Role of renal nerves in stimulation of renin, COX-2, and nNOS in rat renal cortex during salt deficiency

Pollutants and nutrition: Are methylmercury effects on blood pressure and lipoprotein profile comparable to high-fat diet in mice?

Human exposure to methylmercury (MeHg) due to contaminated fish intake as part of a high-fat (HFD), high-carbohydrate diets is a reality today for many populations. HFD is associated with hypertension and hyperlipidemia, primary cardiovascular disease (CVD) risk factors. Some studies suggest that MeHg induces those risk factors. We evaluated the effect of MeHg exposure in mice fed with HFD or control diet for eight weeks. In the last experimental 15 days, the half group received a MeHg solution (20 mg/L) replacing water. Blood pressure (BP), heart rate, lipoprotein concentrations, and paraoxonase activity were evaluated. Liver cholesterol, triacylglycerol, and IBA-1+ cells, as well as transcriptional levels of genes related to lipid metabolism and inflammatory response, were also assessed. HFD and both MeHg groups presented increased BP and total cholesterol (TC). In the liver, HFD but not MeHg was related to an increase in TC. Also, MeHg intoxication reduced paraoxonase activity regardless of diet. MeHg intoxication and HFD increased steatosis and the number of IBA-1+ cells and modified some gene transcripts associated with lipid metabolism. In conclusion, we demonstrated that MeHg effects on CVD risk factors resemble those caused by HFD.

Fetal Programming of Renal Dysfunction and High Blood Pressure by Chronodisruption

Adverse prenatal conditions are known to impose significant trade-offs impinging on health and disease balance during adult life. Among several deleterious factors associated with complicated pregnancy, alteration of the gestational photoperiod remains largely unknown. Previously, we reported that prenatal manipulation of the photoperiod has adverse effects on the mother, fetus, and adult offspring; including cardiac hypertrophy. Here, we investigated whether chronic photoperiod shifting (CPS) during gestation may program adult renal function and blood pressure regulation. To this end, pregnant rats were subjected to CPS throughout pregnancy to evaluate the renal effects on the fetus and adult offspring. In the kidney at 18 days of gestation, both clock and clock-controlled gene expression did not display a daily pattern, although there were recurrent weaves of transcriptional activity along the 24 h in the control group. Using DNA microarray, significant differential expression was found for 1,703 transcripts in CPS relative to control fetal kidney (835 up-regulated and 868 down-regulated). Functional genomics assessment revealed alteration of diverse gene networks in the CPS fetal kidney, including regulation of transcription, aldosterone-regulated Na+ reabsorption and connective tissue differentiation. In adult offspring at 90 days of age, circulating proinflammatory cytokines IL-1β and IL-6 were increased under CPS conditions. In these individuals, CPS did not modify kidney clock gene expression but had effects on different genes with specific functions in the nephron. Next, we evaluated several renal markers and the response of blood pressure to 4%NaCl in the diet for 4 weeks (i.e., at 150 days of age). CPS animals displayed elevated systolic blood pressure in basal conditions that remained elevated in response to 4%NaCl, relative to control conditions. At this age, CPS modified the expression of Nhe3, Ncc, Atp1a1, Nr3c1 (glucocorticoid receptor), and Nr3c2 (mineralocorticoid receptor); while Nkcc, Col3A1, and Opn were modified in the CPS 4%+NaCl group. Furthermore, CPS decreased protein expression of Kallikrein and COX-2, both involved in sodium handling. In conclusion, gestational chronodisruption programs kidney dysfunction at different levels, conceivably underlying the prehypertensive phenotype observed in the adult CPS offspring.

The Role of Angiotensin II Infusion on the Baroreflex Sensitivity and Renal Function in Intact and Bilateral Renal Denervation Rats

Background

The role of renin-angiotensin system (RAS) in communication between renal system and cardiovascular system is extremely important. Baroreflex sensitivity (BRS) index defines as heart rate (HR) alteration versus mean arterial pressure (MAP) change ratio . Sympathetic nerve is arm of the baroreflexes and any change in its activity will lead to change in the BRS. The role of angiotensin II (Ang II) infusion in systemic circulation accompanied with bilateral renal denervation (RDN) on BRS index and renal function was studied.

Materials and Methods

Seventy-two male and female Wistar rats in 12 groups were anesthetized and catheterized. The alteration of MAP and HR responses to phenylephrine infusion compared to control groups was determined in bilateral RDN rats subjected to treat with Ang II (300 or 1000 ng/kg/min) administration.

Results

The BRS index was elevated in Ang II-treated non-RDN (normal) male rats gradually and dose dependently (P < 0.05), while this index was significantly different when compared with RDN male rats (P < 0.05). Accordingly, the BRS index was significantly lower in RDN than non-RDN male rats, and such observation was not observed in female rats. The creatinine clearance (insignificantly) and urine flow (significantly; P < 0.05) were decreased in both non-RDN and RDN male and female rats treated with Ang II. In RDN model, the serum nitrite levels were decreased in male and increased in female by Ang II infusion when compared with vehicle infusion.

Conclusion

The Ang II infusion could increase the BRS index in non-RDN (normal) male rats which is significantly greater than BRS index in RDN rats.

Effects of renal denervation on renal pelvic contractions and connexin expression in rats

AIMS

The renal pelvis shows spontaneous rhythmic contractile activity. We assessed to what extent this activity depends on renal innervation and studied the role of connexins in pelvic contractions.

METHODS

Rats underwent unilateral renal denervation or renal transplantation. Renal pelvic pressure and diuresis were measured in vivo. Spontaneous and agonist-induced contractions of isolated renal pelves were investigated by wire myography. Rat and human renal pelvic connexin mRNA abundances and connexin localization were studied by real-time PCR and immunofluorescence respectively.

RESULTS

Renal denervation or transplantation increased renal pelvic pressure in vivo by about 60 and 150%, respectively, but did not significantly affect pelvic contraction frequency. Under in vitro conditions, isolated pelvic preparations from innervated or denervated kidneys showed spontaneous contractions. Pelves from denervated kidneys showed about 50% higher contraction frequencies than pelves from innervated kidneys, whereas contraction force was similar in pelves from denervated and innervated kidneys. There was no denervation-induced supersensitivity to noradrenaline or endothelin-1. Renal denervation did not increase pelvic connexin37, 40, 43 or 45 mRNA abundances. Gap junction blockade had no effect on spontaneous pelvic contractile activity.

CONCLUSIONS

The denervation-induced effect on pelvic pressure may be the consequence of the enhanced diuresis. The mechanisms underlying the denervation-induced effects on pelvic contraction frequency remain unknown. Our data rule out a major role for two important candidates, by showing that renal denervation neither induced supersensitivity to contractile agonists nor increased connexin mRNA abundance in the pelvic wall.

Role of the renin-angiotensin system, renal sympathetic nerve system, and oxidative stress in chronic foot shock-induced hypertension in rats

OBJECTIVE

The renin-angiotensin system (RAS) and renal sympathetic nerve system (RSNS) are involved in the development of hypertension. The present study is designed to explore the possible roles of the RAS and the RSNS in foot shock-induced hypertension.

METHODS

Male Sprague-Dawley rats were divided into six groups: control, foot shock, RSNS denervation, denervation plus foot shock, Captopril (angiotensin I converting enzyme inhibitor, ACE inhibitor) plus foot shock, and Tempol (superoxide dismutase mimetic) plus foot shock. Rats received foot shock for 14 days. We measured the quantity of thiobarbituric acid reactive substances (TBARS), corticosterone, renin, and angiotensin II (Ang II) in plasma, the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and renal noradrenaline content. RAS component mRNA and protein levels were quantified in the cerebral cortex and hypothalamus.

RESULTS

The two week foot shock treatment significantly increased systolic blood pressure, which was accompanied by an increase in angiotensinogen, renin, ACE1, and AT1a mRNA and protein expression in the cerebral cortex and hypothalamus, an increase of the plasma concentrations of renin, Ang II, corticosterone, and TBARS, as well as a decrease in plasma SOD and GSH-Px activities. Systolic blood pressure increase was suppressed by denervation of the RSNS or treatment with Captopril or Tempol. Interestingly, denervation or Tempol treatment both decreased main RAS components not only in the circulatory system, but also in the central nervous system. In addition, decreased antioxidant levels and increased TBARS and corticosterone levels were also partially restored by denervation or treatment with Tempol or Captopril.

CONCLUSIONS

RAS, RSNS and oxidative stress reciprocally potentiate to play important roles in the development of foot shock-induced hypertension.

Renal nerve stimulation leads to the activation of the Na+/H+ exchanger isoform 3 via angiotensin II type I receptor

Renal nerve stimulation at a low frequency (below 2 Hz) causes water and sodium reabsorption via α1-adrenoreceptor tubular activation, a process independent of changes in systemic blood pressure, renal blood flow, or glomerular filtration rate. However, the underlying mechanism of the reabsorption of sodium is not fully understood. Since the sympathetic nervous system and intrarenal ANG II appear to act synergistically to mediate the process of sodium reabsorption, we hypothesized that low-frequency acute electrical stimulation of the renal nerve (ESRN) activates NHE3-mediated sodium reabsorption via ANG II AT1 receptor activation in Wistar rats. We found that ESRN significantly increased urinary angiotensinogen excretion and renal cortical ANG II content, but not the circulating angiotensinogen levels, and also decreased urinary flow and pH and sodium excretion via mechanisms independent of alterations in creatinine clearance. Urinary cAMP excretion was reduced, as was renal cortical PKA activity. ESRN significantly increased NHE3 activity and abundance in the apical microvillar domain of the proximal tubule, decreased the ratio of phosphorylated NHE3 at serine 552/total NHE3, but did not alter total cortical NHE3 abundance. All responses mediated by ESRN were completely abolished by a losartan-mediated AT1 receptor blockade. Taken together, our results demonstrate that higher NHE3-mediated proximal tubular sodium reabsorption induced by ESRN occurs via intrarenal renin angiotensin system activation and triggering of the AT1 receptor/inhibitory G-protein signaling pathway, which leads to inhibition of cAMP formation and reduction of PKA activity.

Orthostatic function after renal sympathetic denervation in patients with resistant hypertension

INTRODUCTION

Catheter-based renal denervation (RDN) reduces local and whole-body sympathetic activity and blood pressure (BP) in patients with resistant hypertension. However, safety concerns exist concerning the development of orthostatic dysfunction after RDN.

METHODS AND RESULTS

In 36 patients (65 ± 7.6 years, 75% male) with resistant hypertension (office BP 162 ± 24/91 ± 14 mm Hg) treated with 4.8 ± 1.7 antihypertensive drugs, tilt table testing (TTT) was performed before and three months after RDN. Response to RDN was defined as a reduction in office systolic BP (SBP) ≥ 10 mm Hg three months after RDN. Responders (n=26; 72.2%) and non-responders (n=10; 27.8%) were evaluated separately. After RDN, office SBP and diastolic BP (DBP) were reduced by 29 ± 6.2/14 ± 3.6 mm Hg (p<0.0001; p=0.0002) only in responders. During TTT, SBP and DBP in supine position were only reduced in responders. Resting heart rate (HR) decreased in responders but not in non-responders by 5.9 ± 1.7beats/min (p=0.0016). Mean and minimal SBP were not altered during passive tilting. In the responder group, ∆SBP was reduced in the initial phase of tilting. The adaptive increase of HR was preserved in both groups after RDN, while only in responders mean and minimal HR were reduced after passive tilting. Following drug provocation, mean and minimal SBP during all phases of passive tilting remained unchanged. ∆SBP, ∆HR and total number of (pre-)syncopes were neither influenced by RDN nor differing between responders and non-responders.

CONCLUSIONS

In patients with resistant hypertension, RDN reduced office BP, supine BP and HR during TTT without causing orthostatic dysfunction or (pre-)syncopes three months after treatment.

Flutamide increases aldosterone levels in gonadectomized male but not female Wistar rats

BACKGROUND

Sex-specific differences in blood pressure (BP) suggest an important modulating role of testosterone in the kidney. However, little is known about the interaction between androgens and the mineralocorticoid aldosterone. Our objective was to determine the effects of testosterone in gonadectomized male and female rats on a low-salt diet, and to determine the effect of androgen receptor (AR) blockade by flutamide on BP and on aldosterone levels.

METHODS

Normotensive male and female Wistar rats were gonadectomized and put on a low-salt diet. They were treated for 16 days with testosterone or placebo. In addition, the animals received the AR antagonist flutamide or placebo, respectively. BP was measured by tail-cuff method, 24-h urine samples were collected in metabolic cages and blood was collected for hormonal measurements.

RESULTS

Testosterone increased BP in males and females, and this effect could be blocked by flutamide. Flutamide treatment itself significantly increased aldosterone levels in male but not in female rats. These elevated aldosterone levels could be lowered by testosterone treatment during AR blockade. Accordingly to aldosterone levels, flutamide increased in males the serum sodium/potassium to urinary sodium/potassium ratio, an in vivo indicator of renal aldosterone action.

CONCLUSIONS

Testosterone regulates BP in male and female gonadectomized rats via the AR. Flutamide by itself exerts influence over aldosterone in the absence of gonadal steroid replacement suggesting AR involvement in renal sodium handling. These flutamide effects were sex-specific and not seen in female rats.

The complex interplay between cyclooxygenase-2 and angiotensin II in regulating kidney function

PURPOSE OF REVIEW

Cyclooxygenase-2 (COX-2) plays a critical role in modulating deleterious actions of angiotensin II (Ang II) where there is an inappropriate activation of the renin-angiotensin system (RAS). This review discusses the recent developments regarding the complex interactions by which COX-2 modulates the impact of an activated RAS on kidney function and blood pressure.

RECENT FINDINGS

Normal rats with increased COX-2 activity but with different intrarenal Ang II activity because of sodium restriction or chronic treatment with angiotensin-converting enzyme (ACE) inhibitors showed similar renal hemodynamic responses to COX-2-selective inhibition (nimesulide) indicating independence from the intrarenal Ang II activity. COX-2-dependent maintenance of medullary blood flow was consistent and not dependent on dietary salt or ACE inhibition. In contrast, COX-2 influences on sodium excretion were contingent on the prevailing RAS activity. In chronic hypertensive models, COX-2 inhibition elicited similar reductions in kidney function, but COX-2 metabolites contribute to rather than ameliorate the hypertension.

SUMMARY

The maintenance of renal hemodynamics reflects direct and opposing effects of Ang II and COX-2 metabolites. The antagonism in water and electrolyte reabsorption is dependent on the prevailing intrarenal Ang II activity. The recent functional experiments demonstrate a beneficial modulation of Ang II by COX-2 except in the presence of inflammation promoted by hypertension, hyperglycemia, and oxidative stress.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Normotensive sodium loading in conscious dogs: regulation of renin secretion during β-receptor blockade

Renin secretion is regulated in part by renal nerves operating through β1-receptors of the renal juxtaglomerular cells. Slow sodium loading may decrease plasma renin concentration (PRC) and cause natriuresis at constant mean arterial blood pressure (MAP) and glomerular filtration rate (GFR). We hypothesized that in this setting, renin secretion and renin-dependent sodium excretion are controlled by via the renal nerves and therefore are eliminated or reduced by blocking the action of norepinephrine on the juxtaglomerular cells with the β1-receptor antagonist metoprolol. This was tested in conscious dogs by infusion of NaCl (20 μmol·kg−1·min−1for 180 min, NaLoad) during regular or low-sodium diet (0.03 mmol·kg−1·day−1, LowNa) with and without metoprolol (2 mg/kg plus 0.9 mg·kg−1·h−1). Vasopressin V2receptors were blocked by Otsuka compound OPC31260 to facilitate clearance measurements. Body fluid volume was maintained by servocontrolled fluid infusion. Metoprolol per se did not affect MAP, heart rate, or sodium excretion significantly, but reduced PRC and ANG II by 30–40%, increased plasma atrial natriuretic peptide (ANP), and tripled potassium excretion. LowNa per se increased PRC (+53%), ANG II (+93%), and aldosterone (+660%), and shifted the vasopressin function curve to the left. NaLoad elevated plasma [Na+] by 4.5% and vasopressin by threefold, but MAP and plasma ANP remained unchanged. NaLoad decreased PRC by ∼30%, ANG II by ∼40%, and aldosterone by ∼60%, regardless of diet and metoprolol. The natriuretic response to NaLoad was augmented during metoprolol regardless of diet. In conclusion, PRC depended on dietary sodium and β1-adrenergic control as expected; however, the acute sodium-driven decrease in PRC at constant MAP and GFR was unaffected by β1-receptor blockade demonstrating that renin may be regulated without changes in MAP, GFR, or β1-mediated effects of norepinephrine. Low-sodium diet augments vasopressin secretion, whereas ANP secretion is reduced.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

COX-2 activity determines the level of renin expression but is dispensable for acute upregulation of renin expression in rat kidneys

The role of cyclooxygenase 2 (COX-2) in the control of renin is still a matter of debate, since studies with COX-2-deficient mice or with COX-2 inhibitors produced conflicting findings. Therefore, we studied the effect of the COX-2 inhibitor SC-58236 on the regulation of the renin system in adult rat kidneys. Renocortical tissue levels and urinary excretion of PGE(2) were reduced to 65 and 40% of control values, respectively, after a single gavage of SC-58236 and did not further decrease on prolonged treatment. Plasma renin activity (PRA) and renin mRNA levels began to decrease after 3 days and reached a constant level of approximately 60% of control values after 5 days of treatment. Isoproterenol or left renal artery clipping for 2 days increased PRA and renin mRNA to similar levels in both vehicle- and SC-58236-treated rats after 2 days. Pretreatment with SC-58236 for 5 days, however, reduced the absolute increase in PRA and renin mRNA levels. Notably, the relative increases were not different between vehicle- and SC-58236-treated rats. Similar findings were observed for the stimulation of the renin system by angiotensin II inhibition and low salt intake. These findings indicate that COX-2 inhibition attenuates renin secretion and renin gene expression stimulated by a variety of parameters in proportion to the lowering of basal renin activity, while it does not interfere with the different stimulatory mechanism per se. As a consequence, it appears as if COX-2 activity relevantly determines the set point of the activity of the renin system in rat kidneys.

COX-2 and the kidney

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used for the treatment of pain and inflammation. Nonselective NSAIDs inhibit both cyclooxygenase (COX)-1 and COX-2. Nephrotoxicity of nonselective NSAIDs has been well documented. The effects of selective COX-2 inhibitors on renal function and blood pressure are attracting increasing attention. In the kidney, COX-2 is constitutively expressed and is highly regulated in response to alterations in intravascular volume. COX-2 metabolites have been implicated in the mediation of renin release, regulation of sodium excretion, and maintenance of renal blood flow. Similar to nonselective NSAIDs, inhibition of COX-2 may cause edema and modest elevations in blood pressure in a minority of subjects. COX-2 inhibitors may also exacerbate preexisting hypertension or interfere with other antihypertensive drugs. Occasional acute renal failure has also been reported. Caution should be taken when COX-2 inhibitors are prescribed, especially in high-risk patients (including elderly patients and patients with volume depletion).

Neonatal sympathectomy reduces NADPH oxidase activity and vascular resistance in spontaneously hypertensive rat kidneys

Neonatal sympathectomy reduces arterial pressure in spontaneously hypertensive rats (SHR). In SHR transplanted with a kidney from sympathectomized SHR, arterial pressure was lower and less Na+ sensitive than in SHR transplanted with a kidney from hydralazine-treated SHR. This study was performed to identify underlying renal mechanisms. Tests for differential renal mRNA expression of nine a priori selected genes revealed robust differences for renal medullary expression of the NADPH oxidase subunit p47phox. Therefore, we investigated the effects of neonatal sympathectomy on renal mRNA expression of NADPH oxidase subunits, NADPH oxidase activity, and renal function. In 10-wk-old sympathectomized SHR fed a 0.6% NaCl diet, medullary p47phox and gp91phox expression was 40% less than in hydralazine-treated SHR. Also, after a 1.8% NaCl diet, medullary p47phox mRNA expression was lower in sympathectomized than in hydralazine-treated SHR. We found lower cortical (-30%, P<0.01) and medullary (-30%, P<0.05) NADPH oxidase activities in sympathectomized than in hydralazine-treated or untreated SHR. Glomerular filtration rate, renal blood flow, medullary blood flow, and fractional Na+ excretion in kidney grafts from sympathectomized and hydralazine-treated donors (n=8 per group) were similar at baseline and in response to a 20-mmHg rise in renal perfusion pressure. Renal vascular resistance was lower in kidneys from sympathectomized than hydralazine-treated donors (25+/-2 vs. 32+/-4 mmHg.min.ml-1, P<0.05). The results indicate that the sympathetic nervous system contributes to the level of renal NADPH oxidase activity and to perinatal programming of alterations in renal vascular function that lead to elevated renal vascular resistance in SHR.

Update on cyclooxygenase-2 inhibitors

Nonsteroidal anti-inflammatory drugs represent the most commonly used medications for the treatment of pain and inflammation, but numerous well-described side effects can limit their use. Cyclooxygenase-2 (COX-2) inhibitors were initially touted as a therapeutic strategy to avoid not only the gastrointestinal but also the renal and cardiovascular side effects of nonspecific nonsteroidal anti-inflammatory drugs. However, in the kidney, COX-2 is constitutively expressed and is highly regulated in response to alterations in intravascular volume. COX-2 metabolites have been implicated in mediation of renin release, regulation of sodium excretion, and maintenance of renal blood flow. This review summarizes the current state of knowledge about both renal and cardiovascular side effects that are attributed to COX-2 selective inhibitors.

Everolimus treatment downregulates renocortical cyclooxygenase-2 expression in the rat kidney

Based on recent evidence that renal cyclooxygenase-2 (COX-2) gene expression is suppressed by immunosuppressive agents such as cyclosporin A (CsA), tacrolimus and dexamethasone, this study aimed to characterize the effect of the new immunosuppressant everolimus on COX-2 expression in the rat kidney. Oral application of everolimus (3 mg kg(-1) day(-1)) to male Sprague-Dawley rats (175-200 g; n=8) for 7 days lowered COX-2 expression in the rat renal cortex and outer medulla, while COX-2 expression in the inner medulla as well as COX-1 expression remained unaltered. Furthermore, everolimus decreased renocortical prostaglandin (PG) E(2) concentration. Everolimus also attenuated the stimulation of renocortical COX-2 expression by furosemide (12 mg day(-1) for 7 days; s.c. via osmotic minipumps), by low salt intake (0.02% NaCl, wt wt(-1)) or by a combination of low salt intake with the AT(1)-receptor antagonist valsartan (30 mg kg(-1) day(-1); oral). In line with these findings, everolimus decreased renocortical PGE(2) concentration during these treatment maneuvers. Everolimus moderately increased natriuresis and diuresis, while the urinary excretion of PGE(2), 6-keto PGF(1alpha) and thromboxane B(2) was decreased. These findings suggest that everolimus inhibits basal and also stimulated expression of renocortical COX-2 and of tissue prostanoid formation. Since inhibition of renal prostanoid formation by everolimus was associated by an increased rather than decreased natriuresis and diuresis, it appears as if everolimus also inhibits tubular salt and water resorption.

Low- and high-level expressions of heme oxygenase-1 in cultured cells under uninduced conditions

Heme oxygenase-1 (HO-1) degrades heme into biliverdin, iron, and CO. The enzyme participates in adaptive and protective responses to oxidative stress and various inflammatory stimuli. We examined the regulation of HO-1 expression in culture cells under uninduced conditions. Observations by in situ hybridization and immunostaining showed that in cultured mouse fibroblast Balb/3T3 cells not subjected to treatment, 10-15% of cells highly expressed HO-1. The similar pattern of the expression of HO-1 was observed with mouse embryo liver BNL-CL2 cells and Chinese hamster ovary cells. The marked expression of HO-1 was related to the activation of stress-activated protein kinase and to the expression of cyclooxygenase (Cox)-2. When the cells were treated with arachidonic acid, a precursor of prostaglandin, induction of HO-1 in the HO-1-expressing cells but not in the low-expressing cells occurred. This increase was abrogated by the treatment with the Cox inhibitors, indomethacin, and dexamethasone. Neither prostaglandin H2, E2 nor F2a induced HO-1 expression. These results suggest that some cells respond to the cellular stress and intermediates of prostaglandin biosynthesis may act as endogenous stressors to induce HO-1.

Cyclooxygenase-2 and the renal renin-angiotensin system

In the kidney, cyclooxygenase-2 (COX-2) is expressed in the macula densa/cTALH and medullary interstitial cells. The macula densa is involved in regulating afferent arteriolar tone and renin release by sensing alterations in luminal chloride via changes in the rate of Na(+)/K(+)/2Cl(-) cotransport, and administration of non-specific cyclooxygenase inhibitors will blunt increases in renin release mediated by macula densa sensing of decreases in luminal NaCl. High renin states [salt deficiency, angiotensin converting enzyme (ACE) inhibitors or angiotensin receptor blockers, diuretic administration or experimental renovascular hypertension] are associated with increased macula densa/cTALH COX-2 expression. Furthermore, there is evidence that angiotensin II and/or aldosterone may inhibit COX-2 expression. In AT1 receptor knockout mice, COX-2 expression is increased similar to increases with ACE inhibitors or AT1 receptor blockers. Direct administration of angiotensin II inhibits macula densa COX-2 expression. Previous studies demonstrated that alterations in intraluminal chloride concentration are the signal for macula densa regulation of tubuloglomerular feedback and renin secretion, with high chloride stimulating tubuloglomerular feedback and low chloride stimulating renin release. When cultured cTALH or macula densa cells were incubated in media with selective substitution of chloride ions, COX-2 expression and prostaglandin production were significantly increased. A variety of studies have indicated a role for COX-2 in the macula densa mediation of renin release. In isolated perfused glomerular preparations, renin release induced by macula densa perfusion with a low chloride solution was inhibited by a COX-2 inhibitor but not a COX-1 inhibitor. In vivo studies in rats indicated that increased renin release in response to low-salt diet, ACE inhibitor, loop diuretics or aortic coarctation could be inhibited by administration of COX-2-selective inhibitors. In mice with genetic deletion of COX-2, ACE inhibitors or low-salt diet failed to increase renal renin expression, although renin significantly increased in wild type mice. In contrast, in COX-1 null mice there were no significant differences in either the basal or ACE inhibitor-stimulated level of renal renin activity from plasma or renal tissue compared with wild type mice. In summary, there is increasing evidence that COX-2 expression in the macula densa and surrounding cortical thick ascending limb cells is regulated by angiotensin II and is a modulator of renal renin production. These interactions of COX-2 derived prostaglandins and the renin-angiotensin system may underlie physiological and pathophysiological regulation of renal function.

Upregulation of macula densa cyclooxygenase-2 expression is not dependent on glomerular filtration

Although the regulation of cyclooxygenase-2 (COX-2) expression in the kidney cortex has been extensively characterized, the physiological control mechanisms of COX-2 expression at the level of the kidney and at the level of the tubular cells are not well understood. Based on the current hypothesis that tubular salt transport might be a crucial regulator of COX-2 expression, this study aimed to determine the impact of salt delivery to the tubules (glomerular filtration) for the regulation of COX-2 in the kidney cortex in vivo. To this end, glomerular filtration of the right kidney was abrogated by the ligation of the right ureter of male Sprague-Dawley rats. After 1 wk of ligation, the animals were treated with subcutaneous infusions of furosemide (12 mg·kg−1·day−1) or with a low-salt or a high-salt diet (0.02% wt/wt; 8% wt/wt), and COX-2 as well as renin mRNA expression were determined in the ligated and the nonligated contralateral kidney. During ureteral ligation, hydronephrosis developed with a reduction of medullary mass, while the cortex was preserved. Expressions of the Na-K-2Cl cotransporter isoforms A and B were both reduced in the hydronephrotic cortex to 70 and 35% of the corresponding contralateral intact kidney. Despite the abrogation of glomerular filtration, detected by inulin clearance measurements, renocortical COX-2 mRNA abundance was stimulated by furosemide treatment (3.2-fold) or low-salt diet (2.9-fold) to similar degrees compared with the intact contralateral kidney (2.7-fold for both treatments), whereas a high-salt diet did not significantly suppress COX-2 mRNA in the macula densa region of either kidney. Renin mRNA expression was regulated strictly in parallel in both kidneys, a low-salt diet or furosemide treatment stimulating and a high-salt diet suppressing it. We conclude from these findings that salt delivery to the tubules is not an essential requirement for the upregulation of COX-2 by salt deficiency or by loop diuretics in the rat kidney cortex nor is it for chronic stimulation of renin mRNA expression.

Cyclooxygenases, the kidney, and hypertension

Selective cyclooxygenase (COX)-2 inhibitors that are in widespread clinical use were developed to avoid side effects of conventional NSAIDs, including gastrointestinal and renal toxicity. However, COX-2 is constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. COX-2 metabolites have been implicated in maintenance of renal blood flow, mediation of renin release, and regulation of sodium excretion. COX-2 inhibition may transiently decrease urine sodium excretion in some subjects and induce mild to moderate elevation of blood pressure. Furthermore, in conditions of relative intravascular volume depletion and/or renal hypoperfusion, interference with COX-2 activity can have deleterious effects on maintenance of renal blood flow and glomerular filtration rate. In addition to physiological regulation of COX-2 expression in the kidney, increased renal cortical COX-2 expression is seen in experimental models associated with altered renal hemodynamics and progressive renal injury (decreased renal mass, poorly controlled diabetes), and long-term treatment with selective COX-2 inhibitors ameliorates functional and structural renal damage in these conditions.

Role of macula densa cyclooxygenase-2 in renovascular hypertension

Upregulation of the inducible cyclooxygenase (COX-2) in the macula densa accompanies the activation of the juxtaglomerular apparatus in many high-renin conditions. The functional role of COX-2 in these disease states is poorly understood. We tested whether COX-2 is required to increase renin in renovascular hypertension. Rats with established two-kidney, one-clip (2K1C) hypertension were treated for 2 wk with two different inhibitors of COX-2, NS-398 and rofecoxib, respectively. Hypertension in 2K1C rats was not affected or slightly enhanced by COX-2 inhibition, as measured intra-arterially in conscious animals. The increase in plasma renin activity was also unchanged by both rofecoxib and NS-398. The number of glomeruli with a renin-positive juxtaglomerular apparatus was elevated in clipped kidneys and decreased in contralateral kidneys of 2K1C rats. This pattern was unaltered by COX-2 inhibition. To test the effects of COX-2 blockade on a primarily macula densa-mediated stimulus, we studied salt depletion for comparison. A low-salt diet induced a significant increase in plasma renin activity, which was partially inhibited by treatment with NS-398. We conclude that inhibition of COX-2 in established renovascular hypertension does not affect renin synthesis or release. Thus either COX-2 is not necessary for the macula densa mechanism or the macula densa is not important for maintaining high renin in renovascular hypertension.

Role of nitric oxide and cyclooxygenase-2 in regulating the renal hemodynamic response to norepinephrine

We have reported that the renal hemodynamic effects of norepinephrine (NE) are modulated by cyclooxygenase-2 (COX-2)-derived metabolites. Our main objective was to examine whether there is an interaction between nitric oxide (NO) and COX-2 in modulating the renal hemodynamic effects of NE. NE was infused at three doses to anesthetized dogs pretreated with vehicle (n = 8), a selective COX-2 inhibitor (nimesulide) (n = 6), an NO synthesis inhibitor [NG-nitro-l-arginine methyl ester; l-NAME] (n = 8), or with nimesulide and l-NAME (n = 5). During NE infusion, PGE2 excretion increased (125%) in the control group and did not change in the l-NAME-treated dogs. The simultaneous inhibition of NO and COX-2 potentiated to a greater extent the NE-induced renal vasoconstriction than inhibition of either NO or COX-2. The NE-induced renal vasoconstriction during NO and COX-2 inhibition was reduced (P < 0.05) by infusing an AT1 receptor antagonist (n = 6). These results suggest that there is an interaction between NO and COX-2 in protecting the renal vasculature from the NE effects and that angiotensin II partly mediates the NE-induced renal vasoconstriction when NO synthesis and COX-2 activity are reduced.

Cyclooxygenase-2 inhibition attenuates lipopolysaccharide-induced cardiovascular failure

The present study aimed to determine the relevance of cyclooxygenase-2 (COX-2)-derived prostanoids for the adverse effects of lipopolysaccharides (LPSs) on cardiovascular function. For this goal, male Sprague-Dawley rats received a single intravenous dose of LPS (10 mg/kg) and were treated with different cyclooxygenase inhibitors. Injection of LPS caused a marked decrease of systolic arterial pressure, from 128 to 79 mm Hg, and a concomitant increase of heart rate, from 380 to 530 minutes(-1). Both the decrease of systemic arterial pressure and the increase of heart rate induced by LPS were almost absent if the animals also received the COX-2 blocker rofecoxib (20 mg/kg), regardless whether the drug was given 1 hour before or 1 hour after LPS. Although plasma and organ levels of prostanoids were lowered by rofecoxib, the characteristic LPS-induced increases of NO synthase II and COX-2 gene expression, as well as of plasma and tissue nitrate/nitrite concentrations, were not affected by rofecoxib. Although rofecoxib treatment did also not change LPS-induced tissue cytokine concentrations, it markedly improved LPS-induced liver damage, as indicated by the decrease of transaminases. Moreover, the overall well-being of the LPS-injected animals improved on concomitant treatment with the COX-2 inhibitor. Taken together, our data suggest that COX-2-derived prostanoids are major mediators for the detrimental effects of LPS on cardiovascular and organ function.