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Cardiovascular Research 1998 37(3):791-798; doi:10.1016/S0008-6363(97)00262-9
© 1998 by European Society of Cardiology
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Copyright © 1998, European Society of Cardiology

Enhancement of the vasorelaxant potency of nicorandil by metabolic inhibition and adenosine in the pig coronary artery

Christina S Davie and Nicholas B Standen*

Ion Channel Group, Department of Cell Physiology and Pharmacology, University of Leicester, PO Box 138, Leicester LE1 9HN, UK

* Corresponding author. Tel. (+44-116) 252 3302; Fax (+44-116) 252 5045; E-mail: nbs@le.ac.uk

Received 10 June 1997; accepted 7 October 1997


   Abstract
Top
 Abstract
1 Introduction
 2 Materials and methods
 3 Results
 4 Discussion
 References
 
Objective: Nicorandil is used clinically to treat angina and acts in part by opening ATP-sensitive K+ channels whose opening is also enhanced by metabolic compromise. We have therefore investigated whether treatments that mimic conditions in ischaemia can increase the potency of nicorandil to dilate coronary arteries. Methods: Ring segments from pig small coronary arteries were mounted on a myograph, contracted with 20 mM K+ Krebs solution containing 200 nM BAYK 6844, and relaxations to cumulative doses of nicorandil were measured. Results and Conclusions: Nicorandil produced a dose-dependent relaxation with a mean pEC50 (–log EC50, M) of 4.76±0.02. Inhibition of metabolism with carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 100 nM) or by removal of extracellular glucose significantly increased the potency of nicorandil (pEC50s of 5.11±0.08 and 5.08±0.06, p<0.05 in each case). The adenosine analogue 2-chloroadenosine (2-CA, 300 nM) had a similar effect (pEC50=5.17±0.06, p<0.05). Reducing extracellular pH to 6.8 also significantly increased the potency of nicorandil, but to a smaller extent. Glibenclamide reduced the potency of nicorandil (pEC50=3.81±0.01, n=7), and abolished its enhancement by CCCP, zero glucose, 2-CA or pH 6.8 solution. 2-CA did not affect the potency of nicorandil in relaxing contractions to 80 mM K+ or the potency of glyceryl trinitrate. We conclude that the potency of nicorandil to cause coronary vasorelaxation is increased under conditions of metabolic inhibition. This effect appears to result from the K+ channel opening action of the drug, and may have significant consequences for its therapeutic effectiveness.

KEYWORDS Coronary artery; Nicorandil; Potassium channel; Adenosine; Pig; Ischaemia


   1 Introduction
Top
 Abstract
 1 Introduction
2 Materials and methods
 3 Results
 4 Discussion
 References
 
The vasodilator nicorandil is a nicotinamide derivative used clinically for the treatment of angina [1, 2]. Nicorandil has at least two mechanisms of action; it acts as a K+ channel opener and also has a nitrovasodilator action, activating guanylyl cyclase and so increasing guanosine 3':5'-cyclic monophosphate (cGMP) [3, 4]. The contribution of these two pathways to vasorelaxation appears to vary according to the tissue under study and the concentration of nicorandil used, the relative importance of the K+ channel opening mechanism being greater in small vessels and at lower concentrations of nicorandil [4–6]. Clinically, in contrast to other nitrovasodilators, tolerance does not develop on continued application of nicorandil, and this lack of tolerance seems to be due its K+ channel opening action [1].

In common with the ‘pure’ K+ channel openers such as levcromakalim and pinacidil, nicorandil activates glibenclamide-sensitive channels of the ATP-sensitive K+ channel (KATP) family (see [7]for review). Such channels in vascular smooth muscle respond to the metabolic state of the cell, and may be activated by metabolic inhibitors or by hypoxia [8–11]. Further, adenosine, which may play a role in matching blood flow to tissue demand in the coronary circulation can also activate these channels [12, 13]. The metabolic sensitivity of KATP channels suggests that K+ channel openers might be more potent in ischaemic tissue, where metabolism is compromised. Evidence for this has been obtained by Randall and Griffith [14]who showed that the vasorelaxant action of levcromakalim in the rabbit ear artery was enhanced by inhibition of oxidative metabolism or by hypoxia, an effect that may be mediated by activation of receptors by adenosine released as a consequence of hypoxia [15]. Interestingly, however, the potency of another opener of KATP channels, pinacidil, was unaffected by either hypoxia or adenosine receptor activation, suggesting that the effect depends on the exact mechanism of interaction of the opener with the KATP channel [14, 15]. In contrast to these findings in the rabbit ear, hypoxia and low extracellular glucose have been reported to reduce the effectiveness of levcromakalim in relaxing strips of rat aorta [16].

Since nicorandil has the dual action described above, it is possible that its potency might also be enhanced by metabolic compromise, provided that K+ channel opening contributes significantly to its relaxant action in the tissue concerned, and that its K+ channel opening action resembles that of levcromakalim rather than pinacidil. In view of the current clinical use of nicorandil as an anti-anginal agent, we have therefore investigated whether its potency in relaxing coronary arteries is increased by treatments designed to mimic those that might occur in ischaemia. Using rings from small branches of pig coronary artery we find that the relaxant action of nicorandil is enhanced in the presence of the inhibitor of oxidative phosphorylation carbonyl cyanide m-chlorophenyl hydrazone, by removal of extracellular glucose, by the adenosine analogue 2-chloroadenosine, or by lowering the extracellular pH to 6.8. Our results are consistent with nicorandil acting as a more effective vasodilator in ischaemic tissue.


   2 Materials and methods
Top
 Abstract
 1 Introduction
 2 Materials and methods
3 Results
 4 Discussion
 References
 
2.1 Myography
Pig hearts were obtained from a local abattoir, after animals had been slaughtered in accordance with EEC regulations, and second or third order branches (mean outer diameter 358±21 µm, n=36) were dissected from the left anterior descending coronary artery and cut into 2 mm ring segments. Dissection was done in ice cold saline solution containing (mM): 125 NaCl, 5 KCl, 1.8 CaCl2, 1 MgCl2, 10 HEPES, 10 glucose. Two rings were mounted in a small vessel wire myograph [17, 18](JP Trading). The rings were placed in a 10 ml bath maintained at 37°C. All chemicals were added directly to the bath. All the solutions contained 20 µM L-nitro arginine methyl ester (L-NAME) to eliminate endogenous nitric oxide activity. The vessels were contracted using 20 mM K+ Krebs solution (in mM: 20 KCl, 125 NaCl, 1.8 CaCl2, 1 MgCl2, 10 HEPES, 10 glucose, pH 7.4 with NaOH) with 200 nM of the calcium channel agonist BAYK 8644 (20K/BAYK). This solution causes activation of voltage-dependent Ca2+ channels both pharmacologically and as a consequence of the modest depolarization resulting from the increase in [K+] to 20 mM, while keeping the K+ equilibrium potential sufficiently negative for K+ channel activation to cause hyperpolarization and relaxation. We induced tonic contractions in this way because voltage-dependent Ca2+ channels provide the major contribution to tonic force in small arteries [19]and because the contractions elicited by 20K/BAYK were very stable, making it easy to analyse the relaxing effects of nicorandil. The maximal tonic force achieved was 14.5±1.0 mN (n=36), and all measurements were calculated as the reduction in tone. Once a stable contraction was achieved (~30 min) cumulative concentration-response curves to nicorandil (1–300 µM) or glyceryl trinitrate (0.1–30 µM) were constructed. For experiments in which the KATP channel blocker glibenclamide was used, maximal tone was achieved with 20K/BAYK and 10 µM glibenclamide was applied prior to measurement of the cumulative nicorandil concentration-response curve. In some experiments we used an 80 mM K+ solution, made by equimolar substitution of KCl for NaCl in the 20 mM K+ solution described above. Where used, carbonyl cyanide m-chlorophenylhydrazone (CCCP) or 2-chloroadenosine were added to the bath to give final concentrations of 100 and 300 nM respectively. For 0 glucose solution, glucose was omitted from the 20 mM K+ solution. The pH 6.8 solution was made by addition of HCl to 20 mM K+ solution.

2.2 Data analysis
The decrease in tone (relaxation) was measured from the maximal contraction generated with 20K/BAYK. The difference between the maximum tone and the baseline just prior to the contraction was calculated and used to convert the values to a percentage relaxation. All values are given as means±S.E.M. Individual concentration response curves were fitted with the expression


Formula 1

(1)
where y is the % relaxation, x the nicorandil concentration, M the maximum response, k the EC50 for nicorandil, and H the Hill coefficient, using the least squares algorithm in Sigmaplot (Jandel Scientific). EC50 values were obtained as the concentration at which half maximal reduction in tone occurred, and pEC50s calculated as –log EC50 (M). pEC50 values are given throughout as mean±S.E.M., and were used for statistical analysis. Statistical significance was assessed using Student's t-test for simple comparisons, and analysis of variance with Duncan's post hoc test for multiple comparisons.

2.3 Drugs
Nicorandil was a gift from Chugai Pharmaceuticals or Rhône-Poulenc Rorer. Sources of other drugs were as follows: glibenclamide, CCCP and L-NAME, Sigma; 2-chloroadenosine and BAYK 8644, RBI; glyceryl trinitrate, Lipha.


   3 Results
Top
 Abstract
 1 Introduction
 2 Materials and methods
 3 Results
4 Discussion
 References
 
3.1 Effects of metabolic inhibition on the potency of nicorandil
Fig. 1 illustrates the effect of inhibition of oxidative phosphorylation on the vasorelaxant action of nicorandil on pig coronary arterial rings. Under control conditions, nicorandil produced a concentration-dependent vasorelaxation, with a maximum at 300 µM of 96.1±0.6% of the 20K/BAYK contraction, a pEC50 of 4.79±0.02 and a Hill coefficient of 2.52±0.13 (n=30). The KATP channel inhibitor glibenclamide (10 µM) had no effect on the tonic contractile force elicited by 20K/BAYK, but caused a rightward shift in the concentration-effect curve for nicorandil, reducing its pEC50 to 3.81±0.11 and its Hill coefficient to 1.48±0.41 (n=7, Fig. 1A, Table 2).


Figure 1
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  		Fig. 1 Effects of metabolic inhibition on the vasorelaxant action of nicorandil. A. Concentration-response curves for the relaxation of pig coronary arterial rings by nicorandil under control conditions ({circ}, n=30) and in the presence of 10 µM glibenclamide ({blacksquare}, n=7). Rings were contracted with 20K/BAY K as described in the text, and the points show mean±S.E.M. * In this and Figures 1B and 1C indicate responses significantly different (P<0.05) from those under control conditions. The curves are drawn using Eq. (1). B. Concentration-response curves for the relaxation of pig coronary arterial rings by nicorandil under control conditions in the presence of 100 nM CCCP (bullet, n=8), and in the presence of 100 nM CCCP+10 µM glibenclamide ({blacksquare}, n=6). The broken line shows the control concentration-response curve of Figure 1A for comparison. C. Concentration-response curves for nicorandil relaxation in zero glucose solution ({blacktriangleup}, n=7), and in zero glucose solution +10 µM glibenclamide ({blacksquare}, n=6). The broken line shows the control concentration-response curve.

 

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  		Table 2 Values of pEC50s and Hill coefficients (H) for nicorandil relaxations of 20K/BAYK contractions in the presence of glibenclamide (10 µM) alone and together with CCCP (100 nM), zero glucose external solution, 2-chloroadenosine (2-CA, 300 nM), and pH 6.8 external solution. pEC50 values are also shown for nicorandil relaxations of 80 mM K+ contractions in the absence and presence of 2-chloroadenosine

 
To inhibit oxidative metabolism, we used carbonyl cyanide m-chlorophenylhydrazone (CCCP, 100 nM) which uncouples electron transfer in the respiratory chain. CCCP caused a small reduction in the maximum contractile force elicited by 20K/BAYK, from a control value of 14.1±2.7 mN to 12.6±2.5 mN (n=8, P=0.06, paired t-test). In the presence of CCCP, nicorandil was more potent at causing vasorelaxation, its concentration-response curve being shifted to the left, with a pEC50 of 5.11±0.08 and a Hill coefficient of 1.82±0.17 (n=8, Fig. 1B, Table 1), while the maximal relaxation was unaffected (96.2±1.1%). pEC50 and Hill coefficient values for nicorandil under control conditions and in the presence of CCCP and other agents and are given in Table 1. In common with the other experimental conditions we used that increased the potency of nicorandil (zero glucose, reduced pH and 2-chloroadenosine, see below), the effect of CCCP was abolished by glibenclamide (10 µM). Glibenclamide shifted the concentration-effect curve for nicorandil to the right, reducing the pEC50 to 3.95±0.07, a value not significantly different from that in the absence of CCCP (Fig. 1B, Table 2), consistent with the extra potency of nicorandil in the presence of CCCP resulting from its action on KATP channels.


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  		Table 1 Values of pEC50s and Hill coefficients (H) for nicorandil relaxation of 20K/BAYK contractions under control conditions and in the presence of CCCP (100 nM), zero glucose external solution (0 gluc.), 2-chloroadenosine (2-CA, 300 nM), and pH 6.8 external solution

 
We also investigated the effect of reducing glycolysis by removing glucose from the extracellular solution. In rat coronary arteries inhibition of glycolysis has been reported to cause vasodilation which can be reversed with glibenclamide [20]. Rings were bathed with glucose-free solution for 30–40 min before the application of nicorandil. Glucose-free solution did not affect the maximal contractile force elicited by 20K/BAYK, which was 15.6±2.0 mN under control conditions and 15.9±1.5 mN in zero glucose (n=7, P=0.83). However, like CCCP, zero glucose solution significantly increased the potency of nicorandil, raising its pEC50 to 5.08±0.06 (n=7, Fig. 1C, Table 1).

3.2 Decreased extracellular pH
A fall in extracellular pH is a feature of hypoxia or ischaemia. Decreased internal pH is a powerful activator of KATP channels in skeletal muscle [21], and extracellular acidosis has been shown to produce dilations of pig coronary resistance arteries that are inhibited by glibenclamide, implying that KATP channel activation is also involved in this vasodilation [22]. We therefore examined the effects of nicorandil under conditions where extracellular pH was reduced from its normal value of pH 7.4 to pH 6.8. This reduction in pH led to a reduction in maximal contractile force, from 14.2±2.2 mN to 11.2±1.8 mN (n=9, P<0.05, paired t-test). Fig. 2A and Table 1 show that, though not as effective as metabolic inhibition, a reduction in external pH to 6.8 caused a small but significant increase in the potency of nicorandil, raising its pEC50 to 4.95±0.06 (n=9).


Figure 2
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  		Fig. 2 Increase in the potency of nicorandil by reduced pH and by adenosine receptor activation. A. Concentration-response curves for nicorandil relaxation at pH 6.8 ({square}, n=9), and at pH 6.8+10 µM glibenclamide ({blacksquare}, n=6). Rings were contracted with 20K/BAY K, and the points show mean±S.E.M. * In this and in Figure 2 B indicates responses significantly different (P<0.05) from those under control conditions. The curves are drawn using Eq. (1). The broken line shows the control concentration-response curve. B. Concentration-response curves for the relaxation of pig coronary arterial rings by nicorandil in the presence of 300 nM of the adenosine agonist 2-chloroadenosine ({circ}, n=7), and in 2-chloroadenosine +10 µM glibenclamide ({blacksquare}, n=7). The broken line shows the control concentration-response curve.

 
3.3 Adenosine receptor activation
Adenosine released from cardiac myocytes may serve to match blood delivery to the metabolic needs of the cardiac tissue, and extracellular adenosine would be expected to increase under conditions of ischaemia [23, 24]. Adenosine receptor activation can activate KATP channels of coronary smooth muscle [12, 25], and activation of such channels by adenosine can play a role in hypoxic vasodilation in the coronary circulation [26]. We used the stable analogue of adenosine, 2-chloroadenosine, which acts at both A1 and A2 adenosine receptors to investigate the effect of adenosine receptor activation on the vasorelaxant potency of nicorandil. 2-Chloroadenosine caused a small reduction in the maximum contractile force elicited by 20K/BAYK from 15.1±2.1 mN to 12.9±1.8 mN (n=7, P<0.05, paired t-test). Fig. 2B shows that 2-chloroadenosine (300 nM) was an effective enhancer of the potency of nicorandil to relax coronary arterial rings, raising the pEC50 for nicorandil to 5.17±0.06 (n=7, Table 1).

3.4 Relaxations in the absence of K+ channel opening effects
The increases in the potency of nicorandil under conditions of metabolic inhibition, reduced pH, and adenosine receptor activation described above are consistent with such conditions increasing the activity of KATP channels so that the K+ channel opening activity of nicorandil is enhanced. Evidence that the enhancement of the potency of nicorandil is associated with its KATP opening action comes from the observation that this effect was abolished when KATP channels were blocked by glibenclamide (10 µM). Figs. 1 and 2Go include concentration-effect curves for nicorandil measured in the presence of glibenclamide. Nicorandil was considerably less potent in the presence of glibenclamide, and its potency was unaffected by CCCP, zero glucose, 2-chloroadenosine or by reduced pH under these conditions (Table 2). To further investigate whether the increased potency of nicorandil described above could be attributed to its K+ channel opening action, we investigated the effect of 2-chloroadenosine on the nicorandil relaxations of contractions to 80 mM K+ external solution, rather than the 20K/BAYK used in the previous experiments. In 80 mM K+, the K+ equilibrium potential should be about –16 mV, so that opening K+ channels will no longer cause hyperpolarization and relaxation. Thus in 80 mM K+, K+ channel opening should not contribute to the relaxant action of nicorandil, which is likely to result from the nitrovasodilator action of the drug under these conditions. Fig. 3A shows that, as expected, nicorandil was less potent in 80 mM K+ than in 20 mM K+, with a pEC50 of 3.97±0.11 (n=6). Further, in 80 mM K+ 2-chloroadenosine (300 nM) did not affect the potency of nicorandil (pEC50=3.96±0.04, n=5, P=0.96), a finding consistent with the increase in potency of nicorandil seen with 2-chloroadenosine at lower external [K+] resulting from an action on K+ channels.


Figure 3
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  		Fig. 3 Nicorandil relaxations in high potassium and relaxations to glyceryl trinitrate. A. Concentration-response curves for nicorandil relaxations of contractions to 80 mM extracellular K+ in the absence ({triangleup}, n=6), and in the presence ({blacktriangleup}, n=5) of 300 nM 2-chloroadenosine. The curves are drawn using Eq. (1). B. Concentration-response curves for the relaxation of pig coronary arterial rings by glyceryl trinitrate under control conditions ({square}, n=4) and in the presence of 300 nM 2-chloroadenosine ({blacksquare}, n=5). Rings were contracted with 20K/BAY K, and the points show mean±S.E.M.

 
3.5 Relaxations to glyceryl trinitrate
We also studied the effect of adenosine receptor activation on relaxations elicited by the nitrovasodilator glyceryl trinitrate. If the enhancement of the potency of nicorandil in the presence of 2-chloroadenosine described above were related to its nitrovasodilator action, then it might be expected that the potency of drug that acts solely as a nitrovasodilator would be increased in the same way. Fig. 3B shows that this was not the case: 300nM 2-chloroadenosine did not affect the potency of glyceryl trinitrate. The pEC50 for glyceryl trinitrate was 5.79±0.14 (n=4) in the absence of 2-chloroadenosine, and 5.56±0.33 (n=5) in its presence (P=0.57).


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Materials and methods
 3 Results
 4 Discussion
References
 
Our results show that in relatively small porcine coronary arteries the potency of nicorandil to cause vasodilation was enhanced by a number of manoeuvres designed to mimic changes that might occur under conditions where tissue perfusion is compromised. Thus inhibition of oxidative metabolism with CCCP or of glycolysis with zero glucose, a reduction in pH, or activation of adenosine receptors with 2-chloroadenosine all caused significant shifts of the dose-response curve for nicorandil to lower concentrations. The mechanism by which these effects occur seems independent of effects on endogenous NO, since all of our experiments were done in the presence of L-NAME to inhibit NO synthesis. Some of the manoeuvres described led to small decreases in the tonic contractile force elicited by 20K/BAYK, but these do not account for the enhanced vasodilator potency. Thus glucose free solution did not decrease contractile force, but significantly enhanced the potency of nicorandil. Further, 2-chloroadenosine, which caused a 15% decrease in tonic force and increased the potency of nicorandil, had no effect on the vasodilator potency of glyceryl trinitrate.

Nicorandil is known to cause vasodilation by two mechanisms, acting both as a K+ channel opener and as a nitrovasodilator [3, 4]. Our evidence suggests that the changes in the potency of nicorandil that we measure under conditions of metabolic compromise result from its K+ channel opening action. First, the increases in potency were abolished by the KATP channel blocker glibenclamide. Glibenclamide blocks channels of the KATP family in smooth muscle with a Ki of 20–200 nM and is an effective and relatively selective blocker of these channels at 10 µM [7, 27]. Glibenclamide shifted the dose-response curve for nicorandil to the right, and rendered it insensitive to the metabolic manoeuvres described above. Secondly, 2-chloroadenosine (300 nM), which caused a marked shift in the potency of nicorandil, was ineffective when extracellular [K+] was raised to 80 mM, which should abolish the ability of K+ channel opening to cause hyperpolarization and so relaxation. Finally, the potency of a compound that acts only as a nitrovasodilator, glyceryl trinitrate, was unaffected by 2-chloroadenosine. It can also be seen from Figs. 1 and 2Go and Table 1 that manoeuvres that increased the potency of nicorandil also reduced the Hill coefficient for its action, an effect that was significant with CCCP and 2-chloroadenosine. Such an effect would be expected if K+ channel opening contributes a relatively greater proportion of the vasorelaxant action of nicorandil at low concentrations, as has been described previously [4, 5]. Conditions that increase the degree of K+ channel opening at a given concentration of nicorandil would then have a greater effect on relaxation at low concentrations, leading to the observed slight flattening of the concentration-effect curve. The Hill coefficient for nicorandil was also reduced in the presence of glibenclamide, or in 80 mM K+ (Table 2). Under these conditions the coefficient presumably reflects only that for the nitrovasodilator action of nicorandil.

The precise mechanism of action of KATP channel opening drugs remains unknown [28, 29]. However in cardiac muscle single channel studies have shown that K+ channel openers reduce the sensitivity of KATP channels to inhibition by intracellular ATP [28, 30]. Conversely, a decrease in [ATP] has been shown to shift the concentration-effect curves for channel activation by RP 49356 or cromakalim to the left [30, 31], while a rise in the concentration of nucleoside diphosphates, which also increases KATP channel activity, can increase the potency of several K+ channel opening drugs [29]. Such direct evidence for the interaction of opening drugs and nucleoside phosphates has not yet been obtained in vascular smooth muscle, but all of the manoeuvres we have used in the present study have been shown, either directly or by implication, to cause activation of KATP channels, which are characterized by their sensitivity to cellular metabolism. Metabolic inhibition similar to that used in our experiments has been shown to reduce ATP levels by about a third in rabbit mesenteric artery and by 59% in uterine smooth muscle [32, 33], and changes in other metabolites, such as an increase in ADP concentration, may also contribute to channel activation under these conditions. Adenosine receptor activation and hypoxia can both activate KATP channels in smooth muscle cells isolated from pig coronary artery [10, 12]. A fall in intracellular pH increases KATP channel activity in skeletal and cardiac muscle, acting by reducing the affinity for ATP to cause channel inhibition [21]. It seems likely that the glibenclamide-sensitive dilation of coronary arterioles reported in response to acidosis [22]may result from a similar mechanism; a fall in intracellular pH occurring as a result of the acidosis and leading to channel activation. We therefore think it likely that the various procedures we have used in the present study lead to an increase in the activity of KATP channels. Under these conditions a given concentration of nicorandil causes more substantial channel opening and so hyperpolarization and vasorelaxation than it does under normoxic conditions.

It is clear, though, that the sensitivity of the action of K+ channel openers in vascular smooth muscle to enhancement by metabolic compromise may depend both on the tissue and on the precise mechanism of interaction between the drug and the channel. Our results with nicorandil on coronary arteries are consistent with the findings using levcromakalim in the rabbit ear described by Randall and Griffith, who found that metabolic inhibition, hypoxia, and adenosine A1 agonists all increased the vasorelaxant potency of levcromakalim [14, 15]. However pinacidil, which also acts by opening K+ channels of the KATP family, differed from levcromakalim in that its potency was unaffected by metabolic inhibition or adenosine, suggesting different mechanisms of interaction between these two agents and their target channel [14, 15]. Further, the potency of levcromakalim is not enhanced by metabolic inhibition in all vascular tissues, since in rat aorta neither hypoxia nor low glucose caused such an increase [16], a finding that may correspond to a relatively greater role for KATP channels in smaller arteries, or to differences in the cellular signalling pathways that contribute to their regulation [7]. The results we report here suggest that in terms of its ability to interact with mechanisms that are activated during ischaemia, the K+ channel opening action of nicorandil on coronary arteries resembles that of levcromakalim rather than pinacidil.

Our results suggest both that nicorandil activates K+ channels by a mechanism that can be enhanced when their activity is increased under conditions of metabolic compromise, and that K+ channel opening makes a large enough contribution to the effects of nicorandil in the coronary arteries we studied for this effect to be reflected in an increased relaxant potency for the drug. Although we have not been able to study resistance-sized vessels in these experiments, others have reported that the contribution of K+ channel opening to relaxation by nicorandil in the coronary circulation becomes greater with decreasing arterial size [4–6]. We would therefore expect the increase in the potency of nicorandil under ischaemic conditions to be at least as great in coronary resistance vessels as those we report here.

Our findings of enhanced potency under conditions designed to simulate those in ischaemia may have significant consequences for the therapeutic actions of nicorandil in angina. Such sensitivity to the metabolic state of tissue may result in nicorandil being effectively targeted to metabolically compromised tissues, so that it exerts its maximum vasodilator effects on regions within the heart where ischaemia is most pronounced.

Time for primary review 21 days.


   Acknowledgements
 
We thank Dr. J.M. Quayle for discussions and for comments on the manuscript, Mrs. D. Everitt for skilled technical support, Chugai Pharmaceuticals and Rhône-Poulenc Rorer for the gift of nicorandil, and the staff of J. Morris and Son and Dawkins Ltd. for supplying pig hearts. The work was supported by the Medical Research Council.


   References
Top
 Abstract
 1 Introduction
 2 Materials and methods
 3 Results
 4 Discussion
 References
 

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