Antiarrhythmic and electrophysiological effects of the novel KATP channel opener, rilmakalim, in rabbit cardiac cells

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Pro- and antiarrhythmic effects of ATP-sensitive potassium current activation on reentry during early afterdepolarization-mediated arrhythmias

BACKGROUND

Under conditions promoting early afterdepolarizations (EADs), ventricular tissue can become bi-excitable, that is, capable of wave propagation mediated by either the Na current (INa) or the L-type calcium current (ICa,L), raising the possibility that ICa,L-mediated reentry may contribute to polymorphic ventricular tachycardia (PVT) and torsades de pointes. ATP-sensitive K current (IKATP) activation suppresses EADs, but the effects on ICa,L-mediated reentry are unknown.

OBJECTIVE

To investigate the effects of IKATP activation on ICa,L-mediated reentry.

METHODS

We performed optical voltage mapping in cultured neonatal rat ventricular myocyte monolayers exposed to BayK8644 and isoproterenol. The effects of pharmacologically activating IKATP with pinacidil were analyzed.

RESULTS

In 13 monolayers with anatomic ICa,L-mediated reentry around a central obstacle, pinacidil (50 μM) converted ICa,L-mediated reentry to INa-mediated reentry. In 33 monolayers with functional ICa,L-mediated reentry (spiral waves), pinacidil terminated reentry in 17, converted reentry into more complex INa-mediated reentry resembling fibrillation in 12, and had no effect in 4. In simulated 2-dimensional bi-excitable tissue in which ICa,L- and INa-mediated wave fronts coexisted, slow IKATP activation (over minutes) reliably terminated rotors but rapid IKATP activation (over seconds) often converted ICa,L-mediated reentry to INa-mediated reentry resembling fibrillation.

CONCLUSIONS

IKATP activation can have proarrhythmic effects on EAD-mediated arrhythmias if ICa,L-mediated reentry is present.

Organotypic slice culture from human adult ventricular myocardium

AIMS

Cardiovascular research requires complex and functionally intact experimental models. Due to major differences in the cellular and subcellular composition of the myocardium between species, the use of human heart tissue is highly desirable. To enhance the experimental use of the human myocardium, we established methods for the preparation of vital tissue slices from the adult ventricular myocardium as well as conditions for their long-term preservation in organotypic culture.

METHODS AND RESULTS

Human ventricular heart samples were derived from surgical specimens excised during a therapeutic Morrow myectomy and cut into 300 μm thick slices. Slices were either characterized in acute experiments or cultured at a liquid-air interface. Viability and functionality were proven by viability staining, enzyme activity tests, intracellular potential recordings, and force measurements. Precision-cut slices showed high viability throughout 28 days in culture and displayed typical cardiomyocyte action potential characteristics, which enabled pharmacological safety testing on the rapid component of the delayed rectifier potassium current (I(Kr)) and ATP-dependent potassium channels throughout the whole culture period. Constant expression of major ion channels was confirmed by quantitative PCR. Acute slices developed excitation-dependent contractions with a clear preload dependency and a β-adrenergic response. Contractility and myosin light chain expression decreased during the first days in culture but reached a steady state with reactivity upon β-adrenergic stimulation being preserved.

CONCLUSION

Organotypic heart slices represent a multicellular model of the human myocardium and a novel platform for studies ranging from the investigation of molecular interactions to tissue engineering.

Nicorandil-associated ulcerations

This leading article refers to the paper by Abdelrazeq AS, Owen C, Smith L, McAdam JG, Pearson HJ, Leveson SH. Nicorandil-associated para-stomal ulceration: case series Eur J Gastroenterol Hepatol 2006; 18:1293-1295. We apologise to all concerned for the dissociation between the two papers, which was due to an administrative error. Nicorandil is used widely in patients with coronary artery disease. Nicorandil is well tolerated with only minor side effects. Nicorandil's association with oral, anal, gastrointestinal ulceration, and more recently para-stomal ulceration has been reported. Medical awareness of nicorandil association with ulcerations should be high to help avoid unnecessary and harmful treatment as only cessation of the drug would heal the ulceration. Nicorandil is an antianginal drug used for the treatment of symptomatic coronary artery disease. It is characterized by an arterial and venous vasodilator effect with dual mechanism of action. Nicorandil is not a first-line agent in the management of angina but it is used in combination with other antianginal medications in stable and unstable angina. It is generally well tolerated with minor side effects such as headache, nausea, flushing and dizziness. The association of nicorandil with mouth and anal ulcers as well as the association with ulceration throughout the gastrointestinal tract has been reported, and recently, an association with para-stomal ulceration has also been described. Medical awareness of the association of nicorandil with ulceration in any part of the gastrointestinal tract should be highlighted among all medical professionals to help avoid delays in withdrawing the treatment and to avoid unnecessary and sometimes invasive and costly interventions.

Action Potential Duration and QT Interval During Pinacidil Infusion in Isolated Rabbit Hearts

INTRODUCTION

Acute myocardial ischemia, which opens K(ATP) channel, is associated with shortened action potential duration (APD) but prolonged QT interval. This discrepancy has not been adequately explained. We hypothesize that the duration of intracellular calcium (Ca(i)) transient (DCaT) may play a role in determining QT interval.

METHODS AND RESULTS

We performed simultaneous optical mapping of voltage and Ca(i) in 15 isolated rabbit hearts during a K(ATP) channel opener (pinacidil) infusion. Anterior epicardial mapping (n = 7) showed no difference of APD(90), QT interval, and the DCaT(90) at baseline. When perfused with 80 microM pinacidil, the APD(90), the QT interval, and the DCaT(90) were 105 +/- 10 msec, 199 +/- 14 msec, and 189 +/- 13 msec, respectively, during right ventricular (RV) pacing (P < 0.05). Posterior epicardial mapping (n = 4) showed that the APD(90) was significantly (P < 0.05) shorter than QT interval and DCaT(90) during pinacidil infusion. The results of the transmural mapping studies (n = 4) showed that the QT interval during RV pacing was not different than the DCaT(90) in the epicardium, midmyocardium, and endocardium, but was significantly (P < 0.01) longer than the APD(90) in epicardium, midmyocardium, and endocardium, respectively. There was a good correlation between the DCaT(90) and QT interval at baseline (r = 0.92, P < 0.0001) and during pinacidil infusion (r = 0.74, P < 0.0001).

CONCLUSION

We conclude that K(ATP) channel opening shortened APD but not the QT interval. Because Ca(i) did not return to diastolic level at the end of action potential, it may have created a heterogeneous membrane potential distribution that determined the QT interval.

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.