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Cardiovascular Research 1997 33(1):188-195; doi:10.1016/S0008-6363(96)00197-6
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Copyright © 1997, European Society of Cardiology

Ramipril therapy improves arterial dilation in experimental hypertension

Nina Hutri-Kähönena,b, Mika Kähönena,c,*, Jari-Petteri Tolvanena, Wu Xiumina, Kirsimarja Sallinena and Ilkka Pörstia,d

aMedical School, University of Tampere, PO Box 607, FIN-33101 Tampere, Finland
bDepartment of Clinical Chemistry, Tampere University Hospital, PO Box 2000, FIN-33521 Tampere, Finland
cDepartment of Clinical Physiology, Tampere University Hospital, PO Box 2000, FIN-33521 Tampere, Finland
dDepartment of Internal Medicine, Tampere University Hospital, PO Box 2000, FIN-33521 Tampere, Finland

Received 3 June 1996; accepted 29 August 1996


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: Angiotensin-converting enzyme (ACE) inhibition has been shown to restore impaired endothelial function in hypertension, but the roles of different mediators in enhanced endothelium-dependent dilation have not been fully characterized. Methods: The effects of ACE inhibition with ramipril (1 mg · kg–1 · day–1) on relaxation responses of mesenteric arterial rings in vitro were studied in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Results: The 12-week-long therapy effectively reduced blood pressure in SHR. In noradrenaline (NA)-precontracted arterial rings, endothelium-dependent relaxations to acetylcholine (ACh) as well as endothelium-independent dilations to isoprenaline and nitroprusside were more pronounced in WKY and ramipril-treated SHR than in untreated SHR. The cyclo-oxygenase inhibitor, diclofenac, which reduces the synthesis of dilating and constricting prostanoids, clearly enhanced the relaxation to ACh in untreated SHR, but was without effect in the other groups. The nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), attenuated the relaxations to ACh more effectively in untreated SHR than in the ramipril-SHR and WKY groups. However, when endothelium-dependent hyperpolarization was prevented by precontracting the preparations with potassium chloride (KCl), no significant differences were found in relaxations to ACh between the study groups. In addition, in NA-precontracted rings the diclofenac- and L-NAME-resistant relaxations to ACh were partially prevented by glibenclamide and apamin, inhibitors of ATP-dependent and Ca2+-activated K+ channels, respectively. Conclusion: Long-term ACE inhibition normalized blood pressure and enhanced arterial dilation in SHR. The improved endothelium-mediated relaxation following ramipril therapy could be attributed to reduced release of cyclo-oxygenase-derived constricting factors and augmented endothelium-dependent hyperpolarization in this type of experimental hypertension.

KEYWORDS ACE inhibitors; Rat, arteries; L-NAME; Blood pressure; Endothelium; Rat, spontaneously hypertensive


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
The antihypertensive action of angiotensin-converting enzyme (ACE) inhibitors is primarily based on the inhibition of angiotensin II (Ang II) formation. In addition to reduced Ang II generation, ACE inhibitors diminish the degradation of bradykinin, which in turn stimulates the synthesis of nitric oxide (NO) and prostacyclin (PGI2) in endothelial cells [1], and enhances endothelium-mediated hyperpolarization of smooth muscle [2]. Interestingly, long-term ACE inhibitor therapy has been shown to augment endothelium-mediated relaxation of arteries in hypertensive experimental animals [3] and also in hypertensive humans [4]. The improved endothelial function following ACE inhibition has been attributed to enhanced endothelium-dependent hyperpolarization [5], increased release of NO from the endothelium [6], and enhanced formation of vasodilatory prostaglandins [7].

Since the roles of different endothelium-derived mediators in the improved arterial responses after ACE inhibition in hypertension remain somewhat obscure, the present study was designed to examine in detail the effects of long-term treatment with ramipril on vascular contractile and relaxation responses in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY).


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1. Animals and experimental design
Male SHR (Okamoto-Aoki strain) and WKY were obtained from Møllegaard's Breeding Centre, Ejby, Denmark. The rats were housed in an experimental animal laboratory (illuminated 06.00–18.00 h, temperature +22°C) with free access water and chow (Ewos, Södertälje, Sweden). The systolic blood pressures of conscious animals were measured at +28°C by the tail-cuff method (Model 129 Blood Pressure Meter; IITC Inc., Woodland Hills, Ca., USA). At 8 weeks of age both SHR and WKY were divided into two groups of equal mean systolic blood pressures. Thereafter, SHR (n = 12) and WKY (n = 12) were given ramipril in drinking water in light-proof bottles (1 mg · kg–1 · day–1, fresh solutions daily prepared), while untreated SHR (n = 12) and normotensive WKY (n = 12) were kept on normal drinking fluid. Ramipril therapy and blood pressure measurements continued for 12 weeks. Thereafter ramipril administration was withdrawn 1 day before the rats were decapitated and exsanguinated. The hearts were removed and weighed, and the superior mesenteric arteries excised. The experimental design was approved by the Animal Experimentation Committee of the University of Tampere, Finland, and performed in accordance with the Home Office Guidance on the operation of the Animals (Scientific Procedures) Act 1986, published by HMSO, London.

2.2. Mesenteric arterial responses in vitro
Five successive sections (3 mm in length) of the mesenteric artery from each animal were cut. In the 3 most distal rings the endothelium was left intact, and from the first two pieces vascular endothelium was gently removed [8]. The rings were placed between hooks (diameter 0.3 mm) and suspended in an organ bath chamber (volume 20 ml) in physiological salt solution (PSS) (pH 7.4) of the following composition (mM): NaCl 119.0, NaHCO3 25.0, glucose 11.1, CaCl2 1.6, KCl 4.7, KH2PO4 1.2, MgSO4 1.2, and aerated with 95% O2 and 5% CO2. The rings were initially equilibrated for 1 h at +37°C with a resting force of 1.5 g. The force of contraction was measured with an isometric force-displacement transducer and registered on a polygraph (FT 03 transducer and Model 7 E Polygraph; Grass Instrument Co., Quincy, Ma., USA). Normally, the presence of intact endothelium is confirmed by an almost complete relaxation to 1 µM acetylcholine (ACh) in 1 µM noradrenaline (NA)-precontracted rings, while no relaxation is observed in endothelium-denuded rings [8]. However, in the study reported here the responses to ACh in the SHR group hardly attained 50% relaxation. Therefore, no vascular preparations were excluded form the study.

Endothelium-independent relaxation: The responses of endothelium-denuded preparations to nitroprusside and isoprenaline were cumulatively determined. The relaxations were elicited after precontraction with 1 µM NA, which resulted in approximately 60% of the maximal contraction in each group. The next drug concentration was added only after the previous level of relaxation had become stable.

Potassium relaxation: The endothelium-denuded ring was contracted with 125 mM KCl (reference response). After 30 min the rings were exposed to K+-free solution (pH 7.4; KH2PO4 and KCl were substituted with NaH2PO4 and NaCl, respectively). The omission of K+ induced gradual contractions, and after the response had reached a plateau, 1 mM K+ was re-added and the subsequent relaxation evaluating the activity of Na+,K+-ATPase was registered [8]. The K+-free contractions and relaxations to K+ repletion were repeated in the presence of 1 mM ouabain.

Arterial contractions and relaxations to ACh after precontraction with KCl: Concentration-response curves for NA and serotonin were determined in endothelium-intact rings. Thereafter relaxations to ACh were examined in rings precontracted with 60 mM KCl. The responses to ACh were repeated in the presence of 3 µM diclofenac, and in the presence of diclofenac and 0.1 mM NG-nitro-L-arginine methyl ester (L-NAME).

Relaxations to ACh after precontraction with NA: Responses to ACh were examined in endothelium-intact rings. The responses to ACh were repeated in the presence of 3 µM diclofenac; in the presence of diclofenac and 0.1 mM L-NAME; in the presence of diclofenac, L-NAME and 1 µM glibenclamide; and in the presence of diclofenac, L-NAME, glibenclamide and 1 µM apamin (inhibitors of cyclo-oxygenase, NO synthase, ATP-dependent K+ channels and Ca2+-activated K+ channels, respectively).

Relaxations to adenosine 5'-diphosphate (ADP): An endothelium-intact ring was used to study responses to ADP after precontraction with 60 mM KCl. Then the relaxations were examined in the presence of 3 µM diclofenac, and in the presence of diclofenac and 0.1 mM L-NAME.

The contractions were expressed in grams and the EC50 for serotonin in each ring was calculated as a percentage of maximal response. The relaxations in response to K+ repletion, ACh, isoprenaline and nitroprusside were presented as a percentage of pre-existing contractile force. The EC25 or EC50 values for the 3 latter relaxants were calculated as percentages of 1 µM NA-induced precontraction by the use of a computer programme and presented as the negative logarithm (pD25, pD50), which values were also used in the statistical analysis.

2.3. Drugs
The following drugs were used: ramipril hydrochloride (Astra Pharmaceutical Company, Sweden), acetylcholine chloride, adenosine diphosphate, apamin, diclofenac, glibenclamide, isoprenaline, NG-nitro-L-arginine methyl ester hydrochloride, serotonin (Sigma Chemical Co., St. Louis, Mo., USA), L-noradrenaline L-hydrogen tartrate and nitroprusside (Fluka Chemie AG, Buchs SG, Switzerland). Ramipril was dissolved directly in tap water. The stock solutions of the compounds used in the in vitro studies were dissolved in distilled water, with the exception of glibenclamide (in dimethylsulfoxide). All solutions were freshly prepared before use and protected from light.

2.4. Analysis of results
Statistical analysis was carried out by one-way analysis of variance (ANOVA) supported by the Bonferroni test when carrying out pairwise comparisons between the test groups. When appropriate, ANOVA for repeated measurements with Greenhouse-Geisser adjustment was applied for data consisting of repeated observations at successive time points. All results are expressed as means with s.e. means. Differences were considered significant when P < 0.05.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1. Blood pressure, heart weight and body weight
The systolic blood pressure of SHR was already higher at the beginning of the study than in WKY, and during the 12-week-long follow up it continued to increase in untreated SHR. Ramipril treatment beginning at the age of 8 weeks effectively reduced blood pressure in SHR, the values being comparable to those in normotensive WKY during the follow-up period. The treatment also somewhat reduced blood pressure in WKY (Fig. GR1). Cardiac hypertrophy was totally prevented in SHR by ramipril, relative heart weights of ramipril-treated SHR not differing from those of WKY (Table 1). WKY on oral ramipril gained somewhat less weight than untreated WKY (Table 1). However, no signs of compromised well-being of the animals were observed by our experienced experimental animal laboratory staff. Chow intakes were comparable in all study groups (data not shown).


Figure 1
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  		Fig. GR1 Systolic blood pressures in untreated spontaneously hypertensive rats (SHR, {square}), ramipril-treated SHR ({blacksquare}, 1 mg·kg–1·day–1), untreated Wistar-Kyoto (WKY, {circ}) rats and ramipril-treated WKY rats (bullet). Symbols indicate means with s.e.m.; n = 10–12 in each group; * P < 0.05, ANOVA for repeated measurements.

 

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  		Table 1 Experimental group data at close of the study

 
3.2. Mesenteric arterial responses
The relaxations induced by ACh in endothelium-intact NA-precontracted (1 µM) mesenteric arterial rings were impaired in untreated SHR when compared with WKY (Fig. GR2). These responses were clearly improved in SHR by the ramipril treatment, the relaxations not differing from those of WKY. Cyclo-oxygenase inhibition with diclofenac (3 µM) markedly improved relaxation to ACh in untreated SHR, but not in the other groups. The NO synthase inhibitor, L-NAME (0.1 mM) (in the presence of diclofenac), diminished the relaxations of NA-precontracted rings in both strains, the influence on ACh response being more pronounced in SHR than WKY (Fig. GR2). Glibenclamide (1 µM), a blocker of ATP-dependent K+ channels, somewhat reduced the diclofenac- and L-NAME-resistant relaxations to ACh in ramipril-SHR and the WKY groups. The addition of apamin (1 µM), an inhibitor of Ca2+-activated K+ channels, induced a moderate further reduction in the remaining relaxations to ACh in the study groups (Fig. GR2, Table 2). The relaxations to ACh were not affected in the study groups in the presence of ramiprilat (the active metabolite of ramipril) or the kinin β2-receptor antagonist, icatibant (Hoe 140; data not shown).


Figure 2
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  		Fig. GR2 Relaxations to acetylcholine (ACh) in isolated endothelium-intact mesenteric arterial rings from untreated spontaneously hypertensive rats (SHR, {square}), ramipril-treated SHR ({blacksquare}), untreated Wistar-Kyoto (WKY, {circ}) rats and ramipril-treated WKY rats (bullet). The relaxations were induced after precontraction with 1 µM noradrenaline in the absence and in the presence of 3 µM diclofenac; in the presence of diclofenac and 0.1 mM NG-nitro-L-arginine methyl ester (L-NAME); in the presence of diclofenac, L-NAME and 1 µM glibenclamide; in the presence of diclofenac; L-NAME, glibenclamide and 1 µM apamin. The insert in panel 3 and panel 6 show the changes in relaxation to ACh induced by diclofenac and L-NAME, respectively, when compared with the responses without these compounds. Symbols indicate means with s.e.m.; n = 10–12 in each group; * P < 0.05, ANOVA for repeated measurements.

 

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  		Table 2 Parameters of contractile and relaxation responses of isolated endothelium-intact arterial rings

 
Interestingly, the relaxations to ACh during precontraction with KCl (60 mM) (i.e., under conditions of prevented endothelium-derived hyperpolarization) were comparable in all 4 study groups (Fig. GR3). Again, diclofenac clearly improved the relaxation to ACh in untreated SHR. In addition, the responses to ACh in KCl-precontracted rings were effectively reduced by L-NAME in all groups (Fig. GR3). Very similar results were observed with ADP, another endothelium-dependent vasodilator, when studied under precontractions induced by 60 mM KCl: the relaxations were comparable in the study groups, and the responses to ADP were correspondingly influenced by diclofenac and L-NAME when compared with the results obtained with ACh (data not shown). The endothelium-intact vascular rings of untreated SHR and WKY showed comparable sensitivity (i.e., pD50 values) and maximal force generation to serotonin, and the ramipril treatment was without significant effect on arterial contractions to serotonin (Table 2).


Figure 3
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  		Fig. GR3 Relaxation to acetylcholine in endothelium-intact mesenteric arterial rings form untreated spontaneously hypertensive rats (SHR, {square}), ramipril-treated SHR ({blacksquare}), untreated Wistar-Kyoto (WKY, {clubsuit}) rats and ramipril-treated WKY rats (bullet). The relaxations were induced after precontraction with 60 mM KCl in the absence and in the presence of 3 µM diclofenac, and in the presence of diclofenac and 0.1 mM NG-nitro-L-arginine methyl ester (L-NAME). Symbols indicate means with s.e.m.; n = 10–12 in each group; * P < 0.05, ANOVA for repeated measurements.

 
The relaxations to the endothelium-independent agents isoprenaline and nitroprusside in endothelium-denuded arterial rings were impaired in untreated SHR when compared with the WKY groups. Ramipril therapy markedly enhanced also these responses in SHR, the relaxations in the ramipril-SHR group not differing from those of normotensive controls (Fig. GR4, Table 2). Interestingly, the relaxations to isoprenaline and nitroprusside were also augmented in WKY by ramipril (Fig. GR4). The maximal contractions elicited by K+-free solution were not statistically different between the study groups (maximal forces in SHR, Rami-SHR, WKY, and Rami-WKY being 1.3 ± 0.2, 0.9 ± 0.2, 0.9 ± 0.2, 0.7 ± 0.1 g, respectively), with the exception of the comparison between untreated SHR and ramipril-WKY (P = 0.005). After the return of K+ to the organ bath upon the K+-free precontractions the rate of the subsequent relaxation was faster in WKY groups than in untreated SHR, and the ramipril therapy clearly enhanced the rate of K+ relaxation in both SHR and WKY (Fig. GR4). Furthermore, K+ relaxation was effectively inhibited by the Na+,K+-ATPase inhibitor, ouabain, in all groups (Fig. GR4).


Figure 4
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  		Fig. GR4 Relaxations of enothelium-denuded mesenteric arterial rings from untreated spontaneously hypertensive rats (SHR, {square}), ramipril-treated SHR ({blacksquare}), untreated Wistar-Kyoto (WKY, {circ}) rats and ramipril-treated WKY rats (bullet). The relaxations to nitroprusside and isoprenaline were induced after precontraction with 1 µM noradrenaline, and the relaxations to re-addition of 1 mM K+ after precontraction induced by K+-free buffer solution in the absence and in the presence of 1 mM ouabain. Symbols indicate means with s.e.m.; n = 10–12 in each group; * P < 0.05, ANOVA for repeated measurements.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
Impaired endothelium-dependent relaxation has been repeatedly observed in experimental hypertension [9]. and also in the present study the relaxations to ACh and ADP in NA-precontracted rings were attenuated in SHR, whereas these responses were clearly enhanced by ramipril. One explanation for the attenuated endothelium-mediated relaxations in hypertension is enhanced release of endothelium-derived contracting factor(s) (EDCF) [10]. Previously, the endothelium-dependent vasoconstrictor responses in SHR have been shown to be blocked by cyclo-oxygenase inhibition[11]. In the present study, the cyclo-oxygenase inhibitor, diclofenac, enhanced the relaxations to ACh in untreated SHR, suggesting that constricting prostanoids were indeed involved in these responses. However, diclofenac was without significant effect on the relaxations to ACh in the other groups, suggesting that products of the cyclo-oxygenase pathway were not playing a significant role in the responses to ACh in the WKY groups and ramipril-treated SHR. Therefore, the release of contractile factors from the endothelium appeared to be reduced in SHR by ramipril.

Recent studies have suggested that a prostanoid-mediated, endothelium-dependent mechanism contributes to the vasoconstrictor effect of angiotensin II (Ang II) in experimental hypertension [12]. Both Ang II and ACh have been found to increase the formation of EDCF, most likely prostaglandin H2 (PGH2) or thromboxane A2 (TXA2), in the mesenteric artery of SHR [13]. In addition, treatment with a TXA2-prostaglandin endoperoxide (TP)-receptor blocker in vivo completely restored endothelium-dependent dilation in SHR [14]. Ang II has also been found to activate enzyme systems which generate another cyclo-oxygenase-dependent contractile factor—the superoxide anion—in cultured aortic smooth muscle cells of the rat[15]. Interestingly, the production of EDCF has been shown to parallel closely the increase in blood pressure in SHR[16]. Therefore, inhibition of Ang II formation and lowering of blood pressure provide possible mechanisms by which the release of endothelial contractile factors was reduced in SHR by ramipril. Nevertheless, since the relaxations to ACh still remained impaired after cyclo-oxygenase inhibition in untreated SHR when compared with the other groups, endothelial factors other than prostanoids were probably involved in the enhanced endothelium-mediated dilations in ramipril-SHR.

ACE inhibition has been suggested to potentiate endothelium-dependent dilation in normotensive [6] and hypertensive animals by enhancing the availability of NO [3]. In the present study, however, the ramipril-SHR group showed distinct L-NAME-resistant dilations to ACh, suggesting that enhanced NO release did not explain these responses. Indeed, ACh has been shown to cause hyperpolarization of arterial smooth muscle which remains resistant to both NO synthase and cyclo-oxygenase inhibition[17]. Therefore a substance termed endothelium-derived hyperpolarizing factor (EDHF) has been proposed as a vasoactive autacoid of endothelial origin [18]. The action of EDHF can be eliminated by membrane depolarization with high concentrations of KCl, and under these conditions the relaxation to ACh thus largely reflects the effects of NO. In contrast, during agonist-induced precontractions EDHF remains operative [19]. Interestingly, no significant differences were found between the present study groups in response to ACh and ADP when the precontractions were induced by KCl. The fact that ramipril therapy did not affect the relaxations to ACh in KCl-precontracted rings, while those induced in NA-precontracted rings were markedly enhanced, suggests that hyperpolarization induced by ACh was augmented by ACE inhibition in SHR.

EDHF has been described to be an endogenous K+ opener, but the nature of K+ channels opened by EDHF has not been fully characterized. Glibenclamide has been reported to inhibit hyperpolarization to ACh in rabbit cerebral artery [20], and antagonise relaxation to ACh in rat aorta [21], which findings suggest the involvement of ATP-sensitive K+ channels. However, apamin, a blocker of Ca2+-activated K+ channels, has been found to reduce the L-NAME-insensitive relaxation in rat mesenteric artery, and apamin together with charybdotoxin to completely abolish these responses [22], whereas glibenclamide was found to be ineffective in blocking the hyperpolarization to ACh [17]. These findings suggest that EDHF relaxes mesenteric arteries mainly by activating Ca2+-activated K+ channels. In the present study, glibenclamide slightly inhibited the responses to ACh in WKY and ramipril-treated SHR, and further inhibition was observed when apamin was added to the medium. These findings support the view of augmented endothelium-dependent hyperpolarization by ramipril in SHR, mediated at least partially via arterial K+ channels.

The endothelium-independent dilations induced by isoprenaline and nitroprusside were also attenuated in untreated SHR, and were enhanced by ramipril. The normalization of vasodilation of β-adrenoceptor activation and exogenous NO, which most likely reflected enhancement of general vascular dilatory properties, may have contributed to the enhanced endothelium-dependent relaxations in ramipril-SHR. Moreover, exogenous NO has been shown to hyperpolarize guinea pig uterine artery [23] and rat mesenteric artery [17], while the blockers of Ca2+-activated K+ channels, charybdotoxin and tetraethyl-ammonium, have been shown to decrease relaxation to NO in guinea pig pulmonary arterial and tracheal smooth muscle [24]. Isoprenaline has also been reported to open ATP-dependent K+ channels in canine saphenous vein[25] and to cause endothelium-independent hyperpolarization in porcine coronary artery [26]. Thus, augmented function of K+ channels in smooth muscle could partially explain the enhanced relaxations to the endothelium-independent agonists as well as the improved endothelium-mediated hyperpolarization in ramipril-SHR in this study.

Vascular Na+,K+-ATPase function was evaluated indirectly by K+ repletion upon K+-free medium-induced precontractions [8] since the return of K+ activates the Na+,K+-ATPase, which repolarizes the cell membrane and thus relaxes smooth muscle [27]. The rate of K+ relaxation also reflects general smooth muscle relaxation mechanisms (e.g., contractile protein dephosphorylation, calcium sequestration and extrusion) [28], but previous results suggest that K+ relaxation rate is indicative of Na+,K+-ATPase activity in rat mesenteric artery [8]. In the present study, the K+ relaxation rate was markedly slower in SHR than WKY, in concert with earlier observations [8], and was markedly enhanced after ramipril therapy. The enhanced K+ relaxation suggests increased recovery rate of ionic gradients across the cell membrane in ramipril-SHR, probably via improved function of Na+,K+-ATPase. This conclusion is supported by the fact that K+ relaxation was effectively inhibited by the Na+,K+-ATPase inhibitor, ouabain, in all study groups. Enhanced function of Na+,K+-ATPase would also favour hyperpolarization of arterial smooth muscle.

In conclusion, ramipril therapy normalized blood pressure in SHR, an effect which was associated with improved endothelium-dependent and -independent arterial relaxation, while vascular contractile responses were not significantly affected. Since the endothelium-mediated relaxations in ramipril-SHR were augmented in the absence and presence of NO synthase inhibition but not under conditions of prevented hyperpolarization, and cyclo-oxygenase inhibition markedly enhanced these relaxations in untreated SHR but not in the other groups, augmented endothelium-dependent relaxation after ACE inhibition could be attributed to enhanced endothelium-dependent hyperpolarization and diminished endothelium-derived contraction.


   Acknowledgements
 
This study was supported by the Finnish Cultural Foundation, Pirkanmaa Fund, the Ida Montin Foundation, the Medical Research Fund of Tampere University Hospital, and the Paavo Ilmari Ahvenainen Foundation, Finland, and by Astra Pharmaceutical Company, Sweden.


   Notes
 
* Corresponding author. Tel. +358 3 2156111; Fax +358 3 2156170. Back


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

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P. M. Vanhoutte and E. H. C. Tang
Endothelium-dependent contractions: when a good guy turns bad!
J. Physiol., November 15, 2008; 586(22): 5295 - 5304.
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