Question: 

Depressed heart rate variability (HRV) is a hallmark of severe sepsis. It was speculated that it may partially be explained by an impairment of cardiac pacemaker channel activity mediated by lipopolysaccharide (LPS). Therefore, we attempted to investigate the effect of LPS on pacemaker channel activity and the underlying molecular mechanisms.

Methods: 

We conducted electrophysiological experiments using a HEK293 cell line that stably expressed the human hyperpolarization-activated cyclic nucleotide-gated channel 2 (hHCN2).

Results and Conclusion: 

In whole-cell experiments, we showed that bath application of LPS reduces channel conductance and shifts voltage-dependence of channel activation to more negative potentials indicative of a reduction in channel activity. In the cell-attached mode, activity of channels is recorded that are located within the patched membrane area. These channels are protected from the bath solution by the patch pipette. In this configuration, LPS applied to the bath had no effect on hHCN2 channel activity. This indicates that modulation of channel activity requires a direct interaction of LPS with the channel protein and does not depend on activation of intracellular signaling cascades.

In order to elucidate the channel-interacting part of the LPS molecule, we also used mutant LPS as well as isolated Lipid A and O-chain. We found that only the full-length molecule was able to modulate hHCN2 channel activity. Thus, we conclude that the modulation of pacemaker channels by LPS critically depends on the presence of both the O-chains and Lipid A. Taken together, the mechanism of pacemaker channel modulation seems to be different from those responsible for the pro-inflammatory properties of LPS.