Pharmacological interaction experiments differentiate between glibenclamide-sensitive K+ channels and cyclic GMP as components of vasodilation by nicorandil

Cardioprotective and Antianginal Efficacy of Nicorandil: A Comprehensive Review

ABSTRACT

Angina pectoris remains a significant burden despite advances in medical therapy and coronary revascularization. Many patients (up to 30%) with angina have normal coronary arteries, with coronary microvascular disease and/or coronary artery vasospasm being major drivers of the myocardial demand-supply mismatch. Even among patients revascularized for symptomatic epicardial coronary stenosis, recurrent angina remains highly prevalent. Medical therapy for angina currently centers around 2 disparate goals, viz secondary prevention of hard clinical outcomes and symptom control. Vasodilators, such as nitrates, have been first-line antianginal agents for decades, along with beta-blockers and calcium channel blockers. However, efficacy in symptoms control is heterogenous, depending on underlying mechanism(s) of angina in an individual patient, often necessitating multiple agents. Nicorandil (NCO) is an antianginal agent first discovered in the late 1970s with a uniquely dual mechanism of action. Like a typical nitrate, it mediates medium-large vessel vasodilation through nitric oxide. In addition, NCO has adenosine triphosphate (ATP)-dependent potassium channel agonist activity (K ATP ), mediating microvascular dilatation. Hence, it has proven effective in both coronary artery vasospasm and coronary microvascular disease, typically challenging patient populations. Moreover, emerging evidence suggests that cardiomyocyte protection against ischemia through ischemic preconditioning may be mediated through K ATP agonism. Finally, there is now fairly firm evidence in favor of NCO in terms of hard event reduction among patients with stable coronary artery disease, following myocardial infarction, and perhaps even among patients with congestive heart failure. This review aims to summarize the mechanism of action of NCO, its efficacy as an antianginal, and current evidence behind its impact on hard outcomes. Finally, we review other cardiac and emerging noncardiac indications for NCO use.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Nicorandil: A drug with ongoing benefits and different mechanisms in various diseased conditions

Nicorandil is a well-known antianginal agent, which has been recommended as one of the second-line treatments for chronic stable angina as justified by the European guidelines. It shows an efficacy equivalent to that of classic antianginal agents. Nicorandil has also been applied clinically in various cardiovascular diseases such as variant or unstable angina and reperfusion-induced damage following coronary angioplasty or thrombolysis. Different mechanisms have been involved in the protective effects of nicorandil in various diseases through either opening of adenosine triphosphate-sensitive potassium (KATP) channel or donation of nitric oxide (NO). The predominance or participation of any of these proposed mechanisms depends on the dose of nicorandil used, the location of diseased conditions, and if this mechanism is still functioning or not. The protection afforded by nicorandil has been shown to be mainly attributed to KATP channel opening in experimental models of myocardial and pulmonary fibrosis as well as renal injury or glomerulonephritis, whereas NO donation predominates as a mechanism of protection in hepatic fibrosis and inflammatory bowel diseases. Therefore, in different diseased conditions, it is important to know which mechanism plays the major role in nicorandil-induced curative or protective effects. This can bring new insights into the proper use of selected medication and its recommended dose for targeting certain disease.

Nicorandil directly and cyclic GMP-dependently opens K+ channels in human bypass grafts

As we previously demonstrated the role of different K(+) channels in the action of nicorandil on human saphenous vein (HSV) and human internal mammary artery (HIMA), this study aimed to analyse the contribution of the cGMP pathway in nicorandil-induced vasorelaxation and to determine the involvement of cGMP in the K(+) channel-activating effect of nicorandil. An inhibitor of soluble guanylate cyclase (GC), ODQ, significantly inhibited nicorandil-induced relaxation, while ODQ plus glibenclamide, a selective ATP-sensitive K(+) (KATP) channel inhibitor, produced a further inhibition of both vessels. In HSV, ODQ in combination with 4-aminopyridine, a blocker of voltage-gated K(+) (KV) channels, did not modify the concentration-response to nicorandil compared with ODQ, whereas in HIMA, ODQ plus iberiotoxin, a selective blocker of large-conductance Ca(2+)-activated K(+) (BKCa) channels, produced greater inhibition than ODQ alone. We showed that the cGMP pathway plays a significant role in the vasorelaxant effect of nicorandil on HSV and HIMA. It seems that nicorandil directly opens KATP channels in both vessels and BKCa channels in HIMA, although it is possible that stimulation of GC contributes to KATP channels activation in HIMA. Contrary, the activation of KV channels in HSV is probably due to GC activation and increased levels of cGMP.

Cardioprotective effects of single oral dose of nicorandil before selective percutaneous coronary intervention

Objective:

Nicorandil, an opener of ATP-sensitive K+ channels, was used to treat angina in patients with coronary artery disease. In this study, we aim to investigate the cardioprotective effects of single oral dose of nicorandil in patients undergoing selective percutaneous coronary intervention (PCI).

Methods:

One hundred and thirty-eight patients with acute coronary syndrome undergoing PCI from July 2011 to October 2012 were randomly divided into control group (group 1, n=47), 10 mg oral nicorandil group (group 2, n=45), and 20 mg oral nicorandil group (group 3, n=46) about 2 hours before procedure, respectively. Cardiac troponin I (cTnI) levels were determined at 20 ~ 24 hours after PCI.

Results:

There was a significant difference in the rate of any cTnI elevation among the three groups (group 1: 36.17%, group 2: 20.00%, group 3: 15.22%, p=0.0176). With respect to the frequency of cTnI elevation ≥3 and 5×the upper limit of normal (ULN), there also had statistical difference among the three groups (17.02% in group 1, 8.89% in group 2, and 4.35% in group 3, respectively for cTnI elevation ≥3× ULN, p=0.0428; 12.77% in group 1, 6.67% in group 2, and 2.17% in group 3, respectively, for cTnI elevation ≥5× ULN, p=0.0487). Logistic regression analysis showed that LVEF (OR=0.915, 95% CI=0.853-0.981) and the use of nicorandil (OR=0.516, 95% CI=0.267-0.996) before PCI were independent protective factors of myocardial injury.

Conclusion:

Single oral dose of nicorandil (10 mg, 20 mg) 2 hours before the PCI procedure could decrease the incidence of peri-procedure myocardial injury and PCI-related myocardial infarction.

Stable angina pectoris: the medical management of symptomatic myocardial ischemia

Coronary artery disease (CAD) remains an important cause of morbidity and mortality and is a serious public health problem. Over the last 4 decades there have been dramatic advances in the both the prevention and treatment of CAD. The management of CAD was revolutionized by the development of effective surgical and percutaneous revascularization techniques. In this review we discuss the importance of the medical management of symptomatic, stable angina. Medical management approaches to both the treatment and prevention of symptomatic myocardial ischemia are summarized. In Canada, organic nitrates, β-adrenergic blocking agents, and calcium channel antagonists have been available for the therapy of angina for more than 25 years. All 3 classes are of proven benefit in the improvement of symptoms and exercise capacity in patients with stable angina. Although there is no clear first choice within these classes of anti-anginal agents, the presence of prior or concurrent conditions (for example, prior myocardial infarction and/or hypertension) plays an important role in the choice of anti-anginal class in individual patients. For some patients, combinations of different anti-anginal agents can be effective; however it is recommended that this approach be individualized. Although not currently available in Canada, other classes of anti-anginal agents have been developed; their mechanism of action and clinical efficacy is discussed. Patients with stable angina have an excellent prognosis. Patients in this category who obtain relief from symptomatic myocardial ischemia may do well without invasive intervention.

Cell signalling-mediating insulin increase of mRNA expression for cationic amino acid transporters-1 and -2 and membrane hyperpolarization in human umbilical vein endothelial cells

Insulin induces vasodilatation in human subjects and increases L-arginine transport and NO synthesis in human umbilical vein endothelial cells (HUVEC). Cell signalling events associated with insulin effects on activity and mRNA expression of the human cationic amino acid transporters 1 (hCAT-1) and 2B (hCAT-2B) are unknown. L-arginine transport and eNOS activity were determined in HUVEC exposed to insulin. mRNA levels for hCAT-1, hCAT-2B and eNOS were quantitated by real time RT-PCR and endothelial NO synthase (eNOS) protein was identified by Western blot analysis. Intracellular Ca2+, L-arginine and L-citrulline levels, L-[3H]citrulline formation from L-[(3)H]arginine, cGMP formation, nitrite level, ATP release and membrane potential were determined. Insulin increased L-arginine transport and the mRNA levels for hCAT-1 and hCAT-2B and eNOS expression and activity. Insulin also induced membrane hyperpolarization and increased intracellular Ca2+, L-[3H]citrulline, cGMP and nitrite formation. Insulin-mediated stimulation of the L-arginine/NO pathway is thus associated with increased hCAT-1 and hCAT-2B mRNA, and eNOS expression, via mechanisms involving membrane hyperpolarization, mitogen-activated protein kinases p42 and p44, phosphatidylinositol 3-kinase, NO and protein kinase C. We have characterized a cell signalling pathway by which hyperinsulinaemia could lead to vasodilatation in human subjects, and which could have implications in patients in whom plasma insulin levels are altered, such as in diabetes mellitus.

Nicorandil inhibits the release of TNFα from a lymphocyte cell line and peripheral blood lymphocytes

Modulation of cytokine release may be of interest in modulating inflammatory diseases. This study determined whether nicorandil, a potassium channel opener, and nitric oxide (NO) donor could inhibit the release of tumour necrosis factor alpha (TNFalpha) from lymphocytes. Nicorandil significantly and dose-dependently inhibited the TNFalpha release from a human Epstein Barr virus-transformed B lymphocyte cell line (EBV-B) and peripheral blood B and T lymphocytes. The inhibition was reversed by the addition of both potassium channel inhibitor glibenclamide and the guanylyl cyclase inhibitor 1H-(1,2,4) oxadiazolo (4,3) quinoxalin-1-one (ODQ). Other potassium channel openers, pinacidil, or the nicorandil analogue SG-209, however, failed to demonstrate inhibition of TNFalpha release. The NO scavenger haemoglobin was unable to reverse the nicorandil-induced TNFalpha inhibition, but in contrast to this, sodium nitroprusside (SNP) partially inhibited the release, which was reversed by haemoglobin. Nicorandil is able to inhibit TNFalpha release from lymphocytes, which requires the dual modes of both potassium channel opening and the nitrate moiety. Moreover, NO donation mechanism appears to be more dominant in the nicorandil inhibitory activity in lymphocytes.The dual mechanism involved in the inhibition of this cytokines may represent a novel therapeutical approach in the modulation of inflammatory disease.

Nicorandil preserves mitochondrial function during ischemia in perfused rat heart

A possible mechanism for the action of nicorandil on the improvement of energy metabolism of ischemic/reperfused hearts was examined. Perfused rat hearts were subjected to 35-min ischemia/60-min reperfusion. The heart was treated with nicorandil at concentrations of 10 to 100 microM for the last 30-min of pre-ischemia. Nicorandil preserved the mitochondrial oxygen consumption rate during ischemia and attenuated the decrease in mitochondrial function during reperfusion in association with the enhanced post-ischemic recovery of the left ventricular developed pressure. To assess the direct effect on mitochondria, myocardial saponin-skinned bundles were incubated under hypoxic conditions in vitro. Hypoxia-induced decrease in the mitochondrial oxygen consumption rate was attenuated by treatment of the bundles with 100 microM nicorandil. This attenuation was abolished by the combined treatment with the K(ATP) channel blocker, 5-hydroxydecanoate (5-HD). These results suggest that nicorandil is capable of attenuating ischemia/reperfusion injury of isolated perfused hearts through preservation of mitochondrial function during ischemia.

Interactions of sildenafil with various coronary vasodilators in isolated porcine coronary artery

There are reports of serious hypotension or circulatory shock when sildenafil citrate, a selective cyclic nucleotide phosphodiesterase type 5 inhibitor, which was developed for the treatment of erectile dysfunction, is given to patients taking certain coronary vasodilators. We thus examined the interaction of sildenafil with various coronary vasodilators including nitric oxide (NO) donors in isolated porcine coronary artery. Sildenafil caused concentration-dependent relaxations of the artery precontracted with U46619 (9,11-dideoxy-9 alpha,11 alpha-methanoepoxy-prostaglandin F(2alpha)). Incubation with the NO synthase inhibitor NG-nitro-L-arginine or the soluble guanylate cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one) significantly shifted the concentration-response curve for sildenafil to the right without affecting the maximum response, indicating that some part of the relaxant response to sildenafil may be the result of the inhibition of phosphodiestrase type 5-induced degradation of cyclic GMP (cGMP) that is produced through guanylate cyclase activation by NO released spontaneously. The relaxant effects of the vasodilators with an NO donor property, isosorbide dinitrate, sodium nitroprusside, nicorandil and nipradilol, were significantly enhanced by sildenafil, as shown by a significant leftward shift of their concentration-response curves. In contrast, the relaxant responses to the drugs without a property as an NO donor, diltiazem, celiprolol and pinacidil, were not affected by sildenafil. The cGMP level of the tissue was elevated after adding sildenafil, and the cGMP-generating effect of a combination of sildenafil and sodium nitroprusside was higher than that of each drug alone. The cyclic AMP level determined simultaneously was not changed by sildenafil. These results suggest that sildenafil potentiates specifically the relaxant responses of porcine coronary artery to the drugs which behave as an NO donor, providing basic evidence that the benefit of sildenafil in the treatment of erectile dysfunction can be limited by a risk of marked vasodilation when used together with NO-related coronary vasodilators.

Hypotensive interaction of sildenafil and nicorandil in rats through the cGMP pathway but not by K(ATP) channel activation

The possibility that sildenafil citrate can potentiate nicorandil-induced hypotension by increasing cGMP levels of vascular smooth muscle cells was examined using anesthetized rats and isolated aortas. In pentobarbital-anesthetized rats, more than 0.3 mg/kg of sildenafil (i.v.) potentiated intra-aortic (i.Ao.) administration of nitroglycerin-induced hypotension. Hypotension due to nicorandil (100 microg/kg, i.Ao.) was potentiated by sildenafil (1 mg/kg, i.v.), even after glibenclamide treatment, although pinacidil-induced hypotension was not reinforced. Hypotensive responses to neither nitroglycerin (3 microg/kg, i.v.) nor nicorandil (100 microg/kg, i.v.) were potentiated by sildenafil, however. Increases in femoral blood flow due to nitroglycerin (0.1-3 microg, i.a.) were potentiated significantly by sildenafil, but those due to nicorandil (1-30 microg, i.a.) were not. Isolated rat aortas precontracted with phenylephrine were dilated dose-dependently using nicorandil, nitroglycerin, pinacidil or sildenafil. The relaxant effect due to nicorandil and nitroglycerin was reinforced significantly by pretreatment with an ineffective concentration of sildenafil (10(-8) M), but pinacidil was not. After ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) completely blocked relaxation by nicorandil, sildenafil did not increase relaxation. These findings suggest that combination of sildenafil with nicorandil, as well as with nitroglycerin, potentiates the hypotensive response by augmentation of vasodilatation. Synergism of vasodilatation may be linked with NO action, but not with K(ATP) channel-activation.

Increase in the vasorelaxant potency of K(ATP) channel opening drugs by adenosine A(1) and A(2) receptors in the pig coronary artery

Myograph recording from ring segments of pig small coronary arteries was used to investigate the effects of adenosine receptor activation on the vasorelaxant potency of ATP-sensitive K(+) channel opening drugs. Receptor activation with 2-chloroadenosine (2-CA, 300 nM) increased the potency of both nicorandil and levcromakalim, shifting the pEC(50)s from 4.68+/-0.03 to 5.05+/-0.04 and from 6.34+/-0.06 to 6.72+/-0.06, respectively (P<0.05 in each case). Experiments with selective adenosine receptor agonists (2-chloro-N(6)-cyclopentyladenosine (CCPA), 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680)) and antagonists (8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), 4-(2-[7-amino-2-(2-furyl)[1,2, 4]triazolo[2,3-a] [1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385)) suggest that both A(1) and A(2a) receptors can increase the potency of nicorandil, while that of levcromakalim is increased only by A(2) receptors. Adenosine receptor activation did not affect the potency of pinacidil. Thus, adenosine receptor activation can increase the potency of some K(+) channel opening drugs to relax coronary arteries, but the details of the interaction with adenosine receptors depend on the particular drug.

Cyclic GMP regulates cromakalim-induced relaxation in the rat aortic smooth muscle: role of cyclic GMP in K(ATP)-channels

Recent studies have shown that nitric oxide (NO) modulates K+-channel activity which play an important role in controlling vascular tone. The formation of cyclic guanosine 3',5'-monophosphate (cyclic GMP) has also been recognized to be associated with the vasodilatory effect of NO. Both cyclic GMP and NO increase whole-cell K+-current by activating Ca2+-activated K+-channels (K(Ca)-channels). Here, we show evidence that activators of soluble guanylyl cyclase sodium nitroprusside or 3-morpholino-sydnonimine (SIN-1), and an analogue of cyclic GMP 8-bromo-cyclic GMP enhance the relaxation induced by cromakalim which is blocked by glibenclamide (a specific inhibitor of ATP-sensitive K+-channels [K(ATP)-channels]), and partially attenuated by methylene blue (an inhibitor of cyclic GMP formation). However, this is not due to the increase of cyclic GMP level by cromakalim itself because the relaxation induced by cromakalim is not associated with the changes of cyclic GMP level formed in the aortic smooth muscle. Thus, it is most likely that cyclic GMP also modulates activity of K(ATP)-channels, in addition to K(Ca)-channels, in the rat aorta.

Attenuation of anti-ischemic efficacy during chronic therapy with nicorandil in patients with stable angina pectoris

After 2 weeks of nicorandil therapy, time to ischemia on stress testing was significantly less than on day 1 and not different from placebo. These data are consistent with attenuation of the anti-ischemic effects of this drug and suggest that the potassium channel-opening properties do not compensate for development of attenuation to the nitrate component of nicorandil.

Cytosolic myocardial calcium modulation by ATP-dependent potassium channel openers and NO donors

The goal of this study was to evaluate, in rat cardiomyocytes, the effects on cytosolic calcium of a pure K-adenosine triphosphate (ATP) channel opener, aprikalim, and those of nicorandil, a dual-acting agent that increases cyclic guanosine monophosphate (cGMP) levels and opens K-ATP channels. These effects were compared with those of a pure NO donor, 3-morpholino-sydnonimine (Sin-1). Ventricular myocytes were isolated from the hearts of adult rats. Changes in cytosolic calcium concentration ([Ca2+]i) were measured by using a Ca2+ indicator, indo-1/AM. Alterations in indo-1 fluorescence were recorded during regular electrical stimulation. After 10 min of pacing, end-diastolic [Ca2+]i was significantly increased as compared with control without significant changes in calcium transient. For doses of 10(-7) to 10(-4) M, aprikalim and nicorandil did not affect significantly the calcium transient. Sin-1 produced a significant decrease in calcium transient (by approximately 20%), which was already maximal at 10(-7) M. When given with the potassium channel antagonist glibenclamide (10(-5) M), nicorandil induced the same effects as those observed with Sin-1. We conclude that potassium channel openers aprikalim and nicorandil do not not decrease calcium transient. Thus the NO-donor properties of nicorandil are not apparent when given alone but appear when ATP-dependent potassium channels are blocked.

Potassium channel activation and relaxation by nicorandil in rat small mesenteric arteries

1. We used whole-cell patch clamp to investigate the currents activated by nicorandil in smooth muscle cells isolated from rat small mesenteric arteries, and studied the relaxant effect of nicorandil using myography. 2. Nicorandil (300 microM) activated currents with near-linear current-voltage relationships and reversal potentials near to the equilibrium potential for K+. 3. The nicorandil-activated current was blocked by glibenclamide (10 microM), but unaffected by iberiotoxin (100 nM) and the guanylyl cyclase inhibitor LY 83583 (1 microM). During current activation by nicorandil, openings of channels with a unitary conductance of 31 pS were detected. 4. One hundred microM nicorandil had no effect on currents through Ca2+ channels recorded in response to depolarizing voltage steps using 10 mM Ba2+ as a charge carrier. A small reduction in current amplitude was seen in 300 microM nicorandil, though this was not statistically significant. 5. In arterial rings contracted with 20 mM K+ Krebs solution containing 200 nM BAYK 8644, nicorandil produced a concentration-dependent relaxation with mean pD2 = 4.77+/-0.06. Glibenclamide (10 microM) shifted the curve to the right (pD2 = 4.32+/-0.05), as did 60 mM K+. LY 83583 caused a dose-dependent inhibition of the relaxant effect of nicorandil, while LY 83583 and glibenclamide together produced greater inhibition than either alone. 6. Metabolic inhibition with carbonyl cyanide m-chlorophenyl hydrazone (30 nM), or by reduction of extracellular glucose to 0.5 mM, increased the potency of nicorandil. 7. We conclude that nicorandil activates KATP channels in these vessels and also acts through guanylyl cyclase to cause vasorelaxation, and that the potency of nicorandil is increased during metabolic inhibition.

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

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.

Double-blind comparison of pramipexole and bromocriptine treatment with placebo in advanced Parkinson's disease. International Pramipexole-Bromocriptine Study Group

Pramipexole is a new, selective, nonergoline dopamine agonist that acts on D2 and preferentially on D3 dopamine receptors. Phase II and III clinical trials have shown this drug to be useful in treating both early and advanced Parkinson's disease (PD) patients. A double-blind, randomized, multicenter study was performed to compare the safety, tolerance, and efficacy of pramipexole versus placebo in patients with advanced PD with motor fluctuations. A bromocriptine treatment group was included to enable comparisons between bromocriptine and placebo groups, but the study was not powered to show statistical differences between the active treatment groups. The study included 247 patients with "wearing off." Patients were Hoehn and Yahr stages II to IV during "on" times. The trial included three phases: dose escalation, 6 months' maintenance, and dose reduction. The primary end points were the Unified Parkinson's Disease Rating Scale (UPDRS) parts II and III. Up to 4.5 mg per day of pramipexole and 30 mg per day of bromocriptine were used. The results of the study showed that the UPDRS part II improved by 26.7% for pramipexole (p = 0.0002) and 14% for bromocriptine (p = 0.02) versus 4.8% for placebo. The UPDRS part III showed improvements of 34% for pramipexole (p = 0.0006) and 23.8% for bromocriptine (p = 0.01) versus 5.7% for placebo. There were no major differences in safety data. In the active treatment groups there were more reports of dyskinesia and nausea compared with placebo. In regard to comparison of the Global Clinical Assessment of Efficacy between active treatment groups, there was a trend to significance (p = 0.056) in favor of pramipexole. We conclude that pramipexole-treated patients with advanced PD improved significantly more than placebo for both primary end points.

Efficacy of pramipexole, a novel dopamine agonist, as monotherapy in mild to moderate Parkinson's disease. The Pramipexole Study Group

A total of 335 patients with early Parkinson's disease (PD) were enrolled in a multicenter, randomized, double-blind trial designed to assess the efficacy and safety of pramipexole. Entry was restricted to patients with idiopathic PD who were not receiving levodopa. Pramipexole was administered according to an ascending dose schedule up to 4.5 mg/d. During the 7-week dose-escalation phase, each subject was titrated to his or her maximally tolerated dose of study medication. This was followed by a 24-week period of maintenance therapy. The mean daily dose during the maintenance period was 3.8 mg. Pramipexole significantly reduced the severity of PD symptoms and signs compared with placebo, as measured by decreases in parts II (Activities of Daily Living) and III (Motor Examination) of the Unified Parkinson's Disease Rating Scale at week 24 compared with baseline (p < or = 0.0001). Differences between the active drug and placebo groups emerged at week 3 (1.5 mg/d) in the ascending-dose interval and persisted throughout the maintenance phase (p < or = 0.0001). The majority of patients completed the study (pramipexole 83%, placebo 80%). In the assessment of adverse events, nausea, insomnia, constipation, somnolence, and visual hallucinations occurred more frequently in the pramipexole treatment group compared with placebo patients. No clinically significant changes were noted in blood pressure or pulse rate. Overall, these results indicate that pramipexole is safe and effective in the treatment of early PD.

Effects of chronic oral administration of a high dose of nicorandil on in vitro contractility of rat arterial smooth muscle

Nicorandil, which is structurally a nitrate and also a nicotinamide, has a vasodilator action by stimulating cyclase and ATP-sensitive K+ channel. The aim of present study was to examine the effects of chronic oral administration of a high dose of nicorandil on in vitro vascular reactivity. Nicorandil (30 mg/kg), at a dose 6-10-times higher than to decrease blood pressure in rat, was orally administered 2-times daily for a 2-4 weeks to the rats. At the end of the administration period, thoracic aorta was isolated for in vitro study. Treatment with nicorandil for 4 weeks markedly reduced the relaxant effect of nicorandil itself and other vasodilators including sodium nitroprusside, nitric oxide, endothelium-derived relaxing factor released by carbachol, 8-Br-cyclic guanosine 3',5'-monophosphate (cGMP), a K+ channel opener, levcromakalim, and forskolin. Increase in cGMP content induced by nicorandil and sodium nitroprusside was less in the aorta from nicorandil-treated rat than in the vehicle-control rat. Chronic administration of nicorandil altered neither the contractile responses to norepinephrine nor the vasodilator effect of verapamil. On the other hand, a 4-week treatment with a dose of nicorandil (2 mg/kg) sufficient to decrease blood pressure in rat showed no change in aortic response. These results suggest that in vivo chronic treatment with a high dose of nicorandil inactivates not only the guanylate cyclase activity but also the mechanism mediated by cGMP; it also attenuates the sensitivity of K+ channels to levcromakalim. Prolonged activation of the specific site may desensitize its site of action.

Primary structure and functional expression of a cGMP-gated potassium channel

Cyclic nucleotides modulate potassium (K) channel activity in many cells and are thought to act indirectly by inducing channel protein phosphorylation. Herein we report the isolation from rabbit of a gene encoding a K channel (Kcn1) that is specifically activated by cGMP and not by cAMP. Analysis of the deduced amino acid sequence (725 amino acids) indicates that, in addition to a core region that is highly homologous to Shaker K channels, Kcn1 also contains a cysteine-rich region similar to that of ligand-gated ion channels and a cyclic nucleotide-binding region. Northern blot analysis detects gene expression in kidney, aorta, and brain. Kcn1 represents a class of K channels that may be specifically regulated by cGMP and could play an important role in mediating the effects of substances, such as nitric oxide, that increase intracellular cGMP.

Role of potassium channels in cerebral blood vessels

BACKGROUND

Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. In cerebral blood vessels, four different potassium channels have been described: ATP-sensitive potassium channels, calcium-activated potassium channels, delayed rectifier potassium channels, and inward rectifier potassium channels.

SUMMARY OF REVIEW

Activation of ATP-sensitive and calcium activated potassium channels appears to play a major role in relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, and hypoxia. Both calcium-activated and delayed rectifier potassium channels may contribute to a negative feedback system that regulates tone in large cerebral arteries. The influence of ATP-sensitive and calcium-activated potassium channels is altered in disease states such as hypertension, diabetes, and atherosclerosis.

CONCLUSIONS

Activation of potassium channels is a major mechanism of cerebral vasodilatation. Alteration of activity of potassium channels and impairment of vasodilatation may contribute to the development or maintenance of cerebral ischemia or vasospasm.

Calculating slope and ED50 of additive dose-response curves, and application of these tabulated parameter values

Comparing dose-response curves (DRCs) of a compound A in the absence and presence of a fixed dose of an antagonist B is standard in pharmacology and toxicology. When B qualitatively resembles A in its action, it is often useful to construct theoretical DRCs of additive and independent combinations. Theoretical curves are calculated from experimental values by the program ALLFIT, which uses the four parameter logistic equation. DRCs of theoretical, additive DRCs are obtained by using the respective values for slope and ED50, which were taken from tables presented here compiled on the basis of the slope of the DRC of A alone (0.6-14) and of the effect of B alone (1-75%). These tables are unnecessary for the construction of theoretical curves if A acts by an independent mechanism, giving values for slope and ED50 identical to those of the DRC of A alone. Experimental DRCs of antiviral and other effects (the latter taken from data in the literature) are compared with theoretical curves by an F-test analysis provided by ALLFIT. The method can be used successfully for the construction of theoretical curves for additive and independent DRCs and comparison with experimental curves. This comparison may help clarify the mode of interaction of A with B.

The inhibitory mechanisms of nicorandil in isolated rat urinary bladder and femoral artery

Nicorandil or cromakalim inhibited contractile responses to acetylcholine and KCl in detrusor muscles of rat urinary bladder, whereas nitroglycerin inhibited only the responses to acetylcholine. In the detrusor muscles contracted by electrical stimulations, relaxations caused by nicorandil and cromakalim were inhibited by glyburide, but not by nitroglycerin or apamin. Methylene blue slightly potentiated the nicorandil-relaxation without affecting the cromakalim-relaxation. NG-Monomethyl-L-arginine also did not affect the relaxation induced by nicorandil. The level of cGMP was increased by both nicorandil and nitroglycerin. In rat femoral arteries contracted by phenylephrine, the relaxation induced by nicorandil was inhibited by methylene blue, glyburide and apamin. The relaxation induced by cromakalim was inhibited by glyburide, but not by apamin or methylene blue. These results suggest that the effect of nicorandil is due to activation of KATP channels in rat detrusor muscles and is due to the activation of guanylate cyclase, KATP and KCa channels in rat femoral arteries. The effect of cromakalim is due to the activation of KATP channels in both smooth muscles.

Effects of nicorandil on human isolated corpus cavernosum and cavernous artery

Nitric oxide (NO) released from nonadrenergic-noncholinergic (NANC) nerves seems to be a principal mediator of the relaxation of penile erectile tissue necessary for erection, and drugs acting by release of NO have been shown to produce erection when injected intracorporeally into impotent patients. By producing hyperpolarization, K+ channel openers are effective in relaxing isolated penile erectile tissue from rabbit and man, and can produce tumescence and erection when injected intracorporeally into animals. Nicorandil is classified as a K+ channel opener, but it also acts as a donor of NO. In the present study, the effects of nicorandil on isolated preparations from human corpus cavernosum (CC) and deep cavernous artery (Acc) were compared with those of cromakalim (K+ channel opener) and SIN-1 (NO donor). Nicorandil produced a concentration-dependent relaxation of CC and Acc preparations. The relaxations obtained at the highest nicorandil concentration used (10(-4) M.) were 75 +/- 3% and 66 +/- 4% in CC preparations contracted by noradrenaline and endothelin-1, respectively. The corresponding effects in Acc preparations were 70 +/- 14% and 73 +/- 5%. Glibenclamide (blocking ATP-dependent K+ channels) significantly reduced the nicorandil-induced relaxation in CC, but not in Acc. Methylene blue (believed to block soluble guanylate cyclase) reduced nicorandil's relaxant effect in CC, although statistical significance was not obtained. NG-nitro-L-arginine 10(-4) M. (NO synthase inhibitor) did not significantly influence the effect of nicorandil on precontracted preparations in either tissue. In CC preparations contracted by electrical field stimulation, nicorandil and cromakalim concentration dependently inhibited the responses. This effect was significantly counteracted by glibenclamide. It is concluded that nicorandil is effective in relaxing human CC chiefly by its K+ channel opening action, and to some extent by its ability to release NO. For nicorandil's relaxing effect on Acc, ATP dependent K+ channels seem to be of limited importance. If effective in impotent patients, the drug may represent a new, interesting approach to the treatment of erectile dysfunction.