Medizinische Universität Graz Austria/Österreich - Forschungsportal - Medical University of Graz
Gewählte Publikation:
Scheruebel, S.
The Combined Effect of Ivabradine and LPS on the Human Pacemaker Current
Doktoratsstudium der Medizinischen Wissenschaft; Humanmedizin; [ Dissertation ] Medical University of Graz; 2014. pp. 120
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- Autor*innen der Med Uni Graz:
- Scherübel-Posch Susanne
- Betreuer*innen:
- Pelzmann Brigitte
- Zorn-Pauly Klaus
- Altmetrics:
- Abstract:
- Background; The human pacemaker current (carried by HCN channels), which is composed of the time- and voltage-independent component Iinst and the time- and voltage-dependent component If, plays a distinct role in the sinoatrial action potential. Through If-inhibition the steepness of the diastolic depolarization decreases and hence, the beating frequency is reduced. This knowledge is applied in the pharmaceutical agent ivabradine, a pure heart rate lowering agent selectively inhibiting If. Currently, the MODIfY-trial investigates potential benefits of heart rate reduction in septic patients, since a lower heart rate is associated with better survival rates. Sepsis can be triggered through lipopolysaccharides (LPS) which are part of the outer cell wall of gram-negative bacteria and can be found naturally in two forms: The S-form (S-LPS) contains all three possible moieties of the molecule, namely lipid A, oligo- and polsaccharides (O-chain). R-form LPS (R-LPS) lacks the O-chain. S-LPS, like ivabradine, inhibits If, but the mechanism of S-LPS action on HCN channels is not known - contrary to ivabradine, which is known to bind to the channel pore thus blocking If. Since both substances, S-LPS and ivabradine, decrease If, investigations on the combined action are of clinical and therapeutical relevance. Furthermore, the second component of the pacemaker current, Iinst, is likely to play a role in the generation and stabilization of pacemaker potentials. Therefore, effects of LPS and ivabradine (also in combination) on this current component are of importance too. Aims of the study; Investigation of 1) acute and chronic effects of S-LPS and R-LPS on If and Iinst, 2) the interaction between LPS and HCN channels, 3) the effect of ivabradine on both pacemaker current components under elevated endotoxin levels, 4) the combined effect of S-LPS and ivabradine on the beating rate in a computational model of a sinoatrial cell. Methods; Electrophysiological recordings were carried out in the whole-cell patch-clamp technique in human atrial cardiomyocytes isolated from the right atrial appendage. A dot blot was carried out to investigate the interaction between LPS and HCN proteins (isolated from mice hearts). Intracellular ivabradine concentrations were determined by HPLC analysis using isolated guinea pig myocytes. Obtained electrophysiological data were implemented in an updated computer model of a rabbit sinoatrial cell (Matlab). Key results; Only S-LPS significantly inhibited If (acute and chronic treatment, 10 µg/mL for both). Dot-blot analysis revealed that the O-chain moiety is necessary for endotoxin binding to HCN channels. In contrast to If, Iinst reduction was O-chain independent and occurred only under chronic LPS treatment. Ivabradine (1 µmol/L) inhibited If in LPS incubated cells to a lesser extent than in control cells. In case of S-LPS this was associated with a significantly lowered intracellular ivabradine concentration. Computational analyses revealed that ivabradine is still able to slow down the cellular sinoatrial pacemaking process under septic conditions. Conclusions; Experimental and modeling data indicate that ivabradine might represent a promising therapeutic approach for heart rate reduction in sepsis related diseases and that O-chain dependent binding of endotoxin to HCN channels is the primary mechanism of the S-LPS effect on If.