K+ channel opening: a new drug principle in cardiovascular medicine

7-Hydroxycoumarin Induces Vasorelaxation in Animals with Essential Hypertension: Focus on Potassium Channels and Intracellular Ca2+ Mobilization

Cardiovascular diseases (CVD) are the deadliest noncommunicable disease worldwide. Hypertension is the most prevalent risk factor for the development of CVD. Although there is a wide range of antihypertensive drugs, there still remains a lack of blood pressure control options for hypertensive patients. Additionally, natural products remain crucial to the design of new drugs. The natural product 7-hydroxycoumarin (7-HC) exhibits pharmacological properties linked to antihypertensive mechanisms of action. This study aimed to evaluate the vascular effects of 7-HC in an experimental model of essential hypertension. The isometric tension measurements assessed the relaxant effect induced by 7-HC (0.001 μM–300 μM) in superior mesenteric arteries isolated from hypertensive rats (SHR, 200–300 g). Our results suggest that the relaxant effect induced by 7-HC rely on K⁺-channels (K_ATP, BK_Ca, and, to a lesser extent, K_v) activation and also on Ca²⁺ influx from sarcolemma and sarcoplasmic reticulum mobilization (inositol 1,4,5-triphosphate (IP3) and ryanodine receptors). Moreover, 7-HC diminishes the mesenteric artery’s responsiveness to α1-adrenergic agonist challenge and improves the actions of the muscarinic agonist and NO donor. The present work demonstrated that the relaxant mechanism of 7-HC in SHR involves endothelium-independent vasorelaxant factors. Additionally, 7-HC reduced vasoconstriction of the sympathetic agonist while improving vascular endothelium-dependent and independent relaxation.

Direct Vasodilators and Sympatholytic Agents

Direct vasodilators and sympatholytic agents were some of the first antihypertensive medications discovered and utilized in the past century. However, side effect profiles and the advent of newer antihypertensive drug classes have reduced the use of these agents in recent decades. Outcome data and large randomized trials supporting the efficacy of these medications are limited; however, in general the blood pressure-lowering effect of these agents has repeatedly been shown to be comparable to other more contemporary drug classes. Nevertheless, a landmark hypertension trial found a negative outcome with a doxazosin-based regimen compared to a chlorthalidone-based regimen, leading to the removal of α-1 adrenergic receptor blockers as first-line monotherapy from the hypertension guidelines. In contemporary practice, direct vasodilators and sympatholytic agents, particularly hydralazine and clonidine, are often utilized in refractory hypertension. Hydralazine and minoxidil may also be useful alternatives for patients with renal dysfunction, and both hydralazine and methyldopa are considered first line for the treatment of hypertension in pregnancy. Hydralazine has also found widespread use for the treatment of systolic heart failure in combination with isosorbide dinitrate (ISDN). The data to support use of this combination in African Americans with heart failure are particularly robust. Hydralazine with ISDN may also serve as an alternative for patients with an intolerance to angiotensin antagonists. Given these niche indications, vasodilators and sympatholytics are still useful in clinical practice; therefore, it is prudent to understand the existing data regarding efficacy and the safe use of these medications.

Minoxidil-associated anorexia in an infant with refractory hypertension

Minoxidil is a potent antihypertensive used as an adjunctive agent in refractory hypertension. It exerts an antihypertensive effect through two mechanisms: selective arterial vasodilation by activation of potassium channels in the vascular smooth muscle and stimulation of carotid and aortic baroreceptors, leading to downstream release of renin and norepinephrine. Although frequently cited in reviews of antihypertensive agents, limited data about the use of minoxidil in neonates are available. We describe an infant girl, born at 35 weeks of gestation, who was diagnosed with idiopathic hypertension after extensive diagnostic evaluation. Adequate blood pressure control was not achieved with captopril, amlodipine, and clonidine. Oliguria secondary to captopril and rapid-onset congestive heart failure due to persistent hypertension led to the introduction of intravenous agents labetalol and nitroprusside. Although adequate blood pressure control was achieved, attempts to transition back to oral agents were unsuccessful, prompting the use of minoxidil as an alternative agent. Although good blood pressure control was achieved, the infant's oral intake plummeted from 210 to 63 ml/kg/day. The anorexia quickly resolved after stopping minoxidil, and she was discharged home at 5 months of age receiving propranolol, amlodipine, and doxazosin. Use of the Naranjo adverse drug reaction probability scale indicated a definite relationship (score of 10) between the patient's development of anorexia and minoxidil therapy. To our knowledge, there have been no previous reports of minoxidil-associated anorexia in preterm or term infants. Clinicians should be aware that anorexia is a possible adverse effect of minoxidil in this patient population when initiating the drug in similar patients.

ATP-dependent potassium channels as a key target for the treatment of myocardial and vascular dysfunction

PURPOSE OF REVIEW

The aim of this review is to highlight the most recent and interesting articles on the physiologic properties and functions of ATP-dependent potassium channels in the cardiovascular system and on the role of the potassium channel openers for the treatment of cardiovascular dysfunction.

RECENT FINDINGS

The initial efforts in the development of potassium channel openers focused on the management of systemic hypertension. Lately, the range of possible indications for potassium channel openers has increased to include pulmonary hypertension and stable angina pectoris. The discovery of a connection between the mitochondrial ATP-dependent potassium channels and the phenomenon of cardiac preconditioning created potential new uses for potassium channel openers in myocardial ischemia, inn unstable angina, in preoperative and perioperative settings, and for the preservation of organs for transplant.

SUMMARY

The most recent data on the physiologic roles of sarcolemmal and mitochondrial ATP-dependent potassium channels and the pharmacology of potassium channel openers in the cardiovascular system are summarized and discussed. Finally, the effects of potassium channel opener drugs including minoxidil, nicorandil, pinacidil, bimakalin, and levosimendan, a dual-action potassium channel opener and calcium sensitizer with inodilator and cardioprotective activity, are discussed.

A new ATP-sensitive potassium channel opener reduces blood pressure and reverses cardiovascular remodeling in experimental hypertension

Some potassium channel openers (KCOs) are potent vasodilators that mainly target the ATP-sensitive potassium channels in vascular smooth muscle cells. Their lack of tissue selectivity limits their clinical use in hypertension therapy. Iptakalim [2,3-dimethyl-n-(1-methylethyl)-2-butylamine], which belongs to a novel chemical type of KCO, possesses unique pharmacological characteristics. In vitro experiments have shown that iptakalim could limit its vasorelaxing actions to resistance vessels. In this study, we investigate the antihypertensive effects of iptakalim on two different experimental hypertensive models: stroke-prone, spontaneously hypertensive rats (SHRsps) and two-kidney with one-clip renal hypertensive dogs (2K1C RHD). In acute hypotensive tests, iptakalim showed stable, long-lasting antihypertensive effects in SHRsps and 2K1C RHDs. Mean-while, it had little effect on heart rate when compared with pinacidil, nifedipine, captopril, or bisoprolol. In experimental therapeutic tests, repeated doses in SHRsps for 30 days or in 2K1C RHDs for 14 days produced consistent antihypertensive effects without causing tolerance. In separate experiments, chronic administration of iptakalim resulted in reversing hypertensive vascular remodeling in spontaneously hypertensive rats and hypertensive cardiac remodeling in SHRsps. These results suggest that iptakalim is a promising antihypertensive drug.

KATP-channel-induced vasodilation is modulated by the Na,K-pump activity in rabbit coronary small arteries

The purpose of the study was to evaluate the importance of the Na,K-pump in relaxations induced by K(ATP)-channel openers in rabbit coronary small arteries. Arterial segments were mounted in myographs for recording of isometric tension. Whole-cell patch clamp was used to assess K(ATP)-channel currents in isolated smooth muscle cells from the arteries. In arteries preconstricted with the thromboxane A(2) analogue U46619 pinacidil and cromakalim induced concentration-dependent relaxations. In arteries preconstricted with potassium (124 mM) only high concentrations of pinacidil had a small relaxant effect. In arteries preconstricted with U46619 pinacidil-induced relaxations were unaffected by pretreatment with N(omega)-nitro-L-arginine (L-NNA) and only slightly reduced after mechanical removal of the endothelium. Pinacidil induced relaxations were not significantly affected by 1 microM glibenclamide. However, the relaxations were partly inhibited in potassium-free media and by 1 microM ouabain. In contrast, the concentration-dependent relaxation to cromakalim was partly blocked by 1 microM glibenclamide and partly by 1 microM ouabain and when both drugs were present the inhibition increased. Ouabain (1 microM) and glibenclamide (1 microM) each partly inhibited an ATP-sensitive current induced by pinacidil and cromakalim. In the presence of both inhibitors a greater inhibition was seen. When the solution in the patch pipette was sodium-free the current was reduced and ouabain had no effect. The study suggests that the relaxation to cromakalim and most likely pinacidil is mediated through opening of K(ATP) channels. Inhibition of the Na,K-pump, however, may change the local environment for the K(ATP) channels (i.e. increases the ATP/ADPratio and/or decreases the transmembrane potassium gradient), which partly prevents the activation of the K(ATP)-channel current.

Minoxidil: an underused vasodilator for resistant or severe hypertension

Minoxidil is a direct vasodilator introduced in the early 1970s for the treatment of hypertension. It is capable of reducing blood pressure in most persons with resistant hypertension where therapy has failed with multidrug regimens. Minoxidil's effect can be limited because of an increase in pulse rate and/or sodium (and water) retention. The latter may prove quite debilitating in some patients. Thus, minoxidil is generally administered with both a diuretic and an agent that can keep pulse rate in check, such as a beta blocker or a combined alpha-beta blocker. The prominent tachycardia with minoxidil can aggravate myocardial ischemia and, if long-standing, leads to left ventricular hypertrophy. Minoxidil has a particularly annoying side effect of hypertrichosis that may limit its use, particularly among women. Minoxidil use is infrequently associated with the idiosyncratic onset of a pericardial effusion. If a patient's hypertension is severe enough to warrant minoxidil therapy, a hypertension specialist should probably become involved in the patient's care. The use of this medication should be limited in view of the availability of effective agents with fewer side effects. There is, however, a place for minoxidil in the treatment of resistant hypertension especially in patients with advanced renal disease.

Levosimendan: a unique approach to the treatment of heart failure

Levosimendan is one of the first agents of a new class of drugs known as calcium sensitizers. These drugs are believed to increase cardiac contractility by sensitizing cardiac myofibrils to calcium, and may therefore be of clinical benefit in the treatment of low-cardiac output states, particularly congestive heart failure. In addition to sensitizing troponin to intracellular calcium, levosimendan has been shown to inhibit phosphodiesterase III, which may contribute to its positive inotropic effect, and open adenosine triphosphate (ATP)-sensitive potassium channels (K(ATP)), which may produce vasodilation. Unlike currently available intravenous inotropes, levosimendan does not increase myocardial oxygen utilization, has not been shown to be proarrhythmic, and has been used effectively in the presence of beta-blocking medications. Levosimendan also has not been shown to impair ventricular relaxation, which was an initial concern with this class of drugs. Clinical studies of levosimendan have demonstrated short-term hemodynamic benefits of levosimendan over both placebo and dobutamine. While large-scale, long-term morbidity and mortality data are scarce, the Levosimendan Infusion versus Dobutamine in severe low-output heart failure (LIDO) study suggested a mortality benefit of levosimendan over dobutamine up to 180 days after treatment. Clinical studies comparing levosimendan with other positive inotropes, namely milrinone, are lacking. Levosimendan treatment appears to be well-tolerated, with the primary adverse events being headache and hypotension. No clinically significant drug-drug interactions have been reported with levosimendan to date. The clinical future of levosimendan will depend on the results of larger, ongoing clinical trials.

Levosimendan increases diastolic coronary flow in isolated guinea-pig heart by opening ATP-sensitive potassium channels

Levosimendan, a novel calcium sensitizer developed for the treatment of acute heart failure, is an inodilator that increases coronary flow. Because it was recently shown that levosimendan stimulates potassium current through K(ATP) channels in isolated rat arterial cells, our aim was to assess whether the levosimendan-induced increase in coronary flow is due to the opening of the K(ATP) channels in coronary smooth muscle. The effect of levosimendan on the diastolic coronary flow velocity (DCFV) was measured in the Langendorff perfused spontaneously beating guinea-pig heart in the absence and presence of glibenclamide. Pinacidil was used as a reference compound, and the protein kinase C inhibitor bisindolylmaleimide was used to study the dilatory effect of levosimendan when the K(ATP) channels in smooth muscle are not inhibited by PKC-dependent phosphorylation. Levosimendan (0.01-1 microM) increased DCFV concentration-dependently and was noncompetitively antagonized by 0.1 microM glibenclamide, whereas pinacidil was inhibited competitively by glibenclamide. In the presence of glibenclamide the positive inotropic and chronotropic effects of levosimendan were unaltered. The effect of bisindolylmaleimide and levosimendan on DCFV was additive. The results indicate that levosimendan induced coronary vasodilation through the opening of the K(ATP) channels. Levosimendan and pinacidil probably have different binding sites on the K(ATP) channels. The additive effect of bisindolylmaleimide and levosimendan on the increase of DCFV suggests that the latter binds to the unphosphorylated form of the channel.

Direct vasodilators and their role in hypertension management: minoxidil

Minoxidil is a direct vasodilator that has been in use for over two decades. It is used primarily to reduce blood pressure in hypertensives who have been poorly controlled on various multidrug regimens. Although minoxidil is extremely effective, its usefulness is limited by its tendency to increase the pulse rate and to trigger salt and water retention. The latter may be incapacitating in some patients. Therefore, minoxidil is typically administered with both a diuretic and an agent that can control the pulse rate, such as a beta blocker. Minoxidil has several other side effects that may limit its use, including hypertrichosis, aggravation of myocardial ischemia and/or left ventricular hypertrophy, and (infrequently) pericardial effusions. If a patient's hypertensive pattern is sufficiently severe to warrant contemplation of minoxidil therapy, referring the patient to a hypertension specialist should be strongly considered. (c) 2001 by LeJacq Communications, Inc.

Hydralazine-induced vasodilation involves opening of high conductance Ca2+-activated K+ channels

The purpose of this study was to investigate whether high conductance Ca2+-activated K+ channels (BK(Ca)) are mediating the vasodilator action of hydralazine. In isolated porcine coronary arteries, hydralazine (1-300 microM), like the K+ channel opener levcromakalim, preferentially relaxed contractions induced by K+ (20 mM) compared with K+ (80 mM). In addition, concentration-relaxation curves for hydralazine (pD2 = 5.38 +/- 0.06; Emax = 85.9 +/- 3.6%) were shifted 10-fold to the right by the BK(Ca) blockers tetraethylammonium (1 mM) and iberiotoxin (0.1 microM). In contrast, nimodipine (a Ca2+-entry blocker), relaxed contractions induced by K+ (20 mM) and K+ (80 mM) equally and nimodipine-induced relaxations were neither antagonized by tetraethylammonium nor by iberiotoxin. In isolated perfused rat hearts, hydralazine (1 microM) increased coronary flow by 28.8 +/- 2.7%. Iberiotoxin (0.1 microM) suppressed this response by 82% (P < 0.05). In conscious, chronically catheterized rats the hypotensive response to hydralazine (0.6 mg kg(-1) min(-1)) was significantly reduced by 41% during infusion of iberiotoxin (0.1 mg kg(-1)). It is concluded, that opening of BK(Ca) takes part in the mechanism whereby hydralazine produces vasodilation.

Coronary vasorelaxant effect of levosimendan, a new inodilator with calcium-sensitizing properties

We examined the action of levosimendan, a new Ca2+-sensitizing inodilator, on isolated porcine coronary arteries. Vessel rings were studied in isometric myographs. Arterial cyclic adenosine monophosphate (cAMP) levels were determined by radioimmunoassay. Levosimendan (10(-7)-10(-3) M) completely relaxed arteries preconstricted by prostaglandin F2alpha (PGF2alpha) with a pD2 (-logEC50) value of 3.99 +/- 0.05 (n = 6-9 in all experiments). Pretreatment with levosimendan also prevented contraction induced by PGF2alpha. The vasorelaxation produced by levosimendan (10(-7)-10(-3) M) was not attenuated by removal of the endothelium. Levosimendan (10(-7)-10(-3) M) relaxed contractions induced by 30 mM K+ as well as 80 mM K+, whereas the K+ channel opener levcromakalim selectively relaxed contraction induced by 30 mM K+. Neither the cyclooxygenase inhibitor indomethacin nor the beta-adrenoceptor blocker propranolol influenced levosimendan-induced vasorelaxation. The Ca2+-entry blocker isradipine failed to relax arteries precontracted by endothelin-1 in Ca2+-free/EGTA medium. However, levosimendan (10(-7)-3 x 10(-3) M) completely relaxed endothelin-1-induced contractions in this medium. Levosimendan potentiated the relaxant effect of a cAMP-stimulating drug, isoprenaline, but also that of nitroglycerin and isradipine. At a maximal effective concentration, it increased arterial tissue contents of cAMP twofold. In conclusion, levosimendan produces coronary vasorelaxation by a mechanism that seems to be endothelium independent and not mediated by K+ channel opening, Ca2+-entry blockade, release of cyclooxygenase products, or beta-adrenoceptor stimulation. Accumulation of cAMP may possibly participate in vasorelaxation at high concentrations of levosimendan, but a cAMP-independent mechanism seems to be involved at lower concentrations.

The effects of potassium channel openers on saphenous vein exposed to arterial flow

OBJECTIVES

To assess the sensitivity of saphenous vein to potassium channel opening drugs (KCOs).

METHODS

Saphenous vein, harvested at bypass surgery or high ligation for correction of varicose veins, was exposed to an in vitro flow circuit and vasomotor responses assessed by organ bath pharmacology.

OUTCOME MEASURES

Effective drug concentrations for 50% reduction in vein ring tension (IC50).

RESULTS

Vein rings pre-contracted with phenylephrine showed a concentration-dependent relaxation to all the KCOs tested with a potency ranking of HOE 234 > cromakalim > pinacidil > diazoxide. The relaxation to cromakalim was endothelium-independent and was inhibited by glibenclamide (an ATP-sensitive K+ channel blocker). The sensitivity of vein rings to cromakalim increased after exposure to arterial flow conditions for 90 minutes (IC50 before 1.7 +/- 0.25 microM and after 0.25 +/- 0.08 microM, p > 0.001). This effect was not evident after 90 min of venous flow conditions, 2.19 +/- 0.49 microM. When the workload on vein, exposed to arterial flow conditions, was reduced mechanically by external stenting with PTFE the increased sensitivity to cromakalim was abolished.

CONCLUSIONS

Saphenous vein has ATP-sensitive K+ channels responsive to KCOs. The increased sensitivity to cromakalim, induced by arterial flow conditions, may represent an endogenous protective mechanism limiting ischaemic damage resulting from the higher workload imposed on grafted vein.