Gating properties of ATP-sensitive K+ channels in the heart

Drug profiling using planar microelectrode arrays

Microelectrode arrays (MEAs) with evenly distributed multiple sensor spots have been designed for specific applications. Using the MEAs, we determined the relative profiles of potassium channel openers (KCOs) on cultured embryonic Sprague-Dawley rat cardiac myocytes. KCO, pinacidil (PIN), cromakalim (CROM), SDZ PCO400 (SDZ), or its vehicle, was added to the myocytes cumulatively. The action potential signal shapes in the presence of PIN and SDZ show that the changes in voltage over time and the magnitudes of the associated voltage change were reduced concentration-dependently. CROM affected sodium influx more than PIN and SDZ. The comparisons of changes in the rate of beating and propagation speed in the presence of KCOs were made using their corresponding pD(2) values (the negative log of EC(50)). All KCOs caused concentration-dependent reductions in the rate of beating and propagation speed, with SDZ being the most potent. In addition to the signal shapes, rate of beating, and propagation speed, the origin of excitation and the excitation pattern inside the culture can be also extracted. The results show that the present system can differentiate the effects of different KCOs on myocytes. It might be possible to utilise the MEA as a means to classify drug action based upon a combined interpretation of the three different datasets gained from the extracellular recordings. The combination of these observations might be used as 'drug signatures' when profiling drugs in the future.

Sarcoplasmic ATP-sensitive potassium channel blocker HMR1098 protects the ischemic heart: implication of calcium, complex I, reactive oxygen species and mitochondrial ATP-sensitive potassium channel

The aim of this study was to investigate the effects of HMR1098, a selective blocker of sarcolemmal ATP-sensitive potassium channel (sarcK(ATP)), in Langendorff-perfused rat hearts submitted to ischemia and reperfusion. The recovery of heart hemodynamic and mitochondrial function, studied on skinned fibers, was analyzed after 30-min global ischemia followed by 20-min reperfusion. Infarct size was quantified on a regional ischemia model after 2-h reperfusion. We report that the perfusion of 10 microM HMR1098 before ischemia, delays the onset of ischemic contracture, improves recovery of cardiac function upon reperfusion, preserves the mitochondrial architecture, and finally decreases infarct size. This HMR1098-induced cardioprotection is prevented by 1 mM 2-mercaptopropionylglycine, an antioxidant, and by 100 nM nifedipine, an L-type calcium channel blocker. Concomitantly, it is shown that HMR1098 perfusion induces (i) a transient and specific inhibition of the respiratory chain complex I and, (ii) an increase in the averaged intracellular calcium concentration probed by the in situ measurement of indo-1 fluorescence. Finally, all the beneficial effects of HMR1098 were strongly inhibited by 5-hydroxydecanoate and abolished by glibenclamide, two mitoK(ATP) blockers. This study demonstrates that the HMR1098-induced cardioprotection occurs indirectly through extracellular calcium influx, respiratory chain complex inhibition, reactive oxygen species production and mitoK(ATP) opening. Taken together, these data suggest that a functional interaction between sarcK(ATP) and mitoK(ATP) exists in isolated rat heart ischemia model, which is mediated by extracellular calcium influx.

Cardiac ATP-sensitive K+ channels: regulation by intracellular nucleotides and K+ channel-opening drugs

ATP-sensitive K+ (KATP) channels are present at high density in membranes of cardiac cells where they regulate cardiac function during cellular metabolic impairment. KATP channels have been implicated in the shortening of the action potential duration and the cellular loss of K+ that occurs during metabolic inhibition. KATP channels have been associated with the cardioprotective mechanism of ischemia-related preconditioning. Intracellular ATP (ATPi) is the main regulator of KATP channels. ATPi has two functions: 1) to close the channel (ligand function) and 2) in the presence of Mg2+, to maintain the activity of KATP channels (presumably through an enzymatic reaction). KATP channel activity is modulated by intracellular nucleoside diphosphates that antagonize the ATPi-induced inhibition of channel opening or induce KATP channels to open. How nucleotides will affect KATP channels depends on the state of the channel. K+ channel-opening drugs are pharmacological agents that enhance KATP channel activity through different mechanisms and have great potential in the management of cardiovascular conditions. KATP channel activity is also modulated by neurohormones. Adenosine, through the activation of a GTP-binding protein, antagonizes the ATPi-induced channel closure. Understanding the molecular mechanisms that underlie KATP channel regulation should prove essential to further define the function of KATP channels and to elucidate the pharmacological regulation of this channel protein. Since the molecular structure of the KATP channel has now become available, it is anticipated that major progress in the KATP channel field will be achieved.