Effect of captopril and angiotensin II receptor blockade on pressure natriuresis in transgenic TGR(mRen-2)27 rats

Evidence That Binding of Cyclic GMP to the Extracellular Domain of NKA (Sodium-Potassium ATPase) Mediates Natriuresis

BACKGROUND

Extracellular renal interstitial guanosine cyclic 3',5'-monophosphate (cGMP) inhibits renal proximal tubule (RPT) sodium (Na+) reabsorption via Src (Src family kinase) activation. Through which target extracellular cGMP acts to induce natriuresis is unknown. We hypothesized that cGMP binds to the extracellular α1-subunit of NKA (sodium-potassium ATPase) on RPT basolateral membranes to inhibit Na+ transport similar to ouabain-a cardiotonic steroid.

METHODS

Urine Na+ excretion was measured in uninephrectomized 12-week-old female Sprague-Dawley rats that received renal interstitial infusions of vehicle (5% dextrose in water), cGMP (18, 36, and 72 μg/kg per minute; 30 minutes each), or cGMP+rostafuroxin (12 ng/kg per minute) or were subjected to pressure-natriuresis±rostafuroxin infusion. Rostafuroxin is a digitoxigenin derivative that displaces ouabain from NKA.

RESULTS

Renal interstitial cGMP and raised renal perfusion pressure induced natriuresis and increased phosphorylated SrcTyr416 and Erk 1/2 (extracellular signal-regulated protein kinase 1/2)Thr202/Tyr204; these responses were abolished with rostafuroxin coinfusion. To assess cGMP binding to NKA, we performed competitive binding studies with isolated rat RPTs using bodipy-ouabain (2 μM)+cGMP (10 µM) or rostafuroxin (10 µM) and 8-biotin-11-cGMP (2 μM)+ouabain (10 μM) or rostafuroxin (10 µM). cGMP or rostafuroxin reduced bodipy-ouabain fluorescence intensity, and ouabain or rostafuroxin reduced 8-biotin-11-cGMP staining. We cross-linked isolated rat RPTs with 4-N3-PET-8-biotin-11-cGMP (2 μM); 8-N3-6-biotin-10-cAMP served as negative control. Precipitation with streptavidin beads followed by immunoblot analysis showed that RPTs after cross-linking with 4-N3-PET-8-biotin-11-cGMP exhibited a significantly stronger signal for NKA than non-cross-linked samples and cross-linked or non-cross-linked 8-N3-6-biotin-10-cAMP RPTs. Ouabain (10 μM) reduced NKA in cross-linked 4-N3-PET-8-biotin-11-cGMP RPTs confirming fluorescence staining. 4-N3-PET-8-biotin-11-cGMP cross-linked samples were separated by SDS gel electrophoresis and slices corresponding to NKA molecular weight excised and processed for mass spectrometry. NKA was the second most abundant protein with 50 unique NKA peptides covering 47% of amino acids in NKA. Molecular modeling demonstrated a potential cGMP docking site in the ouabain-binding pocket of NKA.

CONCLUSIONS

cGMP can bind to NKA and thereby mediate natriuresis.

Effects of empagliflozin and target-organ damage in a novel rodent model of heart failure induced by combined hypertension and diabetes

Type 2 diabetes mellitus and hypertension are two major risk factors leading to heart failure and cardiovascular damage. Lowering blood sugar by the sodium-glucose co-transporter 2 inhibitor empagliflozin provides cardiac protection. We established a new rat model that develops both inducible diabetes and genetic hypertension and investigated the effect of empagliflozin treatment to test the hypothesis if empagliflozin will be protective in a heart failure model which is not based on a primary vascular event. The transgenic Tet29 rat model for inducible diabetes was crossed with the mRen27 hypertensive rat to create a novel model for heart failure with two stressors. The diabetic, hypertensive heart failure rat (mRen27/tetO-shIR) were treated with empagliflozin (10 mg/kg/d) or vehicle for 4 weeks. Cardiovascular alterations were monitored by advanced speckle tracking echocardiography, gene expression analysis and immunohistological staining. The novel model with increased blood pressure und higher blood sugar levels had a reduced survival compared to controls. The rats develop heart failure with reduced ejection fraction. Empagliflozin lowered blood sugar levels compared to vehicle treated animals (182.3 ± 10.4 mg/dl vs. 359.4 ± 35.8 mg/dl) but not blood pressure (135.7 ± 10.3 mmHg vs. 128.2 ± 3.8 mmHg). The cardiac function was improved in all three global strains (global longitudinal strain - 8.5 ± 0.5% vs. - 5.5 ± 0.6%, global radial strain 20.4 ± 2.7% vs. 8.8 ± 1.1%, global circumferential strain - 11.0 ± 0.7% vs. - 7.6 ± 0.8%) and by increased ejection fraction (42.8 ± 4.0% vs. 28.2 ± 3.0%). In addition, infiltration of macrophages was decreased by treatment (22.4 ± 1.7 vs. 32.3 ± 2.3 per field of view), despite mortality was not improved. Empagliflozin showed beneficial effects on cardiovascular dysfunction. In this novel rat model of combined hypertension and diabetes, the improvement in systolic and diastolic function was not secondary to a reduction in left ventricular mass or through modulation of the afterload, since blood pressure was not changed. The mRen27/tetO-shIR strain should provide utility in separating blood sugar from blood pressure-related treatment effects.

Study on screening potential allergenic proteins from infant milk powders based on human mast cell membrane chromatography and histamine release assays

Cow's milk allergy is mainly observed in infants and young children. Most allergic reactions affect the skin, followed by the gastrointestinal and respiratory systems. Conventional diagnosis is based on positive allergy studies and evaluation of parameters including IgE and IgG1 levels, acute allergic skin response and anaphylactic shock reactions. We developed a cell membrane chromatographic (CMC) method based on human mast cells (HMC-1) for screening potential allergens in infant formula milk powders (IFMP). HMC-1 cell membranes were extracted and mixed with silica to prepare cell membrane chromatography columns (10 mm × 2 mm i.d., 5 µm). Under the conditions of 0.2 mL/min flow rate and 214 nm detection wavelength, human breast milk showed no retention. However, IFMP showed clear retention. The retained fractions were collected and analyzed through matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). Four major milk proteins, i.e., α-casein, β-casein, α-lactalbumin, and β-lactoglobulin A, were identified. Furthermore, these proteins and β-lactoglobulin B showed clear retention on HMC-1/CMC columns. To test the degranulation effects of the five proteins, histamine and β-hexosaminidase release assays were carried out. All five proteins induced HMC-1 cells to release histamine and β-hexosaminidase. Also, we established a reversed phase liquid chromatographic (RPLC) method for the determination of the five proteins in IFMP and the results showed that 90% proteins in IFMP were α-casein and β-casein. We concluded that cow's milk proteins may be potential allergens and caseins cause more β-casein allergic risk than other proteins. This conclusion was consistent with other studies.

Angiotensin II type 1 receptor antagonists in animal models of vascular, cardiac, metabolic and renal disease

We have reviewed the effects of angiotensin II type 1 receptor antagonists (ARBs) in various animal models of hypertension, atherosclerosis, cardiac function, hypertrophy and fibrosis, glucose and lipid metabolism, and renal function and morphology. Those of azilsartan and telmisartan have been included comprehensively whereas those of other ARBs have been included systematically but without intention of completeness. ARBs as a class lower blood pressure in established hypertension and prevent hypertension development in all applicable animal models except those with a markedly suppressed renin-angiotensin system; blood pressure lowering even persists for a considerable time after discontinuation of treatment. This translates into a reduced mortality, particularly in models exhibiting marked hypertension. The retrieved data on vascular, cardiac and renal function and morphology as well as on glucose and lipid metabolism are discussed to address three main questions: 1. Can ARB effects on blood vessels, heart, kidney and metabolic function be explained by blood pressure lowering alone or are they additionally directly related to blockade of the renin-angiotensin system? 2. Are they shared by other inhibitors of the renin-angiotensin system, e.g. angiotensin converting enzyme inhibitors? 3. Are some effects specific for one or more compounds within the ARB class? Taken together these data profile ARBs as a drug class with unique properties that have beneficial effects far beyond those on blood pressure reduction and, in some cases distinct from those of angiotensin converting enzyme inhibitors. The clinical relevance of angiotensin receptor-independent effects of some ARBs remains to be determined.

Renal autoregulation in health and disease

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

Impaired pressure natriuresis is associated with interstitial inflammation in salt-sensitive hypertension

PURPOSE OF REVIEW

Impairment of the pressure natriuresis relationship is a central event in the pathogenesis of hypertension. Renal tubulointerstitial inflammation results in salt-sensitive hypertension and, until recently, the changes in pressure natriuresis induced by renal inflammation received little attention.

RECENT FINDINGS

Oxidative stress and increased intrarenal angiotensin II activity, in association with rarefaction and loss of peritubular vascular network, may be involved in the inflammation-induced blunting of the natriuresis resulting from increments in renal perfusion pressure.

SUMMARY

Here, we review the mechanisms for the impairment in pressure natriuresis resulting from renal tubulointerstitial inflammation in reference to the normal physiologic mechanisms involved in this response.

Recovery from renal ischemia-reperfusion injury is associated with altered renal hemodynamics, blunted pressure natriuresis, and sodium-sensitive hypertension

The present studies evaluated intrarenal hemodynamics, pressure natriuresis, and arterial blood pressure in rats following recovery from renal ischemia-reperfusion (I/R) injury. Acute I/R injury, induced by 40 min of bilateral renal arterial occlusion, resulted in an increase in plasma creatinine that resolved within a week. Following 5 wk of recovery on a 0.4% NaCl diet, the pressure-natriuresis response was assessed in anesthetized rats in which the kidney was denervated and extrarenal hormones were administered intravenously. Increasing renal perfusion pressure (RPP) from 107 to 141 mmHg resulted in a fourfold increase in urine flow and sodium excretion in sham control rats. In comparison, pressure diuresis and natriuresis were significantly attenuated in post-I/R rats. In sham rats, glomerular filtration rate (GFR) averaged 1.6 +/- 0.2 mlxmin(-1)xg kidney weight(-1) and renal blood flow (RBF) averaged 7.8 +/- 0.7 mlxmin(-1)xg kidney weight(-1) at RPP of 129 mmHg. Renal cortical blood flow, measured by laser-Doppler flowmetry, was well autoregulated whereas medullary blood flow and renal interstitial hydrostatic pressure increased directly with elevated RPP in sham rats. In contrast, GFR and RBF were significantly reduced whereas medullary perfusion and interstitial pressure demonstrated an attenuated response to RPP in post-I/R rats. Further experiments demonstrated that conscious I/R rats develop hypertension when sodium intake is increased. The present data indicate that the pressure-natriuretic-diuretic response in I/R rats is blunted because of a decrease in GFR and RBF and the depressed pressure-dependent increase in medullary blood flow and interstitial pressure.

The in vivo effects of Tulbhagia violacea on blood pressure in a salt-sensitive rat model

AIM OF THE STUDY

The in vivo effects of Tulbhagia violacea on systemic arterial blood pressure and on the renin-angiotensin system in a Dahl salt-sensitive rat model were investigated.

MATERIALS AND METHODS

Animals were treated for 14 days intraperitoneally as follows: Tulbhagia violacea (Tvl) (50mg/kg b.w.), captopril (Cap) (10mg/kg b.w.) or DMSO (Con). Baseline blood pressures were recorded prior to the commencement of the study and biweekly during the experimental period. Urine volume and sodium concentration were measured during the experimental period. On day 15, animals were anaesthetized (sodium thiopentane, 50mg/kg, i.p.), blood samples for aldosterone levels were taken and the kidneys removed for determining AT1a mRNA expression.

RESULTS

Cap and Tvl groups showed significantly reduced AT1a mRNA expressions by 3.11- and 5.03-fold, respectively, when compared to the Con group (p<0.05). When compared to baseline blood pressures (day 0); Cap and Tvl showed reductions in systolic blood pressure (SBP) of 7.76+/-0.41% and 9.12+/-0.31%, respectively (mean% decrease from day 0 to day 14). In contrast, in the Con group the systolic blood pressure increased from day 0 to day 14 by 4.66+/-0.56%. Blood pressure changes in all treated groups differed from Con significantly. Systolic blood pressure decreased with the decrease in AT1a mRNA expressions in these groups. When comparing day 0 to day 14, urine output increased in the Cap and Tvl groups. In the Con group, urinary volume was reduced by day 14 as compared to day 0. Urinary sodium excretion was increased in the treated groups by day 14.

CONCLUSION

It can be concluded that Tulbhagia violacea reduces systemic arterial blood pressure in the Dahl rat by decreasing renal AT1 receptor gene expression and hence modulating sodium and water homeostasis.

Superoxide and its interaction with nitric oxide modulates renal function in prehypertensive Ren-2 transgenic rats

OBJECTIVE

The present study was performed to examine the role of superoxide (O2*) and its interaction with nitric oxide (NO) in the regulation of renal function in prehypertensive heterozygous Ren-2 transgenic rats (TGR).

METHODS

Renal responses to the O2* scavenger, tempol (150 microg/min per 100 g), and/or the NO synthase inhibitor, nitro-L-arginine methylester (L-NAME; 5 microg/min per 100 g), infused alone or in combination directly into the renal artery were evaluated in anesthetized heterozygous male TGR and aged-matched Hanover Sprague-Dawley rats (HanSD).

RESULTS

There were no differences in arterial pressure (122 +/- 3 versus 115 +/- 2 mmHg), renal plasma flow (RPF; 2.09 +/- 0.1 versus 2.07 +/- 0.1 ml/min per g), glomerular filtration rate (GFR; 0.73 +/- 0.1 versus 0.74 +/- 0.1 ml/min per g) or sodium excretion (0.63 +/- 0.13 versus 0.67 +/- 0.16 micromol/min per g) between TGR and HanSD. Tempol alone caused significant increases in RPF and GFR (10 +/- 4% and 12 +/- 2%, respectively) in TGR but not in HanSD. Tempol also caused greater sodium excretory responses in TGR compared to HanSD (112 +/- 16% versus 43 +/- 7%; P < 0.05). 8-Isoprostane excretion was significantly higher in TGR than in HanSD (10.2 +/- 0.8 versus 6.5 +/- 0.7 pg/min per g), which was attenuated by tempol. L-NAME caused greater decreases in RPF and GFR in TGR (-34 +/- 4% and -22 +/- 4%, respectively) than in HanSD (-19 +/- 3% and -10 +/- 4%, respectively). Co-infusion of tempol partially attenuated the renal hemodynamic and excretory responses to L-NAME in TGR.

CONCLUSIONS

These data suggest that the enhanced O2* activity and its interaction with NO during the prehypertensive phase in TGR modulates renal hemodynamic and excretory function, which may contribute to the development of hypertension in this transgenic rat model.

Effects of Dietary Salt Load and Salt Depletion on the Course of Hypertension and Angiotensin II Levels in Male and Female Heterozygous Ren-2 Transgenic Rats

BACKGROUND

In the present study we evaluated plasma and kidney angiotensin II (ANG II) levels in female and male Ren-2 transgenic rats (TGR) in comparison to age-matched female and male normotensive Hannover Sprague-Dawley rats.

METHODS

The rats were maintained on a normal sodium (NS) diet (0.6% NaCl) or fed a high sodium (HS) diet (2% NaCl) for 4 days or were sodium depleted by administration of 40 mg furosemide per liter drinking water overnight followed by 3 days of low sodium diet (0.01% NaCl) (LS + F). ANG II levels were determined by radioimmunoassay.

RESULTS

Female TGR at the age of 38 days were already hypertensive and had developed cardiac hypertrophy, whereas male TGR at this age still exhibited a normotensive phenotype. HS diet increased the blood pressure (BP) but did not alter the ANG II levels in TGR at any age. LS + F decreased the BP without significant change in ANG II concentrations in TGR. Female TGR responded to salt loading and salt depletion by more pronounced changes in BP than male TGR.

CONCLUSIONS

Female TGR develop hypertension more rapidly and the salt-sensitive component of hypertension is more pronounced in female than in male TGR.

Estrogen and salt sensitivity in the female mRen(2).Lewis rat

The present study determined whether early loss of estrogen influences salt-sensitive changes in blood pressure, renal injury, and cardiac hypertrophy as well as the effects on the circulating renin-angiotensin-aldosterone system (RAAS) in the hypertensive female mRen( 2 ).Lewis strain. Ovariectomy (OVX) of heterozygous mRen( 2 ).Lewis rats on a normal salt (NS) diet (0.5% sodium) increased systolic blood pressure from 137 ± 3 to 177 ± 5 mmHg ( P < 0.01) by 15 wk but did not show any changes in cardiac-to-body weight index (CI), proteinuria, or creatinine clearance. Maintenance with a high-sodium (HS) diet (4%) increased blood pressure (203 ± 4 mmHg, P < 0.01), proteinuria (3.5 ± 0.3 vs. 6.4 ± 0.7 mg/day, P < 0.05), and CI (4.0 ± 0.1 vs. 5.2 ± 0.1 mg/kg, P < 0.01) but decreased creatinine clearance (0.89 ± 0.15 vs. 0.54 ± 0.06 ml/min, P < 0.05). OVX exacerbated the effects of salt on the degree of hypertension (230 ± 5 mmHg), CI (5.6 ± 0.2 mg/kg), and proteinuria (13 ± 3.0 mg/day). OVX increased the urinary excretion of aldosterone approximately twofold in animals on the NS diet (3.8 ± 0.5 vs. 6.6 ± 0.5 ng·mg creatinine−1·day−1, P < 0.05) and HS diet (1.4 ± 0.2 vs. 4.5 ± 1.0 ng·mg creatinine−1·day−1, P < 0.05). Circulating renin, angiotensin-converting enzyme, and angiotensin II were also significantly increased in the OVX group fed a HS diet. These results reveal that the protective effects of estrogen apart from the increase in blood pressure were only manifested in the setting of a chronic HS diet and suggest that the underlying sodium status may have an important influence on the overall effect of reduced estrogen.

Effects of changes in sodium balance on plasma and kidney angiotensin II levels in anesthetized and conscious Ren-2 transgenic rats

OBJECTIVE

Since there is as yet no general agreement regarding the role of plasma and kidney angiotensin II (ANG II) in the development of hypertension in Ren-2 transgenic rats (TGR), in the present study we evaluated plasma and kidney ANG II levels in anesthetized and conscious TGR and in normotensive Hannover-Sprague-Dawley rats (HanSD) fed a normal salt diet (NS). Given the importance of ANG II in the development of salt-sensitive hypertension, and the fact that hypertensinogenic actions of ANG II are mediated via ANG II type 1 (AT1) receptors, the effects of high salt (HS) intake and of sodium depletion on blood pressure (BP), ANG II levels and kidney AT1 receptor protein expression in TGR and HanSD were also examined.

METHODS

Rats were maintained on a NS diet (0.6% NaCl) or fed a HS diet (2% NaCl) for 4 days or were sodium depleted (40 mg/l furosemide for 1 day followed by 3 days of 0.01% NaCl diet). They were sacrificed either by an overdose of anesthetic (thiopental sodium) or by decapitation (without anesthetic) and plasma and kidney ANG II levels were determined by radioimmunoassay during the prehypertensive (32 days old), the early (52 days) and the maintenance (90 days) phases of hypertension. Total kidney AT1 receptor protein levels were assessed by Western blot analysis.

RESULTS

In anesthetized animals fed the NS diet, plasma ANG II levels were lower in 32-day-old TGR than in HanSD, but at 52 and 90 days of age no significant differences were noted. ANG II concentrations in kidney tissue were similar in 32- and 90-day-old TGR and HanSD, but were higher in 52-day-old TGR than in HanSD. In contrast, in conscious animals immediately after decapitation, plasma and kidney ANG II levels were higher in TGR than in HanSD at all ages. HS diet did not change BP but suppressed ANG II levels in HanSD at all ages. In contrast, HS diet increased BP but did not decrease plasma and kidney ANG II levels in TGR at all ages. Sodium restriction did not alter BP and resulted in a marked increase in ANG II levels in HanSD, but caused a significant decrease in BP in TGR without altering plasma or tissue ANG II concentrations. There were no significant differences in renal AT1 receptor protein expression between HanSD and TGR at any age of any of the experimental groups.

CONCLUSIONS

On the basis of our present results we conclude that TGR exhibit a disrupted interaction between sodium homeostasis and the regulation of the renin-angiotensin system (RAS) activity which results in the loss of BP regulation in this model.

The lord of the ring: mandatory role of the kidney in drug therapy of hypertension

Strong evidence supports the idea that total peripheral resistance (TPR) is increased in all forms of human and experimental hypertension. Although the etiological participation of TPR in the origin and long-term maintenance of hypertension has been extensively debated, it now seems clear that the renal, nonadaptive, infinite gain-working, pressure-sensitive natriuresis and diuresis is the main mechanism of blood pressure control in the long term. The tissue, cellular, biochemical, and genetic sensors and executors of this process have not been fully identified yet, but the role of the renal medulla has gained growing attention as the physiopathological scenario in which the key regulatory elements reside. Specifically, the functionality of the renomedullary vasculature seems to be highly responsible for blood pressure control. The vasculature of the renal medulla becomes a new and more specific target for the therapeutic intervention of hypertension. Recent data on the effect of baroreceptor-controlled renal sympathetic activity on the long-term regulation of blood pressure are integrated. The renomedullary effects of the main antihypertensive drugs are discussed, and new perspectives for the therapeutic intervention of hypertension are outlined. Comparison of the genetic program of the renal medulla before and after the development of hypertension in spontaneously hypertensive and experimentally induced animal models might provide a mechanism for identifying the key genes that become activated or suppressed in the development of high blood pressure. These genes, their encoded proteins, or other elements related to their signalling and genetic pathways might serve as new and more specific targets for the pharmacological treatment of abnormally elevated blood pressure. Besides, proteins specifically located to the luminal side of the renomedullary vascular endothelium may serve as potential targets for site-directed drug and gene therapy.

The role of intrarenal angiotensin II in the development of hypertension in Ren-2 transgenic rats

OBJECTIVE

We investigated the responses of mean arterial pressure and renal blood flow to intravenous and intrarenal angiotensin II, plasma and kidney angiotensin II concentrations and renal angiotensin receptor subtype 1 protein expression, and renal functional responses to intravenous and intrarenal angiotensin receptor 1 blockade with candesartan.

METHODS

In male anaesthetized transgenic rats and Hannover Sprague-Dawley rats aged 36-38 days mean arterial pressure and renal blood flow were determined after intravenous and intrarenal boluses of angiotensin II. Mean arterial pressure, renal blood flow and sodium excretion after intravenous or intrarenal candesartan were studied. Plasma and kidney angiotensin II concentrations were determined by radioimmunoassay. Renal angiotensin receptor subtype 1 protein levels were analysed by immunoblotting.

RESULTS

The responses of mean arterial pressure and renal blood flow to angiotensin II were significantly greater in transgenic than in Hannover Sprague-Dawley rats. The administration of candesartan resulted in comparable decreases in mean arterial pressure and increases in renal blood flow and sodium excretion in both groups of rats. Renal angiotensin receptor subtype 1 protein levels were no different between Hannover Sprague-Dawley and transgenic rats.

CONCLUSIONS

Plasma and kidney angiotensin II levels were lower in anaesthetized transgenic rats but, in contrast, were higher in decapitated transgenic rats when compared with Hannover Sprague-Dawley rats, suggesting that the kidney function of prehypertensive transgenic rats is under inappropriately high angiotensin II-dependent influence.

Roles of Nitric Oxide and Oxidative Stress in the Regulation of Blood Pressure and Renal Function in Prehypertensive Ren-2 Transgenic Rats

AIMS

The present study was performed to evaluate the role of nitric oxide (NO) and its interaction with superoxide anion (O2-) in the regulation of blood pressure (BP) and renal function during the developmental phase of hypertension in Ren-2 transgenic rats (TGR). The first aim was to compare BP and renal functional responses to acute NO synthase (NOS) inhibition achieved by intravenous (i.v.) infusion of Nomega-nitro-L-arginine-methyl ester (L-NAME) in prehypertensive heterozygous TGR and in transgene-negative Hannover Sprague-Dawley (HanSD) rats. The second aim was to evaluate whether scavenging of O2- by infusion of the superoxide dismutase mimetic tempol increases NO bioavailability which therefore should augment BP and renal functional responses to L-NAME.

METHODS

Rats were anesthetized, prepared for clearance experiments and BP and renal functional responses were evaluated in response to i.v. L-NAME administration (20 microg.100 g(-1).min(-1)) without or with tempol pretreatment (i.v., 300 microg.100 g(-1).min(-1)). In renal cortical tissue, nitrotyrosine protein expression was assessed by immunoblotting as marker of O2- production and urinary 8-epi-PGF(2alpha) excretion as marker of intrarenal oxidative stress was assessed by enzyme immunoassay.

RESULTS

BP, glomerular filtration rate (GFR), renal plasma flow (RPF) and sodium excretion were similar in TGR and HanSD. L-NAME infusion induced greater increases in BP in TGR than in HanSD (+42 +/- 4 vs. +25 +/- 3 mmHg, p < 0.05). In the absence of a significant change in GFR, L-NAME caused similar decreases in RPF (-32 +/- 6 and -25 +/- 4%, p < 0.05) in TGR and HanSD. Despite significantly higher renocortical expression of nitrotyrosine and urinary 8-epi-PGF2alpha excretion in TGR than in HanSD, pretreatment with tempol did not augment the rise in BP and the decrease in RPF induced by L-NAME.

CONCLUSIONS

The greater BP response to L-NAME in TGR suggests that prehypertensive TGR exhibit an enhanced NO activity in the systemic vasculature as compared with HanSD. Despite increased intrarenal oxidative stress in TGR, the dependency of the intrarenal vascular tone on NO appears to be similar in TGR and HanSD. The lack of a compensatory increase in renal NO activity may partially account for the enhanced renal vascular response to ANG II present in TGR.

Acute Effects of Cyclooxygenase-2 Inhibition on Renal Function in Heterozygous Ren-2-Transgenic Rats on Normal or Low Sodium Intake

BACKGROUND/AIMS

Since there are no data available so far on the role of renal cyclooxygenase-2 (COX-2) in hypertensive Ren-2-transgenic rats (TGR), in the present study we evaluated renal cortical COX-2 protein expression and prostaglandin E2 (PGE2) concentrations as well as renal functional responses to acute COX-2 inhibition in male heterozygous TGR and in normotensive Hannover Sprague-Dawley (HanSD) rats fed either a normal-sodium (NS) or a low-sodium (LS) diet.

METHODS

In rats fed either the NS or the LS diet for 12 days and prepared for clearance experiments with left ureteral catheterization, the renal functional responses of the left kidney were evaluated after intrarenal COX-2 inhibition with DuP-697 or NS-398. In renal cortical tissue, COX-2 protein expression was assessed by immunoblotting, and the concentration of PGE2 as a marker of COX-2 activity was determined by enzyme immunoassay. Mean arterial pressure in the right femoral artery was monitored by means of a pressure transducer.

RESULTS

In heterozygous TGR, to our surprise, the LS diet normalized the mean arterial pressure. Despite significantly higher renocortical expression of COX-2 and PGE2 concentrations as well as urinary PGE2 excretion in TGR as compared with HanSD rats kept on the NS diet, selective intrarenal COX-2 inhibition did not influence renal function either in TGR or in HanSD rats. The LS diet increased renocortical COX-2 expression and PGE2 concentrations as well as urinary PGE2 excretion significantly stronger in TGR than in HanSD rats. Regardless of these increases, the intrarenal COX-2 inhibition caused comparable decreases in glomerular filtration rate, in absolute and fractional sodium excretion, as well as in urinary PGE2 excretion in TGR and HanSD rats kept on the LS diet.

CONCLUSIONS

The present data show that a LS diet normalizes the mean arterial pressure in heterozygous male TGR. This first study on the role of renal COX-2 in TGR also demonstrates that COX-2-derived vasodilatory prostanoids do not act as renal compensatory vasodilator and natriuretic substances in this model of hypertension.

Plasma and kidney angiotensin II levels and renal functional responses to AT1 receptor blockade in hypertensive Ren-2 transgenic rats

OBJECTIVE

The first aim of the present study was to assess plasma and kidney angiotensin II (ANG II) levels and renal cortical ANG II receptor subtype 1A (AT1A) mRNA expression in hypertensive Ren-2 transgenic rats (TGR) and in normotensive Hannover Sprague-Dawley (HanSD) rats. The second aim was to investigate potential differences between TGR and HanSD in blood pressure (BP) and renal functional responses to either intravenous (i.v.), i.e. systemic, or intrarenal (i.r.) AT1 receptor blockade with candesartan.

METHODS

Rats were anesthetized and prepared for clearance experiments. In series 1, ANG II concentrations were assayed by radioimmunoassay and renal cortical AT1A mRNA expression by semiquantitative reverse transcriptase-polyacrylamide gel electrophoresis. In series 2, BP and renal functional responses were evaluated after either i.v. or i.r. bolus administration of candesartan.

RESULTS

Plasma and kidney ANG II levels were significantly lower in TGR than in HanSD (39 +/- 5 versus 107 +/- 19 fmol/ml and 251 +/- 41 versus 571 +/- 95 fmol/g, respectively, P < 0.05). Renal AT1A mRNA expression was not different between TGR and HanSD. Intravenous candesartan caused comparable decreases in BP in TGR and HanSD and did not change renal plasma flow (RPF) or absolute and fractional sodium excretion in HanSD. In contrast, i.v. candesartan significantly increased RPF (+27 +/- 6%, P < 0.05) and absolute and fractional sodium excretion (+49 +/- 10 and + 42 +/- 9%, respectively P < 0.05) in TGR without changing glomerular filtration rate (GFR). Acute i.r. candesartan increased RPF by +36 +/- 6% (P < 0.05) in TGR but not in HanSD with a greater rise in absolute and fractional sodium excretion in TGR (+124 +/-8 and 97 +/- 9%, respectively) than in HanSD (+81 +/- 9 and +69 +/- 8%, respectively) (P < 0.05).

CONCLUSIONS

The enhanced responses of RPF and sodium excretion to AT1 receptor blockade in TGR suggest that renal hemodynamics and sodium excretion in TGR are under strong ANG II influence. The compromised ability of the kidney to respond to BP elevations by appropriate increases in sodium excretion may contribute to the maintenance of high BP in TGR. Thus, the present findings provide new insights into the pathophysiology of hypertension in this model.

Transgenic TGR(mREN2)27 rats as a model for disturbed circadian organization at the level of the brain, the heart, and the kidneys

In transgenic hypertensive TGR(mREN2)27 rats (TGR) harboring the murine Ren-2 gene an inverse 24h blood pressure (BP) profile was described in relation to a normal pattern in heart rate (HR) and motility (MA), normotensive Sprague-Dawley rats (SDR) were used as controls. Transgenic rats as an animal model of human secondary hypertension (non-dipper) was studied in detail at different levels: (1) Radiotelemetry was applied to document gross circadian rhythms/rhythm disturbances in cardiovascular functions, MA and body temperature under normal LD conditions, under DD and after a light pulse. (2) Signal transduction of the overexpressed renin-angiotensin in TGR was studied by determation of AT1-receptors in kidney glomeruli together with kidney functions. (3) Expression of key processes involved in increased sympathetic regulation in TGR, mRNAs, the tyrosine-hydroxylase (TH) and norepinephrine (NE) reuptake1-carrier were determined. (4) In the SCN mRNA of c-fos and c-jun were determined under LD and after light pulse. (5) In primary cultures of pinealocytes the effects of adrenergic agonists and antagonists were evaluated on second messenger (cAMP, cGMP) accumulation and melatonin release. The results of these studies clearly demonstrate that the additional mouse renin genin in TGR greatly affected not only the renin-angiotensin-system and led--as expected--to an increased BP in this rat but also disturbed circadian rhythms from the BP pattern down to the level of hormones, processes of signal transduction, and expression of transcription factors and clock genes. In conclusion, the expression of a single additional gene is able to disturb the circadian system of an animal in a highly complex way. These findings are importance for chronobiologic as well as pharmacologic research.

Pressure natriuresis in AT(2) receptor-deficient mice with L-NAME hypertension

AT(2) receptor-disrupted (AT(2) -/-) mice provide a unique opportunity to investigate the cardiovascular and BP-related effects of NO depletion. This study compared the pressure-diuresis-natriuresis relationship in (AT(2) -/-) and wild-type (AT(2) +/+) mice after treating the animals with L-NAME (130 mg/kg body wt per day) for 1 wk. L-NAME increased mean arterial pressure (MAP) more in AT(2) -/- than in AT(2) +/+ mice (118 +/- 2 versus 108 +/- 4 mmHg). This difference occurred even though L-NAME-treated AT(2) +/+ mice had a greater sodium excretion than AT(2) -/- mice (10.9 +/- 0.5 versus 8.0 +/- 1.0 micro mol/h). The pressure-natriuresis relationship in conscious AT(2) -/- mice was shifted rightward compared with controls. RBF was decreased in AT(2) -/- compared with AT(2) +/+ mice. L-NAME decreased RBF in these mice further from 4.08 +/- 0.43 to 2.79 +/- 0.15 ml/min per g of kidney wt. GFR was not significantly different between AT(2) +/+ and AT(2) -/- mice (1.09 +/- 0.08 versus 1.21 +/- 0.09 ml/min per g of kidney wt). L-NAME reduced GFR in AT(2) -/- to 0.87 +/- 0.07 ml/min per g of kidney wt. Fractional sodium (FE(Na)) and water (FE(H2O)) curves were shifted more strongly to the right by L-NAME in AT(2) -/- mice than in AT(2) +/+ mice. AT(1) receptor blocker treatment lowered BP in both L-NAME-treated strains to basal values. It is concluded that the AT(1) receptor plays a key role in the impaired renal sodium and water excretion induced by NO synthesis blockade. Changes in RBF, GFR, and tubular sodium and water reabsorption are involved and may be also responsible for the greater BP increase in L-NAME-treated AT(2) -/- mice.

Angiotensin-(1-7) is involved in the endothelium-dependent modulation of phenylephrine-induced contraction in the aorta of mRen-2 transgenic rats

1. The contribution of the local vascular production of angiotensin-(1-7) [Ang-(1-7)] to the control of alpha-adrenergic-induced contractions in the aorta of Sprague-Dawley (SD) and TGR(mRen-2)27 [mRen-2] rats was studied. 2. In mRen-2 rats, contractile responses to phenylephrine were diminished as compared to control SD rats in endothelium containing but not in endothelium-denuded vessels. L-NAME increased contractile responses to phenylephrine in mRen-2 rats and, after nitric oxide synthase blockade, responses to phenylephrine became comparable in both strains. 3. Inhibition of angiotensin-converting enzyme (ACE) by captopril potentiated contractile responses in mRen-2 rats and diminished contractile responses in SD rats, both effects being dependent on the presence of a functional endothelium. The effect of captopril in mRen-2 rats was abolished in vessels pre-incubated with Ang-(1-7). 4. Blockade of Ang-(1-7) and bradykinin (BK) receptors by A-779 and HOE 140 respectively, increased phenylephrine-induced contraction in mRen-2, but not in SD rats. This effect was seen only in endothelium-containing vessels. 5. Angiotensin II AT(1) and AT(2) receptor blockade by CV 11974 and PD 123319 did not affect the contractile responses to phenylephrine in aortas of transgenic animals but diminished the response in SD rats. This effect was only seen in the presence of a functional endothelium. 6. It is concluded that the decreased contractile responses to phenylephrine in aortas of mRen-2 rats was dependent on an intact endothelium, the local release and action of Ang-(1-7) and bradykinin.

Influence of circadian time and age on glomerular angiotensin II receptors in normotensive Sprague-Dawley and transgenic hypertensive TGR(mREN2)27 rats

In male heterozygous transgenic hypertensive rats, TGR(mREN2)27 (TGR), exhibiting an inverse blood pressure profile and in normotensive Sprague-Dawley (SPRD) controls, the density and affinity of angiotensin II receptors were determined at six circadian times in glomeruli of animals 11 weeks old kept under light-dark 12h:12 (LD 12:12) conditions. Angiotensin II receptors were also studied in rats 18-20 weeks old of both strains at 2h after light onset. As a measure of renal excretory functions, diuresis, creatinine, and protein excretion were monitored using metabolic cages. The expression of angiotensin II receptor mRNA was determined in renal arteries 2h-4h after light onset. The following results were obtained: (1) Renal excretory functions showed significant daily variation, with higher excretion rates in the dark span in both TGR and SPRD rats. (2) No circadian phase dependency was found in the glomerular angiotensin II receptors in both rat strains. However, receptor density was significantly lower in TGR than in SPRD rats. In both strains, receptor number increased with aging. (3) In renal arteries, the angiotensin II receptor mRNA of the main receptor subtype AT1A was neither strain nor age dependent, AT1B- and AT2-receptor mRNAs were significantly lower in TGR than SPRD rats. In conclusion, the results demonstrate that the overactive renin-angiotensin system in TGR rats led to a down-regulation of glomerular angiotensin II receptors that was not accompanied by a down-regulation of the mRNA of the dominant AT1A- receptor subtype. Circadian short-term variations in blood pressure in both TGR and SPRD rats are not reflected by daily variation in angiotensin II receptor density of renal glomeruli or by variation in receptor expression in renal vascular tissue.

Increased AT(1) receptor expression and mRNA in kidney glomeruli of AT(2) receptor gene-disrupted mice

The proposed feedback between angiotensin II AT(2) and AT(1) receptors prompted us to study AT(1) receptor expression in kidneys of male AT(2) receptor-gene disrupted mice (agtr2 -/y). In wild-type (agtr2 +/y) mice, AT(1) receptor binding and mRNA is abundant in glomeruli, and AT(1) receptor binding is also high in the inner stripe of the outer medulla. AT(2) receptors are scarce, primarily associated to cortical vascular structures. In agtr2 -/y mice, AT(1) receptor binding and mRNA were increased in the kidney glomeruli, and AT(1) receptor binding was higher in the rest of the cortex and outer stripe of the outer medulla, but not in its inner stripe, indicating different cellular regulation. Although AT(2) receptor expression is very low in male agtr 2 +/y mice, their gene disruption alters AT(1) receptor expression. AT(1) upregulation alone may explain the AT(2) gene-disrupted mice phenotype such as increased blood pressure, higher sensitivity to angiotensin II, and altered renal function. The indirect AT(1)/AT(2) receptor feedback could have clinical significance because AT(1) antagonists are widely used in medical practice.

Impairment of pressure-natriuresis and renal autoregulation in ANG II-infused hypertensive rats

Chronic infusions of initially subpressor doses of angiotensin II (ANG II) lead to progressive hypertension over a 2-wk period and to augmented intrarenal ANG II levels. The present study was performed to investigate total renal blood flow (RBF) and medullary blood flow (MBF) autoregulatory behavior and pressure-natriuresis in ANG II-infused hypertensive rats and how these are modified by concomitant treatment with an ANG II AT(1) receptor antagonist. ANG II-infused rats (n = 27) were prepared by administration of ANG II at 60 ng/min via osmotic minipump for 13 days. Twelve of the ANG II-infused hypertensive rats were treated with losartan in the drinking water (30 mg. kg.(-1) day(-1)). Rats were anesthetized with pentobarbital sodium (50 mg/kg, ip) and prepared for renal function measurements. An aortic clamp was placed above the junction of the left renal artery to reduce renal arterial pressure. Autoregulatory responses for renal plasma flow, overall RBF, and glomerular filtration rate were impaired in ANG II-infused hypertensive rats; however, MBF autoregulation was not disrupted. Most strikingly, pressure-natriuresis was markedly suppressed in ANG II-infused hypertensive rats. Chronic treatment with losartan prevented the impairment of the pressure-natriuresis relationship caused by chronic ANG II infusion. These findings demonstrate that chronic ANG II infusion leads to marked impairment of sodium excretion and suppression of the pressure-natriuresis relationship, which may contribute to the progressive hypertension that occurs in this model. These renal effects are prevented by simultaneous treatment with an AT(1) receptor blocker.

Inhibition of pressure natriuresis in mice lacking the AT2 receptor

Inhibition of pressure natriuresis in mice lacking the AT2 receptor.

BACKGROUND

Angiotensin II type 2 (AT2) receptor knockout mice have higher blood pressures than wild-type mice; however, the hypertension is imperfectly defined. We tested the hypothesis that renal mechanisms could be contributory.

METHODS

We conducted pressure-natriuresis-diuresis experiments, measured renal cortical and medullary blood flow by laser Doppler methods, and explored cytochrome P450-dependent arachidonic acid metabolism by means of reverse transcription-polymerase chain reaction.

RESULTS

Blood pressure was 15 mm Hg higher in AT2 receptor knockout mice than in controls, and pressure diuresis and natriuresis curves were shifted rightward. At similar renal perfusion pressures (113 to 118 mm Hg), wild-type mice excreted threefold more sodium and water than AT2 receptor knockout mice. Fractional sodium and water excretion curves were shifted rightward in parallel. Renal blood flow ranged between 6.72 and 7.88 mL/min/g kidney wet weight (kwt) in wild-type and between 5.84 and 6.15 mL/min/g kwt in AT2 receptor knockout mice. Renal vascular resistance was increased in AT2A receptor knockout mice. Cortical blood flow readings leveled at 2.5 V in wild-type and 1.5 V in AT2 receptor knockout mice. Medullary blood flow readings ranged between 0.8 and 1.0 V and increased 116% in wild-type mice as renal perfusion pressure was increased. This increase did not occur in AT2 receptor knockout mice. The glomerular filtration rate (GFR) was similar in both groups at approximately 1 mL/min/g kwt. Renal microsomes from AT2 receptor knockout mice had less activity in hydroxylating arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-meter) than controls, whereas renal AT1 receptor gene expression was increased in AT2 receptor knockout mice.

CONCLUSIONS

Hemodynamic and tubular factors modify renal sodium handling in AT2 receptor knockout mice and may cause hypertension. AT2 receptor disruption induces alterations of other regulatory systems, including altered arachidonic acid metabolism, that may contribute to the intrarenal differences observed between AT2 receptor knockout and wild-type mice.

Angiotensin-converting enzyme inhibition and AT1 receptor blockade modify the pressure-natriuresis relationship by additive mechanisms in rats with human renin and angiotensinogen genes

The intrarenal factors responsible for hypertension in double-transgenic rats (dTGR) harboring human renin and human angiotensinogen genes are unclear. The pressure-natriuresis and -diuresis relationships in response to chronic angiotensin-converting enzyme (ACE) inhibition and AT1 receptor blockade were evaluated. Renal renin-angiotensin and nitric oxide (NO) system gene expression was also investigated. Six-week-old dTGR were treated for 3 wk with submaximal doses of cilazapril (10 mg/kg, orally) or losartan (10 mg/kg, orally) or with the drug combination. In untreated dTGR, pressure-natriuresis relationships were maximally shifted rightward by approximately 70 to 80 mmHg, and both renal blood flow (RBF) and GFR were markedly decreased. Submaximal cilazapril and losartan dosages both decreased systolic BP by 30 mmHg and shifted the pressure-natriuresis curves leftward by 25 to 30 mmHg. Cilazapril increased RBF and GFR to values observed in normotensive control animals but did not significantly affect fractional sodium excretion (FENa) or fractional water excretion (FEH2O) curves. In contrast, losartan had no significant effect on RBF or GFR but shifted the FENa and FEH2O curves leftward. The cilazapril and losartan combination completely normalized BP and shifted the pressure-natriuresis curves leftward more than did either drug alone. When cilazapril and losartan were administered at higher doses (30 mg/kg, orally), the two drugs equally shifted the pressure-natriuresis curves leftward, by 50 mmHg. Both drugs increased RBF and GFR; however, only losartan shifted FENa and FEH2O curves leftward. Human and rat renin and angiotensinogen genes were downregulated in dTGR and were increased by losartan and cilazapril treatments, whereas no changes in the expression of rat ACE and AT1A receptor genes were observed. Endothelial NO synthase expression was increased by cilazapril but not by losartan. Neither inducible NO synthase nor neural NO synthase gene expression was affected by drug treatments. Therefore, submaximal ACE inhibition enhanced sodium excretion mainly by increasing RBF and GFR, whereas submaximal AT1 receptor blockade decreased tubular sodium and water reabsorption. The combination of the two drugs produced an additive effect. The ACE inhibitor effects may involve increased endothelial NO synthase expression, perhaps related to the inhibition of bradykinin degradation.

Enhanced renal vascular responsiveness to angiotensin II in hypertensive ren-2 transgenic rats

The present study was performed to evaluate renal vascular responsiveness (RVR) to ANG II in hypertensive transgenic rats [TGR; strain TGR(mRen2)27] harboring the mouse ren-2 renin gene. Renal blood flow (RBF) responses to either intravenous or intrarenal arterial administration of ANG II were assessed in pentobarbital sodium-anesthetized female heterozygous TGR (9-12 wk old) and age-matched transgene-negative Hanover Sprague-Dawley rats (HanSD). Intravenous bolus injections of 15 and 30 ng ANG II elicited dose-dependent increases in mean arterial blood pressure (AP) and decreases in RBF in both TGR and HanSD. However, the magnitude of the increases in AP was greater in TGR than in HanSD (24 +/- 1 vs. 17 +/- 2 mmHg and 33 +/- 2 vs. 25 +/- 1 mmHg, respectively, P < 0.05 in both cases). Similarly, the magnitude of the decrease in RBF elicited by intravenous administration of 15 ng of ANG II was greater in TGR than HanSD (-62 +/- 3 vs. -52 +/- 5%, P < 0.05). Intrarenal arterial administration of 1.5 and 3 ng ANG II did not alter mean AP in either group but elicited larger decreases in RBF in TGR than in HanSD (-24 +/- 2 vs. -13 +/- 1% and -41 +/- 5 vs. -30 +/- 2%, respectively, P < 0.05 in both cases). In contrast, intrarenal arterial administration of norepinephrine (40 and 80 ng) elicited smaller decreases in RBF in TGR than in HanSD (-24 +/- 3 vs. -40 +/- 6% and -51 +/- 9 vs. -71 +/- 8%, respectively, P < 0.05 in both cases), indicating that TGR do not exhibit a generalized increase in RVR to endogenous vasoconstrictors. Furthermore, the enhanced RVR to ANG II does not appear to reflect an impaired RVR to endogenous vasodilator factors since intrarenal administration of bradykinin and acetylcholine elicited larger increases in RBF in TGR than in HanSD. The present findings indicate that hypertensive TGR exhibit exaggerated renal and peripheral vascular responses to ANG II, which likely contributes to an increased renal and peripheral vascular resistance and thereby to the hypertension in TGR.

Transgenic animal models for the functional analysis of vasoactive peptides

The interplay of vasoactive peptide systems is an essential determinant of blood pressure regulation in mammals. While the endothelin and the renin-angiotensin systems raise blood pressure by inducing vasoconstriction and sodium retention, the kallikrein-kinin and the natriuretic-peptide systems reduce arterial pressure by eliciting vasodilatation and natriuresis. Transgenic technology has proven to be very useful for the functional analysis of vasoactive peptide systems. As an outstanding example, transgenic rats overexpressing the mouse Ren-2 renin gene in several tissues become extremely hypertensive. Several other transgenic rat and mouse strains with genetic modifications of components of the renin-angiotensin system have been developed in the past decade. Moreover, in recent years gene-targeting technology was employed to produce mouse strains lacking these proteins. The established animal models as well as the main insights gained by their analysis are summarized in this review.

ICAM-1 antisense oligodesoxynucleotides prevent reperfusion injury and enhance immediate graft function in renal transplantation

BACKGROUND

Ischemia-reperfusion injury after organ transplantation is a major cause of delayed graft function. We showed earlier that antisense oligodesoxynucleotides (ODN) for intercellular adhesion molecule-1 (ICAM-1) ameliorate reperfusion injury after acute ischemia. This study tested the hypothesis that antisense ODN for ICAM-1 prevents ischemia-reperfusion injury and facilitates immediate graft function in a rat autotransplantation model.

METHODS

Both kidneys were removed from male Lewis rats and re-implanted the left kidney after 30 minutes of cold ischemia time. The warm ischemia time was 60 minutes. Sham operated, uninephrectomized animals served as controls for renal function and histology. ICAM-1 antisense ODN (5 mg/kg), reverse ODN, or saline-vehicle were administered to donor animals i.v. six hours before autotransplantation. Glomerular filtration rate (insulin clearance), and serum creatinine concentrations were measured 24 hours post-transplantation. Tubular necrosis severity was assessed by histological grading scale. ICAM-1 expression was determined by immunohistochemistry and Western blot.

RESULTS

Antisense ODN decreased ICAM-1 expression and leukocyte infiltration significant. Antisense ODN-treated animals showed significantly less tubular necrosis, than controls. Serum creatinine of antisense ODN-treated animals (N = 6) was 0.55 +/- 0.02 mg/dl compared to 1.92 +/- 0.07 mg/dl in reverse ODN-treated controls (N = 6; P < 0.01), 24 hours after transplantation. Antisense ODN-treated animals had normal GFR (0.93 +/- 0.07 ml/min/kidney wt) compared to sham-operated animals (0.95 +/- 0.09 ml/min/kidney wt), while autotransplanted animals treated with reverse ODN or saline-vehicle were all anuric. The ischemia-reperfusion-induced up-regulation of MHC class II was totally prevented by antisense ODN.

CONCLUSIONS

ICAM-1 inhibition ameliorates ischemia-reperfusion injury and prevents delayed graft function. Antisense ODN-treatment of donors or donor organs for ICAM-1 may be useful for the prevention of reperfusion injury in human renal transplantation.

Mouse and rat plasma renin concentration and gene expression in (mRen2)27 transgenic rats

The (mRen2)27 transgenic rat [TGR(mRen2)27] is said to have low plasma levels of active renin. We used a direct radioimmunoassay (RIA) for mouse submaxillary renin, as well as an indirect enzyme-kinetic assay based on the generation of angiotensin I with modification of the pH optimum, to measure rat and mouse plasma renin activity (PRA), plasma renin concentration (PRC), and plasma prorenin in TGR before and after lisinopril. The relationship between rat PRC and % rat kidney extract was steepest at pH 6.0 and flat at pH 8.5, whereas the relationship between mouse PRC and purified mouse renin was steepest at pH 8.5 and flat at pH 6.0. Mouse PRC was highly correlated with direct RIA measurements (r = 0.93). PRA before lisinopril was little influenced by pH, whereas the increase with lisinopril was greatest at pH 6.5. PRC before lisinopril was fourfold higher at pH 8.5 compared with that at pH 6.0. Lisinopril increased both PRC values but reversed the pH dependency. Prorenin was fourfold higher at pH 8.5 compared with that at pH 6.0 and decreased slightly with lisinopril. Renal renin concentration was higher at pH 6.0 than at pH 8.5. With lisinopril, renal renin concentration increased at both pH values. Mouse PRC was not changed by lisinopril. Ribonuclease protection assay showed both rat and mouse renin gene expression in the kidney, which increased with lisinopril. Thus TGR have circulating active rat and mouse renin and prorenin. The notion that TGR are a "low renin" model should be revised.

Effect of nitric oxide inhibition on blood pressure and renal function in TGR(mRen2)27 rats

We examined the effect of acute systemic blockade of nitric oxide synthesis on blood pressure and renal function in the monogenetically hypertensive TGR(mRen2)27 rat strain. Untreated conscious transgenic rats had significantly (p < 0.01) higher systolic blood pressures (185 +/- 9 versus 130 +/- 5 mm Hg) and urinary albumin excretion (32 +/- 5 versus 6 +/- 2 mg/day) than did control animals without evidence of renal insufficiency. Plasma and urinary nitric oxide metabolite levels did not differ between transgenic and control rats. i.v. administration of NG-nitro-L-arginine methyl ester (10 mg/kg) to both groups caused similar elevations in systemic blood pressure (transgenic 25 +/- 3 versus control 24 +/- 3 mm Hg). NG-Nitro-L-arginine methyl ester induced reductions in whole kidney (1.4 +/- 0.2 versus 0.7 +/- 0.1 mL/min), and single nephron (23 +/- 3 versus 11 +/- 2 nL/min) glomerular filtration rates were significantly (p < 0.05) larger in transgenic than in control rats. This greater loss of GFR in transgenic animals was caused by a larger reduction in glomerular ultrafiltration coefficient (1.8 +/- 0.2 versus 1.1 +/- 0.1 nL x min[-1] x mmHg[-1], p < 0.05), a larger increase in afferent arteriole resistance (3.4 +/- 0.2 versus 1.4 +/- 0.1 dyne x s x cm[-5], p < 0.05), and a subsequently smaller rise in glomerular transcapillary pressure (10 +/- 1 versus 5 +/- 1 mmHg, p < 0.05). These results indicate that the renal microvasculature and glomerular hydraulic conductivity or surface area of transgenic rats are more sensitive to nitric oxide inhibition and are consistent with an important role for nitric oxide in TGR(mRen2)27 kidney function.

Angiotensin II mediates cell hypertrophy in vascular smooth muscle cultures from hypertensive Ren-2 transgenic rats by an amiloride- and furosemide-sensitive mechanism

We have reported that cultured vascular smooth muscle cells (VSMC) from hypertensive Ren-2 transgenic rats (TGR) are constitutively hypertrophic when compared to matched cells from normotensive Sprague-Dawley rats (SD), by a mechanism involving the endogenous production of angiotensin II (AII). In the present work, we analyzed the possible involvement of two Na+ transport mechanisms in TGR-VSMC hypertrophy. In both SD- and TGR-VSMC, AII increased both cell size, by a furosemide- and amiloride-sensitive mechanism, and Na+/K+/2Cl- cotransport activity, by an amiloride-sensitive mechanism. Under basal unstimulated conditions, TGR-VSMC showed higher cell size and Na+/K+/2Cl- cotransport activity than SD-VSMC. Under these same conditions, losartan, furosemide, or amiloride reduced cell size only in TGR-VSMC. Similarly, basal cotransport activity was reduced by losartan and amiloride to levels similar to those observed in SD-VSMC. We conclude that hypertrophy of TGR-VSMC is dependent on the endogenous production of AII and mediated by increased Na(+)-H+ exchange and Na+/K+/2Cl- cotransport activities.

Pressure diuresis and natriuresis in DOCA-salt mice

The increasing use of mice in renal and cardiovascular studies has necessitated adapting physiological methods used for rats to mice, which are far smaller in size. We have adapted measurements of continuous renal blood flow, pressure natriuresis and diuresis, and laser-Doppler cortical and medullary flow to 40 g mice with DOCA-salt hypertension. We demonstrated a rightward shift in the pressure-natriuresis-diuresis curve. We conclude that with current, commercially-available equipment, sophisticated renal physiology can be conducted in the mouse. These methods will be important to investigations of gene-targeted mice.

The kidney in blood pressure regulation and development of hypertension

Systemic arterial pressure is a dynamic and responsive physiologic parameter that can be influenced by many different factors. In particular, short-term changes in arterial pressure are caused by a myriad of mechanisms that affect cardiac output, total peripheral resistance, and cardiovascular capacitance. In the long run, however, most of these actions can be buffered or compensated by appropriate renal adjustments of sodium balance, ECFV, and blood volume. As long as the mechanisms regulating sodium excretion can maintain sodium balance by appropriately modulating the sensitivity of the pressure-natriuresis relationship, normal arterial pressure can be sustained. Derangements that compromise the ability of the kidneys to maintain sodium balance, however, can result in the kidney's need for an elevated arterial pressure to reestablish net salt and water balance.

Exaggerated natriuresis in transgenic (mRen2)27 rats

BACKGROUND

Hypertension features an exaggerated natriuresis after acute volume expansion. In humans, the degree of exaggerated natriuresis appears to be correlated inversely to the level of angiotensin (Ang) II.

OBJECTIVE

To test the hypothesis that the degree of exaggerated natruresis is correlated to the level of Ang II by studying two rat models, transgenic rats (TGR) with and extra renin gene (TGR mRen2)27 and desoxycorticosterone acetate (DOCA)-salt rats, in comparison with Sprague-Dawley Hannover (SDH) rat controls.

METHODS

All of the rats were uninephrectomized for 1 month. DOCA-salt rats were implanted with a DOCA pallet and drank 1% saline. Rats were anesthetized and their left kidneys were instrumented with renal sympathetic nerve activity (RSNA) electrodes and laser-Doppler cortical and medullary flow probes. The glomerular filtration rate, diuresis, and natriuresis were measured for 120 min after sodium loading (5% body weight 0.9% saline administered during 3 min). Kidneys were examined histologically.

RESULTS

The blood pressure in TGR and DOCA-salt rats was 40-50 mmHg higher than that in SDH rats, and decreased briefly after volume expansion for all groups. The diuresis and natriuresis of TGR and DOCA-salt rats were greater than those of SDH rats. The medullary blood flow increased and the cortical blood flow in SDH decreased, whereas the cortical blood flow in TGR and DOCA-salt rats remained high. The RSNA in rats of all groups decreased; however, this decrease was greater in SDH than it was in TGR and DOCA-salt rats. The histology was affected most severely for the DOCA-salt rats.

CONCLUSIONS

Exaggerated natriuresis occurred in hypertensive rats regardless of their Ang II status. Both strains were characterized by a smaller decrease in RSNA and a preserved cortical blood flow in the face of volume expansion. These data do not support the notion that exaggerated natriuresis is a function of renin-level suppression for rats.

Effects of captopril, losartan, and nifedipine on cell hypertrophy of cultured vascular smooth muscle from hypertensive Ren-2 transgenic rats

1. We hypothesized that tissular renin-angotensin system (RAS) induces vascular hypertrophy in hypertensive Ren-2 transgenic rats (TGR; strain name TGR(mRen2)L27). This assumption was tested in cell cultures of vascular smooth muscle (VSMC) from both hypertensive TGR and control normotensive Sprague-Dawley (SD) rats. Planar cell surface area, protein synthesis, and protein content per cell were studied, the role for locally produced angiotensin II (AII) was evaluated and the possible pharmacological interference by different drugs was analysed. 2. By use of radioimmunoassay techniques, AII could be determined in TGR cultures (10.25 +/- 0.12 pg per 10(7) cells) while it could not be detected in SD ones. 3. Under serum-free conditions, VSMC from hypertensive TGR were hypertrophic when compared to SD VSMC, as they presented a higher protein content per cell (335 +/-18 and 288 +/- 7 pg per cell respectively; P<0.05) and increased mean planar cell surface area, as determined by image analysis (4,074 +/- 238 and 4,764 +/- 204 microm2, respectively; P < 0.05). 4. When exogenously added to cultured SD and TGR VSMC, AII (100 pM to 1 microM) promoted protein synthesis and protein content in a concentration-dependent manner without affecting DNA synthesis. Maximal effects were observed at 100 nM. At this concentration, AII effectively increased planar cell surface area in both SD and TGR cultures by approximately 20%. 5. Treatment of TGR cultures, in the absence of exogenous AII, with the angiotensin-converting enzyme inhibitor captopril or the angiotensin AT1 receptors antagonist losartan (100 nM to 10 microM) reduced planar cell surface area in a concentration-dependent manner. In addition, both captopril and losartan (10 microM), decreased protein synthesis by approximately 15%. 6. Treatment of SD VSMC, in the absence of exogenous AII, with both captopril and losartan had no effect either on planar cell surface area or protein synthesis. 7. Treatment with the Ca2+ antagonist nifedipine (100 nM to 10 microM) reduced cell size in both SD and TGR cultures. Maximal cell reduction reached by nifedipine averaged 906 +/- 58 and 1,292 +/- 57 microm2, in SD and TGR, respectively (P<0.05). In addition, nifedipine, nitrendipine and nisoldipine (all at 10 microM) decreased protein synthesis in both cell types by 15-25%. 8. We concluded that cultured VSMC from TGR are hypertrophic in comparison with those from SD. This cell hypertrophy can be the consequence of the expression of the transgene Ren-2 that activates a tissular RAS and locally produces AII, which acts in a paracrine, autocrine, or intracrine manner. Cell hypertrophy in TGR cultures could be selectively reduced by RAS blockade, while nifedipine decreased cell size and protein synthesis in both hypertrophic and non hypertrophic cells.

Nifedipine, losartan and captopril effects on hyperplasia of vascular smooth muscle from Ren-2 transgenic rats

Vascular smooth muscle cells from hypertensive transgenic rats for the mouse Ren-2 gene exhibited radioimmunoassayable angiotensin II and hyperplasia in comparison with cells from Sprague-Dawley rats. However, neither captopril, losartan, saralasin, nor PD123319 (all at 10 microM) modified DNA synthesis or cell number observed in 4-day growth curves with 10% fetal calf serum. Nifedipine reduced DNA synthesis in both cell types, the concentration required being significantly higher in Sprague-Dawley- (1 microM) than in transgenic-derived cultures (100 nM). The EC50 values were of 2.43 +/- 0.32 and 1.0 +/- 0.17 microM, respectively (P < 0.05). In both cell types, only 10 microM nifedipine reduced serum-induced cell proliferation, but inhibition percentage was higher in transgenic-derived cultures. In conclusion, hyperplasia of transgenic-derived vascular smooth muscle cells is not blocked by angiotensin-converting enzyme inhibitors or angiotensin receptor antagonists, but these cells are more sensitive to the antiproliferative effects of nifedipine.

Reduction of cardiac hypertrophy in TGR(mREN2)27 by angiotensin II receptor blockade

TGR(mREN2)27 is a transgenic rat harboring the murine Ren-2 gene and exhibit fulminant hypertension and marked heart hypertrophy. In order to study the role of angiotensin II in the increase of cardiac mass, these animals were treated with antihypertensive and non-antihypertensive doses of the angiotensin II receptor AT1 antagonist Telmisartan for 9 weeks. All doses led to significant reductions of heart hypertrophy detected by the evaluation of the diameter of cardiac muscle bundles. We conclude from this study that cardiac hypertrophy in TGR(mREN2)27 is characterized by an increased volume of cardiomyocytes and an unchanged amount of fibrous tissue and that angiotensin II plays an important role in the mechanisms leading to this phenotype.