ATP dependent potassium current IK(ATP) in isolated human atrial and ventricular cardiomyocytes

The aim of this project is to analyze the ATP dependent potassium current I K(ATP) in human atrial and ventricular myocytes. Since there are species dependent differences in the electrophysiological characteristics of cardiomyocytes, studies on human cardiomyocytes are of particular interest. The function of the cardiac K(ATP) channel under physiological conditions is still not completely understood. However, its important contribution to the increase of outward K + conductance and shortening of the action potential duration during conditions of metabolic inhibition such as ischemia and hypoxia is well established. The physiological role of this action potential shortening is likely to decrease Ca2+ entry during the plateau phase of the action potential. This leads to a rapid decrease of contractile function, an energy sparing effect that reduces the rate of ATP depletion. In addition intracellular Ca2+ accumulation, eventually leading to cell death, is prevented. Moreover, I K(ATP) is generally believed to play an essential role in ischemic preconditioning (i.e. the effect that short periods of mild ischemia protect the heart against subsequent periods of long ischemia and arrhythmias). Human cardiomyocytes will be isolated from heart tissue obtained during open heart surgery and heart transplantation. Electrophysiological experiments will be performed with the patch clamp technique in the voltage clamp and current clamp configuration which allow to study transmembrane ionic currents and action potentials. All experiments will be performed at a physiological temperature allowing a more reliable determination of the role of I K(ATP) in the human heart. The research activities consist of three parts (i) analysis of the basic properties of I K(ATP) (e.g. effects of openers, blockers, metabolic inhibition); (ii) role of I K(ATP) in preconditioning (effects of factors involved in preconditioning on human I K(ATP); (iii) side effects of I K(ATP) blocking agents (sulfonylureas like glibenclamide). A more detailed knowledge of the characteristics of I K(ATP) in isolated human cardiomyocytes, of its intracellular regulation and its modification by blockers and agonists would be, without doubt, an important step to understanding of the role of I K(ATP) in the genesis and prevention of arrhytmias, in the human heart.

It was the aim of project P13111 MED to analyze the ATP dependent potassium current I K(ATP) in human atrial and ventricular cells. The results of these studies are published in two full papers (Schaffer et al., 1999 Brit. J. Pharmacol. 128: 1175 - 1180, Pelzmann et al., 2001 Naunyn-Schmiedeberg`s Arch Pharmacol. 363: 125-132), a further paper is submitted for publication (Zorn-Pauly et al., 2002), several findings are published at present in abstract form and were presented as oral communications or posters at several international conferences. A publication comparing I K(ATP) in human and guinea pig ventricular myocytes is at present in preparation. One of the main results of our studies (Schaffer et al., 1999) is that the sulfonylurea glibenclamide (GLIB) which is commonly used for treatment of non-insulin dependent diabetes mellitus (NIDDM) in human atrial myocytes not only blocks specifically cardiac I K(ATP) but also the transient outward current I to1 and the ultrarapid delayed rectifier current I Kur . In human ventricular myocytes a similar inhibition of I to1 was seen. In principle these findings could have implications for patients treated with GLIB. Inhibition of I to1 and I Kur occurred at concentrations above the therapeutical plasma level, but may play a role in iatrogenic induction of long QT- syndrome under certain conditions. In another study (Pelzmann et al., 2001) the electrophysiological characteristics of the ATP-dependent potassium current I K(ATP) and the effects of the potassium channel openers cromakalim and pinacidil were studied in human atrial myocytes. I K(ATP) did not contribute to basal electrical activity in these cells. Cromakalim and pinacidil activated I K(ATP) concentration dependently and it could be shown that that repetitive exposure to pinacidil potentiated the activation of I K(ATP). The study of ionic currents linked to cell metabolism like I K(ATP) raises the question on possible changes of ionic currents related to cell culture duration and metabolic state. A study performed on calcium currents in guinea pig ventricular myocytes showed that between day 0 and 5 of culture I Ca,T was significantly reduced whereas peak I Ca,L revealed no significant difference. The data were presented at the Annual Meeting of the Biophysical Society in Boston, MA, USA and were summarized in a paper which has been submitted for publication (Zorn-Pauly et al., 2001). In his doctoral thesis Klaus Zorn-Pauly established mathematical models of electrical excitability of cardiac myocytes (c.f. Luo & Rudy, 1994) which yield important information on the interplay of ion currents underlying the cardiac excitation process.