Pharmacological analysis of the inhibitory effects of KRN2391 on endothelin-1-induced contraction in isolated large coronary artery of the pig

1. The relaxant effect of KRN2391, N-cyano-N′-(2-nitroxyethyl)-3-pyridine-carboximidamide-monomethanesulfonate (with both K⁺ channel opener and nitrate actions), nifedipine (Ca²⁺ channel blocker), nitroglycerin (nitrate) and cromakalim (K⁺ channel opener) were investigated in isolated porcine large coronary arteries contracted by endothelin-1. These drugs inhibited endothelin-1-induced contraction in a concentration-dependent manner.2. The relaxation induced by KRN2391 was nearly complete at their maximum effects, but nifedipine and cromakalim could not produce complete relaxation.3. The concentration-relaxation curves for KRN2391 underwent a rightward shift in the presence of methylene blue or glibenclamide. The concentration ratios of KRN2391 calculated based on EC₅₀ values were 2.8 and 3.7 in the presence of methylene blue and glibenclamide, respectively.4. The concentration-relaxation curves for nitroglycerin and cromakalin underwent a rightward shift in the presence of methylene blue and glibenclamide, respectively, and the concentration ratios of nitroglycerin and cromakalim were 12.0 and 6.3.5. These relaxant effects of KRN2391 and nitroglycerin on endothelin-1-induced contraction of porcine coronary artery were greater than those of cromakalim and nifedipine. This potent relaxant action of KRN2391 on endothelin-induced contraction is thought to be based on both a nitrate action and a K⁺ channel opening action.     

Comparison of the effects of KRN2391 and other coronary dilators on porcine isolated coronary arteries of different sizes

Abstract— The present study was performed to determine whether KRN2391 (N-cyano-N′-(2-nitroxyethyl)-3-pyridinecarboximidamide monomethanesulphonate), a novel vasodilator, shows different effects on porcine isolated coronary arteries of different sizes. The vasodilating effects of KRN2391 on porcine large (2·5–3·0 mm outer diam.) and small (0·8–1·0 mm) coronary arteries were also compared with those of cromakalim, nicorandil, nifedipine, nitroglycerin and adenosine. The relaxant effects of these drugs were examined in coronary arteries contracted by 25 Mm KCl. Nitroglycerin caused greater relaxation in large vessels than in small vessels. In contrast, adenosine, nifedipine and cromakalim caused greater relaxation in small vessels. However, there was no difference between large and small vessels in the relaxing effects of KRN2391 and nicorandil. These unique features of KRN2391 and nicorandil appear to be beneficial in ischaemic heart disease.     

Dissimilarity in the mechanisms of action of KRN2391, nicorandil and cromakalim in canine renal artery

Abstract— In the present study, we examined the mode of action of KRN2391 (N-cyano-N′-(2-nitroxyethyl)-3-pyridinecarboximida-mide monomethanesulphonate) in isolated canine renal artery compared with those of nicorandil and cromakalim. KRN2391 (10^?8-3 × 10^?5 m ), nicorandil (10^?7-3 × 10^?4 m ) and cromakalim (10^?8-3 × 10^?5 m ) relaxed renal arteries contracted by 25 Mm KCl in a concentration-dependent manner. KRN2391-induced relaxation was inhibited by methylene blue (10^?5 m ) and glibenclamide (10^?6 m ). Nicorandil-induced relaxation was inhibited by methylene blue, but not by glibenclamide. The concentration-relaxation curve for cromakalim displayed a rightward parallel shift in the presence of glibenclamide. In the control observation, KRN2391 and nicorandil also produced full relaxation, but cromakalim did not. The present results suggest that KRN2391 acts as both a nitrate and a potassium channel opener, and nicorandil acts only as a nitrate and only in canine renal artery.     

Contribution of cyclic GMP formation to KRN2391-induced relaxation in coronary artery of the pig.

1. In the present study, we investigated the relationship between relaxation and guanosine 3':5'-cyclic monophosphate (cyclic GMP) formation induced by KRN2391, compared with those induced by nicorandil and nitroglycerin, in the coronary artery of the pig. 2. KRN2391 (10(-8)-3 X 10(-5) M), nicorandil (10(-8)-3 X 10(-4) M) and nitroglycerin (10(-9)-10(-5) M) antagonized the contraction caused by 25 mM KCl in a concentration-dependent manner. 3. The concentration-relaxation curves for KRN2391, nicorandil and nitroglycerin shifted rightward in the presence of methylene blue (10(-5) M). 4. KRN2391 (10(-6) M), nicorandil (10(-4) M) and nitroglycerin (10(-6) M) induced an increased in cyclic GMP. 5. The magnitude of the shift of the concentration-relaxation curve caused by methylene blue and the increase in cyclic GMP with KRN2391 were lower than those with nicorandil and nitroglycerin. 6. The adenosine 3':5'-cyclic monophosphate (cyclic AMP) level was not increased by KRN2391 even at a concentration that produced full relaxation. 7. The present results suggest that KRN2391-induced relaxation in the coronary artery of the pig is partly due to the increase in cyclic GMP formation through the stimulation of guanylate cyclase.     

Differential vasodilator properties of KRN2391, cromakalim, nitroglycerin and nifedipine in rabbit isolated femoral artery and vein.

1. The selectivity for artery and vein of KRN2391, cromakalim, nitroglycerin and nifedipine was examined in isolated femoral artery and vein preparations of the rabbit. 2. All drugs produced a concentration-dependent relaxation in both femoral artery and vein. 3. Nitroglycerin was more potent in femoral vein than in femoral artery at all concentrations. The EC50 value obtained in the vein was about 14 times smaller than that obtained in artery. 4. Cromakalim and nifedipine were almost equipotent on both vascular preparations. Cromakalim at the highest concentration (10(-5) M) produced 88 and 78% relaxation in femoral artery and vein, respectively. The maximum relaxation induced by nifedipine (10(-6) M) was less than 50% in both preparations. 5. KRN2391 was active at a lower concentration in the vein than in the artery and its maximum relaxation at 10(-5) M was about 90% in both preparations. 6. Glibenclamide (10(-6) M) inhibited the vasorelaxation caused by KRN2391 in both artery and vein. Methylene blue (10(-5) M) also inhibited the relaxant action of KRN2391 but this action was slight in the artery. 7. These results suggest that KRN2391 and nitroglycerin are more potent in the vein than in the artery and cromakalim and nifedipine are equipotent in both. It is considered that the relaxation induced by low concentrations of KRN2391 reflects predominantly its action as a nitrate and that at high concentrations it acts as a K+ channel opener in addition to its nitrate action. The different vascular selectivities of these drugs are thought to relate to the differences in their mechanisms of action in vascular smooth muscle.     

Vasodilator action in the nitroxylated cyanoamidine derivative, KRN2391, in rabbit basilar artery.

KRN2391 is a cyanoamidine derivative with a pyridine ring and a nitroxyl group. This gives the molecule a dual pharmacological action as both an ATP-sensitive K channel (K(ATP)) opener and an organic nitrate. In cerebrovascular disease with endothelial dysfunction, such a compound could be advantageous to prevent the negative consequences of a reduced synthesis of endogenous nitric oxide and endothelium-derived hyperpolarizing factor. The objective of this study was to characterise the vasodilator action of KRN2391 in a cerebral artery. As shown in the rabbit basilar artery contracted by endothelin-1 KRN2391 elicited a concentration-dependent relaxation. KRN2391 was unable to relax arteries contracted by a 60 mM K solution. The KRN2391-induced relaxation of endothelin-1-contracted arteries was unaffected by N(G)-nitro-L-arginine (0.1 mM), indomethacin (10 microM) or removal of the endothelium. The guanylate cyclase inhibitors ODQ (10 microM) and LY53583 (10 microM), and the cGMP phosphodiesterase inhibitor zaprinast (10 microM) each had no effect on the KRN2391-induced relaxation. Glibenclamide (1 microM), a blocker of K(ATP), caused a rightward shift of the concentration-response curve for KRN2391. The relaxation induced by the prototype K(ATP) opener levcromakalim was inhibited to a similar extent by glibenclamide. Addition of ODQ or LY53583, or the calcium-sensitive K channel blockers apamin (0.1 microM) and charybdotoxin (0.1 microM) in the presence of glibenclamide did not produce a significant further inhibition of the KRN-induced relaxation. KRN2391 (10 microM) did not influence the content of cGMP in the basilar artery, whereas the nitric oxide donor 3-morpholino-sydnonimine (0.1 mM) increased the cGMP level three-fold. Thus, KRN2391 is an effective vasodilator of the rabbit basilar artery, acting mainly through opening of KATP . The nitro-moiety of the molecule does not seem to contribute to the relaxant effect in this artery.     

Vasodilator Action of the Nitroxylated Cyanoamidine Derivative,KRN2391, in Rabbit Basilar Artery

Abstract: KRN2391 is a cyanoamidine derivative with a pyridine ring and a nitroxyl group. This gives the molecule a dual pharmacological action as both an ATP-sensitive K channel (K_ATP) opener and an organic nitrate. In cerebrovascular disease with endothelial dysfunction, such a compound could be advantageous to prevent the negative consequences of a reduced synthesis of endogenous nitric oxide and endothelium-derived hyperpolarizing factor. The objective of this study was to characterise the vasodilator action of KRN2391 in a cerebral artery. As shown in the rabbit basilar artery contracted by endothelin-1 KRN2391 elicited a concentration-dependent relaxation. KRN2391 was unable to relax arteries contracted by a 60 mM K solution. The KRN2391-induced relaxation of endothelin-1-contracted arteries was unaffected by N^G-nitro-L-arginine (0.1 mM), indomethacin (10 μM) or removal of the endothelium. The guanylate cyclase inhibitors ODQ (10 μM) and LY53583 (10 μM), and the cGMP phosphodiesterase inhibitor zaprinast (10 μM) each had no effect on the KRN2391-induced relaxation. Glibenclamide (1 μM), a blocker of KATP, caused a rightward shift of the concentration-response curve for KRN2391. The relaxation induced by the prototype K_ATP opener levcromakalim was inhibited to a similar extent by glibenclamide. Addition of ODQ or LY53583, or the calcium-sensitive K channel blockers apamin (0.1 μM) and charybdotoxin (0.1 μM) in the presence of glibenclamide did not produce a significant further inhibition of the KRN-induced relaxation. KRN2391 (10 μM) did not influence the content of cGMP in the basilar artery, whereas the nitric oxide donor 3-morpholino-sydnonimine (0.1 mM) increased the cGMP level three-fold. Thus, KRN2391 is an effective vasodilator of the rabbit basilar artery, acting mainly through opening of K_ATP. The nitro-moiety of the molecule does not seem to contribute to the relaxant effect in this artery.     

Vasospasmolytic effect of KRN2391 on 3,4-diaminopyridine-induced rhythmic contraction of porcine coronary artery

1. In the present study, we examined the vasospasmolytic effect of KRN2391 on rhythmic contractions of porcine coronary artery caused by 3,4-diaminopyridine (3,4-DAP) compared with cromakalim and nitroglycerin.2. KRN2391 at 10⁻⁷ showed a tendency to prolong the cycle length and at 10⁻⁶ M completely eliminated rhythmic contractions in all preparations. The elimination by 10⁻⁶ M KRN2391 was antagonized by either oxyhemoglobin (10⁻⁵ M) or glibenclamide (3 × 10⁻⁶) although not completely.3. Cromakalim at 10⁻⁵ M and nitroglycerin at 10⁻⁷ M completely eliminated 3,4-DAP-induced rythmic contractions in all preparations. The elimination by cromakalim and nitroglycerin was completely antagonized by glibenclamide and oxyhemoglobin, respectively.4. The present study suggests that the vasospasmolytic effect of KRN2391 on 3,4-DAP-induced rhythmic contractions is based on its nitrate action and K channel opening action.     

Spasmolytic action of nicorandil in canine conductive coronary arteries in vivo is not modified by glibenclamide.

The spasmolytic effects of nicorandil, cromakalim, and nitroglycerin on coronary arteries were investigated by angiographic technique in anesthetized dogs. With intracoronary arterial (i.a.) U 46619, a thromboxane A2 mimetic, the diameter of coronary arteries decreased in a sustained manner by 36.1 +/- 1.6% from control levels (coronary spasm). With a successive i.a. injection of nicorandil (300 micrograms), cromakalim (30 micrograms), or nitroglycerin (3 micrograms), the diameter recovered control levels (102.9 +/- 3.9, 96.8 +/- 5.6, and 100.1 +/- 4.3%, respectively). In dogs treated intravenously (i.v.) with glibenclamide, a pharmacologic antagonist of K-channel openers, the spasmolytic effect of cromakalim was significantly reduced, whereas the activity of nicorandil or nitroglycerin remained unaffected. We also investigated a possible modification by glibenclamide of the increase in coronary blood flow (CBF) induced by i.a. nicorandil and cromakalim in anesthetized dogs. The dose-dependent blood flow responses to cromakalim and nicorandil were significantly attenuated by glibenclamide, whereas the response to nitroglycerin remained unaffected. These results suggest that the spasmolytic effect of nicorandil on canine conductive coronary vessels is not mediated by K-channel opening but by a nitroglycerin-like action and that the dilatation of resistive coronary vessels induced by nicorandil may be largely due to its action as a K-channel opener.     

Pharmacological analysis of the vasodepressor effect of KRN2391 in pithed rats

• 1.1. The antagonism by glibenclamide of the vasodepressor effects of KRN2391, cromakalim and nitroglycerin was compared in pithed rats with blood pressure supported by an infusion of phenylephrine.
• 2.2. Cumulative administration of KRN2391 (3–300 μg/kg, i.v.), cromakalim (3–300 μg/kg, i.v.) and nitroglycerin (1–300 μg/kg, i.v.) produced dose-dependent decreases in diastolic blood pressure.
• 3.3. In rats given glibenclamide (20 mg/kg, i.v.), the dose-vasodepressor curves for KRN2391 and cromakalim were shifted to the right. However, glibenclamide had no effect on the vasodepressor effect of nitroglycerin.
• 4.4. The ED_{50 mmHg} values increased about 5.9 fold for KRN2391 and 9.5 fold for cromakalim in glibenclamide-treated rats.
• 5.5. These results suggest that the vasodepressor effect of KRN2391 is due to both glibenclamide-sensitive and insensitive mechanisms. This glibenclamide-insensitive effect of KRN2391 is thought to reflect its nitrate action.

     

Pharmacological analysis of the effect of KRN2391 on coronary vasculature in perfused rat heart

• 1.1. The antagonism by glibenclamide of the effects of KRN2391, cromakalim and nifedipine on coronary flow (CF) were compared in isolated perfused rat hearts.
• 2.2. KRN2391, cromakalim and nifedipine increased CF in a concentration-dependent manner. In the presence of glibenclamide, the concentration-effect curves on CF for KRN2391 and cromakalim shifted to the right but that for nifedipine did not change.
• 3.3. The EC₅₀ values increased about 6.6-fold for KRN2391 and 19.0-fold for cromakalin in the presence of glibenclamide.
• 4.4. These results suggest that the increase in CF induced by KRN2391 is due to both glibenclamide-sensitive and insensitive mechanisms.

     

Differential antagonism by glibenclamide of the relaxant effects of cromakalim,pinacidil and nicorandil on canine large coronary arteries

Summary The relaxant mechanisms of action of cromakalim, pinacidil and nicorandil, potassium channel openers, on large epicardial coronary arteries were investigated in isolated canine left circumflex arteries contracted by 10^–7 mol/l U46619, a thromboxane A₂ analogue, or addition of 25 mmol/l KCl in comparison with nitroglycerin.Cromakalim (3 × 10^–8–3 × 10^–5 mol/l), pinacidil (10^–6–10^–4 mol/l), nicorandil (3 × 10^–6–10^–3 mol/l) and nitroglycerin (3 × 10^–8–10^–5 mol/l) all produced a concentration-dependent relaxation in both U46619- or KCl-contracted arteries. At their maximum effects pinacidil, nicorandil and nitroglycerin produced full relaxation in arteries contracted by either means. In contrast, cromakalim produced about a 73% relaxation in KCl-contracted arteries, although it caused full relaxation in U46619-contracted ones. In the presence of glibenclamide the concentration-relaxation curves for cromakalim in U46619- or KCl-contracted arteries underwent rightward parallel shifts. Schild regression had a slope of 1.00 and yielded a pA₂ of 7.47 for glibenclamide in U46619-contracted arteries, and corresponding values obtained in KCl-contracted arteries were 0.86 (not significantly different from unity) and 7.28. The concentration-relaxation curves for pinacidil in U46619-contracted arteries also underwent rightward parallel shifts in the presence of glibenclamide, however, Schild regression had a slope of 0.60. The concentration-relaxation curves for pinacidil in KCl-contracted arteries underwent rightward parallel shifts only to a limited extent in the presence of glibenclamide. The concentration-relaxation curves for nicorandil and nitroglycerin in U46619- or KCl-contracted arteries were not affected by glibenclamide in concentrations which antagonized cromakalim. The concentration-relaxation curves for nicorandil or nitroglycerin in U46619-contracted arteries were shifted by methylene blue (10^–5 mol/l) to the right without suppression of the their maximum effects. Similar curves for cromakalim were not affected at all by this concentration of methylene blue. The concentration-relaxation curves for nicorandil in U46619-contracted arteries determined in the presence of methylene blue (10^–5 mol/l) and glibenclamide (3 × 10^–7, 10^–6 and 3 × 10^–6 mol/l) were not significantly different from those in the presence of methylene blue alone.These results indicate the following: In canine large epicardial coronary arteries (1) cromakalim produced relaxation by the mechanism antagonized by glibenclamide, probably opening ATP-sensitive potassium channels, (2) pinacidil did so by the mechanism shared with cromakalim and by one not antagonized by glibenclamide as well, and (3) nicorandil did so exclusively by the mechanism of action as a nitrate. Send offprint requests to N. Taira at the above address     

Cardioprotective effects of KRN2391 and nicorandil on ischemic dysfunction in perfused rat heart.

The cardioprotective effect of KRN2391 (N-cyano-N-(2-nitroxymethyl)-3- pyridinecarboximidamide methanesulfonate), a novel vasodilator, was studied in the isolated perfused rat heart and compared with that of nicorandil. The isolated buffer-perfused rat heart was subjected to 25 min ischemia followed by 30 min reperfusion. The heart was pretreated with 0.1-10 microM KRN2391, 10-1000 microM nicorandil or vehicle. Before ischemia, KRN2391 (1-10 microM) and nicorandil (10-1000 microM) increased coronary flow, but did not modify the cardiac mechanical function. KRN2391 (1-10 microM) and nicorandil at high doses (300-1000 microM) resulted in significant improvements of cardiac functions and coronary flow during reperfusion and significantly reduced the release of cytosolic enzymes. The protective effects of 3 microM KRN2391 and 300 microM nicorandil were completely reversed by 3 microM glibenclamide, a blocker of ATP-sensitive potassium channels. Thus, KRN2391 and nicorandil at high doses have a direct cardioprotective effect, which may be related to activation of ATP-sensitive potassium channels.     

Analysis of relaxation and repolarization mechanisms of nicorandil in rat mesenteric artery.

1. The mechanisms by which nicorandil causes relaxation of rat isolated small mesenteric arteries mounted on a Mulvany myograph was investigated by use of a combination of putatively mechanism-specific antagonists. 2. In arteries precontracted by the thromboxane-mimetic, U46619, the EC50 for cromakalim and levcromakalim-induced relaxation curves were raised by 36 and 17 fold by glibenclamide (3 microM) while the EC50 for nicorandil-induced relaxation was unaffected by either glibenclamide or methylene blue (10 microM). A combination of these antagonists raised the EC50 for nicorandil by 8 fold. 3. In U46619-contracted arteries, nifedipine (100 nM) did not affect the cromakalim relaxation curve but it raised the EC50 for nicorandil by 5 fold. The combination of methylene blue, glibenclamide and nifedipine further inhibited the maximum relaxation to nicorandil. 4. In separate experiments, membrane potential (Em) and force responses were measured simultaneously. In arteries depolarized and contracted by U46619 both nicorandil and cromakalim repolarized (delta Em, 35 mV) and relaxed (100%) the vessels. Glibenclamide (3 microM) did not alter the relaxation-concentration curve to nicorandil but reduced the maximum repolarization to delta 10.8 mV. In contrast to Em and relaxation-response curves to cromakalim were shifted to the right by glibenclamide by 30-100 fold. 5. In unstimulated arteries, nicorandil (but not cromakalim) -induced hyperpolarization was significantly antagonized by methylene blue (10 microM) (6.6 fold rightward shift) or nifedipine (100 nM) (2.6 fold). In depolarized arteries (U46619), nifedipine but not methylene blue inhibited the nicorandil-induced hyperpolarization. 6. In arteries precontracted to 50% tissue maximum by either KCl or U46619, nifedipine (100 nM) relaxed the artery but failed to repolarize the Em. Presumably voltage-operated calcium channels (VOCC) were blocked preventing contraction but the artery remained depolarized, presumably through non VOCC mechanisms. 7. These data suggest that nicorandil may relax small arteries through 3 parallel pathways, (i) NO-donor mediated stimulation of guanylate cyclase and increase in cyclic GMP, (ii) K+ATP channel opening, and (iii) nifedipine-sensitive VOCC inhibition. Em data suggest that nicorandil-induced repolarization is caused principally through opening K+ATP channels. Blockade of this hyperpolarization by glibenclamide is not sufficient to alter the relaxation, indicating dissociation of nicorandil-induced changes in membrane potential and relaxation. 8. These results highlight the ''chameleon'' actions of nicorandil where there is no apparent association of Em repolarization with relaxation, in contrast to the parallel responses for cromakalim.     

KRN2391: dual action on rat pulmonary artery and no loss of potency in pulmonary hypertension

1. The pulmonary vasorelaxant properties of KRN2391 (N-cyano-N'-(2-nitroxyethyl)-3-pyridinecarboximidamide) were examined in isolated ring preparations of main (MPA) and intralobar (IPA) pulmonary artery from control and pulmonary hypertensive rats (exposure to hypoxia, 10% oxygen, for 1 week). 2. On both MPA and IPA, pulmonary vasorelaxant responses were inhibited by methylene blue (10 micromol/L) or glibenclamide (1 or 10 micromol/L). Thus, KRN2391 has the properties of both a nitric oxide (NO) donor and a K(ATP) channel opener on rat pulmonary arteries. 3. KRN2391 was more potent and gave a greater maximum relaxation on MPA (-log EC(50) 6.47; maximum 92% reversal of induced contraction) than on IPA (-log EC(50) 6.09; maximum 58% reversal of induced contraction). Comparable differences between MPA and IPA were seen for SIN-1 (NO donor) and levcromakalim (K(ATP) channel opener). 4. KRN2391 was equipotent in MPA from control and pulmonary hypertensive rats but, when glibenclamide (10 micromol/L) was present, KRN2391 was six-fold less potent in preparations from pulmonary hypertensive than control rats. An eight-fold reduction in potency was seen for SIN-1 (no glibenclamide) in arteries from pulmonary hypertensive rats, confirming previous findings with other NO donors. 5. It is concluded that the dual mechanism of action of KRN2391 accounts for the finding that this drug is equally potent in pulmonary arteries from pulmonary hypertensive and control rats. In the context of pulmonary hypertension, this property of the drug could give it an advantage over drugs that act solely as NO donors because these decline in potency, at least in animal models of this disease.     

Comparative analysis of vasodilating Mechanisms of Kil769, Ki3315 and KRN2391, pyridinecarboximidamide derivatives,in porcine isolated coronary artery

1. The vasodilating mechanisms of pyridinecarboximidamide derivatives which have a nitroxyl group (KRN2391), a phenyl group (Ki1769) or a hydroxyl group (Ki3315) were studied in porcine isolated coronary artery.2. KRN2391 (10⁻⁶ M) increased cyclic GMP formation but did not affect intracellular cyclic AMP level. Ki1769 (10⁻⁵ M) and Ki3315 (10⁻³ M) had no effect on intracellular cyclic GMP and cyclic AMP levels.3. Despite producing submaximal relaxation at KRN2391 (10⁻⁶ M) and nitroglycerin (10⁻⁶ M), the increase in cyclic GMP caused by KRN2391 was lower than that caused by nitroglycerin.4. Methylene blue (10⁻⁵ M) inhibited KRN2391- and nitroglycerin-induced relaxations but did not affect Ki1769- and Ki3315-induced relaxation.5. Glibenclamide (10⁻⁶ M) inhibited KRN2391-, Ki1769- and Ki3315-induced relaxation but did not affect nitroglycerin-induced relaxation.6. These results suggest that the nitroxyl group of KRN239 contributes to its nitrate action and the pyridinecarboximidamide moiety plays an important role of developing a K channel opening action.     

Comparison of KRN2391 with nicorandil and nifedipine on canine coronary blood flow: antagonism by glibenclamide.

The mode of action of KRN2391 [N-cyano-N'-(2-nitroxyethyl)-3- pyridinecarboximidamide monomethanesulfonate] in coronary circulation was examined in anesthetized dogs in comparison with those of nicorandil and nifedipine. Administration of KRN2391 (10 micrograms/kg i.v.), nicorandil (300 micrograms/kg i.v.), and nifedipine (3 micrograms/kg i.v.) caused an increase in coronary blood flow (CBF) and decreases in mean blood pressure (MBP) and in coronary vascular resistance (CVR). Heart rate (HR) was slightly and simultaneously increased by the drugs. Glibenclamide (5 mg/kg i.v.) blocked the changes of these parameters caused by KRN2391 and nicorandil, but not those caused by nifedipine. The present study suggests that the mechanism of action through ATP-sensitive K channels which are blocked by glibenclamide may contribute, at least in part, to the effects of KRN2391 and nicorandil on CBF and blood pressure (BP).     

Effect of Selective Inhibition of Potassium Channels on Vasorelaxing Response to Cromakalim,Nitroglycerin and Nitric Oxide of Canine Coronary Arteries

A comparative study was performed on the sensitivity of in-vitro vasorelaxation by nitroglycerin and cromakalim to block glibenclamide, a blocker of ATP-sensitive potassium channels, and iberiotoxin, a selective inhibitor of large-conductance calcium-activated potassium channels. In isolated canine coronary arteries preconstricted with 25 μM prostaglandin F_2α, nitroglycerin (0.005–1.8 μM) and cromakalim (0.15–9.6 μM) produced dose-dependent vasodilations. Glibenclamide (30 μM) had no significant effect on relaxation of the dose-response curve to nitroglycerin and almost completely abolished the relaxation by cromakalim, a known opener of ATP-sensitive potassium channels. Iberiotoxin (90 nM) decreased the maximal response to nitroglycerin and had no effect on the vasodilation induced by cromakalim. The effect of iberiotoxin on the vasorelaxing action of nitric oxide, the active metabolite of nitroglycerin, was also examined. In a low potassium chloride (14.4–20.4 mM) medium, as a contractile stimulus, iberiotoxin inhibited relaxations by exogenous nitric oxide (100–200 nM). Enhancement of potassium concentrations to 35.4–40.4 mM significantly decreased relaxation by nitric oxide and under these conditions the inhibitory action of iberiotoxin disappeared. The present study demonstrated that in canine coronary arteries, the functional role of two potassium channels can be separated by pharmacological means. Nitroglycerin-induced vasorelaxation may be mediated, at least in part, by its enzymatic breakdown product, nitric oxide that activates large-conductance calcium-activated potassium channels.     

Effects of a new antianginal agent, nicorandil, on normoxic and anoxic contractions in isolated miniature pig coronary arteries exposed to 5-hydroxytryptamine and norepinephrine: comparison with nitroglycerin and diltiazem

Nicorandil, nitroglycerin and diltiazem effects on normoxic and anoxic contractile responses to 5-hydroxytryptamine (5-HT) and norepinephrine (NE) were compared in isolated miniature pig coronary artery strips. Anoxia augmented contractile responses of the strips to 5-HT and NE. Nicorandil and nitroglycerin inhibited the normoxic contractile responses to 5-HT and NE and the further contraction induced by subsequently imposed anoxia. Diltiazem inhibited only the responses to 5-HT and to subsequently imposed anoxia. Whereas nicorandil and nitroglycerin produced relaxation of both potassium- and lanthanum-induced contractions, diltiazem was an effective relaxant of the potassium-induced contraction. The present experiments demonstrate that anoxia can augment the contractions of isolated large coronary arteries to 5-HT and NE. Nicorandil as well as nitroglycerin attenuate normoxic and anoxic contractions to 5-HT and NE. This is probably accomplished by inhibition of intracellular Ca2+ mobilization.     

Cardiovascular effects of KRN2391, nitroglycerin and cromakalim in dihydroergotamine-treated pithed rats

1. The effects of KRN2391 on the cardiovascular system were compared with those of nitroglycerin and cromakalim in pithed rats treated with dihydroergotamine (DHE) in order to examine the effects of these drugs on venous blood vessels.2. DHE (100 μg/kg, i.v.) produced increases in mean blood pressure (MBP), cardiac output (CO) and central venous pressure (CVP) without changes in total peripheral vascular resistance (TPR) and heart rate (HR) based on venoconstriction. The DHE-treated pithed rats, nitroglycerin (30 μg/kg, i.v.) decreased CO and CVP whereas cromakalim (30 μg/kg, i.v.) produced a slight increase in CO followed by a decrease and did not affect CVP. KRN2391 (30 μg/kg, i.v.) produced a decrease in CVP without affecting CO. Decreases in MBP and TPR were induced by all drugs.3. These results suggest that nitroglycerin acts predominantly as a venodilator and KRN2391 and cromakalim showed a venodilating action in addition to an arterial dilating action in DHE treated pithed rats. However, the venodilating action of KRN2391 in this condition is more potent than that of cromakalim.