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(Hypertension. 2006;47:552.)
© 2006 American Heart Association, Inc.
Part 2 Original Articles |
From the Inserm U36 (C.A.B., G.N., J.M.G.), Paris, France; Collège de France (C.A.B., G.N., J.M.G.), Experimental Medicine Unit, Paris, France; Max Delbrück Center for Molecular Medicine (M.B., C.A.B., E.P., R.P.) and Franz Volhard Clinic (D.N.M.), Berlin, Germany; Institute for Physiology (J.P.), University Greifswald, Greifswald, Germany; and Department of Pharmacology (A.H.J.D., I.M.G.), Erasmus MC, Rotterdam, the Netherlands.
Correspondence to Michael Bader, Max-Delbrück-Center for Molecular Medicine, Robert-Rössle Strasse 10, 13125 Berlin-Buch, Germany, E-mail mbader{at}mdc-berlin.de
| Abstract |
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Key Words: renin rats, transgenic blood pressure
| Introduction |
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To analyze the functional relevance of renin receptor in vivo we generated transgenic rats [TGR(SMMHC-HRR)] overexpressing the human renin receptor (HRR) in smooth muscle cells under the control of a 16-kb fragment of the mouse smooth muscle myosin heavy chain gene. TGR(SMMHC-HRR) showed strong transgene expression in the arterial wall, making the model valuable for studying the contribution of the renin receptor to circulating and tissue RAAS. TGR(SMMHC-HRR) spontaneously developed slow and progressive cardiovascular changes, marked by increased blood pressure and heart rate, accompanied by aldosterone elevation.
| Methods |
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Detection of HRR mRNA Expression
RPA
Tissue-specific mRNA expression of the transgene was verified by RPA from total tissue RNA. A 32P-labeled antisense probe was prepared with T7 RNA polymerase transcription of the plasmid SMMHC-HRR-230 after SpeI digestion. Total mRNA was isolated with TRIZOL from brain, lung, heart, aorta, adrenal gland, kidney, liver, intestine, bladder, uterus, skeletal muscle, and aortic smooth muscle cells, from both male and female rats at 8 weeks of age. RPA was performed according to the manufacturers protocol (AMBION, RPA kit II). For each hybridization reaction, 40 µg of RNA and 50 000 cpm of purified 32P-labeled transcript were incubated at 42°C overnight.
Integrity of the HRR mRNA
We checked the integrity of the transgenic mRNA by 5' and 3' rapid amplification of cDNA ends (RACE) PCR, using the RACE XLM PCR kit (Ambion). We followed the instructions of the manufacturer, starting from a uterus mRNA sample and using the following internal specific primers: HRR25, 5'-AACGAGTTTAGTATATTAAAT-3', and HRR43, 5'-TTCCTCAGAAAAATAAGGAGTG-3'.
In Situ Hybridization
Correct expression pattern of the transgene was assessed by in situ hybridization, as described previously. Briefly a 35S-UTPlabeled mRNA probe was synthesized using a 600-bp fragment of the AF291814 as template.12
Prorenin Binding to Aortic Segments
Rat aorta segments (3 to 4 mm) were removed and incubated overnight in cold (4°C), oxygenated Krebs bicarbonate solution13 containing 100 U/L human recombinant prorenin. The next morning, the vessels were washed 5 times with cold Krebs bicarbonate buffer and homogenized. Prorenin was activated by acidification,2 and the acidified homogenates were incubated with excess sheep angiotensinogen to quantify the total amount of sequestered prorenin by enzyme-kinetic assay. Values were corrected for background angiotensin Igenerating activity using acidified homogenized segments that had not been exposed to human prorenin.
Measurement of Blood Pressure and Heart Rate
Systolic blood pressure, diastolic blood pressure, and heart rate were recorded in conscious, freely moving animals by a telemetric pressure transducer implanted in the aorta as described.14
Measurement of Plasma RAS, Electrolytes, and Kidney Function
Rats were anesthetized briefly with ketamine and xylazine. Blood obtained from jugular vein puncture was collected into ice-cold microcentrifuge tubes containing EDTA and immediately centrifuged to isolate plasma. Plasma renin activity (PRA) and plasma renin concentration (PRC) were determined as described before.15 Plasma aldosterone was measured by radioimmunoassay. For the evaluation of kidney function, rats were placed for 3 days in metabolic cages, and urine was collected over a 24-hour period. Blood was collected in tubes containing heparin and centrifuged. K+, Na+, creatinine, albumin, and/or urea were measured in urine and blood plasma.
Statistical Analysis
The results are expressed as mean±SEM. Hemodynamic parameters are presented as the mean of value of 3 days of sampling. Student t test was used for comparisons between groups. P<0.05 was considered significant.
| Results |
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After overnight incubation with human prorenin, homogenized aortic segments of TGR(SMMHC-HRR) contained 3 times as much prorenin (measured as angiotensin Igenerating activity after prorenin activation by acidification) as aortic segments from control rats (n=5 for each; Figure 3; P=0.07).
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Cardiovascular Phenotype in TGR(SMMHC-HRR)
Transgenic animals are fertile and develop normally. Male heterozygous animals of 2 lines (7154 and 7329) and control littermates were submitted to telemetry recording of blood pressure and heart rate in freely moving conscious animals for a time period covering 6 months to 1 year. Transgenic rats revealed a spontaneous progressive cardiovascular phenotype. Onset of the phenotype showed variability between the 2 lines, with earlier appearance in line 7154 presenting the highest expression level of the transgene. Before 3 months of age, line 7154 and control rats had comparable systolic blood pressure. After 6 months of age, a progressive increment in blood pressure, initially predominantly systolic, was observed (Figure 4A). At 8 months of age, the systolic blood pressure was 137.8 (±5) mm Hg, as compared with 118.7 (±3.7) mm Hg (P=0.008) in control rats. Simultaneously, a 20% increase in heart rate was observed in line 7154 (P=0.023; Figure 4B).
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RAAS and Kidney Function in TGR(SMMHC-HRR)
PRA (3.6±1.1 versus 2.5±1.0 ng angiotensin I/mL per hour) and PRC (126±50 versus 104±69 ng angiotensin I/mL per hour) were identical in TGR(SMMHC-HRR) and control rats. No differences were observed in plasma or urinary electrolytes, creatinine, or albumin (Table). Plasma aldosterone was doubled in TGR(SMMHC-HRR) (Figure 5), and this correlated with a trend to lower plasma K+ in TGR(SMMHC-HRR) (Table). The aldosterone/renin ratio was significantly higher in TGR(SMMHC-HRR) (Figure 5).
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| Discussion |
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In TGR(SMMHC-HRR), HRR mRNA expression is, as expected, restricted to organs with a high smooth muscle cell content and is very strong in smooth muscle cells of the aortic vascular wall. The smooth muscle cell layer location does not hinder (pro)renin accessibility as the enzyme diffuses freely by a paracellular pathway through the endothelial cell layer.1 Therefore, TGR(SMMHC-HRR) constitutes a valuable model for unraveling the impact of HRR overexpression on systemic and, particularly, on local RAS. We observed a clear tendency for increased uptake of prorenin by the TGR(SMMHC-HRR) aortic wall in vitro, hence confirming cell culture data, which showed increased binding of renin and prorenin at the cell surface after transfection of human or rat renin receptor cDNA.4,6
Interestingly, TGR(SMMHC-HRR) rats develop a delayed cardiovascular phenotype, defined by high blood pressure and heart rate. The increased blood pressure was accompanied by significant elevation of plasma aldosterone levels, a marked increase of the aldosterone/renin ratio, and a tendency to hypokalemia. Because PRC, PRA, and angiotensin I generation were comparable in transgenic and control rats, aldosterone hyperproduction possibly results from a primary adrenal cause. Consistently, there is no other strong stimulating systemic signal for aldosterone synthesis, such as hyperkalemia. Moreover, the likelihood of exaggerated corticotropin secretion seems to be very low, because the adrenal glands of the transgenic rats were macroscopically normal (data not shown).
In vitro, the renin receptor has been described as a stimulating cofactor for renin enzymatic activity.4 Because the HRR is expressed in the adrenal gland of TGR(SMMHC-HRR) with a vascular localization (data not shown), the elevated aldosterone level could be related to an intraadrenal RAS activation. However, it is not completely clear whether aldosterone elevation is the culprit for blood pressure increase. This will be additionally evaluated by administration of an aldosterone antagonist and adrenalectomy. Nevertheless, we cannot exclude a locally increased angiotensin II generation in resistance vessels as the cause for the hypertensive phenotype.
Another striking phenomenon in TGR(SMMHC-HRR) is the increase in heart rate. With a functioning baroreflex, one would expect normal or low heart rate in response to higher blood pressure. This may indicate a baroreflex dysfunction with sympathetic activation and may be caused by a stimulated central RAS.
Perspectives
A progressive cardiovascular phenotype is associated with VSMC renin receptor overexpression in TGR(SMMHC-HRR). In vitro, (pro)renin binding to the receptor increases its catalytic activity. In vivo, local activation of the intraadrenal RAS may contribute to aldosterone hyperproduction and to elevated blood pressure. Thus, inhibiting (pro)renin binding to the receptor may limit local RAS activation and may be considered as a potential new therapeutic target.
| Acknowledgments |
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Received October 1, 2005; first decision October 25, 2005; accepted November 29, 2005.
| References |
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2. Danser AHJ, van Kats JP, Admiraal PJJ, Derkx FHM, Lamers JMJ, Verdouw PD, Saxena PR, Schalekamp MADH. Cardiac renin and angiotensins. Uptake from plasma versus in situ synthesis. Hypertension. 1994; 24: 3748.
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8. Luetscher JA, Kraemer FB, Wilson DM, Schwartz HC, Bryer-Ash M. Increased plasma inactive renin in diabetes mellitus. A marker of microvascular complications. N Engl J Med. 1985; 312: 14121417.[Abstract]
9. Regan CP, Manabe I, Owens GK. Development of a smooth muscle-targeted cre recombinase mouse reveals novel insights regarding smooth muscle myosin heavy chain promoter regulation. Circ Res. 2000; 87: 363369.
10. Madsen CS, Regan CP, Hungerford JE, White SL, Manabe I, Owens GK. Smooth muscle-specific expression of the smooth muscle myosin heavy chain gene in transgenic mice requires 5'-flanking and first intronic DNA sequence. Circ Res. 1998; 82: 908917.
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13. Tom B, Garrelds IM, Scalbert E, Stegmann APA, Boomsma F, Saxena PR, Danser AHJ. ACE- versus chymase-dependent angiotensin II generation in human coronary arteries: a matter of efficiency? Arterioscler Thromb Vasc Biol. 2003; 23: 251256.
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