered signaling modules and the possible therapeutic applications of EP4-selective agonists and antagonists.

The PGE(2)-EP4 receptor is necessary for stimulation of the renin-angiotensin-aldosterone system in response to low dietary salt intake in vivo

Increased cyclooxygenase-2 (COX-2) expression and PGE(2) synthesis have been shown to be prerequisites for renal renin release after Na(+) deprivation. To answer the question of whether EP4 receptor type of PGE(2) mediates renin regulation under a low-salt diet, we examined renin regulation in EP4(+/+), EP4(-/-), and in wild-type mice treated with EP4 receptor antagonist. After 2 wk of a low-salt diet (0.02% wt/wt NaCl), EP4(+/+) mice showed diminished Na(+) excretion, unchanged K(+) excretion, and reduced Ca(2+) excretion. Diuresis and plasma electrolytes remained unchanged. EP4(-/-) exhibited a similar attenuation of Na(+) excretion; however, diuresis and K(+) excretion were enhanced, and plasma Na(+) concentration was higher, whereas plasma K(+) concentration was lower compared with control diet. There were no significant differences between EP4(+/+) and EP4(-/-) mice in blood pressure, creatinine clearance, and plasma antidiuretic hormone (ADH) concentration. Following salt restriction, plasma renin and aldosterone concentrations and kidney renin mRNA level rose significantly in EP4(+/+) but not in EP4(-/-) and in wild-type mice treated with EP4 antagonist ONO-AE3-208. In the latter two groups, the low-salt diet caused a significantly greater rise in PGE(2) excretion. Furthermore, mRNA expression for COX-2 and PGE(2) synthetic activity was significantly greater in EP4(-/-) than in EP4(+/+) mice. We conclude that low dietary salt intake induces expression of COX-2 followed by enhanced renal PGE(2) synthesis, which stimulates the renin-angiotensin-aldosterone system by activation of EP4 receptor. Most likely, defects at the step of EP4 receptor block negative feedback mechanisms on the renal COX system, leading to persistently high PGE(2) levels, diuresis, and K(+) loss.

Hypertonicity increases sodium transporters in cortical collecting duct cells independently of PGE2

Cyclooxygenase-2 (COX-2) expression is increased by hypertonicity. Therefore we hypothesized that hypertonicity increased PGE(2) can modulate the sodium transporters (Na(+)/K(+)-ATPase: NKA, epithelial sodium channel: ENaC, and sodium hydrogen exchanger: NHE) in M1 cortical collecting duct (CCD) cells. We demonstrated by immunoblotting a 2-fold increase in NKA expression and activity following hypertonic treatment. α-ENaC was also increased, however sgk1, an ENaC activator, decreased in response to hypertonicity. Other CCD sodium transporters (β-ENaC, NHE) were unchanged. Hypertonicity also increased PGE(2) but EP(4) receptor mRNA was unaltered. PGE(2) increased intracellular Na(+) and cAMP production in M1 cells, but PGE(2)-stimulated cAMP response was attenuated by hypertonicity. Overall, PGE(2) had no effect on sodium transporter levels. Since neither COX inhibition nor EP(4) siRNA altered the induction of NKA, we propose that sodium transporter regulation by hypertonicity is independent of PGE(2). Altogether, these data indicate that despite a concomitant increase in PGE(2) production and sodium transporter expression in hypertonicity, both pathways are acting independently of each other.

COX-2 disruption leads to increased central vasopressin stores and impaired urine concentrating ability in mice

It was hypothesized that cyclooxygenase-2 (COX-2) activity promotes urine concentrating ability through stimulation of vasopressin (AVP) release after water deprivation (WD). COX-2-deficient (COX-2(-/-), C57BL/6) and wild-type (WT) mice were water deprived for 24 h, and water balance, central AVP mRNA and peptide level, AVP plasma concentration, and AVP-regulated renal transport protein abundances were measured. In male COX-2(-/-), basal urine output and water intake were elevated while urine osmolality was decreased compared with WT. Water deprivation resulted in lower urine osmolality, higher plasma osmolality in COX-2(-/-) mice irrespective of gender. Hypothalamic AVP mRNA level increased and was unchanged between COX-2(-/-) and WT after WD. AVP peptide content was higher in COX-2(-/-) compared with WT. At baseline, plasma AVP concentration was elevated in conscious chronically catheterized COX-2(-/-) mice, but after WD plasma AVP was unchanged between COX-2(-/-) and WT mice (43 ± 11 vs. 70 ± 16 pg/ml). Renal V2 receptor abundance was downregulated in COX-2(-/-) mice. Medullary interstitial osmolality increased and did not differ between COX-2(-/-) and WT after WD. Aquaporin-2 (AQP2; cortex-outer medulla), AQP3 (all regions), and UT-A1 (inner medulla) protein abundances were elevated in COX-2(-/-) at baseline and further increased after WD. COX-2(-/-) mice had elevated plasma urea and creatinine and accumulation of small subcapsular glomeruli. In conclusion, hypothalamic COX-2 activity is not necessary for enhanced AVP expression and secretion in response to water deprivation. Renal medullary COX-2 activity negatively regulates AQP2 and -3. The urine concentrating defect in COX-2(-/-) is likely caused by developmental glomerular injury and not dysregulation of AVP or collecting duct aquaporins.

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.

Regulation of renal proximal tubule Na-K-ATPase by prostaglandins

Prostaglandins (PGs) play a number of roles in the kidney, including regulation of salt and water reabsorption. In this report, evidence was obtained for stimulatory effects of PGs on Na-K-ATPase in primary cultures of rabbit renal proximal tubule (RPT) cells. The results of our real-time PCR studies indicate that in primary RPTs the effects of PGE(2), the major renal PG, are mediated by four classes of PGE (EP) receptors. The role of these EP receptors in the regulation of Na-K-ATPase was examined at the transcriptional level. Na-K-ATPase consists of a catalytic α-subunit encoded by the ATP1A1 gene, as well as a β-subunit encoded by the ATP1B1 gene. Transient transfection studies conducted with pHβ1-1141 Luc, a human ATP1B1 promoter/luciferase construct, indicate that both PGE(1) and PGE(2) are stimulatory. The evidence for the involvement of both the cAMP and Ca(2+) signaling pathways includes the inhibitory effects of the myristolylated PKA inhibitor PKI, the adenylate cyclase (AC) inhibitor SQ22536, and the PKC inhibitors Gö 6976 and Ro-32-0432 on the PGE(1) stimulation. Other effectors that similarly act through cAMP and PKC were also stimulatory to transcription, including norepinephrine and dopamine. In addition to its effects on transcription, a chronic incubation with PGE(1) was observed to result in an increase in Na-K-ATPase mRNA levels as well as an increase in Na-K-ATPase activity. An acute stimulatory effect of PGE(1) on Na-K-ATPase was observed and was associated with an increase in the level of Na-K-ATPase in the basolateral membrane.

Potential involvement of P2Y2 receptor in diuresis of postobstructive uropathy in rats

AVP resistance of the medullary collecting duct (mCD) in postobstructive uropathy (POU) has been attributed to increased production of PGE2. P2Y2 receptor activation causes production of PGE2 by the mCD. We hypothesize that increased P2Y2 receptor expression and/or activity may contribute to the diuresis of POU. Sprague-Dawley rats were subjected to bilateral ureteral obstruction for 24 h followed by release (BUO/R, n = 17) or sham operation (SHM/O, n = 15) and euthanized after 1 wk or 12 days. BUO/R rats developed significant polydipsia, polyuria, urinary concentration defect, and increased urinary PGE2 and decreased aquaporin-2 protein abundance in the inner medulla compared with SHM/O rats. After BUO/R, the relative mRNA expression of P2Y2 and P2Y6 receptors was increased by 2.7- and 4.9-fold, respectively, without significant changes in mRNA expression of P2Y1 or P2Y4 receptor. This was associated with a significant 3.5-fold higher protein abundance of the P2Y2 receptor in BUO/R than SHM/O rats. When freshly isolated mCD fractions were challenged with different types of nucleotides (ATPgammaS, ADP, UTP, or UDP), BUO/R and SHM/O rats responded to only ATPgammaS and UTP and released PGE2, consistent with involvement of the P2Y2, but not P2Y6, receptor. ATPgammaS- or UTP-stimulated increases in PGE2 were much higher in BUO/R (3.20- and 2.28-fold, respectively, vs. vehicle controls) than SHM/O (1.68- and 1.30-fold, respectively, vs. vehicle controls) rats. In addition, there were significant 2.4- and 2.1-fold increases in relative mRNA expression of prostanoid EP1 and EP3 receptors, respectively, in the inner medulla of BUO/R vs. SHM/O rats. Taken together, these data suggest that increased production of PGE2 by the mCD in POU may be due to increased expression and activity of the P2Y2 receptor. Increased mRNA expression of EP1 and EP3 receptors in POU may also help accentuate PGE2-induced signaling in the mCD.

ATP and adenosine in the local regulation of water transport and homeostasis by the kidney

Regulation of body water homeostasis is critically dependent on the kidney and under the control of AVP, which is released from the neurohypophysis. In the collecting duct (CD) of the kidney, AVP activates adenylyl cyclase via vasopressin V2 receptors. cAMP-dependent activation of protein kinase A phosphorylates the water channel aquaporin-2 and increases water permeability by insertion of aquaporin-2 into the apical cell membrane. However, local factors modulate the effects of AVP to fine tune its effects, accelerate responses, and potentially protect the integrity of CD cells. Nucleotides like ATP belong to these local factors and act in an autocrine and paracrine way to activate P2Y2 receptors on CD cells. Extracellular breakdown of ATP and cAMP forms adenosine, the latter also induces specific effects on the CD by activation of adenosine A1 receptors. Activation of both receptor types can inhibit the cAMP-triggered activation of protein kinase A and reduce water permeability and transport. This review focuses on the role and potential interactions of the ATP and adenosine system with regard to the regulation of water transport in the CD. We address the potential stimuli and mechanisms involved in nucleotide release and adenosine formation, and discuss the corresponding signaling cascades that are activated. Potential interactions between the ATP and adenosine system, as well as other factors involved in the regulation of CD function, are outlined. Data from pharmacological studies and gene-targeted mouse models are presented to demonstrate the in vivo relevance to water transport and homeostasis.

PGE2 potentiates tonicity-induced COX-2 expression in renal medullary cells in a positive feedback loop involving EP2-cAMP-PKA signaling

Cyooxygenase-2 (COX-2)-derived PGE2 is critical for the integrity and function of renal medullary cells during antidiuresis. The present study extended our previous finding that tonicity-induced COX-2 expression is further stimulated by the major COX-2 product PGE2 and investigated the underlying signaling pathways and the functional relevance of this phenomenon. Hyperosmolality stimulated COX-2 expression and activity in Madin-Darby canine kidney (MDCK) cells, a response that was further increased by PGE2-cAMP signaling, suggesting the existence of a positive feedback loop. This effect was diminished by AH-6809, an EP2 antagonist, and by the PKA inhibitor H-89, but not by AH-23848, an EP4 antagonist. The effect of PGE2 was mimicked by forskolin and dibutyryl-cAMP, suggesting that the stimulatory effect of PGE2 on COX-2 is mediated by a cAMP-PKA-dependent mechanism. Accordingly, cAMP-responsive element (CRE)-driven reporter activity paralleled the effects of PGE2, AH-6809, AH-23848, H-89, forskolin, and dibutyryl-cAMP on COX-2 expression. In addition, the stimulatory effect of PGE2 on tonicity-induced COX-2 expression was blunted in cells transfected with dominant-negative CRE binding (CREB) protein, as was the case in a COX-2 promoter reporter construct in which a putative CRE was deleted. Furthermore, PGE2 resulted in PKA-dependent phosphorylation of the pro-apoptotic protein Bad at Ser155, a mechanism that is known to inactivate Bad, which coincided with reduced caspase-3 activity during osmotic stress. Conversely, pharmacological interruption of the PGE2-EP2-cAMP-PKA pathway abolished Ser155 phosphorylation of Bad and blunted the protective effect of PGE2 on cell survival during osmotic stress. These observations indicate the existence of a positive feedback loop of PGE2 on COX-2 expression during osmotic stress, an effect that apparently is mediated by EP2-cAMP-PKA signaling, and that contributes to cell survival under hypertonic conditions.

Activation of the PGI(2)/IP system contributes to the development of circulatory failure in a rat model of endotoxic shock

Prostacyclin levels are increased in septic patients and several animal models of septic shock, and selective inhibition of cyclooxygenase-2 improved cardiovascular dysfunction in rats treated with lipopolysaccharide (LPS). Here, we examine the specific role of prostacyclin and of the receptor for prostacyclin (IP) in the development of LPS-induced circulatory failure. Intravenous injection of LPS (10 mg/kg) into male Sprague-Dawley rats caused a strong increase in plasma prostacyclin levels, which was paralleled by a decrease in blood pressure and an increase in heart rate. Moreover, LPS injection increased the mRNA expression of the IP receptor in the heart, aorta, lung, liver, adrenal glands, and kidneys. Cotreatment with the IP antagonist CAY-10441 (1, 10, 30, and 100 mg/kg) dose-dependently moderated the LPS-induced changes in mean arterial blood pressure, heart rate, cardiac output, and systemic vascular resistance. The development of cardiovascular failure was ameliorated by CAY-10441 in spite of the typical LPS-induced increases in plasma levels of cytokines and NO. In vitro, cytokines dose- and time-dependently induced IP expression in rat vascular smooth muscle cells. Incubation of cells with the stable IP agonist iloprost in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-mehylxanthine resulted in higher cAMP levels in cytokine-treated cells compared with untreated cells. Taken together, our data demonstrate a prominent role of the prostacyclin/IP system in the development of LPS-induced cardiovascular failure.

Dietary salt induces transcription of the prostaglandin transporter gene in renal collecting ducts

Prostaglandin E(2) (PGE(2)) plays an important role in maintaining body fluid homeostasis by activating its receptors on the renal collecting duct (CD) to stimulate renal Na(+) and water excretion. The PG carrier prostaglandin transporter (PGT) is expressed on the CD apical membrane, where it mediates PG reuptake as part of the termination of autocrine PG signaling. Here we tested the hypothesis that dietary salt loading regulates PGT gene transcription in renal CDs. We placed green fluorescence protein (GFP) under control of 3.3 kb of the mouse PGT promoter and injected this construct into the pronuclei of fertilized FVB mouse eggs. Four of thirty-eight offspring were GFP positive by genotyping. We extensively characterized one (no. 29) PGT-GFP transgenic mouse line. On microscopic examination, GFP was expressed in CDs as determined by their expression of aquaporin-2. We fed mice a low (0.03% NaCl)-, normal (0.3% NaCl)-, or high-salt (3% NaCl) diet for 2 wk and quantified CD GFP expression. The average number of GFP-positive CD cells per microscopic section varied directly with dietary salt intake. Compared with mice on the control (0.3% sodium) diet, mice on a low-sodium (0.03%) diet had reduced numbers of GFP-positive cells (71% of control, P < 0.001), whereas mice on a high-sodium (3%) diet had increased numbers of GFP-positive cells (139% of control, P < 0.001). This increase in apparent CD PGT transcription resulted in a 51-55% increase (P < 0.001) in whole kidney PGT mRNA levels as determined by real-time PCR. The regulation of PG signal termination via reuptake represents a new pathway for controlling renal Na(+) balance.

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.

Direct vasoconstrictor effect of prostaglandin E2on renal interlobular arteries: role of the EP3 receptor

Evidence indicates that prostaglandin E2(PGE2) preferentially affects preglomerular renal vessels. However, whether this is limited to small-caliber arterioles or whether larger vessels farther upstream also respond to PGE2is currently unclear. In the present study, we first investigated the effects of PGE2along the preglomerular vascular tree and subsequently focused on proximal interlobular arteries (ILAs). Proximal ILAs in hydronephrotic rat kidneys as well as isolated vessels from normal kidneys constricted in response to PGE2, both under basal conditions and after the induction of vascular tone. By contrast, smaller vessels, i.e., distal ILAs and afferent arterioles, exhibited PGE2-induced vasodilation. Endothelium removal and pretreatment of single, isolated proximal ILAs with an EP1 receptor blocker (SC51322, 1 μmol/l) or a thromboxane A2receptor blocker (SQ29548, 1 μmol/l) did not prevent vasoconstriction to PGE2. Furthermore, in the presence of SC51322, responses of these vessels to PGE2and the EP1/EP3 agonist sulprostone were superimposable, indicating that PGE2-induced vasoconstriction is mediated by EP3 receptors on smooth muscle cells. Immunohistochemical staining of proximal ILAs confirmed the presence of EP3 receptor protein on these cells and the endothelium. Adding PGE2to normal isolated kidneys induced a biphasic flow response, i.e., an initial flow increase at PGE2concentrations ≤0.1 μmol/l followed by a flow decrease at 1 μmol/l PGE2. Thus our results demonstrate that PGE2affects multiple segments of the preglomerular vascular tree in a different way. At the level of the proximal ILAs, PGE2had a direct vasoconstrictor action mediated by EP3 receptors.

Urine concentrating defect in prostaglandin EP1-deficient mice

We investigated the role of the prostaglandin E2(PGE2) EP1receptor in modulating urine concentration as it is expressed along the renal collecting duct where arginine-vasopressin (AVP) exerts its anti-diuretic activity, and in the paraventricular and supraoptic nuclei of the hypothalamus where AVP is synthesized. The urine osmolality of EP1-null mice (EP1−/−) failed to match levels achieved by wild-type (WT) counterparts upon water deprivation (WD) for 24 h. This difference was reflected by higher plasma osmolality in WD EP1−/−mice. Along the collecting duct, the induction and subapical to plasma membrane translocation of the aquaporin-2 water channel in WD EP1−/−mice appeared equivalent to that of WD WT mice as determined by quantitative RT-PCR and immunohistochemistry. However, medullary interstitial osmolalities dropped significantly in EP1−/−mice following WD. Furthermore, urinary AVP levels of WD EP1−/−mice were significantly lower than those of WD WT mice. This deficit could be traced back to a blunted induction of hypothalamic AVP mRNA expression in WD EP1−/−mice as determined by quantitative RT-PCR. Administration of the AVP mimetic [deamino-Cys1,d-Arg8]-vasopressin restored a significant proportion of the urine concentrating ability of WD EP1−/−mice. When mice were water loaded to suppress endogenous AVP production, urine osmolalities increased equally for WT and EP1−/−mice. These data suggest that PGE2modulates urine concentration by acting at EP1receptors, not in the collecting duct, but within the hypothalamus to promote AVP synthesis in response to acute WD.

Dominant role of prostaglandin E2 EP4 receptor in furosemide-induced salt-losing tubulopathy: a model for hyperprostaglandin E syndrome/antenatal Bartter syndrome

Increased formation of prostaglandin E2 (PGE2) is a key part of hyperprostaglandin E syndrome/antenatal Bartter syndrome (HPS/aBS), a renal disease characterized by NaCl wasting, water loss, and hyperreninism. Inhibition of PGE2 formation by cyclo-oxygenase inhibitors significantly lowers patient mortality and morbidity. However, the pathogenic role of PGE2 in HPS/aBS awaits clarification. Chronic blockade of the Na-K-2Cl co-transporter NKCC2 by diuretics causes symptoms similar to HPS/aBS and provides a useful animal model. In wild-type (WT) mice and in mice lacking distinct PGE2 receptors (EP1-/-, EP2-/-, EP3-/-, and EP4-/-), the effect of chronic furosemide administration (7 d) on urine output, sodium and potassium excretion, and renin secretion was determined. Furthermore, furosemide-induced diuresis and renin activity were analyzed in mice with defective PGI2 receptors (IP-/-). In all animals studied, furosemide stimulated a rise in diuresis and electrolyte excretion. However, this effect was blunted in EP1-/-, EP3-/-, and EP4-/- mice. Compared with WT mice, no difference was observed in EP2-/- and IP-/- mice. The furosemide-induced increase in plasma renin concentration was significantly decreased in EP4-/- mice and to a lesser degree also in IP-/- mice. Pharmacologic inhibition of EP4 receptors in furosemide-treated WT mice with the specific antagonist ONO-AE3-208 mimicked the changes in renin mRNA expression, plasma renin concentration, diuresis, and sodium excretion seen in EP4-/- mice. The GFR in EP4-/- mice was not changed compared with that in WT mice, which indicated that blunted diuresis and salt loss seen in EP4-/- mice were not a consequence of lower GFR. In summary, these findings demonstrate that the EP4 receptor mediates PGE2-induced renin secretion and that EP1, EP3, and EP4 receptors all contribute to enhanced PGE2-mediated salt and water excretion in the HPS/aBS model.

COX-2 inhibition prevents downregulation of key renal water and sodium transport proteins in response to bilateral ureteral obstruction

Bilateral ureteral obstruction (BUO) is associated with marked changes in the expression of renal aquaporins (AQPs) and sodium transport proteins. To examine the role of prostaglandin in this response, we investigated whether 24-h BUO changed the expression of cyclooxygenases (COX-1 and -2) in the kidney and tested the effect of the selective COX-2 inhibitor parecoxib (5 mg·kg−1·day−1via osmotic minipumps) on AQPs and sodium transport. Sham and BUO kidneys were analyzed by semiquantitative immunoblotting, and a subset of kidneys was perfusion fixed for immunocytochemistry. BUO caused a significant 14-fold induction of inner medullary COX-2 (14.40 ± 1.8 vs. 1.0 ± 0.4, n = 6; P < 0.0001) and a reduction in medullary tissue osmolality, whereas COX-1 did not change. Immunohistochemistry confirmed increased COX-2 labeling associated with medullary interstitial cells. COX isoforms did not change in cortex/outer medulla after 24-h BUO. In BUO kidneys, inner medullary AQP2 expression was reduced, and this decrease was prevented by parecoxib. In the inner stripe of outer medulla, the type 3 Na+/H+exchanger (NHE3) and apical Na+-K+-2Cl−cotransporter (BSC-1) were significantly reduced by BUO, and this decrease was significantly attenuated by parecoxib. Immunohistochemistry for AQP2, NHE3, and BSC-1 confirmed the effect of parecoxib. Parecoxib had no significant effect on the Na-K-ATPase α1-subunit, type II Na-Picotransporter, or AQP3. In conclusion, acute BUO leads to marked upregulation of COX-2 in inner medulla and selective COX-2 inhibition prevents dysregulation of AQP2, BSC-1, and NHE3 in response to BUO. These data indicate that COX-2 may be an important factor contributing to the impaired renal water and sodium handling in response to BUO.

Prostaglandin E2 EP2 and EP4 receptor activation mediates cAMP-dependent hyperpolarization and exocytosis of renin in juxtaglomerular cells

PGE(2) and PGI(2) stimulate renin secretion and cAMP accumulation in juxtaglomerular granular (JG) cells. We addressed, at the single-cell level, the receptor subtypes and intracellular transduction mechanisms involved. Patch clamp was used to determine cell capacitance (C(m)), current, and membrane voltage in response to PGE(2), EP2 and EP4 receptor agonists, and an IP receptor agonist. PGE(2) (0.1 micromol/l) increased C(m) significantly, and the increase was abolished by intracellular application of the protein kinase A antagonist Rp-8-CPT-cAMPS. EP2-selective ligands butaprost (1 micromol/l), AE1-259-01 (1 nmol/l), EP4-selective agonist AE1-329 (1 nmol/l), and IP agonist iloprost (1 micromol/l) significantly increased C(m) mediated by PKA. The EP4 antagonist AE3-208 (10 nmol/l) blocked the effect of EP4 agonist but did not alter the response to PGE(2). Application of both EP4 antagonist and EP2-antagonist AH-6809 abolished the effects of PGE(2) on C(m) and current. EP2 and EP4 ligands stimulated cAMP formation in JG cells. PGE(2) rapidly stimulated renin secretion from superfused JG cells and diminished the membrane-adjacent granule pool as determined by confocal microscopy. The membrane potential hyperpolarized significantly after PGE(2), butaprost, AE1-329 and AE1-259 and outward current was augmented in a PKA-dependent fashion. PGE(2)-stimulated outward current, but not C(m) change, was abolished by the BK(Ca) channel inhibitor iberiotoxin (300 nmol/l). EP2 and EP4 mRNA was detected in sampled JG cells, and the preglomerular and glomerular vasculature was immunopositive for EP4. Thus IP, EP2, and EP4 receptors are associated with JG cells, and their activation leads to rapid PKA-mediated exocytotic fusion and release of renin granules.

Over-the-counter analgesics in older adults: a call for improved labelling and consumer education

The use of analgesics increases with age and on any given day 20-30% of older adults take an analgesic medication. Over-the-counter (OTC) analgesics are generally well tolerated and effective when taken for brief periods of time and at recommended dosages. However, their long-term use, use at inappropriately high doses, or use by persons with contraindications may result in adverse effects, including gastrointestinal haemorrhage, cardiovascular toxicity, renal toxicity and hepatotoxicity. Many OTC drugs are also available through a prescription, for a broader range of indications and for longer durations of use and wider dose ranges, under the assumption that healthcare providers will help patients make safe choices about analgesics. Safe and effective use of medications is one of the greatest challenges faced by healthcare providers in medicine. More than 60% of people cannot identify the active ingredient in their brand of pain reliever. Additionally, about 40% of Americans believe that OTC drugs are too weak to cause any real harm. As a result of a recent US FDA policy, the conversion of prescription to OTC medications will result in a 50% increase of OTC medications. To reduce the risks of potential adverse effects from OTC drug therapy in older adults, we propose that the use of analgesics will be enhanced through the use of patient and healthcare provider education, as well as improved labelling of OTC analgesics. Improved labelling of OTC analgesics may help consumers distinguish common analgesic ingredients in a wide variety of preparations and facilitate informed decisions concerning the use of OTC drugs.

Activation of EP4 receptors contributes to prostaglandin E2-mediated stimulation of renal sensory nerves

Induction of cyclooxygenase-2 (COX-2) in the renal pelvic wall increases prostaglandin E(2) (PGE(2)) leading to stimulation of cAMP production, which results in substance P (SP) release and activation of renal mechanosensory nerves. The subtype of PGE receptors involved, EP2 and/or EP4, was studied by immunohistochemistry and renal pelvic administration of agonists and antagonists of EP2 and EP4 receptors. EP4 receptor-like immunoreactivity (LI) was colocalized with calcitonin gene-related peptide (CGRP)-LI in dorsal root ganglia (DRGs) at Th(9)-L(1) and in nerve terminals in the renal pelvic wall. Th(9)-L(1) DRG neurons also contained EP3 receptor-LI and COX-2-LI, each of which was colocalized with CGRP-LI in some neurons. No renal pelvic nerves contained EP3 receptor-LI and only very few nerves COX-2-LI. The EP1/EP2 receptor antagonist AH-6809 (20 microM) had no effect on SP release produced by PGE(2) (0.14 microM) from an isolated rat renal pelvic wall preparation. However, the EP4 receptor antagonist L-161,982 (10 microM) blocked the SP release produced by the EP2/EP4 receptor agonist butaprost (10 microM) 12 +/- 2 vs. 2 +/- 1 and PGE(2), 9 +/- 1 vs. 1 +/- 0 pg/min. The SP release by butaprost and PGE(2) was similarly blocked by the EP4 receptor antagonist AH-23848 (30 microM). In anesthetized rats, the afferent renal nerve activity (ARNA) responses to butaprost 700 +/- 100 and PGE(2).780 +/- 100%.s (area under the curve of ARNA vs. time) were unaffected by renal pelvic perfusion with AH-6809. However, 1 microM L-161,982 and 10 microM AH-23848 blocked the ARNA responses to butaprost by 94 +/- 5 and 78 +/- 10%, respectively, and to PGE(2) by 74 +/- 16 and 74 +/- 11%, respectively. L-161,982 also blocked the ARNA response to increasing renal pelvic pressure 10 mmHg, 85 +/- 5%. In conclusion, PGE(2) increases renal pelvic release of SP and ARNA by activating EP4 receptors on renal sensory nerve fibers.

Molecular response to cytotoxic injury: role of inflammation, MAP kinases, and endoplasmic reticulum stress response

Nephrotoxicants have varied direct and indirect effects on the vasculature, tubules, and interstitium of the kidney. In most cases the molecular components of the toxic insult are poorly understood. In this review some common themes of injury, repair, and adaptive protective responses that represent characteristic responses of the cells and kidney tissue that transcend the specifics of a particular toxin are presented. Particular attention is paid to the vascular and inflammatory aspects of nephrotoxicity as well as the activation of the MAP kinase families and the endoplasmic reticulum stress response by the tubular epithelial cell.

Role of prostaglandin E receptor EP1 subtype in the development of renal injury in genetically hypertensive rats

One of the major causes of end-stage renal diseases is hypertensive renal disease, in which enhanced renal prostaglandin (PG) E2 production has been shown. PGE2, a major arachidonic acid metabolite produced in the kidney, acts on 4 receptor subtypes, EP1 through EP4, but the pathophysiological importance of the PGE2/EP subtypes in the development of hypertensive renal injury remains to be elucidated. In this study, we investigated whether an orally active EP1-selective antagonist (EP1A) prevents the progression of renal damage in stroke-prone spontaneously hypertensive rats (SHRSP), a model of human malignant hypertension. Ten-week-old SHRSP, with established hypertension but with minimal renal damage, were given EP1A or vehicle for 5 weeks. After the treatment period, vehicle-treated SHRSP showed prominent proliferative lesions in arterioles, characterized by decreased alpha-smooth muscle actin expression in multilayered vascular smooth muscle cells. Upregulation of transforming growth factor-beta expression and tubulointerstitial fibrosis were also observed in vehicle-treated SHRSP. All these changes were dramatically attenuated in EP1A-treated SHRSP. Moreover, EP1A treatment significantly inhibited both increase in urinary protein excretion and decrease in creatinine clearance but had little effect on systemic blood pressure. These findings indicate that the PGE2/EP1 signaling pathway plays a crucial role in the development of renal injury in SHRSP. This study opens a novel therapeutic potential of selective blockade of EP1 for the treatment of hypertensive renal disease.

COX-2 inhibition and prostaglandin receptors in experimental nephritis

BACKGROUND

Renal cyclooxygenases (COX) produce the prostaglandins (PG) E2, I2 and thromboxane (TxA2), which interact with distinct G protein-coupled receptors. We investigated the expression of the three EP receptors EP2, EP3 and EP4 and the receptors for PGI2 (IP) and TxA2 (TP) in rats with passive Heymann nephritis (PHN). We studied their regulation by COX-2 inhibition with celecoxib.

MATERIALS AND METHODS

Four groups of Wistar rats were studied: healthy rats (group A), healthy rats treated with celecoxib (group B), rats with PHN (group C), and rats with PHN receiving celecoxib (group D). Expression of the mRNA for all receptors in the renal cortex and for the EP3 receptor in cultured mesangial cells (MCs) was determined by semiquantitative reverse transcriptase polymerase chain reaction. Stable prostaglandin metabolites were measured in the urine by radioimmunoassay.

RESULTS

Rats with PHN (group C) showed an 1.8-fold increase of cortical EP3 receptor mRNA expression as compared with controls (group A). In celecoxib-treated PHN rats (group D) the mRNA expression of the EP3 and EP4 receptors was significantly reduced to 1.0-fold and 0.7-fold induction, respectively. Furthermore, the excretion of bicyclo-prostaglandin E2 (PGE2) was inhibited by celecoxib. No changes were observed in the expression of the other PG-receptors. In cultured MC, PGE2 enhanced the EP3 mRNA expression.

CONCLUSIONS

These data suggest a predominant role of the EP3 receptor in the transduction of PGE2-actions in PHN. It was concluded that COX-2-dependent PGE2 is able to potentiate its effects in the kidney by up-regulating its own receptors.

Regulation of renal cortical cyclooxygenase-2 in young rats

Cyclooxygenase-2 (COX-2) is involved in kidney morphogenesis and is transiently elevated in the immature kidney. In adult rats, renal cortical COX-2 expression is tonically suppressed by mineralocorticoids (MC) and glucocorticoids (GC) and induced by chronic salt restriction. Young rats have low levels of GC and are in a state of relative volume depletion. The present study was designed to investigate the mechanisms underlying elevated cortical COX-2 expression in the immature kidney. Supplementation of GC or MC suppressed cortical COX-2 expression in suckling rats. GC suppression was significantly, but not completely, prevented by either an MC receptor antagonist or a GC receptor antagonist. MC suppression was completely prevented by a mineralocorticoid receptor antagonist. Salt supplementation suppressed cortical COX-2 expression in a dose- and time-dependent pattern in the suckling rats. Cortical COX-2 expression in the weanling rats was upregulated by a low-salt diet and downregulated by a high-salt diet. These results suggest that relative volume depletion and reduced GC levels are involved in elevated cortical COX-2 expression in the immature rodent kidney.

Prostaglandins that increase renin production in response to ACE inhibition are not derived from cyclooxygenase-1

It is well known that nonselective, nonsteroidal anti-inflammatory drugs inhibit renal renin production. Our previous studies indicated that angiotensin-converting enzyme inhibitor (ACEI)-mediated renin increases were absent in rats treated with a cyclooxygenase (COX)-2-selective inhibitor and in COX-2 -/- mice. The current study examined further whether COX-1 is also involved in mediating ACEI-induced renin production. Because renin increases are mediated by cAMP, we also examined whether increased renin is mediated by the prostaglandin E(2) receptor EP(2) subtype, which is coupled to G(s) and increases cAMP. Therefore, we investigated if genetic deletion of COX-1 or EP(2) prevents increased ACEI-induced renin expression. Age- and gender-matched wild-type (+/+) and homozygous null mice (-/-) were administered captopril for 7 days, and plasma and renal renin levels and renal renin mRNA expression were measured. There were no significant differences in the basal level of renal renin activity from plasma or renal tissue in COX-1 +/+ and -/- mice. Captopril administration increased renin equally [plasma renin activity (PRA): +/+ 9.3 +/- 2.2 vs. 50.1 +/- 10.9; -/- 13.7 +/- 1.5 vs. 43.9 +/- 6.6 ng ANG I x ml(-1) x h(-1); renal renin concentration: +/+ 11.8 +/- 1.7 vs. 35.3 +/- 3.9; -/- 13.0 +/- 3.0 vs. 27.8 +/- 2.7 ng ANG I x mg protein(-1) x h(-1); n = 6; P < 0.05 with or without captopril]. ACEI also increased renin mRNA expression (+/+ 2.4 +/- 0.2; -/- 2.1 +/- 0.2 fold control; n = 6-10; P < 0.05). Captopril led to similar increases in EP(2) -/- compared with +/+. The COX-2 inhibitor SC-58236 blocked ACEI-induced elevation in renal renin concentration in EP(2) null mice (+/+ 24.7 +/- 1.7 vs. 9.8 +/- 0.4; -/- 21.1 +/- 3.2 vs. 9.3 +/- 0.4 ng ANG I x mg protein(-1) x h(-1); n = 5) as well as in COX-1 -/- mice (SC-58236-treated PRA: +/+ 7.3 +/- 0.6; -/- 8.0 +/- 0.9 ng ANG I x ml(-1) x h(-1); renal renin: +/+ 9.1 +/- 0.9; -/- 9.6 +/- 0.5 ng ANG I x mg protein(-1) x h(-1); n = 6-7; P < 0.05 compared with no treatment). Immunohistochemical analysis of renin expression confirmed the above results. This study provides definitive evidence that metabolites of COX-2 rather than COX-1 mediate ACEI-induced renin increases. The persistent response in EP(2) nulls suggests involvement of prostaglandin E(2) receptor subtype 4 and/or prostacyclin receptor (IP).

Prostanoid receptor subtypes

Prostanoids are a group of lipid mediators that include the prostaglandins (PG) and thromboxanes (TX). Upon cell stimulation, prostanoids are synthesized from arachidonic acid via the cyclooxygenase (COX) pathway and released outside the cells to exert various physiological and pathological actions in a variety of tissues and cells. The activities of prostanoids are mediated by specific G protein-coupled receptors, which have been classified on the basis of pharmacological experiments into eight types and subtypes according to their responsiveness to selective agonists and antagonists. These prostanoid receptors have been cloned from various species including human, and their distinct binding properties and signal transduction pathways have been characterized by analyses of cells expressing each receptor. Furthermore, the distribution patterns of prostanoid receptor mRNAs have been determined in tissues and cells for various species. This information is useful for understanding the molecular basis of the pathophysiological actions of prostanoids.

PGE(2) increases release of substance P from renal sensory nerves by activating the cAMP-PKA transduction cascade

Increasing renal pelvic pressure increases afferent renal nerve activity (ARNA) by a PGE(2)-mediated release of substance P (SP) from renal pelvic nerves. The role of cAMP activation in the PGE(2)-mediated release of SP was studied by examining the effects of the adenylyl cyclase (AC) activator forskolin and AC inhibitor dideoxyadenosine (DDA). Forskolin enhanced the bradykinin-mediated release of SP from an isolated rat renal pelvic wall preparation, from 7.3 +/- 1.3 to 15.6 +/- 3.0 pg/min. PGE(2) at a subthreshold concentration for SP release mimicked the effects of forskolin. The EP(2) receptor agonist butaprost, 15 microM, and PGE(2), 0.14 microM, produced similar increases in SP release, from 5.8 +/- 0.8 to 17.0 +/- 2.3 pg/min and from 8.0 +/- 1.3 to 21.6 +/- 2.7 pg/min. DDA blocked the SP release produced by butaprost and PGE(2). The PGE(2)-induced release of SP was also blocked by the PKA inhibitors PKI(14-22) and H-89. Studies in anesthetized rats showed that renal pelvic administration of butaprost, 10 microM, and PGE(2), 0.14 microM, resulted in similar ARNA responses, 1,520 +/- 390 and 1,170 +/- 270%. s (area under the curve of ARNA vs. time) that were blocked by DDA. Likewise, the ARNA response to increased renal pelvic pressure, 7,180 +/- 710%. s, was blocked by DDA. In conclusion, PGE(2) activates the cAMP-PKA pathway leading to a release of SP and activation of renal pelvic mechanosensory nerve fibers.

G protein-coupled prostanoid receptors and the kidney

Renal cyclooxygenase 1 and 2 activity produces five primary prostanoids: prostaglandin E2, prostaglandin F2alpha, prostaglandin I2, thromboxane A2, and prostaglandin D2. These lipid mediators interact with a family of distinct G protein-coupled prostanoid receptors designated EP, FP, IP, TP, and DP, respectively, which exert important regulatory effects on renal function. The intrarenal distribution of these prostanoid receptors has been mapped, and the consequences of their activation have been partially characterized. FP, TP, and EP1 receptors preferentially couple to an increase in cell calcium. EP2, EP4, DP, and IP receptors stimulate cyclic AMP, whereas the EP3 receptor preferentially couples to Gi, inhibiting cyclic AMP generation. EP1 and EP3 mRNA expression predominates in the collecting duct and thick limb, respectively, where their stimulation reduces NaCl and water absorption, promoting natriuresis and diuresis. The FP receptor is highly expressed in the distal convoluted tubule, where it may have a distinct effect on renal salt transport. Although only low levels of EP2 receptor mRNA are detected in the kidney and its precise intrarenal localization is uncertain, mice with targeted disruption of the EP2 receptor exhibit salt-sensitive hypertension, suggesting that this receptor may also play an important role in salt excretion. In contrast, EP4 receptor mRNA is predominantly expressed in the glomerulus, where it may contribute to the regulation of glomerular hemodynamics and renin release. The IP receptor mRNA is highly expressed near the glomerulus, in the afferent arteriole, where it may also dilate renal arterioles and stimulate renin release. Conversely, TP receptors in the glomerulus may counteract the effects of these dilator prostanoids and increase glomerular resistance. At present there is little evidence for DP receptor expression in the kidney. These receptors act in a concerted fashion as physiological buffers, protecting the kidney from excessive functional changes during periods of physiological stress. Nonsteroidal anti-inflammatory drug (NSAID)-mediated cyclooxygenase inhibition results in the loss of these combined effects, which contributes to their renal effects. Selective prostanoid receptor antagonists may provide new therapeutic approaches for specific disease states.

Localization of prostaglandin E(2) EP2 and EP4 receptors in the rat kidney

We investigated the localization of cAMP-coupled prostaglandin E(2) EP2 and EP4 receptor expression in the rat kidney. EP2 mRNA was restricted to the outer and inner medulla in rat kidney, as determined by RNase protection assay. RT-PCR analysis of microdissected resistance vessels and nephron segments showed EP2 expression in descending thin limb of Henle's loop (DTL) and in vasa recta of the outer medulla. The EP4 receptor was expressed in distal convoluted tubule (DCT) and cortical collecting duct (CCD) in preglomerular vessels, and in outer medullary vasa recta. Butaprost, an EP2 receptor-selective agonist, dose dependently raised cAMP levels in microdissected DTL and outer medullary vasa recta specimens but had no effect in EP2-negative outer medullary collecting duct segments. Dietary salt intake did not alter EP2 expression in the kidney medulla. These results suggest that PGE(2) may act in the resistance vessels and in the DTL and DCT-CCD segments as a paracrine, cAMP-dependent regulator of vascular resistance and tubular transport, respectively.

Altered growth response to prostaglandin E2 and its receptor signaling in mesangial cells from stroke-prone spontaneously hypertensive rats

OBJECTIVE

Prostaglandin (PG) E2, a major arachidonic acid metabolite in the kidney, acts on four receptor subtypes (EP1, EP2, EP3 and EP4). One of major causes of end-stage renal failure is hypertensive renal disease, in which enhanced renal PGE2 production has been shown. In this study, to explore the pathophysiological significance of EP subtypes in the kidney, we examined the role of EP subtypes on proliferation of mesangial cells (MCs) from stroke-prone spontaneously hypertensive rats (SHRSPs), which show faster growth than those from normotensive Wistar-Kyoto rats (WKYs).

DESIGN AND METHODS

Using MCs from SHRSPs and WKYs, we investigated DNA synthesis and its upstream event, the phosphorylation of extracellular signal-regulated kinase (ERK), together with the gene expression of EP subtypes.

RESULTS

Sulprostone, an EP1 agonist, dose-dependently increased DNA synthesis and the phosphorylation of ERK in MCs from both strains. The EP4 agonist, 11-deoxy-PGE1, inhibited sulprostone-induced phosphorylation of ERK in WKY-MCs. In contrast, 11-deoxy-PGE1 failed to inhibit the ERK activity in SHRSP-MCs. Interestingly, cAMP production mediated by EP4 was markedly attenuated in SHRSP-MCs as compared with that in WKY-MCs, despite the overproduction of endogenous PGE2 in SHRSP-MCs. Similar gene expressions of EP1 and EP4 and only faint expression of EP3 were detected in MCs from both strains.

CONCLUSIONS

These results indicate that the PGE2/EP4 system counteracts the PGE2/EP1 system at the level of the intracellular signaling pathway. The altered EP4 signaling may play a critical role in the exaggerated mesangial growth in SHRSPs.

Involvement of prostaglandin E2 in clearance of aggregated protein via protein kinase A in glomeruli

Recently we immunohistochemically demonstrated that prostaglandin E2 (PGE2) promoted the clearance of aggregated bovine serum albumin (a-BSA) deposited in glomeruli. Herein, we investigated the role of PGE2 and its signal transduction in the disposal of macromolecules in glomeruli. EP2 and EP4 receptor mRNA was detected in glomeruli by RT-PCR analysis. A-BSA was injected twice into mice. Glomeruli were then isolated and incubated. A-BSA gradually disappeared from isolated glomeruli. PGE2 increased the intracellular cyclic AMP and decreased a-BSA level in glomeruli. Additionally, 8-bromocyclic AMP evoked a loss of a-BSA in isolated glomeruli. The effect of 8-bromo-cyclic AMP on the clearance of a-BSA was abolished by KT 5720 in glomeruli. PGE2 and 8-bromo-cyclic AMP also prompted disposal of a-BSA in cultured mesangial cells. These findings indicate that PGE2 positively regulates the removal of macromolecules via cyclic AMP and protein kinase A in glomeruli, and they provide insight into how to prevent the development of glomerulonephritis and glomerulosclerosis.

Conducted vasoconstriction in rat mesenteric arterioles: role for dihydropyridine-insensitive Ca(2+) channels

The aim of this study was to evaluate the role of voltage-operated Ca(2+) channels in the initiation and conduction of vasoconstrictor responses to local micropipette electrical stimulation of rat mesenteric arterioles (28 +/- 1 microm, n = 79) in vivo. Local and conducted (600 microm upstream from the pipette) vasoconstriction was not blocked by TTX (1 micromol/l, n = 5), nifedipine, or nimodipine (10 micromol/l, n = 9). Increasing the K(+) concentration of the superfusate to 75 mmol/l did not evoke vasoconstriction, but this depolarizing stimulus reversibly abolished vasoconstrictor responses to current stimulation (n = 7). Addition of the T-type Ca(2+) antagonist mibefradil (10 micromol/l, n = 6) to the superfusate reversibly blocked local and conducted vasoconstriction to current stimulation. With the use of RT-PCR techniques, it was demonstrated that rat mesenteric arterioles <40 microm do not express mRNA for L-type Ca(2+) channels (alpha(1C)-subunit), whereas mRNA coding for T-type subunits was found (alpha(1G)- and alpha(1H)-subunits). The data indicate that L-type Ca(2+) channels are absent from rat mesenteric arterioles (<40 microm). Rather, the vasoconstrictor responses appear to rely on other types of voltage-gated, dihydropyridine-insensitive Ca(2+) channels, possibly of the T-type.

Role of EP(2) and EP(3) PGE(2) receptors in control of murine renal hemodynamics

The kidney plays a central role in long-term regulation of arterial blood pressure and salt and water homeostasis. This is achieved in part by the local actions of paracrine and autacoid mediators such as the arachidonic acid-prostanoid system. The present study tested the role of specific PGE(2) E-prostanoid (EP) receptors in the regulation of renal hemodynamics and vascular reactivity to PGE(2). Specifically, we determined the extent to which the EP(2) and EP(3) receptor subtypes mediate the actions of PGE(2) on renal vascular tone. Renal blood flow (RBF) was measured by ultrasonic flowmetry, whereas vasoactive agents were injected directly into the renal artery of male mice. Studies were performed on two independent mouse lines lacking either EP(2) or EP(3) (-/-) receptors and the results were compared with wild-type controls (+/+). Our results do not support a unique role of the EP(2) receptor in regulating overall renal hemodynamics. Baseline renal hemodynamics in EP(2)-/- mice [RBF EP(2)-/-: 5.3 +/- 0.8 ml. min(-1). 100 g kidney wt(-1); renal vascular resistance (RVR) 19.7 +/- 3.6 mmHg. ml(-1). min. g kidney wt] did not differ statistically from control mice (RBF +/+: 4.0 +/- 0.5 ml. min(-1). 100 g kidney wt(-1); RVR +/+: 25.4 +/- 4.9 mmHg. ml(-1). min. 100 g kidney wt(-1)). This was also the case for the peak RBF increase after local PGE(2) (500 ng) injection into the renal artery (EP(2)-/-: 116 +/- 4 vs. +/+: 112 +/- 2% baseline RBF). In contrast, we found that the absence of EP(3) receptors in EP(3)-/- mice caused a significant increase (43%) in basal RBF (7.9 +/- 0.8 ml. min(-1). g kidney wt(-1), P < 0.05 vs. +/+) and a significant decrease (41%) in resting RVR (11.6 +/- 1.4 mmHg. ml(-1). min. g kidney wt(-1), P < 0.05 vs. +/+). Local administration of 500 ng of PGE(2) into the renal artery caused more pronounced renal vasodilation in EP(3)-/- mice (128 +/- 2% of basal RBF, P < 0.05 vs. +/+). We conclude that EP(3 )receptors mediate vasoconstriction in the kidney of male mice and its actions are tonically active in the basal state. Furthermore, EP(3) receptors are capable of buffering PGE(2)-mediated renal vasodilation.

Expression and regulation of prostaglandin E receptor subtype mRNAs in rat sensory ganglia and spinal cord in response to peripheral inflammation

Prostaglandins are known to act via seven transmembrane domain receptors to exert actions on both peripheral and central neurons resulting in changes in neuronal excitability. Prostaglandin E2, the prostaglandin most often associated with inflammation, itself acts on a family of closely related receptors, the EP receptors. Using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), we have shown that rat primary afferent neurons express the mRNA for all EP receptor subtypes, and that some, but not all EP receptor subtype mRNAs are down-regulated in sensory neurons in response to an acute peripheral inflammation. We also show for the first time that all EP receptor subtype mRNAs are expressed in rat lumbar spinal cord. Spinal cord EP receptor subtype mRNAs are also regulated in acute inflammation in a pattern distinct from the changes seen in sensory ganglia in response to the same inflammatory stimulus.

The alpha(1G)-subunit of a voltage-dependent Ca(2+) channel is localized in rat distal nephron and collecting duct

The molecular type and localization of calcium channels along the nephron are not well understood. In the present study, we assessed the distribution of the recently identified alpha(1G)-subunit encoding a voltage-dependent calcium channel with T-type characteristics. Using a RNase protection assay, alpha(1G)-mRNA levels in kidney regions were determined as inner medulla >> outer medulla congruent with cortex. RT-PCR analysis of microdissected rat nephron segments revealed alpha(1G) expression in the distal convoluted tubule (DCT), in the connecting tubule and cortical collecting duct (CT+CCD), and inner medullary collecting duct (IMCD). alpha(1G) mRNA was expressed in the IMCD cell line mIMCD-3. Single- and double-labeling immunohistochemistry and confocal laser microscopy on semithin paraffin sections of rat kidneys by using an anti-alpha(1G) antibody demonstrated a distinct labeling at the apical plasma membrane domains of DCT cells, CT principal cells, and IMCD principal cells.

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.