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(Hypertension. 2004;43:471.)
© 2004 American Heart Association, Inc.
Scientific Contribution |
From the Medical Faculty of the Charité, The Clinical Research Center, Franz Volhard Clinic and the Max Delbrück Center for Molecular Medicine (S.B., O.T., C.S., G.F., A.B., A.A., Y.N. A.M., H.R.T., M.G., J.J., F.C.L.), HELIOS Klinikum, Berlin, Germany; the Department of Genetics (A.R.), University of Erlangen, Germany; and the Department of Internal Medicine VI (C.H., H.S., W.E.H.), Clinical Pharmacology and Pharmacoepidemiology, University of Heidelberg, Germany.
Correspondence to Friedrich C. Luft, MD, Franz Volhard Clinic, Wiltberg Strasse 50 13125 Berlin, Germany. E-mail luft{at}fvk-berlin.de
| Abstract |
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Key Words: hypertension genetics gene expression
| Introduction |
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| Methods |
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150-mmol sodium and 80-mmol potassium.3,4 They refrained from smoking and consumed no alcohol.
Interphase Fluorescent In Situ Hybridization
We prepared lymphoblastoid cell lines (LCL) and selected five genomic clones prepared as bacterial artificial chromosomes (BAC; CITB library, Research Genetics) or P1-derived artificial chromosomes (PAC; Genome Systems, Inc) spread over the linkage region. The clones were physically mapped and fluorescent probes were prepared. To analyze their order, a series of three-color interphase fluorescence in situ hybridization (FISH) studies were performed.11 Briefly, air-dried slides from cell lines were pretreated with RNAse and pepsin, denatured in 70% formamide/2x SSC at 75°C, and dehydrated with an ethanol series. We isolated genomic DNA from clones that we labeled by nick translation with either FluoroX (green), or Cy3 (red), or both (yellow), to generate 500-bp fragments. Probes were denatured in formamide containing hybridization solution for 5 minutes at 75°C and hybridized to slides overnight at 37°C. Slides were washed and stained with DAPI and examined under a fluorescence microscope. In assessing interphase cells, we scored only those chromosomes in which all 3 probes were visualized in close alignment with each other. We scored at least 20 chromosomes for each individual.
RNA Extraction and Reverse Transcription Polymerase Chain Reaction
Buttocks biopsies were performed under local anesthesia as described elsewhere.12 Small vessels were isolated for physiological studies and mRNA extraction. Arteries were homogenized and centrifuged through a QIAshredder column (Qiagen). Total RNA was extracted with the RNeasy Mini Kit (Qiagen). cDNA synthesis was performed using Superscript II (Gibco BRL) and hexanucleotide primers. Quantitative RNA extraction and reverse transcription polymerase chain reaction (RT-PCR) was performed with the TaqMan Universal Master Mix (Applied Biosystems) in a 25-µL reaction. The sequences of primers and probes are available on request.
Intrabrachial Artery Infusions
We tested the hypothesis that affected family members were less sensitive to the vasodilatory effect of isoproterenol because of increased PDE3A activity, compared with controls. We cannulated the brachial artery of the nondominant arm. Intraarterial blood pressure and heart rate (ECG) were recorded. Forearm blood flow was determined by strain gauge plethysmography (Filtrass 2001, Domed, Munich, Germany) in both arms.13 The hand circulation was excluded by a wrist cuff inflated to
20 mm Hg above systolic blood pressure. After a baseline period, 5 incremental isoproterenol doses (Isuprel, Abbott, Germany) were infused for 4 minutes each through the arterial catheter at 0.5, 1.5, 3.0, 6.0, and 12 ng/min/100 mL.14,15 After a 45-minute washout, the phosphodiesterase inhibitor milrinone (Corotrop, Sanofi-Synthelabo, Germany) was infused at 0.1, 0.3, 1.0, 2.5, and 5 µg/min/100 mL.15 To assess the interaction between beta-adrenergic stimulation and phosphodiesterase inhibition, we repeated testing with isoproterenol during milrinone infusion at 5 µg/min/100 mL. The cumulative dose, based on a forearm blood volume of 1500 mL, was 2x1725 ng isoproterenol and 2415 µg milrinone. After washout, we tested the effect of incremental intraarterial diazoxide (Hypertonalum, Essex Pharma, Germany) at 0.125, 0.25, 0.5, and 1 mg/min/100 mL for 5 minutes each.16
Systemic Pharmacological Testing
We compared nitroprusside, isoproterenol, and diazoxide in affected and nonaffected subjects. Blood pressure was determined by automated brachial cuff and finger photoplethysmography (Finapres); ECG measured heart rate. After a 45-minute baseline, nitroprusside at 0.1, 0.2, 0.4, 0.8, and 1.6 µg/kg/min for 5 minutes each was initiated. The infusion was increased to decrease systolic blood pressure by 20 mm Hg. After the next 30-minute baseline, incremental isoproterenol at 0.25 µg/min was initiated and increased at 5-minute intervals until heart rate increased 25 bpm or until systolic blood pressure decreased by 20 mm Hg. Then, after another baseline, diazoxide bolus doses at 12.5, 25, 50, and 75 mg were given.
Isolated Vessels
Small arteries (150 to 300 µm) were dissected from buttock biopsies and were bathed in cold physiological saline solution (PSS) gassed with 5% CO2 in 95% O2. The PSS contained (all in mM): 120 NaCl, 5 KCl, 1.6 CaCl2, 1.2 MgSO4, 25 NaHCO3, 1.2 NaH2PO4, 0.02 EDTA, and 5.5 glucose. Two-mm segments were threaded onto two stainless wires and suspended in 5-mL microvascular myograph baths (Danish Myotechnology, Aarhus, Denmark) according to Mulvany and Halpern.17 The arteries equilibrated for 60 minutes and were then exposed to isomolar 60-mmol/L KCl-containing solution (KPSS) to ensure their viability and contractility. KPSS was similar to PSS except that NaCl was exchanged with equimolar KCl. After PSS washing, the vessels were subjected to one of the following protocols. A cumulative concentration-effect curve to norepinephrine (3x10-8 to 3x10-6 M) was generated for each vessel. After tension had stabilized, the vessels were exposed to increasing doses of forskolin (10-8 to 3x10-5) to increase the intracellular cAMP concentration via activation of adenylate cyclase in the presence or absence of the ATP-dependent K+ channel (SUR2) inhibitor glibenclamide (3x10-6 M) or the ATP-dependent K+ channel opener cromakalim (10-8 to 10-4 M). In the second set of experiments, vessel rings were contracted by KPSS. After tension had stabilized, the vessels were exposed to the PDE3 inhibitor milrinone (3x10-7 to 10-4 M).18 In all experiments, a separate vessel segment from each individual was simultaneously monitored as control vessel.
Statistical Analysis
We used analysis of variance, repeated measures where indicated, and Bonferroni corrected t tests. We also used regression analysis, multiple stepwise if indicated; probability value <0.05 was considered significant.
| Results |
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mRNA Expression of Kir6.1, SUR2, PDE3A, and SOX5 in Arterial Vessels
TaqMan assays showed no difference in the candidate genes Kir6.1, SUR2, and LSOX5 between affected and nonaffected family members (data not shown). The TaqMan experiments showed, on average, a 147-fold higher PDE3A expression in 3 older (43±4 years), affected, severely hypertensive subjects, compared with 3 younger (23±2 years), less hypertensive-affected subjects. The expression in these 3 older subjects was 176-fold higher than in the 4 nonaffected (39±4 years) normotensive family members (Figure 2).
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Isolated Vessels
The response to KCl was similar in vessels from affected and nonaffected persons (Figure 3). The force generated by exposure to norepinephrine was not different. In precontracted vessels, the responses to cromakalim, forskolin, or forskolin in the face of glibenclamide were no different. The responses to milrinone were suggestive. We successfully performed experiments in 2 affected persons. Both showed enhanced relaxation to milrinone, compared with 4 nonaffected persons. One of these subjects had high PDE3A expression, whereas one did not.
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Pharmacological Testing
The forearm blood flow experiments showed no consistent differences between the groups (Figure 4). The response to intraarterial diazoxide was not increased in affected persons (Figure 4, upper). We also did not find a major difference in the responsiveness to intraarterial isoproterenol (Figure 4, middle) and milrinone infusions between nonaffected and affected family members (Figure 4, lower). The systemic infusions showed greater responses in affected persons (not shown). Affected subjects were highly sensitive to the depressor effect of systemic diazoxide. A cumulative dose of 137.5-mg diazoxide lowered systolic blood pressure 1.8±4 mm Hg in nonaffected and 18±5 mm Hg in affected family members (P=0.03). Affected family members were also more sensitive to nitroprusside. Systemic isoproterenol infusion at a rate of 1 µg/kg/min increased heart rate 16±2.9 bpm in nonaffected and 24±3.3 bpm in affected family members (P<0.05). The blood pressure response was similar in both groups.
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| Discussion |
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We believed our candidate genes were reasonable investigative targets. Potassium channels have a major function in regulating smooth muscle tone. SUR2 was particularly attractive because that ATP-dependent K channel regulator has been investigated extensively in gene-disrupted mice. Sur2-/-mice feature hypertension and episodic coronary spasm.20 We had sequenced the gene earlier and had found no mutations. However, the rearrangement raised the possibility of faulty potassium channel regulation. We used the potassium channel openers diazoxide and cromakalim to test the physiology of this system in the patients and their vessels. The vessels were tested with glibenclamide, a SUR2 blocker. However, we found no consistent differences between affected and nonaffected subjects or their vessels. The gene expression studies showed no differences in these candidates. Affected family members were hypersensitive to systemic diazoxide. However, responsiveness in forearm blood flow to intraarterial diazoxide was similar in both groups. This observation suggests that systemic diazoxide hypersensitivity is explained by impaired baroreflex blood pressure buffering rather than vascular hypersensitivity.7
PDE3A was an attractive candidate in our view. We had preliminary observations from Affymetrix chip experiments that PDE3A might be upregulated in affected persons (unpublished). We were unable to substantiate those findings here. Nevertheless, there was a tendency towards higher mRNA expression in older affected family members. The younger affected persons showed PDE3A transcription values in the range of the nonaffected subjects. A strong age effect on blood pressure was shown in affected persons in our earlier studies.3 PDE3A regulates cAMP and cGMP-mediated intracellular signaling.21 We used milrinone to inhibit PDE in these investigations. We found that vessels from two affected subjects differed in their responses, compared with vessels from nonaffected subjects. However, only one of these subjects had high PDE3A gene expression, an inconsistency that remains unexplained. We were limited in the material that we had available and by the small number of subjects. Interestingly, affected family members developed a greater tachycardia with isoproterenol than nonaffected subjects regardless of age. We cannot attribute this observation with certainty to changes in PDE3A function. We speculate that higher vascular PDE3A expression is possibly not the cause for the hypertension, but rather a result of the marked increase in blood pressure with age.
Perspectives
Delineating the rearrangement has advanced our search even though our 3 candidates failed under scrutiny. Our strategy is to determine the rearrangements in our remaining families, to recruit more families, and thereby determine the aspect of the rearrangement that all families have in common. Southern blotting will follow to show a different pattern at the breakpoint regions between affected and nonaffected subjects. We will explore the EST in the region. We suggest that this Mendelian hypertension syndrome is likely to be relevant to primary hypertension.
| Acknowledgments |
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Received September 30, 2003; first decision October 31, 2003; accepted November 26, 2003.
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