Pinacidil opens ATP-dependent K+ channels in cardiac myocytes in an ATP- and temperature-dependent manner

Involvement of SUR2/Kir6.1 channel in the physiopathology of pulmonary arterial hypertension

Aims

We hypothesized that the ATP-sensitive K⁺ channels (KATP) regulatory subunit (ABCC9) contributes to PAH pathogenesis. ABCC9 gene encodes for two regulatory subunits of KATP channels: the SUR2A and SUR2B proteins. In the KATP channel, the SUR2 subunits are associated with the K⁺ channel Kir6.1. We investigated how the SUR2/Kir6.1 channel contributes to PAH pathogenesis and its potential as a therapeutic target in PAH.

Methods and results

Using in vitro, ex vivo, and in vivo approaches, we analyzed the localization and expression of SUR2A, SUR2B, and Kir6.1 in the pulmonary vasculature of controls and patients with PAH as in experimental pulmonary hypertension (PH) rat models and its contribution to PAH physiopathology. Finally, we deciphered the consequences of in vivo activation of SUR2/Kir6.1 in the monocrotaline (MCT)-induced PH model. We found that SUR2A, SUR2B, and Kir6.1 were expressed in the lungs of controls and patients with PAH and MCT-induced PH rat models. Organ bath studies showed that SUR2 activation by pinacidil induced relaxation of pulmonary arterial in rats and humans. In vitro experiments on human pulmonary arterial smooth muscle cells and endothelial cells (hPASMCs and hPAECs) in controls and PAH patients showed decreased cell proliferation and migration after SUR2 activation. We demonstrated that SUR2 activation in rat right ventricular (RV) cardiomyocytes reduced RV action potential duration by patch-clamp. Chronic pinacidil administration in control rats increased heart rate without changes in hemodynamic parameters. Finally, in vivo pharmacological activation of SUR2 on MCT and Chronic-hypoxia (CH)-induced-PH rats showed improved PH.

Conclusion

We showed that SUR2A, SUR2B, and Kir6.1 are presented in hPASMCs and hPAECs of controls and PAH patients. In vivo SUR2 activation reduced the MCT-induced and CH-induced PH phenotype, suggesting that SUR2 activation should be considered for treating PAH.

Congenital long QT syndromes and Brugada syndrome: the arrhythmogenic ion channel disorders

Congenital long QT syndromes (LQTS) and Brugada syndrome are hereditary disorders of cardiac ion channels which result in life-threatening cardiac arrhythmias or sudden cardiac death in patients with anatomically normal hearts. The pathogenesis of these dramatic events has been partially elucidated with the identification of the individual ion channels involved and understanding of the effect of some disease-causing mutations on the membrane currents and action potential. The clinical spectrum of congenital LQTS is broader than previously thought and involves certain patients previously diagnosed with idiosyncratic drug-induced proarrhythmia. The initial treatment for congenital LQTS patients involves beta-blockers in most cases. Indications for implantable cardioverter-defibrillator (ICD) or pace-maker (PM) implantation in selected individuals continue to evolve.

Effects of KRN4884 (a novel K+ channel opener), levcromakalim, nilvadipine and propranolol on endothelin-1-induced heart disorders in anesthetized rats

The effects of KRN4884 (5-amino-N-[2-(2-chrolophenyl)ethyl]-N'-cyano-3-pyridinecarboxa midine), a novel K+ channel opener, on the electrocardiogram changes caused by the intracoronary administration of endothelin-1 (ET-1) were studied in anesthetized rats and compared with the effects of levcromakalim, a K+ channel opener; nilvadipine, a Ca2+ antagonist; and propranolol, a beta-adrenoceptor antagonist. KRN4884 (50 microg/kg, i.v.) and levcromakalim (300 microg/kg, i.v.) inhibited the ST segment elevation and the development of arrhythmias induced by ET-1 (5 microg, i.c.) and decreased the incidence of death. Nilvadipine (300 microg/kg, i.v.) and propranolol (1000 and 3000 microg/kg, i.v.) each prevented the ST segment elevation, but the suppressions of the occurrence of arrhythmias produced by nilvadipine and propranolol were less than that shown by KRN4884. KRN4884 (30 and 50 microg/kg, i.v.), levcromakalim (100 and 300 microg/kg, i.v.) and nilvadipine (100 and 300 microg/kg, i.v.) significantly decreased the mean blood pressure in a dose-dependent manner, but propranolol did not. The heart rate was decreased by nilvadipine (100 and 300 microg/kg, i.v.) and propranolol (1000 and 3000 microg/kg, i.v.), but was not affected by KRN4884 (30 and 50 microg/kg, i.v.) or levcromakalim (100 and 300 microg/kg, i.v.). These results suggest that pretreatments with KRN4884 and levcromakalim are more effective on ET-1-induced electrocardiogram changes than those with nilvadipine and propranolol.

Myocardial protection in the acutely injured heart: hyperpolarizing versus depolarizing hypothermic cardioplegia

OBJECTIVES

The superiority of hyperpolarized arrest with adenosine triphosphate-sensitive potassium channel openers over standard hyperkalemic depolarizing cardioplegia during normothermic ischemia has been documented. This study examined the hypothesis that pinacidil would provide superior protection in a more clinically relevant model of an acutely injured heart and hypothermic cardioplegic arrest.

METHODS

In a blood-perfused, parabiotic, rabbit heart Langendorff model, hearts underwent 15 minutes of unprotected global normothermic ischemia before the administration of 50 ml of cardioplegic solution at 4 degrees C, followed by 50 minutes of hypothermic (15 degrees C) ischemia and 30 minutes of reperfusion. The cardioplegic solutions administered consisted of Krebs-Henseleit solution alone (N = 6), Krebs-Henseleit solution with pinacidil (50 mumol/L; N = 10), Krebs-Henseleit solution with pinacidil (50 mumol/L) and glibenclamide (a potassium channel blocker, 10 mumol/L; N = 8), or St. Thomas' Hospital solution (N = 8). The percent recovery of developed pressure, linear diastolic pressure-volume relationships, and coronary blood flow were compared.

RESULTS

The percent recovery of developed pressure was 32.8% +/- 2.8%, 43.0% +/- 4.3%, 46.5% +/- 2.2%, and 49.3% +/- 2.7% for the Krebs-Henseleit, the Krebs-Henseleit with pinacidil and glibenclamide, the St. Thomas' Hospital, and the Krebs-Henseleit with pinacidil groups, respectively. No hearts had ventricular fibrillation on reperfusion.

CONCLUSIONS

During hypothermic hyperpolarized arrest, as opposed to normothermic ischemia as in our previous studies, there was neither an increased incidence of ventricular fibrillation nor prolonged electrical activity when compared with results during traditional hyperkalemic arrest. Myocardial protection by St. Thomas' Hospital solution and pinacidil was superior (p = 0.009) to that with Krebs-Henseleit solution alone. The protection provided by pinacidil was lost with the addition of glibenclamide, indicating that the drug has adenosine triphosphate-sensitive potassium channel activity during hypothermia.

ATP-sensitive potassium channels are altered in ventricular myocytes from diabetic rats

Hypoxia-induced shortening of the action potential duration, attributed to activation of the ATP-sensitive potassium (KATP) channels, occurs to a much greater extent in ventricular cells from diabetic rats. This study examined whether the KATP channels are altered in streptozotocin-diabetic myocardium. In inside-out patches from ventricular myocytes (with symmetrical 140 mM [K+]), inward KATP currents (at potentials negative to the K+ reversal potential) were similar in amplitude in control and diabetic patches (slope conductances: 69 and 74 pS, respectively). However, outward single-channel currents were larger for channels from diabetic heart cells than from control cells (e.g., at +75 mV the diabetic channel currents were 3.7 +/- 0.3 pA vs. 2.7 +/- 0.1 pA for control currents, p < 0.05), due to reduced inward rectification of diabetic channel currents. There was no difference in open and closed times between control and diabetic channels. The IC50 for ATP inhibition of the KATP channel single-channel currents was 11.4 microM for control currents and 4.7 microM for diabetic channel currents. Thus, the major difference found between KATP channels from control and diabetic hearts was the greater outward diabetic single-channel current, which may contribute to the enhanced sensitivity to hypoxia (or ischemia) in diabetic hearts.

Protective Effects of Ranolazine on Ventricular Fibrillation Induced by Activation of the ATP-Dependent Potassium Channel in the Rabbit Heart

BACKGROUND: The authors studied the antifibrillatory effects of the adenosine-triphosphate (ATP)-sparing metabolic modulator ranolazine in a rabbit isolated heart model in which ventricular fibrillation occurs under conditions of hypoxia/reoxygenation in the presence of the ATP-dependent potassium channel opener pinacidil. METHODS AND RESULTS: Ten minutes after ranolazine or vehicle administration, addition of pinacidil (1.25 µM) to the buffer was followed by a 12-minute hypoxic period and 40 minutes of reoxygenation. At a reduced concentration of ranolazine (10 µM), ventricular fibrillation occurred in 60% of the hearts, compared to 89% in the control group (P = NS). In contrast, only three of nine hearts (33%) treated with 20 µM ranolazine developed ventricular fibrillation (P <.05 vs vehicle). Hemodynamic parameters including coronary perfusion pressure, left ventricular developed pressure, and +/-dP/dt were not affected by the presence of ranolazine in the perfusion medium. Ranolazine did not prevent or modify the negative inotropic or coronary vasodilator actions of pinacidil, suggesting a mechanism of action independent of potassium channel antagonism. CONCLUSIONS: Ranolazine significantly reduced the incidence of ventricular fibrillation in the hypoxic/reoxygenated heart exposed to the ATP-dependent potassium channel opener, pinacidil. The reported ability of ranolazine to prevent the decrease in cellular ATP during periods of a reduced oxygen supply may account for its observed antifibrillatory action. By maintaining intracellular ATP, ranolazine may modulate or prevent further opening of the ATP-dependent potassium channel in response to hypoxia and/or pinacidil.

Effect of pinacidil on rat hearts undergoing hypothermic cardioplegia

We studied the effect of pinacidil, a potassium-channel opener, on the hemodynamic, biochemical, and ultrastructural changes in rat hearts undergoing hypothermic cardioplegia. Fifty-four male Wistar rats weighing 250 to 300 g were used. Isolated hearts were prepared for modified Langendorff circulation in the working mode using modified Krebs-Henseleit bicarbonate solution bubbled with a 95% O2 and 5% CO2 gas mixture. Eighty minutes of cardioplegia at 25 degrees C was followed by normothermic reperfusion for 30 minutes. Pinacidil, 5, 10, or 50 mumol/L added to the cardioplegic solution, did not affect heart rate, but is significantly improved the recovery of aortic flow as compared with controls (88.1% +/- 4.3 [5 mumol/L]; 83.2% +/- 8.5% [10 mumol/L]; 90.3% +/- 5.3% [50 mumol/L] compared with 55.6 +/- 4.3% [control]; p < 0.05). Administration of pinacidil during reperfusion did not further enhance the recovery of aortic flow. The dose-response curve of aortic flow to the pinacidil concentrations was flat from 5 to 50 mumol/L. However, preservation of myocardial adenosine triphosphate and calcium concentrations and mitochondrial morphology suggested that the optimal concentration of pinacidil cardioplegia is 10 mumol/L.

Pinacidil attenuates positive inotropic but not chronotropic responses to norepinephrine in isolated dog atrial and ventricular preparations

We investigated whether pinacidil, a K+ATP channel opener like acetylcholine and adenosine, attenuated the positive chronotropic and inotropic responses to norepinephrine in isolated, blood-perfused dog atrial and ventricular preparations. Pinacidil (0.01-0.3 mumol) decreased atrial and ventricular contractile force to a much greater extent than sinus rate in a dose-related manner. Pinacidil dose-dependently attenuated increases in atrial and ventricular forces induced by norepinephrine but not increases in sinus rate. Pinacidil similarly attenuated the positive atrial and ventricular inotropic responses to Bay k 8644 and CaCl2. The pinacidil doses producing a fifty percent decrease (ED50) of the atrial and ventricular contractile force were not significantly different from the respective pinacidil doses producing a fifty percent inhibition (ID50) of the positive inotropic responses to norepinephrine, Bay k 8644 and CaCl2. Ouabain (5 and 15 nmol) did not affect the decreases in atrial and ventricular contractile force in response to pinacidil. These results suggest that the K+ATP-channel activator pinacidil, unlike acetylcholine or adenosine, functionally attenuates increases in ventricular and atrial contractile force in the responses to norepinephrine and other cardiotonics due to shortening of the action potential duration induced by K+ATP-channel activation in the dog heart.

Antifibrillatory effects of clofilium in the rabbit isolated heart

1. This study was designed to determine whether clofilium exhibits antifibrillatory activity in a pinacidil + hypoxia-induced model of ventricular fibrillation (VF) in Langendorff-perfused hearts. 2. Ten minutes after exposure to vehicle or clofilium (0.1, 1.0 and 10.0 microM), hearts were exposed to pinacidil (1.25 microM), then subjected to 12 min of hypoxia and reoxygenated. Onset to VF was recorded. Additional groups of hearts were pretreated with UK-68,798 (1.0, 3.0 and 10.0 microM), a delayed rectifier channel blocker, and 5-hydroxydecanoate (10 microM), a known ATP-dependent K+ channel blocker, and subjected to an identical protocol. 3. Clofilium decreased the incidence of VF in a concentration-dependent manner; 7/9 control hearts developed VF vs 1/9 hearts (P = 0.007, Fisher's Exact) treated with 10.0 microM clofilium. In addition, 5-hydroxydecanoate protected hearts from VF, while UK-68,798 pretreatment did not. 4. In a separate group of hearts, electrically-induced VF was converted to sinus rhythm in 10/11 hearts after clofilium was introduced as a bolus. 5. Clofilium is capable of preventing VF in the rabbit isolated heart in a concentration-dependent manner. We have data to suggest that the ability of clofilium to attenuate the effects of pinacidil+hypoxia in our model may include blockade of metabolically active K+ channels, i.e., KATP (glibenclamide-sensitive) channel.

Negative inotropic effect of pinacidil on the rat left atria

Cromakalim at 50 microM and pinacidil at 0.1-10 microM had no effect, but pinacidil at 0.1 mM had a negative inotropic effect on the rat electrically-driven left atria without altering the positive inotropic responses to isoprenaline or phenylephrine alone. Glibenclamide had no effect but 4-aminopyridine, procaine (30 microM) and tetraethylammonium (3 mM) augmented the cardiac stimulation response. The ability of pinacidil to attenuate the cardiac stimulation response was not altered by glibenclamide, 4-aminopyridine or procaine but was prevented by pretreatment with tetraethylammonium. Thus, on the rat left atria, pinacidil has a negative inotropic effect which is unrelated to the opening of ATP-sensitive potassium channels, but may be due to opening the inward rectifying potassium channels.

Effects of KRN2391, nicorandil and diltiazem on the changes in the electrocardiogram caused by endothelin-1 in anaesthetized rats

1. The effect of KRN2391, a novel vasodilator, on the changes of electrocardiogram caused by endothelin-1 (ET-1) was studied in anaesthetized rats and compared with the effects of nicorandil and diltiazem. In addition, the effect of KRN2391 on the action potential of guinea-pig papillary muscle was studied. 2. The intracoronary administration (i.c.) of ET-1 (5 micrograms) induced not only ST segment elevation of the electrocardiogram due to contraction of the coronary artery, but also arrhythmias involving atrioventricular block (A-V block), ventricular premature contraction (VPC) and ventricular fibrillation (VF), and resulted in death in most animals. However, the administration of methacholine (3 micrograms, i.c.) produced ST segment elevation alone without developing arrhythmias. 3. Pretreatment with intravenous administration of KRN2391 (30 micrograms kg-1) inhibited the ST segment elevation and the development of arrhythmias induced by ET-1, and decreased the incidence of death. 4. Nicorandil (1000 micrograms kg-1) prevented the ST segment elevation without suppression of the occurrence of VF. Diltiazem (100 micrograms kg-1) suppressed both the ST segment elevation and the occurrence of VF but not other arrhythmias. Nicorandil at 3000 micrograms kg-1 and diltiazem at 300 micrograms kg-1 produced not only a suppression of ST segment elevation and VF incidence but also a decrease in the occurrence of arrhythmias. These doses of nicorandil and diltiazem produced a decrease in death in a dose-dependent manner. 5. KRN2391 (10 and 30 micrograms kg-1), nicorandil (1000 and 3000 micrograms kg-1) and diltiazem (100 and 300 micrograms kg-1) significantly decreased mean blood pressure in a dose-dependent manner. Heart rate was decreased by nicorandil (3000 microg kg-1) and diltiazem (100 and 300 microg kg-1) but was not affected byKRN2391 (10 and 30 microg kg-1).6. KRN2391 (30 microM) significantly shortened the action potential duration of guinea-pig ventricle at 50% and 90% repolarization (APD50 and APD90). The effect of KRN2391 was inhibited by a K+channel blocker, glibenclamide (30 microM).7. These results suggest that the occurrence of ST segment elevation and arrhythmias induced by ET-1 are due to a dual direct action on both coronary vascular smooth muscle and myocardium. Therefore,the protective effects of KRN2391, nicorandil and diltiazem on ET-l-induced heart disorders appear to be due to their direct actions on coronary vascular smooth muscle and the myocardium.

Effects of potassium channel openers and calcium channel blockers on the force responses of the electrically driven rat right ventricle strip

1. The effects of potassium channel openers (cromakalim and pinacidil) and of calcium channel blockers (verapamil, diltiazem and flunarizine) on the contractile responses of the rat right ventricle have been determined. 2. On the electrically driven rat right ventricle cromakalim at 3 x 10(-7), 10(-6), 3 x 10(-6) M and 10(-5) M, and pinacidil at 10(-6), 10(-5) and 3 x 10(-5) M had no effect on the force responses to cardiac stimulation or to isoprenaline. 3. Pinacidil at 10(-4) M in a vehicle of 0.7% ethanol reduced the force responses to cardiac stimulation. This inhibitory effect was solely due to the ethanol. Pinacidil in 5 x 10(-4) M HCl (which had no effect alone) potentiated some of the responses to isoprenaline. 4. Verapamil, greater than or equal to 10(-6) M, diltiazem, greater than or equal to 10(-5) M, and flunarizine, greater than or equal to 10(-6) M, reduced the force responses to cardiac stimulation and to isoprenaline. 5. The present study has shown that calcium channel blockers, but not potassium channel openers, have inhibitory effects on the rat right ventricle.

Effects of ATP-sensitive K+ channel blockers on the action potential shortening in hypoxic and ischaemic myocardium

1. In order to determine whether activation of adenosine triphosphate (ATP)-sensitive K+ channels exclusively explains the hypoxia- and ischaemia-induced action potential shortening, effects of tolbutamide and glibenclamide on changes in action potential duration (APD) during hypoxia, metabolic blockade or experimental ischaemia were examined in guinea-pig and canine isolated myocardium by standard microelectrode techniques. 2. With use of patch clamp techniques, activity of ATP-sensitive K+ channels was recorded from open cell-attached patches of guinea-pig isolated ventricular myocytes. The probability of opening of the K+ channels was decreased by 2 mM tolbutamide and 20 microM glibenclamide to almost the same extent, whereas it was increased by 100 microM pinacidil. 3. In guinea-pig papillary muscles a marked shortening of the action potential produced by 100 microM pinacidil was completely antagonized by 2 mM tolbutamide or 20 microM glibenclamide. 4. In guinea-pig papillary muscles exposed to hypoxic, glucose-free solution or dinitrophenol (10 microM)-containing, glucose-free solution, APD declined gradually and twitch tension decreased. Pretreatment with glibenclamide partially but significantly inhibited the action potential shortening, whereas tolbutamide failed to improve it during hypoxia or metabolic blockade. 5. When in canine isolated myocardium, experimental ischaemia was produced by the cessation of coronary perfusion, APD was gradually shortened. The action potential shortening was partially but not completely inhibited by pretreatment with 20 microM glibenclamide. 6. These results suggest that changes in membrane current(s) other than the outward current through ATP-sensitive K+ channels also contribute to the action potential shortening in hypoxic or ischaemic myocardium.

Potassium channel openers and vascular smooth muscle relaxation

Potassium channel openers comprise a diverse group of chemical agents which open plasma-lemmal K-channels. They show selectivity for smooth muscle, although K-channels in cardiac and skeletal muscle, neurones and the pancreatic beta-cell are also affected at relatively high concentrations. In addition, at least one endogenous K-channel opener of vascular origin--endothelium-derived hyperpolarizing factor--exists and in man plays a role in modulating blood vessel tone. The type of K-channel involved in the actions of both exogenous and endogenous K-channel openers is still uncertain, although a prime candidate in smooth muscle seems similar to the [ATPi]-modulated K-channel in the pancreatic beta-cell. This review focuses attention on the action of these agents in vascular smooth muscle and on the possible clinical exploitation of their powerful vasorelaxant properties.