The inotropic consequences of selective Na+/Ca2+ exchanger inhibition is controlled by the actual transport balance

Abstract

The major goal of the cardiac Ca2+ handling is to control the actual magnitude and kinetics of the cell contractions and therefore contribute to the regulation of the cardiac output. The Na+/Ca2+ exchanger has a crucial role in the beat-to-beat Ca2+ balance by extruding a relatively small fraction of the released Ca2+. Since the exchanger works Ca2+ influx, as well as Ca2+ efflux mode in the same heart cycle, its inhibition theoretically could lead to Ca2+ loss or Ca2+ gain. The functional consequence of these effects could be manifested in negative inotropy which may have antiarrhythmic effects during Ca2+ overload, or could cause positive inotropy which is desired in heart failure. However the exact electrophysiological mechanism which determines these two outcomes is not clarified because of the lack of selective inhibitors. In this thesis we would like to address these issues by using novel, selective NCX compounds. The main results can be summarized as follows: 1) The novel NCX inhibitor GYKB-6635 effectively suppressed both mode of the NCX current measured by conventional ramp protocol. Our study showed the GYKB-6635 did not influence the kinetics of the L-type Ca2+ current and major K+-currents therefore it could be considered a promising experimental tool for future NCX research. 2) The selective NCX inhibition is able to cause both positive and negative inotropy by ORM-10962 in the cardiac cells, depending on the experimental condition. When the reverse mode is facilitated Ca2+ loss, in the case of forward mode enhancement Ca2+ gain occurs. The major underlying mechanism is the actual Ca2+ level of the sarcoplasmic reticulum which strongly depends on the function of the NCX. 3) The selective NCX inhibition by ORM-10962 reverts the hypokalaemia induced positive inotropy: the low [K+]o increases the intracellular Na+ level of the cells which shifts the actual reversal potential of the NCX facilitating reverse mode. The selective NCX inhibition may inhibit preferentially the reverse mode of the NCX under this setting which decreases the intracellular Ca2+ and cell shortening.