Does the opening of ATP-sensitive K+ channels modify ischaemia-induced ventricular arrhythmias in anaesthetised dogs?

Effects of Verapamil and Pinacidil on Extracellular K+, pH, and the Incidence of Ventricular Fibrillation during 60 Minutes of Ischemia

Ca⁺⁺-channel antagonist verapamil and ATP-sensitive K⁺-channel opener pinacidil are known to decrease the rise in extracellular K⁺ ([K⁺]_e) level and pH (pH_e) that occurs during reversible acute myocardial ischemia and to lessen the accompanying activation delay. Verapamil is also known to decrease the incidence of ventricular tachycardia (VT)/fibrillation (VF) during acute myocardial ischemia; however, the effects of ATP-sensitive K⁺-channel opener on the incidence of VT/VF are controversial. We studied, in an in vivo pig model, the effects of verapamil and pinacidil on the changes in [K⁺]_e level and pH_e, local activation, and the incidence of VT/VF during 60 minutes of ischemia. Thirty-one pigs were divided into 2 groups: a verapamil group (9 control pigs and 8 verapamil-treated pigs) and pinacidil group (5 control pigs and 9 pinacidil-treated pigs). In the verapamil group, VF developed in 1 of the 9 control pigs, whereas no VF developed in 8 verapamil-treated pigs. In the pinacidil group, VF developed in 3 of the 5 control pigs and all 9 pinacidil-treated pigs. Under verapamil treatment (versus the control condition), onset of the second rise in [K⁺]_e level was delayed, and the maximum rise in [K⁺]_e level was decreased. Under pinacidil treatment (versus the control condition), time to the onset of VT/VF was shorter than that under the control condition, and VT/VF developed at lower [K⁺]_e level and higher pH_e. In conclusion, VF may develop at a lesser [K⁺]_e rise and pH_e fall in the presence of pinacidil during acute myocardial ischemia.

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.

The role of mitochondrial K(ATP) channels in antiarrhythmic effects of ischaemic preconditioning in dogs

1. In the canine a single brief (5 min) coronary artery occlusion protects the myocardium against the severe ventricular arrhythmias and reduces the ischaemic changes that result from a subsequent, more prolonged (25 min) occlusion. The main purpose of the present study was to examine whether mitochondrial K(ATP) channels are involved in this protection. 2. In chloralose-urethane anaesthetized dogs, preconditioning (PC) was induced by a single 5 min period occlusion of the left anterior descending (LAD) coronary artery, 20 min prior to a 25 min occlusion of the same artery. In some of these PC dogs 5-hydroxydecanoate (5-HD; 150 micro g kg(-1) min(-1) by intracoronary infusion) was given over a period of 30 min either before, or after PC. In other dogs the mitochondrial K(ATP) channel opener diazoxide (1 mg kg(-1); i.c.) was given, either alone or in the presence of 5-HD. Control dogs (infused with saline) were simply subjected to a 25 min occlusion and reperfusion. 3. Compared to controls, both PC and diazoxide significantly reduced the number of ventricular premature beats (VPBs; 295+/-67 to 89+/-28 and 19+/-11, respectively; P<0.05), the number of episodes of ventricular tachycardia (VT; 8.3+/-4.2 to 1.6+/-0.9 and 0.2+/-0.1; P<0.05) and the incidences of VT (100 to 43 and 33%; P<0.05) and ventricular fibrilation (VF; 60 to 0 and 17%; P<0.05) during the 25 min occlusion of the LAD. Further, 43% of the PC dogs and 58% of the diazoxide treated dogs survived the combined ischaemia-reperfusion insult (cp. 0% in the controls; P<0.05). The protection afforded by PC and diazoxide was abolished by 5-HD, especially when it was given prior to the PC occlusion. In the presence of 5-HD, three out of 10 dogs fibrillated during the PC occlusion and another three dogs died following reperfusion. Furthermore, there were no survivors in this group from the prolonged ischaemia/reperfusion insult. 5-HD given after PC only attenuated the antiarrhythmic protection. 4. Opening of mitoK(ATP) channels prior to ischaemia by preconditioning and diazoxide protects the myocardium against ischaemia and reperfusion-induced arrhythmias. This protection is abolished if the opening of these channels is prevented by the prior administration of 5-HD but only attenuated if 5-HD is given after preconditioning. The results indicate that opening of mitoK(ATP) channels prior to ischaemia is mandatory for protection against ischaemia and reperfusion-induced arrhythmias.

Anti-arrhythmic effects of levcromakalim in the ischaemic rat heart: a dual mechanism of action?

The action of pharmacological openers of K(ATP) channels depends on the availability and levels of various intracellular nucleotides. Since these are subject to change during myocardial ischaemia, K(ATP) channel openers may affect ischaemic and non-ischaemic tissue differentially. Using a recently developed dual coronary perfusion method, we investigated the effects on arrhythmias of the prototypical K(ATP) channel opener levcromakalim when applied selectively to ischaemic and/or non-ischaemic tissue. A novel perfusion cannula was used to independently perfuse the left and right coronary beds of hearts isolated from rats. Selective infusion of levcromakalim (3, 10 or 30 microM) into the left coronary bed in the absence of ischaemia did not induce ventricular arrhythmias. Regional zero-flow ischaemia was induced by cessation of flow to the left coronary bed and hearts received levcromakalim selectively into either the left, right, or both coronary beds. When applied selectively to the ischaemic left coronary bed, levcromakalim (3, 10 or 30 microM; n=10/group) delayed the onset of ventricular tachycardia in a dose-dependent manner (by 21*, 43* and 112%* at 3, 10 and 30 microM; *P<0.05 vs. control). When applied only to the non-ischaemic right coronary bed, levcromakalim reduced the incidence of ventricular tachycardia during later phases of ischaemia (from 100% in controls to 30%*). When present in both coronary beds, levcromakalim had a striking anti-arrhythmic effect--the overall incidence of ventricular tachycardia being reduced from 100% in controls to 20%*. We conclude that levcromakalim may have an anti-arrhythmic effect when applied either to ischaemic or non-ischaemic tissue but that the mechanisms may differ depending on the metabolic state of the heart.

ATP-Sensitive potassium channels: a review of their cardioprotective pharmacology

ATP-sensitive potassium channels (K(ATP)) have been thought to be a mediator of cardioprotection for the last ten years. Significant progress has been made in learning the pharmacology of this channel as well as its molecular regulation with regard to cardioprotection. K(ATP)openers as a class protect ischemic/reperfused myocardium and appear to do so by conservation of energy. The reduced rate of ATP hydrolysis during ischemia exerted by these openers is not due to a cardioplegic effect and is independent of action potential shortening. Compounds have been synthesized which retain the cardioprotective effects of first generation K(ATP)openers, but are devoid of vasodilator and cardiac sarcolemmal potassium outward currents. These results suggest receptor or channel subtypes. Recent pharmacologic and molecular biology studies suggest the activation of mitochondrial K(ATP)as the relevant cardioprotective site. Implications of these results for future drug discovery and preconditioning are discussed.