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Cardiovascular Research 1999 42(1):214-223; doi:10.1016/S0008-6363(98)00316-2
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Copyright © 1999, European Society of Cardiology

Pharmacological characterisation of endothelium-dependent relaxation in human radial artery: comparison with internal thoracic artery

C.A Hamilton*, R Williams, V Pathi, G Berg, K McArthur, A.R McPhaden, J.L Reid and A.F Dominiczak

Departments of Medicine and Therapeutics and Cardiothoracic Surgery, Western Infirmary, Glasgow G11 6NT, UK

* Corresponding author. Tel.: +44-141-211-2042; fax: +44-141-339-2800.

Received 16 June 1998; accepted 5 October 1998


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objective: The aim of this study was to investigate the contribution of nitric oxide/prostanoid-independent pathways to endothelium-dependent vasorelaxation in human conduit arteries. Methods: Rings of internal thoracic artery (ITA) and radial artery (RA) taken from patients undergoing coronary artery bypass graft surgery were suspended in 10-ml organ baths and relaxation to carbachol and bradykinin studied in the presence and absence of nitric oxide synthase (NOS) inhibitors and potassium channel blockers. Results: No significant relaxation to carbachol or bradykinin was observed in ITA after NOS inhibition. In contrast, in RA less than 40% attenuation of relaxation to carbachol or bradykinin was achieved with any of the NOS inhibitors. In the presence of 20 mM K+ relaxation to carbachol and bradykinin was inhibited by 28±9% and 42±9% while in the presence of L-NAME 200 µM+20 mM K+ relaxation was inhibited by 66±6% and 70±4% respectively in this artery. Tetraethylammonium, glibenclamide, apamin and iberiotoxin had little effect on relaxation to carbachol but charybdotoxin alone and charybdotoxin plus apamin attenuated relaxation to carbachol by 23±4% and 49±9% in RA. In the presence of L-NAME 200 µM attenuation of these relaxations were increased to 60±4% and 78±4%. Conclusion: In ITA relaxations to carbachol and bradykinin were mediated via nitric oxide. In contrast in RA, a conduit vessel of similar diameter, both nitric oxide-dependent and independent pathways appeared to contribute to vascular relaxation. This nitric oxide-independent relaxation involved opening of Ca2+ activated potassium channel(s). The existence of alternative pathways mediating endothelium-independent relaxation could be important under pathological conditions and may contribute to the long term survival of radial artery grafts.

KEYWORDS Human radial arteries; Potassium channels; Relaxation


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
The endothelium plays a major role in the regulation of vascular tone. Endothelium-derived relaxing factor, prostacyclin and endothelium-derived hyperpolarizing factors (EDHFs) released from the endothelium can all cause vasorelaxation. Endothelium-derived relaxing factor has been identified as nitric oxide (NO) [1, 2], however the identity of EDHFs remains uncertain [3]. Recently, pharmacological tools have become available allowing the characterisation of the potassium channels involved in mediating this relaxation. There is evidence for both large and small conductance calcium-sensitive potassium channels being opened by EDHFs in different vascular beds from a range of animal species [4–6].

Less is known about the relative contributions of NO and EDHFs to relaxation in human vessels. NO-dependent relaxation is believed to predominate in internal thoracic artery (ITA) and saphenous veins (SV) [7, 8]. Both NO and EDHF-dependent relaxations have been demonstrated in coronary arteries [9, 10]and pial arteries from some subjects [6]. EDHF-dependent relaxations which were sensitive to tetraethylammonium have been reported in gastroepiploic and omental arteries [11, 12]. Little is known of the role of hyperpolarization and potassium channel activation in any other human arteries.

Internal thoracic artery (ITA) and saphenous vein (SV) are the most commonly used conduit vessels in coronary artery bypass graft surgery. While long term patency and survival are good for ITA grafts the prognosis for SV grafts is less promising [13, 14]. This has led to the search for other suitable conduit vessels. The radial artery (RA) as a conduit for coronary artery bypass grafting (CABG) was first suggested in 1971 [15], however its use was discontinued after early problems with occlusion of the vessel thought to be due in part to spasm [16]. In the late eighties interest was revived when some of the original patients were found at repeat angiography to have perfectly patent grafts. At this time the benefit of ITA use in CABG was apparent [17]which together with pharmacological means of reducing vasospasm led to renewed interest in arterial revascularisation. Recent reports of RA as a conduit in CABG have shown good early and medium term results [18–20]. It has been suggested that lower levels of nitric oxide production may contribute to the reduced patency of SV compared to ITA grafts [7, 8, 21]. Despite the increasing use of RA in CABG surgery little is known about the mechanisms regulating vasorelaxation in this vessel. The aim of the present study was to assess the contribution of EDHF and NO to endothelium-dependent relaxation in isolated radial arteries and internal thoracic arteries.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
Human ITA and RA were obtained from male patients undergoing CABG. Approval to use discarded tissues was granted by the Western Infirmary Ethical Committee, Glasgow UK. Clinical characteristics of the patients are summarised in Table 1. Human ITA was dissected as a pedicle with its venae commitantes from the thoracic wall by a no-touch technique leaving the vessels surrounded by internal thoracic fascia. The discarded distal end (1–2 cm) was placed in physiological salt solution.


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  		Table 1 Patient characteristicsa

 
Radial arteries were harvested from the non-dominant arm. A pedicle of RA with two venae commitantes was dissected without touching the artery. Branches were controlled with ligaclips and divided with scissors. No vasodilators were administered systematically or topically during the harvest periods. Following heparinization the proximal end was divided to assess backflow. If satisfactory the distal end was divided and 1–2 cm immediately placed in physiological salt solution.

The vessel segments were immediately transferred to the laboratory. The vessels were cleaned of connective tissue and cut into 2–3 mm rings. Rings were suspended on wires in 10-ml organ chambers filled with physiological salt solution (PSS), maintained at 37°C, and aerated with a mixture of 95% O2–5% CO2. The rings were connected to force transducers and changes in isometric tension were recorded. The PSS (pH=7.4±0.1) had the following composition in (mM): 130 NaCl, 4.7 KCl, 14.9 NaHCO3, 1.18 KH2PO4, 5.5 glucose, 1.17 MgSO4·7H2O, 1.6 CaCl2·2H2O, 0.03 CaNa2EDTA. In one series of experiments relaxation was studied first in the absence and then in the presence of indomethacin (0.02 mM dissolved in DMSO). The indomethacin was added to inhibit prostanoid synthesis. Based on the results from this study, indomethacin was added to the Krebs buffer for all subsequent work.

Based on preliminary experiments both vessels were set to a resting tension of 1.5 g. An equilibrium period of one and a half hours for ITA and 2–3 hours for RA were allowed. A longer equilibrium period was required for RA as it is a very muscular artery and took longer to achieve a stable baseline tension than ITA. Cumulative dose–response curves to phenylephrine (10–8–3x10–5 M) were then constructed. Baths were washed out and tissues allowed to relax. Rings were then constricted to their individual EC50 values for phenylephrine and relaxation to carbachol (10–8–10–5 M) or bradykinin (10–10–3x10–7 M) studied. The baths were again washed out, inhibitors of nitric oxide synthase (NOS) or potassium channel blockers added. Fifteen minutes later the rings were again constricted to the EC50 for phenylephrine. If necessary, the dose of phenylephrine was adjusted so that the tone was comparable to that previously achieved. A second dose–response curve to carbachol or bradykinin was then constructed. In some cases a third carbachol dose–response curve was also examined in the presence of a combination of NOS and potassium channel blockers. As for the second dose–response curve the third dose–response curve to carbachol was preceded by washout and reconstriction of the ring to its EC50 for phenylephrine. The duration of the experiment from constriction of the initial dose–response curve to phenylephrine to completion was 4–6 h. Some rings, which acted as controls, had neither nitric oxide synthase nor potassium channel blockers added before construction of the second and third dose–response curves.

The NOS inhibitors used were L-NAME 200 µM, 500 µM and 2 mM and L-NMMA 1 mM.

The following potassium channel blockers were used: tetraethylammonium (TEA) (3 mM) which is a relatively non-specific blocker of calcium-activated potassium channels. Charybdotoxin (50 nM) and Iberiotoxin (100 nM) which preferentially block large conductase KCa channels (BKCa), Apamin (0.5 µM) which blocks small conductance channels (SKCa) and Glibenclamide (10 µM) which blocks ATP-regulated channels. In addition relaxation to carbachol was also studied in the presence of the cytochrome P450 inhibitor miconazole (10 µM).

ITA and RA segments from patients were fixed in 10% phosphate buffered formalin followed by dehydration and embedding in paraffin wax. Four-micrometre thick sections were stained using haematoxylin and eosin. These sections were assessed for vessel wall structure integrity and endothelial continuity.

2.1 Materials
L-NMMA was supplied by Calbiochem, Nottingham, UK and glibenclamide by Alexis Corporation, Nottingham, UK. All other chemicals were obtained from Sigma–Aldrich Company, Poole, Dorset, UK.

2.2 Statistical analysis
The maximal relaxation to carbachol or bradykinin (Emax) and the concentration of agonist required to produce 50% relaxation (EC50) were calculated for each individual ring using Microsoft Excel (R). Groups contained 5–12 rings. Individual groups contained no more than two rings from any one patient. As the initial relaxation varied greatly between rings, the effects of NOS and K+ channel blockers were expressed as a percentage change as well as absolute values.

There were marked differences between patients with respect to age, plasma cholesterol levels, drug treatment and underlying disease which could all influence relaxation to carbachol resulting in marked differences between responses in rings from different patients. To allow for such variations, statistical analysis was normally only undertaken when it was possible to compare pre- and post-treatment values in the same rings. For comparison of one treatment with the control pre-treatment values the paired t-tests were used. Similarly, paired t-tests were used to compare the effects of charybdotoxin with those of charybdotoxin+apamin. Analysis of variance with bonferroni correction for multiple comparisons was used to compare the effects of K+ channel inhibitor+L-NAME with those of L-NAME alone. The Chi-squared test was used to examine differences between subjects donating RA and ITA with respect to history of hypertension, diabetes and smoking. All results are expressed as mean±SE, p<0.05 was taken as significant.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Patient characteristics
Patient characteristics are shown in Table 1. There were no significant differences in the ages of the patients donating RA and ITA. For patients donating RA the age range was 41–75, for those donating ITA 45–79. There were no significant differences between the groups in cardiovascular risk factors such as smoking, plasma cholesterol, hypertension or diabetes. One ITA patient was on only 2 drugs while one RA patient was taking 8. Apart from these two exceptions, the number of drugs taken ranged from 3–6 in both groups with a mean of 4±1 in each group. No particular drug combination was favoured by either group.

3.2 Histological appearance of RA and ITA
Light microscopic examination of haematoxylin and eosin stained paraffin sections of RA and ITA showed both to be muscular arteries with an average external diameter of 2.1 mm and 1.9 mm respectively (Fig. 1). There was a prominent fibrointimal layer in all RA examined, this layer measuring up to 58 µm in thickness. The ITA lacked this prominent fibrointimal layer with the endothelium often appearing to be closely applied to the luminal aspect of the internal elastic lamina. No other structural differences were apparent. In both ITA and RA, relaxation to carbachol and bradykinin was dependent on the presence of the endothelial layer.


Figure 1
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  		Fig. 1 Photomicrographs of the complete thickness of the wall of a typical RA (A) and ITA (B). The vessel sections were stained with haematoxylin+eosin. The luminal surface is at the top with an intact endothelial lining. The prominent fibrointimal layer of the RA is seen above the internal elastic lamina. Scale bar represents 40 µM.

 
3.3 Contractile responses to phenylephrine
Maximal contractile responses to phenylephrine were greater in RA than ITA 3.06±0.38g compared to 1.64±0.17 g. Similarly overall the PE50 values used to study relaxation to carbachol were higher in RA than ITA, 1.62±0.08 in RA and 1.03±0.08 in ITA. However, within RA and ITA subgroups there were no significant differences between the PE50 values.

3.4 Effect of indomethacin on relaxation to carbachol in ITA and RA
Relaxation to carbachol was not modified by indomethacin in either ITA or RA. There was a trend towards increased relaxation in the presence of the prostanoid synthesis inhibitor, however, this did not reach significance. In ITA, relaxation was increased in the presence of indomethacin in 6/9 rings in RA relaxation was increased in 8/9 rings (p=0.090, CI=–13.66, 1.22).

3.5 Effects of nitric oxide synthase inhibitors on relaxation to carbachol in ITA and RA. Relaxations to carbachol and bradykinin in control rings of RA and ITA
In control rings, to which neither nitric oxide or potassium channel blockers were added, relaxations to carbachol and bradykinin were similar in the initial and subsequent dose–response curves. No significant differences with respect to either Emax or EC50 were found between the first and second or third relaxation curves (Table 2).


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  		Table 2 Effect of NOS and prostanoid inhibitors on relaxation in ITA and RA

 
Relaxation in response to carbachol was significantly greater in RA than ITA. In ITA, L-NAME virtually abolished carbachol-dependent relaxation; in the presence of L-NAME 200 µM relaxation was reduced from 59±9% to 10±9% while in the presence of L-NAME 500 µM, relaxation was reduced from 57±12% to 10±3%. In contrast in RA, these concentrations of L-NAME only attenuated relaxation to carbachol by 24±6% and 27±6% respectively (Fig. 2, Table 2). Similarly in this vessel, L-NMMA 1 mM only attenuated relaxation to carbachol by 23±4%. In RA, increasing the concentration of L-NAME to 2 mM had only a small additional effect on relaxation (as shown in Table 2).


Figure 2
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  		Fig. 2 Relaxation to carbachol in internal thoracic artery (left hand panel) and radial artery (right hand panel). Rings were constricted to their individual EC50 values for phenylephrine and dose–response curves to carbachol 10–8–10–5 M constructed. Results are shown as mean±SE of each data point. {circ}, control; bullet, in the presence of L-NAME 200 µM; ITA n=6; RA, n=7.

 
Relaxation to bradykinin in RA also showed a large L-NAME resistant component, whereas in ITA no clear relaxation to bradykinin was observed in the presence of L-NAME (Table 2). However, it must be noted that only small relaxations to bradykinin were observed under control conditions in ITA.

3.6 Effect of high extracellular K+ on relaxation in RA
When the concentration of K+ in the Krebs buffer was raised to 20 mM maximum relaxation to carbachol was significantly reduced from 101±10% to 72±8%. In the presence of 20 mM K+ and L-NAME 200 µM or 500 µM further reductions in relaxation were observed. Raising the extracellular potassium concentration to 20 mM caused a similar attenuation of relaxation to bradykinin (Table 3).


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  		Table 3 Effect of raising extracellular K+ on relaxation in RA

 
3.7 Effect of potassium channel blockers on relaxation to carbachol in RA
Neither TEA, apamin, glibenclamide or iberiotoxin had any significant effect on the maximum relaxation to carbachol. Iberiotoxin caused a significant increase in the EC50 and although all curves were shifted to the right in the presence of TEA, the increase in EC50 just failed to reach significance (p=0.059; Table 4). In contrast, charybdotoxin caused a significant decrease of 24±3% in relaxation. The combination of charybdotoxin and apamin resulted in a 53±12% attenuation of relaxation which was significantly greater than that observed with charybdotoxin alone. Other combinations such as iberiotoxin plus apamin or glibenclamide plus charybdotoxin were much less effective (Table 4, Fig. 3).


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  		Table 4 Effect of K+ channel blockers on relaxation to carbachol in RA

 

Figure 3
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  		Fig. 3 Inhibition of relaxation to carbachol in RA by potassium channel blockers. Dose–response curves to carbachol were constructed in the absence and presence of potassium channel blockers. The reduction in the maximum relaxation in the presence of the potassium channel blocker was calculated and expressed as a percentage of the maximum relaxation prior to treatment. Results are expressed as mean±SE. The number of rings in each group is as indicated. Glib, glibenclamide (10 µM) n=6; TEA, tetraethylammonium (3 mM) n=5; Ibx, iberiotoxin (100 nM) n=6; Ap, Apamin (0.5 µM) n=6; Cx, Charbdotoxin (50 nM) n=12; Ib+Ap, n=6; Cx+Ap, n=7; Cx+Glib, n=5. * Significant attenuation of relaxation to carbachol p<0.05.

 
3.8 Effects of potassium channel blockers on relaxation in the presence of L-NAME in RA
In the presence of L-NAME plus TEA, apamin, iberiotoxin or glibenclamide, relaxation to carbachol was similar to that observed in the presence of L-NAME alone (Table 4, Fig. 4). In contrast, the attenuation of relaxation in the presence of L-NAME+charybdotoxin and L-NAME+charybdotoxin and apamin was significantly greater than that achieved in the presence of L-NAME alone. Charybdotoxin+L-NAME 200 µM attenuated relaxation by 60±4% while charybdotoxin+apamin and L-NAME 200 µM caused a 78±4% reduction in relaxation. Increasing the concentration of L-NAME to 500 µM appeared to have a small additional effect on relaxation but this did not reach statistical significance. In the presence of L-NAME 500 µM+charybdotoxin+apamin relaxation was reduced by 85±5%.


Figure 4
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  		Fig. 4 Inhibition of relaxation to carbachol in RA by potassium channel blockers in the presence of L-NAME. Dose–response curves to carbachol were constructed prior to any treatment and again in the presence of L-NAME or L-NAME+a potassium channel blocker. The reduction in the maximum relaxation after treatment was calculated and expressed as a percentage of the maximum relaxation prior to treatment. Results are expressed as mean±SE. The number of rings in each group is as indicated/L-NAME 200 µM, n=7; Glib, n=6; TEA, n=6; Ibx, n=6; Ap, n=6; Cx, n=6; Ib+Ap, n=6; Cx+Ap, n=7; Cx+Glib, n=7. * Significantly greater attenuation of relaxation to carbachol in the presence of a K+ channel blocker+L-NAME compared to L-NAME alone. Groups compared using ANOVA with Bonferroni correction for multiple comparisons.

 
3.9 Effect of miconazole on relaxation
Miconazole, which can act as an inhibitor of cytochrome P450-dependent enzymes, also attenuated relaxation to carbachol in RA. Maximum relaxation was 90±13% and 74±10% in the absence and presence of miconazole. The combination of miconazole+L-NAME 200 µM reduced relaxation from 93±13% to 38±9%.


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
In these studies we have shown that two human arteries of similar diameter have very different nitric oxide dependencies for endothelium-dependent relaxation. While relaxation in ITA was almost entirely nitric oxide-dependent both nitric oxide-dependent and independent pathways contributed to relaxation in RA. These nitric oxide-independent relaxations were inhibited by calcium-dependent potassium channel blockers and had characteristics consistent with endothelium-dependent hyperpolarizing factor-mediated relaxation.

Indomethacin did not attenuate relaxation in either vessel suggesting that prostanoids do not contribute to endothelium-dependent relaxation to carbachol. Indeed, there was a tendency for relaxation to improve in the presence of indomethacin and it is possible that carbachol caused contraction by the stimulated release of prostanoids such as PGH2 [22]. All subsequent work was therefore carried out in the presence of indomethacin. The lack of prostanoid-mediated relaxation in this study is consistent with in vivo observations in RA. Using an echotracking system coupled to a Doppler device to measure radial blood flow Joannides and colleagues [23]found aspirin to be without any haemodynamic effect.

Relaxation was reduced in the presence of K+ 20 mM and to a greater extent in the presence of K+ 20 mM+L-NAME 200 µM suggesting that an endothelium-dependent hyperpolarizing factor could contribute to relaxation in RA.

Relaxations dependent on potassium channel opening have been extensively reported in a range of animal vessels [24]but the role of endothelium-dependent hyperpolarization and potassium channel opening in mediating relaxation in human vessels is more uncertain. In man, nitric oxide-independent relaxation to bradykinin and substance P have been observed in coronary arteries [9, 10]and in some cases in pial arteries [6]. These relaxations were suggested to be due to EDHF release but the nature of the potassium channels involved has not been fully elucidated. In gastroepiploic and omentum arteries endothelium-dependent nitric oxide prostanoid-independent relaxations which were blocked by the non selective calcium-dependent potassium channel blocker TEA have been demonstrated [11, 12]. There are few other reports of endothelium-dependent nitric oxide/prostanoid-independent relaxation in human vessels.

There have been some studies looking at contractile responses in RA. Our findings are similar to those reported by others [25]. However, very little has been published on relaxation in this vessel. He and Yang [26]compared relaxation in RA and ITA but did not investigate the underlying mechanisms. There is some evidence from in vivo studies using combined echotracking/Doppler techniques to support the hypothesis that substance(s) additional to nitric oxide contribute to vasorelaxation in radial arteries [27, 28]. Joannides and colleagues [28]speculate that nitric oxide synthase inhibition may be associated with a compensatory increase in the release of EDHF. The current work appears to substantiate this contention.

A number of animal studies have led to the supposition that nitric oxide is the predominant vasodilator in large vessels whereas EDHFs are more important in small resistance vessels [29, 30]. Urakami-Harasawa and co-workers [11]came to the same conclusion in their study with human gastroepiploic arteries. However, the RAs and ITAs used for our study were of similar diameter. The consistent presence of a distinct fibrointimal layer or fibrointimal hyperplasia in RA compared with ITA represented the only morphological difference between these two muscular arteries. Thus, in man, the vessel diameter does not necessarily appear to predict the pathway(s) regulating vasodilatation. Animal studies have provided no concensus on the identity of the K+ channel opened by EDHF. In rabbit middle cerebral artery and abdominal aorta glibenclamide-sensitive relaxations have been reported suggesting the involvement of ATP-sensitive potassium channels [31, 32]. However other studies examining different vascular beds were unable to show any effect of glibenclamide on endothelium-dependent hyperpolarization [9, 33]. In the majority of studies either charybdotoxin, apamin or a combination of charybdotoxin plus apamin inhibited EDHF-mediated relaxation [33–35]consistent with the involvement of large and/or small conductance calcium-sensitive potassium channels.

In human radial arteries we found the nitric oxide/prostanoid-independent relaxation to be resistant to glibenclamide and apamin but attenuated by charybdotoxin and more strongly by charybdotoxin plus apamin. However iberiotoxin, which like charybdotoxin blocks large conductance calcium-sensitive potassium channels, had a much smaller effect on relaxation. Thus, the pattern of inhibition of vasorelaxation by the potassium channel blockers does not exactly correspond to that expected for any of the potassium channels identified to date. However, a similar profile for inhibition of EDHF-mediated relaxation has recently been described in guinea pig carotid artery and rat hepatic artery [35–37]. In the latter publication, the authors suggest that the target potassium channels for EDHF may be similar to one of the apamin-insensitive subtypes of small conductance calcium-sensitive potassium channels recently described by Kohler et al. [38].

If the potassium channel mediating relaxation in RA is a calcium-dependent potassium channel, the lack of attenuation of relaxation to carbachol in the presence of TEA requires consideration. Dose–response curves to carbachol were shifted to the right in the presence of TEA in all cases although this just failed to reach statistical significance (p=0.059). It is possible that a significant effect would have been observed if the number of vessels studied or the dose of TEA was increased. In other human vessels in which TEA or TBA have been shown to attenuate agonist-mediated relaxation studies were carried out in the presence of 10 mM inhibitor compared to the 3 mM used in this study [11, 12].

The nature of EDHFs remains uncertain. Evidence from several groups suggest the involvement of arachadonic acid metabolites formed via the cytochrome P450 pathway [39, 40]. Epoxyeicosatrienoic acids (EETs) are P450-derived metabolites of arachadonic acid and have recently been identified as EDHFs in bovine coronary vessels. Others have suggested that an endogenous cannabinoid represents EDHF [41]although this has been disputed [42]. In our studies of radial arteries miconazole, a cytochrome P450 enzyme inhibitor attenuated relaxation. Inhibition of nitric oxide/prostanoid-independent relaxation by miconazole and/or other inhibitors of cytochrome P450 have also been demonstrated in rat mesenteric arteries and procine and bovine coronary arteries [5, 39, 40]although not in human omental and gastroepiploic arteries or guinea pig carotid artery [11, 12, 43]. Attenuation of nitric oxide-independent relaxation by cytochrome P450 inhibitors has been taken as evidence for involvement of cytochrome P450 enzymes in synthesis of EDHFs. However, most of these compounds have additional effects [44, 45]which have the potential to modify endothelium-dependent relaxations.

In our studies in radial arteries, attenuation of relaxation in the presence of L-NAME and K+ channel blockers appeared to be more than additive in a number of instances. A similar phenomenon has been reported in rat isolated superior mesenteric bed [5]. The authors of this paper suggested that EDHF activity may become up-regulated on inhibition of nitric oxide production, compensating for the loss of nitric oxide. Similar suggestions have been made by Kilpatrick and Cocks [46]and Olmos et al. [47]. Moreover, an increase in non nitric oxide-mediated relaxation has been observed in vessels from cholesterol fed rabbits [48], stenosed canine coronary arteries [49]and from sheep with pulmonary hypertension [50]. However, Urakami-Harasawa and co-workers [11]reported that ageing and hypercholesterolaemia impair EDHF-mediated relaxations in human gastroepiploic arteries. The patient numbers in our study in radial arteries were too low to permit any attempt to relate risk factors (smoking, plasma cholesterol etc) to the relative contributions of nitric oxide-dependent and independent mechanisms to vasorelaxation.

All the vessels used in this study were from subjects with severe coronary artery disease. Despite the fact that many were taking HMG CoA reductase inhibitors, the average plasma cholesterol was high. The results obtained in this study may not reflect the pattern of relaxation in healthy individuals. However, human radial arteries are used increasingly for coronary grafts and the precise understanding of mechanisms of vasorelaxation in these vessels harvested from individuals with atherosclerotic intravascular milieu is of indisputed clinical importance. Knowledge of the factors regulating endothelium-dependent vasodilatation in such vessels may be important in optimising treatment of cardiovascular disease.

Time for primary review 30 days.


   Acknowledgements
 
This work has been funded by IRIS, Paris, France, The British Heart Foundation and Chest, Heart and Stroke (Scotland).


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

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