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(Hypertension. 2004;44:952.)
© 2004 American Heart Association, Inc.
Scientific Contributions |
From the Physiologisches Institut (C.d.W.), Universität Lübeck, Germany; Physiologisches Institut (M.K., D.S., U.P.), Ludwig-Maximilians-Universität München, Germany; and Institut für Pharmakologie und Toxikologie (R.F., S.F., F.H.), Technische Universität München.
Correspondence to Dr Cor de Wit, Physiologisches Institut, Universität Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany. E-mail dewit{at}physio.uni-luebeck.de
| Abstract |
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105 mm Hg). Pressure drops in response to intracarotid bolus application of the NO donor sodium nitroprusside (SNP) were almost abolished in cGKI/ mice, whereas ACh-induced pressure decreases remained intact in cGKI/ and eNOS/ mice. The direct observation of arterioles in the cremaster muscle by intravital microscopy showed impaired SNP-induced dilations in cGKI/ mice (by
80%) and normal ACh-induced dilations in cGKI/ and eNOS/ mice. ACh-induced dilations in eNOS/ mice were attenuated by iberiotoxin (by
50%), indicating that they were mediated in part by Ca2+-activated K+ channels, but not by inhibitors of cyclooxygenase or p450-monooxygenases. We conclude that cGMP and cGKI are the major effectors of NO to induce acute dilations of murine resistance vessels. However, the NO/cGMP/cGKI pathway is not essential for ACh-induced dilation of arterioles and for basal blood pressure regulation in mice.
Key Words: nitric oxide endothelium microcirculation
| Introduction |
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| Materials and Methods |
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Microcirculatory Studies
Intravital microscopy was performed in the cremaster muscle of anesthetized mice (droperidol [20 mg/kg], fentanyl [0.1 mg/kg], midazolam [2 mg/kg] IV) as described.10 In each animal, 7 to 12 arterioles were observed using a microscope (Metallux; Leitz) equipped with a video camera. Diameters were measured shortly before and during the local superfusion of ACh (0.1 to 10 µmol/L), SNP (0.1 to 10 µmol/L), or S-nitroso-N-acetyl-D,L-penicillamine (SNAP; 1 to 10 µmol/L). Increasing concentrations of vasoactive drugs were applied consecutively, with a recovery period of 5 minutes after washout. In some experiments, dilator responses were restudied in the presence of the cyclooxygenase inhibitor indomethacin (3 µmol/L), the reversible inhibitor of p450-monooxygenases sulfaphenazole (20 µmol/L), the suicide inhibitor of the p450-monooxygenase 17-octadecynoic acid (ODYA; 100 µmol/L), or the specific blocker of large-conductance Ca2+-dependent K+-channels (BKCa) iberiotoxin (0.1 µmol/L). The applied concentrations of these inhibitors have been shown previously to be effective in hamster microcirculation.11 The inhibitors were added continuously to the superfusion 30 minutes before the protocol was repeated, except for iberiotoxin, which was applied for only 15 minutes before restudying the dilator responses. Experiments lasted typically between 3 and 5 hours. The maximal diameter of the arterioles was measured during superfusion of a combination of different vasodilators (adenosine [100 µmol/L], SNP [100 µmol/L], and ACh [100 µmol/L]) at the end of the experimental protocol, and the animal was euthanized by an overdose of anesthesia.
Statistics and Calculations
Vascular tone is given as the quotient of the resting diameter of the vessel divided by its maximum. Changes of the inner diameter of the vessels were normalized to the maximal possible constriction or dilation according to the following relationship. Percentage of maximal response=(DTrDCo)/(DMDCo)x100, where DTr is the diameter observed after treatment and DCo is the control diameter before treatment. DM is (for dilator responses) the diameter at maximal dilation or (for constrictions) the minimal luminal diameter (zero). Comparisons within groups were performed using paired t tests, and for multiple comparisons, P values were corrected according to Bonferroni. Data between groups were compared by ANOVA followed by post hoc analysis of the means, with P<0.05 considered significant. Data are presented as mean±SEM.
| Results |
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Dilations in the Cremaster Muscle Microcirculation In Vivo
Arterioles with a diameter between 14 and 94 µm were studied (wt 31±3, n=42 in 4 animals; cGKI/ 27±2 µm, n=67 in 6 animals; P=0.22). Arteriolar resting tone was not different between genotypes (wt 0.34±0.03; cGKI/ 0.35±0.02) reflecting the normotension in cGKI/ mice. SNP induced a concentration-dependent arteriolar dilation in wt mice (Figure 2A). Significant dilations were already observed at the lowest SNP concentration used (0.1 µmol/L). In contrast, SNP-induced dilations in cGKI/ mice were absent at low (
0.3 µmol/L) and strongly reduced at higher (
1.0 µmol/L) SNP concentrations compared with wt controls (Figure 2A). Similar results were obtained using the NO donor SNAP. Dilations in response to 1 µmol/L and 10 µmol/L SNAP in wt mice were 30±5% and 63±5%, and in cGKI/ mice, 11±3 and 15±3% (P<0.05 versus wt). Endothelium-dependent dilations on ACh application were preserved in cGKI/ and eNOS/ arterioles (Figure 2B and 2C).
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Effect of Different Inhibitors on ACh-Induced Dilations
Indomethacin (3 µmol/L) did not induce significant diameter changes in cGKI/ mice or attenuate ACh-induced dilations (data not shown). Likewise, indomethacin did not attenuate ACh-induced dilations in eNOS/ mice (Figure 2C). Additional application of inhibitors of p450-monooxygenases (20 µmol/L sulfaphenazole or 100 µmol/L ODYA) did not affect arteriolar resting diameter (data not shown) or ACh-induced dilations in eNOS/ mice (Figure 2D). However, iberiotoxin (0.1 µmol/L), a specific blocker of large-conductance Ca2+-activated K+ (BKCa) channels, significantly attenuated ACh dilations in eNOS/ mice (Figure 2C), whereas vascular tone (before 0.24±0.02; after 0.23±0.03) or dilations in response to SNP (10 µmol/L) were not altered (71±4 versus 62±4%).
| Discussion |
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32 days) but normotensive at older age (
43 days). Thus, cGKI is apparently not essential for basal blood pressure regulation in 6- to 8-week-old mice. Second, endothelium-dependent dilations of resistance vessels triggered by ACh do not require eNOS-derived NO or cGKI because dilations initiated by ACh were preserved in eNOS/ and cGKI/ mice in the microcirculation, and only a shortening of the pressure drop after systemic ACh application was observed. This is in marked contrast to larger vessels, such as the aorta and arteria tibialis, in which ACh-induced relaxation is mediated exclusively by a pathway involving eNOS5 and cGKI.2,3 Our findings that ACh-induced decreases in MAP as well as ACh-induced dilations of cremaster arterioles were almost intact in eNOS/ and cGKI/ mice, together with previous reports showing normal ACh responses in various vascular beds of eNOS/ mice,16 strongly suggest that the NO/cGMP/cGKI pathway is not essential for ACh-triggered dilation of resistance vessels. The ACh-induced dilation of cremaster arterioles in eNOS/ mice was not affected by indomethacin but attenuated by iberiotoxin. This indicates that the ACh response was not mediated by prostaglandins but at least in part by an EDHF-like activity resulting in the opening of BKCa channels, hyperpolarization, and smooth muscle relaxation. In contrast to coronary17 and skeletal muscle18 arteries, this EDHF was apparently not related to metabolites of the cytochrome p450 pathway because 2 chemically distinct blockers of this pathway were without effect. The identity of the EDHF-like activity in cremaster arterioles remains unknown.
Perspectives
This study shows that hypotheses about vasodilator pathways cannot be generalized and need critical evaluation in different vascular beds, especially in vessels of different sizes. As in larger arteries,2 cGKI is an important effector of the acute effects of NO in resistance vessels. However, the NO/cGMP/cGKI pathway in arterioles is not essential for ACh-induced dilations and basal blood pressure control in mice. These results support the notion that cGMP-dependent pathways in smooth muscle may be more important in acute versus chronic blood pressure regulation19 and that the systemic hypertension of eNOS/ mice might be caused not only by a defect in endothelium-dependent vasorelaxation but also by other changes in the knockout animals, such as a disturbed baroreceptor response or an increased release of renin from the kidney into the bloodstream.5,20
| Acknowledgments |
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Received August 23, 2004; first decision September 7, 2004; accepted October 4, 2004.
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