Differential effects of endothelin-1 on the vasorelaxant properties of benzopyran and non-benzopyran potassium channel openers

KATP channel openers: structure-activity relationships and therapeutic potential

ATP-sensitive potassium channels (K(ATP) channels) are heteromeric complexes of pore-forming inwardly rectifying potassium channel subunits and regulatory sulfonylurea receptor subunits. K(ATP) channels were identified in a variety of tissues including muscle cells, pancreatic beta-cells, and various neurons. They are regulated by the intracellular ATP/ADP ratio; ATP induces channel inhibition and MgADP induces channel opening. Functionally, K(ATP) channels provide a means of linking the electrical activity of a cell to its metabolic state. Shortening of the cardiac action potential, smooth muscle relaxation, inhibition of both insulin secretion, and neurotransmitter release are mediated via K(ATP) channels. Given their many physiological functions, K(ATP) channels represent promising drug targets. Sulfonylureas like glibenclamide block K(ATP) channels; they are used in the therapy of type 2 diabetes. Openers of K(ATP) channels (KCOs), for example, relax smooth muscle and induce hypotension. KCOs are chemically heterogeneous and include as different classes as the benzopyrans, cyanoguanidines, thioformamides, thiadiazines, and pyridyl nitrates. Examples for new chemical entities more recently developed as KCOs include cyclobutenediones, dihydropyridine related structures, and tertiary carbinols.

Effects of potassium channel opener KRN4884 on human conduit arteries used as coronary bypass grafts

AIMS

The effects of a new potassium channel opener KRN4884 on human arteries have not been studied. This study was designed to investigate the effects of KRN4884 on the human internal mammary artery (IMA) in order to provide information on possible clinical applications of KRN4884 for preventing and relieving vasospasm of arterial grafts in coronary artery bypass grafting.

METHODS

IMA segments (n = 140) taken from patients undergoing coronary surgery were studied in the organ chamber. Concentration-relaxation curves for KRN4884 were established in the IMA precontracted with noradrenaline (NA), 5-hydroxytryptamine (5-HT), angiotensin II (ANG II), and endothelin-1 (ET-1). The effect of glibenclamide (GBC) on the KRN4884-induced relaxation was also examined in NA or 5-HT-precontracted IMA. Concentration-contraction curves for the four vasoconstrictors were constructed without/with pretreatment of KNR4884 (1 or 30 microM) for 15 min.

RESULTS

KRN4884 induced less relaxation (P < 0.05) in the precontraction induced by ET-1 (72.9 +/- 5.5%) than by ANG II (94.2 +/- 3.2%) or NA (93.7 +/- 4.1%) with lower EC50 (P < 0.05) for ANG II (-8.54 +/- 0.54 log M) than that for NA (-6.14 +/- 0.15 log M) or ET-1 (-6.69 +/- 0.34 log M). The relaxation in the IMA pretreated with GBC was less than that in control (P < 0.05). KRN4884-pretreatment significantly reduced the contraction (P < 0.05) induced by NA (151.3 +/- 18.4% vs 82.7 +/- 8. 7%), 5-HT (82.7 +/- 12.2% vs 30.1 +/- 7.3%), and ANG II (24.3 +/- 6. 3% vs 5.4 +/- 1.6%), but did not significantly reduce the contraction induced by ET-1 (P > 0.05).

CONCLUSION

KRN4884 has marked vasorelaxant effects on the human IMA contracted by a variety of vasoconstrictors and the effect is vasoconstrictor-selective.

Effect of verapamil on responses to endothelin-1 in aortic rings from streptozotocin-induced diabetic rats

In this study, we investigated the constrictor responsiveness to endothelin-1 (ET-1, 10-30 n m) of aortic rings (under 1 g resting tension in Krebs-Bicarbonate solution) from 8-weeks streptozotocin (STZ, 65 mg kg-1, i.p)-induced diabetic rats and vehicle-treated control rats. The maximum ET-1-induced contraction of the aorta in diabetic rats was increased by 150%, but the EC50 of ET-1 remained unchanged. Although in both groups, verapamil reduced the constrictor responses to ET-1 (diabetic group P<0.001, control group P<0.05), there were not any significant differences between PD2 values. These results suggest that verapamil inhibits ET-1-induced Ca2+ entry through the L-type channel and this effect did not change in diabetes mellitus.

Basal nitric oxide release differentially modulates vasodilations by pinacidil and levcromakalim in goat coronary artery

In the current investigation, the role of basal nitric oxide (NO) in modulating the vasorelaxant responses to pinacidil and levcromakalim was examined in goat isolated coronary artery. Pinacidil (10(-8) 10(-4) M) elicited concentration-dependent relaxations of the coronary artery ring segments (with intact endothelium) constricted with 30 mM K+ saline solution. The EC50 of the vasodilator was 2.57 x 10(-6) M (95% CL, 1.9-3.46 x 10(-6) M). The removal of endothelium by mechanical rubbing caused a rightward shift in the concentration-response curve of pinacidil with a corresponding increase in EC50 value (1.90 x 10(-5) M; 95% CL, 1.12-3.23 x 10(-5) M). Similar to endothelium removal, treatment of endothelium-intact rings either with the NO synthesis inhibitor L-NAME (NG-nitro-L-arginine methyl ester; 3 x 10(-5) M) or the guanylate cyclase inhibitor, methylene blue (3 x 10(-6) M) resulted in a marked inhibition in the relaxant responses to pinacidil. Hence, the EC50 values of the potassium channel opener were significantly higher in tissues treated either with L-NAME (7.41 x 10(-6) M; 95% CL, 6.02-9.12 x 10(-6) M) or methylene blue (2.29 x 10(-5) M; 95% CL, 1.58-3.31 x 109-5) M) as compared to untreated controls. The ATP-sensitive potassium (KATP) channel blocker glibenclamide, which caused a significant rightward shift in the concentration-relaxation curve of pinacidil in control tissues, was found to be less potent in antagonising the relaxant responses of the KATP channel opener in endothelium-denuded rings and in rings with intact endothelium but treated with either L-NAME or methylene blue. In contrast to the observations made with pinacidil, the vasodilator responses to another KATP channel opener, levcromakalim, were potentiated in the absence of basal NO. Thus, the EC50 of levcromakalim was 1.33 x 10(-8) M (95% CL, 0.8-2.21 x 10(-8) M) in control tissues with intact endothelium, which was significantly higher than those obtained in endothelium-deprived rings (4.81 x 10(-9) M; 95% CL, 4.04-5.73 x 10(-9) M) or endothelium intact rings treated either with L-NAME (2.63 x 10(-9) M; 95% CL, 1.58-4.36 x 10(-9) M) or methylene blue (2.82 x 10(-9) M; 95% CL, 1.7-4.68 x 10(-9) M). The selective modulation by basal NO of the arterial relaxations elicited with the KATP channel openers was evident from the findings that papaverine-induced relaxations were not affected in the absence of basal NO. Taken together, the results of the present study suggest that basal NO differentially modulates the interaction of pinacidil and levcromakalim with the KATP channels in goat coronary artery through a cGMP-dependent pathway.

The pulmonary vasodilator properties of potassium channel opening drugs

1. This article reviews the effects of potassium channel opening drugs (KCOs) on blood vessels of the pulmonary circulation. KCOs are effective pulmonary vasodilators in vitro (isolated arteries and perfused lungs) and in vivo in a variety of animal species. They prevent or reverse pulmonary vasoconstriction/contraction induced by a range of vasoconstrictor spasmogens or by alveolar hypoxia. 2. The pulmonary vasorelaxant effects of the KCO drugs are blocked by glibenclamide, do not depend on the endothelium, are dependent on the vasoconstrictor spasmogen used to contract the preparations and are enhanced in preparations taken from pulmonary hypertensive rats. 3. Selectivity for pulmonary compared with systemic vessels is seen in vessels from pulmonary hypertensive rats but not in the absence of pulmonary hypertension. 4. The pulmonary vasodilatation that is induced by (a) endothelium derived hyperpolarising factor, (b) endothelin, (c) increased pulmonary blood flow or (d) prolonged, severe hypoxia is probably due to potassium efflux through the same population of potassium channels as those on which the KCOs act. 5. Acute hypoxic pulmonary vasoconstriction, and also the depolarisation seen in arteries from chronically hypoxic rats, each involve inhibition of potassium efflux through glibenclamide-insensitive potassium channels. 6. It is suggested that the KCOs warrant investigation as possible therapeutic agents in the treatment of pulmonary hypertension.

Effects of calcium antagonists on endothelin-1-induced myocardial ischaemia and oedema in the rat

1. The effects of the calcium channel blockers, verapamil and nifedipine on myocardial ischaemia and oedema evoked by endothelin-1 (ET-1) or IRL 1620, an ETB receptor-selective agonist were studied in anaesthetized and conscious rats. 2. Bolus injection of ET-1 (1 nmol kg-1, i.v.) or IRL 1620 (1 nmol kg-1, i.v.) to conscious chronically catheterized rats evoked a transient depressor response followed by a prolonged pressor effect. Corresponding to changes in blood pressure, a transient tachycardia and a sustained bradycardia were observed. Pretreatment of the animals with verapamil (1 mg kg-1, i.v.) or nifedipine (200 micrograms kg-1, i.v.) produced on average 5 mmHg decrease in mean arterial blood pressure. Both verapamil and nifedipine inhibited by 63 and 44% the pressor actions of ET-1 or IRL 1620 (1 nmol kg-1), respectively, and the accompanying bradycardia. Both verapamil and nifedipine potentiated the magnitude of the depressor action of ET-1 and IRL 1620 without affecting the accompanying tachycardia. Decreasing mean arterial blood pressure with hydralazine (0.2 - 0.3 micromol kg-1, i.v.) to levels comparable to those observed after verapamil or nifedipine had no significant effects on the haemodynamic responses to ET-1 or IRL-1620. 3. Intravenous bolus injection of ET-1 or IRL 1620 (0.1-2 nmol kg-1) into anaesthetized rats produced dose-dependent ST segment elevation of the electrocardiogram without causing arrhythmias. ST segment elevation developed within 30-50s and persisted for at least 10-20 min following injection of the peptides. 4. Pretreatment of the animals with verapamil (1 mg kg-1, i.v.) or nifedipine (200 micrograms kg-1, i.v.) inhibited on average by 79 and 76% the ST segment elevation elicited by ET-1 (1 nmol kg-1), respectively. Verapamil and nifedipine also attenuated IRL 1620 (1 nmol kg-1)-induced ST segment elevation on average by 71 and 74%, respectively. In contrast, no significant inhibition was observed with hydralazine (0.2-0.3 mumol kg-1). 5. Both ET-1 and, to a lesser extent, IRL 1620 (0.1-2 nmol kg-1) evoked albumin accumulation in cardiac tissues in a dose-dependent fashion as measured by the local extravascular accumulation of Evans blue dye in conscious rats. ET-1 and IRL 1620 (1 nmol kg-1) enhanced albumin extravasation by 109 and 82%, and 34 and 44% in the left ventricle and right atrium, respectively. ET-1 or IRL 1620-induced albumin extravasation was completely prevented by verapamil (1 mg kg-1) or nifedipine (200 micrograms kg-1) in these vascular beds. In contrast, hydralazine (0.2-0.3 mumol kg-1) failed to modify the effects of ET-1 or IRL 1620 on albumin extravasation. 6. These results show that verapamil and nifedipine are highly effective in protecting the myocardium against the pro-ischaemic and microvascular permeability enhancing effects of ET-1 and suggest that ETA and constrictor ETB (tentatively termed ETB2) receptors mediating these actions of ET-1 are coupled to calcium influx through dihydropyridine-sensitive calcium channels.

In vitro hypoxia on rat pulmonary artery: effects on contractions to spasmogens and role of KATP channels

The effect of in vitro hypoxia for 1 h on concentration-response curves to vasoconstrictor spasmogens was examined in rat isolated pulmonary arteries. Hypoxia, like levcromakalim (KATP channel opener), did not affect contractions to endothelin-1 but attenuated contractions to U46619 ((1,5,5,)-hydroxy-11 alpha, 9 alpha-epoxymethano) prosta 5Z, 13E-dienoic acid; thromboxane-mimetic), angiotensin II, noradrenaline and 5-hydroxytryptamine. The attenuation was prevented by glybenclamide. In pre-contracted arteries, subsequent exposure to hypoxia caused a response consisting of four phases (transient relaxation due to endothelium-derived nitric oxide; transient contraction; slow relaxation; sustained contraction). Glybenclamide, if added before hypoxia, did not eliminate either of the relaxant phases but, if added during the sustained contractile phase, caused further contraction. We conclude that exposure of pulmonary arteries to prolonged hypoxia causes KATP channels to open, as in systemic arteries; this diminishes contractions to some, but not all, vasoconstrictor spasmogens. The data suggest that endothelin-1, unlike other vasoconstrictors, would remain a highly effective pulmonary vasoconstrictor under severe hypoxic conditions.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Modulation of vasodilatation to levcromakalim by adenosine analogues in the rabbit ear: an explanation for hypoxic augmentation

1. We have used a rabbit isolated ear, buffered-perfused preparation to investigate the effects of adenosine analogues on the vasodilatation to the potassium channel opener, levcromakalim (the active (-)-enantiomer of cromakalim). We have examined the effects of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 antagonist, on vasodilatation to levcromakalim under hypoxic conditions and also following inhibition of nitric oxide synthesis. 2. Levcromakalim relaxed preconstricted preparations with an EC50 = 369 +/- 48 nM and maximum relaxation of tone (Rmax) = 81.0 +/- 3.2%. In the presence of 1 microM N6-cyclohexyladenosine (CHA) a selective adenosine A1 agonist, there was a significant (P < 0.01) leftward shift in the concentration-response curve with an EC50 = 194 +/- 54 nM and Rmax = 93.2 +/- 2.0%. Conversely, the presence of CHA did not influence vasodilatation to either pinacidil or sodium nitroprusside. 3. Hypoxia also significantly (P < 0.001) increased the vasodilator potency of levcromakalim (EC50 = 134 +/- 22 nM), and this enhancement was completely reversed (EC50 = 380 +/- 107 nM, P < 0.01) by pretreatment of the preparations with 5 microM DPCPX, a selective A1 adenosine antagonist. However, under normoxic conditions DPCPX did not influence vasodilatation to levcromakalim. 4. Inhibition of nitric oxide synthesis with 100 microM NG-nitro-L-arginine methyl ester (L-NAME) caused a significant (P < 0.001) leftward shift in the concentration-response curve to levcromakalim (EC50 = 73.0 +/- 7.6 nM). Pretreatment of preparations with DPCPX partially reversed the increase in potency found in the absence of nitric oxide synthesis (EC50 = 153 +/- 18 nM, P < 0.001). 5. We have shown that an adenosine Al agonist may increase the potency of levcromakalim indicating that adenosine receptor activation may augment the vasodilator activity of levcromakalim. That responses to levcromakalim but not those to pinacidil were affected by CHA points to further differences in the pharmacology of these potassium channel openers. The reversal by the adenosine Al antagonist of the hypoxic-potentiation of vasodilatation to levcromakalim, and also augmentation following inhibition of nitric oxide synthesis, suggests that under these conditions there is an endogenous release of adenosine which may enhance responses to levcromakalim. The findings of this study suggest that levcromakalim may selectively dilate vessels where there is elevated adenosine release.

Modulation of vasodilatation to levcromakalim by hypoxia and EDRF in the rabbit isolated ear: a comparison with pinacidil, sodium nitroprusside and verapamil

1. We have used an isolated buffer-perfused preparation of the rabbit ear to investigate the effects of hypoxia and inhibition of endothelium-derived relaxing factor (EDRF) synthesis on the vasodilator responses to the potassium channel opener, levcromakalim (the active (-)-enantiomer of cromakalim). The results obtained with levcromakalim have been compared with those for pinacidil, sodium nitroprusside and verapamil. 2. Levcromakalim relaxed preconstricted preparations with an EC50 = 343 +/- 41 nM and Rmax = 80.3 +/- 6.4%. Under hypoxic conditions the concentration-response curve was significantly (P < 0.01) shifted to the left with an EC50 = 118 +/- 16 nM and Rmax = 89.9 +/- 2.7%. Hypoxia did not influence relaxation to either pinacidil, sodium nitroprusside or verapamil. 3. Inhibition of EDRF synthesis with 100 microM NG-nitro-L-arginine methyl ester (L-NAME) also significantly (P < 0.001) increased the vasodilator potency of levcromakalim (EC50 = 56 +/- 5 nM), and caused a similar shift in the concentration-response curve to sodium nitroprusside. It did not influence vasodilation to either verapamil or pinacidil. The potentiation of vasodilator responses to levcromakalim by L-NAME was reversed by an excess of L-arginine. 4. Impairment of oxidative phosphorylation with 400 nM carbonyl cyanide m-chlorophenylhydrazone significantly (P < 0.05) increased the potency of levcromakalim (EC50 = 120 +/- 20 nM) but did not influence vasodilation to pinacidil or endothelium-dependent relaxations to acetylcholine. 5. Vasodilatation to levcromakalim was augmented both by hypoxia and by inhibition of EDRF activity. Since impairment of oxidative phosphorylation increased the potency of levcromakalim but did not alter EDRF activity then the mechanism responsible for hypoxic facilitation of responses to levcromakalim is likely to be due to reduced ATP levels in hypoxic smooth muscle cells rather than a change in EDRF activity. These results suggest that levcromakalim may selectively dilate both hypoxic vessels and vessels with impaired EDRF activity. The results also point to important differences in the pharmacology of levcromakalim and pinacidil.